Seismicity and fault geometry of the San Andreas fault around Parkfield, California and their implications

• Large seismic velocity anomalies along the San Andreas fault around Parkfield, California at shallow depths.
• A spiral geometry in the San Andreas fault around Parkfield, California
• Localized tectonic-loading may cause the periodic M6 events in the Parkfield region.

Fault geometry is a consequence of tectonic evolution, and it provides important information on potential seismic hazards. We investigated fault geometry and its properties in Parkfield, California on the basis of local seismicity and seismic velocity residuals refined by an adaptive-velocity hypocentral-parameter inversion method. The station correction terms from the hypocentral-parameter inversion present characteristic seismic velocity changes around the fault, suggesting low seismic velocities in the region east of the fault and high seismic velocities in the region to the west. Large seismic velocity anomalies are observed at shallow depths along the whole fault zone. At depths of 3–8 km, seismic velocity anomalies are small in the central fault zone, but are large in the northern and southern fault zones. At depths > 8 km, low seismic velocities are observed in the northern fault zone. High seismicity is observed in the Southwest Fracture Zone, which has developed beside the creeping segment of the San Andreas fault. The vertical distribution of seismicity suggests that the fault has spiral geometry, dipping NE in the northern region, nearly vertical in the central region, and SW in the southern region. The rapid twisting of the fault plane occurs in a short distance of approximately 50 km. The seismic velocity anomalies and fault geometry suggest location-dependent piecewise faulting, which may cause the periodic M6 events in the Parkfield region.