Earthquake Hazard Information – Hazard, Risk, Magnitude, Intensity, Earthquake Statistics (Information for interpreting the results of building contest and shake table testing; L. Braile, 03/12/03)

Earthquake ground shaking and damage are related to the size (magnitude) of the earthquake, the distance from the epicenter, the local geological conditions and the characteristics of buildings. Assessment of earthquake effects involves evaluating the hazard and the risk. Definitions of these two concepts (from the USGS, http://earthquake.usgs.gov/image_glossary/) are:

 

Earthquake hazardEarthquake hazard is anything associated with an earthquake that may affect the normal activities of people. This includes surface faulting, ground shaking, landslides, liquefaction, tectonic deformation, tsunamis, and seiches.

Earthquake riskEarthquake risk is the probable building damage, and number of people that are expected to be hurt or killed if a likely earthquake on a particular fault occurs. Earthquake risk and earthquake hazard are occasionally incorrectly used interchangeably.

Earthquake Magnitude: Several magnitude scales have been developed for measuring the size of an earthquake. Magnitude is a measure of the energy released by the earthquake. The earliest magnitude scale was Richter

magnitude and news reports still often refer to magnitudes as Richter magnitude. However, today, the most reliable magnitude scale is the moment magnitude, now referred to simply as M. For well-recorded, shallow, moderate to large earthquakes, estimates of the earthquake size using the various magnitude scales usually results in approximately the same numerical result. Current earthquake information, including magnitude, can be found at: http://earthquake.usgs.gov/ and http://www.iris.edu/seismon/. A more complete description of earthquake magnitude is given below (from http://neic.usgs.gov/neis/general/handouts/measure.html):

Measuring the Size of an Earthquake

Earthquakes range broadly in size. A rock-burst in an Idaho silver mine may involve the fracture of 1 meter of rock; the 1965 Rat Island earthquake in the Aleutian arc involved a 650 kilometer length of the Earth's crust. Earthquakes can be even smaller and even larger. If an earthquake is felt or causes perceptible surface damage, then its intensity of shaking can be subjectively estimated. But many large earthquakes occur in oceanic areas or at great focal depths and are either simply not felt or their felt pattern does not really indicate their true size.

Today, state of the art seismic systems transmit data from the seismograph via telephone line and satellite directly to a central digital computer. A preliminary location, depth-of-focus, and magnitude can now be obtained within minutes of the onset of an earthquake. The only limiting factor is how long the seismic waves take to travel from the epicenter to the stations - usually less than 10 minutes.

Magnitude

Modern seismographic systems precisely amplify and record ground motion (typically at periods of between 0.1 and 100 seconds) as a function of time. This amplification and recording as a function of time is the source of instrumental amplitude and arrival-time data on near and distant earthquakes. Although similar seismographs have existed since the 1890's, it was only in the 1930's that Charles F. Richter, a California seismologist, introduced the concept of earthquake magnitude. His original definition held only for California earthquakes occurring within 600 km of a particular type of seismograph (the Woods-Anderson torsion instrument). His basic idea was quite simple: by knowing the distance from a seismograph to an earthquake and observing the maximum signal amplitude recorded on the seismograph, an empirical quantitative ranking of the earthquake's inherent size or strength could be made. Most California earthquakes occur within the top 16 km of the crust; to a first approximation, corrections for variations in earthquake focal depth were, therefore, unnecessary.

2

Richter's original magnitude scale (ML) was then extended to observations of earthquakes of any distance and of focal depths ranging between 0 and 700 km. Because earthquakes excite both body waves, which travel into and through the Earth, and surface waves, which are constrained to follow the natural wave guide of the Earth's uppermost layers, two magnitude scales evolved - the mb and MS

scales.

The standard body-wave magnitude formula is

mb = log10(A/T) + Q(D,h) ,

where A is the amplitude of ground motion (in microns); T is the corresponding period (in seconds); and Q(D,h) is a correction factor that is a function of distance, D (degrees), between epicenter and station and focal depth, h (in kilometers), of the earthquake. The standard surface-wave formula is

MS = log10 (A/T) + 1.66 log10 (D) + 3.30 .

There are many variations of these formulas that take into account effects of specific geographic regions, so that the final computed magnitude is reasonably consistent with Richter's original definition of ML. Negative magnitude values are permissible.

A rough idea of frequency of occurrence of large earthquakes is given by the following table:

MS Earthquakes per year ---------- ----------- 8.5 - 8.9 0.3 8.0 - 8.4 1.1 7.5 - 7.9 3.1 7.0 - 7.4 15 6.5 - 6.9 56 6.0 - 6.4 210

This table is based on data for a recent 47 year period. Perhaps the rates of earthquake occurrence are highly variable and some other 47 year period could give quite different results.

The original mb scale utilized compressional body P-wave amplitudes with periods of 4-5 s, but recent observations are generally of 1 s-period P waves. The MS scale has consistently used Rayleigh surface waves in the period range from 18 to 22 s.

