Characterizing crustal structure with passive seismic methods [electronic resource] : applications to the western Himalaya and assessment of alternate methodologies
Keywords: Geophysics.

This work comprises three research projects, each with the aim of illuminating crustal structure, and each employing different means to do so. Two of the projects are based on field data, and in those I also explore the implications that the observed structure holds for collisional tectonics in the western Himalaya. Chapter 1 introduces the three projects. In Chapter 2, I use inversions of surface-wave dispersion to observe an anomalous low-velocity layer in the mid-crust of the Himalaya in northwest India (7-17% velocity reduction centered at ~30 km depth). This layer is continuous beneath the Tethyan Himalaya, Indus Tsangpo Suture Zone, Ladakh Arc Complex and southern Tibetan plateau, in total extending more than 200 km across the strike of the Himalaya. Comparison of these results with laboratory measurements and theoretical models suggests that the observed velocity reduction is indicative of 3-7% partial melt in the mid-crust, an interpretation consistent with spatially-coincident observations of low resistivity from magnetotelluric studies along the same profile. The results of this work provide additional evidence in support of previous speculations about a low-viscosity layer beneath the NW Himalaya, and suggest that the physical conditions required to enable active mid-crustal channel flow may be present in this area at the present day. In Chapter 3, I apply receiver-function common conversion-point (CCP) stacking to data from a seismic array crossing the Himalaya, from the Main Boundary Thrust to the South Tibet Detachment. I image the Moho deepening from 35-45 km depth beneath the Subhimalaya to > 50 km depth beneath the High Himalaya. I also image the Main Himalayan Thrust (MHT), the detachment between the top of the subducting Indian crust and the base of the Himalayan Thrust wedge. The MHT has a flat-ramp-flat geometry, with the ~10 km high and ~16° dipping ramp located beneath the surface trace of the Munsiari Thrust (or MCT-I). My interpreted MHT exhibits a complex polarity signature that varies down-dip. A magnetotelluric survey along the same profile shows high electrical conductivities on the thrust at locations coincident with my observation of a negative impedance contrast in the seismic section, down-dip of the ramp, indicating the presence of free fluids. Local seismicity recorded in this region is concentrated at a mean depth of 15 km just north of the surface projection of the Munsiari Thrust/MCT-I, up-dip of the negative-polarity segment of the MHT, and overlapping with my inferred ramp location. This location is thus the probable location for the locking line of the MHT in Garhwal, the concentration of local seismicity at this location being the result of stress accumulation, which focuses microseismic activity at the down-dip edge of locked faults. In Chapter 4, I apply the technique of wave-equation migration to synthetic teleseismic data with the aim of determining the conditions under which this technique can offer improvements over the ubiquitous approach of back-projection and stacking of one-dimensional receiver functions to create a two-dimensional CCP image. I test migrations under a variety of differing parameters relevant to teleseismic field data, including source period, scattering mode, receiver spacing, and signal-to-noise ratio, and show clear improvements in resolution over receiver-function CCP stacking. This method, which utilizes both back-scattered and forward-scattered energy, and requires fewer assumptions about the geometry of discontinuities than receiver functions, can be a powerful imaging tool for experimentalists analyzing teleseismic data from broadband arrays. However, most recent passive-seismic imaging experiments fail to use the close station spacing or record the high frequencies necessary for successful implementation of wave-equation migration.