St. Louis Area Earthquake Hazards Mapping Project— 

A Progress Report-November 2008 

By D. Karadeniz, J.D. Rogers, R.A. Williams, C.H. Cramer, R.A. Bauer, D. Hoffman, J. Chung, G.L. Hempen, P.J. 

Steckel, O.L. Boyd, C.M. Watkins, N.S. McCallister, and E. Schweig 

In collaboration with the St. Louis Area Earthquake Hazards Mapping Project (SLAEHMP) 

Open-File Report 2009–1059 

U.S. Department of the Interior 

U.S. Geological Survey 
ii

U.S. Department of the Interior 

Ken Salazar, Secretary 

U.S. Geological Survey 

Suzette M. Kimball, Acting Director 

U.S. Geological Survey, Reston, Virginia 2009 

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod 

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its natural and living resources, natural hazards, and the environment: 

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Suggested citation: 

Karadeniz, D., Rogers, J.D., Williams, R.A., Cramer, C.H., Bauer, R.A., Hoffman, D., Chung, J., Hempen, G.L., 

Steckel, P.J., Boyd, O.L., Watkins, C.M., McCallister, N.S., and Schweig, E., 2009, St. Louis area earthquake hazards 

mapping project—A Progress report-November 2008: U.S. Geological Survey Open-File Report 2009-1059, 14 p. 

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endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 

Cover: St. Louis metropolitan area encompasses 29 USGS quadrangles shown on shaded relief map. 
iii

Contents 

Executive Summary....................................................................................................................................................... 1 

Abstract ......................................................................................................................................................................... 2

Introduction.................................................................................................................................................................... 3

Methodology .................................................................................................................................................................. 5

Bedrock Geology ............................................................................................................................................ 5

Surficial Geology and Depth to Bedrock ......................................................................................................... 6

Shear-Wave Velocity Investigations in the St. Louis Metropolitan Area .......................................................... 7

Reference Vs Profiles ..................................................................................................................................... 8

Site Amplification ............................................................................................................................................ 8

Project Accomplishments .............................................................................................................................................10

Conclusions ..................................................................................................................................................................11

References Cited..........................................................................................................................................................12

Figures 

1. Seismicity of Midwestern United States and the areal extent of the New Madrid and 

 Wabash Valley Seismic Zones .............................................................................................................3 

2. The St. Louis metropolitan area encompasses 29 USGS quadrangles...........................................................4 

3. Schematic cross-section showing the surficial geology in the study area.........................................................6 

4. Probabilistic seismic hazard map showing 0.2 s spectral acceleration(g) with 2 percent probability 

 50 of exceedance in years...................................................................................................................10 
St. Louis Area Earthquake Hazards Mapping Project— 

A Progress Report-November 2008 

By D. Karadeniz, J.D. Rogers, R.A. Williams, C.H. Cramer, R.A. Bauer, D. Hoffman, J. Chung, G.L. Hempen, P.J. 

Steckel, O.L. Boyd, C.M. Watkins, N.S. McCallister, and E. Schweig 

Executive Summary 

St. Louis has experienced minor earthquake damage at least 12 times in the past 200 years. 

Because of this history and its proximity to known active earthquake zones, the St. Louis Area 

Earthquake Hazards Mapping Project (SLAEHMP) is producing digital maps that show variability of 

earthquake hazards, including liquefaction and ground shaking, in the St. Louis area. The maps will be 

available free via the internet. Although not site specific enough to indicate the hazard at a house-byhouse resolution, they can be customized by the user to show specific areas of interest, such as 

neighborhoods or transportation routes. 

