Aftershock duration of the 1976 MS7.8 Tangshan earthquake and implication for seismic hazard estimation

The disastrous MS7.8 earthquake in Tangshan, China, on 28 July 1976 caused great damage and significant casualties. The mainshock was followed by two large aftershocks: the MS7.1 eventto the northeast near Luanxian 15 h after the mainshock, and the MS6.9 event on 15 Novembersouthwest of Tangshan near Ninghe. The aftershock sequence lasts to date, making regional seismic activity around the Tangshan main fault much higher than that of before the mainshock. It is always difficult to accurately determine the duration time of aftershock sequences and identify the aftershocks from main event catalog. In this study, by using theoretical inference and empirical relation, we tried to derive the time length of aftershock sequences of the MS7.8 Tangshan earthquake. The estimated aftershock duration from a log-log regression is about 120 years according to the empirical Omorirsquo;s relation (Toda et al). Based on Dietrich theory, it has been claimed that the aftershock duration is a function of remote shear stressing rate, normal stress acting on the fault plane, and fault frictional constitutive parameters (Dieterich). In general, shear stressing rate could be estimated in three ways: ① Shear stressing rate could be written as a function of static stress drop and a mean earthquake recurrence time (Dieterich). In this case, the time length of aftershock sequences is about 70mdash;100 years. However, the recurrence time inherits a great deal of uncertainty. ② Ziv and Rubinderived a general function between shear stressing rate, fault slip speed and fault width with a consideration that aftershock duration does not scale with mainshock magnitude. Therefore, from Zivrsquo;s consideration, the aftershock duration is about 80 years. ③ Shear stressing ratecan also be described as a function of background seismicity rate (Console et al). From this approach, a two-dimensional spatial distribution of the aftershock duration can be obtained, and the resultant time length of aftershock sequences is about 130mdash;160 years, which is much longer than that given by previous two approaches. The conclusive noting gives that, for the 1976 MS7.8 Tangshan earthquake, the triggered aftershocks would last at least 70 years, implying that the current earthquakes which occurred in Tangshan region after the 1976 mainshock are the aftershocks of the main event.