HR: 1330h
AN: T42B-0295    [PDF]
TI: Source Mechanisms, Velocity Structures and Himalaya Tectonics
AU: * Wu, F T
EM: francis@binghamton.edu
AF: SUNY Binghamton, Vestal Parkway E, Binghamton, NY 13902 United States
AU: Sheehan, A F
EM: afs@cires.colorado.edu
AF: Univ of Colorado, Broadway, Boulder, CO 80309
AU: Huang, G
EM: dino@geol.binghamton.edu
AF: SUNY Binghamton, Vestal Parkway E, Binghamton, NY 13902 United States
AU: Monsalve, G
EM: gaspar.monsalvemejia@colorado.edu
AF: Univ of Colorado, Broadway, Boulder, CO 80309
AB: The Himalayan Nepal-Tibet Seismic Experiment (HIMNT; in a region bounded by 26.7 and 29.5 degrees N latitudes and 85 and 88 degrees E longitudes) produced, for the first time, broadband seismic data appropriate for determining earthquake and lithospheric characteristics with a local network astride as well as along the high Himalaya. A suite of studies are being conducted in an attempt to gain subsurface information for a better understanding of the orogenic processes that produced the mountain range. First, it is somewhat surprising that of the 20 earthquakes (M$>$3.5) for which we are able to use waveform inversion to derive focal mechanisms the great majority of them are tensile types with generally EW oriented T-axes. Some of these events are shallow (~20 km) crustal events and a few are deeper crustal or even upper mantle ones (50-80 km). Several of them appear to be related to the N-S trending graben structures at the surface. Initial attempts at tomography yield structures that generally agree with the the results of receiver function analyses (Schulte-Pelkum, this meeting), with a crust of about 45 km under southern Himalaya and much thicker under the Nothern Himalaya in Tibet. The relocated seismicity using hypoDD (this paper and Monsalve et al., this meeting) lie in well-defined zones. In the region around Mount Everest a zone dips at shallow angle ($<$ 10 degrees) from the Greater Himalaya toward the south can be seen. Under the low foothills in south Nepal steep dipping zones between 30 and 60 km are found in some sections. The seismicity in the upper crust ($<$30 km) under the high Himalayan range is notable in places but under the eastern part of our network deeper crustal (60 $<$ h $<$ 80 km) concentrate without much shall seismicity. While the shallow (~20 km) reflector from the receiver function analyses can be interpreted as the presence of Indian lithosphere and lends support to the INDEPTH model, the interpretation of seismicity and focal mechanisms indicate that a more complicated model may be needed. Since we have only recorded relatively small (M $<$ 4.5) events in the region, are we just taking a glimpse of the ongoing processes in between large events? Would the large events produce major underthrusting consistent with some of the current models?
DE: 8102 Continental contractional orogenic belts
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting