Seismic tomography in the Tengchong volcanic area and its tectonic implication

Using earthquake data from a temporary network and other permanent stations in southwestern Yunnan, we inverted the P-wave velocity structure of Tengzhong and adjacent areas, with the aim to analyze the crustal structure of the Tengchong volcanoes and Longling M > 7 earthquake areas. Our results indicate that the Tengchong volcanic area is underlain by a prominent low-velocity zone at 10 similar to 20 km depth and with a lateral extent of 20 similar to 30 km. We infer that this low-velocity zone represents an active magma source and the heat flow channel may extend into the deep crust through the Tengchong fault. The W-E variation in the crustal structure is correlated with the volcanic activities in different periods. The high-velocity structure in eastern Tengchong reflects the cooled and solidified magma intrusions or high density remnants within the volcanic channels during Miocene-Pliocene, whereas the low-velocity zone of western Tengchong reveals the magmatic and heat flow activities from Pleistocene to present day. A strong lateral heterogeneity is observed in the crust structure of the Longling M > 7 earthquake area. The occurrence of the M7.3 and M7.4 earthquakes are related to the structural variation across the Nu River fault and the Longling fault, both of which are located near velocity boundaries. The upper crust east of the Nu River fault and west of the Longling fault is characterized by high velocities, which is considered as a primary location for stress accumulation in the earthquake area; however, low velocities are observed in the region between the Nu River fault and the Longling fault. It implies that multiple magma intrusions probably decreased the rheologic strength in the crust. It is the distinct variation in the structure that made the Nu River fault and the Longling fault easy to produce seismic ruptures for the intense earthquakes under the effect of tectonic stress. We infer that these two earthquakes occurred at 10 similar to 12 km depth.