Characteristics of the fault-related rocks, fault zones and the principal slip zone in the Wenchuan Earthquake Fault Scientific Drilling Project Hole-1 (WFSD-1)

Scientific drilling in active faults after a large earthquake is ideal to study earthquake mechanisms. The Wenchuan earthquake Fault Scientific Drilling project (WFSD) is an extremely rapid response to the 2008 Ms 8.0 Wenchuan earthquake, which happened along the Longmenshan fault, eastern margin of the Tibetan Plateau. In order to better understand the fault mechanism and the physical and chemical characteristics of the rocks, the WFSD project will eventually drill 5 boreholes along the two main faults. This paper focuses on the first hole (WFSD-1), which started just 178 days after the earthquake, down to a final depth of 1201.15 m. Petrological and structural analyses of the cores allowed the identification of fault-related rocks in the Yingxiu–Beichuan fault (fault gouge, cataclasite, and fault breccia), and the Principle Slip Zone (PSZ) location of the Wenchuan earthquake was determined.We found 12 fault zones in the entire core profile, with at least 10, including the Yingxiu–Beichuan fault zone, with a multiple cores structure and minimum width of ~ 100 m. The co-seismic slip plane of the Wenchuan earthquake at depth (corresponding to the Yingxiu–Beichuan fault zone at the outcrop), as well as its PSZ, was expected to be located at the bottom of the fault zone (at 759 m-depth). Instead, it was found at ~590 m-depth with 1 cm-wide fresh fault gouge, as determined by logging data such as temperature, natural gamma ray, p-wave velocity and resistivity, combined with the fresh appearance, magnetic susceptibility, and microstructure of the gouge. The Wenchuan earthquake slip plane has a dip angle of ~ 65°, showing the high-angle thrust feature. The distribution of fault gouge with several meters thick, the location of the Wenchuan earthquake's PSZ and the thickness of fresh gouge all imply a correlation between the width of the fault zone and the number of seismic events.

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► WFSD-1 borehole started shortly (178 days) after the Wenchuan earthquake down to 1201 m-depth
► The Wenchuan earthquake alone produced ~1 cm thick fresh fault gouge
► At least 12 fault cores/zones were discovered and constitute the Yingxiu–Beichuan fault zone with multiple cores structure (~ 100 m-wide)
► The principal slip zone is located at ~590 m-depth, with a slope of 65°
► The fault gouge is 3.79 m-thick, attesting the long-term high seismic activity of the fault