3-D P and S wave velocity of crust and Poisson’s ratio structures and their influence on seismic activity in Shandong area

Shandong area is located in eastern China, where faults develop and earthquakes occur frequently, so high resolution velocity structure is quite necessary for better understanding causal mechanism of earthquakes. Based on abundant P and S wave arrival times recorded at stations in and around Shandong area, high resolution 3-D P wave velocity, S wave velocity and Poisson’s ratio structures are obtained. The results reveal that, the hypocenter of the 2020 Changqing M_S4.1 earthquake is located in the transition zone where seismic velocity and Poisson’s ratio change drastically in the horizontal direction. This suggests that the 2020 Changqing M_S4.1 earthquake may be caused by the sinistral strike-slip movement of the Changqing fault affected by the regional stress field. Occurrence of the 2003 Laoshan M_L4.1 earthquake and Laoshan, Rushan and Changdao earthquake swarms may be strongly affected by fluids. These fluids invaded along pre-existing faults or cracks in relatively intact rock bodies, and induced faults to act or cracks to break up, which caused the occurrence of middle and strong earthquakes and earthquake swarms.