Earthquakes currently cannot be predicted, but scientists can estimate how strongly the ground is 

likely to shake as the result of an earthquake. Earthquake hazard maps provide one way of conveying 

such estimates. The U.S. Geological Survey (USGS), which produces earthquake hazard maps for the 

Nation, is working with local partners to develop detailed maps for urban areas vulnerable to strong 

ground shaking. These partners, which along with the USGS comprise the SLAEHMP, include the 

Missouri University of Science and Technology-Rolla (Missouri S&T), Missouri Department of Natural 

Resources (MDNR), Illinois State Geological Survey (ISGS), Saint Louis University, Missouri State 

Emergency Management Agency, and URS Corporation. Preliminary hazard maps covering a test 

portion of the 29-quadrangle St. Louis study area have been produced and are currently being evaluated 

by the SLAEHMP. A USGS Fact Sheet summarizing this project was produced and almost 1000 copies 

have been distributed at several public outreach meetings and field trips that have featured the 

SLAEHMP (Williams and others, 2007). In addition, a USGS website focusing on the SLAEHMP, 

which provides links to project results and relevant earthquake hazard information, can be found at: 

http://earthquake.usgs.gov/regional/ceus/urban_map/st_louis/index.php. This progress report 

summarizes the methodology and data used to generate these preliminary maps. For more details about 

many of the topics in this summary the reader is referred to the Karadeniz (2007) and Chung (2007) 

Ph.D. theses. 

1
Abstract 

Draft probabilistic and deterministic seismic hazard maps for the St. Louis metropolitan area, 

which include the effects of surficial geology on site response, have been prepared for three pilot USGS 

7.5-minute quadrangles of the 29-quadrangle study area. The computer codes used in this study, which 

are similar to those used in the generation of the USGS national seismic hazard maps (Frankel, 2002), 

were modified for use in this study by Cramer (2003). These codes, which were implemented by 

Karadeniz (2007) in his Ph.D. thesis on St. Louis area seismic hazards, account for the fullyprobabilistic approach in developing the maps and apply the median of site amplification estimates to 

the hard-rock ground motion attenuation relations in the deterministic maps. All of the seismic hazards 

were calculated based on a grid of 0.005°, or about every 500 m, the same spacing employed in the 

amplification distribution calculations. To account for some of the uncertainty found in St. Louis area 

shear-wave velocity measurements, shear modulus proxies, depth to bedrock calculations, earthquake 

time histories, and so on, a Monte Carlo randomization procedure was used to generate siteamplification distributions and provide an estimate of the uncertainty, in terms of mean, median, and 

standard deviation. These distributions were assumed to be lognormal in form. 

In this study one-dimensional equivalent-linear response analysis was used to evaluate siteamplifications because of the following reasons: 1) high strain levels are not expected; 2) high excess 

water pressure development is not expected, and 3) the bedrock structure and overlying soft-sediment 

layering is near-horizontal in the St. Louis area. When compared to the USGS National Maps, the new 

probabilistic hazard levels calculated in the pilot study area for upland sites (loess covered residuum, 

drift, till) show zero to 300 percent greater ground motion levels for peak ground acceleration (PGA), 

200 to 250 percent greater ground motion levels for 0.2-s spectral acceleration (SA), and 0 to 175 

percent greater ground motion levels for 1.0-s SA. Probabilistic hazard levels for lowland (alluvial) 

sites, generally the modern Mississippi and Missouri River floodplains, exhibit zero to 200 percent 

greater ground motion levels for PGA, between 20 percent smaller to 150 percent greater ground motion 

levels for 0.2-s SA, and 100 to 260 percent greater ground motion levels for 1.0-s SA, when compared 

to the National Hazard Map. 

A complimentary project in the St. Louis area by William Lettis & Associates (funded by the 

National Earthquake Hazard Reduction Program—NEHRP) has been the generation of liquefaction 

hazard maps in 12 USGS 7.5-minute quadrangles of the St. Louis metropolitan area (Pearce and 

Baldwin; 2008). Their study assesses the relative susceptibility of Quaternary geologic deposits to 

earthquake-induced liquefaction. Based on this analysis they constructed 12 GIS-based, 1:24,000-scale, 

liquefaction susceptibility maps. Their work differentiated four primary geologic units: artificial fill, 

Holocene alluvium, Pleistocene glacio-fluvial outwash, and Pleistocene loess. The results of the 

integrated analysis show that Holocene alluvial units in the lowlands and in a few stream valleys of the 

uplands are the most susceptible to liquefaction. Late Pleistocene glacio-fluvial outwash has a 

moderate-to-low susceptibility; the loess deposits in the uplands have a very low susceptibility. 

