JumpStart - Earth Science

Earthquakes*

Frequently Asked Questions

Introduction One of the most frightening and destructive phenomena of nature is a severe earthquake and its terrible aftereffects. An earthquake is a sudden movement of the Earth, caused by the abrupt release of strain that has accumulated over a long time. For hundreds of millions of years, the forces of plate tectonics have shaped the Earth as the huge plates that form the Earth's surface slowly move over, under, and past each other. Sometimes the movement is gradual. At other times, the plates are locked together, unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free. If the earthquake occurs in a populated area, it may cause many deaths and injuries and extensive property damage. Today we are challenging the assumption that earthquakes must present an uncontrollable and unpredictable hazard to life and property. Scientists have begun to estimate the locations and likelihoods of future damaging earthquakes. Sites of greatest hazard are being identified, and definite progress is being made in designing structures that will withstand the effects of earthquakes. Introduction by Kaye M. Shedlock & Louis C. Pakiser Page is URL:http://pubs.usgs.gov/gip/earthq1/intro.html Maintained by John Watson and Kathie Fraser Last modified 10-23-97 (jmw)

Frequently asked questions about Earthquakes

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Beliefs

• Is there a time pattern to when an EQ occurs?
  No. EQs occur at all times of day. There have been damaging EQs both in the a.m. and p.m.
• Is there such a thing as EQ weather?
  No. Many people believe EQs are more common in certain kinds of weather. In fact, no correlation with weather has been found. BECAUSE EQs begin many kilometers below the region affected by surface weather. [People tend to notice EQs that fit the pattern and forget the ones that don't.]
• Will California eventually fall into the ocean?
  No. The motion of plates will not make California sink--California is moving horizontally along the San Andreas fault and up around the Transverse Ranges.
• Are EQs really on the increase?
  We continue to hear from people throughout the world that EQs are on the increase, and it may seem that we are having more EQs--EQs of magnitude 7.0 or greater have remained fairly constant throughout this century and according to records have actually seemed to decrease in recent years.

  EXPLANATION: In the last 20 years we have been able to locate more EQs yearly because of the increase in the number of seismograph stations in the world and improved global communications. [e.g., 1931 there were 350 stations operating in the world, today more than 4,000 stations and the data comes in rapidly from these stations by telex, computer and satellite. This increase has help us and other seismological centers to locate many small EQs which were undetected in earlier years.
  

• What is a fault?
  A fault is a thin zone of crushed rock between two blocks of rock, and can be any length, from centimeters to thousands of kilometers.

  It is a fracture in the crust of the earth along which rocks on one side have moved relative to those on the other side. Most faults are the result of repeated displacements over a long period of time.

• Name the 3 different kinds of faults?
  (1) Normal, dip-slip fault. The fault plane of a normal fault dips away from the uplifted crustal block. Faulting occurs in response to extension.
  (2) Reverse, dip-slip fault. The fault plane of a reverse fault dips beneath the uplifted crustal block. Faulting occurs in response to compression.
  (3) Strike-slip fault. Crustal blocks slide past each other. The slip may be left lateral or right lateral. This example shows a left-lateral, strike-slip fault.
• What is a thrust fault?
  A thrust fault is a reverse fault with a gently incline, or low angle of dip. If the slip on a fault is partly strike-slip and partly dip-slip, the fault is called an oblique-slip fault.
• What happens to a fault when an EQ occurs?
  EQs occur on faults. When an EQ occurs on one of these faults, the rock on one side of the fault slips with respect to the other. The fault surface can be vertical, horizontal, or at some angle to the surface of the earth. The slip direction can also be at any angle.
• What are the names of the 2 basic EQs?
  Strike-slip EQ--occurs on an approximately vertical fault plane as the rock on one side of the fault slide horizontally past the other. Dip-slip EQ--The fault is at an angle to the surface of the earth and the movement of the rock is up or down.
• How do we know a fault exists?
  (1) if the EQ left surface evidence, such as surface ruptures or fault scarps (cliffs made by EQs);
  (2) if a large EQ has broken the fault since we began instrumental recordings in 1932;
  and (3) if the faults produces small EQs that we can record with the denser seimographic network established in the 1970s.
• What is "surface rupture" in an EQ?
  Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. NOT ALL EQs result in surface rupture.