When initially developed, these magnitude scales were considered to be equivalent; in other words, earthquakes of all sizes were thought to radiate fixed proportions of energy at different periods. But it turns out that larger earthquakes,

3

which have larger rupture surfaces, systematically radiate more long-period energy. Thus, for very large earthquakes, body-wave magnitudes badly underestimate true earthquake size; the maximum body-wave magnitudes are about 6.5 - 6.8. In fact, the surface-wave magnitudes underestimate the size of very large earthquakes; the maximum observed values are about 8.3 - 8.7. Some investigators have suggested that the 100 s mantle Love waves (a type of surface wave) should be used to estimate magnitude of great earthquakes. However, even this approach ignores the fact that damage to structure is often caused by energy at shorter periods. Thus, modern seismologists are increasingly turning to two separate parameters to describe the physical effects of an earthquake: seismic moment and radiated energy.

Fault Geometry and Seismic Moment, MO

The orientation of the fault, direction of fault movement, and size of an earthquake can be described by the fault geometry and seismic moment. These parameters are determined from waveform analysis of the seismograms produced by an earthquake. The differing shapes and directions of motion of the waveforms recorded at different distances and azimuths from the earthquake are used to determine the fault geometry, and the wave amplitudes are used to compute moment. The seismic moment is related to fundamental parameters of the faulting process.

MO = µS‹d› ,

where µ is the shear strength of the faulted rock, S is the area of the fault, and <d> is the average displacement on the fault. Because fault geometry and observer azimuth are a part of the computation, moment is a more consistent measure of earthquake size than is magnitude, and more importantly, moment does not have an intrinsic upper bound. These factors have led to the definition of a new magnitude scale MW, based on seismic moment, where

MW = 2/3 log10(MO) - 10.7 .

The two largest reported moments are 2.5 X 1030 dyn·cm (dyne·centimeters) for the 1960 Chile earthquake (MS 8.5; MW 9.6) and 7.5 X 1029 dyn·cm for the 1964 Alaska earthquake (MS 8.3; MW 9.2). MS approaches it maximum value at a moment between 1028 and 1029 dyn·cm.

Energy, E

The amount of energy radiated by an earthquake is a measure of the potential for damage to man-made structures. Theoretically, its computation requires summing the energy flux over a broad suite of frequencies generated by an earthquake as it ruptures a fault. Because of instrumental limitations, most

4

estimates of energy have historically relied on the empirical relationship developed by Beno Gutenberg and Charles Richter:

log10E = 11.8 + 1.5MS

where energy, E, is expressed in ergs. The drawback of this method is that MS

is computed from an bandwidth between approximately 18 to 22 s. It is now known that the energy radiated by an earthquake is concentrated over a different bandwidth and at higher frequencies. With the worldwide deployment of modern digitally recording seismograph with broad bandwidth response, computerized methods are now able to make accurate and explicit estimates of energy on a routine basis for all major earthquakes. A magnitude based on energy radiated by an earthquake, Me, can now be defined,

Me = 2/3 log10E - 2.9.

For every increase in magnitude by 1 unit, the associated seismic energy increases by about 32 times.

Although Mw and Me are both magnitudes, they describe different physical properites of the earthquake. Mw, computed from low-frequency seismic data, is a measure of the area ruptured by an earthquake. Me, computed from high frequency seismic data, is a measure of seismic potential for damage. Consequently, Mw and Me often do not have the same numerical value.

Intensity

The increase in the degree of surface shaking (intensity) for each unit increase of magnitude of a shallow crustal earthquake is unknown. Intensity is based on an earthquake's local accelerations and how long these persist. Intensity and magnitude thus both depend on many variables that include exactly how rock breaks and how energy travels from an earthquake to a receiver. These factors make it difficult for engineers and others who use earthquake intensity and magnitude data to evaluate the error bounds that may exist for their particular applications.

An example of how local soil conditions can greatly influence local intensity is given by catastrophic damage in Mexico City from the 1985, MS 8.1 Mexico earthquake centered some 300 km away. Resonances of the soil-filled basin under parts of Mexico City amplified ground motions for periods of 2 seconds by a factor of 75 times. This shaking led to selective damage to buildings 15 - 25 stories high (same resonant period), resulting in losses to buildings of about $4.0 billion and at least 8,000 fatalities.

The occurrence of an earthquake is a complex physical process. When an earthquake occurs, much of the available local stress is used to power the

5

earthquake fracture growth to produce heat rather that to generate seismic waves. Of an earthquake system's total energy, perhaps 10 percent to less that 1 percent is ultimately radiated as seismic energy. So the degree to which an earthquake lowers the Earth's available potential energy is only fractionally observed as radiated seismic energy.

by William Spence, Stuart A. Sipkin, and George L. ChoyEarthquakes and VolcanoesVolume 21, Number 1, 1989

Earthquake intensity (usually described with the Modified Mercalli Intensity Scale) is a measure of earthquake effects and level of ground shaking at a particular location. A description of earthquake intensity is given below (from http://neic.usgs.gov/neis/general/handouts/mercalli.html):

The Modified Mercalli Intensity Scale

The effect of an earthquake on the Earth's surface is called the intensity. The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction. Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli (MM) Intensity Scale. It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.

The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place. After the occurrence of widely-felt earthquakes, the Geological Survey mails questionnaires to postmasters in the disturbed area requesting the information so that intensity values can be assigned. The results of this postal canvass and information furnished by other sources are used to assign an intensity within the felt area. The maximum observed intensity generally occurs near the epicenter.

