Graduate Thesis Or Dissertation
 

Seismotectonics of the Himalayas and the Tibetan Plateau : moment tensoranalysis of regional seismograms

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  • This thesis presents a detailed seismotectonic investigation of the Himalayan region and the Tibetan plateau as part of project HiCLIMB to explore the state of stress and the kinematics of the world’s largest continental collision zone. Using full regional waveforms for moment tensor inversion, source parameters for 107 earthquakes were determined with moment-magnitudes (Mw) ranging from 3.5 to 6.3. The significant decrease in magnitude threshold with respect to previous studies was accomplished through the usage of broadband data from the HiCLIMB, HIMNT, and Bhutan temporary networks. Combining the results from this study with previously published earthquake source parameters, the investigation focuses on three topics: (1) Deformation along the front of the Himalayan arc associated with the Main Himalayan Thrust (2) Extension in the southern Tibetan plateau, and (3) Location and stress orientation of intermediate-depth earthquakes. Thrust event epicenters along the Himalayan front closely coincide with the 3500 m topography contour. These earthquakes can be associated with elastic strain accumulation near the lower tip of the locked part of the MHT due to tectonic loading from its creeping down-dip extension. Centroid depths and nodal plane dips of these thrust events are inconsistent with slip merely on the main detachment and indicate significant deformation in the vicinity of the MHT. Especially in far western Nepal, nodal plane dips are systematically steeper and slip during these events might play a role in the formation of asperities and barriers on the detachment surface. The P-axes azimuths of the thrust events along the Himalayan arc deviate considerably from a mere circular geometry. Spatial filtering of the regional topography reveals that slip of events in the footwall as well as the hanging wall aligns perpendicular to the mountain range on a 50 km wavelength scale. The topography-perpendicular alignment of the slip direction on planes with significant inclination suggests that these thrust events contribute considerably to the mountain building process and to the formation of the local shape of the arc. Deformation on the southern Tibetan plateau is dominated by shallow normal faulting in the upper 15 km of the crust. The extensional direction, while generally trending east-west, shows an apparent transition from arc parallel in the Tethyan Himalaya to northward convex in the southern Lhasa terrane. North of about N31º, deformation changes to strike-slip prevalence. The locations of changes in faulting patterns coincide with changes in geometry of the underthrusting Indian crust revealed by receiver function images. This correlation indicates a significant influence of basal traction on shallow crustal faulting processes. This study provides additional evidence that most intermediate-depth seismicity occurs beneath the Moho, signifying a strong upper mantle. Faulting in the upper mantle is dominated by strike-slip faulting with northerly trending P-axes. The maximum horizontal compressive stress axes of mantle earthquakes align with the direction of the India-Eurasia convergence and imply a relation of this deformation to the subduction process.
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