3D Finite Difference Tomography for the Crustal Structure Beneath the Khövsgöl Rift System, Mongolia

Abstract The Khövsgöl rift system of northern Mongolia demarks a transitional tectonic regime between the northward convergence of the Indian plate and Asia into the Siberian craton to the north. We deployed a temporary network of 26 broadband seismic stations encompassing 200km2 of the Khövsgöl rift system from August 2014 to June 2016. Data from our dense network were analyzed to provide a first high-resolution look at the crustal structure of this transtensional setting. Impulsive first motions and full waveform inversion of sources with magnitudes ranging between ML=5.16 and ML=3.02 revealed extensional faulting at 7 – 18 kilometer depths along the southeastern boundary of Khövsgöl basin and northeastern Darkhad valley while earthquakes 15-18 kilometers beneath Lake Khövsgöl were left-lateral strike slip events. We used finite difference tomography to invert arrival times from bodywave data to solve for both local hyprocentral location as well as the 3D velocity structure of the crust. Hypocenters outline a previously unmapped active fault along the eastern border of the Darkhad basin. Earthquake clustering beneath Lake Khövsgöl contain a MW=4.79 mainshock-aftershock sequence that initiated December 5, 2014. Results from the 3D velocity inversion show that the seismogenic depths do not exceed the basin depths of 20 kilometers, seismicity and source characterizations are well aligned with rift structures within the velocity model, and rifting has not thinned the 50 kilometer thick crust. Velocity values range from 6.13 – 7.3 km/s for VP, 3.52 – 4.09 km/s for VS , and the VP/VS ratio values range from 1.7 – 1.82. Variation in the Khövsgöl crustal structure is attributed to sediment fill within rift structures, we did not find evidence of magma nor a thinned crust.