The lower numbers of the intensity scale generally deal with the manner in which the earthquake is felt by people. The higher numbers of the scale are based on observed structural damage. Structural engineers usually contribute information for assigning intensity values of VIII or above.

6

The following is an abbreviated description of the 12 levels of Modified Mercalli intensity.

I. Not felt except by a very few under especially favorable conditions.

II. Felt only by a few persons at rest, especially on upper floors of buildings.

III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.

IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.

VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.

VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

7

Abridged from The Severity of an Earthquake, a U. S. Geological Survey General Interest Publication.

U.S. GOVERNMENT PRINTING OFFICE: 1989-288-913

This publication is one of a series of general interest publications prepared by the U.S. Geological Survey to provide information about the earth sciences, natural resources, and the environment. To obtain a catalog of additional titles in the series "General Interest Publications of the U.S. Geological Survey," write:

U.S. Geological SurveyInformation ServicesBox 25286Denver, CO 80225

Earthquake Facts and Statistics (from http://neic.usgs.gov/neis/eqlists/eqstats.html)

Frequency of Occurrence of Earthquakes Based on Observations since 1900

Descriptor Magnitude Average Annually

Great 8 and higher 1

Major 7 - 7.9 18

Strong 6 - 6.9 120

Moderate 5 - 5.9 800

Light 4 - 4.9 6,200 (estimated)

Minor 3 - 3.9 49,000 (estimated)

Very Minor < 3.0 Magnitude 2 - 3: about 1,000 per day Magnitude 1 - 2: about 8,000 per day

The USGS estimates that several million earthquakes occur in the world each year. Many go undetected because they hit remote areas or have very small magnitudes. The NEIC now locates about 50 earthquakes each day, or about 20,000 a year.

8

Number of Earthquakes Worldwide for 1990 - 2002 Located by the US Geological Survey National Earthquake Information

Center

Magnitude 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

8.0 to 9.9 0 0 0 1 2 3 1 0 2 0 1 1 0

7.0 to 7.9 12 11 23 15 13 22 21 20 14 23 14 15 13

6.0 to 6.9 115 105 104 141 161 185 160 125 113 123 158 126 133

5.0 to 5.9 1635 1469 1541 1449 1542 1327 1223 1118 979 1106 1345 1243 1037

4.0 to 4.9 4493 4372 5196 5034 4544 8140 8794 7938 7303 7042 8045 8084 8034

3.0 to 3.9 2457 2952 4643 4263 5000 5002 4869 4467 5945 5521 4784 6151 6542

2.0 to 2.9 2364 2927 3068 5390 5369 3838 2388 2397 4091 4201 3758 4162 5884

1.0 to 1.9 474 801 887 1177 779 645 295 388 805 715 1028 944 1068

0.1 to 0.9 0 1 2 9 17 19 1 4 10 5 5 1 6

No Magnitude 5062 3878 4084 3997 1944 1826 2186 3415 2426 2096 3120 2807 2687

Total 16612 16516 19548 21476 19371 21007 19938 19872 21688 20832 22256 23534 *25404

EstimatedDeaths 51916 2326 3814 10036 1038 7949 419 2907 8928 22711 231 21357 1711

Number of Earthquakes in the United States for 1990 - 2002 Located by the US Geological Survey National Earthquake Information

Center

Magnitude 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

8.0 to 9.9 0 0 0 0 0 0 0 0 0 0 0 0 0

7.0 to 7.9 0 1 2 0 1 0 2 0 0 2 + 1 0 1 1

6.0 to 6.9 3 6 9 9 5 7 6 6 3 5 10 5 5

5.0 to 5.9 72 50 84 69 67 49 109 63 62 52 60 45 52

4.0 to 4.9 283 255 404 269 331 355 621 362 411 360 287 294 401

3.0 to 3.9 621 701 1713 1115 1543 1050 1042 1072 1053 1388 913 834 831

9

2.0 to 2.9 411 555 996 1007 1194 820 652 759 742 814 657 646 659

1.0 to 1.9 1 3 5 7 2 0 0 2 0 0 0 2 2

0.1 to 0.9 0 0 0 0 0 0 0 0 0 0 0 0 0

No Magnitude 877 599 368 457 444 444 375 575 508 381 415 434 440

Total 2268 2170 3581 2933 3587 2725 2807 2839 2779 3003 2342 2261 *2391

EstimatedDeaths 0 2 3 2 60 1 0 0 0 0 0 0 0

Red values indicate the earthquakes occurred in Alaska. Blue values indicate the earthquakes occurred in California.

* As of 08 January 2003

Earthquakes Located by the USGS NEIC 1980-1989.

Earthquakes Located by the USGS NEIC 1970-1979.

As more and more seismographs are installed in the world, more earthquakes can be and have been located. However, the number of large earthquakes (magnitude 6.0 or greater) have stayed relatively constant.

Graphs

TABLE 4 - Magnitude vs. Ground Motion and Energy

Magnitude

Change

Ground Motion Change(Displacement)

Energy Change

1.0 10.0 times about 32 times

0.5 3.2 times about 5.5 times

0.3 2.0 times about 3 times

0.1 1.3 times about 1.4 times

TABLE 4 shows, for example, that a magnitude 7.2 earthquake produces 10 times more ground motion that a magnitude 6.2 earthquake, but it releases about

10

32 times more energy. The energy release best indicates the destructive power of an earthquake.

