HR: 1340h
AN: S43B-1308    [Abstracts]
TI: Using Small-Aperture Arrays to Characterize the Far-Regional P-Wavefield in Central Asia
AU: * Ferris, A
EM: aferris@westongeophysical.com
AF: Weston Geophysical Corp., 181 Bedford St. suite 1, Lexington, MA 02420, United States
AU: Reiter, D
EM: delaine@westongeophysical.com
AF: Weston Geophysical Corp., 181 Bedford St. suite 1, Lexington, MA 02420, United States
AB: Accurate knowledge of waveform characteristics observed at far-regional and near-teleseismic distances is critically important for nuclear monitoring in regions in which observations are sparse. However, seismograms of far-regional earthquakes are often difficult to interpret because of complex wave interactions with upper mantle structure. At these distances (14-22°), seismic discontinuities near 220, 410 and 660 km depth, as well as other mantle heterogeneity, produce interference phenomena that result in phase triplication, amplitude modulations and travel-time anomalies. For example, in central Asia, travel-time bulletins exhibit large residuals (1 - 8 sec) for the direct P arrival from far-regional events. These residuals are likely caused by complicated multi-pathing along the upper mantle propagation path and frequent phase misidentification due to the complexity of the seismograms. To improve the usefulness of far-regional arrivals in seismic event location and discrimination, we have modified some well-known array-processing techniques to increase the accuracy of primary and early coda phase identification recorded by small aperture (2 - 3 km) borehole arrays. To date the high fidelity recordings provided by these small aperture arrays have been primarily utilized for accurate picks of primary phase onset times and local earthquake studies. However, we have found that adequate determinations of other arrival characteristics are possible by modifying two well- known τ-p filtering (or velocity spectra analysis) methods: Nth-root and phase-weighted stacking. We have applied our modified methods to vertical-component data recorded at distances between 14- 18° by 9-element small-aperture arrays in central Asia. Our results indicate that in many cases we can identify closely-spaced arrivals by their slowness values in a time window that includes the direct P arrival, depth phases and arrivals from upper- mantle discontinuities.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7219 Seismic monitoring and test-ban treaty verification
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Seismology [S]
MN: 2007 Fall Meeting