Artificial-fill deposits are common, and are assigned a conservative value of “very high” liquefaction 

susceptibility because of the difficulties associated with estimating their geotechnical properties, and 

thus the ability to forecast their response to seismic shaking. Since many transportation routes, power 

and gas transmission lines, and population centers exist in or on the highly susceptible Holocene 

alluvium, the St. Louis region is at significant potential risk from seismically induced liquefaction and 

liquefaction-related ground deformation. 

2
Introduction 

The SLAEHMP is a key part of the USGS Earthquake Hazards Program, which seeks to reduce 

the Nation’s risk from earthquake hazards; other metropolitan areas that are a part of this national 

program include Seattle, Wash., Evansville, Ind., Memphis, Tenn., Reno, Nev., and the Wasatch Front, 

Utah urban corridor. The project addresses earthquake hazards throughout the St. Louis metropolitan 

area, a densely populated urban zone, which is split between Missouri and Illinois (fig. 1). The region 

has experienced strong ground shaking as a result of pre-historic and contemporary seismicity 

associated with the major neighboring seismic source areas, including the Wabash Valley Seismic Zone 

(WVSZ) and New Madrid Seismic Zone (NMSZ) (fig. 1). The goal of the St. Louis project is to 

produce practical hazards maps and an internet-accessible database that can be used by those in the 

geosciences, design communities, and city and county planning agencies and would allow these groups 

to more accurately plan for events of this nature. 

Figure 1. Seismicity of Midwestern United States and the areal extent of the New Madrid and Wabash Valley Seismic 

Zones relative to the St. Louis area. Dots represent the seismic activity recorded during historic time. The diameters of the 

circles represent earthquake epicenters, with increasing magnitude. 

In response to earthquake hazard potential in other parts of the Midwest, in 2004 the USGS 

Memphis office organized the SLAEHMP. The project is guided by a Technical Working Group 

(TWG) consisting of earth scientists and engineers from local firms, universities, and government 

agencies. The TWG convenes four times a year to discuss mutual goals and assignments focusing on 

evaluating relative seismic risks and ground shaking hazards posed to the St. Louis Metropolitan area. 

3
The study area encompasses about 4,000 square km across 29 USGS 7.5-minute quadrangles (fig. 2). 

The objectives of this project are to: 1) create detailed maps of earthquake hazards in the St Louis area, 

Figure 2. The St. Louis metropolitan area encompasses 29 USGS quadrangles as shown on this shaded relief map. The 

three pilot quadrangles highlighted in this report (Monks Mound, Granite City, and Columbia Bottom) are outlined (heavy 

black line). The general outline of NMSZ seismicity is shown as the red area in the upper left (after Hoffman 2005, personal 

commun.; Chung, 2007). 

2) create a three-dimensional database of geologic and geotechnical information (largely 

completed although ongoing and focusing on sparse data areas), and 3) enlist practical input from 

stakeholder and end users of the hazard maps. A principal short-term goal, which is largely 

accomplished and being evaluated by the TWG, is to produce earthquake hazard maps for three pilot 

quadrangles (Granite City, Monks Mound, and Columbia Bottom) that will allow assessment of the 

4
methodology before proceeding to the remaining 26 quadrangles in the study area. The array of hazard 

maps and their format will be very similar to the format established by the USGS for the 

Memphis/Shelby County Seismic Hazard Mapping project completed in 2004. Detailed surficial 

geologic mapping of the entire study area will probably be completed over the next 3-4 years, but a 

complete set of hazard maps for a larger subset of the 29-quadrangle study area adjoining the three pilot 

quads should be completed in about 2 years. 

This region is, for the most part, located on unconsolidated Quaternary deposits which consists 

of: 1) lowlands of alluvial deposits in floodplains along four major rivers (Mississippi, Missouri, 

Illinois, and Meramec), and 2) uplands of loess over residuum or over glacial till or drift deposits 

(Goodfield, 1965; Grimley and others, 2007; Grimley and Phillips, 2006). Bedrock shallowly underlies 

the Quaternary deposits in the St Louis region and largely consists of flat-lying sedimentary rock 

formations, mostly Mississippian-age limestone and Pennsylvanian-age shale (Harrison, 1997). 

According to borehole data provided by the Missouri and Illinois Geological Surveys, the depths to 

bedrock are generally about 30 to 40 m and about 0 to 15 m in lowlands and uplands, respectively. 

Methodology 

The physical properties of the bedrock and Quaternary deposits in the St. Louis area will 

strongly influence seismic wave transmission through the region. It is well known that the bedrock of 

the Central and Eastern United States (CEUS) is older and more indurated than that in the western 

United States, such as California, and partly as a result of this, seismic energy is transmitted much more 

efficiently in the CEUS. These properties of CEUS bedrock also lead to higher seismic wave speeds, 

less energy attenuation, and permit crustal earthquake waves to spread laterally over much larger areas 

(Nuttli, 1973). 

The site response analyses used in this study considered the time-histories at the base of the 

Quaternary deposits and assume a flat (horizontal) boundary between the Quaternary and the underlying 

bedrock. In the St. Louis study area, the Quaternary-rock interface is nearly horizontal, so focusing 

effects are not expected to be a strong influence. Past experiences have demonstrated that the intensity 

of ground shaking may vary considerably during any given earthquake, depending on the underlying 

geology. Our seismic hazard analyses depend on ground shaking estimates which, in turn, are based on 

accurate subsurface characterization of the geology. Some fundamental uncertainties always exist, 

however, with the accuracy of the subsurface models, which are estimates based on data from borings or 

other subsurface measurements that may be located some distance away. The uncertainty in estimated 

depths and thicknesses of geologic layers increases with increasing offset distances from the points of 

subsurface measurement. In this study the most important factors affecting ground shaking estimates 

appear to be the physical properties and thickness of the Quaternary surficial materials overlying the 

bedrock. 

In this section, information on the compiled bedrock and surficial geology are briefly 

summarized, as well as the methods employed to estimate the depth and thickness of the surficial units. 

Bedrock Geology 

The term “bedrock” in the St. Louis area is used to describe those geologic formations of 

Paleozoic Era in the 29-quadrangle study area, which range in age between the Ordovician and 

5
Pennsylvanian Periods. This bedrock is only sparsely exposed in the three pilot quadrangles. Most of 

the information about the underlying bedrock was developed from available borehole logs and 

previously mapped and interpreted areas. Information concerning the bedrock geology is referenced 

from the studies and maps prepared by ISGS, USGS, and MDNR (Denny, 2003; Denny and Devera, 

2001; Harrison, 1997; Grimley and others, 2007; Lutzen and Rockaway, 1971). The bedrock is 

composed mainly of limestone, dolostone, chert, and sandstone. Pockets of individual sinkholes and 

karstic features in these limestones were identified by Goodfield (1965). In the hazard maps, however, 

karstic features were not taken into account because of the limited knowledge about the extent of these 

features. 

Surficial Geology and Depth to Bedrock 

The surficial geology of the St. Louis area varies widely, from thick alluvium in the broad 

Mississippi River valley to thin glacial drift (usually < 15 m), and to thick loess in the areas east of the 

Mississippi River, in Illinois. The loess is thickest (up to 29 m) at the bluffs immediately east of the 

Mississippi River valley and thins to the east and northeast (Fehrenbacher and others, 1986; Grimley 

and others, 2007). The modern river flood plains are comprised of alluvial sediment, mostly silts and 

clays, and a thick sequence of sands and gravels extending down to the bedrock (Goodfield, 1965). 

About 50 to 80 percent of the area in the Granite City, Columbia Bottom, and Monks Mound 

Quadrangles are contained within this Mississippi River alluvial valley (fig. 3). 

Figure 3. Schematic cross-section showing the surficial geology in the study area (after Grimley and others, 2007; Phillips 

and others, 2001). Thickness of Quaternary clay, sand, and gravel in the river lowland, overlying the Paleozoic bedrock, is 

constrained by borehole data (vertical black lines with horizontal bar at maximum depth). 

