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SURFACE OF THE EARTH: NORTH AMERICA 2006 <strong>IRIS</strong> 5-YEAR PROPOSAL High-Resolution Surface Wave Tomography From Ambient Seismic Noise Nikolai M. Shapiro, Michael H. Ritzwoller • University of Colorado at Boulder Michel Campillo, Laurent Stehly • Laboratoire de Géophysique Interne et de Tectonophysique Université Joseph Fourier, Grenoble, France Rayleigh wave Green functions, extracted by cross-correlating long sequences of ambient seismic noise (Shapiro and Campillo, 2004) normally discarded as part of traditional seismic data processing, contain information about the structure of the shallow and middle crust. Cross-correlating one month of ambient seismic noise recorded at USArray stations in California yields hundreds of short period (6 – 20 s) surface-wave group-speed measurements on inter-station paths. These measurements are used to construct dispersion maps of the principal geological units of California, with low-speed anomalies corresponding to the main sedimentary basins and high-speed anomalies corresponding to the igneous cores of the major mountain ranges (Shapiro et al., 2005). The use of ambient seismic noise as the source for seismic observations addresses several shortcomings of traditional surface wave methods. The method is particularly advantageous in the context of temporary seismic arrays such as the Transportable Array component of USArray or PASSCAL experiments because it can return useful information without earthquakes. The short-period dispersion maps provide homogeneous information about shear wave speeds in the crust which are hard to acquire with traditional methods. The new method enhances resolution because measurements are made between regularly spaced receivers, which may lie much closer to one another than to earthquakes. Group velocity maps constructed by cross-correlating 30 days of ambient noise between USArray stations in California. Black solid lines show known active faults. White triangles show the locations of USArray stations used in this study. Shapiro, N.M. and M. Campillo, Emergence of broadband Rayleigh waves from correlations of the ambient seismic noise, Geophys. Res. Lett., 31, L07614, doi:10.1029/2004GL019491, 2004. Shapiro, N.M, M. Campillo, L. Stehly, and M.H. Ritzwoller, High-Resolution surface wave tomography from ambient seismic noise, Science, 307, 1615- 1618, 2005. 70
2006 <strong>IRIS</strong> 5-YEAR PROPOSAL SURFACE OF THE EARTH: NORTH AMERICA Imaging the Fine Structure of the San Andreas Fault at Seismogenic Depths Using an Archived PASSCAL Dataset Jeff McGuire • Woods Hole Oceanographic Institution Yehuda Ben-Zion • University of Southern California Detailed imaging of fault-zone material properties at seismogenic depths is a difficult seismological problem owing to the short length scales of the structural features. Seismic energy trapped within a low-velocity damage zone has been utilized to image the fault core at shallow depths, but these phases appear to lack sensitivity to structure in the depth range where earthquakes nucleate. Major faults that juxtapose rocks of significantly different elastic properties generate a related phase termed a fault-zone head wave that spends the majority of its path refracting along the fault. We utilize data from a dense temporary PASSCAL array of seismometers in the Bear Valley region of the San Andreas Fault to demonstrate that head waves have sensitivity to fault-zone structure throughout the seismogenic zone. Measured differential arrival times between the head waves and direct P arrivals and waveform modeling of these phases provide high-resolution information on the velocity contrast across the fault. The obtained values document along-strike, fault-normal, and downdip variations in the strength of the velocity contrast, ranging from 20 to 50% depending on the regions being averaged by the ray paths. The complexity of the fault-zone waveforms increases dramatically in a region of the fault that has two active strands producing two separate bands of seismicity. Synthetic waveform calculations indicate that geological observations of the thickness and rocktype (granite) of the layer between the two strands are valid also for the subsurface structure of the fault. The results show that joint analysis of fault zone head waves and direct P arrivals can resolve important small scale elements of the fault zone ruptures is necessary for evaluating theoretical predictions of the effects that these structures have on rupture propagation. Map of the Bear Valley section of the San Andreas Fault (black line) and the seismometer locations (black triangles) of the 1994 PASSCAL deployment by Thurber, Rocker, et al. The portion of the fault southeast of station SUM has two strands mapped at the surface with a sliver of granite (light blue) inbetween them. Our analysis of the fault zone head-waves indicates that this granite sliver extends throughout the seismogenic zone (middle panel with black dots for earthquake locations). The right panel shows the headwaves recorded at station SUM, aligned on the direct P arrival (0 time). The headwave (HW) moves out with distance. The reverberations between the HW and the direct wave result from the presence of the intermediate velocity granite sliver (blue region in map). 71
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