The Precursory Significance of Tectonic Stress Field Transformation Before the Wenchuan MW7.9 Earthquake and the Chi‐Chi MW7.6 Earthquake

The 2008 Wenchuan earthquake and the 1999 Chi-chi earthquake are both intraplate thrust fault earthquakes occurred under the tectonic setting of thrust-nappe. 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 type focal mechanism of the Chi-Chi mainshock coincides with the 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 mainshock region, the strike slip type 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 fault type focal mechanism of 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 Wenchuan mainshock is consistent with the tectonic stress field. When the local stress field is consistent with the thrust fault type stress field, the Wenchuan mainshock occurred immediately. With the stress accumulation near the source region, there are similar local stress field transformation phenomena before the thrust fault mainshock. When the local stress state transformed to the stress state of mainshock, the stress level is up to very high near the source region. It may be a precursor of large earthquake and must arouse further concerns.