COST IMPACT OF THE NEHRP RECOMMENDED PROVISIONS ON THEDESIGN AND CONSTRUCTION OF BUILDINGS
STEPHEN F. WEBER
ABSTRACT
This paper provides some information on the approximate cost impacts resulting from implementation of the NEHRP (National Earthquake Hazards Reduction Program) Recommended Provisions (Building Seismic Safety Council 1984 a) and proposes research to obtain improved estimates of cost impacts. The information is derived from the 52 case studies of the Building Seismic Safety Council (BSSC) trial design program conducted in1983-84 and based on an amended version of the Applied Technology Council's Tentative Provisions for the Development of Seismic Regulations for Buildings (ATC Tentative Provisions). The NEHRP Recommended Provisions are the result of the revisions and amendments to the ATC Tentative Provisions that were recommended during the trial design program. For the 29 trial designs conducted in the 5 cities (Chicago, Ft. Worth, Memphis, New York, and St. Louis) whose local building codes currently have no seismic design provisions, the average projected increase in total building construction costs was 2.1 percent. For the 23 trial designs conducted in the 4 cities (Charleston, Los Angeles, Phoenix, and Seattle) whose local codes currently do have seismic design provisions, the average projected increase in total building construction costs was 0.9 percent. The average increase in cost for all 9 cities combined was 1.6 percent. Although these case study results cannot be directly projected to the U.S. building population, they do reflect the order of magnitude of the cost impacts.
INTRODUCTION 
This paper provides information on the approximate cost impacts resulting from implementation of the National Earthquake Hazards Reduction Program NEHRP Recommended Provisions and proposes research to obtain improved estimates of these cost impacts. The information presented here summarizes the results of 52 case studies which compared the costs of constructing the structural components of a wide variety of buildings designed according to two distinct criteria: (1) the prevailing local 
Dr. Weber is an Economist for the Center for Applied Mathematics, National Bureau of Standards, Gaithersburg, Maryland. He developed this paper for the BSSC Study of Societal Implications and presented this information at the BSSC meetings in Charleston, Memphis, St. Louis, and Seattle.
building code; and (2) a proposed set of improved seismic safety provisions similar to the NEHRP Recommended Provisions. Some of the case studies also compared the structural engineering design time required for the two design criteria. The case studies included multifamily residential, office, industrial, and commercial building designs in nine U.S. cities.
The case studies that serve as the primary data source for this paper are the result of the Building Seismic Safety Council (BSSC) trial design program that was conducted in 1983-84. This trial design program was established to evaluate the usability, technical validity, and cost impact of the application of a somewhat amended version the Applied Technology Council (ATC) Tentative Provisions for the Development of Seismic Regulations for Buildings. The NEHRP Recommended Provisions,  which currently are being balloted by the 8SSC membership, include additional amendments made in response to the results of the trial designprogram.1 It is important to note, therefore, that the trial design program data on potential cost impacts of seismic design summarized here are based on the amended Tentative Provisions and not directly on the NEHRP Recommended Provisions themselves and that, as noted by the BSSC: "Some buildings showing high cost impacts will be significantly affected by new amendments to the amended Tentative Provisions that should tend to reduce the impact (BSSC, 1984 b)."
The framework for selecting the specific building designs included in the trial design program is first described. The major factors considered in that selection framework include building occupancy type, structural system, number of stories, and the cities for which the designs were developed. The types of cost data reported by the participating engineering firms also are described. The cost impact data results of the trial designs then are presented in summary form by building occupancy type and by city as well as in detail for each of the four cities visited by the BSSC Committee on Societal Implications (Charles-
ton, South Carolina; Memphis, Tennessee; St. Louis, Missouri; and Seattle, Washington). In presenting the cost data, a distinction will be made between two separate cases: (I) building communities not currently using a seismic code of any kind (e.g., Memphis and St. Louis) and (2) building communities that currently are using a seismic code (e.g., Charleston and Seattle). The paper closes with some conclusions regarding the cost impact of seismic design and suggestions for further research.
DESCRIPTION OF THE TRIAL DESIGN DATA 
The construction cost impact of the amended Tentative Provisions generally depends on two major groups of factors: those related to characteristics of the building itself and those related to the location in which the building is to be constructed. The first group includes such 
See Volume 1, Overview of Phase I and 11, of the 1984 BSSC report, BSSC Program on Improved Seismic Safety Provisions, for a full description of the trial design effort.
factors as the planned occupancy of the building, the structural system used to support the building, the general shape of the building in terms of number of stories and floor plan, and the total size of the building. The second group includes such factors as the seismic hazard of the building site and the degree to which that hazard is reflected in the current local building code. Because each of these six cost impact factors can assume several different values, the number of potentially unique trial designs is very large indeed. A statistically valid experimental design that would adequately sample from each of these unique cases (combinations of cost impact factors) would have required a total sample size that was well beyond the budget and time available for the trial design program.
Framework for Selecting Trial Designs 
Because of the necessary limit on the number of trial designs, the case study approach was used as an alternative to statistical sampling. In order to make the case studies as representative as possible, a framework was developed distributing the trial designs over the broad range of values for each of the cost impact factors mentioned above. This overall framework used for selecting the specific building designs included in the trial design program is best illustrated by referring to Table 1. Beginning with the left-hand column, there are four types of building occupancy included in the framework: residential, office industrial, and commercial. As the next four columns show, the structural system was divided into four elements, each of which has a number of different types: vertical load system, seismic resisting system components, other vertical components, and floor or roof components. For example, the vertical load system could use either bearing walls or a complete vertical load carrying frame. The method of resisting seismic forces could employ such systems as plywood walls, concrete masonry walls, brick walls, precast concrete walls, reinforced concrete shear walls, prestressed moment frame, or steel braced frame. The number of stories varied from single-story to a high-rise building with 40 stories. Between these extremes there were buildings with 2, 3, 5, 10,20, and 30 stories. As indicated in the far right-hand columns, the trial designs were distributed over nine cities: Los Angeles, Seattle, Memphis, Phoenix, New York, Chicago, Ft. Worth, Charleston, and St. Louis. These cities cover the range of seismic hazard levels found in the United States and they vary in the degree to which seismic provisions are contained in their local building code. For example, Los Angeles is in a very high seismic hazard area while New York City is in a low hazard area. Similarly, Seattle has adopted the Uniform Building Code (1979) seismic provisions while the city of Memphis, although exposed to considerable seismic hazard, has no seismic provisions in its building code.
There are a total of 468 possible combinations of the 9 cities with the 52 building types. Each of these combinations constituted a potential candidate for inclusion in the trial design program. Each candidate is represented by one of the cells in the nine columns on the right-hand side of Table 1. From all these potential candidates, 46 were selected as the building design/city combinations used in the trial design program. These selected combinations are represented by dots that appear in the cells of Table 1. For 6 of these 46 buildings, alternative designs were also developed to provide 6 additional cost impact estimates. As a result, there are 52 data points for which cost impact estimates are available.
For each of the 52 building designs included in the trial design program,
a set of building requirements or general specifications was developed and provided to the responsible design engineering firm. An example of such building requirements specifications is presented in Table 2. Within these requirements designers were given latitude to assure that building design parameters such as bay size were compatible with local construction practice. The designers were not permitted, however, to change the basic structural type. For example, they could not change from a reinforced concrete frame system specified in the building requirements to a reinforced concrete shear wall system. Such changes were not permitted even if an alternative structural type would have cost less under the amended Tentative Provisions than the specified type. This constraint may have prevented the designer from selecting the most economical system for the amended Tentative Provisions, and consequently may have resulted in overestimates of the cost impacts for some of the trial designs. The 17 design firms involved in the trial design program and the building designs for which each was responsible are identified by city in Table 3.
Data Reported for Trial Designs 
For each of the trial designs, the engineering firms developed two individual designs for the structural components of the buildings. One design was based on the prevailing local building code and the other was based on the amended Tentative Provisions for the city in which the building was to be located. The former will be referred to as the Local Code Design and the latter,-will be referred to as the Tentative Provisions Design. Both of these designs are described in considerable detail for each trial design in the engineering reports submitted by the firms(BSSC, 1984c). It should be noted that only structural components were included in the analysis for the 52 trial designs summarized here. Consequently, the Tentative Provisions Design did not include those requirements for nonstructural elements described in Chapter 8 of the amended Tentative Provisions. The engineering reports also include detailed estimates of the construction costs for the structural components of each of the two designs (Local Code Design and Tentative Provisions Design). These cost estimates were derived using standard, nationally recognized cost estimating guides that take into account local cost factors. The estimates were made on the basis of current construction costs at the time the designs were completed, which ranged from early 1983 through the middle of 1984. The percentage differences in these structural component cost estimates for the two designs (i.e., cost of the Tentative Provisions Design minus cost of the Local Code Design divided by cost of the Local Code Design times 100) provide the1-5
TABLE 2 Typical Building Requirements 
* Plan Form -as per that shown for each building type
* Number of Stories -20
Clear Structural Height -11 feet except that: (a) the first story shall have a 20 -foot clear structural height, and (b) the clear structural height does not apply along the perimeter
* Plan Story Area -79500 to 25,000 sq ft
* Plan Aspect Ratio -1:1 to 2:1
* Bay Size -20 foot minimum dimension; 600 sq ft minimum area (mini-
mum bay size does not apply to perimeter column spacing)
* Roof -nominally flat but with a 1/4 in 12 slope for drainage
* Window Areas -30 to 40 percent of exterior wall areas
* Core Size -proportional to the building height
* Core Walls and Floors -include openings for doorways, stairs, and elevators; core wall may be structural
* Foundation Conditions -selected as representative of those that could be anticipated in the local, consistent for all designs, and included in design presentations
* Vertical Load Systems -complete vertical load-carrying frames 
* Seismic Resisting Systems Components -dual system -steel moment frame (Special) and braced frame 
* Other Vertical Components -steel framing
* Floor and Roof Components -steel beams and reinforced concrete slabs 
* Similarity should be maintained in paired studies, such as local requirements for live loads and assumed dead loads 
* Other -not applicable
Requirements vary with building type.
As defined in Chapter 2 amended Tentative Provisions.
TABLE 3 Design Firms and Types of Building Designs 
City/Design firm Type of Building/No.
Seattle 
Abam Engineers, Inc. 10-Story Steel Frame with RC Shear Wall (O)/S-24
Bruce C. Olsen 3-Story Wood with Plywood Walls(R)/S-1o I-Story Long Spa Steel, 30' Clear Height-MF and Braced Frames(1)/5-40
Skilling, Ward, Rogers,
Barkshire Los Angeles. B. Barnes & Associates Johnson & Nielsen Wheeler & Gray Phoenix Magadini-Alagia Associates Read, Jones,
Christoffersen Inc.
o 20-Story Steel Frame-Dual Special & Braced Frames (0)S-30o 3-Story Wood with Plywood Walls(R)LA-I:
o 1-Story Wood Frame with Precast Concrete Tilt-Up Walls (1)/LA-37,
o I-Story Steel with Moment and Braced Frames (1)LA-39 o 2-Story Steel Frame with RC Block Walls (C)/LA-41o 20-Story Steel Moment Frame with Shear Walls (Dual) (O)LA-34: 12-Story Reinforced Brick Bearing Wall with RC Slabs (R)LA-5o 5-Story RC Bearing Wall (R)/P-10o 20-Story RC Bearing Wall with Core Shear Walls (O)P-22 Eo 10-Story RC Frame (Ordinary)(0)/P-32o 3-Story RC Block Bearing Wall(R)/P-2o 5-Story RC Block Bearing Wall(R)/P-3o 1-Story Steel Frame with RC Block Shear Walls (I)/P-3541-7-e
TABLE 3 Continued 
City/Design Firm Allen & Hoshall, Inc.
Ellers, Oakley, Chester& Rike, Inc.
Type of Building/No.
o 5-Story Bearing Wall (R)M-8o I-Story Steel Frame with RC Ti It-Up Exterior Shear Walls (1)/M-38o 2-Story Steel Frame with Non-Bearing RC Block Walls (C)M-42
o 20-Story Steel Moment and Braced Frame with RC Floors (R)/M-14o 10-Story RC Moment Frame(Perimeter) (R)/M-18o 10-Story Steel Moment Frame(Special) with RC Slabs (O)/M-27Ft. Worth, Texas Datum-Moore Partnership 5-Story RC Block Walls with Pre-
stressed Slabs (R)/FW-3o 10-Story RC Frame with RC Shear Walls (R)FW-15o 5-Story Steel Moment Frame(O)FW-27ASt. Louis Theiss Engineering 10-Story Clay Brick Bearing Wall(R)/SL-5Ao 20-Story RC Frame with RC Shear Walls (R)SL-16o 5-Story Steel Frame with Braced Framed at Core (O)/SL-26A Chicago Alfred Benesche & Co.
Klein & Hoffman 3-Story Brick and RC Block Bearing Walls with Plywood Floor & Roof Diaphragms (R)/C-2Ao 20-Story RC Frame with RC Shear Walls (R)/C-16o 12-Story RC Bearing Wall (R)/C-9o Parametric Study of Steel Moment and/or Braced Frames (O)C-26, C-27, & C-30o I-Story Precase RC Bearing Walls with PC Double Tee Roof (I)/C-36A1-8
TABLE 3 Continued 
City/Design Firm Klein & Hoffman Type of Building/No.
o 12-Story RC Bearing Wall (R)/C-9o Parametric Study of Steel Moment and/or Braced Frames (O)/C-26, C-27, & C-30o I-Story Precast RC Bearing Walls with PC Double Tee Roof (I)/C-36ANew York City Weidlinger Associates Robertson and Fowler 12-Story Brick Bearing Wall(R)/NY-5o 30-Story RC Moment Frame and Non-
Bearing Shear Wall (Dual) (R)/NY
20Ao 10-Story RC Moment Frame (O)/NY-32o 20-Story RC Bearing Wall (O)/NY-22o 5-Story Steel Moment Frame (0)/NY
27Ao 30-Story Steel Moment Frame (0)/NY
28Ao 2-Story Steel Frame with RC Block Walls (I)/NY-41ACharleston* S.C.
Enright Associates 5-Story Brick and RC Block Bearing Walls (R)/CSC-6: o 10-Story Steel Frame with RC Shear1.Walls (0)/CSC-24o I-Story Steel Moment and Braced Frame (I)/CSC-39R = Residential0 = Office = Industrial = Commercial
primary raw data on which this paper is based. Because the focus of this paper is on percentage cost differences rather than absolute estimates, the slight changes in construction costs during the study period can be reasonably ignored.
In addition to the estimates of the construction costs for the structural components of the two designs, the engineering firms also submitted rough estimates of the additional design time that would be required to use the amended Tentative Provisions. Typically these estimates were reported as percentage changes in design time required for the structural components assuming the design engineer was already familiar with the amended Tentative Provisions. These design time cost percentage change estimates are also summarized below.
SUMMARY OF COST IMPACTS 
This section summarizes the cost impact data reported by the 17 design engineering firms that participated in the trial design program. The first subsection provides an overview of the construction cost impacts organized first by type of building occupancy and then by city. In the overview by city, the data are presented in two groups: cities not currently using any seismic provisions In their local building codes and cities currently using seismic provisions in their codes. The first subsection also summarizes the design time percentage change estimates provided by the engineering firms. The second subsection reports the construction cost impacts for each individual trial design in the four cities that were visited by the BSSC Committee on Societal Implications(Charleston, Memphis, St. Louis, and Seattle).
Overview of Cost Impacts 
Table 4 presents an overview of the construction cost impacts by type of building occupancy. The five classes of buildings were derived from the original four classes found in the framework for selecting trial designs by dividing the residential designs into low-rise (five stories or fewer) and high rise (more than five stories). Because only three of the office building designs have fewer than ten stories (and those three have five stories), the office building class is not divided. Similarly, all seven of the industrial building designs have just one story and the three commercial designs all have two stories. The third column in Table 4 presents the percentage change in construction costs for the structural components of the building, with the Local Code Design as the base, as estimated by the BSSC trial design engineering firms. As can be seen, the average change for the structural costs is 5.6 percent, with by far the largest change (11.2 percent) reported for the high-rise residential designs. This high average for residential buildings is significantly influenced by the extremely high estimates reported for four of these building designs: LAIB (17 percent); M14 (16 percent);
M18 (46 percent); and NY20A (20 percent).
TABLE 4 Percentage Changes in Structural Cost and Total Building Cost for the Trial Designs by Building Occupancy Type 
Building Number of Estimated Change In Projected Change Occupancy Designs Structural Cost (%)2 in Total Cost (7) Low-rise residential High-rise residential d Office Industrial Commercial 93.6120.73.311.2214.771.31.53 Average Percentage Change 0.55.61.75.61.6
Percentage change in structural construction cost from the local code to Amended Tentative Provisions, as estimated by the BSSC trial design engineering firms, 1983-1984.
Projected percentage change in total building construction cost from the local code to Amended Tentative Provisions, derived from estimated structural cost changes by using the following McGraw-Hill's, Dodge Construction Systems Cost (1984) data on structural cost as a percent of4----1 k... 1Aln nc *I..LQa I Liu I I Ul IdI Uul X.
Low-rise residential High-rise residential Office Industrial Commercial Five or fewer stories.
more than five stories.
18. 1%
30.0%
28. 17A-; 33.77.29.5%
The fourth column of Table 4 presents the projected percentage change in total building construction costs for each building occupancy type. These total cost changes were projected from the structural cost percentage. changes by using data on structural cost as a percentage share of total building cost for each building occupancy type. The percentage shares are based on data from McGraw-Hill's, Dodge Construction System Costs (1984), which reports the structural percentage share of total building cost for a large number of typical building designs. The shares for three of these typical building designs were averaged for each of the building occupancy types to derive the percentage shares used in Tables 4 and 5 and reported in the footnotes to the tables. The average projected change in the total construction cost over all 52 of the trial designs is 1.6 percent. The high-rise residential building designs have the highest total building cost impact with 3.3 percent, both because of the four outliers mentioned above and the relatively high structural percentage share used for this type of building (30.0 percent).
Table 5 presents the same type of data as Table 4 but reported for each city grouped according to whether the city currently has a seismic building code or not. As expected, the average estimated change in the structural cost is considerably higher (more than twice as high) for those cities with no seismic provisions in their local codes than for those with seismic provisions: 7.6 percent versus 3.1 percent. A similar relationship holds for the projected change in total building cost: 2.1 percent for cities without seismic provisions versus 0.9 percent for those already having some seismic provisions in their local codes.
Table 6 summarizes the estimates made by the engineering firms of the change in structural design time that is expected to be required once the firms are familiar with the amended Tentative Provisions. The 52responses are divided into the four categories: negligible change, positive but unspecified change, positive specified change, and negative specified change. The fourth category means that the amended Tentative Provisions, once adopted and familiar to the design firms, would require fewer design hours than the current codes do. The first response category of negligible change was the most common with 28 designs.
Detailed Cost Impacts for Selected Cities 
Tables 7 through 10 present the cost impact data for each of the individual trial designs in the four cities visited by the BSSC Committee on Societal Implications. The first two cities (presented in Tables 7 and8), Memphis and St. Louis, are examples of cities with no seismic provisions in their current building code even though the amended Tentative Provisions place them in relatively high seismic hazard zones. The last two cities (presented in Tables 9 and 10), Charleston and Seattle, are two examples of cities that do have seismic provisions in their local building codes. The point made in reference to Table 6 regarding greater cost impact for the cities without seismic codes can also be1-TABLE 5 Percentage Changes in Structural Cost and Total Building Cost for the Trial Designs, by City and City Group With and Without Seismic Provisions in Current Local Codes Number Of Estimated Change In Project Change in City Designs Structural Cost (7) Total Cost (7) cities Without Seismic Provisions Chicago 10 2.5 0.7 Fort Worth 3 6.1 1.5 Memphis 6 18.9 5.2 New York 7 7.3 2.1St. Louis 3 4.5 1.3 Average Percentage 7.6 2.1 ChangeCities With Seismic Provisions Charleston 3 -2.5 -0.6 Los Angeles 10 4.2 1.3 Phoenix 6 6.9 1.9 Seattle 4 -1.1 -0.3 Average Percentage 3.1 0.9 ChangeOverall Average Percentage Change 5.6 1.6! Percentage change in structural construction cost from the local code to the amended Tentative Provisions, as estimated by the BSSC Trial Design engineering firms, 1983-1984.
Projected percentage change. in total building construction cost from the local code to Amended Tentative Provisions, derived from estimated structural cost changes by using the following McGraw-Hi1I's,Dodge Construction Systems Costs (1984) data on structural cost as percent of total building costs:
Low-Rise Residential 18.1%
High-Rise Residential 30.0%
Office 28.1%
Industrial 33.7%
Commercial 29.5%
TABLE 6 Possible Effects of the Amended Tentative Provisions on Structural Engineering Design Time as Reported by the Trial Design Firms ao For these 28 building designs negligible change was reported:
LAI, SI, P2, P3, LA5, SL5A, CSC6, C9, P10, LA15, FW15, SL16, LA18,
NY20a, 524, CSC24, SL26A, LA27, FW27A, NY28A, NY32, P35, C36A, LA37,
CSC39, S40, LA41o For these 11 building designs positive but unspecified change was reported:
C2A, FW3, NY5, C26A, C26, C27, C27A, S30, C30A, C30, NY41Ao For these 11 building designs positive specified change ranging from 5% to 50% was reported:
M8, M14, C16, MIS, P22, NY22, M27, NY27A, P32, M38, M42o For these 2 building designs negative specified change of -57,wasreported:
LA29, LA34
For descriptions of the individual building designs listed here, see Table 3.1-
TABLE 7 Design Description and Percentage Changes in Structural Cost and Total Building Cost for the Trial Designs of Memphis 
Design Structural Total Building Design Code Code Stories Cost Change (%)a Cost Change (M)a DescriptionM8 5M14 20M18 104.54.813.83.11.83.0Residential,
reinforced concrete wall and slab Residential,
steel frame/
moment frame,
composite floor Residential,
reinforced concrete .
moment frame,
flat plate Office, steel moment frame,
composite floor Industrial,
tilt-up shear wall, steel framing Masonry shear wall, steel framing
See note on Tables 4 and 5 for definition.
