Mantle Earthquakes Underneath The Indian-Tibetan Plateau Collision Zone

Earthquakes below the crust are often associated with oceanic subduction.However,earthquakes that occurred at the mantle depth are also discovered below the continental lithosphere.In general,these subcrustal earthquakes are referred to as mantle earthquakes.As a typical area for the occurrence of mantle earthquakes beneath the continental lithosphere,the India-Tibet Plateau collision zone is the collision boundary between the Eurasian plate and the India plate.A good understanding of mantle earthquakes in this region will help us to study the mechanism of mantle earthquakes and provide better constraints on the evolution of the lithosphere of the Tibetan Plateau.In my thesis,I use the CAP method to invert the focal mechanism and depth of mantle earthquakes that have occurred beneath the Indian-Tibetan Plateau collision zone since 2000.I further discuss the effects of different crustal velocity models on the inversion results.For some earthquakes having very different event depths from previous studies or whose focal depths are just near the Moho,we generated synthetic waveforms for events with different depths and compare them with observed waveforms.By comparing with the data,I am able to accurately determine whether the earthquakes are located in the mantle or not.Our results suggest that mantle earthquakes do exist beneath the Indian-Tibetan Plateau collision zone,and these mantle earthquakes are mainly strike-slip faults and normal faults.The strike-slip faults indicate compression in a north to south direction,while the normal faults show extension from east to west,which is consistent with the internal stress of the Tibetan Plateau.Based on the surface structure and the tomographic images of the lithosphere,we find that these mantle earthquakes mostly occur beneath rift,suture zone,or large faults,which correspond to the edge of the tearing of the subducted Indian mantle lithosphere.The stress caused by such tear is consistent with the stress inferred from mantle earthquakes.Therefore,we believe that the mantle earthquakes in this area are likely to be caused by the tearing of the Indian mantle lithosphere.