In the study area the alluvial floodplains, being generally the area of lowest elevation, are 

identified as lowlands. Areas outside the floodplains generally covered by silt, till, loess, and some 

bedrock exposures are identified as upland regions. A Quaternary thickness map (or depth to bedrock) 

of the uplands and lowlands was prepared for the study area and used as input data in ground motion 

hazard calculations. Stratigraphic interpretations and geologic cross sections, used to estimate depth to 

bedrock, were prepared by MDNR and ISGS based on information gleaned from field exposures, 

geophysical surveys, and well logs (geotechnical, water wells, mining, and environmental; Missouri and 

6
Illinois Department of Transportation (MoDOT and IDOT) and other private geotechnical agencies; 

Palmer and others, 2006, written commun.; Palmer 2006, personal commun.; Bauer 2006, personal 

commun.). 

The Quaternary thickness map shows that the bedrock surface is thickest in the lowlands—

generally about 30- to 40-m deep below the ground surface, and 0- to 10-m deep in the uplands. The 

bedrock surface changes most abruptly at the lateral margins of the modern day floodplain, which 

coincides with the boundary between lowlands and uplands. In limited sampling from seismic and 

boreholes the bedrock surface beneath the Mississippi River floodplain appears to be an even planated 

to slightly undulating surface, with an extensive area at an elevation of between 90 and 100 m. The 

greatest uncertainties about depth to bedrock exist in the deep alluvial valleys bordering major rivers, 

where there are few borings piercing the bedrock. 

Shear-Wave Velocity Investigations in the St. Louis Metropolitan Area 

The seismic hazard maps require accurate site-amplification calculations that largely depend on 

the estimated shear-wave velocity (Vs) of the materials underlying the site. In most site response hazard 

studies Vs has emerged as the one index property that is well correlated, inversely, with earthquake 

ground motion (Borcherdt, 1970, Wills and others, 2000). Researchers at Missouri S&T, Saint Louis 

University, MDNR, ISGS, and USGS have collaborated to collect, analyze, and interpret Vs 

measurements in the upper 50 m at over 100 locations in the St. Louis metropolitan area. These data 

constrained the St. Louis area seismic velocity model used in the earthquake hazard map calculations. 

For these seismic studies we used small hammers to gently shake the ground either vertically to 

generate surface waves or horizontally to produce shear waves. Vs data was extracted from a variety of 

field methods including primarily seismic reflection/refraction and multi-channel analysis of surface 

waves (MASW) measurements, a few seismic cone penetrometer tests, one (Horseshoe Lake) site with 

downhole data, and one ultrasonic lab test of St. Louis limestone. As described above, bedrock depth 

around St. Louis was supplemented by existing drillers logs, some older oil exploration wells, and a 

detailed map of bedrock in downtown St. Louis compiled by URS-St. Louis. Subsurface seismic 

velocity models at each test site were constructed which were then used to develop the generalized 

shear-wave reference profiles for the St. Louis area. 

Validation of the results by the different methodologies was achieved by comparing the surfacebased methods (MASW and reflection/refraction) against existing borehole and downhole data from the 

same location or a nearby site. Seismic velocities of the soft sediments determined by surface-based 

methods differed by 5 to 30 percent from the downhole data. Discussions within the SLAEHMP about 

the thickness of highly weathered bedrock around St. Louis indicates that it is generally about 1 m thick 

for the limestone bedrock and that it is probably non-existent at the top of bedrock within the Missouri 

and Mississippi River floodplains where it has been scoured off. Thus, for the purposes of the 

earthquake hazard maps in St. Louis, the depth to the “distinct” alluvium or residuum contact with 

bedrock by reflection/refraction methods was determined to within about 10 percent of the downhole or 

borehole result. Vertical resolution limits of the surface-based methods are on the order of 1.5 to 3 m 

and thus capable of detecting layering that would influence ground motions at the 1- to 5-Hz frequencies 

of the proposed hazard maps. 

Uplands on the Illinois side of the Mississippi River and part of the bedrock below the river on 

the east side is Pennsylvanian strata, which consists mostly of shales and claystones and thin layers of 

coal and limestone. The TWG has placed a 20-m thick weathered zone in these shales based on other 

weathering zones in shales in other parts of the state of Illinois. 

