HR: 1340h
AN: S13B-1064    [Abstracts]
TI: Seismicity and Velocity Structure of the Himalayan Collision Zone: Mantle Earthquakes and Fast Velocity Zone in the Lower Crust of Southern Tibet
AU: * Monsalve, G
EM: monsalve@colorado.edu
AF: University of Colorado at Boulder, 2200 Colorado Ave, Boulder, CO 80309 United States
AU: Sheehan, A
EM: afs@cires.colorado.edu
AF: University of Colorado at Boulder, 2200 Colorado Ave, Boulder, CO 80309 United States
AU: Schulte-Pelkum, V
EM: vera_sp@cires.colorado.edu
AF: University of Colorado at Boulder, 2200 Colorado Ave, Boulder, CO 80309 United States
AU: Wu, F
EM: francis@binghamton.edu
AF: SUNY at Binghamton, PO Box 6000, Binghamton, NY 13902 United States
AU: Rowe, C
EM: char@lanl.gov
AF: Los Alamos National Laboratory, EES-11 M.S. D408, Los Alamos, NM 87545 United States
AB: P and S-wave arrival data from 28 broadband seismic stations deployed in eastern Nepal and southern Tibet were timed to determine hypocenters of local earthquakes, velocity structure of the crust and upper mantle, and geometry of subsurface interfaces. After picking first arrivals of P and S phases, we estimated hypocenters for over 1600 earthquakes in the area of our network for the time period between October 2001 and April 2003. Locations were determined using a weighted least squares algorithm and a priori 1-D velocity models for Nepal and southern Tibet. We relocated the earthquakes using a double-difference algorithm, recovering about 70% of the events. Cross-sections of relocated hypocenters show remarkable alignment of crustal earthquakes, with depths between 15 and 25 km, along the region of highest relief of the Himalayan Front. In addition to these shallow concentrations, two groups of upper mantle earthquakes clusters stand out: seismicity in the 1988 Udaypur Earthquake zone in southern Nepal and a belt of deep seismicity in southern Tibet, nearly 200 km long and approximately parallel to the Himalayan front. With the depth of the crust-mantle boundary constrained reasonably well via receiver functions, these groups of deep earthquakes can be associated with the existence of a strong lithospheric mantle in the Himalayan collision zone. We found conclusive evidence that at least one of the Udaypur aftershocks occurred in the mantle. The depths of the Tibetan earthquakes are more difficult to constrain, but irrespective of the velocity model used, their mantle depths are robust, even when noise is added to the seismic signals. We suggest the presence of a fast velocity layer in the lower crust of southern Tibet based on Wadati diagrams of deep earthquakes, the variations of travel-time residuals with depth, preliminary 3-D joint tomographic models and the receiver function profile. This fast layer could be a result of eclogitization of the Indian crust
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2004 AGU Fall Meeting