HR: 0800h
AN: S51A-0127    [Abstracts]
TI: Focal Mechanisms From Moment Tensor Solutions and First Motion Polarities of Shallow to Deep Local Earthquakes in Eastern Nepal and Southern Tibet
AU: * de la Torre, T L
EM: tomd@lithos.colorado.edu
AF: Univesity of Colorado at Boulder, Campus Box 399, Boulder, CO 80309 United States
AU: Sheehan, A F
EM: afs@cires.colorado.edu
AF: Univesity of Colorado at Boulder, Campus Box 399, Boulder, CO 80309 United States
AU: Monsalve, G
EM: monsalve@colorado.edu
AF: Univesity of Colorado at Boulder, Campus Box 399, Boulder, CO 80309 United States
AU: Wu, F
EM: wu@geol.binghamton.edu
AF: SUNY Binghamton, P.O. Box 6000, Binghamton, NY 13902 United States
AB: We determined focal mechanisms using waveforms and first motion polarities from local earthquakes recorded during the Himalayan Nepal Tibet Seismic Experiment (HIMNT). The HIMNT experiment included the deployment of 28 broad band seismometers in eastern Nepal and southern Tibet from September 2001 to April 2003. Using a regional moment tensor method (Ammon and Randall, 2001) and first motion polarities for displaying double-couple focal mechanisms (Snokes, 2003), we analyzed the fault plane solutions at three distinct zones of seismicity. Characteristic focal mechanisms in seismically concentrated areas may indicate the presence of fault ramps or a decollement in the Himalayan collision zone. Previous studies of focal mechanisms on the Tibetan Plateau predominantly indicate east-west extension and shallow thrusting at the Himalayan collision zone for shallow to intermediate earthquakes (Ni and Barazangi, 1984; Molnar and Lyon-Caen, 1989; Randall et al., 1995) and east-west extension for intermediate to deep earthquakes (Zhu and Helmberger, 1996; Chen and Yang, 2004). The first zone in southeast Nepal between the Main Boundary and Main Frontal faults consist of earthquakes < Mw 4.0 at depths 40 - 60 km near the epicenter of the 1988 Udaypur earthquake, Mb 6.1, depth 57 km. The second zone north of the Main Central Thrust outcrop in eastern Nepal consists of 14 earthquakes 3.0 - 5.0 Mw at depths < 30 km that indicate north-south strike normal faulting and east-west strike thrust faulting. The third zone is an arc parallel to the Himalayas in southern Tibet and a cluster in northeast Nepal. This zone consists of 45 earthquakes < 4.0 Mw at depths > 50 km. Four earthquakes indicate northwest-southeast compression resulting in northeast strike strike-slip faulting while one earthquake in the northeast cluster indicates east-west compression at a source depth below the crust-mantle boundary. Focal mechanisms from full waveform moment tensor inversions are cross checked with first motion solutions for selected events. Source depths as determined from normalized error of the sum of the squared differences between the data and synthetic seismogram coincide with the source depths determined from the travel time residual inversion.
DE: 7205 Continental crust (1242)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting