Tectonic Tremor along the Parkfield-Cholame Section of the San Andreas Fault Triggered by the 2014 M6.0 South Napa and Other Regional Earthquakes

Large distant earthquakes are known to trigger deep tectonic tremor along the Parkfield-Cholame section of the San Andreas Fault. The triggered tremors are mostly modulated by dynamic stresses from large-amplitude surface waves, although sometimes teleseismic body waves are also capable of triggering tremor. However, there are relatively few observations of triggering from regional-distance earthquakes. The 2014 M6.0 South Napa earthquake is the largest earthquake occurring in the Bay Area since the 1989 M6.9 Loma Prieta earthquake. It has triggered an increase of microearthquake activities in the Geysers geothermal field during and immediately following the mainshock waves (Meng et al., this meeting). Although we did not observe any obvious modulated tremors at Parkfield-Cholame during the Napa wavetrain, a small tremor episode occurred just NW of Parkfield coincident with the arrival of seismic waves at the 2.5 km-deep seismometer in the SAFOD main hole. A major tremor episode began about 10 hours later near Cholame (SE of Parkfield). This episode is one of the largest seen over the past several years, containing intense activity for ~3 days and taking more than 3 weeks to return to background levels. While it is impossible to entirely rule out random coincidence at this stage, minor activity beneath Cholame started only 90 minutes after the Napa event in a zone with significant episodes only every few months, suggesting that the major tremor episode may have been triggered. In addition, we plan to systematically examine both tremor catalogs and continuous waveforms following the occurrence of other recent earthquakes in California to better understand the ambient conditions and factors (including amplitude and frequency of the incoming waves) controlling tremor triggering in this region.