Crustal Structure in Southeastern Gansu from Regional Seismic Waveform Inversion

Abstract

AbstractThe 3‐component full‐waveform data from the August 1, 2008 aftershock of the Wenchuan earthquake recorded by 24 permanent broadband seismograph stations in southeast Gansu and adjacent areas were used to study the crustal velocity structure of southeast Gansu by the waveform inversion method which combines the niching genetic algorithm and reflectivity method. The western and eastern region of southeast Gansu belongs respectively to the Tibetan Plateau and the transition zone between Tibetan Plateau and Ordos block. The inversion result indicates that the upper and middle crusts of the two regions have very similar average P‐wave velocities and consist of acid and intermediate rocks. However, the lower crusts differ considerably in P‐wave velocity (6.41 km/s in the east and 6.96 km/s in the west) and thickness (9.3 km in the east and 19.8 km in the west). This indicates that the lower crust of the western region is composed of basic rock, while that of eastern region is intermediate rock, supporting the view that crustal thickening in the northeast margin of Tibetan Plateau occurs mainly in the lower crust. The P‐wave velocity at the top of upper mantle in the western region is 7.73 km/s, corresponding to that of a young, tectonically active area; while that in the eastern region is 8.05 km/s, corresponding to a stable ancient landmass. On the top of upper crust of the eastern region there is a sedimentary layer about 6.5 km thick and with a P‐wave velocity of 5.78 km/s, which is missing in the western region. Compared with the global average crust velocity of PREM, the overall crustal velocities of both regions are somehow low.