Another example: How much bigger is a magnitude 9.7 earthquake than a 6.8 earthquake?

A magnitude 9.7 earthquake is 794 times BIGGER on a seismogram than a magnitude 6.8 earthquake. The magnitude scale is logarithmic, so

(10**9.7)/(10**6.8) = (5.01*10**9)/(6.31*10**6) = .794*10**3 = 794 OR = 10**(9.7-6.8) = 10**2.9 = 794.328

Another way to get about the same answer without using a calculator is that since 1 unit of magnitude is 10 times the amplitude on a seismogram and 0.1 unit of magnitude is about 1.3 times the amplitude, we can get,

10 * 10 * 10 / 1.3 = 769 times [not exact, but a decent approximation]

The magnitude scale is really comparing amplitudes of waves on a seismogram, not the STRENGTH (energy) of the quakes. So, a magnitude 9.7 is 794 times bigger than a 6.8 quake as measured on seismograms, but the 9.7 quake is about 23,000 times STRONGER than the 6.8! Since it is really the energy or strength that knocks down buildings, this is really the more important comparison. This means that it would take about 23,000 quakes of magnitude 6.8 to equal the energy released by one magnitude 9.7 event. Here's how we get that number:

One whole unit of magnitude represents approximately 32 times (actually 10**1.5 times) the energy, based on a long-standing empirical formula that says log(E) is proportional to 1.5M, where E is energy and M is magnitude. This means that a change of 0.1 in magnitude is about 1.4 times the energy release. Therefore, using the shortcut shown eartlier for the amplitude calculation, the energy is,

32 * 32 * 32 / 1.4 = 23,405 or about 23,000

The actual formula would be:

((10**1.5)**9.7)/((10**1.5)**6.8)

= 10**(1.5*(9.7-6.8)) = 10**(1.5*2.9) = 22,387

11

This explains why big quakes are so much more devastating than small ones. The amplitude ("size") differences are big enough, but the energy ("strength") differences are huge. The amplitude numbers are neater and a little easier to explain, which is why those are used more often in publications. But it's the energy that does the damage.

Maps of intensity of ground shaking can be prepared for specific earthquakes. Today, color maps are prepared very quickly for significant events from predictions based on the earthquake location and magnitude or from reports of ground shaking (“felt reports”) and the maps displayed on the USGS web page (http://earthquake.usgs.gov/shakemap/). Examples of these shake maps for the January 17, 1994 Northridge earthquake, the October 17, 1989 Loma Prieta earthquake, and the February 9, 1971 San Fernando earthquake are shown below. The table and color code beneath the maps illustrates the correlation between potential damage, peak ground acceleration and Modified Mercalli Intensity. The intensity of ground shaking (and therefore damage) is usually greatest near the earthquake epicenter and decreases rapidly with distance from the epicenter. Local geology (near-surface ground conditions) and building characteristics also influence the intensity distribution.

12

13

14

A map of peak ground accelerations, an acceleration versus distance plot and an intensity map for the January 17, 1995 Kobe, Japan earthquake are shown below (from http://www.eqe.com/publications/kobe/kobe.htm):

15

16

17

Earthquake Hazards: The US Geological Survey recently released a publication (Fact Sheet 017-03, http://geopubs.wr.usgs.gov/fact-sheet/fs017-03/, shown below) that discusses various aspects of earthquake hazard assessment in the United States.

U.S. Geological Survey Fact Sheet 017-03

The USGS Earthquake Hazards Program in NEHRP—Investing in a Safer Future

In 1977, Congress authorized the creation of

18

the National Earthquake Hazards Reduction Program (NEHRP) to improve the Nation’s understanding of earthquake hazards and to mitigate their effects. Earthquakes are the most costly natural hazard faced by the United States. Twenty-five years of work by the U.S. Geological Survey (USGS), in close cooperation with the three other NEHRP agencies, has yielded major advances in earthquake preparedness and monitoring, as well as a vastly improved understanding of earthquake hazards, processes, and effects. The USGS is poised to build on these accomplishments, helping through NEHRP to protect lives and property in the future earthquakes that are certain to strike the United States.

 

19

As shown by this U.S. Geological Survey (USGS) national seismic-hazard map, earthquake hazards exist throughout the United States. USGS national and regional seismic-hazard maps forecast the amount of shaking expected over specified time periods. Many parts of the Central and Eastern United States have moderate to high long-term hazard, even though they have not experienced recent large quakes. Successive updates of USGS seismic-hazard maps are used to revise building codes and are also widely used by structural engineers and government agencies. The next generation of such maps will provide time-dependent probabilities that take into account the effects of prior quake occurrence on future earthquake likelihood.

The U.S. Geological Survey (USGS) Earthquake Hazards Program monitors the Nation’s earthquakes, studies why they occur and how they shake the ground, provides quantitative earthquake-hazard assessments, helps promote loss-reduction measures using these results, and provides crucial scientific information to assist emergency responders when earthquakes occur. The USGS Earthquake Hazards Program operates under the National Earthquake Hazards Reduction Program (NEHRP), created by Congress in 1977. To meet the challenges and potential of NEHRP, activities supported by the USGS

20

Earthquake Hazards Program are managed under four broad interrelated objectives:

• Improve quantification of seismic hazards—The USGS produces quantitative hazard-assessment products that enable the public and private sectors to assess earthquake hazards and implement effective mitigation strategies.

