EARTHQUAKE FAQ

Q: What is a fault?

A: A fault is a discontinuous displacement of rocks along a surface. A fault therefore separates two adjacent rock masses into blocks with a thin zone of crushed rock called gouge in between. A fault must have observable shear displacement between the adjacent geological units, otherwise we call the fracture a joint. Faults can be any length, from centimeters like in laboratory deformed rock samples to thousands of kilometers like the San Andreas or Anatolian fault zones. Fault surfaces can be at any angle to the surface of the earth, and the direction of motion along the fault can also be at any direction. A fault that breaks the surface of the Earth creates a line across the surface called the fault trace. The angle in a clockwise direction between the fault trace and a line on the surface pointing North is called the fault strike. The fault dip is the angle the fault makes with the surface of the Earth, the fault slip is the amount of displacement, and the rake is the clock-wise angle between the direction of slip along the fault and horizontal. The throw along the fault is the total vertical offset between the top and bottom of an offset geological unit such as a layer of limestone. Seismology inherited the terms hanging wall and footwall from mine engineering. Mine tunnels were often excavated along faults; when the fault surface was overhead the miners called it a hanging wall since they would hang lanterns from that wall; when the miners walked on the fault surface they called it a footwall.

Q: What is an earthquake?

A: Earthquakes occur when stresses in the earth reach a level greater than the strength of the rock, causing the rocks on opposite sides of the fault to suddenly and violently slip past one another. Stresses acting perpendicular to the fault push the rocks on either side of the fault together. The strength of a fault is related to the size of these stresses and the coefficient of friction of the material forming the fault. At the same time, other stresses act parallel to the fault plane to move the rocks past each other. When enough stress accumulates to overcome the strength of the fault, an earthquake occurs as the rocks snap back toward equilibrium, and release the stored energy in the form of seismic waves, which shake the surrounding rocks. H. F. Ried first developed this hypothesis of how earthquakes occur, called the elastic rebound theory, following the (Great) 1906 San Fransisco earthquake.

    Whenever a noteworthy earthquake occurs newspapers, television broadcasts, and scientists quote many terms and quantities not included in the average person's vocabulary. For example, the rupture surface is the portion of the fault which slips when the earthquake occurs. The earthquake rupture begins at one point on the rupture surface called the focus or hypocenter specified with a latitude, a longitude, and a depth. The epicenter is the point on the earth's surface above the hypocenter, specified with only a latitude and longitude. The rupture progresses from the hypocenter along the rupture surface at a finite speed until, for some reason, it stops. The total time of shaking caused by an earthquake is related to the length of time needed for the rupture to progress along the entire rupture surface.

    A rupture may stop because all of the accumulated stress is released, because it reaches a stronger section of the fault, or because it reaches the end of the fault. What physical conditions allow a rupture to begin, and what causes it to stop are important questions earthquake researchers are now attempting to answer. Generally, but not in all cases, earthquake rupture begins at some point many kilometers deep within the lithosphere and progresses updip along the fault plane over the entire rupture surface.

    If the earthquake occurs underneath the sea and continues almost to the sea floor, then the earthquake may create a tsunami. Tsunamis are popularly and erroneously called 'tidal waves,' but have nothing to do with tides or weather. More information on Tsunamis can be found at The WWW Tsunami Information Resource.

Q: Which came first, the fault or the earthquake?

A: People often wonder whether earthquakes cause faults or faults cause earthquakes. In laboratory experiments we can measure the mechanical strength of rock samples by continuously increasing stress on the sample until it breaks. Many people think of the break as an earthquake; however, usually this is quite unlike real-world earthquakes, which occur on preexisting faults. The Earth's lithosphere is riddled with faults, joints, buried ancient weathering surfaces, and other zones of weakness. It is much easier to slip on an existing zone of weakness, even if it is not optimally oriented, than to create a new fault. Therefore, on the physical Earth, faults always exist before we measure earthquakes occurring along them.

Q: What is fault creep?

A: Fault creep occurs when rock units on opposite sides of a fault slip past each other slowly and steadily. Large earthquakes are not as likely on a fault section undergoing creep as along a "locked" section of a fault.

Q: What are the different types of faults?

A: Scientists classify faults into strike-slip or dip-slip types according to the motion along the fault. Strike-slip faults are approximately vertically dipping with horizontal displacement along the strike of the fault. When looking across the fault, if objects on the opposite side move to your right the fault is a right-lateral strike-slip fault, and if objects on the opposite side move to your left the fault is a left-lateral strike-slip fault.

    Dip-slip faults are inclined at some angle to the surface, and displacement is primarily normal to strike direction, along the dip of the fault. Geologists give more descriptive names to dip-slip faults depending on the direction of the displacement. For example, on a normal fault the foot wall moves up with respect to the hanging wall. Normal faulting occurs in response to lithospheric extension with the fault plane dipping away from the uplifted rocks. Reverse faulting occurs in response to lithospheric shortening, or compression with the fault plane dipping beneath the uplifted rocks. On a reverse fault the foot wall moves down with respect to the hanging wall. When a reverse fault has a small dip angle, geologists call them thrust faults. Blind thrust faults occur at some depth but do not extend to the surface, forcing the layers of rock above the fault to bend instead of break.

Q: What are the different types of earthquakes?

A: Seismologists classify earthquakes according to the motion on the fault and fault type as strike- slip and dip-slip earthquakes (see the previous question for the definitions). Earthquakes that are a combination of strike-slip and dip-slip movements, are called oblique-slip earthquakes. Earthquakes may also be classified in terms of the origin of the stresses that produce them; e. g., volcanic earthquakes are caused by stresses associated with a volcanic eruption.

Q: How commonly do earthquakes occur?

A: The answer depends on the size of the earthquake. Over the entire Earth, the number of earthquakes with magnitudes of 8 and greater is less than one each year. However, each year there are about 10 earthquakes of magnitude 7 or greater and 100 earthquakes of magnitude 6 or greater. No matter where they occur, these earthquakes are all powerful enough to be recorded by all or mostly all of the world's sensitive seismograph stations, such as UTIG's station HKT.

    Each year there are probably also about 1,000 earthquakes of magnitude 5 or more, 10,000 of magnitude 4 or more, etc. However, a seismograph station must be very close to the epicenter to recorded smaller earthquakes; thus, many small earthquakes occurring in remote areas of the Earth are never recorded, located, or catalogued.

Q: Are earthquakes on the increase?

A: The number of earthquakes recorded in earthquake catalogs is increasing. However, that does not mean that the rate of earthquake acivity itself is increasing. What has increased in the last three decades is the number and quality of the world's seismograph stations. We can now reliably locate most earthquakes greater than magnitude 5.2 occurring anywhere in the world, and in some regions like parts of Japan and southern California we locate most earthquakes of magnitude 2.0 and greater. Therefore if one examines the earthquake catalogs without knowing of these incredible improvements in detection one might conclude erroneously that the number of earthquakes is rising, when in fact we are just becoming more efficient at detecting and locating earthquakes.

Q: Is there a particular time of day earthquakes are more likely to occur?

A: Earthquakes can occur any time of day or night. There is no correlation between time of day and earthquake occurance, although small earthquakes may be more noticeable at night when there is less cultural noise from heavy equipment and transportation, and from weather.

Q: Can earthquakes occur anywhere?

A: Wherever faults exist an earthquake can occur. That includes places that have never experienced an earthquake in recorded history and places listed as devoid of any reasonable risk on seismic hazard maps. However, earthquakes are much more likely to occur in some places than others; they are most likely along the boundaries between tectonic plates and at particular points of weakness within plates.

Q: What is a seismogram?

A: Seismologists call a record of the ground motion at a particular point on the Earth's surface a seismogram. A seismometer measures the ground motion, usually by observing the behaviour of an appropriately oriented pendulum, or by observing the electromagnetic fields needed to keep a mass stationary. The seismograph is the whole package: a device for measuring ground motions and one for processing those motions so they can be recorded.

    The earliest seismic instruments were seismoscopes, devices designed with an unstable element which changed when an earthquake occurred. For example, one kind of seismogscope was just a bowl of mercury surrounded by little cups. An earthquake's shaking would slosh mercury into the cups; the amount spilled told something about the quake's intensity; which cups were filled told something about direction. The earliest seismographs resembling modern instruments were deployed in Europe and Japan in the last half of the nineteenth century. However, until about 1910 the physics of seismic waves was too poorly understood, seismographs were too crude, and their timing was too inaccurate to study earthquakes the way we do today. Thus, our record of earthquake activity worldwide only is complete back to about 1900, even for very large earthquakes.

    The first truly world wide network was the World Wide Standardized Seismograph Network, or WWSSN, deployed in the early 1960's, The WWSSN actually was meant to detect underground nuclear explosions, but in addition contributed greatly to our knowledge of the structure of the Earth and the physics of earthquakes. WWSSN instruments were relatively simple instruments compared to modern systems; the completely analog WWSSN systems were sensitive only to a narrow frequency band of seismic waves, and used photographic film to create permanent seismograms. Modern digital seismographs such as UTIG's station HKT, record ground motions over a very wide frequency band and have a great dynamic range, meaning they can record both small and great earthquakes.

Q: What is earthquake magnitude?

A: Earthquakes vary broadly in size; from microscopic fractures to slip occurring on a fault hundreds of kilometers long. Earthquakes also vary in location and depth. Individuals would probably notice a small earthquake occurring a kilometer below the surface, but a larger earthquake occurring several hundred kilometers below your feet may not be noticed at all. Therefore scientists have developed procedures for measuring the size of an earthquake independent of location, depth, or damage to structures.

    Charles F. Richter introduced the concept of earthquake magnitude in the 1930's. Basically, his "scale" measured the maximum signal amplitude recorded on a standard seismograph, then corrected this for distance and instrument gain to obtain the magnitude. Richter developed his magnitude scale only for earthquakes occurring in California and measured on one specific type of seismometer, a Wood-Anderson torsion seismometer, designed to record the velocity of ground motion on photographic film. To find the magnitude, one measures the maximum amplitude A from the photographic record using a metric ruler. The formula for determining the Richter magnitude is

Q: What is earthquake moment?

A: Earthquake moment, or seismic moment M₀ is perhaps the most fundamental parameter we can use to measure the strength of an earthquake. While magnitudes are a convenient measure of earthquake size determined directly from one seismogram, M₀ is a more physically meaningful measurement of earthquake size not subject to many of the problems that plague magnitudes. In fact, M₀ is directly related to the fundamental parameters of the faulting process. Seismologists write this relationship as

Q: What is earthquake intensity?

A: Scientists and engineers often describe the effects of ground shaking on humans and man made structures in terms of earthquake intensity. Earthquake intensity is judged on the Modified Mercalli scale and is, by definition, subjective, since it doesn't depend on instrumental measurements, but instead on the observer's assessment of damage or shaking. A level III intensity indicates rattling doors and windows, broken dishes, and cracked plaster. The highest intensity, XII, is reserved for total devastation, with shaking so severe that objects may be thrown vertically into the air. Intensity differs from earthquake magnitude in that the severity of shaking from any earthquake varies from place to place. A map of intensity values from a single earthquake will have isoseismal contour lines drawn on it to provide a representation of the broad variations in shaking over the region surrounding the earthquake epicenter. As distance from the epicenter increases intensity generally decreases.

Modified Mercalli Intensity Scale

I. People do not feel any Earth movement.

II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings.

III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring.

IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.

V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers.

VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage.

VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings.

VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change.

IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage.

X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly.

XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent.

XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.

Note that no where in this list does it say "Huge cracks open in the Earth and swallow people." Falling into a crevasse caused by an Earthquake is more the realm of Hollywood special effects than reality.

     The subjective nature of earthquake intensity creates several problems for comparing the effects of different earthquakes. In particular, intensity depends on quantifying the response of different variables. For example, the low ratings of intensity depend on what people notice, the mid-values depend on the response of structures, and the highest ratings describe what happens when ground failure occurs. In addition, where building standards enforce construction resistant to damage from ground shaking the maximum intensity can be quite low. However, if buildings constructed out of unreinforced masonry, mud and straw, or thatching are subjected to the same amount of shaking they may completely disintegrate. Furthermore, if the epicentral region is sparsely populated then there may not be enough "measurements" to obtain a good idea on the maximum intensity of the earthquake.

Q: How well do we know where an earthquake occurred?

