As we saw in Chapter 2, GPS receivers can record the slow movements of lithospheric plates. These instruments can also measure the strain that builds up from such movements, as well as the sudden slipping on a fault when it ruptures in an earthquake.
Seismologists use GPS observations to study another kind of movement along active faults. It has been known for many years that a section of the San Andreas fault in central California creeps continuously, rather than rupturing suddenly. This creep slowly deforms structures and cracks pavements that cross the fault trace. More recently, GPS receivers have found surface movements at convergent plate boundaries that reflect short-lived creep events, which commonly last days to weeks at a time. They have been named silent earthquakes because these gradual movements do not trigger destructive seismic waves. Nevertheless, they can release large amounts of stored strain energy.
These observations raise many questions that geologists are now trying to answer: What causes faults to stick and slip catastrophically in some places and creep in others? Will the release of strain energy by silent earthquakes make destructive earthquakes in those regions less frequent or less severe? Can silent earthquakes be used to predict potentially destructive earthquakes?