Where an earthquake will occur is the easiest feature to predict. How would you predict this? Scientists know that earthquakes take place at plate boundaries and tend to happen where theyve occurred before (Figure 1.1). Fault segments behave consistently. A segment with frequent small earthquakes or one with infrequent huge earthquakes will likely do the same thing in the future. The probabilities of earthquakes striking along various faults in the San Francisco area between 2003 (when the work was done) and 2032.
a good prediction
Scientists are a long way from being able to predict earthquakes. A good prediction must be detailed and accurate. Where will the earthquake occur? When will it occur? What will be the magnitude of the quake? With a good prediction authorities could get people to evacuate. An unnecessary evacuation is expensive and causes people not to believe authorities the next time an evacuation is ordered.
Signs sometimes come before a large earthquake. Small quakes, called foreshocks, sometimes occur a few seconds to a few weeks before a major quake. However, many earthquakes do not have foreshocks, and small earthquakes are not necessarily followed by a large earthquake. Ground tilting, caused by the buildup of stress in the rocks, may precede a large earthquake, but not always. Water levels in wells fluctuate as water moves into or out of fractures before an earthquake. This is also an uncertain predictor of large earthquakes. The relative arrival times of P-waves and S-waves also decreases just before an earthquake occurs. Folklore tells of animals behaving erratically just before an earthquake. Mostly, these anecdotes are told after the earthquake. If indeed animals sense danger from earthquakes or tsunami, scientists do not know what it is they could be sensing, but they would like to find out. Earthquake prediction is very difficult and not very successful, but scientists are looking for a variety of clues in a variety of locations and to try to advance the field.
When an earthquake will occur is much more difficult to predict. Since stress on a fault builds up at the same rate over time, earthquakes should occur at regular intervals (Figure 1.2). But so far scientists cannot predict when quakes will occur even to within a few years. Click image to the left or use the URL below. URL: Around Parkfield, California, an earth- quake of magnitude 6.0 or higher occurs about every 22 years. So seismologists predicted that one would strike in 1993, but that quake came in 2004 - 11 years late. Click image to the left or use the URL below. URL:
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what do scientists know when it comes to predicting an earthquake?
a) where earthquakes have taken place. b) where plate boundaries are. c) where faults are active. --> d) all of the above.
small quakes that occur before an earthquake.
a) forequake b) aftershock --> c) foreshock d) primary shock
scientists are able to accurately predict when an earthquake will strike.
a) true --> b) false
scientists can predict
--> a) where an earthquake will take place. b) when an earthquake will take place. c) the magnitude of the earthquake. d) all of these.
if an evacuation was ordered and the earthquake didnt happen, a lot of money would be spent and people would be annoyed.
--> a) true b) false
to see whether animals can predict earthquakes, scientists would need to
a) do a survey after a big quake. --> b) be alerted to erratic behavior before a quake. c) let animals predict quakes and base their predictions on them. d) animals cant ever predict quakes and its not worth considering.
since it is now more than 10 years past 2003, the probability of a quake by 2032 on each of the faults in the sf bay area map is
a) less likely. --> b) about as likely. c) more likely. d) impossible to know.
fault segments do not behave the same over time.
--> a) true b) false
fault segments have regular earthquakes; e.g. parkfield always has a quake about every 22 years.
a) true --> b) false
what are some indicators that an earthquake will strike?
a) foreshocks b) ground tilting c) water levels in wells --> d) all of these
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