Exploration of Laoyachen fault in Zheng-zhou and its activity investigation

In the paper, an exploration is made by the techniques of shallow seismic prospecting, borehole composite profile analysis, field morphological investigation, and new geological dating for the Laoyachen fault in Zhengzhou that was originally recognized as an active one. The result from shallow seismic prospecting indicates this fault only existed in the stratum prior to the Neogene, dislocations and activity traces of this fault could not be found in the Neogene stratum. At the same time, the surface morphological investigation reveals that the location of the so-called "fault scarp" is not consistent with that of Laoyachen fault. Moreover, seismic drilling and borehole composite profile analysis also indicate that the scarp only developed in Malan Loess. The stratum beneath it is gentle and no dislocation traces can be seen. Therefore, we propose that this scarp is not relevant to the Laoyachen fault, which might be resulted from the diverted course of Yellow River. Laoyachen fault is not an active one.

Activity of the Laoyachen fault, which was previously recognized as an active fault, is investigated by means of shallow seismic exploration, borehole composite profile analysis, geomorphologic survey and geological dating. The seismic exploration result indicates that this fault only appears in the strata before Neogene, while it has not cut through the Neogene strata themselves. The surface geological and geomorphologic survey show that the position of socalled “fault scarp” is not consistent with the outcrop of the Laoyachen fault. Moreover, the result of drilling and borehole composite profile analysis denotes that the scarp only grew in the Malan Loess, and the strata below kept gentle without offsetting. We think that the formation of this scarp may be associated with the course change of the Yellow River, rather than being related to the motion of the Laoyachen fault. Therefore, the Laoyachen fault is not an active fault.