Finite Difference Tomography of Crustal Velocity Structure Beneath Tengchong, Yunnan Province

Abstract

This work applies the finite difference tomographic method to invert P-wave velocity structure beneath the Tengchong area using earthquake data from a temporary network and other permanent stations, and analyzes the magmatic activity of Tengchong volcanoes and the deep structural genesis of the Longling M 7.3/M 7.4 earthquakes. The results show the velocity structure of the crust in this area has a pronounced heterogeneity. The lower velocities in the shallow subsurface are caused by unconsolidated basin deposits, Cenozoic volcanic deposits, fluid fractures and hot springs. The high-velocity zone at 5∼15 km depth probably represents cooled and solidified magmatic intrusions or less volatile ultramafic remnants. The low-velocity structure in the deep crust indicates molten or semi-molten magma. It is probable that the magmatic activity beneath volcanoes and magma intrusions in the deep crust of the Longling earthquake zone are from the same magmatic hearth area, which is the main area of the present magmatic activities. A strongly lateral heterogeneity is observed in the Longling earthquake zone. The east of the Nujiang fault belt and the west of the Longling fault belt are characterized by high velocity, implying large strain strength of rocks, which are the main carriers of stress accumulation. The low-velocity anomalies between the two faults extend to lower crust, probably caused by magmatic intrusions in the deep crust. The magmatic activity in this area is controlled clearly by the Longling fault and the Nujiang fault. The Longling M 7.3 and M 7.4 earthquakes occurred near the velocity boundary beneath the faults. The main reasons for the strong earthquakes in the Longling area inlcude the heterogeneity of stress accumulation caused by the lateral variation of the crustal medium strength, the deep magmatic aggregation and dynamic actions.