WHY EARTHQUAKES OCCUR
The planet Earth is believed to consist of a thin crust 2–3 mi thick under the oceans and as much as 25 mi thick beneath the continents that covers the large, solid sphere of the rock mantle, which descends to about 1,800 mi. Below the mantle is the fluid outer core, and, at about 3,200 mi depth, the apparently solid inner core. The province of earthquakes recorded thus far is from the crust to a maximum depth of about 450 mi.
Conditions thought to prevail in this hot, dark, high-pressure land cannot be simulated in existing laboratories—at the base of the mantle, pressure is about 11,000 tons per square inch and temperature is 10,000°F. These diamond-smashing pressures produce a rigidity in mantle rock about four times that of ordinary steel, with an average density about that of titanium.
This very solid mantle rock seems to behave, over periods of millions of years, like a very sluggish fluid. Something, perhaps the temperature difference between the white-hot region near the core and the cooler region near the crust, drives slow-moving cycles of rising and descending currents in the mantle rock itself.
Evidently, these currents rise beneath the thin-crusted ocean floor, thrust up the mid-ocean ridges, and generate the stresses that produce their spine-like transverse cracks and shallow earthquakes. This is believed to be the force that causes material to well up through the crust, replacing and spreading the old sea floor, and pushing drifting continents apart.
Where the currents begin their descent at the edges of continents, they produce compressive pressures, and massive folding in the form of trenches and mountain ranges. These regions are the sites of the deeper earthquakes, and of most volcanism.
Earth stresses and strains and then releases
Stresses generated in the crust and upper mantle by convective currents are stored in the form of strain—physical deformation of the rock structure. Under normal circumstances, the "solid" rocks deform plastically, releasing pent-up energy before it builds to catastrophic levels. But, when stresses accumulate too rapidly to be removed by plastic flow, some structural compensation is necessary. Large blocks of material are slowly forced into highly strained positions along faults, and held in place by a supporting structure of stronger materials. These energy-absorbing zones of weakness continue to shift, like longbows being pulled to the breaking point. Finally, more stress causes the supporting rocks to rupture, triggering the "cocked" fracture back toward equilibrium. The sides of the rebounding fault move horizontally with respect to one another (strike-slip), vertically (dip-slip), or in combinations of such motion, as in the large-scale tilting that accompanied the Alaska earthquake in March 1964.
Foreshocks and aftershocks
Sometimes all the energy to be released goes out in one large wrench, followed by trains of smaller tremors, or aftershocks, produced by continuing collapse and slippage along the fracture. Sometimes the fault shift is preceded by the small structural failures we detect as foreshocks. The magnitude 5.9 earthquake that shook Fairbanks, Alaska, on June 21, 1967, was preceded by a magnitude 5.6 foreshock, followed by a magnitude 5.5 aftershock, and then, over the next 24 hours, by more than 2,000 smaller aftershocks. Small tremors were detected for days after the initial event. However, all small tremors or earthquake "swarms" do not necessarily indicate that a big one is on the way. The Matsushiro, Japan, swarm maintained an intermittent tremble for more than a year, probably doing more psychic than physical damage. Of more than 600,000 tremors recorded between August 3, 1965, and the end of 1966, 60,000 were strong enough to be felt, and 400 were damaging. During the most active period, in April and May 1966, Matsushiro felt hundreds of tremors daily, all under magnitude 5.
Whatever the time period involved, the energy of strain flows out through the shifted fault in the form of heat, sound, and earthquake waves.
How earthquake waves travel
There are four basic seismic waves: two preliminary "body" waves that travel through the earth, and two that travel only at the surface. Combinations, reflections, and diffractions produce a virtual infinity of other types. The behavior of these are well-enough understood that wave speed and amplitude have been the major means of describing Earth 's interior. In addition, a large earthquake generates elastic waves that echo through the planet like vibrations in a ringing bell, which actually cause the planet to expand and contract infinitesimally.
The primary (P) wave is longitudinal, like a sound wave, propagates through both liquids and solids, and is usually the first signal that an earthquake has occurred. Where the disturbance is near enough or large enough to be felt, the P wave arrives at the surface like a hammer blow from the inside. This is the swiftest seismic wave, its speed varying with the material through which it passes. In the heterogeneous crustal structure, P-wave velocity is usually less than 4 mi per second—nearly 15,000 mph. Just below the crust, at a layer called the Mohorovicic discontinuity (the Moho), these speeds jump to 5 mi per second and subsequently increase to about 8.5 mi per second (more than 30,000 mph) through the core.
As the compressional phase of the P wave passes through the earth, particles are pushed together and displaced away from the disturbance. The rare factional phase dilates the particles and displaces them toward the earthquake source. For an object embedded in the ground, the result is a series of sharp pushes and pulls parallel to the wave path—motions similar to those that passengers feel when a long train gets under way.
The secondary (S) wave is transverse, like a light or radio wave, and travels about half as fast as the primary wave. Because S waves require a rigid medium to travel in coherent rays, their apparent absence below the mantle gives credence to the theory of a fluid core. About twice the period and amplitude of the associated P waves, these shear waves displace particles at right angles to the direction of wave travel. The vertical component of this movement is somewhat dampened by the opposing force of gravity; but side-to-side shaking in the horizontal can be quite destructive. Where the motion is perceptible, the arrival of the S waves marks the beginning of a new series of shocks, often worse than the P-wave tremor.
Surface waves, named for their discoverers, Love and Rayleigh, are of much greater length and period, e.g., 30 seconds or more, versus less than one second for P waves. Love waves are shear in the horizontal dimension, and the Rayleigh wave induces a retrograde, elliptical motion, something like that in wind-driven ocean waves. The speed of the Love wave is about 2.5 mi per second; the Rayleigh
wave is about 10% slower. Despite the large proportion of earthquake energy represented by these waves, their long period smoothes out the motion they impart, reducing their destructiveness.
Wave motion is not considered in describing the travel of seismic waves through the earth. Instead, the P and S body waves, and their large family of reflected, combined, or resonated offspring, are treated as rays. If the planet were homogeneous, like a ball of wax, these rays would be straight lines. But in the heterogeneous Earth, the rays describe concavely spherical paths away from the earthquake source, and from points of reflection at the surface.
Since they travel at different speeds, seismic waves arrive at a given point on the Earth's surface at different times. Near the source, the ground will shake over a slightly longer interval of time than it took the fault to slip. At great distances, the same energy released by a single event may be detected instrumentally for days.
