Spatiotemporal changes of aftershocks and background seismicity in Tangshan and the rest of North China

Abstract:

The 1976 Great Tangshan earthquake (Ms 7.8) in North China was one of the deadliest earthquakes in modern history. Understandably, a sequence of moderate (M ≥ 4.0) earthquakes in recent years in the Tangshan seismic zone, including the recent Ms 5.1 Guye earthquake (2020/07/12) that occurred within the 1976 Tangshan earthquake rupture zone, raised social and scientific concerns about seismic risk near Tangshan and in North China, a region of intense intraplate seismicity. One controversy is whether or not these recent earthquakes are aftershocks of the 1976 Great Tangshan earthquake. The answer is uncertain, because in continental interiors aftershock sequences tend to last long and the levels of background seismicity are often poorly defined. In this study, we use a statistic method to separate clustered events (i.e., aftershocks) from background earthquakes in Tangshan and the rest of North China. The method is based on the nearest-neighbor distances of events in a space-time-energy domain. We found that the aftershock sequence of the Great Tangshan earthquake, based on relatively large (M ≥ 4) events, statistically ended around 2010, so the 2020 Guye earthquake is unlikely an aftershock. However, the Tangshan aftershock sequences based on smaller events continue to today. The spatial pattern of background seismicity in North China is relatively stable; the zones of relatively high seismicity are mostly tectonic zones and correlate to high strain rates measured by GPS. The temporal changes of background seismicity in North China are complex and not clearly related to the Great Tangshan earthquake except around its rupture zone, where changes of background seismicity seems to correlate with stress changes caused by the Tangshan mainshock and its large aftershocks.