Does the slip rate of the San Jacinto fault vary along strike? Constraints from campaign GPS data

Does the slip rate of the San Jacinto fault vary along strike? Constraints from campaign GPS data Conrad, JP Jconr003@ucr.edu Dept. of Earth Sciences, University of California, Riverside, CA, USA Funning, G J gareth@ucr.edu Dept. of Earth Sciences, University of California, Riverside, CA, USA The historically active San Jacinto fault (SJF) is major component of the plate boundary fault system in southern California. In close proximity to large population centers in California's Inland Empire, the loss of life and property damage that from a large earthquake along the SJF is potentially great. As the SJF is a relatively young fault, morphologically, it is made up of numerous discontinuous strands and segments, leading to disparate geologic slip rates and complicating their estimation. In the most recent Uniform California Earthquake Rupture Forecast, the modeled slip rates for the northern SJF are variable, from 6.0 mm/yr on the San Bernardino section to 14.8 mm/yr in Anza [Field et al., 2009]. Since fault slip rates control the accumulation of moment deficit on a fault, such a reduction should correspond with a proportional reduction in seismic hazard. The San Bernardino segment slip rate was lowered from the previous UCERF forecast and is a factor of 1/3 less than the segment immediately south of it, yet no new data was introduced to substantiate this change. With velocity fields modeled from GPS data collected over 25 years we will validate whether GPS velocities are consistent with the UCERF 2 slip rates. To accomplish this, we process data from over 100 continuous and survey GPS sites with epochs from 1995 to 2013 within the Western United States. Data sources include locally collected data from previous and current UC Riverside campaigns, as well as campaign data archived at the Southern California Earthquake Center, UNAVCO, and SOPAC and continuous data from the IGS and Plate Boundary Observatory. Site positions are estimated using GAMIT 10.5 in the ITRF 2008 reference frame. Upon processing of all data, time series for each site for each year are reviewed for general trends, inconsistencies, and outliers. After this is completed, a multiyear velocity solution is processed and reprocessed to iteratively refine the a priori coordinates and velocities for all sites used. Using the velocity field thus produced with GLOBK, we determine the slip rate at various locations along the SJF using fault-normal velocity profiles of sites at a range of distances from the fault trace. Preliminary results indicate that may be an overall northwestward decrease in fault parallel velocities along the Anza and San Jacinto Valley sections on the order of 0.5 to 1.5 mm/yr although this is equivalent to the uncertainties in our velocities.. We have insufficient data at present to resolve any slip rate changes between the San Bernardino Valley and Claremont strands of the fault, but incorporation of additional archived campaign GPS data will likely improve this situation in future.