Himalayan Seismotectonics at Deep Structure

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The Himalayan Nepal Tibet Seismic Experiment (HIMNT) was the first broadband seismic experiment to simultaneously cover the plains of southern Nepal, the Lesser and Greater Himalaya, and the Southern Tibetan Plateau. The HIMNT project included the deployment of 29 broadband seismc stations in eastern Nepal and southern Tibet in 2001-2003. The first studies with the HIMNT earthquake seismic data revealed upper mantle earthquakes under Nepal and Tibet, a first view of the Indian-Himalayan decollement under Nepal, crust mantle boundary (Moho) structure in the collision zone, and earthquake focal mechanisms suggesting decoupling in the lower Tibetan crust. The current project continues this analysis of HIMNT data to interpret structure and processes associated with the India/Eurasia collision. Teleseismic receiver functions analyzed to date are now supplemented with crustal conversions from local deep earthquakes. A ramp-flat geometry on the decollement has been proposed to explain clustering of earthquakes and uplift patterns at the Himalayan front. While an initial stacked image from teleseismic receiver functions lacked in resolution in the ramp area, modeling of receiver functions and converted phases from deep local earthquakes to show the presence or nonexistence of a decollement ramp is now underway. Changes in focal mechanisms from normal faulting in the upper Tibetan crust to strike-slip mechanisms at subcrustal depths indicate a decoupling zone in the lower Tibetan crust, which may be identified with a lower crustal flow channel suggested elsewhere based on surface wave analysis and geodesy. Attenuation studies combined with velocity tomography will help ascertain the physical state of the crust, and the nature of an apparent midcrustal decoupling zone discovered beneath the Himalayan Plateau. The frequency content of raw seismograms suggests a strong variation in attenuation between Tibet (high attenuation, low Q) and Nepal (low attenuation, high Q). The resulting stress regimes, material velocities derived from local and regional tomography, crustal geometry from receiver functions, new attenuation measurements, and new gravity modeling based on tomographic and receiver function results are combined to explain the behavior of the Tibetan crust in the collision zone. Regional surface wave analysis reveals radical changes in lithospheric thickness in the eastern versus western part of the study area. The obtained lithospheric thicknesses are used as input into flexural modeling to investigate the dynamics of subduction of the Indian subcontinent. The correlation between lithospheric thickness and subduction angle suggests either high rigidity or increased buoyancy for the thicker lithosphere, which can now be tested. Through the joint application of seismic as well as other methods, the investigators are assembling a complete kinematic and dynamic picture of the Himalayan collision zone. Broader impacts associated with the project involve the development of a natural hazards curriculum for the University of Colorado Science Explorers program. This program includes a series of one-day workshops for teams of middle school teachers and students throughout the state of Colorado. The project reaches over 300 teachers and 1500 students in one-day workshops at twenty different locations throughout the state. Other broader impacts are graduate student education and project participation by underrepresented groups. The data are available to the seismological community through IRIS and event information including picks will be made available to the International Seismological Centre for further dissemination.