  Fault rupture almost always follows preexisting faults which are zones of weakness. Rupture may occur suddenly during an EQ or slowly in the form of fault creep. Sudden displacements are more damaging to structures because they are accompanied by shaking.

• What is meant by "fault creep?"
  Fault creep is the slow movement of faults in the earth's crust. [EXAMPLES of creep are well known along the Hayward Fault where it crosses highly developed areas in Contra Costa and Alameda Counties.]

• What is an EQ?
  An EQ is caused by a sudden slip on a fault. Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an EQ.
• What is the cause of an EQ?
  An EQ occurs when plates grind and scrape against each other. In California there are 2 plates the Pacific Plate and the North American Plate. The Pacific Plate consists of most of the Pacific Ocean floor and the California Coast line. The North American Plate comprises most the North American Continent and parts of the Atlantic Ocean floor. These primary boundary between these 2 plates is the San Andreas fault. The San Andreas fault is more than 650 miles long and extends to depths of at least 10 miles. Many other smaller faults like the Hayward (Northern California) and the San Jacinto (Southern California) branch from and join the San Andreas fault zone. The Pacific plate grinds northwestward past the North American Plate at a rate of about 2 inches per year. Parts of the San Andreas fault system adapt to this movement by constant "creep" resulting in many tiny shocks and a few moderate earth tremors. In other areas where creep is NOT constant, strain can build up for hundreds of years, producing great EQs when it finally releases.
• Can we prevent EQs?
  No. An EQ can occur virtually anywhere at any time. They cannot be prevented, however, damage, destruction and loss of life can be significantly reduced but because Mother Nature provides us with regular reminders of the dangers, we can prepare and reduce loss of life, injuries and propery damage if everyone sufficiently prepared themselves, their homes, work places and communities for a major EQ.
• What is the hypocenter?
  The point where the earthquake rupture begins, usually deep down on the fault.
• What is the epicenter?
  The point on the surface directly above the hypocenter.
• What is the name of the zone of EQs surrounding the Pacific Ocean?
  The zone is called the Circum-Pacific belt--about 90% of the world's EQ occur there.

  The next most seismic region (5-6% of Earthquakes) is the Alpide belt (extends from Mediterranean region, eastward through Turkey, Iran, and northern India.

• Earthquakes in the media?
  The press, radio, and television are doing an excellent job of alerting the public to the hazard through news, reports, feature articles and television specials--this has caused an increased awareness of the threat that has prompted many people to seek additional info about EQ and to ask questions about what they should do.

  --the increased awareness will help to reduce death, injuries and property damage in the next big earthquake

• Why are we having so many EQs?
  There is nothing unusual about what's happening, it's perfectly natural. Many parts of the U.S. are known to be highly seismically active. Sometimes there is a short-term increase in seismicity in that region, in an area where we commonly see quakes, just usually not so many at once. These types of increases are not usually alarming or cause for concern. It is a high level of activity that has since slowed and the norm is just an occasional earthquake.
• How many EQs does the National Earthquake Information Center report yearly?
  The NEIC now locates about 12,000 to 14,000 EQs yearly or approximately ~35/day.
• According to long-term records how many EQ are expected yearly?
  According to long-term records (since about 1900) we expect about 18 major EQs (7.0-7.9) and one great EQ (8.0 or above) in any given year. [see National Earthquake Information Center info]

  Records show that 1992 [23 EQs] was the first time that we have reached or exceeded the long-term average number of major EQ since 1971 [19 EQs/1 Major EQ] .

• What kind of damage does ground shaking cause?
  Ground shaking causes 99% of the EQ damage to California homes.
• What system does the USGS use to characterize the severity of ground shaking at different location?
  The USGS uses a scale based on ground acceleration (as a percentage of the acceleration due to the force of gravity) and the Modified Mercalli Intensity Scale (Mercalli Scale). The 12-level intensity scale represents the effects of the shaking on people, manmade structures, and on the landscape by a number.

  Seismologists use the list to represent the severity of ground shaking in a community or part of a community by a number.

• List of 1-12 levels of the Modified Mercalli Intensity Scale of 1931 (Abridged; Wood and Neumann, 1931)?
  See Earthquakes & Volcanoes, v. 25, no. 2, 1994, p. 87 for further details.
• What is an isoseismal map?
  A map showing intensities at individual locations may be contoured with isoseismal lines, to provide a representation of the broad variations of the shaking over the region surrounding the EQ source.

  An isoseismal map shows the different levels of intensity experienced in the affected area.

• How do you measure the shaking that you feel during an earthquake?
  What you feel is very complex--hard, gentle, long, short, jerky or rolling--and not describable with one number. Motions are described by the PEAK VELOCITY (how fast the ground is moving); PEAK ACCELERATION (how quickly the speed of the ground is changing); FREQUENCY (energy is released in waves and these waves vibrate at different frequencies just like sound waves); and DURATION (how long the strong shaking lasts).
• What 3 factors primarily determine what you feel in an EQ?
  (1) Magnitude (you feel more intense shaking from a big EQ than from a small one; big EQ also release their energy over a larger area and for a longer period of time. In most cases, only 10-15 seconds of shaking that originate from the part of the fault nearest you will be very strong;
  (2) distance from the fault (EQ waves die off as they travel through the earth so EQ shaking becomes less intense farther from the fault),
  and (3) local soil conditions (certain soils greatly amplify the shaking in an EQ. Seismic waves travel at different speeds in different types of rocks. Passing from rock to soil, the waves slow down but get bigger. A soft, loose soil will shake more intesely than hard rock at the same distance from the same EQ. The looser and thicker the soil is, the greater the amplification will be, (e.g, Loma Prieta EQ damage area of Oakland and Marina (SF) were 100 km (60 mi) and most of the Bay Area escaped serious damage.

• What is EQ intensity?
  EQ intensity is a measure of the effects of ground shaking at a specific locality. (See the discussion of intensity and magnitude under Measuring Earthquakes above).

• What are the EQ Magnitude classes?
  Great = M > 8
  Major = 7 < M < 7.9
  Strong = 6 < M < 6.9
  Moderate = 5 < M < 5.9
  Light = 4 < M < 4.9
  Minor = 3 < M < 3.9
  

• Northridge, CA
  DATE: Jan. 17, 1994
  TIME: 4:31 a.m.
  LAT 37.036¡ N., Long 121.883¡ W.
  [20 mi west-northwest of Los Angeles]
  MAGNITUDE: 6.7 Mw
  DURATION:
  DEPTH: 11 mi
  DEATHS: 57
  INJURIES: 9,000
  PROPERTY DAMAGE: $13-15 Bil
  COMMENTS: Blind fault (occurred on an unknown "thrust fault"); 7 freeways sites closed due to damage; 2 hospitals closed 150,000 w/o water; 40,000 w/o natural gas; 25,000 people left homeless; 18,480 injuries and 1,533 hospitalized
• Parkfield, CA
  DATE:July 1966
  TIME:
  MAGNITUDE:
  DEATHS:
  INJURIES:
  PROPERTY DAMAGE:
  
• Loma Prieta
  DATE: Oct. 17, 1989 TIME: 5:04 pm PDT (15.24 seconds after)
  MAGNITUDE: 7.1
  DURATION: 15 seconds
  DEATHS: 62
  INJURIES: 3,757
  PROPERTY DAMAGE: >$6 Bil
  COMMENTS: rupture of 40 km fault stretch (25 mi [40 km] segmen of San Andreas fault SW of San Jose slipped ~ 7 ft]; SF Bay Bridge unusable for 1 month; 18,306 homes damaged; 2,575 business damaged; 12,053 people displaced
• Coalinga, CA
  DATE: May 2, 1983
  TIME:
  MAGNITUDE: 6.4
  DEATHS: 0
  INJURIES: 47
  PROPERTY DAMAGE: $31 Mil
  
• San Francisco 1906Q
  DATE: April 18, 1906
  TIME: 5:12 am
  MAGNITUDE: 8.25
  DURATION: 40 seconds
  DEATHS: 700-2,500 people (debated)
  PROPERTY DAMAGE: $
  COMMENTS: 400 km fault segment released; San Andreas fault slipped as much as 15 ft along 270 mi (434 km) segment from south of San Jose to northwestward to Cape Mendocino
• San Fernando, Calif. DATE: 1971
  

• Kobe, Japan
  DATE: Jan. 17, 1995
  TIME:
  MAGNITUDE: 6.9
  
• Mexico City, Mexico
  DATE:
  TIME:
  MAGNITUDE:
  
• El Asnam, Algeria DATE: 1980
  MAGNITUDE: 7.3
  
• Tangshan, China DATE: 1976
  
• Guatemala DATE: 1976
  
• Managua, Nicaragua DATE: 1972
  

• How are EQs recorded?
  By a seismographic network. Each seismic station in the network measures the movement of the ground at the site. The slip of block of rock over another in an EQ releases energy that makes the ground vibrate. That vibration pushes the adjoining piece of ground and cause it to vibrate and thus the energy travel out from the EQ in a wave.
• What is the most common method used to measure an EQ?
  Magnitude is the most common measure of an EQs size (used to tell the public). Measures the energy produced by the EQ and is not what you feel during the event. In the 1930, Beno Gutenberg and Charles Richter borrowed the idea from astronomers.
• What is moment magnitude?
  Moment is a physical quantity proportional to the slip on the fault times the area of the fault surface that slips; it is related to the total energy released in the EQ. The moment can be estimated from seismograms (and also from geodetic measurements). The moment is then converted into a number similar to other earthquake magnitudes by a standard formula. The results is called the moment magnitude. The moment magnitude provides an estimate of earthquake size that is valid over the complete range of magnitudes, a characteristic that was lacking in other magnitude scales.
• What is the Mercalli Scale of EQ intensity based on?
  The Mercalli Scale is based on observable EQ damage. From a scientifc standpoint, the Richter scale is based on seismic records while the Mercalli is based on observable data which can be subjective.

  Thus, the Richter scale is considered scientifically more objective and therefore more accurate.

  For example a level I-V on the Mercalli scale would represent a small amount of observable damage. At this level doors would rattle, dishes break and weak or poor plaster would crack. As the level rises toward the larger numbers, the amount of damage increases considerably. The top number, 12, represents total damage.

• What is the difference between intensity scales and magnitude scales?

  Intensity scales, like the Modified Mercalli Scale and the Rossi-Forel scale, measure the amount of shaking at a particular location. So the intensity of an earthquake will vary depending on where you are. Sometimes earthquakes are referred to by the maximum intensity they produce.

  Magnitude scales, like the Richter magnitude and moment magnitude, measure the size of the earthquake at its source. So they do not depend on where the measurement is made. Often, several slightly different magnitudes are reported for an earthquake. This happens because the relation between the seismic measurements and the magnitude is complex and different procedures will often give slightly different magnitudes for the same earthquake.

• What are seismographs?
  Seismographs are instruments used to record the motion of the ground during an EQ--installed in the ground throughout the world and operate as seismographic network; 1st developed ~1890.
• What are seismograms?
  The records (paper copy) produced by seismographs used to calculate the location and magnitude of an EQ. Shows how the ground moves with the passage of time. SEISMO How do you read seismograms? The HORIZONTAL axis = time (measured in seconds); VERTICAL axis= ground displacement (usually measured in millimeters) When there is NO EQ reading = straight line except for small wiggles caused by local disturbance or "noise."
• What is a seismometer?
  An internal part of the seismograph which may be a pendulum or a mass mounted on a spring.
• How is the movement of the seismometer converted into a seismogram?
  There are several ways:
  (1) a pen drawing an ink line on paper revolving on a drum;
  (2) a light beam making a trace on a moving phographic film
  or (3) electromagnetic system generating a current that is recorded electronically on tape
• What is a terrascope?
  A modern digital seismic station recording a wide range of seismic motions.