1-1525.016.046.0I 1.0M27M38M421025.410.0
TABLE 8 Design Description and Percentage Changes in Structural Cost and Total Building Cost for the Trial Designs of St. Louis Design Structural Total Building Design Code Stories Cost Change (%)a Cost Change (%)_ Description SL5A 10 6.0 1.8 Residential, masonry walls,
reinforced concrete slabSL16 20 3.8 1.1 Residential,
reinforced shear wall,
flat plate SL26A 5 3.6 1.0 Office, steel braced frame,
composite floor-See note on Tables 4 and 5 for definition.
TABLE 9 Design Description and Percentage Changes in Structural Cost and Total Building Cost for the Trial Designs of Charleston, S. C.
Design Structural Total Building Design Code Stories Cost Change (%)a Cost Change (%) DescriptionCSC6 5 -3.5 -0.6 Residential,
masonry walls,
steel joists CSC24 10 -4.0 -1.1 Office, rein-
forced concrete shear wall 1,
composite floor CSC39 I 0.0 Industrial,
steel braced frame/moment frame_See note on Tables 4 and 5 for definition.
TABLE 10 Design Description and Percentage Changes in Structural Cost and Total Building Cost for the Trial Designs of Seattle Design Structural Total Building Design Code Stories Cost Change (%)a Cost Change (%)b Descriptions 3 -1.1 -0.2 Residential,
wood frame,
plywood Diaphragms 524 10 -4.6 -1.3 Office, rein-
forced concrete shear wall,
composite floor S30 20 1.3 0.4 Office, dual steel braced frame/moment frame, composite floor 540 1 0.0 0.0 Industrial,
steel braced frame /mr4nentframe(metal building)
See note on Tables 4 and 5 for definition.
made here by comparing the average projected change in total building costs for Memphis (the highest at 5.2 percent) and St. Louis (1.3 percent) with the corresponding percentages for Charleston and Seattle (both negative).
CONCLUSIONS AND SUGGESTIONS FOR FURTHER RESEARCH 
The results of the BSSC trial design program presented here provide some Idea of the approximate cost impacts expected from implementation of the NEHRP Recommended Provisions. For the 29 trial designs conducted in the 5 cities (Chicago, Ft. Worth, Memphis, New York, 'and St. Louis) whose local building codes currently have no seismic design provisions, the average projected increase in total building construction costs was2.1 percent. For the 23 trial designs conducted in the 4 cities (Charleston, Los Angeles, Phoenix, and Seattle) whose local codes currently do have seismic design provisions, the average projected increase in total building construction costs was 0.9 percent. The average increase in costs for all 9 cities combined was 1.6 percent. Although these case study results cannot be directly projected to the U.S. building population, they do reflect the order of magnitude of the cost impacts of the NEHRP Recommended Provisions.
In spite of the limited sample size of the trial design program, these data do offer several avenues for further research. The first is an analysis of variance test to see whether the difference in the cost impact estimates for the cities with and without current seismic provisions is statistically significant. Because of the rather large variance in the cost impact estimates, it may be that the difference between the two categories (2.1 percent versus 0.9 percent) is not significant. Other analyses could be conducted to see whether the factors such as building occupancy type and number of levels have a significant effect on the cost impact estimates.
Another major effort could be undertaken to normalize the data by controlling for the effect of the local seismic hazard and the presence of seismic provisions in the current code from city to city. If a seismic design value could be established for the Local Code Design cases that is comparable (i.e., on the same numeric scale) to the Seismic Design Coefficient used in the amended Tentative Provisions cases, then such a normalization could be accomplished. This would make possible the use of regression analysis techniques to develop a statistically valid method for estimating seismic design cost impacts for any city.
REFERENCES 
Building Seismic Safety Council. 1984a. NEHRP (National Earthquake Hazards Reduction Program) Recommended Provisions. Volume2 of BSSC Program on Improved Seismic Safety Provisions.
Washington, D.C.: Building Seismic Safety Council.
Building Seismic Safety Council'. 1984b. Overview of Phases I and 11.
Volume 1 of BSSC Program on Improved Seismic Safety Provisions. Washington, D.C.: Building Seismic Safety Council.
Building Seismic Safety Council. 1984c. Trial Designs.
BSSC Program on Improved Seismic Safety Provisions.
D.C.: Building Seismic Safety Council.
Volume 3 of Washington,

CURRENT PRACTICES IN EARTHQUAKE PREPAREDNESS AND MITIGATION FOR CRITICAL FACILITIES
JAMES E. BEAVERS
In this paper an attempt is made to briefly address the broad issues of earthquake preparedness and mitigation for critical facilities. Critical facilities considered herein are divided into two major groups: industrial and public.
Critical industrial facilities are defined as those facilities that, if damaged by an earthquake occurrence, could result in the release of substances harmful to the public, employees, or the environment or that could result in what owners consider as unacceptable financial losses. Examples of such facilities are nuclear power plants, chemical processing plants, research and development facilities, and high-technology manufacturing plants.
Critical public facilities are defined as those facilities that, if damaged by an earthquake occurrence, could result in large numbers of the public experiencing life, life-support systems, or financial losses.
Examples of such facilities are hospitals, schools, stadiums, fire stations, dams, and bridges.
CURRENT PRACTICES 
Practice vs. Hazard 
Current practice today is actually based on the perception of the earthquake hazard. All one has to do to recognize this is to compare earthquake design practice in the State of California to that in the State of Tennessee for example. In California, the perception Is that there is an earthquake hazard, rightfully so. As a result, there are uniformly accepted seismic preparedness and mitigating practices, primarily in the form of accepted seismic design codes. In Tennessee, the perceptions that there is no earthquake hazard, which is wrongfully so. As result, not only are there no uniform seismic preparedness and mitigating practices, they are virtually nonexistent.
Dr. Beavers is Manager, Civil and Architectural Engineering, at Martin Marietta Energy Systems, Inc., Oak Ridge, Tennessee. He presented this paper at the FEMA Earthquake Education Curriculum Workshop held at the National Emergency Training Center, Emmitsburg, Maryland, June 27-29,1984.2-1
Four Levels of Practle 
Regardless of the general perception of the earthquake hazard, todays practice in earthquake preparedness and mitigation for critical facilities from an engineering point of view can be divided into four general levels:
Level l--Complex earthquake hazard evaluation and facility seismic analysis and design as is conducted for nuclear power plants(U.S. Nuclear Regulatory Commission, 1975).
Level II--Earthquake hazard evaluation and seismic analysis and design as is conducted for an important chemical plant or, on occasion, possibly a hospital (Manrod et al., 1981).
Level III--Normal earthquake hazard evaluation and facilities analysis and design procedures as is conducted using the Uniform Building Code (UBC) or similar codes (International Conference of Building Officials, 1982; Structural Engineers Association of California, 1975).
Level IV--No earthquake hazard evaluation or facility seismic analysis or design provisions except for the inherent lateral resistance provided by wind analysis and design requirements.
Level I provides for a thorough evaluation of the earthquake hazard at the location of interest to the point of simulating the expected ground motions. The ground motions are then used as input to a rigorous seismic analysis of the facilities followed by detail design and documentation procedures. In many cases, Level I is considered as a very conservative approach to earthquake preparedness and mitigation.
Level II generally represents an adjusted medium between the approach in Level I and the approach used in Level III. The Applied Technology Council provisions (Applied Technology Council, 1978) represent a Level II approach for buildings. Manrod and co-workers (1981) discuss a Level11 approach for preparedness and mitigation of existing critical industrial facilities.
Unfortunately, the preparedness and mitigation actions taken for most structures built in the United States today, many of which may be considered critical, fall under Level IV.
Except in California and one or two other states, there are virtually no adopted earthquake hazard evaluation or seismic analysis and design guidelines or codes in the cities, counties, or municipalities.
Levels of Application vs. Critical Facilities 
All nuclear power plants being constructed today fall under the strict seismic evaluation, analysis, and design requirements set forth by the U.S. Nuclear Regulatory Commission identified herein as Level I. Other similar critical facilities, such as plutonium facilities, generally fall under the same requirements.
Chemical processing facilities, uranium enrichment facilities, and high technology manufacturing plants usually will fall into the Level III approach and, in some circumstances, Level II at the discretion of the owners--be they government or private industry. However, in many cases, using the minimum requirements of the UBC seismic design provision (the Level III application) may not be adequate for such facilities.
Critical public facilities such as dams and bridges may also fall under Level II and III seismic provisions depending upon the perceived earthquake hazard of the builder/owner. Schools, hospitals, fire stations, and stadiums will fall under the seismic provisions as described in either Level III or IV. Since the mid-1970s, most hospital designs fall under the Level III procedures. However, hospitals built before the mid-1970s and schools (except California), fire stations, and stadiums built today may actually fall under Level IV.
All critical facilities, as a minimum, should meet earthquake preparedness and mitigation requirements as defined in the UBC and, in many cases, go beyond the requirements of the UBC. However, as a cautionary note, It must be remembered when using the UBC, especially for industrial facilities, that it is a building code and judgment must be used where the code does not directly apply.
Today's Application 
Although it was stated above that most structures built in the United States today are not designed to earthquake preparedness and mitigation provisions (a Level IV approach), nor are such provisions required bylaw, a process is occurring in this country where such provision are being applied more and more each day. This process is happening because of the educational program occurring within the professional groups(engineers, architects, scientists, etc.) and the liability responsibilities of such professionals. For example, most engineers are now aware of the need for earthquake hazard preparedness and mitigation practices in the design of any new facility. Although no local enforcement codes may require such procedures, architects and engineers are acutely aware of recent decisions in the courts where following the minimum requirements of building codes is not justification for not using prudent engineering judgment. As a result, many architects and engineers are now applying earthquake hazard preparedness and mitigation provisions in their facility design. For critical facilities, architects and engineers usually have no trouble convincing the builder/owner of the necessity for such provisions and the builder/owner is willing to accept the additional costs. However, for noncritical facilities, it is extremely difficult for the engineer or architect to convince the builder/owner of the long-term cost benefit of applying such provisions, and in many cases, the builder/owner will refuse--creating a professional dilemma for the architect or engineer.
TODAY'S TECHNOLOGY 
Progress 
Today's technology can best be described as a "forever changing state of the art." After each major earthquake, scientists and engineers seem to gain new insights as to how earthquake ground-shaking occurs and how man-made structures respond. The state of the art has advanced tremendously during the past 20 years as a result of the 1964 Alaskan Earthquake, the 1971 San Fernando Earthquake, other large but less notable earthquakes (e.g., Coalinga 1983)9 engineers and scientists' success at obtaining instrumental recordings of earthquake motions and structural response, the "national" emphasis placed on understanding the earthquake phenomena to provide safe nuclear power plants, and the passage of the Earthquake Hazards Reduction Act of 1977.
The nuclear power industry can be contributed with being the catalyst that sparked a strong earthquake and earthquake engineering research program in the mid-1960s that may have peaked as we entered the 1980s.
Although a lot has been learned during the past 20 years, our current understanding of the earthquake phenomena and how man-made structures respond to such events still has many shortcomings.
Understanding the Problem 
We now understand the general phenomena of what causes earthquakes based on the concept of plate tectonics. This concept applies very well on the West Coast of the United States. However, understanding the concept of earthquake occurrences at intra-plate locations like the Midwestern and eastern parts of the United States is extremely lacking. The lack of understanding can be based on two primary reasons: infrequent earthquake occurrences and earthquake occurrences at depth with no surface faulting. We do know enough about intra-plate earthquakes to know that the same design and analysis principles that are used on the West Coast may not be directly applicable in the Midwest and East because of the infrequency of such events and the attenuation rates.
From a purely engineering point of view, a such high state of technology exists regarding our ability to analyze complex structures to great detail. The phenomenal growth of the computer industry has provided us with this capability. However, our understanding of material properties and our ability to construct structures to such precise detail is far behind. In fact, our ability to analyze and design structures to earthquake ground motions far exceeds our ability to understand what the motions might be.
PRACTICE KEEPING PACE WITH TECHNOLOGY 
Lag Times 
As engineers and scientists learn more about preparedness and mitigation of the earthquake hazard and our development of technology, they begin the process of adopting this new found knowledge to practice. Like any industry, when trying to put new technology into practice, there is alag time. However, in the case of nuclear power plants where the Level I approach to preparedness and mitigation occurs, technology has been placed directly into practice with little or no lag time. The Level approach to preparedness and mitigation has been the leader of the earthquake industry." In the Level 11 approach9an assessment would be made of the new developments in the Level I approach and these developments would be either rejected or accepted as deemed appropriate and practical for the particular critical facility under consideration. For those developments deemed appropriate for a Level II application, the lag time was usually relatively short. Those developments not deemed appropriate for a Level II application have been put aside--it may take years before such developments become practice.
The lag time in getting new developments into practice at the Level III stage of application usually is several years unless the development results in the awareness of a serious deficiency in the Level III approach. Even then it would probably take one or two years to get the code bodies changed.
Dynamic Analysis--Practice 
As an example of the difficulty of taking technological development and applying it to practice, let's consider the case of dynamic analysis. Dynamic analysis capability has been around for 30 years and engineers recognize that structures subjected to earthquake loads are more properly analyzed using some form of dynamic analysis. But in the UBC, which is an accepted nationwide Level III type application, there are no provisions for such analyses. This exists for several reasons including, for example, perceived added costs of doing such analyses which are more complex than a simple static analysis, an undergraduate engineering educational level that does not require a dynamic-analysis background(reserving it for graduate students), perceived low earthquake hazards by engineers and the public, and the tendency to keep legislated codes as simple as possible in an attempt to insure more uniform application of such requirements.
Applied Technology Council 
In an attempt to overcome the obstacles to placing current technology into the hands of practice in as practical a way as possible, the Applied Technology Council (1978) developed the Tentative Provisions for the Development of Seismic Regulations for Buildings. This effort began in the early 1970s and when the result was published in 1978, it represented a very good recommendation for earthquake technology transfer to practice. Excellent work is still going on to substantiate and justify the cost benefits of this technology transfer. However, except for isolated cases on a voluntary basis, none of this technology transfer has actually occurred.
EXISTING CRITICAL FACILITIES 
Although earthquake hazard preparedness and mitigation practices have been occurring for new critical facilities during recent years, very little has been done to retrofit existing critical facilities. Most owners are not willing to provide the funds to retrofit such facilities because of the high cost involved. The high costs occur when the retrofit requirements are based on bringing the existing facilities under total compliance of a Level 1, II, or III approach.
To avoid the high costs of total retrofit, much can still be done in costing critical facilities to minimize the earthquake risks. For example, anchoring equipment and piping systems in existing facilities is an effective way to conduct earthquake hazard preparedness and mitigation procedure.
TECHNOLOGY TRANSFER COMMITMENTS 
Several technology initiatives could be developed for the transfer of earthquake hazard preparedness and mitigation technology to practice. However, to be successful, several commitments must be made.
There must be a commitment by government, industry, and the public to appropriate the funds required for such initiatives. In addition, the public, industrial and government managers, and political representatives must have a reasonable understanding of what the earthquake hazards are in their area of concern. As stated earlier, the problem here is that other than in, say, California, the earthquake hazard is perceived by these groups to be no hazard. The professional groups--architects, engineers, and scientists--must do their utmost to understand the earthquake hazard and develop proper preparedness and mitigation procedures--technology transferred to practice. The political and industrial communities must be committed to support and promote the initiatives.
For critical industrial facilities, today's social and political environment in the United States is very conductive for obtaining the commitment of the public and the political community. To get the same level of commitment for many critical public facilities is, and will be, considerably more difficult and will not occur until the public has some understanding of the earthquake hazard. However, because critical faci1ities are "critical," there is an ever-increasing commitment by architects, engineers, builders, and owners to transfer today's earthquake technology to practice.
SUMMARY 
Although scientists and engineers continue to strive for a better understanding of earthquake hazard preparedness and mitigation, the technological state of the art seems far ahead of that technology, except for highly visible and critical facilities, used in current practice.
An education program involving all phases of training is needed. However, public information and awareness programs should be placed at the top of the list. Until the public has a better understanding of what the earthquake hazards are, progress toward earthquake preparedness and mitigation will be slow unless regulation occurs--and regulators arte public.
REFERENCES 
Applied Technology Council. 1978. Tentative Provisions for the Development of Seismic Regulations for Buildings. Washington, D.C.:
U.S. Government Printing Office.
International Conference of Building Officials. 1982. Uniform Building Code. Whittier, California: ICBO.
Manrod, W. E., W. J. Hall, and J. E. Beavers. 1981. "Seismic Evaluation Criteria for Existing Industrial Facilities." In Earthquakes and Earthquake Engineering: The Eastern United States, edited by. E. Beavers. Ann Arbor, Michigan: Ann Arbor Science Publishers.
Structural Engineers Association of California, Seismology Committee.
1975. Recommended Lateral Force Requirements and Commentary. Los Angeles, California: Structural Engineers Association of California.
U. S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation. 1975. Standard Review Plan. Washington, D.C.: U.S. Nuclear Regulatory Commission.

DEVELOPMENT OF SEISMIC SAFETY CODES
ROBERT M. DILLON, AIA, M.ASCE, A.AIC
The history of the codes and standards system in the United States is an interesting one; however, of greater importance in this context is what it can tell us about the likely future course of codes and standards development, and the wisdom of working within that system to effect nationwide change in building hazard mitigation practices.
The first model code, the National Building Code, was prepared in 1905by the National Board of Fire Underwriters, now the American Insurance Association. Concerned about the huge fire losses in American cities and towns, the Board drafted the code with the hope that it would be adopted into law by these cities and towns. Of course, the code dealt with more than fire safety, so it also held the promise of helping reduce the wide variations in the content of building codes--a problem that already was becoming apparent as community after community made a tailored response to perceived public health and safety needs and to public demands for such protection. As early as 1921, a U.S. Senate committee called attention to the high costs of construction that it felt were a consequence of the growing number of municipal codes and the lack of uniformity among those codes. Therefore, the lack of uniformity in building codes, as well as the extent and adequacy of their coverage, is hardly a new concern--just one that is rediscovered from time to time.
In 1927, the first edition of the Uniform Building Code was published by what today is the West Coast headquartered International Conference of Building Officials (ICBO).
In 1939, it was the U.S. National Bureau of Standards that issued are port calling for greater code uniformity. At the same time, it called for the use of nationally recognized building standards in building codes and for the development of means for the acceptance of new materials and methods--the concept of a total system for both regulation and the introduction of technology.
Following World War 11 (in 1946), the Southern Building Code Congress(SBCC), headquartered in Alabama, was formed and its model code, the Standard Building Code, was first published. Then, in 1950, the Building 
Mr. Dillon, AIA, M.ASCE, A.AIC, is Executive Vice President of the American Council for Construction Education, Washington, D.C. He presented this paper at the FEMA Earthquake Education Curriculum Workshop held at the National Emergency Training Center, Emmitsburg, Maryland, on June27-29, 1984.3-1
Officials and Code Administrators (BOCA)9 which was created in 1915 ands headquartered in Chicago, published its model code, the Basic Building Code.
There now were four model codes--the National Building Code, the Uniform Building Code, the Standard Building Code, and the Basic Building Code. The latter three were and are prepared by building officials with input from the building community.
The National Su Building Code was last revised in 1976, and in 1980, the National Conference of States on Building Codes and Standards--a body that received its impetus from the National Bureau of Standards--obtained the rights to the code and proposed to develop it as a consensus document In the manner of standards of the American Society for Testing and Materials (ASTM) and the American National Standards Institute(ANSI). Although the concept of a consensus code--as distant from a document produced building officials as the sole decision-makers--was lauded by many and a degree of progress was made In organizing for the task, the concern for the creation of yet another model code, just as it appeared that the number would be reduced to three, led to the ultimate abandonment of the effort. Today, BOCA has the rights to the national building code name.
The three model code bodies have been quite aggressive and competitive in seeking adoptions of their respective codes. Nevertheless, there still are communities across the country that have no code, particularly communities in rural and newly developing areas, and areas where the code treats only or principally facilities involving public use or occupancy. Also, many of the communities that have adopted one of the model codes have not done so without additions, deletions, and modifications--not infrequently, extensive such deviations. Further, not all codes are up to date by any means, which leads to even further lack of uniformity among various jurisdictions.
The difficulty was compounded by a move in the late 1960s and early1970s to foster more state rather than local codes--leaving us with a greater mixture of both. Finally, many of our nation's largest cities continue to have their own code. Thus, the dream of uniformity or, what is perhaps a better way of phrasing the need, harmony of provisions is far from a reality.
As early as 1949, the model code organizations, together with several national organizations such as ASTM, the American Insurance Association and the Underwriter's Laboratories, several federal agencies, and the National Research Council of Canada formed the Joint Committee on Building Codes (JC8C) to seek greater code uniformity. In 1959, the JCBC became the Model Codes Standardization Council (MCSC) and the design professions became advisory members. The MCSC was further expanded in1970 to include construction industry representatives, also as advisory members.
With all of this, progress was still painfully slow on the issue of uniformity and/or harmonization. The nation and building technology were growing rapidly and there still were strong feelings that codes were growing rapidly and there still were strong feelings that codes were a major deterrent to progress and a cause of increased building costs. As a result, Congress created the National Commission on Urban Problems--more popularly known as the Douglas Commission after its chairman, the late Senator Paul Douglas of Illinois. The Douglas Commission made a rather exhaustive study of the codes and standards situation across the United States. Its findings were detailed in a 1969 report,
and one of those findings was that an entirely new instrument was needed to address the problem--one that would have the backing of the Congress and the clear mission of bringing about a more rational and responsive building regulatory environment and a nationwide system for facilitating the introduction of new technology. The new instrument was designated the National Institute of Building Sciences (NIBS) by the Commission.
