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Using a dense GPS array as a strain meter on the Anza section of the San Jacinto fault

Margaret Grenier, & Yuri Fialko

Published August 14, 2018, SCEC Contribution #8587, 2018 SCEC Annual Meeting Poster #115

The San Jacinto fault is the most actively seismic fault in Southern California and has produced a series of moderately (M>6) sized earthquakes in the last 120 years. Within the main strand of the San Jacinto fault there is a ~20 km long locked section that has not ruptured in over 200 years and is bounded by fault segments with high seismicity rates (Sanders & Kanamori, 1984). This quiescent portion of the San Jacinto fault has been termed the Anza Seismic Gap. Previous geodetic studies have shown a high strain rate of ~0.4 microradians per year due to enhanced deformation within the few km wide fault zone (Lindsey et al., 2014). This high strain rate could be attributed to multiple factors such as a low rigidity fault zone, distributed elastic deformation within the fault zone, or an abnormally shallow locking depth. Understanding the details of strain accumulation within the Anza Seismic Gap can give rise to a better understanding of the fault zone architecture and improve seismic hazard estimates.

We investigate the local interseismic deformation within the fault zone by analyzing GPS data from 21 monuments comprising a ~400 m long alignment array at Anza. The data was collected in three GPS campaigns in 2013, 2015, and 2018. Analysis of the data was done using the GAMIT/GLOBK software package (Herring et al., 2018). We calculated precise baselines between all monuments of the array and used them to calculate velocities of each site. Given the right lateral motion of the San Jacinto fault interseismic deformation should result in an apparent rotation of the linear alignment array. Our result for the 5-year data set show an unexpected dilation signal with average velocities on the order of a few mm/year across the array. Our observations also confirm the lack of measurable localized creep in agreement with previous results that compare the total station observations and GPS data. The apparent fault zone dilation may be due to seasonal variability or residual noise. Further observations will provide more accurate constraints on the deformation style across the fault zone on the San Jacinto fault at Anza.

Key Words
Anza, San Jacinto fault, GPS, Seismic gap

Grenier, M., & Fialko, Y. (2018, 08). Using a dense GPS array as a strain meter on the Anza section of the San Jacinto fault . Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy