On the condition that the velocity ratio of compressive wave to shear wave is stable during the propagating process of seismic wave,this paper develops an estimation method for medium quality factor based on single station's travel time difference between direct S and P waves and the first period signal of direct P wave.8 774 high SNR wave data altogether recorded by 75 stations are analyzed.The results show that:(1)under the normal regional stress field,the quality factors QmSP in the stable tectonic area are higher than that in the active tectonic area around the Weifang-Jiashan section and its surrounding area in the Tanlu fault zone;(2)in the Juxian-Tancheng section the seismic wave attenuation is relatively quick,and the media is relatively broken,suggesting no tectonic stress accumulation;(3)the Xinyi-Sihong section is currently locking and in accumulating elastic strain energy stage,which has the deep environment and conditions of strong earthquake generation similar to those of the 1668 M 8.5 Tancheng strong earthquake.
The deep structure features around Tancheng M8(1/2)earthquake occurred in 1668 have been analyzed. Based on the crustal velocity structure obtained from travel-time tomographic inversion,especially the low velocity zones in middle crust and Moho depths,the deep velocity structure distribution in the area of 34° ~ 36°N,118° ~ 119°E is scanned along the latitude,longitude and oblique directions,and the corresponding crustal velocity profiles are obtained. By comparison,we take the area with velocity features coincident to the deep structure of the 1668 Tancheng M8(1/2) earthquake as the deduced epicenter,which is at 34. 8° ~ 35. 2°N,118. 2° ~ 118. 7°E,and the reasonable location is 35. 1°N,118. 6°E,and the focal depth is 20 km.