MEASURING AN EARTHQUAKE
Intensity is an indication of an earthquake's apparent severity at a specified location, as determined by experienced observers. Through interviews with persons in the stricken area, damage surveys, and studies of Earth movement, an earthquake's regional effects can be systematically described. For seismologists and emergency workers, intensity becomes an efficient shorthand for describing what an earthquake has done to a given area.
The Modified Mercalli Intensity Scale generally used in the United States grades observed effects into 12 classes ranging from I, felt only under especially favorable circumstances, to XII, damage total. The older Rossi Forel Intensity Scale (RF) has 10 categories of observed effects, and is still used in Europe. Still other intensity scales are in use in Japan and the former Soviet Union.
Rating earthquakes by intensity has the disadvantage of being always relative. In recent years, an "objective" scale of earthquake magnitude has supplemented intensity ratings. Magnitude expresses the amount of energy released by an earthquake as determined by measuring the amplitudes produced on standardized recording instruments. The persistent misconception that the "Richter scale" rates the size of earthquakes on a "scale of 10" is extremely misleading, and has tended to mask the clear distinction between magnitude and intensity.
Earthquake magnitudes are similar to stellar magnitudes in that they describe the subject in absolute, not relative, terms, and that they refer to a logarithmic, not an arithmetic, scale. An earthquake of magnitude 8, for example, represents seismograph amplitudes 10 times larger than those of a magnitude 7 earthquake, 100 times larger than those of a magnitude 6 earthquake, and so on. There is no highest or lowest value, and it is possible here, as with temperature, to record negative values. The largest earthquakes of record were rated at magnitude 8.9; the smallest, about minus 3. Preliminary magnitude determinations may vary with the observatory, equipment, and methods of estimating—the Alaska earthquake of March 1964, for example, was described variously as magnitude 8.4, 8.5, and 8.6 by different stations.
Magnitude also provides an indication of earthquake energy release, which intensity does not. In terms of ergs, (in the centimeter-gram-second system, an erg is the unit of work equal to a force of 1 dyne acting through a distance of 1 cm (0.39 in); a dyne is the force required to accelerate a freestanding gram mass 1 cm/second) a magnitude 1 earthquake releases about one billionth the energy of a magnitude 7 earthquake; a magnitude 5, about one thousandth that of a magnitude 7, etc.
UNDERSTANDING EARTHQUAKES & WHY THEY ARE ALWAYS A SURPRISE
Most natural hazards can be detected before they strike. However seisms (from the Greek seismos , earthquake) have no known precursors, and so they come without warning. For this reason, they continue to kill, in some areas, at a level usually reserved for wars and epidemics.
Natural hazards worldwide, such as those caused by storms, earthquakes, volcanoes, and tsunamis cause $1 billion in damages daily. The years 1995–1998 have seen more destructive tsunamis than any other time period since the beginning of the twentieth century. Recent tsunamis alone have caused approximately $2 billion in damage, over 1,820 deaths, 1,500 serious injuries (not including the New Guinea Tsunami), and left more than 135,000 homeless. In addition, there have been at least 540,000 fatalities in 209 tsunamis from the year 684 to 1998.
It is estimated that there are 500,000 detectable earthquakes in the world each year—100,000 of those can be felt and 100 of them cause damage. Worldwide, 1,741,127 people have been killed in earthquakes during the twentieth century.
WHERE EARTHQUAKES OCCUR
Our planet's most active earthquake-producing feature is the circum-Pacific seismic belt, which trends along the major geologic faults and the deep oceanic trenches of island arcs decorated here and there with the volcanic "Ring of Fire." The mid-Atlantic Ridge, with its fish-skeleton figure of transverse cracks, is also quite active. Other major seismic belts branch from the circum-Pacific system and arc across southeastern and southern Asia into southern Europe, through the Indian Ocean up through the eastern Mediterranean, and up through southern Asia into China.
In an average year, these belts will generate several million tremors, ranging in severity from barely detectable wiggles to great earthquakes of the size that ravaged San Francisco in 1906 and tilted a third of Alaska in 1964. There is always an earthquake in progress somewhere.
ACTIVE FAULTS OF CALIFORNIA
The most earthquake-prone areas in the contiguous United States are those that are adjacent to the San Andreas fault system of coastal California and the fault system that separates the Sierra Nevada from the Great Basin. Many of the individual faults of these major systems are known to have been active during the last 150–200 years, and others are believed to have been active since the wane of the last great ice advance about 10,000 years ago. Parts of these earthquake-prone areas are among the most densely populated and rapidly urbanizing sections of the western states. A knowledge of the location of these active faults and an understanding of the nature of the earthquake activity that is related to them is necessary for people to accommodate themselves and their work to these hazards.
Earthquakes in California are relatively shallow and clearly related to movement along active faults. During historical times, at least 25 California earthquakes have been associated with movements that ruptured Earth's surface along these faults. On the San Andreas fault, eight moderate-to-severe earthquakes have been accompanied by movements on the fault at the earth's surface since 1838, and other faults in the California region have also experienced repeated earthquakes. The magnitude of shallow earthquakes can generally be correlated with the amount and length of the associated fault movement. Thus, the largest episode of fault movement (or fault slip) recorded in California accompanied the three great earthquakes of 1857, 1872, and 1906—all of which had estimated magnitudes that were over 8 on the Richter scale.
Many of the California faults have had one or more episodes of sudden slip or of slow movement, called creep, during historical time or a documented history of shallow earthquakes. For other faults, however, recent activity can only be inferred from geologic and topographic relations, which indicate that they have been active during the past several thousand years. Such activity suggests that some of these faults will, and that any of them might, slip or creep again.
In parts of California where relatively little geologic work has been done, evidence of other recently active faults will undoubtedly be found as research progresses. This is particularly true of large areas in northern California where topographic features by which recent fault movements can be recognized are commonly obscured by dense vegetation and rapid erosion. Further study may also reveal that some of the unknown faults have been recently active, and that some parts of faults thought to be active are actually dead.
Most of the active California faults are vertical or nearly vertical breaks, and movement along these breaks has been predominantly horizontal. If the block on the opposite side of the fault from the observer has moved to the right, the movement is termed right-lateral; movement of the opposite block to the left is termed left-lateral. Most of the faults trend northwesterly, and movement on these faults has been right-lateral. Notable exceptions to the predominantly northwesterly trend of faults are the west-trending Garlock and Big Pine faults; movement on these faults has been left-lateral.
