EVIDENCE FOR ASEISMIC DEFORMATION RATE CHANGES PRIOR TO EARTHQUAKES*

For ten earthquakes in nonmagmatic settings, there are credible published accounts of pre-earthquake deformation-rate changes lasting hundreds of seconds to more than a decade. Although most M > 7.5 earthquakes have occurred without detectable pre-earthquake deformation, the detection threshold for aseismic deformation remains high, in that aseismic slip with moment equivalent to an M5 earthquake would in most (although not all) cases have been missed. Interseismic deformation rates vary without being followed by earthquakes, and a strain-rate change prior to the 1989 Mw6.9 Loma Prieta, California, earthquake is shown to be similar in size to many other rate changes that have occurred since that time. Most examples of pre-earthquake aseismic deformation lasting hundreds of seconds or more probably originate adjacent to, or downdip of, the seismic rupture plane, rather than within the zone that undergoes seismic failure.