7
Results of the Vs field studies show that the Paleozoic rock underlying softer alluvium and 

residuum in the St. Louis region is clearly identifiable in the reflection/refraction data at depths of 

generally 10 to 40 m. Indeed, in an analysis of St. Louis Advanced National Seismic System (ANSS) 

station recordings of a small earthquake, this important impedance boundary was shown to generate a 

prominent 2-Hz resonance in the 30-m thick alluvial section overlying bedrock in the Mississippi River 

floodplain (Williams and others, 2007). A clear difference exists in the Vs profiles between lowland 

sites and upland urban areas of St. Louis. Vs30 (average Vs to 30-m depth) values in the lowlands 

range from 200 to 290 m/s (NEHRP category D) and contrast with upland sites which have Vs30 values 

ranging from 410 to 785 m/s (NEHRP categories C and B). The lower Vs30 values and earthquake 

recordings in the floodplains suggest a greater potential for stronger and more prolonged ground shaking 

during an earthquake. 

Despite all these measurements and extensive compilation, there are still ‘holes’ or areas of great 

uncertainty, especially about bedrock depth along the margins of the floodplain, in areas of the urban 

uplands not located near major highways and infrastructure. There is now discussion within SLAEHMP 

about gathering new data using inexpensive methods, to target these areas of uncertainty. 

Reference Vs Profiles 

In this section we summarize the methodology for determining two average Vs profiles for the 

study area. The methodology used is similar to that of Romero and Rix (2001, 2005) and Gomberg and 

others (2003). Based on similar characteristics, an average Vs profile was assigned to the lowlands 

(alluvium) and one to the uplands (loess, residuum, and till). Local analyses were performed to 

ascertain variations, uncertainties, and randomness associated with the Vs profiles. This study used 76 

site-specific Vs profiles to compile characteristic profiles needed for the hazard calculations. In 

summary, characteristic Vs profiles were determined following a three-step procedure: 1) investigation 

of geology from the available borehole logs and estimations of the stratigraphy underlying each point of 

calculation, 2) determination of mean Vs (with uncertainties) from local Vs profiles, in one-meter depth 

increments, and 3) comparing the variations in Vs values with borings and known limiting parameters, 

such as depth-to-bedrock. See Karadeniz (2007) for more details on this methodology. 

Site Amplification 

The method used to calculate site amplification was similar to those employed in the Memphis 

seismic hazard maps, summarized in Cramer and others (2004). Time histories (see below) were input 

into the one-dimensional site-response software program SHAKE91 which calculates the propagation of 

the wave through the soil column and estimates the site-specific amplification factors and other 

parameters. 

Anytime we perform a series of calculations that utilize a series of input variables, uncertainties 

with each of those variables will be compounded, leading to a greater range of uncertainty, bracketing 

the calculated/reported values. In the assessment of site amplification, uncertainties exist in the 

following input parameters: 1) natural variations in shear-wave velocity (for example, horizontal versus 

vertically propagating shear waves, effects of fracture intensity, weathering, and so forth), 2) natural 

variations in bulk density (especially, with preferential weathering), 3) the techniques used to estimate 

the depth and thickness of the soil layers, and 4) the differences in the earthquake time-history records 

8
used in the 1-D shaking analyses. When combined together, these uncertainties may cause large 

differences in amplification calculations. To account for this variability and uncertainties, a random 

sampling method is usually applied. Cramer and others (2004) used amplification distributions to 

account for the uncertainties associated with the amplification calculations. Cramer (2003) asserted that 

this method of calculating the hazard was the most dependable because it incorporates the uncertainties 

in the amplification factor. When a truly probabilistic site-specific ground motion is desired, the stateof-the-art approach should be used to estimate the site specific amplification factor distributions for use 

in the probabilistic calculations (Cramer, 2003; Cramer, and others 2004). 

