Seismic Evidence for a Four-Year Episode of Deep Transient Creep Preceding the 2004 Parkfield Earthquake

Abstract:

We establish a causal relation between tremor, deep creep and off-fault seismicity of the San Andreas fault at Parkfield, CA by analyzing spatiotemporal seismicity data and using coulomb stress modeling. The 2004 M6 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 (Mw<3) 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 to 2000 to 20 events/yr between 2000 and the 2004 earthquake. This increase in the rate of seismicity coincided with an increase in the occurrence of non-volcanic tremor, which suggests that creep may have driven the enhanced seismicity. Using Coulomb stress modeling, 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. Other evidence for a causal link between deep creep and off-fault seismicity is the observation that off-fault seismicity before and after the 2004 earthquake occurred in the same location. This is expected if the foreshocks are driven by an episode of deep creep and the aftershocks are driven by afterslip, both occurring on the same deep extension of the fault plane. Finally, a transient increase in off- fault seismicity at Parkfield was observed to follow the 2010 Maule and the 2011 Tohuku earthquakes. The seismic waves from these events were observed to trigger tremor at Parkfield, which is 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.