HR: 1340h
AN: T43A-1379    [Abstracts]
TI: Evidence for the Decoupling of Stress Above and Beneath the Main Himalayan Thrust
AU: * Burtin, A
EM: arnaud.burtin@ens.fr
AF: Laboratoire de Géologie, Ecole Normale Supérieure, 24 rue Lhomond, Paris, 75231 France
AU: Nábělek, J L
EM: nabelek@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331 United States
AU: Baur, J
EM: jbaur@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331 United States
AU: Vergne, J
EM: vergne@geologie.ens.fr
AF: Laboratoire de Géologie, Ecole Normale Supérieure, 24 rue Lhomond, Paris, 75231 France
AB: We have determined moment-tensors (source mechanisms) and depths for earthquakes in the Himalayan region of Nepal and Bhutan using regionally recorded long-period seismograms. All events greater than magnitude 5 occurring since 1988 have been investigated. The source mechanisms of the regional thrust earthquakes show evidence for the decoupling of the stress above and below the Main Himalayan Trust (MHT). The thrust events on and above the MHT show a well know pattern of radially distributed compression axes (and slip-vectors) perpendicular to the Himalayan arc. This radial pattern is a result of the Indian subcontinent's collision with the Eurasia modified by the crustal extension in southern Tibet. We have now found several well-determined moment tensors for events beneath the detachment (imaged by project Hi-CLIMB) and these show, as one might expect, the compression axes well aligned with the direction of the India-Asia convergence (azimuth ~30° E, from recent GPS studies). One of these events is the anomalously deep 1988 Udayapur earthquake beneath the Main Frontal Thrust, where the MHT reaches the surface. These results indicate that the extension in southern Tibet modifies the stress above the MHT while the deeper deformation is unaffected, and provide constrains on dynamic models of crustal extension in southern Tibet. For example, the results are consistent with the idea of the extension being caused by the shear drag due to the subduction along the MHT and the circular geometry of the collision zone, as proposed by McCaffrey and Nabelek, 1998.
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8108 Continental tectonics: compressional
DE: 8123 Dynamics: seismotectonics
SC: Tectonophysics [T]
MN: Fall Meeting 2005