Table 7.2: (continued)Location Year Magnitude
Africa, Peru (now Chile) 1868 9.0
Cascadia Subduction Zone 1700 9.0(Data from:United States Geological Survey)
Earthquake Prediction
To be valuable, an earthquake prediction must be accurate. A good predication would
anticipatethedate, location, andmagnitudeoftheearthquake. Thepredictionwouldneedto
beaccuratesothatauthoritiescouldconvincepeopletoevacuate. Anunnecessaryevacuation
would be very expensive and would decrease the credibility of authorities who might need
to evacuate the region at a later time. Unfortunately, accurate predictions like these are not
likely to be common for a long time.
The easiest thing to predict is where an earthquake will occur (Figure7.43). Because
nearly all earthquakes take place at plate boundaries, and because earthquakes tend to
happen where they’ve occurred before, scientists know which locations are likely to have
earthquakes. This information is useful to communities because those that are earthquake-
prone can prepare for the event. For example, these communities can implement building
codes to make structures earthquake safe. The added work and expense can be avoided in
areas that are not at risk.
Predicting when an earthquake will occur is much more difficult. Scientists can get a general
idea by looking at the historical and geological records of earthquakes in an area. If stress
on a fault builds up at the same rate over time, then earthquakes should occur at regular
intervals. While this is true, there is a large margin of error in these predictions. Using
this method, scientists cannot even be accurate to within a few years, and evacuation is not
practical.
Seismologists have also used the seismic gap theory for long-term earthquake prediction. In
this theory, scientists assume that, on average, all of the rocks on the same side of a fault
move at the same rate. For example, they say that rocks on the North American plate side
of the San Andreas Fault in California move at the same speed over time. While this may
be true, the frequency and magnitude of earthquakes along the fault is not the same: there
are more quakes in the northern and southern sections, but a relatively inactive zone in the
center.
Seismologists attempted to use the seismic gap theory to predict an earthquake in a seismic
gap. Around Parkfield, California earthquakes occur regularly: an earthquake of magnitude
6.0 or higher occurs about every 22 years. Using this information, seismologists predicted