Tomographic determination of the upper crustal structure in Jiashi strong earthquake swarm region——joint inversion of explosion and earthquake data
伽师强震群区上地壳三维速度层析成像——人工爆破和天然地震的联合反演

Jiashi strong earthquake swarm region is located at the juncture area of three tectonic units,i.e.Tianshan fold zone,Pamir tectonic arc and Tarim block.Many strong earthquakes occurred recently in this area.In order to obtain the fine crustal velocity structures of this area,the data from a 3-D seismic transmission experiment and a portable seismograph network acquired in Jiashi strong earthquake swarm region in 1997 are processed.Joint inversion of explosion and earthquake data is carried out to reconstruct the 3-D images of velocity perturbation of the upper crust under the seismic arrary.The temporaty seismic array was used to record the seismic waves generated by 8 shots fired at different azimaths,and the additional 20 seismic stations were deployed in this area to record local earthquakes.The results show that the structures of the upper crust in the study area is characterized by obvious inhomogeneouities both laterally and vertically.The low P wave velocity zone centered at Saruxi beneath the Jiashi earthquake swarm area is gradually replaced by high P wave velocities with the increase of depth.There is a high P wave velocity anomaly striking NNW at the depth of 12 km under the earthquake swarm area.It extends downwards to the depth of 16 km at least,and is surrounded by relatively low velocities.V_P/V_S ratios are also characterized by high values in this region.This kind of structural differences may be related to the occurrences of Jiashi strong earthquake swarm.It can be found from the P wave velocity images at the depth of 16 km that Jiashi strong earthquake swarm generally occurred in the regions with comparatively high P wave velocity perturbations or in the transition zones between high and low velocity perturbations.The existence of high P wave velocity anomaly in the crust may play an important role in the seismogenic process.