NIBS was a long time coming into being. Not only did the Congress have to be convinced that it was needed--particularly in the form of a private, nongovernmental body authorized by the Congress--but the many diverse and divided public and private interests in the building community itself had to be convinced that NIBS was necessary or at least worth a try.
It took from 1969 until 1974 to be authorized by the Congress, and untilmid-1976 for the President of the United States to appoint its first Board of Directors. NIBS received its first of five start-up capital appropriations from the Congress in late 1977 and effectively began operations at the beginning of 1978. And, during these years, the building community and the code bodies were not idle.
In 1972, the three model code bodies formed the Council of American Building Officials (CABO), and CABO in turn created the Board for the Coordination of Model Codes (BCMC) and the National Research Board (NRB)
to begin a process for reviewing and recognizing building products and systems. This was not the first effort made by the three model codes to find a way to work together but it has been the only one to have withstood the test of time to date. No doubt the creation of NIBS and the events that surrounded it provided considerable impetus to succeed.
One example of CABO achievements is that it succeeded in creating a one-and two-family dwelling code that, because of its adoption by reference by the three parent model code bodies, has become a nationwide model. It must be pointed out at this juncture, however, that there are few who are familiar with the regulatory scene in this country who would like to see a national model code--or, perhaps it would be more to the point to say that there are a few who would want to see a single national model code that could easily become a national building code by legislative action. The building community has gained a healthy respect for the value of divided authority whether private or public. This is not to say, however, that there is not a desire for greater harmonization of the provisions of both model and actual codes. The same can be said for working to eliminate needless overlap, duplication, and conflict among the standards referenced and available for referencing in codes.
For example, when NIBS recommended the gradual phasing-out of the HUD Minimum Property Standards in favor of an improved CABO One-and Two Family Dwelling Code for that type of housing and any of the three nationally recognized model codes or their equivalent for multifamily housing, a great opportunity was created for achieving increased harmonization of code provisions, at least in this one area of building regulation. Both HUD and CABO have followed through with this recommendation. Further, because the One-and Two-Family Dwelling Code process is more open to building community participation than is the case with the model codes themselves, there has been the opportunity to bring a diversity of building industry talents to bear on at least one area of model code formulation in a manner akin to that of voluntary consensus standards development.
With this gradual movement toward greater harmonization of the model codes, there also has been a gradual movement toward the adoption of these model codes by the nation's states and communities. However, it must be stressed again that adoptions are by no means universal and certainly not adoptions without modification; that most of the major cities continue to have a code that is in many ways unique to that city and reflective of its history and political character, that not all jurisdictions keep their codes up to date, and that appeals and resulting variances make it virtually impossible to be able to say that provisions that even appear to be the same are truly the same at any given point in time.
Therefore, with perhaps as many as 16,000 code issuing jurisdictions in the country, some at the state level, some at the local level and some at both, and with all of these forces at work, there remains a great deal of disharmony among the resulting codes and code provisions in force. It also is the case that many federal agencies have their own construction requirements which add to the lack of harmony. As an aside, the relatively recent action of the Office of Management and Budget in issuing a bulletin that calls upon all federal agencies to rely on voluntary consensus standards to the maximum extent possible is helping the cause of harmonization significantly.
It should be clear at this point that there is no one point of entry for effecting code changes even though input through the model code change process can have a significant effect on the whole of code practice. It always must be remembered that ultimately it is the body having political jurisdiction that must decide what performance level will besought and what specific requirements will be imposed to achieve that level of performance. This applies to the location, design, construction, and rehabilitation of its own facilities as well as to those under private ownership.
These decisions--that is, whether and how to provide protection against any potential natural or man-made destructive force--are political simply because determining the level of risk and the costs and benefits that are likely to flow from taking any given set of protective measures is so much a matter of judgment. The challenge to the professional community, then, is to provide, political decision-makers with ever more reliable information and recommendations to assist them in their awesome task of assessing the risks and establishing the costs and benefits of one decision over the other. This implies, of course, that the professional community will be able to reach a reasonable agreement on what information and recommendations are to be provided. And in this regard, the nation is at a turning point with regard to earthquake technology and its proper application.
Today, there is a major debate concerning how realistic the risk of damaging earthquakes is in much of the eastern two-thirds of the country and an even greater debate on what regulatory provisions can best address those perceived risks.
It is important to recognize that perhaps 80 percent of a building code is made up of reference standards or materials that have come from standards. In the United States, most of these standards are either voluntary consensus standards or industry standards; however, there continues to be reliance on a number of government standards as well, particularly standards promulgated by federal agencies for their own use or for regulatory purposes. Therefore, it is to these criteria and standards that one also must look if building practices are to be changed or influenced. It was not too many years ago that the sources of information and data on seismicity and seismic effects were numerous. Today, these sources are fewer.
At this point it might be best to refer to the June 1978 publication, Tentative Provisions for the Development of Seismic Regulations for Buildings, prepared by the Applied Technology Council of the Structural Engineers Association of California. Popularly known as ATC 3-06, this document has become the focus of proposed changes in seismic standards and codes because of its sponsorship by the National Science Foundation and wide participation by design professionals and representatives of code bodies, governmental agencies at all levels, and the materials industry.
The program effectively began with a workshop on disaster mitigation sponsored by NSF and the National Bureau of Standards (NBS) in Boulder, Colorado, in August 1972. Therefore, the current effort to upgrade disaster mitigation through improved codes and standards is already 12years old. After ATC 3-06 was published, there was much debate as to the appropriateness of some of the proposed provisions, as to the extent of the proposed application of the provisions, and as to the usefulness of the document itself for the purpose implied in its title--i.e., as provisions for regulatory purposes--because of its mixture of criteria, design procedures, and commentary. Actually, it is clearly stated in the foreword to the document that:
These provisions are tentative in nature. Their viability for the full range of applications should be established. We recommend this be done prior to their being used for regulatory purposes. Trial designs should be made for representative types of buildings from different areas of the country and detailed comparisons made with costs and hazard levels from existing design regulations.
Concern for a better way to assure consensus among all of the interested parties became a significant issue toward the end of the 1970s; therefore, in 1979, after much discussion among the key building community organizations and federal agencies, the Building Seismic Safety Council(BSSC) was created under the auspices of the aforementioned National Institute of Building Sciences. Today, BSSC operates within NIBS as an independent, voluntary body of some 58 separate organizations. The trial designs recommended by ATC are some 58 separate organizations. The trial designs recommended by ATC are well under way with funding by FEMA--indeed, the second series of these designs is now nearing completion. The next phase of the program will entail getting agreement of the members of the Council on any changes proposed by its committees as a result of previous balloting on the tentative provisions and any changes that seem needed as a result of the trial designs. Publication of the agreed upon seismic safety provisions will follow. It also will include an assessment of the socio-economic impact that could be expected as a consequence of implementing and utilizing the provisions, especially in communities east of the Rocky Mountains that to-date have been largely unconcerned with the seismic safety aspects of building design; a study of the likely impact of the provisions on building regulatory practices; and development of materials and plans for encouraging maximum use of the provisions. Next will come the arduous tasks of seeking changes in the model and actual codes and the appropriate reference standards and educating designers and other building community participants in their use. A good start on this latter task will already have been made because of the involvement of local firms across the country in the trial designs.
In the meantime, the federal government, working through an interagency committee, has been proceeding with applications for federal construction. And, it appears that the National Bureau of Standards, as the Secretariat for an American National Standards Institute standards committee known as A-58.1, already has introduced elements of ATC 3-06into the 1982 edition of A58.1. For example, the A58.1-1982 seismic zone maps--i.e., maps of the 50 states and Puerto Rico which identify geographic areas of differing earthquake hazard (from 0 to 4)--is derived from maps contained in ATC 3-06.
It appears likely that seismic design procedures will be considerably different if the current work stays on course. At present, the seismic force factors used In ANSI A58.1-1982 are quite similar to those used in the 1982 edition of the Uniform Building Code (UBC) and, because the UBC is the model code most used in the West where earthquakes of significant magnitude are a matter of fairly recent memory, the UBC is typically the most responsive to changes in earthquake engineering technology. The Standard Building Code (SBC) simply references the provisions of A58.1 and must be updated to reference new editions or to introduce other provisions. The lateral force factors in the Basic Building Code (BBC) are specified and are somewhat different from those in the UBC and A58.1-1982. The risk maps in the SBC and BBC are different than those in A59.1-1982. It might be reasoned that all of these standard reference works will come into greater harmony if not actually share the same provisions once the work of BSSC is finished and a reasonable consensus has been achieved on the seismic safety provisions thus recommended. However, even if this does occur, that is not to say that all states and communities will readily adopt the provisions appropriate to their area.
It does seem, however, that with the greater acceptance of decision-making processes such as those employed by the Building Seismic Safety Council and A58.1 (which deals with all dead, live, and environmental loads on buildings and not just earthquakes), the opportunity exists to influence those political bodies that ultimately must make the risk taking decisions in the areas of public health, safety, and welfare. By bringing together representatives of all vital interests and expertise, the likelihood of finding adequate authority outside the process to challenge the collective judgments of those involved decreases dramatically.
One would think that concern for the potentially devastating effects of earthquakes would engender an eagerness to apply the regulatory provisions offered by technical experts. This simply has not been the case. Regardless of what the technical experts say, the evidence has not been sufficient to convince a lay public that has never experienced an earthquake or is aware that there has not been an earthquake of significance in their area in recorded history that one of potentially devastating effect could occur tomorrow. And, perhaps more to the point, the lay public may not perceive the odds that such an earthquake will occur in their area during their lifetime to be great enough to justify spending large sums of public and/or private funds to provide or upgrade protection. A finding that the costs of providing adequate protection are minimal or within reason, would go a long way toward allaying these concerns--at least with new construction.
Unfortunately, much the same skepticism can be found with many design professionals and others directly involved with the building community who have never been taught seismic design and who are not required to possess such knowledge to be able to practice or fulfill their other roles in building. Such knowledge simply is of little use in an area where it is not needed for survival in the marketplace.
The answer to the question of whether there are problems that can be addressed by education, therefore, is a resounding yes. There is a big job of public education to be done. There is need to expand the education of building design professionals in seismic design practices.
There is need to educate all those who would participate in housing,
building, and planning on the state of the art in seismic technology.
And, there is need to continue to educate everyone on the importance of achieving a voluntary consensus--one that includes the executive branches of government--on the standards and regulatory provisions that are to be recommended to the appropriate legislative bodies.
It appears that the knowledge and tools will soon be ready for making the next step up on seismic building design, construction, and rehabilitation practice. What is needed is a game plan for bringing those tools into play in an atmosphere of rationality--something that has not been done too well in the building arena in the past. Experience has shown that once a change is perceived as desirable or possible by those directly involved, the federal government has all too frequently agreed to lead the charge--not in a studied manner but in a rush and with an outsized and often frantic program with unreal goals and timetables. I hope I have indicated that the building community and the body politic as it deals with housing, building, and planning issues simply does not respond well to this kind of pressure.
What usually happens after one of these frantic efforts has been tried and fails is that the legislators that voted the resources and the consumers that have been stimulated to great expectations either become convinced that one cannot get from here to there or simply fall back to sleep. The effort is aborted and the goal is farther from achievement than if the program had never been launched--witness Operation Breakthrough and the Building Energy Performance Standards.
A continuation of the cooperative program already under way, with a steady hand on the tiller, will undoubtedly prove in the long run to have been the best course to follow. The old adage "haste makes wastes"
certainly should not be forgotten in the case of the earthquake hazard reduction program. Its going well. Let's not break it.

THE EARTHQUAKE AT CHARLESTON IN 1886
G. A. BOLLINGER
At about 9:50 p.m. on August 31, 1886, a large earthquake occurred in Charleston, South Carolina. Its magnitude (1s) has been estimated at7.5, its modified Mercalli intensity (MMI) was X, and it was sensibly felt by people over an area of some 2 million square miles. There was extensive damage to the city of Charleston ($5 million in 1886 dollars) and death estimates ranged between 60 and 100 (1886 population density). In Milwaukee, Wisconsin, large buildings were shaken violently, windows were broken, and people fled into the streets. At Brooklyn, New York, buildings were also shaken to the extent that people were frightened; chandeliers rattled. On the sixth floor of a Chicago hotel, plastering was thrown from ceilings and guests were nauseated and fled the hotel in terror. The shock was felt as far away as Boston, Massachusetts; Bermuda; and Cuba.
The 1886 earthquake was certainly the largest known for the southeastern United States and one of the largest historic earthquakes in all of eastern North America. The following will first discuss three important factors that can be derived from consideration of the 1886 shock in the context of the historical seismicity of the region. Each of those factors then will be seen to have one or more important, associated questions. Finally, the physical effects from this large earthquake will be presented in some detail.
IMPORTANT FACTORS AND ASSOCIATED QUESTIONS 
The important factors are:
1. The fact that a magnitude 7.5 earthquake occurred in Charleston,
South Carolina, demonstrates the presence in the area of a seismogenic structure capable of generating such a shock. In principle, such a structure could occur elsewhere, but at the present time Charleston is the only locale in the Southeast that has its presence confirmed.
2. The earthquake activity in the eastern United States was at a much higher level prior to the turn of the century than it has been subsequently. In addition to the 1886 shock, there was a 
Dr. Bollinger is a member of the faculty of Virginia Polytechnic Institute and State University. He developed this paper for presentation at the BSSC Meeting in Charleston, South Carolina, on February 13, 1985. 
magnitude 5.7 (Ms) earthquake located in western Virginia in 1897 and a series of magnitude 8-8+ earthquakes in southern Missouri during 1811-1812. None of those three states, South Carolinas Virginia or Missouri, or their neighboring states has experienced such large shocks during the twentieth century. Thus, we have documentation that the level of earthquake energy release in the region can change with time.
3. The decrease of earthquake vibrations with increasing distance from an earthquake epicenter in the eastern United States has been shown by numerous studies during the past decade to be very slow, especially with respect to the western part of the country. What this means is that larger areas of structural damage and other earthquake effects can be expected in the East than in the West. The 1886 Charleston earthquake is a good example of those larger than average affected areas.
Some direct questions that follow from the above factors are:
1. Is the Charleston area the only area in the region capable of generating a 7.5 magnitude earthquake? The answer is that it probably is not since it is geologically reasonable for other such seismogenic structures to be present. Also, there are zones of persistent, low-level earthquake activity in the eastern United States. Those zones are candidates for larger shocks in the future.
2. Although the seismicity of the region is currently at a low level, is it going to continue that quiescence or are we in a lull before another period of increased earthquake occurrences?
3. Can the 1886 Charleston earthquake be used as a 'type example" of what to expect from a future occurrence of a large earthquake in the region? Yes, but the soil and bedrock geology are certainly different in the Appalachian highlands (Valley and Ridge and Blue Ridge provinces) than in the Atlantic Coastal area that was host to the 1886 shock. These differences as well as the difference in construction practices and materials between 1886 and 1985 need to be taken into account. The differences in type and degree of land utilization also are relevant.
The preceding questions cannot be answered in a deterministic fashion. We just do not have enough data of all kinds geologic, geophysical, seismological, and engineering to develop precise answers. What can be done, however, is to approach the problem from a probabilistic point of view. The U.S. Geological Survey (USGS) has been very active in such studies for the past decade. (For summary a overview of the USGS results see the paper by Walter W. Hays.)
DESCRIPTION OF THE EFFECTS FROM THE 1886 EARTHQUAKE 
Epicentral Region 
At least 80 kilometers of railroad track was seriously damaged and more than 1,300 km2 of extensive cratering and fissuring occurred as a result of the 1886 earthquake. In Charleston, the railroad-track damage and cratering were virtually absent, but many buildings on both good and poor ("made") ground were destroyed. Specifically, Dutton (1889) reports:
There was not a building in the city which had wholly escaped injury, and very few had escaped serious injury. The extent of the damage varied greatly, ranging from total demolition down to the loss of chimney tops and the dislodgement of more or less plastering. The number of buildings that were completely demolished and leveled to the ground was not great. But there were several hundred which lost a large portion of their walls. There were very many also which remained standing, but were so badly shattered that public safety required that they be pulled down altogether. There were not, so far as is at present known, a brick or stone building which was not more or less cracked, and in most of them the cracks were a permanent disfigurement and a source of danger or inconvenience. A majority of them, however, were susceptible to repair by means of long bolts and tie-rods.
Also see the reprint of USGS Professional Paper 1028 (1977) that concludes this paper.
At a Distance of 100 Kilometers (60 miles)
Most severely affected at this range from the epicenter of the 1886 shock were coastal locations such as Port Royal and Beaufort to the southwest and Georgetown to the northeast. At Port Royal (MMI of IX), the shock was described by the United Press as "very violent." Houses were moved on their foundations and people were thrown to the ground. At Beaufort (Associated Press) and Georgetown (Dr. M. S. Iseman, M.D.), both with an MMI of VIII, chimneys and chimney tops were thrown down, brick parapets were dislodged, and brick buildings "undulated." Residents fled their houses and remained in the streets and fields all night, many praying. At Beaufort, the Charleston Yearbook described the shock as very severe," lasting 30 seconds, cracking some large buildings, and causing a 2-foot depression over an area some 60 feet in circumference.
Noncoastal location such as Manning to the north and Orangeburg and Bamberg to the northwest were shaken at a MMI level of VII. All reported damage to brick houses and brick walls and the falling of plaster. The response of the populace at these northerly sites was also one of terror and many camped in the open air overnight.
At a Distance of 200 Kilometers (120 miles)
Reports from Augusta, Georgia, 200 kilometers from the epicenter, deal extensively with the response of the citizenry. The Savannah Morning News of September 2, 1886, gave a September I communication from Augusta citing: "...two ladies lie at the point of death from fright," "...an old lady died from fright," and "many ladies fainted and thousands of men were completely unnerved. The citizens remained in the streets all night."
The following paragraphs from Dutton (1889) comment on the pronounced psychological effects at Augusta as well as the structural damages suffered there:
Thus Augusta, in Georgia, just beyond the 100-mile circle, was shaken with great violence. Many buildings were seriously damaged. At the arsenal two heavy walled buildings used as officer's quarters were so badly shattered that reconstruction was necessary. Many cornices were dislodged and it is estimated that more than a thousand chimneys were overthrown. People residing in brick dwellings refused for several days to enter them and found lodgings in wooden houses or camped in the streets and gardens. So great was the alarm felt that business and society were for two days fully paralyzed as in Charleston. Everyone was in a state of apprehension that the worst was yet to come and the only thing to be thought of was safety. Indeed, among all the large cities of the South, the general tenor of the reports indicates that Augusta stands next to Charleston in respect to the degree of violence of the shocks and the consternation of the people.
Augusta is built in close proximity to the contact of the new and older strata, and starting from that city it will be of interest to follow this line of contact northeastward. In detail the course is more or less sinuous. A few miles to the northeast of Augusta is a little railway station named Langley, where a small tributary of the Savannah River has been dammed to secure water power. The ground in this neighborhood, which is a loose soil thinly covering harder rocks below, was in many places fissured by the earthquake and opened in many cracks, some of which were several inches in width. A number of large cracks passed through the dam, opening passage for the water in the reservoir, which quickly enlarged the fissures. The county below was quickly a flood. The railway track was swept (away], and before warning could be given a passenger train ran into the flood and upon the broken track, where it was wrecked, with some loss of life. In this neighborhood the towns of Bath, Graniteville, and Vaucluse, which stand upon outcrops of crystalline rocks, report shocks of very great severity. Still farther to the northeastward, Batesburg, Leesville, and Lexington give similar reports. Passing beyond Columbia along the same line of contact, we find reports of very violent shocks at Blythwood, Camden, Chesterfield, and Cheeraw.
The Savannah Morning News report also noted that "the most severe damage was done on the Sand Hills in Georgia and in Aiken County, South Carolina." Specific localities mentioned were Langley and Bath, just across the Savannah River from Augusta, some 10 kilometers to the east. At Langley, on the South Carolina Railroad, 24 kilometers (15 miles) from Augusta, Georgia, and 200 kilometers (125 miles) from Charleston, "the earthquake destroyed the mill dam and the water washed away the roadbed. A train dashed into the flood, and the engineer and fireman were drowned. The engine is now 40 feet under water."-Dutton (1889) reported: "Houses badly shaken and glasses broken; dams broke loose destroying 1,000 feet of railroad; terrible suffering among the inhabitants." An MMI of X is assigned to the Langley, South Carolina, locale (Bollinger and Stover, 1975).
At a Distance of 400 Kilometers (240 miles)
At an epicentral distance of 400 kilometers, the level of ground-shaking continued to cause panic among the people: "a state of terror and excitement; people left their houses and many stayed in the streets all night (Beaufort, North Carolina); "streets rapidly filled with people, screams of frightened persons could be heard" (Raleigh, North Carolina); "rushed frightened from their houses into the streets; terror-stricken men, women and children, in night dress, crowded the streets in a moment; a number of ladies fainted" (Asheville, North Carolina); and "people rushed into the streets in indescribable confusion, each looking for an explanation from the others; the streets at 10 o'clock are full of people, who fear to return to their houses" (Atlanta, Georgia).
Buildings and household items (mirrors, pictures, lamps, dishes, window glass, etc.) were shaken at a MMI level of VIII or less. Atlanta, in northern Georgia, reported one house (Marrietta Street) "shaken to pieces," all the chimneys fell from the six-story Construction building in the city, window glass was broken, chimneys were knocked down, and dishes and glasses were smashed to pieces. However, Valdosta, to the south southeast and near the Georgia-Florida border, reported only falling of plaster (MM1 VI).
