Ambient Tremor Triggered by the 24 August 2014, M6.0 South Napa Earthquake in the Parkfield-Cholame region of California

The rate of ambient tremor activity in the Parkfield-Cholame area of California increased by a factor of ~10 beginning between 11 and 12 hours after August 24, 2014 M6.0 S. Napa earthquake and lasted for ~ 48 hours. Over the subsequent 26 days, rates diminished but remained elevated, and on Sept. 22, 2014, rates increased again to levels comparable to those immediately following the South Napa event. This second episode has continued through the end of the analysis period (Sept. 24). The proportion of tremors beneath Monarch Peak (NW of Parkfield-Cholame) also increased during this second episode, suggesting a migration of tremor deep slow-slip to the northwest. The rate of tremor since the Napa earthquake is the largest observed in the Parkfield-Cholame area since the ambient tremor triggered by the 2004 Parkfield M6 earthquake and is approximately as energetic. This is in spite of the fact that the South Napa earthquake was over 300 km further away from the Parkfield-Cholame tremor zone. This suggests a heightened sensitivity to stress change in the Parkfield-Cholame tremor zone and that significant changes in conditions within the deep SAF may have taken place. The increased rates of tremor and the general association of changing tremor rates with deformational transients and slow-slip also suggest that the South Napa event may have triggered a large, deep slow-slip episode at a great distance and along a critical section of the San Andreas Fault (i.e., where the 1857 Ft. Tejon M7.8 earthquake apparently nucleated). This presentation will include monitoring results tracking the ongoing unusual tremor activity up through the week prior to the Fall AGU meeting and will compare this tremor evolution with ongoing seismicity and another delayed ambient tremor episode in the Bitterwater area of the SAF that was apparently triggered by the 1998 San Juan Baustista M5.1 earthquake.