Seismicity and the State of Stress in the Lithosphere

Just as in the case of volcanoes, the occurrences of earthquakes strongly correlate with plate boundaries. The worldwide distribution of seismicity is illustrated. Earthquakes occur on all types of plate boundaries; however, subduction zones and areas of continental collision are noted for their very large earthquakes. Large earthquakes also occur in plate interiors but with a much lower frequency. Earthquakes are associated with displacements on preexisting faults. A typical displacement in a very large earthquake is 10 m. If the relative velocity across a plate boundary is 50 mm yr−1, it would take 200 years to accumulate this displacement. Large earthquakes at subduction zones and major transform faults such as the San Andreas reoccur in about this period. Since regular displacements do not have to be accommodated in plate interiors, the period between major intraplate earthquakes is much longer. The near-surface expressions of major faults are broad zones of fractured rock with a width of a kilometer or more. Smaller faults may have zones of fault gouge with widths of a few centimeters or less. Small 86 Plate Tectonics Figure 1.59 Cross sections of (a) a normal fault and (b) a thrust fault and top views of (c) right-lateral and (d) left-lateral strike-slip faults. faults grade down to rock fractures across which there is no offset isplacement. The total offset across major faults may be hundreds of kilometers. A fault zone is a zone of weakness. When the regional stress level becomes sufficiently large, the fault ruptures and an earthquake occurs. There is extensive geological evidence that faults become reactivated. Large stresses can eactivate faults that have been inactive for tens or hundreds of millions of years.

The direction of the displacement on a fault can be used to determine the state of stress responsible for the displacement. Since voids cannot be created in the Earth’s deep interior, displacements on faults are parallel to the faultsurface.