Imaging the Himalayan megathrust in northwest India: wave-equation migration and receiver function CCP-stacking compared

We performed wave-equation migration (WEM) of teleseismic earthquakes from a range of backazimuths to image the main features of the Himalayan thrust belt. To date we have successfully imaged the Main Himalayan Thrust (MHT), and done so with greater resolution than we found using more-common teleseismic imaging methods (namely, CCP stacking of depth-converted receiver functions). At this time our results suggest that the depth of the MHT in the northwest Himalaya (longitude ~79°E) is similar to the depth reported in studies from the central Himalaya (longitude ~90°E, more than 1000 km east of our study site). The wave-equation migration that we employ consists of cross-correlating two passive teleseismic wavefields (source and receiver wavefields, as calculated with a one-way wave equation) to resolve scattering points from subsurface discontinuities into interpretable lithospheric images. This differs from existing methods in that it incorporates both forward-scattered and back-scattered energy. Traditional receiver-function studies utilizing forward-scattered converted phases at single stations yield only one-dimensional models, and although CCP stacking using an array of stations can yield a two-dimensional image, this method still considers only forward-scattered energy, thus utilizing only a portion of the information available in teleseismic arrivals. For validation, we compared our WEM-derived images to images derived from receiver functions. We calculated receiver functions using an iterative time-domain method and depth-converted them using an assumed velocity model to obtain 2-dimensional images. Corresponding features in both images (primarily the MHT) indicate that our WEM method is generating valid images. The data set that we use for this work was acquired in northwest India in 2005-2006 by India's National Geophysical Research Institute (NGRI). The array was situated across the epicenter of a 1999 Mw = 6.6 event, and had a receiver spacing of ~10 km, making it well-suited for migration. The 21 stations in the array spanned the Himalayan orogen between the Main Frontal Thrust (MFT) in the south and the South Tibetan Detachment (STD) in the north, and crossed the surface expressions of Main Boundary Thrust (MBT) and Main Central Thrust (MCT).