• Complete the modernization and expansion of real-time earthquake notification and monitoring systems—The USGS is tasked with collecting, interpreting, and disseminating information on the earthquakes that occur throughout the United States and on significant quakes worldwide in support of disaster response, earthquake preparedness, national security, scientific research, and public hazard awareness.

• Achieve better scientific understanding of earthquake processes and effects—The USGS pursues research on earthquake occurrence and effects for the purpose of developing and improving hazard-assessment methods and loss-reduction strategies.

• Provide national and local leadership to engage communities in earthquake safety practices—The USGS works with user communities to ensure that its products are readily available, easily understood, and appropriately used for earthquake mitigation and response.

The work of the USGS Earthquake Hazards Program focuses both on the Nation as a whole and also on particular regional needs and problems in areas where quake risk is the greatest. The program’s work is carried out by USGS scientific and technical personnel and also through a system of competitive external grants and contracts that is allotted one-quarter of program funds. In the past 25 years, this grants program has funded approximately 2,500 grants and cooperative agreements with state geological surveys, university researchers and research consortia, state and local government agencies, and nonprofit and other organizations in the private sector. USGS also works closely with the other NEHRP agencies (FEMA, NSF, and the National Institute of Standards and Technology) and with non-NEHRP agencies, such as NASA, NOAA, and USAID.

Accomplishments and Impacts of the USGS EarthquakeHazards Program

Over the past 25 years, NEHRP has made the Nation safer from the ravages of earthquakes. One of the flagship products from NEHRP contributing to this achievement are the national seismic-hazard maps

21

that the USGS has produced since 1976. These maps are derived by estimating the likelihood of future earthquakes along active faults throughout different regions and evaluating the ground shaking that these quakes would cause. These seismic-hazard maps are the scientific basis of seismic provisions in building codes enacted throughout the United States to prevent loss of life and limit damage during large earthquakes. The 1996 national seismic-hazard maps are directly included in design maps in the NEHRP Recommended Provisions, published by the Building Seismic Safety Council and the Federal Emergency Management Agency (FEMA). Seismic provisions in both the 2000 International Building Code (the merging of the three major national model codes) and codes have now been adopted by jurisdictions in 37 states. Thus, the USGS-produced national seismic-hazard maps are now being used to make billions of dollars of new construction each year safer from earthquakes.

The national seismic-hazard maps are also used in FEMA’s retrofit guidelines, ensuring that older buildings are strengthened so that they withstand future earthquakes. In addition, these maps and associated products are used in the design of highway bridges, landfills under EPA regulation, and dams, as well as in the setting of earthquake-insurance premiums and the cost of reinsurance. The California Earthquake Authority uses seismic-hazard maps produced jointly by the USGS and the California Geological Survey to set earthquake premiums for the State’s quake-insurance program. Pension funds apply these maps to evaluate the risks to their portfolios of properties. Presidential executive orders specify that new and leased Federal buildings must adhere to the NEHRP Recommended Provisions. The State of Oregon recently raised seismic requirements in construction along the southern part of its coast, largely on the basis of information presented in the USGS seismic-hazard maps.

The advances in earthquake-hazard assessments during the first 25 years of NEHRP have their roots in pioneering USGS field, laboratory, and theoretical research focused on understanding the basic physical processes associated with earthquakes. Key breakthroughs include:

• Improved quantification of regional seismic-energy attenuation with distance from an earthquake.

• Use of Global Positioning System (GPS) stations to determine the rate at which faults are being “loaded” (stressed) by the movements of the tectonic plates that make up the Earth’s outer shell.

• Discovery and documentation of large prehistoric earthquakes through a new field of study known as paleoseismology (identifying

22

evidence of past quakes in trenches dug across faults, in riverbanks, and from drowned coastlines).

• Use of new remote-sensing technology, such as LIDAR (light detection and ranging), to identify active faults in heavily forested regions like the Pacific Northwest.

• Quantification of the effects of soils and near-surface conditions in amplifying strong ground motion.

• Advances in earthquake forecasting through improved understanding of the physics of the fracture and friction of rocks in fault zones.

The USGS Earthquake Hazards Program has also realized major improvements in its ability to provide timely and informative earthquake reports and information. To fulfill its Federal responsibility to monitor seismic activity in the United States, USGS operates the U.S. National Seismograph Network, the National Earthquake Information Center, and the National Strong Motion Program and supports 14 regional seismic networks in areas of moderate to high quake activity. Additionally, the USGS, in cooperation with the National Science Foundation (NSF), operates the Global Seismic Network, which provides the main source of worldwide earthquake information.

USGS has capitalized on the revolution in information technology to achieve dramatic advances in real-time seismic-data analysis and rapid earthquake notification. The most noteworthy result of this is “ShakeMap,” a system for automatically generating, within minutes of an earthquake, maps of areas subjected to strong shaking. ShakeMap, where available, can be delivered in 10 minutes or less and thus forms the basis for emergency response by cities, states, Federal agencies, and lifeline operators.

“SHAKEMAP”—A NEW TECHNOLOGY TO AID EMERGENCY RESPONSE

23

This U.S. Geological Survey (USGS) “ShakeMap” shows shaking intensities during the 2001 magnitude 6.7 Nisqually, Washington, earthquake, which caused $2 billion in damage and economic losses in the Olympia-Seattle area. The capability to automatically generate computer maps of the intensity of ground shaking and to provide them to the public on the Internet within minutes of a quake was developed after the 1994 Northridge, California, earthquake. ShakeMaps help greatly in the quick assessment of the scope of an earthquake emergency and in guiding emergency response. ShakeMap requires data from modern seismic networks with digital strong-motion recording capabilities and real-time telecommunications feeds. Currently, few urban areas in the United States possess such networks. For this reason, ShakeMap is currently available only in the Los Angeles, San Francisco, Seattle, and Salt Lake City areas. Full implementation by the USGS of the

24

Advanced National Seismic System (ANSS) will allow expansion of ShakeMap to all large U.S. metropolitan areas with moderate to high seismic risk.

Complementing ShakeMap are a suite of other real-time earthquake products, such as earthquake paging and e-mail services, real-time earthquake location maps, automatic Web pages with information on significant events, and aftershock probability estimators. Additionally, USGS has created a Web-based interface, called “Did You Feel It?,” to provide Internet users with a means of reporting individual quake experiences that are compiled into maps of ground-shaking intensity. This suite of products provide the general public with rapid and comprehensive information about U.S. and global earthquakes when they need it most.

During the past 10 years, the demand for USGS earthquake-information products, including national and regional hazard maps and data from realtime seismic monitoring, has skyrocketed. USGS information now directly underpins local, state, and national earthquake loss-reduction and emergency response efforts.

HELPING THE PUBLIC PREPARE FOR EARTHQUAKES

25

The U.S. Geological Survey (USGS) and the Southern California Earthquake Center (SCEC) work jointly to produce an array of earthquake-hazard products for southern California. The map above shows how the level of shaking is likely to vary across the Los Angeles Basin because of soft sediments and subsurface geologic structures. In the wake of the frightening 1992 magnitude 7. 3 Landers and 1994 magnitude 6.7 Northridge earthquakes, the USGS and SCEC jointly published a guide (left) to increase public awareness of earthquakes and quake preparedness. This guide has helped residents understand their quake risks and make their homes, workplaces, schools, and families safer in earthquakes. More than 1.8 million copies of the guide were distributed in southern California through schools, libraries, community centers, and local government offices. More than 3.5 million copies of a similar guide were distributed in the San Francisco Bay region after the 1989 magnitude 6.9 Loma Prieta earthquake. To commemorate the 10th anniversary of the Northridge earthquake, updated English and Spanish guides will be published in 2004.

 

The Future of the USGS Earthquake Hazards Program

The USGS Earthquake Hazards Program is poised to build on its accomplishments. The USGS will continue to improve on existing earthquake monitoring, assessment, and research activities, with the ultimate goal of providing the Nation with a new generation of earthquake products that more effectively promote earthquake mitigation and better facilitate earthquake response. At the heart of this effort will be a continued emphasis on delivering information that is useful, accessible, and easily understood. By working closely with policymakers and emergency planners, USGS will ensure that they have the most reliable and accurate information possible about earthquake hazards and that USGS products are tailored to their needs. USGS will participate in local and national earthquake-mitigation planning exercises and help train emergency responders,

26

contingency planners, risk managers, the media, and others in how to use earthquake-hazard assessments and real-time information products. USGS will also continue to interact directly with communities to help them understand their vulnerabilities to earthquakes and to plan mitigation actions. Critical decisions for earthquake preparedness and response, including those that ensure uninterrupted corporate and government operations, are often made far from areas of high seismic hazard. So that informed and appropriate actions can be taken, USGS will continue to work to ensure that earthquake-hazard information and products are useful and familiar to decisionmakers, even in regions of low seismic hazard.

Earthquakes pose a significant threat to urban areas in the Intermountain West. For example, almost 75% of Utah’s population lives near the Wasatch Fault. Movement on this fault in past quakes has uplifted the Wasatch Range to form a spectacular backdrop for the state’s three largest cities, including Salt Lake City, shown here. U.S. Geological Survey and Utah State Geological Survey scientists have shown that this fault has repeatedly generated earthquakes of magnitude 7 or larger and will continue to do so in the future. Efforts to increase public awareness of earthquake hazards in Utah have resulted in residents and community leaders taking actions that will save lives and reduce damage in future quakes. (Photo courtesy of the Salt Lake Convention and Visitors Bureau.)

The USGS is committed to providing the Nation with new and useful products to reduce earthquake losses and improve quake safety. However, many needs cannot be met or opportunities seized under current funding levels. Critical among those needs and opportunities are:

Completing the Advanced National Seismic System—A major obstacle to further reducing loss of life and property in earthquakes is the present scarcity of strong-motion recordings of actual ground-shaking levels in urban areas and of the dynamic performance of structures and lifelines in quakes. The Advanced National Seismic System (ANSS) is intended to address this need by providing a nationwide network of least 7,000 sophisticated shaking monitors placed both on the ground and in buildings, mostly in seismically active urban areas. The closely spaced ANSS stations will be used to identify areas with specific problems, such as high amplification and focusing of seismic waves, and provide data crucial for finding cost-effective seismic-design solutions.