A few reverse faults have also been active in California. The planes of such faults are inclined to the earth's surface, and the rocks above the fault have been thrust upward over the rocks below the fault plane. The magnitude 7.7 Arvin-Tehachapi earthquake of 1952 was associated with such movement along the White Horse reverse fault, and the magnitude 6.6 San Fernando earthquake of 1971 was caused by a sudden rupture along a reverse fault at the foot of the San Gabriel Mountains.
Studies of historical fault movement have shown that they occur in two ways. The first, and better known, is the sudden displacement, or slip, of the ground along a fault. Such displacement is accompanied by earthquakes and occasionally produces spectacular offsets of topographic and even of human-made features. During the 1906 earthquake, the ground was displaced as much as 21 ft along the San Andreas fault in northern California. During the 1857 earthquake, displacement of the ground along this fault was possibly as much as 30 ft in southern California. The second type of fault movement, termed creep, is now taking place on portions of several faults in California. This type of movement was well documented for the first time in 1956, and has since been found to be commonplace. It is characterized by continuous or intermittent slight slip without noticeable earthquakes. Recent fault creep on portions of the Hayward, Calaveras, and San Andreas faults has produced cumulative offsets ranging from a fraction of an inch to almost a foot in curbs, streets, and railroad tracks, and has caused some damage to buildings.
Most of the faults are, in reality, zones made up of a number of subsidiary faults or fault strands. These fault zones range in width from several feet to a mile or more. Slip along them during historical time and the recent geologic past has been found to recur repeatedly on only one or a few of the multiple strands that constitute these zones. Most of the strands commonly show no evidence of recent activity, although slip does at times recur on older strands or on entirely new ones. The strong tendency for fault slip and earthquakes to recur along the most recently active strands makes knowledge of the precise location of these strands essential to land-use planning.
The source of the stresses that cause the Earth's crust to break and slip in the California region is unknown, but the stresses appear to be related to crustal distortion on a global scale. Geologists have found abundant evidence that these stresses have been acting for millions of years. Whatever their source, the result is a continuing history of surface displacements and earthquakes along numerous faults in the California region.
The San Andreas fault
The most important of California's faults is the San Andreas, which is the "master fault" of the intricate network of faults that cuts through rocks of the coastal region of California. It is a fracture in the Earth's crust along which two parts of the crust have slipped with respect to each other.
The presence of the San Andreas fault was dramatically brought to the attention of the world on April 18, 1906, when displacement along the fault resulted in the great San Francisco earthquake and fire. This, however, was but one of many, many earthquakes that have resulted from displacement along the fault throughout its life of possibly 100 million years.
The fault is a huge fracture some 600 mi or more long, extending almost vertically into the earth to a depth of at least 20 mi. In detail, it is a complex zone of crushed and broken rock from a few hundred feet to a mile wide. Many smaller faults branch from and join the San Andreas fault zone, and if almost any road cut in the zone is examined, one will find a myriad of small fractures, fault gouge (pulverized rock), and a few solid pieces of rock.
Where is the San Andreas fault?
The San Andreas fault forms a continuous break from northern California southward to Cajon Pass. From Cajon Pass southeastward the identity of the fault becomes confused, because several branching faults such as the San Jac-into, Mission Creek, and Banning faults have similar characteristics. Nevertheless, the San Andreas type of faulting continues unabated southward to, and under, the Gulf of California.
Over much of its length, a linear trough reveals the presence of the fault; and from an airplane the linear arrangement of the lakes, bays, and valleys appears striking. Undoubtedly, however, many people driving near Crystal Springs Reservoir, along Tomales Bay, through Cajon or Tejon Passes, do not realize they are on the San Andreas fault zone. On the ground, the fault zone can be recognized by long straight escarpments, narrow ridges, and small-undrained ponds formed by the settling of small blocks within the fault zone. Characteristically, stream channels jog sharply along the fault trace.
Essentially, blocks on opposite sides of the San Andreas fault move horizontally, and if one were to stand on one side of the fault and look across it, the block on the opposite side would appear to be moved to the right. Geologists refer to this as a right-lateral strike-slip fault, or wrench fault.
During the 1906 earthquake, roads, fences, and rows of trees and bushes that crossed the fault were offset several
feet, and the road across the head of Tomales Bay was offset 21 ft, the maximum offset recorded. In each case the ground west of the fault moved relatively northward.
Geologists who have studied in detail the fault between Los Angeles and San Francisco have suggested that the total accumulated displacement along the fault may be as much as 350 mi. Similarly, geologic study of a segment of the fault between Tejon Pass and the Salton Sea revealed geologically similar terrains on opposite sides of the fault now separated by 150 mi, indicating that the separation is a result of movement along the San Andreas and branching San Gabriel faults.
It is difficult to imagine this great amount of shifting of Earth's crust; yet the rate represented by these ancient offsets seems consistent with the rate measured in historical time. Precise surveying shows a slow drift at the rate of about 2 in per year. At that rate, if the fault has been uniformly active during its possible 100 million years of existence, over 300 mi of offset is indeed a possibility.
Since 1934, earthquake activity along the San Andreas fault system has been concentrated in the areas of three cities: Eureka, San Francisco, and Los Angeles/San Bernadino. These are areas where historical earthquakes and fault displacements of the Earth's surface have been most common and where fault creep is taking place today. The sections of the state intervening the three areas mentioned above, on the other hand, have had almost no earthquakes or known slip events since the great earthquakes of 1857 in the southernmost segment and 1906 in the segment between Eureka and San Francisco. This implies to some earth scientists that these two segments of the San Andreas fault are temporarily locked, whereas in the other areas stress is being continually relieved by slip, which produces small-to-moderate earthquakes, and by creep. The lack of such activity in the locked segments could mean that these segments are subject to less frequent but larger fault movements and correspondingly more severe earthquakes.
The recorded history of earthquakes along the San Andreas fault is an extremely small sample from which, however, a clear pattern of behavior can be determined. Judging from this short history, great earthquakes seem to occur only a few times a century, but smaller earthquakes recorded only on sensitive seismographs occur much more frequently.
It is a popular misconception that once there has been a small earthquake along a segment of the fault, strain is released and further earthquakes are not to be expected for many years. Seismologists have pointed out, however, that the really great earthquakes have been preceded by numerous strong shocks and that large earthquakes seem to cluster in 10–20 year periods. Furthermore, the energy released during small earthquakes is insignificant compared to that in earthquakes having the same magnitude as the one in 1906.
