We discuss several outstanding aspects of seismograms recorded during >4 weeks by a spatially dense Nodal array, straddling the damage zone of the San Jacinto fault in southern California, and some example results. The waveforms contain numerous spikes and bursts of high-frequency waves (up to the recorded 200 Hz) produced in part by minute failure events in the shallow crust. The high spatial density of the array facilitates the detection of 120 small local earthquakes in a single day, most of which not detected by the surrounding ANZA and regional southern California networks. Beamforming results identify likely ongoing cultural noise sources dominant in the frequency range 1-10 Hz and likely ongoing earthquake sources dominant in the frequency range 20-40 Hz. Matched-field processing and back-projection of seismograms provide alternate event location. The median noise levels during the experiment at different stations, waves generated by Betsy gunshots, and wavefields from nearby earthquakes point consistently to several structural units across the fault. Seismic trapping structure and local sedimentary basin produce localized motion amplification and stronger attenuation than adjacent regions. Cross correlations of high-frequency noise recorded at closely spaced stations provide a structural image of the subsurface material across the fault zone. The high spatial density and broad frequency range of the data can be used for additional high resolution studies of structure and source properties in the shallow crust.

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[1] We use differences between arrival times of teleseismic events at sets of stations crossing the North Anatolian fault east of Ismetpasa, where shallow creep has been observed, to detect and quantify a contrast of seismic velocities across the fault. Waveform cross correlations are utilized to calculate phase delays of P waves with respect to expected teleseismic arrivals with incident angles corresponding to the generating events. Compiled delay times associated with 121 teleseismic events indicate about 4.3% average P wave velocity contrast across the fault over the top 36 km, with faster velocity on the north side. The estimated contrast is about 8.3% if the velocity contrast is limited to the top 18 km. The sense of velocity contrast is consistent with the overall tectonic setting and inference made for the examined fault section based on theoretical expectations for bimaterial ruptures and observed asymmetry of rock damage across the fault. Our data indicate lack of significant microseismicity near the fault, suggesting that creep in the area is limited to the depth section above the seismogenic zone. Citation: Ozakin, Y., Y. Ben-Zion, M. Aktar, H. Karabulut, and Z. Peng (2012), Velocity contrast across the 1944 rupture zone of the North Anatolian fault east of Ismetpasa from analysis of teleseismic arrivals, Geophys. Res. Lett., 39, L08307,

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