Wenchuan Mainshock Research Articles

Static stress triggering effects related with M s8.0 Wenchuan earthquake

In this article, firstly, we calculated and analyzed the patterns of Coulomb stress changes induced by a sequence of strong earthquakes that occurred in Songpan (松潘), Sichuan (四川) Province in 1973 and 1976, and discovered that the Ms8.0 Wenchuan (6C76;川) earthquake of 2008 was epicentered in a relevant Coulomb stress triggering zone. This suggests that the Coulomb stress on the middle and southern segments of the Longmenshan (龙门山) fault zone increased after the Songpan sequence of strong earthquakes, and the stress increment might cause the 2008 Wenchuan earthquake having already occurred somewhat ahead of time. Further, we calculated and analyzed Coulomb stress changes coinduced by both the Songpan sequence and the Ms8.0 Wenchuan mainshock. The result shows that the Ms6.4 Qingchuan (青川) earthquake of May 25, 2008 on the northeastern segment of the Longmenshan fault zone was triggered by the Wenchuan mainshock, and that the southwestern segment of the fault zone is also in the stress triggering zone. Besides, the Maoxian (茂县)-Wenchuan fault (i.e., the back-range fault of the Longmenshan fault zone), which extends parallel to the seismogenic fault of the Wenchuan earthquake, is in a shadow zone of the Coulomb stress changes, and therefore, its potential hazard for producing a strong or large earthquake in the near future could be reduced relatively.

Structural heterogeneity of the Longmenshan fault zone and the mechanism of the 2008 Wenchuan earthquake (Ms 8.0)

We determined detailed 3‐D images of P and S wave velocity (Vp, Vs) and Poisson's ratio (σ) in and around the Longmenshan (LMS) fault zone by using a large number of P and S wave arrival times from the aftershocks of the 2008 Wenchuan earthquake (Ms 8.0) and other local events. Our results show that the structure of the LMS fault zone north of the Wenchuan main shock is very different from that south of the main shock. The southern section of the LMS fault zone contains a broad zone with low‐Vp, low‐Vs, and high‐σ anomalies, while the northern segment exhibits more scattered heterogeneities, corresponding to most of the aftershocks which occurred there. A prominent low‐Vp, low‐Vs, and high‐σ anomaly exists directly beneath the Wenchuan main shock hypocenter, suggesting that in addition to compositional variations, fluid‐filled fractured rock matrices may exist in the LMS fault zone, which may have influenced the generation of the large Wenchuan earthquake. Our tomographic results provide sound seismic evidence for the hypothesis that an upward intrusion of lower crustal flow occurred along the LMS fault zone. In addition, most small earthquakes before the 2008 Wenchuan main shock occurred around the Guanxian‐Jiangyou fault, while the Wenchuan aftershocks were mainly concentrated on the Yingxiu‐Beichuan fault, suggesting that the rupture process of the Wenchuan earthquake may belong to an out‐of‐sequence thrusting event, a suggestion which is in good agreement with the results from geological surveys and also quite similar to the rupture processes of the 1999 Chi‐Chi earthquake (M 7.5) and the 2005 Kashmir earthquake (M 7.6). A few aftershocks occurred close to the blind Guangyuan‐Dayi fault in the Sichuan basin, suggesting that this blind fault was also ruptured by the Wenchuan earthquake, consistent with geological surveys.

Fine Seismic Structure Under the Longmenshan Fault Zone and the Mechanism of the Large Wenchuan Earthquake

AbstractIn this study we used a large number of P‐wave arrival times recorded by the Sichuan permanent and portable seismic stations during January 2001 and June 2008 to obtain a fine three‐dimensional P‐wave tomographic model of the crust around the Longmenshan fault zone. The results show that there exist obvious differences between south and north of the Wenchuan mainshock in the seismic structure. North of the Wenchuan mainshock there exist much more heterogeneities, where many aftershocks occurred. These results have significantly improved over the previous results suggesting that the Longmenshan fault zone is only a transition zone between different blocks. The source area of the Wenchuan mainshock is underlain by obvious low velocity anomalies, suggesting that there exist fluids within the Longmenshan fault zone. These fluids may affect the occurrence of the large Wenchuan earthquake. The present results provide significantly seismological evidence for the upward intrusion of the lower crustal flow along the Longmenshan fault zone.