Different segments of the fault also behave differently. For example, in the vicinity of Hollister, frequent small shocks are recorded, and slow movement at the rate of 0.47 in per year has been recorded. In contrast, the segment near San Francisco, except for an earthquake of magnitude 5.3 in 1957, has been relatively quiet since 1906. Perhaps, as some believe, it is gradually bending or accumulating strain that will be adjusted all at once in one large "snap."
What can be done about the fault?
Much is yet to be learned about the nature and behavior of the San Andreas fault and the earthquakes it generates. Some questions geologists would like to answer are: How old is the fault? Has movement been uniform? What movement has there been on branching faults? What is the fundamental cause of the stresses that produced the San Andreas fault? Until these questions and others have been satisfactorily answered, the question "what can be done about the fault?" is best responded to, according to the U. S. Geological Survey (USGS), in this way: "Though man cannot stop earthquakes from happening, he can learn to live with the problems they cause. Of prime importance are adequate building codes, for experience shows that well-constructed buildings greatly lessen the hazards. In construction projects, greater consideration should be given to foundation conditions. Degree of damage will range widely, between construction on bedrock, water-saturated mud, filled ground, or landslide terrain. For example, in 1906, most buildings on filled or 'made'land near the foot of Market Street in San Francisco suffered particularly intense damage, whereas buildings on solid rock suffered little or no damage. Geologists are horrified to see land developers build rows of houses straddling the trace of the 1906 break."
Maps showing the most recently active strands or breaks along the San Andreas and related active faults are being prepared by the USGS. This governmental agency can be contacted at 804 National Center, Reston, Virginia 20192. The USGS also maintains Public Inquiries Offices in San Francisco and Los Angeles.
RECENT QUAKE EVENTS
Bay Area's 1989 Earthquake Teaches Basic Lessons
There is no substitute for experience in understanding an earthquake. Most of us will never have a significant earthquake experience, but millions had the next thing to it when television was uniquely deployed in San Francisco (for other reasons) and shifted its focus to tell us first-hand how the violent shaking there felt and looked.
At 5:04 P.M ., on a quiet, autumnal afternoon, October 17, 1989, the San Andreas fault upset life beyond description in the San Francisco-Oakland Bay area. It heaved its giant breast in the Santa Cruz Mountains and wrought havoc in widening circles that reached throughout the Bay Area and shook buildings as far as Reno, Nevada, 250 mi to the east, and rattled skyscrapers 400 mi south in Los Angeles. The internationally televised third game of baseball's World Series, about to begin, gave the world the word and picture as it was happening, beginning with views of apprehensive players and fans inside Candlestick Park stadium, and combining them with telephoto visuals of the fires and devastation some 8–10 mi to the north. However, virtually no one in the Bay Area needed the ABC television crew to tell them that the "big one" was happening. Fanning out in every direction from the epicenter, shock waves that reached 6.9 on the Richter scale sundered the quiet afternoon of uncounted
thousands of people in the area, bringing fear, destruction, and, in the next minutes, death as only an earthquake can. There were more than 100 fatalities and 3,000 injuries.
As earthquakes go, the intensity, maintained for a short 15 seconds, was great. But it was nowhere near the 9.2 reading of the 1964 Alaskan quake that destroyed with tidal waves as much as with earth shaking. (Each whole number on the Richter scale equals ten times greater intensity than the previous whole number.) The toll in life and property of this 1989 cataclysm (the third most lethal of all time) resulted because the epicenter was so close to very large concentrations of population and technology-laced living styles. In the cities of San Francisco, Oakland, and other edge communities, the lives of millions, densely packed into a few square miles, involves structures of all types and descriptions. It also involves the steel and concrete double-deck freeway connecting Oakland and the mainland to San Francisco's peninsula.
Structures that survived, and some that did not
The safest structures proved to be high-rise office buildings in San Francisco, constructed since a 1971 tremor had spurred new standards; most vulnerable were the restored
Earthquake Safety Rules
An earthquake strikes your area and for a minute or two the "solid" earth moves like the deck of a ship. What you do during and immediately after the tremor may make life-and-death differences for you, your family, and your neighbors. These rules will help you survive.
Before an Earthquake
At home, bolt down water heaters and gas appliances. Place large, heavy objects and fragile items on securely fastened, lower shelves; brace or anchor heavy objects.
Keep a flashlight and battery-powered transistor radio in the home, ready for use.
During an Earthquake
- Remain calm. Think through the consequences of any action you take. Try to calm and reassure others; prepare them for the certainty of aftershocks.
- If indoors, watch for falling plaster, bricks, light fixtures, and other objects. Watch for high bookcases, china cabinets, shelves, and other furniture that might slide or topple. Stay away from windows, mirrors, and chimneys. If in danger, get under a table, desk, or bed; in a corner away from windows; or in a strong doorway. Encourage others to follow your example. Usually it is best not to run outside.
- If in a high-rise building, get under a desk. Do not dash for exits, since stairways may be broken and jammed with people. Power for elevators may fail.
- If in a crowded store, do not rush for a doorway since hundreds may have the same idea. If you must leave the building, choose your exit as carefully as possible.
- If outside, avoid high buildings, walls, power poles, and other objects which could fall. Do not run through streets. If possible, move to an open area away from all hazards. If in an automobile, stop in the safest place available, preferably an open area.
After an Earthquake
- Check for injuries in your family and neighborhood. Do not attempt to move seriously injured persons unless they are in immediate danger of further injury.
- Check for fires or fire hazards.
- Wear shoes in all areas near debris or broken glass.
- Check utility lines and appliances for damage. If gas leaks exist, shut off the main gas valve. Shut off electrical power if there is damage to your house wiring. Report damage to the appropriate utility companies and follow their instructions. Do not use matches, lighters, or open-flame appliances until you are sure no gas leaks exist. Do not operate electrical switches or appliances if gas leaks are suspected. This creates sparks that can ignite gas from broken lines.
- Do not touch downed power lines or objects touched by the downed wires.
- Immediately clean up spilled medicines, drugs, and other potentially harmful materials.
- If water is off, emergency water may be obtained from water heaters, toilet tanks, melted ice cubes, and canned vegetables.
- Check to see that sewage lines are intact before permitting continued flushing of toilets.
- Do not eat or drink anything from open containers near shattered glass. Liquids may be strained through a clean handkerchief or cloth if danger of glass contamination exists.
- If power is off, check your freezer and plan meals to use foods that will spoil quickly.
- Use outdoor charcoal broilers for emergency cooking.
