Seismic full moment tensors for events in the Indian lower crust beneath the Himalayas and their relevance to eclogitization

We present seismic full moment tensors for intermediate-depth events in the Indian lower crust underthrusting Tibet. These are candidate events that may be related to metamorphic reactions. The events were recorded between 2001-2003 by the Himalaya Nepal Tibet Seismic Experiment (HIMNT) array of 29 broadband stations. We focus on a group of events at depths 50-100 km with epicenters within the array. Two events had magnitudes near Mw 4, and the majority had magnitudes 1-2. The waveforms for these events show some impulsive first motions, body waves above 1 Hz frequencies, and no discernible surface waves. For each event we utilize all possible waveforms and first motion polarities, and apply an algorithm to estimate its optimal moment tensor and uncertainty. The algorithm performs a grid search over the six-dimensional space of moment tensors by generating synthetic waveforms at each grid point and then evaluating a misfit function between the observed and synthetic waveforms. We computed the synthetic waveforms using several 1-D structural models for the region. The results reveal non-double-couple events, and we discuss their significance in light of the structural and petrological models. A preliminary analysis of the largest events points to mechanisms similar to collapsing cracks. If these seismic events are related to mineralogical reactions such as eclogitization, these results would suggest that the reactions occur at previously undocumented spatial scales.