27

One critical component of ANSS is the instrumentation of buildings, bridges, and other structures. It is essential to have multiple structures in high hazard areas instrumented with arrays of seismometers so that engineers can understand how different types of buildings respond to earthquake shaking. These instruments will also provide crucial information on:

• The coupling between building foundations and underlying soils,

• The role of torsion of columns in building shaking,

• The performance of commonly used structural systems, such as shear walls combined with a moment-frame structure, and

• The ability of mathematical models to predict the performance of structures during strong shaking.

INSTRUMENTED BUILDINGS PROVIDE CRUCIAL DATA

To design safer buildings and to provide vital information for strengthening older ones, U.S. Geological Survey earthquake engineers have installed arrays of seismic instruments in several buildings, such as San Francisco’s Transamerica tower (above), in order to capture their motion during earthquakes. The resulting records reveal how structural systems perform in quakes and how shaking is amplified on upper floors, as demonstrated here by records from the tower during the1989 Loma Prieta earthquake. In that quake, 63 people were killed when other structures collapsed. A major objective of the Advanced National Seismic System (ANSS)

28

is to instrument many representative building types and key structures across the country to acquire the data that engineers need to help prevent loss of life in future quakes.

Another urgent need that will be addressed by ANSS is improved reliability, timeliness, and usefulness of USGS real-time earthquake products for emergency-response purposes. ShakeMap, in particular, requires access to a modern seismic network with digital strong-motion recording capabilities and real-time telecommunications feeds. Currently, few urban areas in the United States possess such a network. Full implementation of ANSS will allow expansion of ShakeMap to all large U.S. metropolitan areas with moderate to high seismic risk.

Acquiring essential data for expanded urban hazard assessments—Most current USGS earthquake-hazard assessments are compiled on regional or national scales. These estimates typically assume uniform firm soil conditions, as opposed to the varying actual soil conditions beneath cities and lifelines. At the scales required for urban planning and development, assessments need to account for the amplifying effects of soils, as well as the potential for ground failures, such as liquefaction and landslides.

USGS pilot urban assessments in Oakland, Seattle, and Memphis have shown the usefulness of detailed urban seismic assessments. USGS must expand this effort to include other urban centers at risk from earthquakes. Central to this endeavor will be the acquisition of data on local geology, site conditions, and ground motions needed to produce detailed urban seismic hazard maps. These data-acquisition efforts will require additional resources that will in part be used to help support expanded partnerships with state geological surveys and local government agencies. As these urban hazard assessments evolve, they will allow estimates to be made for potential earthquake losses to buildings and critical lifelines. This is one of the keys to developing cost-effective mitigation strategies to reduce future earthquake losses.

SEISMIC-HAZARD INFORMATION HELPS ENSURE THE SURVIVAL OF STRUCTURES AND LIFELINES

29

RETROFITTED HIGHWAY OVERPASS NON-RETROFITTED OVERPASS

U.S. Geological Survey (USGS) seismic-hazard information is used by structural engineers to design structures and lifelines, such as freeway overpasses, to help ensure their survival in quakes. Over the past 25 years, USGS and other scientists have discovered that quakes can shake the ground much more violently than provided for in earlier building codes. The California Department of Transportation (Caltrans) has nearly completed a program of retrofitting bridges and overpasses throughout the State. In 1994, when the magnitude 6.7 Northridge earthquake struck Los Angeles, the success of this investment was demonstrated. Retrofitted bridge columns in areas of severe to violent shaking (such as those at left) survived with minor or no damage, whereas non-retrofitted overpass supports (such as the one at right) collapsed. Caltrans is now beginning to use USGS “ShakeMaps” to expedite their post-earthquake response, enabling emergency managers to focus field inspections on the most severely shaken areas. (Photos courtesy of Caltrans.)

 Expanding activities in the Eastern United States—The USGS Earthquake Hazards Program devotes approximately 75% of its resources to work in the Western United States, primarily because earthquakes occur more frequently there. However, history demonstrates that a catastrophic quake could also strike a major city in the Eastern United States. Four damaging earthquakes with magnitudes greater than 7 centered in the New Madrid, Missouri, area struck the Mississippi Valley in 1811-12. Charleston, South Carolina, was devastated by a magnitude 6.7 shock in 1886, and a magnitude 6 quake struck the Boston area in 1755.

30

USGS studies show that urban areas in the Eastern United States face far greater damage and far more deaths in a quake of a given magnitude than those in the West for several reasons: (1) For the same magnitude earthquake, severe shaking affects a much larger area, (2) most structures are not designed to resist earthquakes, and (3) population density is high and residents are not routinely educated about seismic safety.

USGS has been developing the understanding and methods that can form the groundwork for a substantial effort in the East, where earthquake faults are rarely exposed at the surface and the subsurface conditions beneath major cities are poorly documented. More thorough and accurate assessment of the seismic risk faced by major urban centers in the East will reveal the greatest vulnerabilities and provide information that is essential to evaluating possible mitigation strategies.