A: Seismologists interpret seismograms to determine the location of earthquakes around the world. When earthquakes occur in areas where there are many, many seismographs (like California), the location of their epicenter can be determined very accurately-- to within less than a kilometer. However, when earthquakes occur far from the nearest seismic stations, the location uncertainties are much larger. For example, for large earthquakes occurring south of New Zealand the location uncertainties may be 30 km or so.

Q: What are those circular diagrams that come with earthquake reports?

A: These diagrams, sometimes informally called "beachball" diagrams, show how seismic waves the earthquake creates vary in different directions as they leave the hypocenter. If you imagine that an earthquake occurs at the center of the sphere, the points plotted represent the lower hemisphere projection of the first motion that an earthquake wave makes as it leaves the hypocenter. The dark colored part of the sphere shows the direction of motions outward from the hypocenter, while the light colored part shows inward motion directions. Seismologist commonly call these diagrams focal mechanisms, and refer to them when discussing the earthquake radiation pattern and plate tectonics.

     When we have many observations, we can determine the p-wave radiation pattern of the earthquake. This provides information on the geometry and displacements on the ruptured fault. Generally compressional first motions (usually a positive swing on a seismogram) are plotted as (+) points or in the black region, and dilatational first motions (usually a negative swing on a seismogram) are plotted as (-) points or in the white region of the diagram. Each plotted point represents the response of the seismometer at at a single location, backprojected onto the focal sphere. To determine where points plot on the focal sphere, we need good estimates of the azimuth and distance of the seismic observatory with respect to the earthquake hypocenter, the depth of the earthquake, and the structure of the Earth.

     The boundaries between the compressional and dilatational regions represent nodal planes, where a seismometer would not measure any motion in a particular direction. When looking at p-wave first motions, the nodal planes represent the projection of two planes, one of which is the fault plane containing the earthquake, and the other which is called the auxiliary plane.

Q: What factors influence intensity?

A: The most important factors influencing intensity are:

• Magnitude - Larger earthquakes occur over longer faults and have longer source durations. Earthquakes start at a single point (the hypocenter) and progress as a front over the rupture surface. The entire rupture surface creates the shaking you feel, and the shaking will continue as long as individual points along the surface continue moving. The duration of shaking is directly proportional to the amount of damage incurred by structures.
• Distance From The Fault - The effects of motion on a fault die off with distance, so that shaking decreases with distance from the fault.
• Soil Conditions - Unconsolidated sediment and soil can amplify the shaking due to an earthquake. Shaking felt over soft, loose soil is more intense than that on hard rock at the same distance and orientation with respect to the epicenter. The amplification of shaking increases with the thickness and looseness of the soil. In cases of very intense shaking, the soil may completely loose cohesion and become liquified.
• Construction - Buildings constructed from unreinforced masonry, adobe, or wattle never fair well during an earthquake. Buildings constructed on a wood or steel frame, such as most modern houses, survive ground shaking fairly well because the frame flexes in response to the shaking. However, moderate shaking can destroy frame buildings that are not well anchored to a foundation.

      Traditional Japanese construction usually features a wooden frame, but often also includes heavy clay tiles for roofing in order to withstand the high winds associated with tropical storms. Unfortunately these tiles proved deadly during the recent Kobe earthquake.

• Radiation Pattern - Earthquakes do not radiate energy equally in all directions. Thus, the amount of shaking a structure undergoes is also related to the orientation of the fault with respect to the building location. See the explanation of earthquake focal mechanisms above.
• Directivity - The direction the earthquake rupture progresses along the fault, or directivity, also affects the duration and intensity of shaking. The total time the shaking lasts is extended or reduced because the source of the earthquake waves is moving as it releases energy, similar to the what happens when sound waves are Doppler shifted.
  Figure 9 demonstrating directivity.

Q: How do we measure shaking?

A: Ground motions need several numbers to be fully quantified:

• Peak Velocity - The fastest the ground moves.
• Peak Acceleration - The fastest the direction of ground motion changes.
• Duration - The total time of shaking.
• Period - The time it takes for one cycle of a seismic wave to pass a point on the earth; the wave period is the inverse of wave frequency.

Q: What is the proper response to an earthquake prediction?

A: That depends on who made the prediction. Many predictions are made by unqualified individuals, and should be ignored. For example, in 1990 a climatologist named Iben Browning announced there was a 50% chance that a big earthquake would occur in the Missouri-Tennessee region on 3 December, at the time when solar and lunar tides were to be especially strong. Browning had no background in seismology, indeed, he had obtained his Ph.D. in biology from The University of Texas at Austin (!!) in 1948. He was unwilling to publish an account of the tidal theories that led to his prediction, and he steadfastly refused to discuss the basis of his prediction with legitimate scientists, preferring instead to communicate only with the press. The entire earthquake community repudiated the prediction except for one seismologist, David Stewart, who, years previously, had to leave his position at the University of North Carolina after endorsing the use of paranormal means for predicting earthquakes. Under these circumstances, how could anyone take Browning's prediction seriously? Unfortunately, many people did. The predicted earthquake never occurred.

    In contrast, in 1985 seismologists at the U.S. Geological Survey forecast that a magnitude 6 earthquake would occur near Parkfield, California, before 1993. When a magnitude 4.7 earthquake did occur on 20 October 1992, they raised an "A-level alert," signifying a 37% probability that the forecast earthquake would occur within 72 hours. In this case the forecasted earthquake never occurred. However, while many seismologists raised technical questions about the basis of the forecasts, all would agree that the forecasters were responsible, legitimate, professional seismologists, and that their forecasts should be taken seriously.

    For any prediction to be considered legitimate it should meet the following criteria:

• Location - A specific geographic location must be named. The region must be limited to within a specific distance of a point and preferably on a specific fault or fault complex. Naming a region like "somewhere in southern California" or "eastern Japan" is not sufficient since the region is too large
• Time - A narrow time window must be specified. For example, a legitimate prediction will give a time range with specific expiration dates. A prediction stating "within the next year or two" is not specific enough.
• Size - A size range for the prediction is very important since the frequency of occurrance of earthquakes depends on their size. For example, if I predicted a magnitude 2 event in the northern Los Angeles basin within two weeks of mm/dd/yy hh:mm:ss no one would take me seriously because such an earthquake will occur in nearly every two-week period.
• Probability - The probability estimate of the earthquake occurring should be specified. Probability estimates are similar to weather forecasters' probabilities of precipitation. Probabilities are most commonly stated as a "??% chance of XX" occurring. Probabilities can not just be guessed; the person releasing the prediction must specify how they estimated the probability.
• Updating - The prediction should be updated as time progresses to account for changes in the data used to predict the event.

Predictions should be released or registered before the earthquake in question, they should specify all methods and reasoning, and the predictor should be available to to answer questions. Some things to watch out for:

• Post Predictions - Predictions announced after the fact
• Predictions Lacking Probability Estimates
• Predictions Lacking Narrow Time Windows
• Predictions Reported To The Press Before Any Other Individuals and Agencies Have Been Informed
• Predictions Lacking Detailed Justification

Q: Can human activity trigger earthquakes?

A: Often individuals wonder whether construction of a reservoir, hydrocarbon production, or the injection of fluids into the ground caused an earthquake at a specific location. Several well-documented cases exist where such large engineering projects and damaging earthquakes are genetically related. In most of these cases, the engineering projects themselves most likely did not create the stresses that caused the earthquake, but rather changed local conditions in such a way as to allow an earthquake to occur. The difference between inducing an earthquake and triggering an earthquake is subtle but important; generally, human activity will not create enough stress in the underlying rocks to cause an earthquake large enough to be recorded by distant seismographs. However, the activity can change the physical conditions at depth and thus allow an earthquake to occur; we call these seismic events triggered earthquakes.

     The different activities that can trigger or induce earthquakes include impounding of deep artificial water reservoirs, underground mining, large-scale surface quarrying, high pressure fluid injection, removal of subterranian fluids for hydrocarbon production, and underground explosions. For example, reservoir-induced seismicity has been documented in China, central Africa, Greece, India, Egypt, Russia, Italy, South Carolina, Arizona, and California (Oroville Reservoir). The first well documented example occurred when Lake Mead, formed by Hoover Dam on the Colorado River, began filling in the late 1930's. The largest and most damaging reservoir-induced earthquake occurred on December 10, 1967, at Koyna, India (M 6.3), and claimed over 200 lives while destroying much of the Koyna Nagar town.

    Pumping fluids into the ground at high pressure plays a major role in geothermal power generation, oil production, solution mining, and hazardous waste disposal. The first well-documented case of triggered seismicity due to fluid injection occurred at the Rocky Mountain Arsenal near Denver, Colorado during the early 1960's, where there was no previously recorded seismicity. Following these earthquakes, a fluid-injection experiment carried out in Rangely, Colorado, in a producing oil field proved that injecting high pressure fluids into geologic formations under the right conditions of stress could trigger earthquakes.

    In several locations, earthquakes and faulting have also accompanied fluid extraction. These earthquakes occur particularly in large shallow hydrocarbon fields from the release of stresses built up throughout the producing region as reservoir fluid pressures are reduced. Contraction of the reservoir rock drives the rock above and below the producing layer into compression, while rocks on the edge of the producing field extend since they are not displaced as much as rocks directly above. Examples of seismicity probably triggered by fluid extraction include Goose Creek, Texas; Buena Vista Hills, California; Rocky Mountain House, Canada; Lacq, France; Fashing, Texas; and probably the most clear example, Willmington oil field, California (M 2.4, M 3.3).

    Mining activities involve the removal of mass from a continuos body and easily can change physical conditions in the surrounding rock. Mine excavations act as stress concentrators, often leading to increases in seismic activity ranging from microseismic acoustic emissions to full scale tremors with magnitude of 5. Mine seismicity includes spallations, tunnel collapse, gas outbursts, "bumps," and rockbursts, as well as real earthquakes. Some examples of mine-induced seismicity include Wappinger's Falls, New York, in a dolomitic limestone quarry reaching a depth of about 50 meters and surface area of about 1 square kilometer (M 3.3); and in the Belchatow trench, Poland, at a large strip mining operation centering on brown coal at about 200 meters depth (M 3.6).

Q: Can we prevent earthquakes?

A: In general we cannot prevent earthquakes from occurring. Preventing earthquakes would require some reasonable control over stress along faults. Engineers and scientists have proposed injecting fluids deep into the ground near active faults, allowing many small earthquakes to relieve stress rather than one large damaging natural event. However, many scientific and legal issues would need to be resolved before this could be happen; it isn't going to happen anytime soon, at least not in the U. S.

Q: How can we avoid earthquake disasters?

A: Unfortunately, we have found no scientifically verifiable way to predict earthquakes. Therefore, it is impossible to take short-term measures to avoid earthquake disasters. In contrast, nowadays weather forecasts allow us to predict when and where a hurricane will come ashore and what wind speeds to expect more than 24 hours in advance. Warned individuals can then make short term preparations such as boarding windows and evacuating coastal areas.

    In comparison, we can only implement long-term preparations to avoid earthquake disasters. We can estimate the size, frequency, and possible location of earthquakes from the historical records of past events. For the largest estimated earthquake we can estimate the magnitude and type of ground shaking at various distances from the epicenter. Knowing what shaking to expect allows engineers and city planners to create structures better able to withstand the shaking, and to enact zoning codes so that hospitals, pipelines, and main service routes are not in locations most likely to be destroyed by an earthquake.

Several other sites on the World Wide Web have more comprehensive information on planning for earthquakes:

Southern California Earthquake Center
USGS Southern California Office
USGS Menlo Park Office

Q: Do earthquakes occur on the Moon?

A: Quakes do occur on the Moon, but we call them moonquakes instead of earthquakes. The Apollo space missions emplaced five seismographs on the Moon; four recorded seismic activity until 1977. Although the Moon doesn't have multiple tectonic plates like the Earth, it does have a lithosphere which acts as a single plate. Thus the tectonic inter-plate earthquakes prevalent on the Earth cannot occur on the moon. However, the Moon does experience shallow intra-plate (interior of a plate) earthquakes. In addition, very deep quakes caused by tidal forces, meteroid impacts, and near-surface quakes caused by the heating and cooling of the Moon's surface occur regularly. Yosio Nakamura , a scientist at the UT Institute for Geophysics, is probably the world's leading authority on the seismic activity of the Moon.

Q: Do other planets or moons have quakes?

A: Yes, maybe. The Viking space mission placed a seismometer on Mars in 1976, but it never measured any quakes, probably because it was a very low-sensitivity instrument. Some of the surface features observed on Venus, Io (a moon of Jupiter), and the icy satellites of the outer planets suggest that seismic activity might occur there. But for now we don't have seismographs in place to be sure.

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