- Do not use your telephone except for genuine emergency calls. Turn on your radio for damage reports and information.
- Check your chimney over its entire length for cracks and damage, particularly in the attic and at the roofline. Unnoticed damage could lead to a fire. The initial check should be made from a distance. Approach chimneys with caution.
- Check closets and storage shelf areas. Open closets and cupboard doors carefully and watch out for objects failing from shelves.
- Do not spread rumors. They often do great harm after disasters.
- Do not go sightseeing immediately, particularly in beach and waterfront areas where seismic sea waves could strike. Keep the streets clear for passage of emergency vehicles.
- Be prepared for additional earthquake shocks called "aftershocks." Although most of these are smaller than the main shock, some may be large enough to cause additional damage.
- Respond to requests for help from police, fire fighting, civil defense, and relief organizations, but do not go into damaged areas unless your help has been requested. Cooperate fully with public-safety officials. In some areas, you may be arrested for getting in the way of disaster operations.
There are no rules that can eliminate all earthquake danger. However, damage and injury can be greatly reduced by following these simple rules.
single homes built 60–90 years ago on landfills in an area known as the Marina district of San Francisco. The former coped with the shocks with well-planned engineering provisions, resulting in minimal damage. Meanwhile, more than 50 of the latter (wood and brick structures) collapsed into their foundations, and many lives were lost as scores were trapped inside. The most shocking element of the catastrophe was reserved, however, for the freeway. Due to reasoning that seems strange in retrospect, the lifeline traffic artery known as the Nimitz Freeway (Interstate Route 80) had been constructed years before as a double-decker, with its supports assumed to be—but never tested to be—earthquake resistant. The supports failed this test. Cars and drivers alike were crushed in mid-cruise as the upper deck first undulated with the shock wave then dropped its millions of tons of steel and concrete on the deck below. Drivers were pinned and vehicles crushed as if made of cardboard. The San Francisco-Oakland Bay bridge fared only slightly better, with one end of an upper section falling to meet the lower level, closing it for days.
As terrifying as this quake was, carnage was in one sense light because the early start of the ball game had drained the streets of much rush-hour traffic. That is small consolation to the families of those who died, but a blessing to thousands of others whose route would have placed them directly under the collapsing concrete that repudiated its supports.
Northridge, California, 1994
Early on the morning of January 17, 1994, Martin Luther King Day, Los Angeles area residents were jolted awake by what was to be the most significant urban earthquake to occur in California since 1906. The initial 10 seconds of trembling ground resulted in massive property damage and loss of life.
The powerful quake struck at its epicenter of Northridge at 4:31 A.M . on January 17, reaching a magnitude of 6.8. Residents within an area of approximately 2,192 mi 2 experienced over 1,000 aftershocks of a magnitude of 1.5 for weeks after the quake.
Nearly 100 deaths and 9,000 injuries resulted from the incident. Over 50,000 people were displaced from their homes; thousands of individuals were forced to move to temporary shelters in schools and churches, or camp in city parks and endure the chilly, less-than-favorable temperatures and rain.
Nearly 114,039 residential and commercial structures were damaged in the area despite the fact that seismic building code provisions and other mitigations had been intensified, especially since the Loma Prieta earthquake in 1989. Total damage was estimated at $17 billion. This most recent seismic event brought to light the difficulties in protecting Californians from their seismically unsafe environment.
The 1999 Colombia Earthquake
On January 25, 1999, at 1:19 P.M ., an earthquake shook the Armenia-Calarca-Pereira area of Colombia, causing extensive damage and killing over 2,000. It was followed by a magnitude 4.5 aftershock at 5:40 P.M . that day.
This region has had many earthquakes above magnitude 5.5 since 1973, most recently a magnitude 6.8 event on June 6, 1994, which killed at least 295 and caused extensive damage in Cauca, Tolima, and Valle Departments.
The January 1999 earthquake caused extraordinary damage for a very moderate earthquake, perhaps due to a phenomenon known as soil amplification, in which thick alluvial layers resonate and amplify seismic energy over what it would be in hard rock.
The quake occurred in the center mountain range of Cordillera Central, of three north-trending mountain ranges in western Colombia. This mountain range has active volcanoes and the earthquake occurred near the Ruiz-Tolima volcanic complex. The volcanoes exist here because the Nazca tectonic plate is subducting beneath South America; most earthquakes in this zone are in the subducting plate and thus at least 62 mi beneath the surface. This earthquake, however, had a source depth of about 11 mi, and was caused by some near-surface tectonic adjustment. This earthquake was a strike slip earthquake; that is, there was essentially no vertical motion in its faulting. Its faulting is similar to what typically occurs on the San Andreas fault.
January 13, 2001, El Salvador Earthquake
A major earthquake, 7.6 magnitude, struck about 105 miles south-southwest of San Miguel, El Salvador, in the Pacific Ocean. The death toll in Central America, as of early March 2001, is approximately 1,300 people, with 8,000 injured. An aftershock occurred in the same area a month later, killing hundreds.
January 26, 2001, southern India Earthquake
A major earthquake, 7.9 magnitude, struck about 65 miles north-northeast of Jamnagar, India. Buildings collapsed in the state of Gujarat. The earthquake was felt at Mumbai (Bombay) and Delhi, as well as Karachi and Peshawar, Pakistan, and in parts of Nepal. As many as 20,000 people were killed by this earthquake. On June 16, 1819, an earthquake in this same general area killed 1,500–2,000 people.
The earthquake occurred along an approximately east-west trending thrust fault at shallow (<15.5 mi) depth. Thrust faults occur when one portion of the Earth's crust is pushed up over an adjacent portion. The strain that caused this earthquake is due to the Indian plate pushing northward into the Eurasian plate.
February 28, 2001, western Washington State Earthquake
A major earthquake of 6.8 magnitude occurred in the Seattle-Tacoma area of Washington state on February 28,2001. At least several hundred people were injured, but no deaths were directly connected to the earthquake. Damage is estimated at about $2 billion.
The location of this western Washington state earthquake is very near the locations of a 1949 magnitude 7.1 earthquake and a 1965 magnitude 6.5 earthquake. These events occurred on a normal fault within the Juan de Fuca plate where it subducts (goes under) the North America plate. The 2001 earthquake is called the "Nisqually earthquake," because of the proximity of the earthquake to the Nisqually River delta in Puget Sound. The name Nisqually is taken from a group of Native Americans who live in the area.
