Appendix F

Earthquakes and How Buildings

F.1 The Nature of Earthquakes 

In a global sense, earthquakes result from motion between plates comprising the 
earth's crust (see Figure F-1). These plates are driven by the convective motion 
of the material in the earth's mantle between the core and the crust, which in 
turn is driven by heat generated at the earth's core. Just as in a heated pot of 
water, heat from the earth's core causes material to rise to the earth's 
surface. Forces between the rising material and the earth's crustal plates 
cause the plates to move. The resulting relative motions of the plates are 
associated with the generation of earthquakes. Where the plates spread apart, 
molten material fills the void. An example is the ridge on the ocean floor, at 
the middle of the Atlantic Ocean. This material quickly cools and, over millions 
of years, is driven by newer, viscous, fluid material across the ocean floor. 

These large pieces of the earth's surface, termed tectonic plates, move very 
slowly and irregularly. Forces build up for decades, centuries, or millennia at 
the interfaces (or faults) between plates, until a large releasing movement 
suddenly occurs. This sudden, violent motion produces the nearby shaking that is 
felt as an earthquake. Strong shaking produces strong horizontal forces on 
structures, which can cause direct damage to buildings, bridges, and other man-
made structures as well as triggering fires, landslides, road damage, tidal 
waves (tsunamis) and other damaging phenomena. 

A fault is like a "tear" in the earth's crust and its fault surface may be from 
one to over one hundred miles deep. In some cases, faults are the physical 
expression of the boundary between adjacent tectonic plates and thus are 
hundreds of miles long. In addition, there are shorter faults, parallel to, or 
branching out from, a main fault zone. Generally, the longer a fault, the larger 
magnitude earthquake it can generate. Beyond the main tectonic plates, there 
are many smaller sub-plates, "platelets" and simple blocks of crust which can 
move or shift due to the "jostling" of their neighbors and the major plates. The 
known existence of these many sub-plates implies that smaller but still damaging 
earthquakes are possible almost anywhere. With the present understanding of the 
earthquake generating mechanism, the times, sizes and locations of earthquakes 
cannot be reliably predicted. Generally, earthquakes will be concentrated in the 
vicinity of faults, and certain faults are more likely than others to produce a 
large event, but the earthquake generating process is not understood well enough 
to predict the exact time of earthquake occurrence. Therefore, communities must 
be prepared for an earthquake to occur at any time. 

Four major factors can affect the severity of ground shaking and thus potential 
damage at a site. These are the magnitude of the earthquake, the type of 
earthquake, the distance from the source of the earthquake to the site, and the 
hardness or softness of the rock or soil at the site. Larger earthquakes will 
shake longer and harder, and thus cause more damage. Experience has shown that 
the ground motion can be felt for several seconds to a minute or longer. In 
preparing for earthquakes, both horizontal (side to side) and vertical shaking 
must be considered. 

There are many ways to describe the size and severity of an earthquake and 
associated ground shaking. Perhaps the most familiar are earthquake magnitude 
and Modified Mercalli Intensity (MMI, often simply termed "intensity"). 
Earthquake magnitude is technically known as the Richter magnitude, a numerical 
description of the maximum amplitude of ground movement measured by a 
seismograph (adjusted to a standard setting). On the Richter scale, the largest 
recorded earthquakes have had magnitudes of about 8.5. It is a logarithmic 
scale, and a unit increase in magnitude corresponds to a ten-fold increase in 
the adjusted ground displacement amplitude, and to approximately a thirty-fold 
increase in total potential strain energy released by the earthquake. 

Modified Mercalli Intensity (MMI) is a subjective scale defining the level of 
shaking at specific sites on a scale of I to XII. (MMI is expressed in Roman 
numerals, to connote its approximate nature.) For example, slight shaking that 
causes few instances of fallen plaster or cracks in chimneys constitutes MMI VI. 
It is difficult to find a reliable precise relationship between magnitude, which 
is a description of the earthquake's total energy level, and intensity, which is 
a subjective description of the level of shaking of the earthquake at specific 
sites, because shaking intensity can vary with earthquake magnitude, soil type, 
and distance from the event. 

The following analogy may be worth remembering: earthquake magnitude and 
intensity are similar to a light bulb and the light it emits. A particular light 
bulb has only one energy level, or wattage (e.g., 100 watts, analogous to an 
earthquake's magnitude). Near the light bulb, the light intensity is very bright 
(perhaps 100 foot-candles, analogous to MMI IX), while farther away the 
intensity decreases (e.g., 10 footcandles, MMI V). A particular earthquake has 
only one magnitude value, whereas it has intensity values that differ 
throughout the surrounding land. 

MMI is a subjective measure of seismic intensity at a site, and cannot be 
measured using a scientific instrument. Rather, MMI is estimated by scientists 
and engineers based on observations, such as the degree of disturbance to the 
ground, the degree of damage to typical buildings and the behavior of people. A 
more objective measure of seismic shaking at a site, which can be measured by 
instruments, is a simple structure's acceleration in response to the ground 
motion. In this Handbook, the level of ground shaking is described by the 
spectral response acceleration. 

F.2 Seismicity of the United States 

Maps showing the locations of earthquake epicenters over a specified time period 
are often used to characterize the seismicity of given regions.  Figures F-2, F-
3, and F-4 show the locations of earthquake epicenters4 in the conterminous 
United States, Alaska, and Hawaii, respectively, recorded during the time 
period, 1977-1997. It is evident from Figures F-2 through F-4 that some parts of 
the country have experienced more earthquakes than others. The boundary between 
the North American and Pacific tectonic plates lies along the west coast of the 
United States and south of Alaska. The San Andreas fault in California and the 
Aleutian Trench off the coast of Alaska are part of this boundary. These active 
seismic zones have generated earthquakes with Richter magnitudes greater than 8. 
There are many other smaller fault zones throughout the western United States 
that are also participating intermittently in releasing the stresses and strains 
that are built up as the tectonic plates try to move past one another. Because 
earthquakes always occur along faults, the seismic hazard will be greater for 
those population centers close to active fault zones. 

In California the earthquake hazard is so significant that special study zones 
have been created by the legislature, and named Alquist-Priola Special Study 
Zones. These zones cover the larger known faults and require special 
geotechnical studies to be performed in order to establish design parameters. 

On the east coast of the United States, the sources of earthquakes are less 
understood. There is no plate boundary and few locations of faults are known. 
Therefore, it is difficult to make statements about where earthquakes are most 
likely to occur. Several significant historical earthquakes have occurred, such 
as in Charleston, South Carolina, in 1886 and New Madrid, Missouri, in 1811 and 
1812, indicating that there is potential for large earthquakes. However, most 
earthquakes in the eastern United States are smaller magnitude events. Because 
of regional geologic differences, specifically, the hardness of the crustal 
rock, eastern and central U.S. earthquakes are felt at much greater distances 
from their sources than those in the western United States, sometimes at 
distances up to a thousand miles.

F.3 Earthquake Effects 

Many different types of damage can occur in buildings. Damage can be divided 
into two categories: structural and nonstructural, both of which can be 
hazardous to building occupants. Structural damage means degradation of the 
building's structural support systems (i.e., vertical- and lateral-force-
resisting systems), such as the building frames and walls. Nonstructural damage 
refers to any damage that does not affect the integrity of the structural 
support systems. Examples of nonstructural damage are chimneys collapsing, 
windows breaking, or ceilings falling. The type of damage to be expected is a 
complex issue that depends on the structural type and age of the building, its 
configuration, construction materials, the site conditions, the proximity of the 
building to neighboring buildings, and the type of non-structural elements. 

When strong earthquake shaking occurs, a building is thrown mostly from side to 
side, and also up and down. That is, while the ground is violently moving from 
side to side, taking the building foundation with it, the building structure 
tends to stay at rest, similar to a passenger standing on a bus that accelerates 
quickly. Once the building starts moving, it tends to continue in the same 
direction, but the ground moves back in the opposite direction (as if 
the bus driver first accelerated quickly, then suddenly braked). Thus the 
building gets thrown back and forth by the motion of the ground, with some parts 
of the building lagging behind the foundation movement, and then moving in the 
opposite direction. The force F that an upper floor level or roof level of the 
building should successfully resist is related to its mass m and its 
acceleration a, according to Newton's law, F= ma. The heavier the building 
the more the force is exerted. Therefore, a tall, heavy, reinforced-concrete 
building will be subject to more force than a lightweight, one-story, wood-frame 
house, given the same acceleration. Damage can be due either to structural 
members (beams and columns) being overloaded or differential movements between 
different parts of the structure. If the structure is sufficiently strong to 
resist these forces or differential movements, little damage will result. If the 
structure cannot resist these forces or differential movements, structural 
members will be damaged, and collapse may occur. 

Building damage is related to the duration and the severity of the ground 
shaking. Larger earthquakes tend to shake longer and harder and therefore cause 
more damage to structures. Earthquakes with Richter magnitudes less than 5 
rarely cause significant damage to buildings, since acceleration levels (except 
when the site is on the fault) and duration of shaking for these earthquakes are 
relatively small. In addition to damage caused by ground shaking, damage can be 
caused by buildings pounding against one another, ground failure that causes the 
degradation of the building foundation, landslides, fires and tidal waves 
(tsunamis). Most of these "indirect" forms of damage are not addressed in this 
Handbook. 

Generally, the farther from the source of an earthquake, the less severe the 
motion. The rate at which motion decreases with distance is a function of the 
regional geology, inherent characteristics and details of the earthquake, and 
its source location. The underlying geology of the site can also have a 
significant effect on the amplitude of the ground motion there. Soft, loose 
soils tend to amplify the ground motion and in many cases a resonance effect can 
make it last longer. In such circumstances, building damage can be accentuated. 
In the San Francisco earthquake of 1906, damage was greater in the areas where 
buildings were constructed on loose, man-made fill and less at the tops of the 
rocky hills. Even more dramatic was the 1985 Mexico City earthquake. This 
earthquake occurred 250 miles from the city, but very soft soils beneath the 
city amplified the ground shaking enough to cause weak mid-rise buildings to 
collapse (see Figure F-5). Resonance of the building frequency with the 
amplified ground shaking frequency played a significant role. Sites with rock 
close to or at the surface will be less likely to amplify motion. The type of 
motion felt also changes with distance from the earthquake. Close to the source 
the motion tends to be violent rapid shaking, whereas farther away the motion is 
normally more of a swaying nature. Buildings will respond differently to the 
rapid shaking than to the swaying motion. Each building has its own vibrational 
characteristics that depend on building height and structural type. Similarly, 
each earthquake has its own vibrational characteristics that depend on the 
geology of the site, distance from the source, and the type and site of the 
earthquake source mechanism. Sometimes a natural resonant frequency of the 
building and a prominent frequency of the earthquake motion are similar and 
cause a sympathetic response, termed resonance. This causes an increase in the 
amplitude of the building's vibration and consequently increases the potential 
for damage. 

Resonance was a major problem in the 1985 Mexico City earthquake, in which the 
total collapse of many mid-rise buildings (Figure F-5) caused many fatalities. 
Tall buildings at large distances from the earthquake source have a small, but 
finite, probability of being subjected to ground motions containing frequencies 
that can cause resonance. 

Where taller, more flexible, buildings are susceptible to distant earthquakes 
(swaying motion) shorter  and stiffer buildings are more susceptible to nearby 
earthquakes (rapid shaking). Figure F-6 shows the effects on shorter, stiffer 
structures that are close to the source. The inset picture shows the interior of 
the house. Accompanying the near field effects is surface faulting also shown in 
Figure F-6. 

The level of damage that results from a major earthquake depends on how well a 
building has been designed and constructed. The exact type of damage cannot be 
predicted because no two buildings undergo identical motion. However, there are 
some general trends that have been observed in many earthquakes. 

. Newer buildings generally sustain less damage Figure F-7 Collapsed chimney 
with damaged roof, than older buildings designed to earlier  codes. 1987 
Whittier Narrows earthquake. 

. Common problems in wood-frame construction are the collapse of unreinforced 
chimneys
(Figure F-7) houses sliding off their foundations
(Figure F-8),collapse of cripple walls
(Figure F-9), or collapse of post and pier foundations
(Figure F-10). Although such damage may
be costly to repair, it is not usually life threatening.

.  The collapse of load bearing walls that support an entire structure is a 
common form of damage in unreinforced masonry structures (Figure F-11)

.  Similar types of damage have occurred in many older tilt-up buildings (Figure 
F-12).

From a life-safety perspective, vulnerable buildings need to be clearly 
identified, and then strengthened or demolished.

F.4 How Buildings Resist Earthquakes 

The collapse of load bearing walls that support As described above, buildings 
experience horizontal an entire structure is a common form of damage distortion 
when subjected to earthquake motion. in unreinforced masonry structures When 
these distortions get large, the damage can be catastrophic (Figure F-11). 

Therefore, most buildings are designed with lateral-force-resisting systems (or 
seismic systems), to resist the effects of earthquake forces. In many cases 
seismic systems make a building stiffer against horizontal forces, and 
thus minimize the amount of relative lateral movement and consequently the 
damage. Seismic systems are usually designed to resist only forces that result 
from horizontal ground  motion, as distinct from vertical ground motion. 

The combined action of seismic systems along the width and length of a building 
can typically resist earthquake motion from any direction. Seismic systems 
differ from building to building because the type of system is controlled to 
some extent by the basic layout and structural elements of the building. 
Basically, seismic systems consist of axial-, shear-and bending-resistant 
elements. 

In wood-frame, stud-wall buildings, plywood siding is typically used to prevent 
excessive lateral deflection in the plane of the wall. Without the extra 
strength provided by the plywood, walls would distort excessively or "rack," 
resulting in broken windows and stuck doors. In older wood frame houses, this 
resistance to lateral loads is provided by either wood or steel diagonal 
bracing. 

The earthquake-resisting systems in modern steel buildings take many forms. In 
moment-resisting steel frames, the connections between the beams and the columns 
are designed to resist the rotation of the column relative to the beam. Thus, 
the beam and the column work together and resist lateral movement and lateral 
displacement by bending. Steel frames sometimes include diagonal bracing 
configurations, such as single diagonal braces, cross-bracing and "Kbracing." In 
braced frames, horizontal loads are resisted through tension and compression 
forces in the braces with resulting changed forces in the beams and columns. 
Steel buildings are sometimes constructed with moment-resistant frames in one 
direction and braced frames in the other. In concrete structures, shear walls 
are sometimes used to provide lateral resistance in the plane of the wall, in 
addition to moment-resisting frames. Ideally, these shear walls are continuous 
reinforced-concrete walls extending from the foundation to the roof of the 
building. They can be exterior walls or interior walls. They are interconnected 
with the rest of the concrete frame, and thus resist the horizontal motion 
of one floor relative to another. Shear walls can also be constructed of 
reinforced masonry, using bricks or concrete blocks. 

Appendix A

ASCE, 1998, Handbook for the Seismic Evaluation of Buildings - A Pre-standard, 
prepared by the American Society of Civil Engineers for the Federal Emergency 
Management Agency, FEMA 310 Report, Washington D.C. 

ASCE, 2000, Prestandard and Commentary for the Seismic Rehabilitation of 
Buildings, prepared by the American Society of Civil Engineers for the Federal 
Emergency Management Agency, FEMA 356 Report, Washington, D.C. 

ATC, 1987, Evaluating the Seismic Resistance of Existing Buildings, Applied 
Technology Council, ATC-14 Report, Redwood City, California. 

ATC, 1989, Procedures for Postearthquake Safety Evaluation of Buildings, Applied 
Technology Council, ATC-20 Report, Redwood City, California. 

ATC, 1992, Procedures for Building Seismic Rehabilitation (Interim), Applied 
Technology Council, ATC-26-4 Report, Redwood City, California 

ATC, 1995, Addendum to the ATC-20 Postearthquake Building Safety Procedures 
Applied Technology Council, ATC-20-2 Report, Redwood City, California. 

ATC, 1988a, Rapid Visual Screening of Buildings for Potential Seismic Hazards:  
A Handbook, prepared by the Applied Technology Council for the Federal Emergency 
Management Agency, FEMA 154 Report, Washington, D.C. 

ATC, 1988b, Rapid Visual Screening of Buildings for Potential Seismic Hazards:  
Supporting Documentation, prepared by the Applied Technology Council for the 
Federal Emergency Management Agency, FEMA 155 Report, Washington, D.C. 

ATC, 1997a, NEHRP Guidelines for the Seismic Rehabilitation of Buildings, 
prepared by the Applied Technology Council for the Building Seismic Safety 
Council, published by the Federal Emergency Management Agency, FEMA 273 Report, 
Washington, D.C. 

ATC, 1997b, NEHRP Commentary on the Guidelines for the Seismic Rehabilitation of 
Buildings, prepared by the Applied Technology Council for the Building Seismic 
Safety Council, published by the Federal Emergency Management Agency, FEMA 274 
Report, Washington, D.C. 

ATC, 2002, Rapid Visual Screening of Buildings for Potential Seismic Hazards:  
Supporting Documentation (2nd edition), prepared by the Applied Technology 
Council for the Federal Emergency Management Agency, FEMA 155 Report, 
Washington, D.C. 

BSSC, 1992, NEHRP Handbook for the Seismic Evaluation of Existing Buildings, 
prepared by the Building Seismic Safety Council for the Federal Emergency 
Management Agency, FEMA 178 Report, Washington D.C. 

BSSC, 1997, NEHRP Recommended Provisions for the Seismic Regulations for New 
Buildings and Other Structures, and Commentary, prepared by the Building Seismic 
Safety Council for the Federal Emergency Management Agency, FEMA 302 and 303 
Reports, Washington, D.C. 

BSSC, 2000, NEHRP Recommended Provisions for the Seismic Regulations for New 
Buildings and Other Structures, and Commentary, 2000 Edition, prepared by the 
Building Seismic Safety Council for the Federal Emergency Management Agency, 
FEMA 368 and 369 Reports, Washington, D.C. 

EERI, 1998, Seismic Rehabilitation of Buildings:  Strategic Plan 2005, prepared 
by the Earthquake Engineering Research Institute for the Federal Emergency 
Management Agency, FEMA 315 Report, Washington, D.C. 

FEMA, 1995, Typical Costs for Seismic Rehabilitation of Buildings, Second 
Edition, FEMA 156 and 157 Reports, Federal Emergency Management Agency, 
Washington, D.C. 

ICBO, 1973, 1997, Uniform Building Code, International Conference of Building 
Officials, Whittier, California. 

NBS, 1980, Development of a Probability Based Load Criterion for American 
National Standard A58.1, NBS Special Publication 577, National Bureau of 
Standards, Washington, D.C. 

NIBS, 1999, Earthquake Loss Estimation Methodology HAZUS, Technical Manual, Vol. 
1, prepared by the National Institute of Building Sciences for the Federal 
Emergency Management Agency, Washington, D.C. 

ROA, 1998, Planning for Seismic Rehabilitation:  Societal Issues, developed for 
the Building Seismic Safety Council, by Robert Olson Associates, Inc., for the 
Federal Emergency Management Agency, FEMA-275 Report, Washington, D.C. 

SAC, 2000, Recommended Seismic Design Criteria for New Steel Moment-Frame 
Buildings, prepared by the SAC Joint Venture, a partnership of the Structural 
Engineers Association of California, the Applied Technology Council, and 
California Universities for Research in Earthquake Engineering, for the Federal 
Emergency Management Agency, FEMA 350 Report, Washington, D.C. 

