Crustal velocity structure beneath the northern North-South Seismic Zone from local seismic tomography and its tectonic implications

We collected and picked the first arrivals of P and S waves of local and regional events recorded by 680 portable broadband seismic stations of the ChinArray-Himalaya Ⅱ and 217 fixed stations of the Chinese Seismic Network located on the northern North-South Seismic Zone （96°E−110°E, 30°N−44°N）. We got a 3D P and S wave velocity （v_P and v_S） structure of the crust beneath the northern North-South Seismic Zone with a horizontal grid space of 0.33°×0.33° by tomography. The tomography results reveal obvious integrated low velocity anomalies at 30 km depth beneath the northeastern Tibetan Plateau. The low velocity zone extends southward to the Longmenshan fault and extends eastward to the 106°E, dividing the Qinling orogenic belt into low velocity zone at westside and high velocity zone at eastside. The low velocity zone also extends northeast along the Yinchuan-Hetao graben, and extends northward through the Hexi Corridor, and it shows low velocity anomalies beneath the Alxa block at 30 km depth. This implicates that the extension of the Tibetan Plateau may be blocked by the relatively strong Sichuan basin and Qinling block in the east, while the extension affects the Hexi Corridor and the Alashan block, as well as the northwestern margin of Ordos along the Yinchuan-Hetao graben. At 50 km depth, it shows high velocity anomalies beneath the Alxa block and eastern Qilian orogenic belt, indicating possible southward subduction of the Alxa block beneath the eastern Qilian orogenic belt. Most crustal earthquakes in the studied region generally occurred along the fault zones between low velocity and high velocity zones where v_P and v_S change drastically over a short distance. The projections of large earthquakes （M≥6.0） at 30 km depth are almost located in the low velocity zone, which may indicate that the background of strong earthquakes is related to the low velocity zone beneath the source area.