Across the entire state of North Carolina, MMI effects ranged from V toV1I. Examples of the highest levels were seen at Beaufort on the coast, Raleigh in central North Carolina and Waynesville in the extreme southwestern part of the state. The seismic waves at those locations caused chimneys to be overthrown or have their tops shaken off, some walls to crack, plastering to be thrown down, buildings to rock, and some floors to break "loose from their supports." Additionally, church bells were rung, clocks stopped, mirrors and pictures were thrown from walls, and lamps were overturned. At Asheville, North Carolina, houses were violently shaken, but no buildings were "shaken down" (MMI of VI). In Black Mountain (20 kilometers to the east of Asheville), the vibrations were accompanied by loud explosive sounds and heavy rumblings, and large masses of rock were dislodged from several steep slopes and rolled into the valleys below.
THROUGHOUT THE COUNTRY The following pages are a reprint of a study of the effects of the 1886earthquake throughout the United States that was published in 1977 apart of Studies Related to the Charleston, South Carolina, Earthquake of 1886A Preliminary Report, USGS Professional Paper 1028, edited by Douglas W. Rankin (Washington, D.C.: U.S. Government Printing Office).
Reinterpretation of the Intensity Data for the 1886 Charleston, South Carolina, Earthquake 
By G. A. BOLLINGER
STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA,
EARTHQUAKE OF 1886-A PRELIMINARY REPORT 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 1028-B
CONTENTS 
Page 
Abstract 17
Introduction 17
Intensity effects in the epicentral region   18
Intensity effects throughout the country 22
Attenuation of intensity with epicentral distance 27
Magnitude estimate 29
Conclusions 31
References cited 31
ILLUSTRATIONS
Page
FIGURE 
1. Epicentral area maps for the 1886 Charleston, S.C., earthquake -20
2. Isoseismal map showing the State of South Carolina for the 1886 Charleston earthquake _22
3-5. Maps of the Eastern United States showing:
3. Distribution of intensity observations for the 1886 Charleston earthquake 23
4. Isoseismal map contoured to show the more localized variations in the reported intensities for the 1886 Charleston earthquake 24
5. Isoseismal map contoured to show the broad regional patterns of the reported intensities for the 1886 Charleston earthquake 25
6. Histogram showing distribution of intensity as a function of epicentral distance for the 1886 Charleston earthquake 28
7, 8. Graphs showing attenuation of intensity with epicentral distance for various fractiles of intensity at given distance intervals for the 1886 Charleston earthquake 28, 29
9. Histogram showing distribution of epicentral distances for given intensity levels of the 1886 Charleston earthquake 30
10. Graph showing body wave magnitude estimates for the 1886 Charleston earthquake based on Nettles technique 31
TABLES
Page
TABLE 1. Variation of intensity effects along the South Carolina Railroad 21
2. Number of intensity observations as a function of epicentral distance intervals for the 1886 Charleston, S.C., earthquake  2 74-8
STUDIES RELATED TO THE CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886-A PRELIMINARY REPORT
REINTERPRETATION OF THE INTENSITY DATA FOR THE1886 CHARLESTON, SOUTH CAROLINA, EARTHQUAKE 
By G. A. BOLLINGER
ABSTRACT 
1889, C. E. Dutton published all his basic intensity data for the 1886 Charleston, S.C., shock but did not list what intensity values he assigned to each report, nor did show the distribution of the locations of these data re-on his isoseismal map. The writer and two other seismologists have each independently evaluated Dutton's 1,300 reports (at least two of the three interpreters on intensity values for 90 percent of the reports), the consensus values were plotted and contoured. One was prepared on which contours emphasized the broad pattern of effects (with results similar to Dutton's); another map. was contoured to depict the more localized variations of intensity. As expected, the latter map shows detail in the 'epicentral region as well as in the field. In particular, intensity VI (Modified Mercalli MM)) effects are noted as far away as central Alabama and the Illinois-Kentucky-Tennessee border area. Dutton's low intensity zone" in West Virginia appears on both maps. A maximum MM intensity of X for the epicentral region IX for. Charleston appears to be appropriate. Epicentral included at least 80 km of railroad track seriously and more than 1,300 km D of extensive cratering and fissuring. In Charleston, the railroad-track damage and were virtually absent, whereas many, but not buildings on both good and poor ground were destroyed. .The epicentral distances to some 800 intensity-observation localities were measured, and the resulting data set was analyzed by least-square regression procedures. The attenuation equation derived is similar to others published for different parts of the eastern half of the United States. The technique of using intensity-distance pairs rather than maps. has the advantages, however, of completely bypassing the subjective contouring step in the data and of being able to specify the particular fractile the intensity data to be considered. When one uses intensities in the VI to X range, and their epicentral distances for this earthquake, body-magnitude estimates of 6.8 (Central United States in-velocity data published by Nuttli in 1976) and 7.1 (Western United States intensity-velocity data published In by Trifunac and Brady in 1975) are obtained.
INTRODUCTION
The problems associated with the description of ports seismic ground motion in a minor seismicity such as the Southeastern United States are well known. In that region, the largest events took place before instruments were available to record them, so map that only qualitative descriptions of their effects regional exist. During the past few decades, when instruments began to be used, no event having mb>5hasconsiderable taken place. Thus we have quantitative data only for far-small events, and we need to analyze the qualitative(data, which are all that is available for larger events. The purpose of this study is to review thoroughly isoseismal the data that do exist and to derive as much information as possible concerning regional seismic and ground motions. Fortunately, the largest earthquake effects known to have occurred in the region, the 1886damaged Charleston, S.C., earthquake, was well studied by cratering Dutton (1889) and his coworkers. An excellent suite most, of intensity information is thus available for that important earthquake. Secondly, the Worldwide Standard Seismograph Network (WWSSN) stations the Eastern United States provide data on the radiation from the regional earthquakes that have occurred since installation of the stations. Finally, intensity-particle-velocity relationships as well as isoseismal been proposed that can be utilized in an attempt to of synthesize the above data types. The initial part of this paper is concerned with a associated reevaluation of the intensity data for the 1886wave Charleston earthquake and the second part with consideration of the attenuation of intensity as distance from the epicenter increases. (The distance the epicenter is hereafter called epicentral distance.) The concluding section presents a magnitude estimate for the 1886 shock. This research was conducted while the author was on study-research leave with the U.S. Geological Survey (U.S.G.S.) in Golden, Colo. Thanks are extended to the members of the Survey, particularly Robin McGuire and David Perkins, for their many helpful discussions. Robin McGuire did the regression analysis presented in this paper, and Carl Stover pro-vided a plot program for the intensity data. Thanks are also due to Rutlage Brazee (National Oceanographic and Atmospheric Administration, N.O.A.A.) and Ruth Simon (U.S.G.S.) for interpreting the sizable amount of intensity data involved in this study. This research was sponsored in part by the National Science Foundation under grant No. DES 75-14691.
INTENSITY EFFECTSIN THE EPICENTRAL REGION
Dutton assigned an intensity X as the maximum epicentral intensity for the 1886 shock. He used the Rossi-Forel scale; conversion to the Modified Mercalli (MM) scale results in a X-XII value. However the revised edition (through 1970) of the "Earthquake History of the United States" (U.S. Environ-mental Data Service, 1973) downgraded Dutton's value to a IX-X (MM). Because of this revision, it is appropriate to compare the scale differences between these two intensity levels (IX and X) with the meizoseismal effects as presented by Dutton. Ground effects, such as cracks and fissures, and damage to structures increase from the intensity IX to the intensity X level, whereas damage to rails is first listed in the MM scale at the X level. Taken literally, rail damage is indicative of at least intensity-X-level shaking. Richter (1958, p. 138) also listed "Rails bent slightly" for the first time at intensity X. However, he instructed (p. 136) that, "Each effect is named: at that level of intensity at which it first appears frequently and characteristically. Each effect may be found less strongly, or. In fewer instances, at the next lower grade of intensity; more strongly or more often at the next higher grade." Thus, widespread damage to rails is a firm indicator of intensity-X shaking. In discussing building damage, it is convenient to use Richter's (1958, p. 136-137) masonry A, B, C, D classification:
-Masonry A. Good workmanship, mortar, and design: reinforced, especially laterally, and bound together by using steel, concrete, etc.; designed to resist lateral forces. 
Masonry B. Good workmanship and mortar: reinforced but not designed in detail to resist lateral forces.
Masonry C. Ordinary workmanship and mortar: no extreme weaknesses like failing to tie in at corners. but neither reinforced nor designed against horizontal forces.
Masonry D. Weak materials, such as adobe: poor mortar: low standards of workmanship; weak horizontally. 
At the IX level, masonry D structures are destroyed. masonry C structures are heavily damaged, some-times completely collapsed, and masonry B structures are seriously damaged. Frame structures, if not bolted, are shifted off their foundations and have their frames racked at IX-level shaking, whereas at intensity X most such structures are destroyed. Nearly complete destruction of buildings up to and including those in the masonry B class is a characteristic of the intensity-X level.
Only in Charleston do we have a valid sample of the range of structural damage caused by the 1886earthquake. It was the only nearby large city, and it contained structural classes up to the range between masonry C and masonry B. Many of the important public buildings, as well as mansions and churches, had thick walls of rough handmade bricks joined with an especially strong oyster-shell-lime mortar. The workmanship was described as excel-lent, but nowhere in Dutton's (1889) account is reference made to special reinforcement or design to resist lateral forces. Structures outside the Charleston area (as in Summerville, see p. 21) were built on piers, some 1-2 m (3-6 ft) high, thereby making the structures inverted pendulums. Dutton's report for Charleston indicates that although the damage was indeed extensive (see below), most masonry buildings and frame structures were not destroyed. This fact plus Dutton's report on the absence of rail damage and extensive ground effects in the Charleston area indicates an intensity level of IX. 
The following quotations from Dutton's report (1889, p. 248-249, 253) contain detailed descriptions of the structural damage in Charleston caused by the earthquake of 1886:
There wan not a building in the city which had wholly escaped injury, and very few had escaped serious injury. The extent of the damage varied greatly, ranging from total demolition down to the loss of chimney tops and the dislodgment of more or less plastering. The number of buildings which were completely demolished and leveled to the ground was not great. But there were several hundred which lost a large portion of their walls. There were very-many also which remained standing, but so badly shattered that public safety required that they should be pulled down altogether. There was not, so far as at present known, a brick or stone building which was not more or less cracked, and in most of them the cracks were a permanent disfigurement and a source of danger or inconvenience. A majority of them however were susceptible of repair by means of long bolts and tie-rods. But though the buildings might be made habitable and safe against any stresses that houses are liable to except fire and earthquake, the cracked walls, warped floors, distorted foundations, and patched plaster and stucco must remain as long as the buildings stand permanent eye-sores and sources of inconveniences. As soon as measures were taken to repair damages the amount of injury disclosed was greater than had at first appeared. Innumerable cracks which had before been unnoticed made their appearance. The bricks had "worked" in the embedding mortar and the mortar was disintegrated. The foundations were found to be badly shaken and their solidity was greatly impaired. Many buildings had suffered horizontal displacement; vertical supports were out of plumb; floors out of level; joints parted in the wood work; beams and joists badly wrenched and in some cases dislodged from their sockets. The wooden buildings in the northern part of the city usually exhibited externally few signs of the shaking they received except the loss of chimney tops. Some of them had been horizontally moved upon their brick foundations, but none were overthrown. Within these houses the injuries were of the same general nature as within those of brick, though upon the whole not quite so severe.
The amount of injury varied much in different sections of the city from causes which seem to be attributable to the varying nature of the ground. The peninsula included between the Cooper and Ashley Rivers, upon which Charleston is built, was originally an irregular tract of comparatively high and dry land, invaded at many points of its boundary by inlets of low swampy ground or salt marsh. These inlets, as the city grew, were gradually filled up so as to be on about the same level as the higher ground. As a general rule, though not without a considerable number of exceptions, the destruction was greater upon made ground than upon the original higher land. [p. 248-249] 
In truth, there was no street in Charleston which did not receive injuries more or less similar to those just described. To mention them in detail would be wearisome and to no purpose. The general nature of the destruction may be summed up in comparatively few words. The destruction was not of that sweeping and unmitigated order which has befallen other cities, and in which every structure built of material other than wood has been either leveled completely to the earth in a chaos of broken rubble, beams, tiles, and planking, or left in a condition practically no better. On the contrary, a great majority of houses were left in a condition shattered indeed, but still susceptible of being repaired. Undoubtedly there were very many which, if they alone had suffered, would never have been repaired at all, but would have been torn down and new structures built in their places; for no man likes to occupy a place of business which suffers by contrast with those of his equals. But when a common calamity falls upon all, and by its very magnitude and universality renders it difficult to procure the means of reconstruction and where thousands suffer much alike, his action will be different. Thus a very large number of buildings were repaired which, if the injuries to them had been exceptional misfortunes instead of part of a common disaster, would have been replaced by new structures. Instances of total demolition were not common.
This is probably due, in some measure, to the stronger and more enduring character of the buildings in comparison with the rubble and adobe work of those cities and villages which are famous chiefly for the calamities which have befallen them. Still the fact remains that the violence of the quaking at Charleston, as indicated by the havoc wrought, was decidedly less than that which has brought ruin to other localities. The number of houses which escaped very serious injuries to their walls was rather large; but few are known to have escaped minor damages, such as small cracks, the loss of plastering, and broken chimney tops. [p. 253]
Damage to the three railroad tracks that extend north, northwest, and southwest from Charleston began about 6 km (3.7 mi) northwest of the city and was extensive (fig. 1A). More than 80 km (62 mi) of these tracks was affected. The effects listed were lateral and vertical displacement, formation of shaped curves, and the longitudinal movement of hundreds of meters of track. A detailed listing of the effects along the South Carolina Railroad tracks, which run northwest from Charleston directly through the epicentral region, is given in table 1.
Ground cracks from which mud or sand are ejected and in which earthquake fountains or sand craters are formed begin on a small scale at intensity VIII, become notable at IX, and are large and spectacular phenomena at X (Richter, 1958, p. 139). The formation of sand craterlets and the ejection of sand were certainly widespread in the epicentral area of the 1886 earthquake. Many acres of ground were overflowed with sand, and craterlets as much as 6.4m (21 ft) across were formed. Dutton (1889, p. 281') wrote: "Indeed, the fissuring of the ground within certain limits may be stated to have been universal, while the extravasation of water was confined to certain belts. The area within which these fissures maybe said to have been a conspicuous and almost universal phenomenon may be roughly estimated at nearly 600 square miles [1,550 sq. km]." By comparison, the elliptical intensity-X contour suggested by the present study encloses an area of approximately 1,300 km.
The distribution of craterlets taken from Dutton (1889, pl. 28) is also shown in figure 1A. In a few localities, the water from the craters probably spouted to heights of 4.5-6 m (15-20 ft), as indicated by sand and mud on the limbs and foliage of trees overhanging the craters. 
Other ground effects indicating the intensity-level are fissures as much as a meter wide running parallel to canal and stream banks, and changes of 
STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886
EXPLANATION
Railroad track damaged Craterlet area X Building destroyed Chimney destroyed Marked horizontal displacement Middleton Place
FIGURE 1. Epicentral area maps for the 1886 Charleston, S.C., earthquake. A, This study. Dashed contour encloses intensity-X effects. B, Dutton's map and C, Sloan's map (modified from Dutton, 1889, pls. 26 and 27, respectively) show contours enclosing the highest intensity zone, although neither Dutton nor Sloan labeled his contours. Base map modified from Dutton (1889). Rivers flowing past the Charleston peninsula are the Ashley River flowing from the northwest and the Cooper River flowing from the north 
the water level in wells (Wood and Neuman, 1931). Dutton (1889, p. 298) reported that a series of wide cracks opened parallel to the Ashley River (see caption, fig. 1) and that the sliding of the bank riverward uprooted several large trees, which fell over into the water. His plate 23 shows a crack along the bank of the Ashley River about a meter wide and some tens of meters long across the field of view of the photograph.
In a belt of craterlets (trend N. 800 E., length -5 km) about 10 km (6.2 mi) southeast of Summerville, Sloan reported (Dutton, 1889, p. 297) that
TABLE 1.-Variation of intensity effects along the South Carolina Railroad [Based on Dutton. 1889. p. 282-287.Refer to fig. I for locations mentioned Distance from Charleston Effect.
<5.8 -<3.66 -Occasional cracks in ground;
marked disturbance of track or roadbed.
5.8-3.66-Rails notably bent and joints between rail opened.
5.8 8-3.66-5 -Ground cracks and small craterlets.
8 5 -Fishplates torn from fastenings by shearing of the bolts; joints between rails opened to 17.5 cm (7 in.).
9.6 -6 Joints opened, roadbed permanently depressed 15 cm(6 in.).
14.4  9 -Lateral displacements of the track more frequent and greater in amount: serious flexure in the track that caused a train to derail;
more and larger craterlets.
16  10 Craterlets seemed to be greater in size (as much as 6.4 m (21 ft) across)
and number; many acres overflowed with sand.
16-17.6 _ 10-11 _ Maximum distortions and dislocations of the track;
often displaced laterally and sometimes alternately depressed and elevated;
occasional severe lateral flexures of double curvature and great amount;
many hundreds of meters of track shoved bodily to the southeast; track parted longitudinally,
leaving gaps of 17.5 cm (7in.) between rail ends; 46cm (18 in.) depression or sink in roadbed over a18-rn (60-ft) length.
17.6-24 11-15 -Many lateral deflections of the rails.
24-25.6 15_16 -Epicentral area-a few wooden sheds with brick chimneys completely collapsed; railroad alignment distorted by flexures; elevations and depressions,
some of considerable amount, also produced.
29-30.6 18.5-19 Flexures in track, one in an8.8-m (29-ft) section of single rails had an S-shape and more than 30 cm (12in.) of distortion.
32 _-____ 20 " a still more complex flexure was found. Beneath it was a culvert which had been strained to the northwest and broken" (p. 286);
a long stretch of the roadbed and track distorted by many sinuous flexures of small amplitude.
TABLE 1.-Variation of intensity effects along the South Carolina Railroad-Continued Distance from Charleston Effects (km) (mi)
33.9 -21 Tracks distorted laterally and vertically for a considerable distance.
34.9 -21.66 At Summerville-many flexures, one of which was a sharp S-shape; broken culvert under tracks in a sharp double curvature.
35.4-44.3 22-27.5. Disturbance to track and roadbed diminishes rapidly.
44.3 27.5 -At Jedburg-a severe buckling of the track.
wells had been cracked in vertical planes from top to bottom, and that the wells had been almost universally disturbed, many overflowing and subsequently subsiding, others filling with sand or becoming muddy.
In Summerville, whose population at that time was about 2,000, the structures were supported on wood posts or brick piers 1-2 m high and, though especially susceptible to horizontal motions, the great majority did not fall. Rather, the posts and piers were driven into the soil so that many houses settled in an inclined position or were displaced as much as 5cm. Chimneys, which were constructed to be independent of the houses, generally had the part above the roofline dislodged and thrown to the ground. Below the roofs, many chimneys were crushed at their bases, both bricks and mortar being disintegrated and shattered, allowing the whole column to sink down through the floors. This absence of overturning in peered structures plus the nature of the damage to chimneys was interpreted by Dutton as evidence for predominantly vertical ground motions.
The preceding discussion indicates an intensity-Level of shaking in the epicentral area. Figure 1Adepicts the approximate extent of this region along with the locations of rail damage, craterlet areas, building damage, and areas of marked horizontal displacements. Dutton and his coworkers did not map the regions of pronounced vertical-motion effects, but they did emphasize the importance of these effects in the epicentral region. Also shown in figure1 (B and C) is the extent of the highest intensity zone, as given by Dutton and by Sloan. Because of the sparsely settled and swampy nature of the region, the meizoseismal area cannot be defined accurately.
STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886
INTENSITY EFFECTS THROUGHOUT THE COUNTRY 
Dutton (1889) published all his intensity reports, some 1,337, but he did not list the intensity values that he assigned to each report, nor did he show the location of the data points on his isoseismal map. By using the basic data at hand, a reevaluation was attempted to present another interpretation of the data (in the MM scale) and to determine whether additional information could be extracted concerning this important earthquake. The writer and two other seismologists (Rutlage Brazee, N.O.A.A., and Ruth Simon, U.S.G.S.) each independently evaluated Duttons intensity data listing according to the MM scale. For the resulting 1,047usable reports, ranging from MM level I to X, at least two of the three interpreters agreed on intensity values for 90 percent of the reports. As would be expected, most of the disagreement was found at the lower intensity levels(II-V). A full listing of the three independent intensity assignments for each location was made by Bollinger and Stover (1976).
The consensus values, or the average intensity values, in the 10 percent of the reports where all three interpreters disagreed were plotted at two different map scales and contoured (figs. 2-5). When multiple reports were involved, for example, those from cities, the highest of the intensity values obtained was assigned as the value for that location.
The greatest number of reports (178) for an individual State was from South Carolina. Figure 2 presents the writer's interpretation of these data, Even
FIGURE 2.-Isoseismal map showing the State of South Carolina for the 1886 Charleston earthquake. Intensity observations are indicated by Arabic numerals, and the contoured levels are shown by Roman numerals.
REINTERPRETATION OF THE INTENSITY DATA
FIGURE 3. Eastern United States showing the distribution of intensity observations for the 1886 Charleston earthquake. Solid circles indicate felt reports; small crosses indicate not-felt reports.
STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886
FIGURE 4.-Isoseismal map of the Eastern United States contoured to show the more localized variations in the reported intensities for the 1886 Charleston earthquake. Contoured intensity levels are shown by Arabic numerals.
