Estimation of a Time-Dependent Strain Rate Field in Southern California Using Continuous GPS Stations in the SCIGN Network.

Advancements in the recognition of fine-scale deformation fluctuations have prompted a great deal of attention to be focused on identifying and characterizing transient strain phenomena. We have developed a tool for recognizing strain rate transients as well as for quantifying the magnitude and style of their temporal and spatial variations. Using time-varying velocity estimates for continuous GPS station data from the Southern California Integrated GPS Network (SCIGN) for the time period between October 1999 and February 2004 [Li et al., 2005]. We determine time-averaged velocity values in 0.05 year epochs for each continuous velocity series. For each velocity field solution we determine a self-consistent model velocity gradient tensor field solution for the region using bi-cubic Bessel interpolation of the GPS velocity vectors. For each epoch solution we plot dilatation strain rates, shear strain rates, and the rotation rates. We also investigate the departures of the model strain rate field and velocity field from a master solution, obtained from a time-averaged solution for the period 1999-2004, as well as estimating the departures of the time variable velocity gradient tensor field from other master solutions, including models that incorporate plate motion constraints and Quaternary fault data. By combining the epoch solution plots, we create movies that allow us to view the spatial and temporal changes in the dilation and shear strain rate field in southern California. In the present solution several time-dependent changes are noteworthy. The Eastern California Shear Zone (ECSZ) region, immediately following the October 1999 Hector Mine earthquake, shows a significant spatial change of relatively high shear strain rate that increases from the immediate area of the earthquake to an area that almost spans the entire ECSZ from east to west. Also following the Hector Mine event, there is a strain rate corridor that extends through the Pinto Mt. fault connecting the ECSZ to the San Andreas Fault segment in the Salton Trough region. This signal slowly decays while shear strain rates on segments of the San Andreas Fault, just east of Palm Springs, and the San Jacinto increase during 2001-2004. During this period shear strain rates increase by roughly 20 nanostrain per year on the San Andreas and 20-30 nanostrain per year on the San Jacinto (over a zone approximately 20 km wide). Moreover, seismicity rates increase along these segments during this same time period in which shear strain rates increase. Results to date are very promising in using this tool for investigating temporal variations in the deformation field. Our results quantify the magnitudes of the strain rate changes and provide some bounds on their spatial coverage. Ultimately this tool should enable us to separate out tectonic signals from other non-tectonic sources, such as hydrologic and atmospheric.