Near surface velocity and Q (S) structure of the Quaternary sediment in Bohai basin, China

Heavily populated by Beijing and Tianjin cities, Bohai basin is a seismically active Cenozoic basin suffering from huge lost by devastating earthquakes, such as Tangshan earthquake. The attenuation (Q (P) and Q (S)) of the surficial Quaternary sediment has not been studied at natural seismic frequency (1-10 Hz), which is crucial to earthquake hazards study. Borehole seismic records of micro earthquake provide us a good way to study the velocity and attenuation of the surficial structure (0-500 m). We found that there are two pulses well separated with simple waveforms on borehole seismic records from the 2006 M (W)4.9 Wen'an earthquake sequence. Then we performed waveform modeling with generalized ray theory (GRT) to confirm that the two pulses are direct wave and surface reflected wave, and found that the average (P) and (S) of the top 300 m in this region are about 1.8 km/s and 0.42 km/s, leading to high (P)/ (S) ratio of 4.3. We also modeled surface reflected wave with propagating matrix method to constrain Q (S) and the near surface velocity structure. Our modeling indicates that Q (S) is at least 30, or probably up to 100, much larger than the typically assumed extremely low Q (10), but consistent with Q (S) modeling in Mississippi embayment. Also, the velocity gradient just beneath the free surface (0-50 m) is very large and velocity increases gradually at larger depth. Our modeling demonstrates the importance of borehole seismic records in resolving shallow velocity and attenuation structure, and hence may help in earthquake hazard simulation.