Abstract:Qaidam Basin is located at the Northeastern margin of Qinghai-Tibet plateau, which is the largest basin on the plateau. The thickness of the Quaternary and Tertiary sediments in the basin vary laterally and reach a maximum of 3,000 m and 8,000 m, respectively. The temporary seismic stations deployed by the INDEPTH IV project recorded three component seismograms of the November 10, 2008 Dachaidan MW6.3 earthquake, which occurred at the margin of Qaidam Basin. Analysis of seismograms at two stations, namely H01 and H02, which have similar epicentral distance and azimuth, show a significant difference. Station H01 is located at the bedrock around the basin and the H02 station is located inside the basin on the Quaternary sediments. Compared with the records at the H01 station, those at the H02 station show significant amplitude amplification, obvious duration extension, and very strong later phases. The amplification of the peak ground velocity reaches 2-3 times, and that for the later phases reaches up to 6 times. Fourier amplitude spectrum analysis for the seismograms show similar frequency range for the two stations. However, the later phases have a lower and concentrated frequency range. Horizontal ground particle motion at the H02 station for the later phases exhibit typical elliptical ground motion, which suggests that the later phases are surface waves. To investigate the mechanism of the ground motion amplification and strong later phases, two 2-dimensional (2D) cross sections through the hypocenter and two stations are constructed. Seismic wave propagation inside the two 2D models are calculated with the fourth-order staggered grid finite difference method. The size of the models is 210 km by 50 km. The model is discretized into 1,000 and 500 grids in the horizontal and vertical direction, respectively. Two-dimensional double-couple line sources are used in the modeling and the dominated period of the source time function is 1.5 s. Modeling results are analyzed with wavefield snapshots and synthetic seismograms. They show that the existence of a low-velocity sedimentary layer in the basin causes amplification of direct waves because of the changes of propagation direction in the basin. Multiple reflections and conversions occurred after the direct waves in the basin. At the edge of the basin, surface waves with large amplitude and low propagation velocity were generated by constructive interference between direct waves and their reflections and conversions. Comparing the observed and synthetic seismograms at the H01 and H02 stations show very similar characteristics. This suggests the existence of the low-velocity layer inside the basin caused the significant difference of observed ground motion at the H01 and H02 stations. The amplification of amplitude of direct waves at the H02 station resulted from the variation of propagation direction in the basin. Seismic phases between the direct waves represent their multiple reflections and conversions in the sedimentary layer. The large amplitude later phases with longer duration are surface waves generated at the basin edge by constructive interference between the direct waves and their reflections and conversions. Results of this study provide new observational examples for seismic ground motion amplification in the sedimentary basin and their possible mechanisms from numerical modeling.
