| The 2008Wenchuan earthquake and the 1999 Chi-Chi earthquake, both of which brought great disasters to human society, occurred in similar regional tectonic settings and have a similar seismogenic fault. The 1999 Chi-Chi earthquake (Mw7.6) is the biggest intraplate earthquake in the Cenozoic orogenic zone of Taiwan Region in recent hundred years. The earthquake surface ruptures along the Chelungpu fault and its focal mechanism show that it is a typical reverse faulting event. The 2008 Wenchuan earthquake (Mw7.9), the biggest earthquake in Sichuan history, occurred on the middle segment of the Longmenshan thrust belt between the Bayan Har and South China blocks at the eastern margin of the Tibetan plateau. The complex earthquake surface ruptures also show that the Wenchuan earthquake is an extraordinary thrust-faulting event in the world.
Due to the particular importance of the two thrust-faulting earthquakes, a lot of researches have been performed in the surface ruptures, coseismic slips, fault structure geometry, focal rupturing process, deep structure background, near-fault strong ground motion, building damage features and so on. By far there is no research on whether or not the tectonic stress field has a similar transformation pattern before these two earthquakes. To some extent the focal mechanism solutions in different phases before a great earthquake may reflect tectonic stress field change over time, which is a clue to find out precursors for an earthquake nucleation. This article will analyze focal mechanism solutions in different periods before Wenchuan and Chi-Chi earthquakes, and discuss the variation rule of tectonic stress field before thrust-faulting earthquakes under compressional tectonic setting and its possible precursor significance. This might be used for predicting occurrence time and location of an impending thrust-faulting earthquake in the future.
Through comparative analysis on CMT solutions, focal mechanism solutions of small and moderate earthquakes and the tectonic stress field derived from focal mechanisms near the mainshcok region, we found that the thrust-faulting focal mechanism of the Chi-Chi mainshock coincides with the regional tectonic stress field from the CMT solutions of earthquakes occurred in Taiwan central and western inland; but for the small and moderate earthquakes randomly occurred near the eipcenter of the mainshock, the strike-slip focal mechanism of regional stress field is not consistent with the tectonic stress field before the Chi-Chi mainshock; the Chi-Chi mainshcok occurred a few months after the local stress field became consistent with the tectonic stress field. Similarly, the compressive stress axis of the thrust faulting focal mechanism of the Wenchuan mainshock coincides with the compressive stress axis of tectonic stress field determined by the CMT solutions of earthquake occurred in the eastern part of Qinghai-Tibet Plateau, the local stress field from the focal mechanisms of small and moderate earthquake near the Wenchuan mainshock is consistent with the tectonic stress field. When the local stress field is consistent with the thrust faulting stress field, the Wenchuan mainshock occurred immediately. With the stress accumulation near the epicenter, there are similar local stress field transformation phenomena before the thrust faulting great earthquake. When the local stress state is transformed to the stress state of the mainshock, the stress level is up to very high near the epicenter. It may be a precursor of large earthquake and must arouse further concerns.
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