November 3, 2002 (UTC), Denali Fault, Alaska
This 7.9 magnitude shock is the largest earthquake on the Denali fault since at least 1912, when an 7.2 magnitude earthquake occurred in the general vicinity of the fault, 50 miles east of this latest epicenter. This 7.9 magnitude shock, one of the largest ever recorded on U.S. soil, occurred on the Denali-Totschunda fault system, which is one of the longest strike-slip fault systems in the world and rivals in size California's famed San Andreas strike-slip fault system that spawned the destructive San Francisco earthquake in 1906.
Earthquake in west central Mexico, January 21, 2003.
A major earthquake occurred in Colima, Mexico, about 30 miles southeast of Manzanillo, Colima, or about 310 miles west of Mexico City at 7:06 P.M . MST, January 21, 2003 (8:06 P.M . CST in Mexico). A preliminary magnitude of 7.8 was computed for this earthquake. The magnitude and location may be revised when additional data and further analysis results are available. There were at least 28 deaths, 300 injured, and considerable damage in the states of Colima, Michoacan, and Jalisco. The earthquake was felt strongly in Mexico City.
This shallow earthquake occurred in a seismically active zone near the coast of central Mexico. The earthquake occurred near the juncture of three tectonic plates: the North American Plate to the northeast, the Rivera Plate to the northwest, and the Cocos Plate to the south. Both the Rivera Plate and the Cocos Plate are being consumed beneath the North American Plate. The slower moving Rivera Plate is moving northwest at about 2 cm per year relative to the North American Plate, and the faster moving Cocos plate is moving in a similar direction at a rate of about 4.5 cm per year.
There have been several significant earthquakes near the recent event. In 1932, a magnitude 8.4 thrust earthquake struck about 100 km to the north-northwest. More recently, on October 9, 1995, a magnitude 8.0 earthquake struck about 50 km to the northwest killing at least 49 people and leaving 1,000 homeless. The most deadly earthquake in the region occurred about 170 km to the southeast on September 19, 1985. This magnitude 8.0 earthquake killed at least 9,500 people, injured about 30,000, and left 100,000 people homeless.
What is a tsunami?
The phenomenon we call "tsunami" is a series of traveling ocean waves of great length and long period, generated by disturbances associated with earthquakes in oceanic and coastal regions. As the tsunami crosses the deep ocean, its length from crest to crest may be 100 mi or more, its height from trough to crest only a few feet. It cannot be felt aboard ships in deep water, and cannot be seen from the air. But in deep water, tsunami waves may reach forward speeds exceeding 600 mph.
As the tsunami enters the shoaling water of coastlines in its path, the velocity of its waves diminishes and wave height increases. It is in these shallow waters that tsunamis become a threat to life and property, for they can crest to heights of more than 100 ft, and strike with devastating force.
The tsunami of the century: Papua New Guinea, 1998
On the evening of Friday, July 17, at 7:30 P.M ., a massive tsunami swept across the sandbar that forms the outer margin of Sissano Lagoon, West Sepik, Papua New Guinea, striking four villages west of the town of Aitape. The wave was reported to be 22.8–33 ft high; up to 3,000 persons were reported killed or missing. This was an unusually damaging tsunami, given the size of the earthquake (a magnitude 7) associated with it.
As of late 1998, scientists were continuing to examine this event, in an attempt to explain the unusually high runups, with the ultimate hope of mitigating such disasters in the future.
Scientists from the USGS participated in the second International Tsunami Survey Team to study the sedimentary deposits left by this tsunami. Animations of the tsunami have also been developed to graphically display how the tsunami evolved from an earthquake source.
Since 1992, the international community has responded to nine major tsunami disasters (Nicaragua, 1992; Flores, 1992; Okushiri, 1993; East Java, 1994; Mindoro, 1994; Kuril Islands, Russia, 1994; Manzanillo, 1995; Irian Jaya, Indonesia, 1996; and Peru, 1996) by dispatching this team of scientists, which has come to be known as the International Tsunami Survey Team (ITST), with more than 30 scientists and 20 students from Indonesia, Korea, Japan, Mexico, Peru, Russia, the United Kingdom, and the United States. The Papua New Guinea survey team was joined by scientists from Australia and New Zealand.
Largest Quakes in the United States (with Magnitudes and Dates)
Prince William Sound, AK (9.2; March 28, 1964)
Andreanof Islands, AK (8.8; March 9, 1957)
Rat Islands, AK (8.7; February 4, 1965)
east of Shumagin Islands, AK (8.3; November 10, 1938)
Lituya Bay, AK (8.3; July 10, 1958)
Yakutat Bay, AK (8.2; September 10, 1899)
Cape Yakataga, AK (8.2; September 4, 1899)
Andreanof Islands, AK (8.0; May 7, 1986)
New Madrid, MO (7.9; February 7, 1812)
Fort Tejon, CA (7.9; January 9, 1857)
Ka'u District, HI (7.9; April 3, 1868)
Kodiak Island, AK (7.9; October 9, 1900)
Gulf of Alaska (7.9; November 30, 1987)
Denali Fault, Alaska (7.9; November 3, 2002)
Owens Valley, CA (7.8; March 26, 1872)
Largest Quakes in the Contiguous United States
New Madrid, MO (7.9; February 7, 1812)
Fort Tejon, CA (7.9; January 9, 1857)
Owens Valley, CA (7.8; March 26, 1872)
Imperial Valley, CA (7.8; February 24, 1892)
New Madrid, MO area (7.7; December 16, 1811)
San Francisco, CA (7.7; April 18, 1906)
Pleasant Valley, NV (7.7; October 3, 1915)
New Madrid, MO (7.6; January 23, 1812)
Landers, CA (7.6; June 28, 1992)
Kern County, CA (7.5; July 21, 1952)
west of Lompoc, CA (7.3; November 4, 1927)
Dixie Valley, NV (7.3; December 16, 1954)
Hebgen Lake, MT (7.3; August 18, 1959)
Borah Peak, ID (7.3; October 28, 1983)
Widely differing magnitudes have been computed for some of these earthquakes; the values differ according to the methods and data used. For example, some sources list the magnitude of the 8.7 Rat Islands earthquake as low as 7.7. On the other hand, some sources list the magnitude of the February 7, 1812, New Madrid quake as high as 8.8. Similar variations exist for most events on this list, although generally not so large as for the examples given.
In general, the magnitudes given in the list above have been determined from the seismic moment, when available. For very large quakes, the moment magnitude is considered to be a more accurate determination than the traditional amplitude magnitude computation procedures. Note that all of these values can be called "magnitudes on the Richter scale," regardless of the method used to compute them.
