Refining interseismic velocity field around the Anza seismic gap with campaign GNSS data

Cornelius Waldhausen, Yuri Fialko, & Yehuda Bock

Submitted September 7, 2025, SCEC Contribution #14902, 2025 SCEC Annual Meeting Poster #TBD

The San Jacinto Fault (SJF), is the most seismically active fault in Southern California. Since 1890 there have been up to 10 earthquakes of magnitude 6 or greater that have ruptured various sections of the fault. One fault section near the town of Anza, however, has remained seismically quiet over the last 2 centuries. Known as the “Anza Seismic Gap”, this section of the SJF is believed to be in the late interseismic stage. Previous studies (Lindsey et al., 2015; Savage and Prescott 1991) have established that the Anza section is locked, and is accumulating elastic strain at a rate of 0.60 ± 0.1 µrad/yr. The best-fit geodetic models suggest a slip rate of 11.7 ± 0.9 mm/yr, an effective locking depth of ~10 km, and the absence of shallow creep. It was also suggested that the interseismic strain rate within a few kilometers from the mapped fault trace is elevated due to the presence of a low-rigidity damage zone. Because the area is moderately vegetated, InSAR measurements of surface deformation are fraught with problems due to phase decorrelation. We take advantage of the dense network of geodetic monuments installed in the Anza area in late 1980s - early 1990s. Most of these monuments have been surveyed with Global Navigation Satellite System (GNSS) in the 1990s and 2000s, however the obtained interseismic velocities have large uncertainties. We conducted several campaigns to re-occupy the existing monuments to extend the time series of displacements and reduce the errors on velocity estimates. We have reoccupied about 50 GNSS monuments located within 25km of the fault trace. With updated positions we extend the geodetic time-series to encompass nearly 30 years of data, significantly reducing the uncertainties for the velocity estimates and the subsequent strain rate estimate. We use the refined interseismic velocity field to improve estimates of the fault slip rate and locking depth, as well as investigate the suggested zone of enhanced strain rate near the fault trace.

Citation
Waldhausen, C., Fialko, Y., & Bock, Y. (2025, 09). Refining interseismic velocity field around the Anza seismic gap with campaign GNSS data. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy