Reviewing data from regional permanent seismic stations and a 2001-2002 deployment of portable broadband seismic stations along the Tibet-Tarim border, we identified five unusually deep earthquakes located beneath the West Kunlun range. Application of regional centroid moment tensor analysis puts these events at depths of 40-60 km, with overall uncertainty ranging between 5-8 km. Previous investigations of the region indicate a Moho depth of ～62 km, implying that these earthquakes occurred near the bottom of the crust. Hence we infer that the lowermost crust beneath the range is brittle and seismically active. Two of these earthquakes are located directly beneath the trace of the Altyn-Tagh fault, with a nodal plane nearly parallel to the local strike of the fault, and a left-lateral sense of slip consistent with the long-term displacement across the fault. The two events suggest that the Altyn-Tagh fault extends through the entire crust in the West Kunlun.
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