Monitoring Velocity Change Over 20 Years at Parkfield

We monitored the time history of the velocity change (dv/v) from 2002 to 2022 to investigate temporal changes in the physical state near the Parkfield Region of the San Andreas Fault throughout the interseismic period. Following the coseismic decrease in dv/v caused by the 2003 San Simeon (SS) and the 2004 Parkfield earthquakes, the dv/v heals logarithmically and shows a net long‐term increase in which the current dv/v level is equivalent to, or exceeding, the value before the 2003 SS earthquake. We investigated this long‐term trend by fitting the model accounting for the environmental and coseismic effects to the channel‐weighted dv/v time series. We confirmed with the metrics of Akaike information criterion and Bayesian information criterion that the additional term of either a linear trend term, or a residual healing term for the case where the healing had not been completed before the SS earthquake occurred, robustly improved the fit to the data. We eventually evaluated the sensitivity of the dv/v time history to the GNSS‐derived strain field around the fault. The cumulative dilatational strain spatially averaged around the seismic stations shows a slight extension, which is opposite to what would be expected for an increase in dv/v. However, the cumulative rotated axial strain shows compression in a range near the maximum contractional horizontal strain (azimuth of N35°W to N45°E), suggesting that the closing of pre‐existing microcracks aligned perpendicular to the axial contractional strains would be a candidate to cause the long‐term increase observed in the multiple station pairs. Plain Language Summary We monitored the temporal change of velocity (dv/v) around the Parkfield Region from 2002 to 2022 to investigate the healing of dv/v after the 2003 San Simeon (SS) and the 2004 Parkfield earthquakes. Following those events, the dv/v recovers logarithmically with time and shows a net long‐term increase in which the current dv/v level is equivalent to, or exceeding, the value before the 2003 SS earthquake. We investigated this long‐term trend by fitting the model accounting for the environmental factors, the coseismic effects, and the additional term explaining the long‐term increase. Models with the additional term provide a statistically robust improved fit to the data, suggesting the long‐term increase observed in the dv/v time history is a non‐negligible factor. We eventually evaluated the sensitivity of the dv/v to the GNSS‐derived strain field a