The site amplification calculations for peak ground acceleration, 0.2 s and 1.0 s spectral 

accelerations (SA) were performed using the site amplification code (siteampunc.f) provided by Chris 

Cramer (written commun., 2007). In this code, input site response parameters are randomly selected 

from a range of Vs profiles, dynamic soil properties, geologic boundaries, and a set of earthquake 

acceleration time-histories. The code then inputs these randomly selected parameters into Shake91 and 

calculates the response. The process for selecting input parameters is explained in the following 

sections and the results are summarized. 

The amplification distributions were calculated based on a grid of 0.005° or for about every 500 

m. There were a total of 1,974 grid points encompassing the three study quadrangles. For every grid 

point the site amplifications and distributions were calculated first, then the seismic hazard calculations. 

The amplification distributions were generated for two distinct geologic units (lowland deposits and 

upland deposits), and the 500-m grid is thought to be sufficient enough to capture the differences 

between these two units. The amplification values were then smoothed using GIS-based software and 

drawn as smooth color contours. 

In this study, 12 recordings from six real earthquakes were selected in an attempt to capture the 

complexity of earthquake-time histories at epicentral distances close to 200 km. These recordings were 

obtained from the PEER strong-motion database, COSMOS strong-motion (http://db.cosmoseq.org/scripts/default.plx), and Turkish General Directorate of Disaster Affairs 

(http://www.deprem.gov.tr/) strong-motion databases. In addition to these earthquake recordings, two 

synthetically generated M7.5 and M8.0 records, from Atkinson and Beresnev (2002), and M7.0 and 

M7.5 records using the SMSIM v. 2.2 code of Boore (2000), were selected. These synthetic recordings 

were chosen because they were felt to be more representative of the CEUS source characteristics and 

attenuation/damping properties. St. Louis is located approximately 200 km from the New Madrid and 

Wabash Valley Seismic Zones and, hence, recordings located at a distance of 180 to 220 km were 

selected, with magnitudes as close to 7.5 as possible. 

To characterize the ground shaking in a fully probabilistic approach, the areal distribution of site 

amplification was required. To capture the amplification distributions, the above mentioned earthquake 

time-histories were scaled. This was accomplished on the actual ground-motion records at ten different 

shaking levels (0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, and 1.0g) at specific frequencies (PGA, 0.2 s 

SA, and 1.0 s SA) to obtain input, or base rock ground-motions. An example of the 0.2 s SA 

probabalistic calculation is shown in figure 4. The Shake91 program was run for each of these shaking 

levels and the predicted site amplifications were determined for each level. In this study we used the 

shear modulus and damping ratio relations published by EPRI (1993). 

9
Figure 4. Probabilistic seismic hazard map showing 0.2-s SA(g) with two percent probability of exceedance in 50 years. 

The L-shape area represents the area of the three pilot quadrangles superimposed on the 2002 USGS National Seismic 

Hazard map for the same ground motion level (after Karadeniz, 2007). 

Project Accomplishments 

This study has so far accomplished the following: 

1. Communication of interim project results occurred at several public outreach meetings, newspaper 

and television pieces, and field trips with an orientation toward St. Louis area earthquake hazard. A 

USGS Fact Sheet and USGS website focusing on the SLAEHMP were prepared. Several NEHRPfunded Final Technical Reports focusing on the St. Louis area have been completed including 

studies of surficial geologic mapping, liquefaction, borehole compilation, database construction, 

shear-wave velocities, and a comparison of 2002 versus 2008 hazard model changes on the three 

pilot quadrangles. A journal article on St. Louis area seismic velocities and site response has also 

been completed. In addition, two NEHRP-assisted Ph.D.’s on St. Louis earthquake hazards were 

completed by Missouri University of Science and Technology students. 

2. Distribution of the surficial material thickness in the St. Louis area was determined with respect to 

recognized (mapped) surficial geologic units. For this purpose subsurface data from 1,500 

10
3. Reference Vs profiles were created from the shear-wave velocity measurements taken in the St. 

Louis region for this study. The shear-wave velocity statistical analysis included an assessment of 

the variations in the measurements and the data interpretation. Reference Vs profiles were prepared 

for the two dominant types of surficial conditions (lowlands and uplands) in the study area using 5-

m depth increments. These reference profiles were the first ones generated for the St. Louis area. 

