Seismic structure of the Tengchong volcanic area southwest China from local earthquake tomography

Abstract

We performed a local earthquake tomography to image the crust of the Tengchong volcanic area in southwestern China, using P-wave arrival data from a temporary network and permanent stations. The objective is to determine the magma sources of the volcanic area and the seismic structure of two M>7 earthquakes in the Longling area south of Tengchong. The result reveals a correlation between the velocity structures and the volcanic activities from late Miocene to Holocene. A prominent low-velocity zone is observed beneath the central volcanic area, that is cored at 10–20km depths and extends down to 30km depth, with a lateral extent of 20–30km. We infer that this unusual low-velocity zone represents a magma source of the heat flows for the volcanic eruption during Pleistocene and Holocene. Close to it, a high-velocity zone appears beneath the eastern volcanic area and it is likely to reflect solidified magma intrusions and high-density remnants within the cooled volcanic channel prior to Pleistocene. In the M>7 earthquake area, the upper crustal structure shows evident variations across the Longling fault and the Nu River fault. Bounded by the two fault zones, the Gaoligong metamorphic belt and the Baoshan block are imaged by high velocities that are consistent with their high strength in the structures; on the contrary, low velocities are imaged in the area between the Longling fault and the Nu River fault, implying a decrease of the strength probably caused by igneous intrusions. It was the lateral variation in the rheologic structure that led to the uneven stress accumulations across these fault zones and created tectonic conditions for the seismic ruptures of the two M>7 earthquakes in the Longling area.