3:45 PM - 4:00 PM
[SCG45-07] Deep Mechanics along the San Andreas Fault through Focal Mechanisms of Low-Frequency Earthquakes near Parkfield
Keywords:Low-frequency earthquakes, Focal mechanisms, San Andreas Fault
Mechanics at the deeper parts of faults is still poorly understood. As direct evidence of current deformation at the depths, we focus on the low-frequency earthquakes (LFEs) near Parkfield on the San Andreas Fault. Although LFEs near Parkfield have been studied in considerable detail, their focal mechanisms have not been determined due to low signal-to-noise ratios.
To determine focal mechanisms, we utilize the catalog of more than a million LFEs since 2001 (Shelly, 2017). Since they are classified into 88 families based on waveform correlations, events in the same family are considered to share the same location and mechanism. After waveform stacking for all events in the same family, we translate the P- and S-wave energies into focal mechanisms. For efficient optimization, we first grid search for the common focal mechanism for all families and then carry out an iterative approach to determine family-specific focal mechanisms and site amplification terms together.
As a result, we find that most of the LFEs are consistent with strike-slip plate motion on the San Andreas Fault (N40ºW), with moderate variations. Regarding strike orientations, southeastern families have focal mechanisms rotated counterclockwise (N50ºW), while northwestern families have clockwise rotation (N30ºW). The southeastern families are strike-slip with a minor reverse-fault component, implying a local compressional stress regime. Indeed, such compression is mechanically expected for the counterclockwise rotation of right-lateral mechanisms. Regarding rake angle variations along the fault, the slip inclination seems to correspond to the spatial gradient of the deeper end of the shallow micro-seismicity, implying that the fault coupling at the shallower part (0–15 km) may control the ductile deformation field at deeper depths (15–30 km) driving LFEs.
To determine focal mechanisms, we utilize the catalog of more than a million LFEs since 2001 (Shelly, 2017). Since they are classified into 88 families based on waveform correlations, events in the same family are considered to share the same location and mechanism. After waveform stacking for all events in the same family, we translate the P- and S-wave energies into focal mechanisms. For efficient optimization, we first grid search for the common focal mechanism for all families and then carry out an iterative approach to determine family-specific focal mechanisms and site amplification terms together.
As a result, we find that most of the LFEs are consistent with strike-slip plate motion on the San Andreas Fault (N40ºW), with moderate variations. Regarding strike orientations, southeastern families have focal mechanisms rotated counterclockwise (N50ºW), while northwestern families have clockwise rotation (N30ºW). The southeastern families are strike-slip with a minor reverse-fault component, implying a local compressional stress regime. Indeed, such compression is mechanically expected for the counterclockwise rotation of right-lateral mechanisms. Regarding rake angle variations along the fault, the slip inclination seems to correspond to the spatial gradient of the deeper end of the shallow micro-seismicity, implying that the fault coupling at the shallower part (0–15 km) may control the ductile deformation field at deeper depths (15–30 km) driving LFEs.