4. Site-amplification factors were estimated for the areas of the three pilot quadrangles, with respect to 

earthquake-shaking level and these data were summarized in map form. These site-amplification 

distributions were calculated for upland and lowland sites. Site-amplification analyses were carried 

out accounting for the uncertainties associated with each of the input parameters. 

5. Hazard analyses were performed for PGA, 0.2 s SA and 1.0 s SA, accounting for the uncertainties 

associated with the results of the site amplification estimates. These results are unique for the area 

because the hazard analyses were based on a fully-probabilistic approach, where site amplification 

distributions were incorporated into the probabilistic calculations instead of just using the median 

site amplification value. 

Conclusions 

The following conclusions have been reached for the preliminary estimations of site 

amplification and the associated hazard estimations. These conclusions are subject to change or 

revision as new data, information, and observations become available. 

1. In the St. Louis area the modern floodplains (lowlands) and the alluvial thickness is fairly uniform, 

between 30 and 40 m, thinning to as little as 5 m at the lateral margins bounding the uplands. The 

thickness of the Quaternary deposits in the uplands varies considerably between 5 and 73 m. 

2. The median shear-wave velocity of Quaternary alluvium (lowland deposits) is 200 m/s, and upland 

deposits have an average velocity of 208 m/s. 

3. Natural variations and physical characteristics of the Quaternary deposits overlying the Paleozoic 

bedrock in the St. Louis area exerts a significant influence on the amplitude and frequency 

characteristics of the earthquake ground motions at the surface. 

4. The measured Vs in lowlands and uplands appear to be a simple function of depth and not their 

geologic age. 

5. Accurate estimates of Quaternary material thickness are essential to allow accurate predictions of 

site amplification, especially for 1.0-s SA. 

The following results apply to the three Pilot Quadrangles study area: Columbia Bottom, Granite City, 

and Monks Mound. 

6. Lowland and upland sites exhibit distinct amplification behaviors for small ground-motion levels 

(<0.01g) for all ground motion parameters: PGA, 0.2-s SA and 1.0-s spectral acceleration. 

7. Although differences in site amplification between lowlands and upland sites tend to decrease with 

increasing ground motion (up to 0.5g), low strain ground motion recordings of earthquakes in 

11
8. Lowland sites exhibited deamplification of ground motions when the rock accelerations exceed 0.5g. 

Upland sites did not exhibit deamplification at any input acceleration (between 0.01 and 1.0g). 

9. The two most important parameters affecting site amplification estimates were the input-time 

histories and the thickness of Quaternary deposits. 

10. Lowlands and uplands exhibit contrasting shaking characteristics for each of the ground motion 

parameters and earthquake scenarios considered. Uplands are characterized by large accelerations at 

0.2-s-period SA for all levels of probability, consistently more than 50 percent greater than in the 

lowlands. This means that on upland sites, earthquake forces may be most severe for short period 

structures (less than five stories high). 

11. Lowlands are characterized by large accelerations at 1.0-s-period SA for all levels of probability. 

This means, in the lowlands, earthquake forces may also be severe for long period structures (greater 

than 10 stories). 

12. The PGAs are similar for the lowlands and uplands sites for all probabilistic ground-motion levels. 

13. When compared to the USGS National Maps, the probabilistic hazard levels calculated in this study 

for lowland sites exhibit zero to 200 percent greater ground motion levels for PGA, and between 20 

percent smaller to 150 percent greater ground-motion levels for 0.2-s SA; and, 100 to 260 percent 

greater ground-motion levels for 1.0-s SA. 

14. When compared to the USGS National Maps, the probabilistic hazard levels calculated in this study 

for upland sites show zero to 300 percent greater ground-motion levels for PGA, 200 to 250 percent 

greater ground-motion levels for 0.2-s SA and 0 to 175 percent greater ground-motion levels for 1.0-

s SA. 

15. The results of the deterministic scenarios suggest that the hazard levels for 0.2-s SA in uplands sites 

are approximately 15 percent higher than for lowland sites. The lowland sites exhibit consistently 

higher acceleration levels at 1.0-s SA, similar to the probabilistic maps. 

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