Background seismicity before and after the MS7.8 Tangshan earthquake in
1976: Is its aftershock sequence still continuing?
Abstract
The aftershock zone of the 1976 7.8 Tangshan earthquake, China, remains
seismically active, experiencing moderate events such as the December 5,
2019,4.5 Fengnan event. It is still debated whether aftershock sequences
following large earthquakes in low seismicity continental regions can
persist for several centuries. To understand the current stage of the
Tangshan aftershock sequence, we analyse the sequence record and
separate background seismicity from the triggering effect using a
finite-source epidemic-type aftershock sequence (FS-ETAS) model. Our
results show that the background rate notably decreases after the
mainshock. The estimated probability that the most recent 4.5 earthquake
(December 5, 2019, Fengnan District, Tangshan) is a background event is
63.8%. This indicates that the contemporary seismicity in the Tangshan
aftershock zone can be characterised as a transition from aftershock
activity to background seismicity. Although the aftershock sequence is
still active in the Tangshan region, it is largely overridden by
background seismicity.