Structural Features And Stress Field Analysis Of The Seismological Faults For The1668M81/2Tancheng Earthquake

Faults causing1668M81/2Tancheng earthquake extend from Yaoshang, Tancheng in the south to guanshuai, Juxian in the north, and is as long as160km. The earthquake faults consist of6NNE-striking active faults, each of which shows as a line in NNE on the remote sensing images. Except the Guanshuai-Chegushan fault segment, the rest coseimic horizonal displacement of the Tancheng earthquake faults shows decreasing trend from the middle to the both sides, however the vertical displacement shows the increasing trend. The southern segment of the Tancheng earthquake faults goes along the F2fault and dips SEE with dip angles of30°60°. The northern segment of the faults is close to the F1fault, and shows varied dips steeply with dip angles mostly higher than70°. The southern segment is dextral thrusting or dextral with thrusting components whereas the northern segment is dextral mostly. The both southern and northern segments are made up of gouge belt, breccia belt and cataclastic belt. Widths of the southern segment range from several meters to more than ten meters while those of the northern segment are from tens of meters to nearly one hundred meters. Stress inversion from measured fault-slip data shows co-seismic ENE-WSW compression for the northern segment and NE-SW compression for the southern segment, and the regional compression stress field is responsible for the earthquake occurrence. The stress orientation variation may result from a geometry change of the earthquake faults. The co-seismic stress field is consistent with present regional stress field, indicating that there was no post-seismic stress adjustment after the Tancheng earthquake or post-seismic stress adjustment after the Tancheng earthquake did not affect the present stress field. Arrangement and geometry of the Tancheng earthquake faults suggest that it originated from reactivity of pre-existing faults rather than newly formed faults, the southern segment originated from reactivity of F2fault, and the northern segment originated from reactivity of F1fault. According to the section of electrical exploration of the Yishu faults zone, and epicenter and hypocenter of the earthquakes along Yishu faults, the fault segments, dipped in SSE, is the action of the thrust fault with the opposite dip in shallow part, which cut the F1faults, and is the action of the F1fault in the deep part.