In the Central and Eastern United States, earthquakes are felt over a broader area than comparable-size quakes in the Western United States because of differences in geology. Although only of magnitude 6, the earthquake that occurred near Saint Louis in 1895 affected a larger area than the 1994 magnitude 6.7

31

Northridge, California, quake, which caused $40 billion in damage and economic losses and killed 67 people. A repeat of the 1895 earthquake could prove disastrous for the Midwest, where structures are not as earthquake resistant as those in California.

 

Expanding activities in Alaska—Alaska has the greatest exposure to earthquake hazard of any state. Because of the relatively small urban population, many people assume that the risk is low in comparison to the rest of the country. However, the impact of a devastating quake in Alaska can extend far beyond its borders, both by generating deadly tsunamis (seismic sea waves) and through economic consequences. Alaska is a major source of natural resources for the rest of the Nation, a major transportation and commercial node of the Pacific Rim, and of significant importance to national defense.

In.

November 2002, the powerful magnitude 7.9 Denali Fault earthquake struck south-central Alaska, rupturing the ground beneath the zigzagging Trans-Alaska Oil Pipeline. Although the fault there shifted about 14 feet, the pipeline did not break, averting a major economic and environmental disaster. This success was largely the result of a design based on geologic and engineering studies done by the U.S.

32

Geological Survey and others. Alaska has the greatest exposure to earthquake hazard of any state. The impact of a devastating quake in Alaska could extend far beyond its borders, both by generating deadly tsunamis and through economic consequences

Capitalizing on new national facilities—As described in the 2003 National Research Council report, Living on an Active Earth: Perspectives on Earthquake Science, continued progress toward evaluating earthquake hazards will increasingly require integrative, physics-based research involving theoretical studies of processes controlling earthquake phenomena, sophisticated numerical modeling, ground- and space-based field observations, and laboratory simulations. Recent and proposed U.S. Government investments in major earth-science and engineering facilities include ANSS, the NSF-coordinated EarthScope program (including the Plate Boundary Observatory, USArray, and the San Andreas Fault Observatory at Depth), the NSF Network for Earthquake Engineering Simulation (NEES), and a future interferometric synthetic aperture radar (InSAR) satellite mission. These facilities will be able to offer, for the first time, the breadth and depth of data required to truly understand the physical nature of earthquakes.

The USGS will take advantage of these new data streams to conduct earthquake-hazard-focused experiments on scales never before possible. To improve long-term hazard assessments, USGS will also use these data together with advanced computational methods to simulate the multiple factors controlling earthquakes within specific fault systems. A major goal will be to understand the criteria for the occurrence of quakes within a fault system and the impact of one quake on the system through the many processes that transfer stresses. To determine the feasibility of short-term prediction of earthquakes, USGS will build new mathematical models of quake likelihood, akin to weather-forecast models.

CREATING TIME-DEPENDENT EARTHQUAKE PROBABILITY MAPS

33

Time-dependent earthquake probability maps take into account the effects of quake occurrence and fault interactions on the likelihood of future quakes. In 1988, the first U.S. Geological Survey (USGS) map of this type correctly identified the future site of the 1989 magnitude 6.9 Loma Prieta earthquake. On the basis of research conducted since that quake, USGS and other scientists conclude that there is a 62% probability of at least one magnitude 6.7 or greater quake, capable of causing widespread damage, striking the San Francisco Bay region before 2031. This periodically

34

updated information is widely disseminated to the public in both the print and broadcast media, as well as in USGS publications, and is being used to focus preparedness efforts throughout the bay region.

Conclusion

After 25 years of NEHRP, the USGS Earthquake Hazards Program is the world scientific leader in seismic-hazard studies. In implementing the results of these studies to mitigate the effects of earthquakes, USGS has actively collaborated with state geological surveys, emergency-response officials, earthquake engineers, local governments, and the public. This collaboration has resulted in dramatic improvements in earthquake preparedness and public safety in the United States, but there is still much work to be done. By integrating USGS earthquake information with data from new national initiatives, such as ANSS, USGS will be able to meet the need for a new generation of earthquake hazard-assessment and mitigation tools. These tools will be used to further reduce losses of life and property in the future earthquakes that are certain to strike the Nation.

By John R. Filson, Jill McCarthy, William L. Ellsworth, and Mary Lou Zoback

Edited by Peter H. Stauffer and James W. Hendley IIGraphics by Sara Boore, Susan Mayfield, and Stephen L. Scott, and

Eleanor Omdahl

For further information contact:U.S. Geological Survey, Mail Stop 905

12201 Sunrise Valley DriveReston, VA 20192(703) 648-6714

Visit the Earthquake Hazards Program website to learn more

PDF version of this fact sheet (4.4 MB)

Download a copy of Acrobat Reader for free

| Help | PDF help | Geopubs main page | Fact Sheets |

| Department of the Interior | U.S. Geological Survey | Geologic Division |

| Privacy Statement | Disclaimer | Accessibility | URL of this page: http://geopubs.wr.usgs.gov/fact-sheet/fs017-03/Maintained by: Carolyn Donlin

35

Created: 2-16-03Last modified: 2-20-03 (cad)

 

36