Relating Transient Seismicity to Episodes of Deep Creep at Parkfield CA

Abstract:

The 2004 M6 Parkfield Ca earthquake was preceded by a four-year period of anomalously high seismicity adjacent to, but not on, the San Andreas Fault. The rate of small events at distances between 1.5 and 20 km from the fault plane and at depths greater than 8 km increased from 6 events/yr prior 2000 to 20 events/yr between 2000 and the 2004 earthquake. This increase in seismicity coincided with an increase in non-volcanic tremor, which, if tremor is indicative of creep on the fault plane, suggests that creep may have driven the enhanced seismicity. A causal relation between creep at the base of the fault zone and off-fault seismicity is supported by Coulomb stress transfer calculations that predict the observed spatial pattern of the seismicity. In particular, an observed SE striking lineation of enhanced seismicity is shown to be a direct consequence of a deepening brittle-ductile transition SE of Parkfield, as evidenced by a deepening of the tremor and low-frequency earthquakes. The observation that off-fault seismicity before and after the 2004 earthquake occurred in the same location suggests that the foreshocks are driven by an episode of deep creep and the aftershocks are driven by after slip, both occurring on the same deep extension of the fault plane. Finally, a transient increase in off-fault seismicity was observed to follow the 2010 Maule and the 2011 Tohuku earthquakes. Since the seismic waves from these events were observed to trigger tremor at Parkfield, this observation can be taken as further evidence for a causal link between deep creep and off-fault seismicity, particularly since the increase in off-fault seismicity was limited to deep events having the same spatial pattern as those that preceded the 2004 earthquakes. A similar anomaly in on-fault seismicity between 1990 and 1994 did not show any evidence of anomalous off-fault seismicity, and did not culminate in a M6 earthquake.