Study Of The Western Kunlunshan Moutain Pass Earthquake Sequency And Its Relative Problems

On November 14, 2001 (at 09:26:14.7 UTC), a Mw7.8 earthquake occurred in theWestern Kunlunshan mountain(at36.2oN,90.9oE) at the border of Qinghai, Xinjiangand Xizang (Tibet). This earthquake has attracted the attention of many seismologists in the World. There are two important questions: one is what is the properties of the seismicity before the earthquake, the other is about the character of the aftershock sequence. The completeness of the catalogue of earthquakes with magnitude over 3.0 can be confirmed by different independent tests such as minimum magnitude distribution, frequency-magnitude distribution and the Rydelek-Sacks test. Using rank-ordering statistical analysis, we can find that the aftershocks sequence could be divided into two parts, with the b-value of the aftershocks of magnitude 3 to 5 being near to 1.0, and that of events of magnitude 5 to 6 being near to 0.5. This observation is, to some extent, similar to that of Knopoff(2000) who found that for the declustered earthquakes in southern California, there exists a characteristic length corresponding to the fault size of a magnitude 5 earthquake, challenging the scale-invariance as proposed by some models such as self-organized criticality(SOC). The frequency-magnitude relation observed here implies that the aftershocks may be divided into two groups. The events with magnitude over 5 are aftershocks, while others with magnitude from 3 to 5 or even smaller can be regarded as the 'aftershocks of aftershocks'. We also discuss the question how the b-value and the Benioff strain change with the time before the great earthquake and its relation with the earthquake. We found that before this great earthquake the seismicity were accelerating in long term, but there is quiescent in short term. And the moderate earthquakes are increasing and the big earthquakes are decreasing. These two trends are consistent.