Coseismic ruptured creeping Guanxian-Anxian fault during the 2008 Mw 7.9 Wenchuan earthquake (China)

The 2008 Mw7.9 Wenchuan earthquake occurred in the eastern margin of the Tibetan plateau (China) and propagated simultaneously along two sub-parallel faults of the Longmen Shan thrust belt: for 270 km along the locked Yingxiu-Baichuan Fault (YBF, max coseismic slip 14 m) and for 80 km along the creeping Guanxian-Anian Fault (GAF, max coseismic slip 4 m).

We carried out multiscale structural analyses of the fault zone rocks from the outcrops of YBF and GAF, and from the cores of the Wenchuan earthquake Fault Scientific Drilling project. Here we focus on the GAF's structure and physical properties in order to understand why did the rupture propagate along a creeping fault.

The GAF has a 40 m thick foliated cataclastic clay-rich fault core made of up to 50% clay minerals (smectite/illite), partly dissolved clasts and mineral grains (e.g., quartz and albite) cut by Riedel shears and pressure solution seams. High magnification (scanning and transmission electron microscopes) microstructural observations combined with X-ray diffraction and X-ray fluorescence element scanning analysis show that most clay minerals formed by pressure solution and precipitation processes. The nucleation and growth of clays in the fault core prevents soluble grains from coalescing, thereby maintaining fast diffusive paths along solution seams and promoting strain accommodation by pressure solution processes. Both frictional sliding along the "weak" and "velocity-strengthening" clays and pressure solution processes may result in long term aseismic creep of the GAF. Moreover, the high concentration of Fe ²⁺ and S in the fault gouge implies that creeping along the GAF occurs at a low temperature and in a reducing environment. This creeping behavior is consistent with GPS velocity data that show that the slip rates of the GAF were respectively 1.5 and 12 mm/yr during preseismic (1998-2008) and postseismic (2009-2011) periods. Based on P wave velocity and strata distribution, we propose a model in which the GAF is creeping at shallow depths and locked at greater depths to explain the large coseismic slip during the 2008 Wenchuan earthquake.