VSP, 1994, Seismic Rehabilitation of Federal Buildings: A Benefit/Cost Model; 
Volume 1: A Users Manual and Volume 2: Supporting Documentation; prepared by VSP 
Associates, Sacramento California, for the Federal Emergency Management Agency, 
FEMA-255 and FEMA-256 Reports, Washington, D.C. 

Web pages 
Sanborn Map Company www.sanbornmap.com www.lib.berkeley.edu/EART/sanborn.html 
References FEMA 154 

Project 
Participants 

Project Management 
Mr. Christopher Rojahn (Principal Investigator) 
Applied Technology Council 555 Twin 
Dolphin Drive, Suite 550 Redwood City, California  94065 

Dr. Charles Scawthorn (Co-Principal Investigator and Project Director) 
ABS Consulting 1111 
Broadway, 10th Floor Oakland, California 94607 

FEMA Management 
Mr. Ugo Morelli Federal Emergency Management Agency 500 C Street, Room 416 
Washington, DC 20472 

Project Advisory Panel 

Prof. Thalia Anagnos
(San Jose State University)
2631 South Court
Palo Alto, California 94306

Mr. John Baals, Seismic Safety Program
Coordinator, U.S. Department of the Interior
Bureau of Reclamation
Denver Federal Center, Building 67
P.O. Box 25007, D-8110
Denver, Colorado 80225-0007

Mr. James Cagley*
Cagley & Associates
6141 Executive Blvd.
Rockville, Maryland 20852

Mr. Melvyn Green
Melvyn Green & Associates
21307 Hawthorne Blvd., Suite 250
Torrance, California 90503

Mr. Terry Hughes, CBO
Code Specialist
Hnedak Bobo Group, Inc.
104 South Front Street
Memphis, Tennessee 38103

Prof. Anne S. Kiremidjian
(Stanford University)
1421 Berry Hills Court,
Los Altos, California 94305

Ms. Joan MacQuarrie
Chief Building Official
City of Berkeley
2120 Milvia Street
Berkeley, California 94704

Mr. Chris D. Poland
Degenkolb Engineers
225 Bush Street, Suite 1000
San Francisco, California 94104

Prof. Lawrence D. Reaveley
(University of Utah)
1702 Cannes Way
Salt Lake City, Utah 84121
Mr. Doug Smits

Chief Building/Fire Official
City of Charleston
75 Calhoun Street, Division 320
Charleston, South Carolina 29401

Mr. Ted Winstead
Winstead Engineering, Inc.
2736 Gerald Ford Drive, East
Cordova, Tennessee 38016

Technical Consultants

Mr. Kent David
ABS Consulting
1111 Broadway, 10th Floor
Oakland, California 94607-5500

Dr. Stephanie A. King
Weidlinger Associates
4410 El Camino Real, Suite 110
Los Altos, California 94022
Mr. Vincent Prabis
ABS Consulting
1111 Broadway, 10th Floor
Oakland, California 94607-5500

Mr. Richard Ranous
ABS Consulting
300 Commerce Drive, Suite 200
Irvine, California 92602

Dr. Nilesh Shome
ABS Consulting
1111 Broadway, 10th Floor
Oakland, California 94607-

Workshop Consultants

Mr. William Holmes (Facilitator)
Rutherford & Chekene
427 Thirteenth Street
Oakland, California 94612

Dr. Keith Porter (Recorder)
California Institute of Technology
1200 E. California Blvd., MC 104-44
Pasadena, California 91125

Report Production and Editing

Dr. Gerald Brady
Applied Technology Council
555 Twin Dolphin Drive, Suite 550
Redwood City, California 94065

Mr. Peter Mork
Applied Technology Council
555 Twin Dolphin Drive, Suite 550
Redwood City, California 94065

Ms. Michelle Schwartzbach
Applied Technology Council
555 Twin Dolphin Drive, Suite 550
Redwood City, California 94065