REINTERPRETATION OF THE INTENSITY DATA
FIGURE 5.-Isoseismal map of the Eastern United States contoured to show the broad regional patterns of the reported intensities for the 1886 Charleston earthquake. Contoured intensity levels are shown in Roman numerals.
in contouring the mode of the intensity values, as I was done here, intensity effects vary considerably with epicentral distance within the State. In particular, two intensity-VI zones are shown that trend northeastward across the State and separate areas of intensity-VIII effects. Although some of this variation may be due to incomplete reporting and (or) population density, it seems more likely that the local effects of surficial geology, soils, and water table level are being seen. Interpreted literally, a very complex behavior of intensity is seen in the epicentral region.
The intensity data base and interpretive, isoseismal lines throughout the Eastern United States are shown in figures 3-5. In figure 4, the data are contoured to emphasize local variations, whereas figure5 depicts the broad regional pattern of effects. Richter (1958, p. 142-145), in discussing the problem of how to allow for or represent the effect of ground in drawing isoseismal lines, suggested that two isoseismal maps might be prepared. One map would show the actual observed intensities; the other map would show intensities inferred for typical or average ground. The procedure followed here was to contour the mode of the intensity values (figs. 2 and 4) so as to portray the observed intensities in a manner that emphasizes local variations. Those isoseismal lines were then subjectively smoothed to produce a second isoseismal map showing the regional pattern of effects (fig. 5). The two maps that result from this procedure seem to the writer to represent reasonable extremes in the interpretation of intensity data. The subjectivity always involved in the contouring of intensity data is well known to workers concerned with such efforts. The purpose of the dual presentation here is to emphasize this subjectivity and to point out that, depending on the application, one form may be more useful than the other. Both local and regional contouring interpretations are to be found in the literature for U.S. earthquakes.
Figures 4 and 5 show that a rather complex isoseismal pattern, including Dutton's low-intensity zone (epicentral distance = A_550 km (341 mi) in West Virginia, was present outside South Carolina. Intensity-VIII effects were observed at distances of250 km (150 mi) and intensity-VI effects were observed 1,000 km (620 mi) from Charleston. Individual reports, given below, are all paraphrased from Dutton (1889). They note what took place in areas affected by intensity VI (MM) or higher at epicentral distances greater than about 600 km (372mi). Some of these reports were ignored in the contouring shown in figure 4.
Intensity VI-VIII in Virginia (600 km (372 mi)):
Richmond (VIII)-Western part of the city: bricks shaken from houses, plaster and chimneys, thrown down, entire population in streets, people thrown from their feet; in other parts of the city, earthquake not generally felt on ground floors, but upper floors considerably shaken.
Charlottesville (VII)-Report that several chimneys were overthrown.
Ashcake (VI)-Piano and beds moved 15 cm (6in.); everything loose moved.
Danville (VI)-Bricks fell from chimneys, walls cracked, loose objects thrown down, a chandelier swung for 8 minutes after shocks.
Lynchburg (VI)-Bricks thrown from chimneys, walls cracked in several houses.
Intensity VII in eastern Kentucky and western West Virginia (A_650 km (404 mi)):
Ashland, Ky. (VIII)-Town fearfully shaken, several houses thrown down, three or four persons injured.
Charleston, W. Va.-"A number of chimneys toppled over" (p. 522).
Mouth of Pigeon, W. Va.-Chimneys toppled off to level of roofs, lamps broken, a house swayed violently.
Intensity VI in central Alabama (.-700 km (434 mi)):
Clanton (VII)-Water level rose in wells, some went dry and others flowed freely; plastering ruined.
Cullman-House wall cracked, lamp on table thrown over.
Gadsden-People ran from houses.
Tuscaloosa-Walls cracked, chimneys rocked, blinds shaken off, screaming women and children left houses.
Intensity VII in central Ohio (800 km (496 mi)):
Lancaster-Several chimneys toppled over, decorations shaken down, hundreds rushed to the streets.
Logan-Bricks knocked from chimney tops, houses shaken and rocked.
Intensity VI in southeastern Indiana and northern Kentucky (A_800 km (496 mi)):
Rising Sun, Ind.-Plaster dislodged, ornaments thrown down, glass broken.
Stanford, Ky.-Some plaster thrown down, hanging lamps swung 15 cm (6 in.).
REINTERPRETATION OF THE INTENSITY DATA 
Intensity VI in southern Illinois, eastern Tennessee, I and Kentucky (A_950 km (590mi)):
Cairo, III.-Broken windows, "houses settled considerably" (p. 430) in one section, ceiling cracked in post office.
Murphysboro, III.-Brick walls shook, firebell rang for a minute, suspended objects swung.
Milan, Tenn.-Cracked plaster, people sitting in chairs knocked over.
Clinton, Ky.-Some bricks fell from chimneys.
Intensity VI in central and western Indiana (1,000 km (620 mi)):
Indianapolis-Earthquake not felt on ground floors; part of a cornice displaced on one hotel, people prevented from writing at desks, clock in courthouse tower stopped, a lamp thrown from a mantle.
Terre Haute-Plaster dislodged, sleepers awakened; in Opera House, earthquake felt by a few on the ground floor, but swaying caused a panic in the upper galleries.
Madison-Several walls cracked, chandeliers swung.
Intensity VI in northern Illinois and Indiana (A_1,200 km (744 mi)):
Chicago, II1.-Plaster shaken from walls and ceilings in one building above the fourth floor; barometer at Signal Office "stood 0.01 inches higher than before the shock for eight minutes"
(p. 432); earthquake not felt in some parts of City Hall, especially noticeable in upper stories of tall buildings, not felt on streets and lower floors.
Valparaiso, Ind.-Plaster thrown down in hotel, chandeliers swung, windows cracked, pictures thrown from walls.
The preceding reports indicate that structural damage extended to epicentral distances of several hundred kilometers and that apparent long-period effects were present at distances exceeding 1,000 km(620 mi). Persons also frequently reported nausea at these greater distances.
Dutton apparently contoured his isoseismal map in a generalized manner, which is an entirely valid procedure. The rationale in that approach is to depict not the more local variations, as was presented in the above discussion, but rather the regional pattern of effects from the event. Figure 5 is the writer's attempt at that type of interpretation, and the resulting map is very similar to Dutton's.
ATTENUATION OF INTENSITY WITHEPICENTRAL DISTANCE 
The decrease of intensity with epicentral distance is influenced by such a multiplicity of factors that it is particularly difficult to measure. The initial task in any attenuation study is to specify the distance (or distance range) associated with a given intensity level. Common selections are: minimum, maximum, or average isoseismal contour distances or the radius of an equivalent area circle. In all these approaches, the original individual intensities are not considered; rather, isoseismal maps are used. Perhaps a better, but more laborious, procedure has been suggested by Perkins (oral commun., 1975), wherein the intensity distribution of observations is plotted for specific distance intervals. In this manner, all the, basic data are presented to the reader without interpretation by contouring. He is then in a position to know exactly how the data base is handled and thereby to judge more effectively the results that follow. Once the intensity-distance data are cast in this format, they are then also available for use in different applications.
The epicentral distances to some 800 different locations affected by the 1886 shock were measured and are listed in table 2. For these measurements, the center of the intensity X (fig. 1) area was assumed to be the epicenter. Figure 6 presents the resulting intensity distributions as functions of epicentral distance. The complexity present in the isoseismal maps (figs. 4 and 5) is now transformed to specific distances, and the difficulty of assigning a single distance or distance interval to a given intensity level is clearly shown. The approach followed here was to perform a regression analysis on the intensity-distance data set, using an equation of the form,
TABLE 2.-Number of intensity observations as a function of epicentral distance intervals for the 1886 Charleston, S. C., earthquake 
Epicentral Number distance 
STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886(16)
Observations (72)
Number of observations40 20 0(68) (95)
EPICENTRAL DISTANCE, IN KM 
FIGURE 6.-Distribution of intensity (Modified Mercalli, MM) as a function of epicentral distance (km) for the1886 Charleston earthquake. Intensity distribution is shown for specific distance intervals.
where a, b, c are constants, A is the epicentral distance in kilometers, Il is the epicentral intensity,
and I is the intensity at distance a. This equation form was selected because it has been found useful by other investigators (for example, Gupta and Nuttli, 1976). The resulting fit for the median, or50-percent fractile, was,
The standard deviation, between the observed and predicted intensities, is 1.2 intensity units for these data. For the 75-percent fractile, the a constant is 3.68; for the 90-percent fractile, the a constant is 4.39. The b term is very small and could perhaps be deleted, as it results in only half an intensity unit at 1,000 km. The minimum epicentral distance at which the equation is valid is probably10-20 km. The intensity-distance pairs extend to within only 50 I am of the center of the epicentral region, but that region (fig. 1) has a diameter of approximately 20 km.
The curves for the 50-, 75-, and 90-percent fractiles are shown in figures 7 and 8 along with other published intensity attenuation curves for the Central and Eastern United States. 
Isoseismal maps-CENTRAL AND EASTERN U.S.
Howell and Schultz (1975) NORTHEASTERN U.S.
Cornell and Merz (1974)
10 20 30 4050 100 200 300 500 EPICENTRAL DISTANCE. IN KM 
FIGURE 7.-Attenuation of intensity (MM) with epicentral distance (km) for various fractiles of intensity at given distance intervals for the 1886 Charleston earthquake (heavy solid curves). Attenuation functions by Howell and Schultz (1975), Gupta and Nuttli (1976), and Cornell and Merz (1974) are shown by light dashed curves.
REINTERPRETATION OF THE INTENSITY DATA_K=1-l_
50lLt10 20 30 4050 100 2EPICENTRALDISTA>\ N 90 percentpercent75percentl I l I l00 300 500 1000 2000kNCE, IN KMFIGURE 8.-Attenuation of intensity (MM) with epicentral distance (k1m) for various fractiles of intensity at given distance intervals for the Charleston earthquake (solid curves). Evernden's attenuation curves (1975) (Rossi Fore l intensity scale; L=10 km, C=25 km, k=1 and 1P4) are shown by dashed curves.
were utilized to develop these latter curves, and the general agreement between the entire suite of curves is remarkable. A direct comparison between curves, which may not be valid because of different data sets and different regions, would suggest that the Howell and Schultz (1975) curve is at about the 85-percentfractile, the Gupta and Nuttli (1976) curve is at the80-percent fractile, and the Cornell and Merz (1974) curve is at the 70-percent fractile. At the intensity VI level and higher, note that there is less than one intensity-unit difference among the Central United States, Central and Eastern United States,, and Northeastern United States curves and the 75-and90-percent fractile curves of this study.
Evernden's (1975) curves (fig. 8) for his k=1 and k=l114 factors lie between the 50-and 90-percentfractile curves of this study. Evernden used k factors to describe the different patterns of intensity decay with distance in the United States. A value of k= 11/4was found for the Gulf and Atlantic Coastal Plains and the Mississippi Embayment and a k=1for the remainder of the Eastern United States. Evernden prefers to work with the Rossi-Forel (RF) intensity scale. The difference between the R-F and MM scales is generally about half an intensity unit, and conversion to R-F values would essentially result in translating the fractile curves of this study upward by that amount. This would put the 75percent fractile curve in near superposition with Evernden's k= 1 curve. Such a result is perhaps not surprising because approximately two-thirds of the felt area from the 1886 shock is in Evernden's k= 1region, and isoseismal lines are often drawn to enclose most of the values at a given intensity level. Although differences in intensity attenuation may exist between various parts of the Eastern United States, it would appear from this study that the dispersion of the data (a1=l.2) could preclude its precise definition. If, indeed, significant differences do exist between the various regions, then the curves given here would apply to large shocks in the Coastal Plain province of the Southeastern United States.
The advantages of the method presented herein are that it allows a prior selection of the fractile of the intensity observations to be considered and that it eliminates one subjective step, the contouring interpretation of the intensity data. Furthermore, the dispersion of the intensity values can be calculated.
Neumann (1954) also presented intensity-versus distance data in a manner similar to that described above. However, Neumann did not consider the intensity distribution for specific distance intervals as was done herein, but rather plotted the distance distribution for each intensity level. To illustrate the difference in the two approaches, the 1886 earthquake data were cast in Neumann's format (fig. 9).
MAGNITUDE ESTIMATE 
Nuttli (1973), in arriving at magnitude estimates for the major shocks in the 1811-1812 Mississippi Valley earthquake sequence, developed a technique| for correlating isoseismal maps and instrumental ground-motion data. Later, he (1976) presented specific amplitude-period (A/T), values for MM intensities IV through X for the 3-second Rayleigh wave.
Basically, Nuttli's technique consists of:
(1) Determination of a relation between (A/T) and intensity from instrumental data and isoseismal maps,
(2) Use of the (A/T), level at 10-km epicentral distance derived from the mb value for the largest well-recorded earthquake in the region.
That level will serve as a reference level from which to scale other mb magnitudes,
(3) For the historical event of interest, assign epicentral distances (i) to each intensity level from the isoseismal map for the event. Convert from intensity to (A/T), according to the relationship of (1) above, then
STUDIES RELATED TO CHARLESTON, SOUTH CAROLINA, EARTHQUAKE OF 1886 
EARTHQUAKE-INTENSITY DISTRIBUTION 
Total observations (5)
Percent of observations at each intensity level (94)
Distribution of epicentral distances (km) for given intensity earthquake.
(MM) levels of the 1886 Charleston 
(4) Plot (A/IT) : versus and fit with a theoretical attenuation curve. Next, scale from (2) above to determine the time, between the historical shock and the reference earthquake.
In the (A T).: versus intensity of (1) and the curve fitting of (4), Nuttli found that surface waves having periods of about 3 seconds (s) were implied.
He justified the use of -,mb(determined from waves having periods of about 1 s) by assuming that the corner periods of the source spectra of the earthquakes involved are no less than 3 s. This implies a constant proportion between the 1-and 3-s energy in the source spectra. Nuttli used Mb, rather than M, because he felt that, for his reference earthquake, the former parameter was the more accurately determined.
If we apply Nuttli's technique to the 1886 earthquake and use the distances associated with the 90percent fractile intensity-distance relationship, the resulting mb, estimate is 6.8 (fig. 10j Nuttli, (1976) obtained a value of 6.5 when he used Dutton's isoseismal map and converted from the Rossi-Forel scale to the MM scale. If the Trifunac and Brady (1975) peak velocity versus MM intensity relationship, derived from Western 'United States data, is taken with the 90-percent fractile distances, then the mb estimate is 7.1 (fig. 10). Because the 90-percentfractile curve is the most conservative, it results in the largest intensity estimate at a given distance.
The magnitude estimates in this study would be upper bound values.
My magnitude estimates, as well as those of Nuttli, are based primarily on three previously mentioned factors: intensity-distance relations, intensity-particle velocity relations, and reference magnitude level (or, equivalently, the reference earthquake, which in this instance is the November 9, 1968. Illinois earthquake with nb=5.5). In the Central and Eastern United States, the data base for the later two factors is very small. It is in this context that the magnitude estimates should be considered.
REINTERPRETATION OF THE INTENSITY DATA 
EXPLANATIONS 
Central U.S. (Nuttli. 1976)
X Western U.S. (Trifunac and Brady, 1975)
0.110 100 1000DISTANCE, IN KM 
FIGURE 10.-Body wave magnitude (mb) estimates for the1886 Charleston earthquake based on Nuttli's (1973, 1976) technique. Nuttli's Central United States particle velocity-intensity data are indicated by solid circles. Trifunac and Brady's (1975) Western United States particle velocity-intensity data are indicated by X's. Distances are from the 90-percent fractile curve of this study. Heavy curve is Nuttli's (1973) theoretical attenuation for the 3s Rayleigh wave. Western United States data fit with a straight line (light curve).
CONCLUSIONS 
The intensity data base published by Dutton (1889) has been studied, and the principal results of that effort are as follows:
1. The maximum epicentral intensity was X (MM), and the intensity in the city of Charleston was IX (MM).
2. The writer verified that Dutton's isoseismal map was contoured so as to depict the broad regional pattern of the effects from ground shaking.
3. When contoured to show more localized variations, the intensity patterns show considerable complexity at all distances.
4. The epicentral distance was measured to each intensity observation point and the resulting data set (780 pairs) was subjected to regression analysis. For the 50-percent fractile of that data set, the equation developed was I=1,+2.87-0.00052. A-2. 88 log A with a standard deviation (a,) of 1.2. For the 90-and 75-percent fractiles, the 2.87 constant is replaced by 4.39 and' 3.68, respectively. This variation of intensity with distance agrees rather closely with relationships obtained by other workers for the central, eastern, and northeastern parts of the United States. It thus appears that the broad overall attenuation of intensities may be very similar throughout the entire Central and Eastern United States.
5. Using intensity-particle velocity data derived from Central United States earthquakes, the writer estimates a body-wave magnitude (me,) of 6.8 for the main shock of August 31, 1886. However, the data base upon which this estimate is made is very small; therefore, the estimated mb should be considered provisional until more data are forthcoming. Use of Western United States intensity-particle velocity data produces an mb estimate of 7.1.
REFERENCES CITED 
Bollinger, G., and Stover. C. W. 1976. List of intensities for the 1886 Charleston. South Carolina. earthquake: U.S. Geol. Survey open-file rept. 76-66, 31 p.
Cornell, C. A., and Merz. H. ., 1974, A seismic risk analysis of Boston: Am. Soc. Civil Engineers Proc. Structural Div. Jour., v. 110, no. ST 10 (Paper 11617), p. 027-2043.
Dutton, C. E. 1889, The Charleston earthquake of August31, 1886: U.S. Geol. Survey, Ninth Ann. Rept. 1887-88, . 203-528.
Evernden, J. F., 1975, Seismic intensities, "size" of earthquakes and related parameters: Seismol. Soc. America Bull., v. 65, no. 5, p. 1287-1313.
Gupta, I. N. and Nuttli, 0. W., 1976, Spatial attenuation of intensities for Central United States earthquake: eismol. Soc. America Bull., v. 66, no. 3, p. 743-751.
Howell, B. F., Jr. and Schultz, T. R., 1975, Attenuation of Modified Mercalli intensity with distance from the epicenter: Seismol. Soc. America Bull., v. 65, no. 3, p. 651665.
Neumann, Frank, 1954. Earthquake intensity and related ground motion: Seattle. Wash., Univ. Washington Press. 7 p. -23
Nuttli, 0. W., 1973, The Mississippi Valley earthquakes of1811 and 1812-Intensities, ground motion and magnitudes: Seismol. Soc. America Bull., v. 63, no. 1, p. 227-48.
1976, Comments on "Seismic intensities, 'size' of earthquakes and related parameters," by Jack F. vernden: Seismol. Soc. America Bull., v. 66, no. 1, p. 31-338. l 
Richter, C. F., 1958, Elementary seismology: San Francisco, I Calif., W. H. Freeman Co., 768 p.
Trifunac, M. D., and Brady, A. G. 1975. On the correlation of seismic intensity scales with the peaks of recorded strong ground motion: Seismol. Soc. America Bull., v. 5, no. 1. p. 139-162.
U.S. Environmental Data Service, 1973. Earthquake history of the United States: U.S. Environmental Data Service Pub. 4 1-1, rev. ed. (through 1970). 208 p.
Wood, H. O., and Neumann, Frank, 1931. Modified Mercalli intensity scale of 1931: Seismol. Soc. America Bull., v. 21, no. 4, p. 277-283.4-24

EARTHQUAKE HAZARD IN THE MEMPHIS, TENNESSEE, AREA
ARCH C. JOHNSTON and SUSAN J. NAVA 
There is a difference to be marked between hazard and risk. The two are most easily distinguished by answering the question: Can the actions of people have any effect on the situation? Hazard cannot be lessened or increased but risk can. The earthquake hazard in Memphis, Tennessee, is an inheritance of geographic location and is due to the city's proximity to the New Madrid seismic zone; it cannot be changed by man. Earthquake risk is the immediate danger posed to the population and it can be substantially altered by a number of actions, most significantly, improved construction and siting of buildings. The purpose of this paper is to give a brief introduction to the seismic hazard in Memphis, Tennessee.
THE NEW MADRID SEISMIC ZONE 
The New Madrid seismic zone is depicted in Figures 1 and 2. Figure I shows the instrumentally located epicenters for the past nine years; the main branches of the seismic zone are delineated by the concentrated pattern of epicenters within the small box of Figure 1. Figure 2 shows the relationship of the zone to Memphis and Shelby County and to the major critical facilities in the surrounding region. The generalized modified Mercalli isoseismal of Algermissen et al. (1983) are superimposed; the contours are estimated as combined effects of maximum magnitude events in the northern and southern portions of the zone. A single event would not produce these estimated intensities at all locations.
The New Madrid seismic zone is regarded by seismologists and disaster response planners as the most hazardous zone east of the Rocky Mountains (Johnston, 1982) There are three basic reasons for this estimation:
1. In the winter of 1811-1812, the zone produced three of the largest earthquakes known to have occurred in North America (Ms 8.5, 8.4, and 8.8) and hundreds of damaging aftershocks (Nuttli, 1983).
2. A major geological structure--an ancient crustal rift--has been identified through a decade of extensive research (McKeown and Pakiser, 1982). The rift underlies the shallow 
The authors are members of the staff of the Tennessee Earthquake Information Center in Memphis. They developed this paper for presentation at BSSC meeting in Memphis on January 22, 1985.
1974 -1983
FIGURE I Map of the central United States with the 1974-1983 instrumental seismicity data set (Stauder and others, 1974-1983). The boundaries of the two source zones used for frequency-magnitude determination are: Large zones, 35.0 -FIGURE 2 The relation of Memphis, Tennessee, and Shelby County to the New Madrid seismic zone. Also shown are major critical facilities in the region and Modified Mercalli isoseismal for a "'composited" maximum magnitude New Madrid earthquake.
sediments of the Mississippi embayment and is of such character and dimension that it could generate major earthquakes.
