Seismic Imaging of the San Andreas Fault at Parkfield, California Using Data From PASO-DOS

In June and July of 2001, the 15 station PASO network (Thurber et al., 2001) was augmented by the installation of 44 additional short-period and broad-band seismic stations within the original aperture (about 15 km) of the network. Each station in the current network (Parkfield Area Seismic Observatory - Densification of Stations or PASO-DOS) is equipped with a three component sensor, of which 29 are Guralp 40T's, 7 are Streckeisen STS-2's, and the remainder are short period (HS-10 or L4C). All data are telemetered to a central recording site in Parkfield, where a subset is transmitted in near real time via satellite to UC San Diego and then to the investigators over the Internet. This form of recording has proven to be extremely advantageous for monitoring data quality and for rapid analysis. Moreover, data return rates have been nearly perfect (99%). Both the original and augmented deployments were motivated by the proposed San Andreas Fault Observatory at Depth (SAFOD). The purpose of the original network was to answer questions about the most probable location of the seismogenic fault at depth and to create first-order images in the region of the proposed borehole. The recent densification of the network (average station spacing of about 1 km) is designed to provide detailed information about the character of seismicity and the structure of the fault zone outside of the vicinity of the borehole. The images produced will help us to interpret the in-situ observations from the Parkfield borehole (downhole observations and cores) in the context of the seismically-imaged variations in structure along the fault. The specific objectives are (a) to derive well-constrained 3D P- and S-wave velocity models that will result in improved accuracy and precision in the location of earthquakes in the Parkfield area, (b) to investigate heterogeneities in seismic velocity structure that may indicate lithologic variations and/or the presence and distribution of fluids in the fault zone, and how the variations along strike and with depth relate to the pattern of seismic behavior (aseismic, background earthquakes, main shock rupture zone). After 45 days of operation of the full network, we have recorded over 100 microearthquakes identified as local events (within the array footprint). About half of these were cataloged by the USGS network, all below magnitude 2.