Source: Stover, C.W., and J.L. Coffman. Seismicity of the United States, 1968-1989 . Revised. U.S. Geological Survey Prof. Paper 1527, 1993.
The tsunami warning system
Development of the National Oceanic and Atmospheric Administration (NOAA) Coast and Geodetic Survey's Pacific Tsunami Warning System was impelled by the disastrous waves of April 1946, which surprised Hawaii and took a heavy toll in life and property. The locally disastrous tsunami caused by the March 1964 Alaska earthquake impelled the development of another type of warning apparatus—the Regional Tsunami Warning System in Alaska.
The Regional Tsunami Warning System is headquartered at the Coast and Geodetic Survey's Seismological Observatory at Palmer, Alaska. This is the nerve center for an elaborate telemetry network linking Palmer with remote seismic and tidal stations along the Alaska coast and in the Aleutian Islands. Seismograph stations in the network are at Palmer Observatory and its two remote stations 25 mi south and west, and at Biorka, Sitka, Gilmore Creek, Kodiak, and Adak. Tide stations are at Seward, Sitka, Kodiak, Cold Bay, Unalaska, Adak, Yakutat, and Shemya. Data from these stations are recorded continuously at Palmer, where a 24-hour watch is kept.
When an earthquake occurs in the Alaska-Aleutian area, seismologists at Palmer Observatory rapidly determine its epicenter (the point on the Earth's surface above the underground source of the earthquake) and magnitude. If the epicenter falls in the Aleutian Island arc or near the Alaskan coastal area, and if the earthquake magnitude is great enough to generate a tsunami, Palmer Observatory issues a tsunami warning through the Alaska Disaster Office, Alaska Command, and Federal Aviation Administration (FAA) covering the area near the epicenter. A tsunami watch is issued for the rest of the Alaskan coastline, alerting the public to the possibility of a tsunami threat. If tide stations detect a tsunami, Palmer Observatory extends the tsunami warning to cover the entire coastline of Alaska. If no tsunami is observed, both the watch and warning bulletins are canceled.
Subsidiary warning centers have been established at Sitka and Adak Observatories. These facilities operate small seismic arrays and have a limited warning responsibility for local areas.
The Pacific Tsunami Warning System has its headquarters at the Coast and Geodetic Survey's Honolulu Observatory. There, seismologists monitor data received from seismic and tidal instruments in Hawaii and around the Pacific Ocean, and provide ocean-wide tsunami watches and warnings. The Pacific system works very closely with its regional counterpart in Alaska. Potentially tsunami-generating earthquakes in the Alaska-Aleutian area are detected and evaluated at Palmer Observatory, and the data relayed directly to the Honolulu Observatory. Where there is tidal evidence of a tsunami, the warning is extended by Honolulu to cover the Pacific Ocean basin. For tsunamis generated elsewhere in the Pacific area, tsunami watch and warning bulletins are prepared at the Honolulu Observatory and disseminated in Alaska by the Alaska Disaster Office, the military, and Federal Aviation Administration (FAA).
FREQUENTLY ASKED QUESTIONS ABOUT EARTHQUAKES
What is the biggest earthquake ever?
Since 1900, the earthquake in Chile on May 22, 1960, is the biggest in the world with magnitude 9.5 Mw.
What is the biggest earthquake in the United States?
Since 1900, the earthquake in Alaska on March 28, 1964, is the biggest earthquake in the United States, with magnitude 9.2 Mw. This earthquake is also the second biggest earthquake in the world.
Tsunami Safety Rules
Tsunamis are generated by some earthquakes. When you hear a tsunami warning, you must assume a dangerous wave is on its way. History shows that when the great waves finally strike, they claim those who have ignored the warning.
- Not all earthquakes cause tsunamis, but many do. When you hear that an earthquake has occurred, stand by for a tsunami emergency.
- A strong earthquake felt in a low-lying coastal area is a natural warning of possible, immediate danger. Keep calm and move to higher ground, away from the coast.
- A tsunami is not a single wave, but a series of waves. Stay out of danger areas until an "all-clear" is issued by competent authority.
- Approaching tsunamis are sometimes heralded by a noticeable rise or fall of coastal water. This is nature's tsunami warning and should be heeded.
- A small tsunami at one beach can be a giant a few miles away. Do not let the modest size of one make you lose respect for all.
- All tsunamis—like hurricanes—are potentially dangerous, even though they may not damage every coastline they strike.
- Never go down to the beach to watch for a tsunami. When you can see the wave you are too close to escape it.
- During a tsunami emergency, your local Civil Defense, police, and other emergency organizations will try to save your life. Give them your fullest cooperation.
Stay tuned to your radio or television stations during a tsunami emergency—bulletins issued through Civil Defense and NOAA offices can help save your life.
Which states in the United States have the most earthquakes?
Alaska and California.
Which state has the most damaging earthquakes?
Which states have the smallest number of earthquakes?
Florida and North Dakota.
What region has the fewest earthquakes?
Antarctica has the fewest earthquakes of any continent, but small earthquakes can occur anywhere in the world.
When will California slide into the ocean?
There is no scientific reason that indicates that California will ever fall into the ocean.
What is the difference between magnitude and intensity?
Magnitude measures the energy released at the source of the earthquake. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded on a seismogram at a certain period. Intensity measures the strength of shaking produced by the earthquake at a certain location. Intensity is determined from effects on people, human structures, and the natural environment. Intensity does not have a mathematical basis, but is based on observed effects.
Where can I buy a Richter scale?
The Richter scale is not a physical device, but a mathematical formula. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded on a seismogram at a certain period.
What is an aftershock?
Smaller earthquakes following the largest earthquake of a series, concentrated in a restricted crustal column.
How long can an earthquake shake?
Two to three minutes.
What is a fault?
A fracture or zone of fractures in rock along which the two sides have been displaced relative to each other parallel to the fracture.
What is liquefaction of soil?
The process of soil and sand behaving like dense fluid rather than a wet solid during an earthquake.
What is the Moho?
The Moho is the abbreviated form of Mohorovicic (pronounced Mo-ho-ro-vish-ich) discontinuity. This is a boundary surface or the sharp seismic velocity discontinuity that separates the Earth's crust from the underlying mantle. Its depth varies from about 3–6 mi beneath the ocean floor to about 20 mi below the continents. The discontinuity probably represents a change in chemical composition. It is named after its Croatian discoverer Andrija Mohorovicic.
When did the first instrument actually record an earthquake?
Probably the earliest seismoscope was invented by the Chinese philosopher Chang Heng in A.D . 132. This was a large urn on the outside of which were eight dragon heads facing the eight principal directions of the compass. Below each dragon head was a toad with its mouth opened toward the dragon. When an earthquake occurred, one or more of the eight dragon-mouths would release a ball into the open mouth of the toad sitting below. The direction of the shaking determined which of the dragons released its ball. The instrument is reported to have detected an earthquake 400 mi away that was not felt at the location of the seismoscope. The inside of the seismoscope is unknown: most speculations assume that the motion of some kind of pendulum would activate the dragons.
Where do earthquakes occur?
Earthquakes can strike any location at any time. But history shows they occur in the same general patterns year after year, principally in three large zones of the earth.
The world's largest earthquake belt, the circum-Pacific seismic belt, is found along the rim of the Pacific Ocean, where about 81% of the world's largest earthquakes occur. The belt extends from Chile, northward along the South American coast through Central America, Mexico, the West Coast of the United States, and the southern part of Alaska, through the Aleutian Islands to Japan, the Philippine Islands, New Guinea, the islands groups of the southwest Pacific, and to New Zealand. This earthquake belt was responsible for 70,000 deaths in Peru in May 1970, and 65 deaths and one billion dollars of damage in California in February 1971.
Why do so many earthquakes originate in this belt?
This is a region of young, growing mountains and deep ocean trenches that invariably parallel mountain chains. Earthquakes necessarily accompany elevation changes in mountains, the higher part of the Earth's crust, and changes in the ocean trenches, the lower part.
The second important belt, the Alpide, extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. This belt accounts for about 17% of the world's largest earthquakes, including some of the most destructive, such as the Iran shock that took 11,000 lives in August 1968, and the Turkey tremors in March 1970 and May 1971 that each killed over 1,000. All were near magnitude 7 on the Richter scale.
The third prominent belt follows the submerged mid-Atlantic Ridge. The remaining shocks are scattered in various areas of the world.
Earthquakes in these prominent seismic zones are taken for granted, but damaging shocks occur occasionally outside these areas. Examples in the United States are New Madrid, Missouri, and Charleston, South Carolina. Many years, however, usually elapse between such destructive shocks.
Can earthquakes be predicted?
It is not possible for scientist to predict earthquakes now and it may never be possible. Some people believe that animals and psychics can predict earthquakes, but that has not been proven.
PROTECTING THE PUBLIC FROM EARTHQUAKE HAZARDS—ADVANCED NATIONAL SEISMIC SYSTEM COMES TO MEMPHIS
October 2002 marked a new milestone in the installation of modern seismic stations in seismically active urban areas across the country. These cities include Memphis, San Francisco, Seattle, Salt Lake City, Anchorage, and Reno. These new instruments are part of a nationwide network of sophisticated ground shaking measurement systems, both on the ground and in buildings, called the Advanced National Seismic System (ANSS). ANSS will become the first line of defense in the war on earthquake hazards—with the ultimate victory being public safety, lives saved, and major losses to the economy avoided.
ANSS stations will assist emergency responders within minutes of an event showing not only the magnitude and epicenter, but where damage is most likely to have occurred.
Ten new ANSS instruments have recently been installed in the Memphis area, 20 have been installed across the mid-America region, and more than 175 have been installed in other vulnerable urban areas to provide real-time information on how the ground responds when a strong earthquake happens.
The ultimate goal of ANSS is to save lives and ensure public safety, said Dr. John Filson, U. S. Geological Survey (USGS) Earthquake Program Coordinator. "This information, already available in Southern California, is generated by data from seismic instruments installed in urban areas and has revolutionized the response time of emergency managers to an earthquake, but its success depends on further deployment of instruments in other vulnerable cities."
In 1997, during the reauthorization of the National Earthquake Hazards Reduction Program, Congress asked for an assessment of the status and needs of earthquake monitoring. The result was the authorization of ANSS to be implemented by the USGS. The system, when implemented, would integrate all regional and national networks with 7,000 new seismic instruments, including 6,000 strong-motion sensors in 26 at-risk urban areas. To date, approximately 350 instruments have been installed.
Earthquakes pose one of the greatest risks for casualties and costly damage in the United States. California's Northridge earthquake in 1994, a magnitude 6.7 quake, took 57 lives when it struck a modern urban environment generally designed for seismic resistance. With losses estimated at $20 billion, this was the most expensive earthquake in U.S. history. During the 1989 World Series, as more than 62,000 fans filled Candlestick Park, a magnitude 7.1 earthquake struck about 60 miles south of San Francisco. The effects of the 20-second quake caused as much as $10 billion in damage. Sixty-two people died.
In March 1964, a magnitude 9.2 earthquake near Anchorage took 125 lives and caused about $311 million in property losses. Thirty blocks of dwellings and commercial buildings were damaged or destroyed in the downtown area of Anchorage. Landslides caused heavy damage, and an area of 130 acres broke the ground into blocks that were collapsed and tilted at all angles.
In 1811 and 1812, the central Mississippi Valley was struck by three of the most powerful earthquakes in U.S. history. Consider what the impact would be if these events happened today in this region that has more earthquakes than any area east of the Rocky Mountains.
The goal of USGS earthquake monitoring is to mitigate risk—using better instruments to understand the damage that shaking causes and to help engineers create stronger and sounder structures that ensure vital infrastructures, and keep utility, water, and communication networks operating safely and efficiently.
The ANSS "strong motion" instruments are critical in giving emergency response personnel real-time maps of severe ground shaking and providing engineers with information about building and site response.
ANSS provides the USGS with the capability to create tools to process earthquake information faster; for example, Shake Map, a rapidly generated computer map that shows the location, severity, and extent of strong ground shaking within minutes after an earthquake. As it modernizes seismic networks, the USGS hopes to be able to provide the ANSS-generated Shake Map capability for every seismically active urban area. A possibility USGS scientists have been keenly aware of throughout the development of ANSS is that an early warning of even a few seconds would give children enough time to get under their desks; could stop trains and subways; shut off pipelines; shut down nuclear facilities; and suspend medical procedures. Another new tool is the "Did you feel it?" website ( http://pasadena.wr.usgs.gov/shake/ ). This allows citizens with internet access to record their observations of shaking. The result is a community intensity map (coded by zip code) across the region.