AFTERSHOCKS AND BACKGROUND SEISMICITY IN TANGSHAN AND THE REST OF NORTH CHINA

The 1976 Great Tangshan earthquake (Ms 7.8) in North China was the deadliest earthquake in the past century. Understandably, a sequence of moderate (M ≥ 4.5) earthquakes in recent years in the Tangshan region, including the Ms 5.1 earthquake on 12 July 2020, raised much social concern and scientific debate about the seismic risk near Tangshan and in North China, a region of intense intraplate seismicity. Are these recent events aftershocks of the 1976 Great Tangshan earthquake or are they background earthquakes? Here we separated clustered events (i.e., aftershocks) from background earthquakes in Tangshan and the entire North China using the nearest neighbor (NN) method, and estimated the duration of the 1976 Tangshan aftershock sequence by fitting the decay of seismicity with respect to the background seismicity. Our results suggest that the recent moderate earthquakes are likely aftershocks. This is consistent with their occurrences in places of increased Coulomb failure stress due to the 1976 Great Tangshan earthquake, which suggests that they are triggered events. The estimated aftershock duration is around 65-100 years for the 1976 Great Tangshan earthquake. The background seismicity in North China, obtained by removing aftershocks identified by the NN method, is relatively stationary in space but varies in time, increasing in recent years in the North China Plain. Major active tectonic zones, including the Shanxi rift and the Zhangjiakou-Penglai fault system, show correlation between relatively high background seismicity, high geodetic strain rates, and large historic earthquakes. Such correlation, however, is poor within the North China Plain, highlighting the complexity of intraplate earthquakes.