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Postearthquake Relaxation and Aftershock Accumulation Linearly Related after the 2003 M 6.5 Chengkung, Taiwan, and the 2004 M 6.0 Parkfield, California, Earthquakes

¹ U.S. Geological Survey, MS977
345 Middlefield Road
Menlo Park, California 94025
 (J.C.S.)
² Institute of Earth Sciences, Acadamia Sinica
P.O. Box 1-55, Nanking
Taipei, Taiwan 115
 (S.-B.Y.)

We treat both the number of earthquakes and the deformation following a mainshock as the superposition of a steady background accumulation and the postearthquake process. The preseismic displacement and seismicity rates r_u and r_E are used as estimates of the background rates. Let t be the time after the mainshock, u(t) + u₀ the postseismic displacement less the background accumulation r_ut, and N(t) the observed cumulative number of postseismic earthquakes less the background accumulation r_Et. For the first 160 days (duration limited by the occurrence of another nearby earthquake) following the Chengkung (M 6.5, 10 December 2003, eastern Taiwan) and the first 560 days following the Parkfield (M 6.0, 28 September 2004, central California) earthquakes u(t) + u₀ is a linear function of N(t). The aftershock accumulation N(t) for both earthquakes is described by the modified Omori Law dN/dt (1 + t/)^–p with p = 0.96 and = 0.03 days. Although the Chengkung earthquake involved sinistral, reverse slip on a moderately dipping fault and the Parkfield earthquake right-lateral slip on a near-vertical fault, the earthquakes share an unusual feature: both occurred on faults exhibiting interseismic fault creep at the surface. The source of the observed postseismic deformation appears to be afterslip on the coseismic rupture. The linear relation between u(t) + u₀ and N(t) suggests that this afterslip also generates the aftershocks. The linear relation between u(t) + u₀ and N(t) obtains after neither the 1999 M 7.1 Hector Mine (southern California) nor the 1999 M 7.6 Chi-Chi (central Taiwan) earthquakes, neither of which occurred on fault segments exhibiting fault creep.