3. The zone is still quite seismically active (Figure 1). More than 2,000 earthquakes (of which 97 percent have been too small to be felt) have been detected in the zone since 1974.
These three observations--past great earthquakes, identified geological structure and continuing activity--constitute the reasons for the high hazard potential with which the New Madrid zone is presently regarded.
EARTHQUAKE PROBABILITY 
Without a doubt, the most frequently asked and least satisfactorily answered question concerning the earthquakes of the New Madrid seismic zones is: When is the next major earthquake going to happen? Seismology cannot now (nor in the near future) answer this question in a deterministic fashion (i.e., accurately predict earthquakes), but a probabilistic assessment is possible. In a recent study, Johnston and Nava (1985) estimated the probability of occurrence of large New Madrid earthquakes for two time periods--by the end of the century and within a representative lifetime (15 and 50 years, respectively) The estimates are based on magnitude: (1) a body-wave magnitude, mb, of 6.0 (or equivalently a surface-wave magnitude, Ms, of 6.3) which could be destructive over an area of one or more counties and (2) a body-wave magnitude of 7.0 (surface-wave magnitude of 8.3) which is considered equivalent to a repeat of one of the great New Madrid events of 1811-1812. Using these magnitude categories, the determined probabilities are as follows:
Probability (M76)
Body Wave Magnitude 1985 to 2000 1985 to 2035mb 6.0 (Ms 6.3) 40-63 86-97mb 7.0 (Ms 8.3) 0.3-1.0 2.7-4
A number of assumptions about the seismic behavior of New Madrid were necessary in order to generate the above probability ranges. The approach used and the assumptions that went into the final probability estimates are described briefly below.
Probability estimates require that the seismic zone behaves in a roughly predictable or period manner. This cannot be proven for large New Madrid events because of an incomplete data set over many seismic cycles, but smaller earthquakes exhibit a well behaved recurrence pattern. Therefore, the authors took instrumentally recorded data from the past nine years (see Figure I) and a historical list of earthquakes of the past 158years, determined the recurrence relationships for this data set, and then extrapolated to large magnitudes. This yielded an estimate of the average recurrence or repeat time in years between New Madrid earthquakes for a given magnitude range. For mb 6.0, the average repeat time is 70years. (The last such event occurred 90 years ago in 1895.) For mb 7.0(Ms 8.3). the average repeat time is 550 years. (The last such event was in 1812, 173 years ago.) These estimates apply to data from the entire region shown in Figure 1. If only the small region is considered (within the rectangle of Figure 1), repeat times approximately double.
There are sound geophysical reasons for choosing the larger source zone.
Once the average repeat time is established, both cumulative and conditional probabilities can be determined. Cumulative probability tells us the likelihood that a quake of a certain magnitude would have occurred by now (the present) given the date of the last occurrence and the average recurrence interval. Conditional probability estimates the likelihood of occurrence during a future specified time period (i.e., 15 and50 years--this study). Obviously, conditional probabilities are of greater interest than cumulative and are therefore emphasized in this study.
In order to make the final probability computations it is necessary to know the manner in which actual earthquake repeat times, for a given magnitude range, are dispersed about the estimated mean repeat time. This is described statistically in terms of a probability distribution with a given standard deviation. Such information for large magnitude New Madrid events is lacking; the authors' approach, therefore, was to take a number of different distributions and a range of standard deviations from the literature of studies of other active earthquake zones and apply these to New Madrid. This approach allowed for a large uncertainty in the actual (but unknown) behavior of New Madrid. This results in a range of probability values as quoted above rather than a single number.
Figures 3-5 are graphs of Gaussian conditional probabilities from mb 6.0, mb 6.6, and mb 7.0 earthquakes (Ms 6.3, Ms 7.6, and Ms 8.3, respectively), graphs on which one can see the effect that the standard deviation exerts on the probability values. The types of probability distribution employed also have an effect but to a lesser degree. The date of last occurrence, the present (1985), and the mean recurrence time are indicated on the horizontal time axis. Shading illustrates the probability range as standard deviation is varied from 33 percent to 50 percent of the mean repeat time. Calculations were done for four different statistical representations--Gaussian, log-normal, Weilbull, and Poisson--but only Gaussian is shown here. Poisson statistics, which yield a constant conditional probability, are not appropriate for this analysis; therefore, only the Gaussian, log-normal, and Weibull distributions were used to obtain the probability ranges quoted above.
In conclusion, the authors estimate that there is a medium probability of a locally destructive New Madrid earthquake in the next 15 years (40percent to 63 percent) and a high probability (86 percent to 97 percent) in the next 50 years. The probability for a great New Madrid event is less than 1 percent by the turn of the century and less than 4.0 percent during the next 50 years. These estimates are of necessity based on a number of unproven assumptions about the New Madrid zone; however, every effort was made to take an appropriate and comprehensive range of estimates in order to bracket the actual probability for future destructive earthquakes in the central United States.
FIGURE 3(a) Gaussian conditional probability computed for magnitude mb 7.0 (Ms 8.3) earthquake. The last such event occurred in 1812 and the mean repeat time (TR) is 550 years. The shaded region represents the range of conditional probability as the standard deviation is varied from 33 percent to 50 percent of TR. Future time intervals (At) of 15and 50 years are depicted.
Within next50 years 2.7%
Within next I %
15 years 03%
FIGURE 3(b)
Figure 3(a).
0 50 100 150 200 250/8/2
Years Since Last Event An expanded view of the circled region near the origin of The probability ranges are those quoted in the text.
EARTHOUAKE Probability Within next 
Years Since Lost Event 
FIGURE 4 Conditional probability representation of earthquake. Graph description follows Figure 3(a).
FIGURE 5 Conditional probability representation of earthquake. Graph description follows Figure 3(a).
REFERENCES 
Hopper, M. G., S. T. Algermissen, E. E. Dobrovolny, 1983. Estimation of Earthquake Effects Associated with a Great Earthquake in the New Madrid Seismic Zone. USGS Open File Report 83-179. Washington,
D.C. U.S. Geological Survey.
Johnston, A. C. 1982. "A Major Earthquake Zone on the Mississippi." Scientific American 246 (4):60-68.
Johnston, A. C., and S. J. Nava. 1982. Investigations of the New Madrid Seismic Zone. Submitted to the Journal of Geophysical Research.
Mekeon, F. A. and L. C. Pakiser, Editors. 1982. Investigation of the New Madrid, Missouri, Earthquake Region. USGS Professional Paper1236. Washington, D.C.: U.S. Geological Survey.
Nuttli, 0. W. 1983. "Average Seismic Source-Parameter Relations for Mid-Plate Earthquake." Bulletin of the Seismic Society of America 73 (2): 519-535.

EVALUATION OF THE EARTHQUAKE GROUND-SHAKING HAZARD FOR EARTHQUAKE-RESISTANT DESIGN 
WALTER W. HAYS
This paper describes current research that can be applied to evaluate the earthquake ground-shaking hazard in any geographic region. Because most of the spectacular damage that takes place during an earthquake is caused by partial or total collapse of buildings as a result of ground shaking or the triggering of geologic effects such as ground failures and surface faulting, an accurate evaluation of the ground-shaking hazard is an important element of: (1) vulnerability studies; (2) specification of seismic design parameters for earthquake-resistant design of buildings, lifeline systems, and critical facilities; (3) assessment of risk(chance of loss); and (4) the specifications of appropriate building codes. Although the physics of ground-shaking, a term used to describe the vibration of the ground during an earthquake, is complex, ground shaking can be explained in terms of body waves (compressional, or P, and shear, or S) and surface waves (Rayleigh and Love) (see Figure 1). Body and surface waves cause the ground and, consequently, a building and its contents and attachments to vibrate in a complex manner. Shear waves, which cause a building to vibrate from side to side, are the most damaging waves because buildings are more susceptible to horizontal vibrations than to vertical vibrations.
The objective of earthquake-resistant design is to construct a building so that it can withstand the vibrations caused by body and surface waves. In earthquake-resistant design, knowledge of the amplitude, frequency composition, and time duration of vibrations is needed. The quantities are determined empirically from strong motion accelerograms recorded in the geographic area or in other areas having similar geologic characteristics.
In addition to ground-shaking, the occurrence of earthquake-induced ground failures, surface faulting, and, for coastal locations, tsunamis also must be considered. Although ground failures induced during earthquakes have caused many thousands of casualties and millions of dollars in property damage throughout the world, the impact in the United States has been limited primarily to economic loss. During the 1969 Prince William Sound, Alaska, earthquake, ground failures caused about 60 percent of the estimated $500 million total loss; landslides, lateral spread failures, and flow failures caused damage to highways, railway grades,
Dr. Hays is Deputy for Research Applications, Office of Earthquakes,
Volcanoes, and Engineering, U.S. Geological Survey, Reston, Virginia.
He prepared this paper as background information for those making presentations at the BSSC meetings in January and February 1985.6-1
bridges, docks, ports, warehouses, and single-family dwellings. In contrast to ground failures, deaths and injuries from surface faulting are unlikely; however, buildings and lifeline systems located in the fault zone can be severely damaged. Tsunamis, long period water waves caused by the sudden vertical movement of a large area of the sea floor during an earthquake, have produced great destruction and loss of life in Hawaii and along the West Coast of the United States. Tsunamis have occurred in the past and are a definite threat in the Caribbean. Historically, tsunamis have not been a threat on the East Coast.
FIGURE I Schematic illustration of the directions of vibration caused by body and surface seismic waves generated during an earthquake. When a fault ruptures, seismic waves are propagated in all directions, causing the ground to vibrate as a consequence of the ground-shaking, and damage takes place if the building is not designed to withstand these vibrations. P and S waves mainly cause high-frequency (greater than I Hertz) vibrations that are more efficient in causing low buildings to vibrate.
Rayleigh and Love waves mainly cause low-frequency vibrations that are more efficient than high-frequency waves in causing tall buildings to vibrate.
EVALUATION OF THE GROUND-SHAKING HAZARD 
No standard methodology exists for evaluating the ground-shaking hazard in a region. The methodology that is used (whether deterministic or probabilistic) seeks answers to the following questions:
1. Where have past earthquakes occurred? Where are they occurring now?
2. Why are they occurring?
3. How big are the earthquakes?
4. How often do they occur?
5. What are the physical characteristics (amplitude frequency composition, duration) of the ground shaking and the physical effects on buildings and other facilities?
6. What are the options for achieving earthquake-resistant design?
The ground-shaking hazard for a community (Figure 2) may be presented in a map format. Such a map displays the special variation and relative severity of a physical parameter such as peak ground acceleration. The map provides a basis for dividing a region into geographic regions or zones, each having a similar relative severity or response throughout its extent to earthquake ground-shaking. Once the potential effects of ground-shaking have been defined for all zones in a region, public policy can be devised to mitigate its effects through appropriate actions such as avoidance, land-use planning, engineering design, and distribution of losses through insurance (Hays, 1981). Each of these mitigation strategies require some sort of zoning (Figure 2). The most familiar earthquake zoning is contained in the Uniform Building Code (UBC) whose aim is to provide a minimum earthquake-resistant design standard that will enable the building to:
1. Resistant minor earthquakes without damage,
2. Resist moderate earthquakes without structural damage but with some nonstructural damage, and
3. Resist major earthquakes with structural and nonstructural damage but without collapse.
HISTORY OF SEISMIC ZONING 
Zoning of the earthquake ground-shaking hazard--the division of a region into geographic areas having a similar relative severity or response to ground-shaking--has been a goal in the contiguous United States for about 50 years. During this period, two types of ground-shaking hazard maps have been constructed. The first type (Figure 3) summarizes the empirical observations of past earthquake effects and makes the assumption that, except for scaling differences, approximately the same physical effects will occur in future earthquakes. The second type (Figures4-6) utilizes probabilistic concepts and extrapolates from regions having past earthquakes as well as from regions having potential earthquake sources, expressing the hazard in terms of either exposure time or return period.
URBAN CELL ORDINANCE 
FIGURE 2 Schematic illustration of a typical community having physical systems (public/community facilities, industrial, transportation, and housing) exposed to earthquake hazards. Evaluation of the earthquake hazards provides policymakers with a sound physical basis for choosing mitigation strategies such as avoidance, land-use planning, engineering design, and distribution of losses through insurance. Earthquake zoning maps are used in the implementation of each strategy, especially for building codes.
FIGURE 3 Seismic hazard zones based on historical modified Mercalli intensity (MMI) data and the distribution of damaging earthquakes (Algermissen, 1969). This map was adopted in the 1970 edition of the UBC and incorporated, with some modifications, in later editions. Zone 3 depicts the greatest hazard and corresponds to MMI VII' and greater.
FIGURE 4 Map showing preliminary design regionalization zones for the contiguous United States proposed by the Applied Technology Council (ATC) in 1978. Contours connect areas underlain by rock having equal values of effective peak acceleration. Mapped values have a 90 percent probability of not being exceeded in a 50-year period. Zone I represents the lowest hazard (0.06 g). Sites located in Zone 4 require site-specific investigations. This map was based on research by Algermissen and Perkins (1976).
FIGURE 5 Graph showing levels of peak horizontal ground acceleration expected at bedrock sites in the Memphis. Tennessee, and the St. Louis, Missouri, areas in various exposure times. The values of peak acceleration have a 90 percent probability of nonexceedance. An exposure time of 50 years corresponds to the useful life of an ordinary building and is typically used in many building codes.
EXPOSURE TIME (YEARS)
FIGURE 6 Graph showing levels of peak horizontal ground acceleration expected at bedrock sites in the Charleston, South Carolina, and the Seattle, Washington, areas in various exposure times. For comparison, San Francisco, California, also is included. The values of peak acceleration have a 90 percent probability of nonexceedance. An exposure time of 50 years corresponds to the useful life of an ordinary building and is typically used in many building codes.
PROCEDURE FOR EVALUATING THE GROUND-SHAKING HAZARD 
Construction of a ground-shaking hazard map requires data on:
1. Seismicity,
2. Earthquake source zones,
3. Attenuation of peak acceleration, and
4. Local ground response.
The procedure for constructing a ground-shaking hazard map is illustrated schematically in Figure 7. Except for probabilistic considerations a deterministic map would follow the same general procedure.
RESEARCH PROBLEMS 
A number of complicated research problems are involved in the evaluation of the ground-shaking hazard (Hays, 1980). These problems must be addressed if more accurate specifications of the ground-shaking hazard are desired. The problems can be categorized in four general areas-seismicity, nature of the earthquake source zone, seismic wave attenuation, and local ground response--with each area having a wide ranger of technical issues. Presented below are representative questions, which generally cannot be answered with a simple "yes" or "no," that illustrate the controversy associated with ground-shaking hazard maps.
Seismicity
* Can catalogs of instrumentally recorded and felt earthquakes (usually representing a regional scale and a short time interval) be used to give a precise specification of the frequency of occurrence of major earthquakes on a local scale?
* Can the seismic cycle of individual fault systems be determined accurately and, if so, can the exact position in the cycle be identified?
* Can the location and magnitude of the largest earthquake that is physically possible on an Individual fault system or in a seismo, tectonic province be specified accurately? Can the recurrence of this event be specified? Can the frequency of occurrence of small earthquakes be specified?
* Can seismic gaps (i.e., locations having a noticeable lack of earthquake activity surrounded by locations having activity) be identified and their earthquake potential evaluated accurately?
* Does the geologic evidence for the occurrence of major tectonic episodes in the geologic past and the evidence provided by current and historic patterns of seismicity in a geographic region agree? If not, can-these two sets of data be reconciled?
ACCELERATION 
FIGURE 7 Procedure for constructing a grounding-shaking hazard map.
The Nature of the Earthquake Source Zone 
* Can seismic source zones be defined accurately on the basis of historic seismicity, on the basis of geology and tectonics, or on the basis of historical seismicity generalized by geologic and tectonic data? Which approach is most accurate for use in deterministic studies? Which approach is most accurate for use in probabilistic studies?
* Can the magnitude of the largest earthquake expected to occur in a given period of time on a particular fault system or in a seismic source zone be estimated correctly?
* Has the region experienced its maximum or upper-bound earthquake?
* Should the physical effects of important earthquake source parameters such as stress drop and seismic moment be quantified and incorporated in earthquake-resistant design even though they are not traditionally used?
Seismic Wave Attenuation 
* Can the complex details of the earthquake fault rupture (e.g., rupture dimensions, fault type, fault offset, fault slip velocity) be modeled to give precise estimates of the amplitude and frequency characteristics of ground motion both close to the fault and far from the fault?
* Do peak ground-motion parameters (e.g., peak acceleration) saturate at large magnitudes?
* Are the data bases adequate for defining bedrock attenuation laws9Are they adequate for defining soil attenuation laws?
Local Ground Response 
* For specific soil types is there a discrete range of peak ground motion values and levels of dynamic shear strain for which the ground response is repeatable and essentially linear? Under what insitu conditions do non-linear effects dominate?
* Can the two-and three-dimensional variations of selected physical properties (e.g., thickness, lithology, geometry, water content,
shear-wave velocity, and density) be modeled accurately? Under what physical conditions do one or more of these physical properties control the spatial variations, the duration, and the amplitude and frequency composition of ground response in a geographic region?
* Does the uncertainty associated with the response of a soil and rock column vary with magnitude?
CONCLUSIONS 
Improved maps of the earthquake ground-shaking hazard will come as relevant geologic and seismological data are collected and synthesized.
The key to progress will be the resolution of the research problems identified above.
REFERENCES 
Algermissen, S. T. 1969. "Seismic Risk Studies in the United States."
In Proceedings of the 4th Conference on Earthquake Engineering,
Vol. 1.
Algermissen, S. T., and D. M. Perkins. 1976. A Probabilistic Estimate of Maximum Acceleration in Rock in the Contiguous United States. USGS Open File Report 76-416. Reston, Virginia: U.S. Geological Survey.
Applied Technology Council. 1978. Tentative Provisions ment of Seismic Regulations for Buildings. Report Alto, California: Applied Technology Council. for the Develop ATC 3-06. Palo 
Hays, W. W. 1980. Procedures for Estimating Earthquake Ground Motions. USGS Professional Paper 1114. Reston, Virginia: U.S. Geological Survey.
Hays, W. W. 1981. Facing Geologic and Hydrologic Hazards-Earth Science Considerations. USGS Professional Paper 1240. Washington, D.C.:
U.S. Government Printing Office.

INTRODUCTION TO SEISMOLOGICAL CONCEPTS RELATED TO EARTHQUAKE HAZARDS IN THE PACIFIC NORTHWEST
STEWART W. SMITH
The objective of this brief discussion is to acquaint you with the general aspects of the earthquake hazards in the Pacific Northwest. We will address the "why," "how big," and "how often" of earthquake occurrence. In addition, some mention will be made of the severity of effects that we may expect in this region. In order to answer the questions concerning "where" and "why," we will call on some general concepts of plate tectonics. Answering the "how big" question will require a discussion of earthquake magnitude and other means of characterizing the "size" of an earthquake. The question of "how often" will cause us to look at some elementary statistics of earthquake distributions and the importance of the historic record. Finally, our discussion of the severity of effects will necessitate the introduction of the idea of how we characterize destructive ground motion and how the severity of motion depends on the local situation.
Whether or not the scientific community is ever able to reliably predict earthquakes, engineering decisions need to be made every day based on our present state of understanding of the earthquake risk. Thus, the principal task of a seismologist interested in reducing the hazards due to earthquake is to develop an understanding of how geologic and seismologic parameters affect motion. This is necessary because we need to predict in advance the nature of ground motion for an earthquake that has not yet occurred and all we have to look at is the geology and the record of past earthquakes.
PLATE TECTONICS AND EARTHQUAKES 
The plate tectonic model of planet Earth is the starting place for understanding the "why" and "where" of earthquake occurrence. In the simplest sense, earthquakes are the "noise" or creaking and grinding disturbances that accompany the motion of tectonic plates. In this view, the plates(with associated continents riding along on top of some of them) do not move smoothly at rates of a few centimeters a year; rather, they move spasmodically, with a jump during each large earthquake, such that the average motion viewed over thousands (or millions) of years is several centimeters per year. Of course, the entire plate does not have to lurch forward during a single earthquake, but significant distortion and 
Dr. Smith is a professor in the Geophysics Program at the University of Washington, Seattle. He prepared this paper for presentation at the BSSC meeting held in Seattle on February 6, 1985.7-1
movement could be expected every time a large portion of any its boundaries slips. That earthquakes are associated with the boundaries of these plates can easily be seen by looking at Figure 1, which illustrates the global pattern of earthquake activity. Narrowing our view to the Pacific Northwest, we have the plate configuration illustrated in Figure 2.
Plate Boundaries 
A plate has three types of boundary--a spreading ridge boundary, a subducting zone boundary, and a transform fault (or edge) boundary. In the simplest view, the ridge has the smallest earthquake occurrences because the lithosphere is thin and hot (weak) near a ridge and, thus, a large area of potential slip (and, thus, a large volume in which to store strain energy) does not exist. In contrast, the subduction zone boundary appears to be the place where the world's largest earthquakes (great earthquakes) occur. This is because the lithosphere is cooler, thicker, and stronger and because a larger area of potential slip exists (the entire interface between the overriding and under thrusting plates). Transform faults or plate edges appear to be intermediate between these two extremes with a limit on the depth extent of faulting, but with a horizontal extent that can be quite large as in the case of Chile, Turkey, and California. It would appear that large earthquakes, but perhaps not truly great earthquakes, are possible on transform faults. The distinction between "large" and "great" for engineering purposes ultimately may be important because of the size of area affected rather than because of distinction in the severity of ground motion. This is true since in recent years it has become clear that even moderate earthquakes can produce very severe ground motion locally.
