S-Wave Triggering of Tremor beneath the Parkfield, California, Section of the San Andreas Fault by the 2011 Tohoku, Japan, Earthquake: Observations and Theory

The dynamic stresses that are associated with the energetic seismic waves generated by the M-w 9.0 Tohoku earthquake off the northeast coast of Japan triggered bursts of tectonic tremor beneath the Parkfield section of the San Andreas fault (SAF) at an epicentral distance of similar to 8200 km. The onset of tremor begins midway through the similar to 100-s-period S-wave arrival, with a minor burst coinciding with the SHSH arrival, as recorded on the nearby broadband seismic station PKD. A more pronounced burst coincides with the Love arrival, followed by a series of impulsive tremor bursts apparently modulated by the 20- to 30-s-period Rayleigh wave. The triggered tremor was located at depths between 20 and 30 km beneath the surface trace of the fault, with the burst coincident with the S wave centered beneath the fault 30 km northwest of Parkfield. Most of the subsequent activity, including the tremor coincident with the SHSH arrival, was concentrated beneath a stretch of the fault extending from 10 to 40 km southeast of Parkfield. The seismic waves from the Tohoku epicenter form a horizontal incidence angle of similar to 14 degrees, with respect to the local strike of the SAF. Computed peak dynamic Coulomb stresses on the fault at tremor depths are in the 0.7-10 kPa range. The apparent modulation of tremor bursts by the small, strike-parallel Rayleigh-wave stresses (similar to 0.7 kPa) is likely enabled by pore pressure variations driven by the Rayleigh-wave dilatational stress. These results are consistent with the strike-parallel dynamic stresses (delta tau(s)) associated with the S, SHSH, and surface-wave phases triggering small increments of dextral slip on the fault with a low friction (mu similar to 0.2). The vertical dynamic stresses delta tau(d) do not trigger tremor with vertical or oblique slip under this simple Coulomb failure model.