Study on Fine Crustal Structure of the Sanhe-Pinggu Earthquake （M8.0） Region by Deep Seismic Reflection Profiling
摘要
Two near-vertical deep seismic reflection profiles (140km-long, 24-fold) were completed in the1679 Sanhe-Pinggu earthquake (M8.0) region. The profiles ran through the Xiadian fault andthe Ershilichangshan fault. The profiling result shows that the crust in this region is dividedinto the upper crust, the lower crust and the crust-mantle transitional zone by two powerfullaminated reflectors: one at the two-way travel-time of about 7.0s (21km), the other at about11.0～ 12.5s (33 ～ 37km). Crustal structure varies significantly in vertical direction. Theshallow part is characterized by obvious stratification, multilayers and complexity. The uppercrust on the whole features reflection transparency, while the lower crust features distinctrefiectivity. Crustal structure also varies a lot in the lateral direction. The main fracture in thisregion is the deep fault under the Xiadian fault. This deep fault is steeply inclined (nearlyvertical), and is supposed to be the causative fault of the Sanhe-Pinggu M8.0 earthquake. Thetwo profiles respectively reveal the existence of local strong refiectivity in the lower crust andthe lower part of the upper crust, which is assumed to be a dike or rock mass formed by theupwelling and cooling down of materials from the upper mantle. Magmatic activity in this partbrought about differences in regional stress distribution, which then gave rise to the formationof the deep fault. That is supposed to be the deep structural setting for the Sanhe-Pinggu M8.0 earthquake.