Abstract

The propagation efficiencies of the regional phases Lg and Pn are indicative of how active and recent tectonics influence crust and uppermost mantle properties, respectively. In this study, regional scale lateral heterogeneity of Lg and Pn attenuation for the region in and around the southern Tibetan Plateau is investigated. The data set is comprised of seismograms recorded at the Chinese Digital Seismogram Network (CDSN) station LSA from regional events with epicentral distances within 11^o. Attenuation of Lg and Pn is calculated using spectral methods and assuming constant Q_Lg and Q_Pn models for the frequency bands 0.3 to 3.0 Hz and 0.5 to 4.0 Hz, respectively. Lateral variation in attenuation is estimated by analyzing data on an event-by-event basis. Significant lateral variation is observed with Q_Lg decreasing from about 520 for events south of LSA to about 340 for events north of LSA and Q_Pn ~ 670 for southern backazimuths, while Q_Pn ~ 240 for northern events. For Pn, this north-south variation is consistent with other observations, indicating partially melted upper mantle beneath north central Tibet. For Lg, the azimuthal variation in attenuation indicates that Lg propagation as observed at LSA is efficient for paths crossing the southern boundary of the Tibetan Plateau relative to paths within the plateau itself.

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