Comment on "P-Wave Back-Azimuth and Slowness Anomalies Observed by an IMS Seismic Array LZDM" by Chunyue Hao and Zhong Zheng

Abstract

At the LZDM seismic array, Hao and Zheng (2010) observed extremely large back-azimuth and slowness deviations for teleseismic P waves, the maximum values of which were 87.1° for back-azimuth deviation and 8.68 s/° for slowness deviation. Hao and Zheng argued that the majority of the observed back-azimuth and slowness deviations could be attributed to a 9°-dip Moho beneath the LZDM array, which seems to be too small to produce the observed large deviations. The purpose of this comment is to show that their forward modeling of the back-azimuth and slowness deviations caused by dipping interface is incorrect, which as a result, gives rise to a seriously underestimated dip angle. The true attitude of the dipping interface is extracted from the observed zero-slowness rays. Assuming the same P -wave velocities as those used by Hao and Zheng (2010), a northeast-dipping interface with a dip of 57° is obtained. The theoretical back-azimuth and slowness deviations for this mode with greater dip calculated by a validated forward modeling program appear to be more consistent with the observed deviations. It is also argued that despite the consistency, previous studies of explosion seismology and receiver function in this area do not support such a steep northeast-dipping Moho interface. The dip interface is more likely to be located in the shallow crust. More information and further investigations are necessary to draw a final conclusion.