In-situ stress measurements along the Beichuan-Yingxiu fault after the Wenchuan earthquake

Using hydraulic fracturing method, we determined in-situ stress after the Wenchuan earthquake at Yingxiu, Leigu, and Yong'an sites along the Beichuan-Yingxiu fault which is one of the three active faults of the Longmenshan fault belt and defines the western margin of the Sichuan Basin. The measurements reveal that the crustal stress level is relatively high in the Yingxiu region but relatively low in the Leigu and Yong'an regions. The stress field along the Beichuan-Yingxiu fault is dominated by horizontal stress within the depth range of most of the measurements. The stress regimes are conducive to thrust faulting in the Yingxiu and Yong'an regions. However, the stress regimes at shallow and deep depths in the Leigu region are conducive to thrust faulting and strike-slip faulting, respectively. The ratios of the maximum to minimum effective stresses in these three sites, as well as their implications for fault activity and geological hazards, are estimated by combining Coulomb friction failure criteria and Byerlee's law with frictional properties of the geological units along the Beichuan-Yingxiu fault. We conclude that most of the measured ratios in the Yingxiu region exceed the expected limit for the reactivation of optimally oriented faults with coefficients of friction ranging from 0.4 to 0.6. This means that sections of the Beichuan-Yingxiu fault in this region show high chance of being reactivated. However, most of the measured ratios at the Leigu and Yong'an sites do not exceed the expected limit with coefficients of friction within the same 0.4-0.6 range, indicating that sections of the Beichuan-Yingxiu fault in the Leigu and Yong'an regions stand less chance of being reactivated in the future. Turning to geological hazards, we suggest that the tectonic faults, the endogenic condition inducing geological hazards, stand less chance of inducing geological hazards in the Leigu and Yong'an regions. The exogenic conditions, such as topography, lithology, and rainfall should be studied in this region. Because fault reactivation in the Yingxiu region is more likely, both endogenic conditions and exogenic conditions should still be studied there.