Postearthquake relaxation and aftershock accumulation linearly related after the 2003 M 6.5 Chengkung, Taiwan, and the 2004 M 6.0 Parkfield, California, earthquakes

Abstract

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 ) + u 0 the postseismic displacement less the background accumulation r u t , and Δ N ( t ) the observed cumulative number of postseismic earthquakes less the background accumulation r E t . 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 ) + u 0 is a linear function of Δ N ( t ). The aftershock accumulation Δ N ( t ) for both earthquakes is described by the modified Omori Law d Δ N / dt ∝ (1 + t / τ ) − p with p = 0.96 and τ = 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 ) + u 0 and N ( t ) suggests that this afterslip also generates the aftershocks. The linear relation between u ( t ) + u 0 and Δ 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.