Subduction Zones 
Looking in more detail at the conditions that affect the potential "size" of earthquakes on subduction zones, we find that the two most important parameters seem to be the age of lithosphere and the rate of plate motion(convergence). A simple model of the down going slab, which progressively grows cooler and thicker as it moves out from its source region at the spreading ridge, is that it is sinking vertically under its own weight while also being subjected to relative horizontal convergence as the overriding plate moves over it. All other things being equal, the faster it tends to sink because of negative buoyancy, the less normal stress there will be between the two plates and the more likely it will be able to move smoothly (without a stick-slip type motion) and, thus, the smaller the earthquakes are likely to be. In the limit of a plate that is sinking so fast that it is actually separating (trying to separate) from the overriding plate, it is unlikely that large earthquakes could occur at all. The single most important parameter that seems to control the density of the down going plate and, thus its buoyancy, is its age. The older and colder the plate, the more dense it is and the faster it will sink. The other parameter is the plate velocity (covergencerate).
FIGURE I World map showing relation between the major tectonics and plates and recent earthquakes and volcanoes. Earthquake epicenters are denoted by the small dots, and the volcanoes by the large dots.
FIGURE 2 Juan de Fuca Plate map.
Here, for a constant sinking rate, the faster the two plates are converging, the more normal stress there will be locking the surface between them. This, in turn, leads to a situation of large stress accumulation and, thus, large earthquakes.
The correlation between lithospheric age and convergence rate shows, for example, that in the Pacific Northwest, where the-Juan de Fuca plate has an age of less than 20 million years off the coast of Washington and a convergence rate of about 3.5 cm/yr., the expected value of moment magnitude for the largest possible earthquake is 8.25. The scatter in the data revealed in the multiple regression work by Heaton and Kanamori would cause one to put an uncertainty of about +0.4. The remarkable thing about this analysis is that here we have a region where the historic record is less than two centuries and there are no reports of earthquakes, larger than around 7.5 and, yet, a model based strictly on geologic data and the plate tectonic hypothesis leads to a prediction of an earthquake as large as 8.5. Transform Faults In trying to apply similar kinds of basic physics to transform faults to see what parameters influence the maximum size of earthquakes, we have much less success. It appears to be only the top 20 or so kilometers of crust-that can support brittle fracture; therefore, the size of the possible slip area is controlled primarily by the length of the fault. Complexity of the fault, lateral in homogenieties and bends or kinks, appears to be important in determining how long a section might rupture in a single earthquake event. Thus, the detailed surface geology is critical and no generalizations can be made. Transform faults or plate boundaries are of several varieties depending on which types of plate boundaries the transforms connect. Plate edges between two offset ridges (RR transform) can be easily modeled with a piece of cardboard in which two slots are cut and through which two pieces of paper (appropriately marked with magnetic stripes) can be pulled. Two lessons are learned from this paper model. First, the relative motion on the transform fault connecting the two ridges is opposite to that which would be expected if one thought that the ridges had been offset by a fault that connected them and that they originally had been a through going feature. More important from the standpoint of assessing possible earthquake size, however, is that the ends of the fault, which extend beyond the ridges and are called fracture zones (FZ), have no relative motion and, thus, can be viewed as fossil faults on which there will be no earthquakes generated. Thus, a fracture zone that is a thousand kilometers long can generate a rupture only as long as the segment joining the two actively spreading ridges. Even in the case of the transform fault, the plate tectonic hypothesis provides some important guidance as to the earthquake potential of this feature. My own view is that we have seen only the beginning of the way in which our understanding of the physics (and chemistry) of the earth will affect our assessment of future earthquake hazards.
FAULT AND EARTHQUAKES 
Up to this point, we have viewed the only source of earthquakes to be plate boundaries, and our view of plates has been one of a grand scale where there are some 17 major plates comprising the entire surface of the planet. Looking closer, we find that this view is only an approximate one and that the earth is very much more complicated. In some instances the plate boundaries are razor sharp and easy to identify, whereas in others the boundary may be spread out over hundreds of kilometers or greatly obscured by the possible subdivision of the plate into many smaller platelets (the term "microplate" is starting to become popular). When we come to the hard question of estimating the future earthquake activity in a region, it sometimes seems that we have simply substituted one crystal ball for another when we try to invoke ideas of plate tectonic models and the plates themselves are not easily understood. Let us leave the simple plate viewpoint for the moment, recognizing that even if we had a simple plate model at depth, what we would see at surface is likely to be obscured by the local geology (e.g., mountains, sedimentary basins). In examining how the surface rocks may deform or fracture (fault) in response to deeper plate movement, we can use some the ideas of fracture mechanics to relate stresses to resulting fault type and pattern.
Normal Faults 
A normal fault is one in which the slip direction is down-dip in such away that you would expect to develop if the region were stretched and the blocks readjusted accordingly. Typically the dip of normal faults is quite steep, between 45 and 90 degrees. (Remember, dip is measured from the horizontal downward). In terms of earthquake potential, one would not expect a great deal of normal stress pressing the two sides of the fault together since the region is undergoing horizontal tension(being pulled apart). Thus, all other things being equal (which in geology they never are ), one would not expect the largest earthquakes to occur on such faults. Substantial earthquakes, however, have been observed on normal faults (e.g., Dixie Valley, Nevada, in 1954 and Hebgen Lake, Montana, in 1959). These faults had vertical displacements of up to 4 or 5 meters over distances of nearly 100 km so they were "big" earthquakes by any measure but they were not "great" earthquakes in the sense of the Alaskan earthquake of 1964. Our 1949 earthquake near Olympia (magnitude 7.1) was apparently on such a fault although it occurred on the deep part of the subducted slab where we cannot directly observe it.
Reverse Faults (and Thrust Faults)
A reverse fault is also a fault on which the slip is in the direction of dip, but in this case it is the upper block (hanging wall) that is pushed up so the sense of motion is opposite to that discussed for the normal fault. Typical dips for reverse faults are 45 degrees or less. When the dip gets to be very shallow, almost horizontal, then the term thrust" fault is used to describe it. There are numerous examples of nearly horizontal thrust sheets where, over geologic time, the upper block has slid many miles on top of the lower sheet. One could expect large normal stresses to develop across such faults (since the two sides of the fault are being pushed together) and, thus, very energetic earthquakes. A recent example of a thrust type earthquake was that in the San Fernando region of southern California in 1971. Since the Juan de Fuca plate is being thrust beneath North America, this is the type of faulting that could conceivably occur beneath western Washington. Should this occur, there would likely be quite severe ground motion over the entire region from the Pacific Coast inland to the Cascade Mountains.
Strike Slip Faults 
Finally, we have the case of nearly vertical fault surfaces with slip in the horizontal direction. Such faults are called "strike slip" and are classified as to right or left lateral depending on the sense of motion with respect to an observer standing on one side of the fault and looking across it. The famous San Francisco earthquake of 1906(magnitude 8.25) occurred on the San Andreas fault, which is a right lateral strike slip fault. During that earthquake the fault slipped as much as 17 feet in some places. The recently noted alignment of earthquakes through Mt. St. Helens extending to the northwest is believed to be a strike slip fault based on indirect seismological evidence although geologic data that would confirm slip on this fault has not yet been uncovered.
Earthquake Potential of Mapped Faults 
Examination of virtually any geologic map will reveal that there are a multitude of faults on a variety of scales present nearly everywhere. In fact, the density of faulting on maps seems to depend largely on how carefully the area has been mapped by geologists and how good the exposures of bedrock are. Areas like the Puget Sound region may not show many faults, for example, if they are covered by a thick blanket of recent glacial material which makes them inaccessible for geologic mapping. The scale of faulting varies from tiny, millimeter-size features you can see in a rock fragment up to global-size features that are best seen in satellite imagery. Obviously not all these features have the same potential for generating earthquakes. Size or length of faulting is an obvious distinction, but perhaps the most important characteristics the age of most recent movement.
Age of Most Recent Movement 
Most observed faults are very old, representing past periods of deformation under stress conditions. that are very different from what we have today. In geology we do our forecasting somewhat like the meteorologist does his when he uses the "strategy of persistence"--i.e., the most likely conditions for tomorrow are more of what we have seen today. In that sense, the faults most likely to cause a problem by generating earthquakes are the ones that have the most recent history of movement. The development of radioactive age dating techniques, particularly those that involve short half life elements like Carbon 14, and can be used to date materials as young as thousands of years, provides the means to distinguish very young and, thus, potentially dangerous faults from those that that are old and no longer active. Investigations are generally made by trenching across the fault trace, or boring through it, with careful mapping of the materials on either sides. The key is to find features that are continuous across the fault and to date these features. For example, an old soil layer that lies uninterrupted across a dip slip fault and has an age of 2,000 years tells us that the fault has not moved in at least 2,000 years. Conversely, if the soil layer were disturbed, it would establish that the fault had moved sometime(exactly when could not be said) in the past 2,000 years.
In western Washington our heavy glacial cover obscures most fault features that might be useful in assessing the record of past earthquakes(and guessing the future ones). Some evidence of ancient fault motion on the Olympic Peninsula was developed a number of years ago by dating trees that were submerged as a possible effect of fault-dammed streams. Some lineaments are visible in air photographs of the Cascade Mountains and in side-looking radar imagery (SLAR), but their significance is not as clearly understood as would be the case in California or Nevada where the overall record of surface geology is much better preserved. In the Mojave Desert of California, fault scarps that moved thousands of years ago are so well preserved they look as if they might have moved yesterday. In contrast, here in the Northwest the rate of growth of vegetation (such as Douglas fir) and the erosion due to heavy rainfall are so great that faults can easily be obscured in a short period of time. In addition, the plate tectonic configuration is basically different in the Pacific Northwest from what it is in California. In California, the boundary between the Pacific and North American plates is an early vertical fault plane (or collection of planes) that intersects the surface of the earth producing obvious features (e.g., the San Andreas fault). In contrast, our plate boundary in the Northwest lies beneath us, the gently dipping interface between the Juan de Fuca plate and the North American plate. Its only intersection with the surface where one might look to see its expression is under water several hundred miles offshore.
Definition of Capable Fault 
The technology for recovering the history of fault movement has developed remarkably during the past decade driven by society's need to assess the "capability" of faults in connection with large dams and nuclear power plants. There are no firm rules to tell us how old a fault has to be before we can classify it as inactive. It seems to be a sliding scale depending on how high the stakes are. In the case of nuclear power plant siting, a specific criteria has evolved in which a fault that has moved at least once in the past 50,000 years must be considered capable." Generally, however, if there is no evidence of movement since ,the last period of glaciation, approximately 10,000 years, it appears unlikely that future movement will occur.
CRUSTAL DEFORMATION 
Obviously, with all the plates stretching, squeezing and colliding with one another, there should be some possibly measurable deformation going on between earthquake occurrences. In the earliest days of seismology, an earthquake was attributed to either explosive action or magma movement deep in the earth. It wasn't until the 1891 earthquake at Mino-Owari in Japan that serious consideration was given to sudden fault slip being the cause of an earthquake. The excellent set of geologic and geodetic data that was collected before and after the 1906 San Francisco earthquake, however, really set the stage for the first rational explanation of earthquake sources, the "elastic rebound" theory.
A number of fundamental questions remain to be answered concerning the slow deformation that precedes (and follows) major earthquakes. The tools to measure these effects are available, primarily laser distance measuring devices both land-based and satellite-based, but since the motions are slow, it is going to take quite a few more years before many of the questions are satisfactorily answered. For example, how does the stress increase in the years (possibly centuries) leading up to the earthquake? Is it rather steady, simply building gradually to a point of failure and then starting over again to produce a periodic recurrence of earthquakes? Alternatively, is the stress quiescent most of the time, with rapid periods of buildup just prior to large earthquakes? These two possible scenarios lead to quite different strategies for predicting future earthquakes.
SEISMIC WAVES We have been using sudden fault slip or rupture as a working model for an earthquake source. The phenomenon that we normally associate with an earthquake, however, is ground-shaking. What's the relation between these two observations? The ground-shaking we notice some distance away from an earthquake (and some time after the faulting occurred back at the hypocenter) is simply the effect of seismic waves that have traveled from the hypocenter to our point of observation. The principal shaking motion that is experienced in an earthquake is due to two broad categories of seismic waves, namely, "body waves" and "surface waves." The term "body wave" means a disturbance that travels directly through a solid medium, choosing a path that is the quickest possible route between source and receiver. There are two general types of body wave,
compressional or P waves and shear or S waves. Surface waves travel along the surface of the earth In a manner somewhat analogous to water waves. They also come in two varieties--Love waves that produce strictly horizontal shaking and Rayleigh waves that cause vertical as well as horizontal shaking.
For a number of fundamental reasons, the frequency of both types of surface waves, Love and Rayleigh, is much lower than that for the direct body waves, P and S. As a result, surface waves are of much more concern for long period structures such as bridges and high-rise building than for more conventional structures. Simple consideration of how the wave energy spreads out in a surface wave (two-dimensional or cylindrical traveling along the surface) compared with body waves (three-dimensional, spherical waves traveling through the medium) tells us that the wave amplitude will die off faster with distance for a body wave than it will for a surface wave. As a result, if a site is near an earthquake, it will most likely be the body waves that do the damage, whereas if the epicenter is a long distance away, it is more likely that the surface waves will present the largest motion.
EARTHQUAKE SIZE 
We have now established that earthquakes are the sudden slip or rupture on a fault plane and that the shaking we observe is a result of seismic waves produced by that fault slip. Intuitively, we might expect more intense shaking from a fault that had a relatively large amount of slip. We also might expect more intense shaking if the fault surface on which slip took place was a large one since that would permit constructive interference effects to occur. As a result, the measure of earthquake size" should somehow include both the amount of slip as well as the size of the fault area.
Now, the observable quantity we have available to measure earthquake size is generally a seismogram. Only very rarely do we have the opportunity to directly measure fault slip and area. Thus, we need a measure of earthquake size that depends on something we can measure on a seismogram, such as the amplitude of some particular seismic wave. In the early development of the magnitude scale, Charles Richter at Caltech simply measured the maximum amplitude on seismograms. To avoid differences in the response of different kinds of instruments, he restricted himself to a particular type, namely, the Wood-Anderson torsion seismograph. This instrument has two attractive attributes for development of a magnitude scale. First, it is a very "broad band" instrument that responds uniformly to vibrations of both very short and very long period. Second, since it is a mechanical-optical device, there are no amplifiers, variable resistors, or, in fact, any knobs at all that can be twiddled to change its sensitivity. Thus, it is nearly "technician proof," and even years after an earthquake has been recorded, one can have confidence in the published sensitivity of the instrument.
Richter Local Magnitude, ML 
Richter noted that the maximum amplitude on seismograms behaved in an organized way. Although there were rapid variations in amplitude and a lot of scatter in data, he found that the maximum amplitude data formed a one-parameter family of curves when the logarithm of the amplitude was plotted versus the logarithm of distance. The free parameter was some kind of arbitrary number which denoted the "size" of the earthquake. He defined that number as the local magnitude and it has been denoted as ML. There is an arbitrary "starting point" for this scale and he chose it such that a magnitude 0 shock would have an amplitude of I mm at a distance of 100 km.
Body Wave Magnitude
Richter didn't specify which seismic wave he was measuring, he simply chose the largest excursion on the record. Since the instrument was measuring horizontal motion and since he was generally dealing with local (nearby) earthquakes, the maximum always corresponded to the SH wave. Subsequent work using earthquakes from further distances showed that this process was inadequate. As waves travel through the earth they preferentially lose their high frequency constituents and, thus,
appear longer in period (lower frequency) the further away you observe them. It was found that dividing the amplitude by the period provided a convenient and useful way to normalize out this effect. It was also necessary to have a scale based on compressional waves as recorded on vertical instruments. The resulting relationship with some empirical corrections added to make it fit reasonably well with the ML scale looked like:
mb = log(A/T) + 0.1l + 5.9,
where A is the amplitude of ground motion, T is the period of the wave,
and A the distance.
Surface Wave Magn1tude
It soon became clear that a single number, either ML for nearby earthquakes or mb for distant ones, wasn't adequate to describe the "size" of an earthquake. Two earthquakes of the same magnitude might produce remarkably different damage effects, and they certainly could write remarkably different looking seismograms. One of the big differences was in the amount of surface waves generated, and this observation soon led to the development of yet another magnitude scale. It utilized the amplitude of Rayleigh waves at a period of 20 seconds. Because of some waveguide effects in the earth, this period usually corresponds to the maximum part of the train of Rayleigh waves and is thus easy to identify. The resulting expression for surface wave magnitude, again adjusted so that it corresponds as closely as possible with the other magnitude scales, is:
Ms = logA + 1.66logL + 2.0.
Seismic moment
In addition to these empirical studies, which led to several magnitude scales that were very useful in classifying earthquakes, there were mathematical developments that led to a characterization of the strength of a seismic source. In the differential equations that describe the motion of an elastic medium, there is a source term expressed as a force. We have no way to describe an earthquake as some kind of force system since we are unable to observe forces directly in the earth and it seemed that there was no apparent way to use an earthquake as the source term in the equations of motion. This situation improved after the development of a mathematical representation theorem that showed how a dislocation (fault slip) model could be expressed as an equivalent force. An important parameter was identified in the resulting equations,
the product of rock strength, fault area, and average slip:
M = pAG.
It has the dimensions of a "moment," (i.e., force times length) so it was called "seismic moment." Here was a parameter that could be measured from a seismogram and could also be directly related to observations that a geologist could make in the field. It also formed the basis of a calculation of energy or work done during an earthquake, and this, in turn, was used to develop yet another (hopefully the last) magnitude scale, the so-called moment magnitude.
STATISTICS AND RECURRENCE CURVE 
One of the first ways of utilizing the magnitude scale was in examining the size distribution of earthquakes. It is immediately clear that there are more small earthquakes than large ones so the question concerns whether the distribution behaves in some organized fashion. The answer,
of course, is yes! If we choose a particular area of the earth and record earthquakes over some specific time, then plotting the log of NM, the cumulative number of earthquakes that exceed magnitude M as a function of magnitude, yields a straight line:
log NM = a -bM.
The intercept "a" is a measure of how active the region is and the slope "b" tells us how many small shocks there are for each large one. We will have only a segment of a straight line because we will run out of data at both ends of the magnitude distribution. There will be some magnitude so small that it will escape detection by our seismic networks,
and there will be some upper limit, namely the largest magnitude shock that has occurred during our time of observation. Within this range of magnitudes, the distribution generally does fit a straight line quite well with the slope ranging from 0.5 to 1.2.

An important question concerns how far we can extrapolate such a line to predict the rate of occurrence of earthquakes larger than those that have already been observed. It would be very convenient if one could record and count earthquake statistics in a region for a short period of time, say several months or even several years, and from this data determine both the maximum magnitude that could be expected in the region and its recurrence period. Unfortunately, this procedure doesn't work because without some additional information about the faults, their behavior, and the age of most recent movement, we do not know how to extrapolate the earthquake statistics to large magnitude. To illustrate this, Figure 3 shows the earthquake distribution for the Puget Sound region. Figure 4 shows a map distribution of the earthquakes that have occurred in Washington since 1841. Note that the largest event shown is the 1949 Olympia earthquake and that if this curve is a fair representation of the long-term seismicity, we should expect a repetition of a shock of this size every 130 years on the average. Can we extend the
FIGURE 3 Frequency of occurrence vs. magnitude for the entire Puget Sound Basis and the one degree area.
FIGURE 4 Larger earthquakes in Washington, 1841-1981.7-14
curve to larger magnitude? If we do, how often would we "expect" a magnitude 9.0 quake and would this make any geologic sense? The Pacific Northwest is a good illustration of the pitfalls of using such curves because we suffer from a very short historic record, a poorly preserved surface geologic record, and a plate geometry not well suited for producing surface fault scarps. Thus, the critical information needed to intelligently use the meager earthquake statistics is simply not available.
GROUND MOTION 
When the ground shakes during a nearby earthquake, we may (it does require some luck) obtain a record (strong motion seismogram) that displays the history of ground-shaking. A considerable amount of information is present in such records, but for our purposes we will mention only a few parameters that can be easily obtained. First, we have the maximum of acceleration, velocity, and displacement. In Figure 5 we illustrate a ground motion recording from the 1949 Olympia earthquake, magnitude7.1, arguably the largest earthquake to have occurred in historic time. Note that acceleration is measured as a percent of the acceleration of gravity (g) or in units of cm/sec2reached a value of 134 cm/sec2or 13percent g for this particular record. Velocity and displacement records are obtained by integrating the original acceleration record once and twice, respectively. Second, we have the duration of strong shaking,
which can be defined, for example, as the length of time during which the shaking exceeded some particular value such as 5 percent g. Finally,
we have some measure of the frequency content, basically a measure to describe how the energy of shaking is distributed between high and low frequencies. A variety of measures are possible ranging between simply the period of ground during which the maximum motion occurred to a response spectrum which displays the maximum motion that would be encountered by hypothetical buildings (single degree of freedom pendulums) of differing resonant frequency.
Intensity 
A completely different way to characterize ground motion is through its damage effects on structures. Earthquake intensity scales are used for this purpose. For the United States, the modified Mercalli scale is the most popular. It characterizes ground motion from I to XII by a series of descriptions ranging from I as barely perceptible through VI where we see the onset of building damage to XII where one has "total destruction." The principal usefulness of such scales is to characterize the "size" of ancient earthquakes for which there are no measurements of actual ground motion. Another useful measure is the area over which the earthquake was felt since this information can often be easily determined from old newspaper reports by simply noting in what localities the shaking was felt.
