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.

S U M M A R YWe determine shear wave splitting parameters of teleseismic SKS and SKKS phases recorded at 43 broad-band seismometers deployed in South Victoria Land as part of the Transantarctic Mountain Seismic Experiment (TAMSEIS) from 2000 to 2003. We use an eigenvalue technique to linearize the rotated and shifted shear wave particle motions and determine the best splitting parameters. The data show a fairly consistent fast direction of azimuthal anisotropy oriented approximately N60 • E with splitting times of about 1 s. Based on a previous study of the azimuthal variations of Rayleigh wave phase velocities that show a similar fast direction, we suggest that the anisotropy is localized in the uppermost mantle, with a best estimate of 3 per cent anisotropy in a layer of about 150 km thickness. We suggest that the observed anisotropy near the Ross Sea coast, a region underlain by thin lithosphere, results either from upper mantle flow related to Cenozoic Ross Sea extension or to edge-driven convection associated with a sharp change in lithospheric thickness between East and West Antarctica. Both hypotheses are consistent with the more E-W fast axis orientation for stations on Ross Island and along the coast, subparallel to the extension direction and the lithospheric boundary. Anisotropy in East Antarctica, which is underlain by cold thick continental lithosphere, must be localized within the lithospheric upper mantle and reflect a relict tectonic fabric from past deformation events. Fast axes for the most remote stations in the Vostok Highlands are rotated by 20 • and are parallel to splitting measurements at South Pole. These observations seem to delineate a distinct domain of lithospheric fabric, which may represent the extension of the Darling Mobile Belt or Pinjarra Orogen into the interior of East Antarctica.

Seismic imaging provides an opportunity to constrain mantle wedge processes associated with subduction, volatile transport, arc volcanism, and back-arc spreading. We investigate the seismic velocity structure of the upper mantle across the Central Mariana subduction system using data from the 2003-2004 Mariana Subduction Factory Imaging Experiment, an 11 month deployment consisting of 20 broadband seismic stations installed on islands and 58 semibroadband ocean bottom seismographs. We determine the three-dimensional V P and V P /V S structure using over 25,000 local and over 2000 teleseismic arrival times. The mantle wedge is characterized by slow velocity and high V P /V S beneath the fore arc, an inclined zone of slow velocity underlying the volcanic front, and a strong region of slow velocity beneath the backarc spreading center. The slow velocities are strongest at depths of 20-30 km in the fore arc, 60-70 km beneath the volcanic arc, and 20-30 km beneath the spreading center. The fore-arc slow velocity anomalies occur beneath Big Blue seamount and are interpreted as resulting from mantle serpentinization. The depths of the maximum velocity anomalies beneath the arc and back arc are nearly identical to previous estimates of the final equilibrium depths of mantle melts from thermobarometry, strongly indicating that the lowvelocity zones delineate regions of melt production in the mantle. The arc and back-arc melt production regions are well separated at shallow depths, but may be connected at depths greater than 80 km.

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