Focal mechanisms and seismicity of LFEs on Parkfield

Miki Aso, Naofumi Aso, & Satoshi Ide

Published August 5, 2018, SCEC Contribution #8279, 2018 SCEC Annual Meeting Poster #275

Tectonic LFEs have been found at plate boundaries worldwide and considered to be associated with regional slow deformation. Therefore, it is natural that the focal mechanisms of LFEs have been determined as low-angle thrust in subduction zones, such as southwest Japan, Mexico, and Cascadia [Ide et al., 2007; Royer and Bostock, 2014; Frank et al., 2013], and strike-slip along the Alpine Fault, New Zealand [Baratin et al., 2018]. However, despite numerous detections of LFE along the San Andreas faults, their focal mechanisms are yet unknown, mainly due to poor S/N ratio of seismic waveforms. Here we try to determine the focal mechanisms of LFEs at Parkfield, CA, by improving the S/N ratio with a stacking method to keep amplitude information of each LFE. We use seismic waveforms of LFEs categorized into 88 families [Shelly, 2017], recorded at 131 stations in five networks near Parkfield, HRSN, NCSN, SCSN, BDSN, and PBOBSN. The quality of seismograms in five networks is not homogeneous in space and time, with some stations eliminated, replaced, and installed during the study period from April 2001 to January 2018. To minimize the effect of the inhomogeneous observation, we adopt a stacking method explained as follows. For each family and each station, we prepare normalized stacked waveforms, by simply stacking all available waveforms normalized by its maximum amplitude. The dot product of the normalized stacked waveform and an event waveform is regarded as the amplitude of each event measured at each station. These amplitude values are inverted into relative magnitude of ~10,000 events and path-site terms at ~100 stations, by solving a least-squares problem. Some stacked waveforms show P and S phases with recognizable polarities, which are mostly consistent with right-lateral strike slip expected along the San Andreas Fault. Nevertheless, there is the non-negligible number of inconsistent observations, and we need further careful investigation.

Key Words
Focal mechanism, Slow earthquake, San Andreas fault

Citation
Aso, M., Aso, N., & Ide, S. (2018, 08). Focal mechanisms and seismicity of LFEs on Parkfield. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)