Recurrence Characteristics of Major Earthquakes in the Tangshan area, North China

The Tangshan area lies in the North China plain where an Ms 7.8 earthquake occurred in 1976, which is associated with a hidden active fault. To reveal the recurrence characteristics of major quakes in this area over a relatively long time, we have conducted a comprehensive study using geological investigations, shallow seismic exploration, boreholes, trench observations and geological dating. Five paleoearthquakes were recognized in a 6.4m-deep trench west to the Tangshan Asylum. Among them, the former three events occurred between 56.78 +/- 4.83ka and 89.39 +/- 7.60 ka, and the fourth event occurred around 6.9 ka, respectively, and then followed by the fifth in 1976. Seven boreholes were deployed crossing the ground fissure formed by the 1976 Tangshan earthquake at the site of No. 10 Middle School, where we have identified 25 liquefaction events in the boreholes TZC65 and 67. By the comprehensive analysis of the trench, the liquefaction events from the boreholes and the depth-time curves of drill cores, we suggest a new recurrence model of major quakes in this area. It is not a constant recurring cycle since 210 ka, instead consisting of six alternating seismically quiet and active stages. Of them, stage I (>177 ka) was a quiescent period in seismicity, stage II (from 143 ka to 177 ka) was an active one, stage III (from 102 ka to 143 ka) was quiescent again, stage IV (from 56 ka to 102 ka) had many quakes, stage V (from 6.9 ka to 56 ka) became quiet, and stage VI (from 6.9 ka to now) was the beginning of a new seismically active period.