WESTERN WASHINGTON EARTHQUAKE APR 13. 1949 -1156 PST
115028 49.002.0 DIST. ENGINEERSOFFICE AT ARMY BASE COMP S02Wa PERK VALUES: RCCEL -66.5 CM/SEC/SEC VELOCITY = 8.2 CO/SEC DISPL = 2.4 CMI0 20 30 40 SO 60 TIME -SECONDS 
FIGURE 5 Western Washington earthquake, April Dist. Engineers Office at Army Base66.5 cm/sec/sec Velocity = 8.2 cm/sec13, 1949, 1156 PST 11B028Comp. 502W Peak Values:
Attenuation Curves 
Obviously, any of these "measures" of ground motion will be more severe for an observation site close to the earthquake than it will be for amore distant location. Attenuation curves are the device we use to display this relation. Any parameter can be used to construct an attenuation curve, even intensity. Typically we display the logarithm of peak horizontal acceleration as a function of distance for one particular size earthquake. The shape of this curve depends critically on a number. of seismologic and geologic parameters such as fault type, depth, crustal thickness, and specific dissipation -(Q-1). This last parameter is a measure of how much of the elastic energy in a wave is converted to heat as the wave passes through the crust. Thus, each region will have its own distinctive curve. Such a curve, when constructed with locally derived ground motion data, together with a recurrence curve, also locally derived, and a map of the potential earthquake source regions are the basic ingredients that one needs to calculate seismic risk.
CONCLUSIONS.
Western Washington lies on top of an active subduction zone. Although the characteristics of this zone are not yet well understood, comparing; it with other subduction zones around the world leads us to predict that an earthquake as large as 8.25 on the moment magnitude scale could happen here. The effects of such an earthquake would not be localized to a narrow fault zone such as is the case for the-San Andreas fault in, California but might be spread widely from the coast inland to the Cascade Mountains and from Vancouver Island to the Columbia River. Although the scientific evidence points toward the possibility of an earthquake of this size, we have not yet been able to determine if such an event, is likely to occur once per century or once per millennium. It is this rate of occurrence that will determine if the risk from such a large earthquake is greater than the risk we know for certain exists due to the repetition of smaller historical earthquakes suchas those of 1949and 1965.7-17:

MANAGEMENT OF EARTHQUAKE SAFETY PROGRAMS BY STATE AND LOCAL GOVERNMENTS
DELBERT B. WARD
This paper deals with fundamental concepts for management of earthquake hazards and associated earthquake safety programs at state and local levels of government. The focus of the paper is upon recognizing and narrowing a gap which the author believes to exist between earthquake hazards information (essentially research data) and applications of the information (public policies for implementation of hazards reduction methodologies).
BACKGROUND 
That natural hazards can be managed for the overall benefit of our society is a notion accepted by most of us. We believe--correctly, I think--that life loss, injuries, and property losses can be reduced through prudent pre-event practices and effective deployment of resources when disasters occur. Emergency management is an institution of government that has evolved over the past two or three decades whose primary purpose is to articulate and carry out a broad array of activities directed to loss prevention and/or loss reduction due to extreme events-both natural and man-made.
Emergency management practices traditionally have separated into several phases, due no doubt to the time-related character of the activities. For this discussion, we refer to four such phrases--preparedness, mitigation, response, and recovery. Other divisions have been used, but the variations have no significance to our purposes here.
Beyond these time-related characteristics that are common to nearly all emergency management activities, the similarities among the risk reduction activities appear to end for the various hazards. Each type of natural hazard--earthquakes, tornadoes, hurricanes, and floods--derives from a different sort of natural phenomenon, has different physical characteristics that create risks to life safety and property, and, consequently, requires different methods for effective control (management) of the risks.
Mr. Ward is an architect with Structural Facilities, Inc., Salt Lake City, Utah. He presented this paper at the FEMA Earthquake Education Curriculum Workshop held at the National Emergency Training Center,
Emmitsburg, Maryland, June 27-29, 1984.
If the reader accepts that there are physical distinctions between the several types of natural hazard named above, then it is useful to examine briefly the implications of these distinctions with respect to the time-related emergency management activities of preparedness, mitigation, response, and recovery. Although management concepts for the hazards may be similar in some cases, the specific risk-reduction activities are quite different for each type of hazard. Moreover, the importance (priority) of the types of action with respect to the end goal of risk reduction seems to be different for each type of hazard.
For example, for a variety of reasons control of losses due to a hurricane requires different emphasis upon preparedness and recovery actions than does control of losses due to an earthquake, In the case of hurricanes, preparedness actions based upon pre-event warning are possible; mitigation is largely a matter of siting considerations; and response activities can be coordinated to occur even during the event. On the assumption that life safety is the paramount objective, preparedness based upon pre-event warning is emphasized.
Riverine flooding, too, requires a different emphasis for effective loss control. Once again, preparedness actions can be based upon pre-event warning, but effective loss control requires that emphasis be placed upon mitigation actions.
Earthquake events, in contrast, say, to hurricanes happen without warning and are of very short duration--a few minutes at most and hardly enough time to do anything more than duck. Current technology does not allow short-term prediction of the events, although regions of greater earthquake potential and even long-term (several years to several decades) speculations about impending events are within current technical state-of-the-art capabilities. 0 Moreover, we presently do not know how to control (eliminate or soften the occurrences) of the earthquake events. Accordingly, emergency management methods presently are limited to (I) reducing the effects of the earthquake upon buildings and people--mitigation--and/or (2) providing recovery services--picking up the pieces,
so to speak--after the events.
Either of the above types of emergency management actions will help to reduce earthquake losses to some extent, but mitigation assuredly can be the most effective of the two types of actions. Mitigation can eliminate losses in some cases and certainly can reduce losses in most cases whereas recovery actions can only attempt to contain the extent of losses and restore essential lost facilities and services.
These differences among the hazards lead to differences in management methods that must be acknowledged and met. This entails, first, recognizing the characteristics of each type of hazard and their consequent effects upon us. The appropriate kinds of management activities and the relative effectiveness of each activity then can be tailored to the type of hazard. We now take the specific case of earthquake safety for elaboration upon this point.
The argument developed above aims essentially at making a strong case for mitigation as the most effective means available to us today to reduce earthquake losses. If this argument is accepted, than we are left with the task of defining mitigation for earthquake safety and, consequently, with describing the implication that a mitigation approaches with respect to emergency management methods.
Mitigation of earthquake risk is accomplished almost entirely through control of the "built environment." Earthquakes themselves rarely if ever kill or injure people directly. Rather, they displace buildings, building components and other elements of the build environment such as highway structures, dams, water and electric systems, etc., which in turn may jeopardize life safety and cause great social and economic inconvenience. By controlling the quality of the things we build and by selecting construction sites less likely to feel hazardous earthquake effects, it is possible to achieve reduced life loss, reduced injuries, and reduced property losses. None of the other emergency management phases accomplish this to any degree even though the phases are necessary parts of a comprehensive emergency operation.
Construction of the built environment is controlled by construction regulations, codes, zoning ordinances, siting evaluations, and good design practices. Most of these controls already are a part of every communitys governance mechanisms. It is through actions that impact upon these processes of control that earthquake mitigation must be accomplished.
The control procedures indicated in the paragraph above are implemented through organizations which have not been dealt with to any great extent by traditional emergency management agencies in the past. Even when emergency management agencies have worked with these existing infrastructures, such as land-use regulatory agencies for flood mitigation efforts, the physical and technical difference between earthquakes and the other hazards allow very little carry-over of learning experiences. It seems clear to this author that effective earthquake hazards mitigation actions will require new liaisons to be forged between emergency management personnel and organizations that control or regulate construction of the built environment.
These new liaisons likely will be somewhat different than the liaison formed in traditional emergency management activities of the past, most notably the civil defense program of the past that dealt with problems not faced by many existing agencies of government. In the case of earthquake mitigation, we find that existing agencies already are in place which have responsibility for controlling the quality of the built environment. It is most likely that these agencies will insist upon preserving their regulatory jurisdictions when earthquake hazards mitigation processes are introduced. Under these circumstances, it is even questionable whether or not the traditional emergency management agency has a role with regard to earthquake hazards mitigation.
Severe flood threat in the State of Utah during the past two years illustrates this point. Having experienced excessive springtime run-off in 1983, with consequent flooding of stream beds and mudslides, Utah counties and cities undertook hurried public works improvements to mitigate similar future problems. Without exception, these projects were managed by existing full-time public works administrators and flood control personnel. These personnel are not part of the state's emergency services agencies and work independently of those agencies. Although coordination between the public works agencies and the emergency services agencies occurred, this was primarily with respect to preparedness and recovery actions. Mitigation actions were carried on by the public works agencies.
Mitigation for earthquake safety seems to have similar restraints in the sense that there are existing governmental agencies responsible for control of the quality of the built environment. Once public policy has been set for earthquake hazards mitigation, as was the case for mitigation of flooding, the existing agencies having jurisdiction will proceed to carry out the policy mandates, I believe.
One implication of the above observation Is that the problem of achieving effective earthquake safety is not so much one of management, but rather is one of persuading a reticent public sector of the need for a sound public policy for earthquake safety. If the public commitment is clear in this regard, the machinery is available In government to carryout the mandate.
THE GAP BETWEEN TECHNOLOGY (RESEARCH) AND APPLICATIONS 
Knowledge about the behavior of earthquakes, although far from adequate for the scientific community, is quite adequate today for applying earthquake risk mitigation techniques to the built environment. The literature on earthquake physical characteristics and on techniques for construction of earthquake-resistant facilities--buildings, transportation systems, dams, utilities systems, etc.--is extensive. Sufficient technical information can be assembled to allow preparation of earthquake risk evaluations which, in turn, allow estimates of possible earthquake losses to be prepared. One also can ascertain the types of likely construction failures associated with the losses.
With such information, one can suggest modifications in siting practices and construction methods that are most effective for saving lives and most cost-effective for the community. Indeed, these kinds of data have been assembled in a variety of forms and for a variety earthquake conditions. As well, some of the data are even assembled for different regional earthquake conditions.
Despite this wealth of information, there has not been widespread application of earthquake risk reduction measures in the private or public sectors of this nation. Except in California, public apathy about earthquake risk prevails, and local governments resist adopting public policies that would encourage application of risk reduction. There is a large gap between the available technical information and application of earthquake mitigation measure.
Credit is due to the federal government which has been actively promoting improved earthquake safety practices and encouraging development of emergency management tools to deal with the hazard. However9theseefforts have aimed largely at making the federal government a helpful partner with state and local government in such matters. In general, mandated federal requirements for earthquake safety do not exist.
Given this present working arrangement, it should come as no surprise that the federal efforts can be no more effective than the efforts of the other half of the partnership--state and local government. It is at these state and local government levels that earthquake safety has failed to receive the attention that I believe is warranted--the exception again being California. Other states and local governments occasionally give verbal support (motherhood statements) to earthquake safety. Rarely have they set forth public policies to bring about the needed changes.
Yet, control and regulation of construction of the built environment lies almost entirely within the domain of state and local government in this nation. The federal government has not usurped this prerogative. State and local governments zone the land; they adopt building codes; and their personnel design many of the public facilities, such as transportation systems, water supply systems, waste systems, and even some utilities systems. Mitigation of earthquake risk, therefore, apparently must be accomplished through these existing institutions and processes of state and local governments. For them to do so, however, the policymaker must be convinced that the public interests are well served. At this time, they do not appear to be convinced.
Some forward motion in improved earthquake safety practices has occurred through the private sector in ways that generally are independent of government. Recognition of this motion is pertinent to our discussion of the gap between technology and applications because it provides further insight into the reasons why the gap occurs.
Construction practices are influenced, sometimes even controlled, by groups besides governmental regulatory agencies. Two such groups are the design professionals and developers of construction codes and standards. The design professional--the architect or engineer--always has the option of specifying construction of a quality that exceeds the minimum requirements of adopted codes and standards. To some extent this has occurred, although randomly, throughout the nation with respect to earthquake-resistant construction. However, without a clear statutory mandate,.designer attentiveness to earthquake hazards mitigation will continue to be random and susceptible to client pressure that the facilities meet only minimum standards of performance.
The national model building code organizations and similar other groups who develop construction codes and standards also have great influence over construction quality. This occurs because the common practice is that state and local governments often adopt these codes as their standards or regulations. Yet, these codes and standards essentially are developed outside of government by mixes of design professionals, building officials acting independently of their agencies, product representatives, and trade organizations. Hence, it is possible to achieve improved earthquake safety practices by including appropriate standards in the codes which eventually get adopted by most, but not all, states and local governments. The process for introducing new concepts into codes and standards is long and tedious, but the avenue is available to us.
Although forward motion in earthquake safety practices has occurred through the two types of groups described above, the efforts have been constrained by inadequate knowledge In application. It is one thing to gain appropriate language in the codes and standards; it is quite another thing to interpret and apply the recommendations in actual construction conditions. Broader and better focused training is essential if the design professionals and the standards are to be a primary means for achieving improved earthquake mitigation practices.
CAN EDUCATION NARROW THE GAP?
In this paper, the existence of a gap between our level of technical knowledge about earthquake hazards and a public willingness to apply the available knowledge to loss reduction practices has been emphasized.
In the author's experience with earthquake safety, this lack of public willingness to utilize available knowledge Is the major reason for the lack of public policies that are needed to promulgate effective earthquake loss reduction actions. Public apathy toward the problem is manifested by the absence of political commitment by state and local governments to deal with the situation in any significant way.
Although the public generally seems to have knowledge about earthquake hazards and associated risks to life and property, albeit sometimes incomplete and inaccurate, this author's view is that there is adequate knowledge and information for the public to take risk reduction actions if only the will to do so were present.
Several conclusions can be drawn from this observation. One can only speculate as to which, if any, of the conclusions are accurate, and, of course, none of the conclusions may be valid if the underlying premise lacks validity--namely, that a public commitment is missing. Five possible conclusions are listed below and then discussed briefly:
1. The risks posed by earthquakes are not believed to be sufficiently great to warrant doing any more than presently is being done to control losses.
2. Earthquake risks are perceived to be too narrowly limited to just a few population centers (earthquake regions) to justify any public policies aimed at abating the problems.
3. In an economic, cost-benefit sense, earthquake risks are perceived (or actually are) lower than the costs of risk reduction.
4. Potential victims of loss believe that governments (federal,
state, and local) will provide the resources to recover any losses. (This conclusion fails to be responsive to the possibility of life loss and injury.)
5. The public simply does not know enough about earthquake risk to give the problem much attention and so does not care.
If Conclusion 1 is accurate, then efforts to broaden the public concern for earthquake safety may be the equivalent of "beating a dead horse." If Conclusion 2 is accurate, then the case can be made for strengthening public information and education programs. If Conclusion 3 is accurate, then some research efforts ought to be shifted to economic analyses to confirm or reject the perceptions. If Conclusion 4 is accurate, then either some changes In governmental assistance policies ought to be made so that individuals and local governments are held accountable for their failure to act prudently or governments should redirect their emergency management functions to preparedness, response, and recovery and abandon mitigation efforts for which the cost is borne by others. If Conclusion 5 is accurate, then intensified efforts in public education seem to be warranted.
This author is not aware of any studies that aim at verifying or rejecting the conclusions suggested above. Until that is done, we can only speculate about which among them may be the more accurate. We therefore cannot direct educational resources to deal with a situation which is inadequately identified.
That the public Is not ready at the present time to make policy commitments to earthquake safety Is the best that can be said. While those of us who seek improvements in earthquake safety can point to a number of individuals and organizations around the nation who feel the same as we do, It is a sad fact that the numbers of us have not grown significantly in recent years nor have we achieved much in the way of public policy changes.
Enough has been said in the negative. The remaining questions are whether or not education and training can help to change this situation and, if so, what might be the form and focus of this education and training. This author's view is that educational efforts in earthquake safety must continue regardless of public receptivity. To do otherwise would reduce, In effect, the level of present knowledge about earthquake hazards and risk reduction for we would fail even to provide an opportunity for follow-up generations to inform themselves, Old timers eventually are replaced by new faces. It Is the natural way of things, We would do a disservice to the younger generations by failing to provide for the transfer of our knowledge.
What kind of education, then, and for whom? Sidestepping for a moment the lack of public commitment to earthquake risk reduction, need for at least three types of education and training can be identified in the comments made in prior portions of this paper: training of emergency management personnel that aims at clarifying the new types of liaisons needed for earthquake risk reduction through mitigation; training for design professionals and governmental regulatory agency personnel that aims at improving their skills in applying mitigation concepts that maybe recommended or mandated in standards and codes; and general public education that aims at advancing the understanding of earthquake risks by the public and their political representatives.
Concurrent with these education and training efforts, it would be helpful to have results from studies of public apathy with respect to earthquake risk--their perceptions, misperceptions, and views--in order to determine whether or not public education is even warranted and, if so, the form it should take to be most effective.

THE NATURE OF THE EARTHQUAKE THREAT IN ST. LOUIS
OTTO W. NUTTLI 
Earthquake hazard in the St. Louis area arises from two causes: nearby earthquakes that produce short-duration, high-frequency ground motion and more distant earthquakes that produce relatively long-duration,
low-frequency ground motion.
Figure I shows my version of the earthquake source zones of the central United States together with my estimates of the surface-wave magnitude of the earthquake with a 1,000-year recurrence time. The source zones closest to the St. Louis area are the St. Francois Mountain uplift to the southwest and the Illinois Basin to the east. The more distant zones are the Wabash Valley fault zone to the southeast and the New Madrid fault zone to the south. On average, St. Louis is 150 to 200 km form the Wabash Valley Zone and 175 to 350 km from the New Madrid Zone.
All four sources zones have produced earthquakes that caused damage in St. Louis. An Ms = 4.4 earthquake in April 1917, which occurred in the St. Francois uplift region about 60 km south of St. Louis, caused modified Mercalli intensity (MMI) V-VI effects in the city. This resulted in bricks being shaken from chimneys, broken windows, cracked plaster, and horses thrown to the pavement.
Two damaging Illinois Basin earthquakes occurred near Centralia, Illinois, about 100 km east of St. Louis. The June 1838 event was of M5= 5.8 and the October 1857 event of Ms = 5.3. Contemporary newspaper accounts and some current earthquake catalogs mistakenly put their epicenters at St. Louis because of the amount of damage that occurred in the city. The former event caused a number of chimneys to be thrown down in St. Louis, corresponding to a MMI of VII. The latter produced only fallen plaster and cracks in walls and chimneys in the St. Louis metropolitan area, corresponding to a MMI of VI.
A Ms = 5.2 earthquake originated in the Wabash Valley region about 150km from St. Louis in November 1968. In St. Louis the MMI was only V(cracked plaster, objects thrown off shelves, etc.) but in the eastern part of the metropolitan area the MMI was at least VI (cracks in walls and chimneys and people thrown to ground).
Dr. Nuttli is Professor of Geophysics in the Department of Earth and Atmospheric Sciences, Saint Louis University, St. Louis, Missouri, He prepared this paper for presentation at the BSSC Meeting in St. Louis on January 23, 1985.
FIGURE I Earthquake source zones of the central United States.
The largest earthquake shaking in the St. Louis area since the citys founding in 1764 was caused by earthquakes of the New Madrid fault zone. Earthquakes in December 1811 and January and February of 1812 (Ms values ranging from 8.0 to 8.7) caused chimneys to be thrown down in St. Louis and 2-foot thick stone building foundations to be badly cracked. There were reports of sand catering and soil liquefaction in Cahokia, Illinois, just across river from St. Louis. The four largest earthquakes caused MMIs of VII to IX in St. Louis area. The October 1895 earthquake (Ms about 6.5) occurred near the northern end of the New Madrid fault and caused MMI VI effects at St. Louis. A few chimneys and old buildings walls were thrown down, suspended objects were thrown from walls, and groceries and other objects were thrown off shelves.
Future earthquake damage in St. Louis can be expected to be more severe than the damage produced by the past earthquakes. In the nineteenth century the population density was low and there were no high-rise structures. There were only 2,000 people living in the metropolitan area in 1811 as opposed to 2,400,000 today. Previously there were no pipelines, bridges, dams, or manufacturing plants with toxic substances to be affected. Furthermore, there was no great dependence on electricity, telephones, highways, and airports, and the economic impact of the disruption of such facilities must be considered.
It is not now possible to make short-term predictions of earthquakes in the Mississippi Valley; however, our knowledge of the earthquake history and the source physics of the New Madrid region permit some generalizations. During the next 50 years MMI VII motion can reasonably be expected in the St. Louis area from earthquake in the St. Francois uplift, the Illinois Basin, or the Wabash Valley region. The shaking will be of relatively short duration (30 seconds or less) and can be expected to cause widespread damage to the walls and chimneys of low-rise structures.
According to my calculations, the maximum earthquake that the New Madrid fault is capable of generating in the near future is one of Ms = 7.6. Figure 2 shows the MMI curves for such an earthquake if it were to occur on the central part of fault. The motion at St. Louis again would be of about MMI of VII, but it would be of relatively low frequency.(about5 to 0.1 Hz), of possibly 2 or more minutes duration, and sinusoidal in character. It would not cause structural damage to well designed, high-rise structures, but it would cause large-amplitude displacements at the upper levels and much nonstructural damage (e.g., fallen ceiling panels and light fixtures, moved and overturned furniture, and fallen debris within and outside the buildings). Widespread chimney damage to low-rise structures also should be expected. Sensitive equipment, including computer facilities, could be put out of operation or damaged. The probability of such an Ms = 7.6 earthquake occurring on the New Madrid fault is about 25 percent in the next 50 years according to Professor Arch Johnston of Memphis State University. However, he finds the probability of occurrence during the next 50 years of the size of the1895 event to be about 90 percent. The extent of damage of this smaller earthquake in the St. Louis area will depend upon whether it occurs near the northern end of the fault as it did in February 1812 and 1895,
near the southern end of the fault as in December 1811 and 1843, or in the central portion as in January 1812.
FIGURE 2 MMI curves for earthquakes generated in the New Madrid fault.