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Observations of direct P-wave slowness and azimuth anomalies for teleseisms recorded in Long Valley caldera, California

U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD RD, MENLO PARK, CA 94025
DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF CALIFORNIA, SANTA BARBBARA, CA 93106

Abstract

Using particle motion, frequency-wavenumber analysis, and planewave fitting, we observe large anomalies in the bearing and apparent velocity of 0.5 to 1.0 Hz teleseismic P-waves incident at stations near the resurgent dome of Long Valley caldera, California. Our 3-component regional and teleseismic data come from three 2-mo deployments: (1) a 10 station, 2.9-km aperture array, (2) a 5 station, 1.9-km aperture array, and (3) a 3 station network, spanning about 5 km. We find stacked particle motion to be the most informative polarization method when the signal-to-noise ratio (SNR) is high and wavefronts are nonplanar. However, the insensitivity of the broadband frequency-wavenumber method (BBFK) to noise makes it more useful as SNR declines. Numerical simulations and data analyses using the BBFK method show that for a 10 station, 3-km aperture array, precision of roughly 8° in bearing and 0.008 sec/km in apparent slowness can be obtained with average teleseismic data in regions having complex structure. Bearing anomalies at Long Valley caldera reach up to 124°, whereas apparent slowness differ from expected values by up to a factor of 2. The anomalies are most likely caused by strong lateral velocity variations in the subcaldera crust between depths of 6 to 35 km.

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