Coseismic velocity drop and postseismic recovery for two moderate earthquakes on the Tangshan fault zone

SUMMARY We employed seismic phase data recorded by the Hebei seismic network to determine the high-precision spatiotemporal distribution of Vp, Vs and Vp/Vs changes in the source area before and following the 2012 Ms4.8 Tangshan and 2020 Ms5.1 Guye earthquakes using a new time-lapse seismic tomography method. This method combines differential P, S and S-P arrival times to reduce errors caused by varying data coverage across periods, enabling robust imaging of seismic velocity variations. The characteristics and mechanisms of seismic velocity variations are investigated by integrating existing geological and geophysical models, yielding the following insights: 1) After the Ms4.8 Tangshan earthquake in 2012 and the Ms5.1 Guye earthquake in 2020, a decrease of the Vp and Vs in the source area was observed, with the region exhibiting this decline exhibiting a spatial correlation with the distribution of aftershocks and subsurface electrical structures. The observed reduction in seismic velocity is attributed to seismic damage and variations in the physical properties of the subsurface media. 2) Eight years following the 2012 Ms4.8 Tangshan earthquake, Vp and Vs exceeded its pre-earthquake level, revealing an observable velocity overshoot following coseismic damage. 3) The Vp/Vs remained largely stable, showing a slight decrease after both earthquakes, potentially pointing to their moderate magnitudes and the post-seismic dominance of dry cracks, although alternative mechanisms and imaging-related noise cannot be entirely ruled out.