Elsevier

Tectonophysics

Volume 584, 22 January 2013, Pages 23-42
Tectonophysics

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)

https://doi.org/10.1016/j.tecto.2012.08.021Get rights and content

Abstract

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.

Graphical abstract

Highlights

► 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

Introduction

The May 2008, Ms 8.0 Wenchuan earthquake occurred in the transition zone between the Tibetan Plateau and the Sichuan Basin, within the Longmenshan, yielding 270 and 80 km-long co-seismic surface ruptures along the Yingxiu–Beichuan and Anxian–Guanxian faults, respectively (Fu et al., 2011, Li et al., 2008, Liu-Zeng et al., 2010, Xu et al., 2009). This large earthquake caused disastrous damages but stimulated numerous research on active faults and their tectonic framework along the SW margin of the Tibetan Plateau. This study aims at helping to understand earthquake mechanisms capable of generating such catastrophic events.

In order to better understand the mechanical, physical and chemical characteristics of the faults that ruptured during the Wenchuan earthquake, the two main strands of the fault are currently being drilled under the auspices of the Wenchuan earthquake Fault Scientific Drilling project (WFSD), funded by the Chinese government alone.

Drilling in active fault zones, especially post-large earthquakes, provides important information on the physical characteristics and the formation of these highly-deformed geological features (Brodsky et al., 2009, Xu et al., 2008, Zoback et al., 2007) and will shed light on the geology of the fault zones and allow to correlate their structure and mineralogy with their seismological behavior during recent earthquakes (Boullier, 2011). The first drilling project following a large earthquake started after the 1995 Kobe earthquake (Nojima Fault Probe Project) (Oshiman et al., 2001). A few years later, the Taiwan Chelungpu fault Drilling Project (TCDP) started after the 1999 Taiwan Chi-Chi earthquake (e.g. Ma et al., 2006, Song et al., 2007).

The Wenchuan Fault Scientific Drilling project (WFSD) is the third drilling project into fault zones. The first borehole drilling (WFSD-1) started just 178 days after the earthquake, on November 6th 2008, and was completed by July 12th 2009, with a total depth of 1201.15 m and 95.4% of core recovery. It represents the most rapid response project to study a large earthquake and its aftershocks. Five boreholes ranging from 550 m to 3400 m-depth will ultimately be drilled along the Yingxiu–Beichuan and Anxian–Guanxian faults, targeting the locations of maximum co-seismic slip. WFSD-1 and WFSD-2, with depths of 1201.15 and 2283.56 m respectively, are located on the hanging wall of the southern Yingxiu–Beichuan fault zone in Bajiaomiao village, where the largest vertical displacement was about 6 m (Fu et al., 2011, Li et al., 2008, Liu-Zeng et al., 2010, Xu et al., 2009). The two other boreholes (WFSD-3P and WFSD-3) were drilled on the hanging wall of the Anxian–Guanxian fault in Qingquan village, Mianzhu City, with depths of 551.65 and 1502.18 m respectively (the maximum vertical displacement was about 4 m). WFSD-4 started to be drilled on August 6th 2012 and will reach a depth of about 3400 m. It is located along the northern part of the Yingxiu–Beichuan fault zone in the Nanba area, where the observed vertical displacement was about 1.5 to 2.5 m (Fu et al., 2011, Li et al., 2008, Li et al., 2010a, Li et al., 2010c, Liu-Zeng et al., 2010, Xu et al., 2009), and the dextral displacement was ~ 2 m (Li et al., 2008, Li et al., 2010a, Li et al., 2010c, Xu et al., 2009). The geological, chemical, and hydrological observations will be combined to provide a better understanding of rupture processes of large earthquakes.

The WFSD scientific goals have been and will be reached through various analysis of rock samples from the boreholes, online fluid geochemical monitoring, borehole geophysical logging, as well as direct stress and seismic observations, to better understand the following four aspects: 1) determination of the composition, texture and structure of the fault zones; 2) reconstruction of the physical and chemical properties of the fault zones, such as including fluid pressure, permeability, seismic velocities, electrical resistivity, density, porosity, etc.; 3) better understanding of the mechanism of the fault that ruptured during the Wenchuan earthquake; 4) long-term monitoring from instruments installed in the boreholes after drilling completion.

This paper reports preliminary data on the lithology, stratigraphy, fault-related rocks and structures of WFSD-1 drill cores, as well as the geometry of observed fault zones and Principal Slip Zone (PSZ) of the Wenchuan earthquake.

Section snippets

Longmenshan tectonics

The Longmenshan lies at the eastern margin of the Tibetan Plateau, west of the Sichuan Basin, and is marked by a steep topographic gradient and a high topographic relief of 3000–4500 m. The Longmenshan is the topographic boundary between eastern and western China, and is also the central segment of the NS-trending seismic belt in China, as well as one of the most active seismic zones (Deng et al., 1994). The Longmenshan fault zone is the easternmost thrust belt of the Songpan–Ganzi orogen (Xu et

Drilling overview

The first hole of the Wenchuan earthquake Fault Scientific Drilling (WFSD-1) (N31.149°, E103.691°) is located in Bajiaomiao village, Hongkou county (Dujiangyan, Sichuan), on Pengguan complex rocks, which represent the hanging wall of the Yingxiu–Beichuan fault zone (Fig. 1, Fig. 2). It is located 385 m west of the Wenchuan earthquake surface rupture (Fig. 2), in order to meet the Wenchuan earthquake fault zone at as shallow depth as possible and to study the characteristics of the fault zone and

Core description

At WFSD-1, above 285 m-depth, the core diameter is 76 mm-wide; from 285 to 810 m-depth, it is 67 mm-wide; and below 810 m-depth, it is 46 mm-wide. The cores consist of diorite and porphyrite, volcanic rocks, pyroclastics, sandstone (including coal-bearing sandstone), siltstone, shale, liquefied breccia (soft sediment deformation, Qiao et al., 2012) (Fig. 4, Fig. 5), and a series of fault-related rocks (Fig. 6, Fig. 7). The diorite, volcanic rocks and pyroclastics make most of the Neoproterozoic

General description

Most of the WFSD-1 cores are typically broken, containing many calcite veins, fault breccias, cataclasites and fault gouge with different orientations. There are also various kinds of slickenside in some fault planes indicating different tectonic features (Fig. 7), and many polished gouge fault surfaces (“mirror structure”) (Fig. 7d, e, f). Calcite veins formed in the fault zones and along fractures, and steps can be seen occasionally on fault planes displaced along the veins (Fig. 7g).

Structure and width of the Yingxiu–Beichuan fault zone

It is generally acknowledged that there are two kinds of typical fault zone structures (Faulkner et al., 2010): (a) a single high-strain core surrounded by a fractured damage zone (Chester and Logan, 1986) and (b) multiple cores model, where many strands of high-strain material enclose lenses of fractured protolith (Faulkner and Rutter, 2003). The fault core generally consists of gouge, cataclasite or ultracataclasite (or a combination of these), and the damage zone generally consists of

Conclusions

The Wenchuan earthquake Fault Scientific Drilling project (WFSD) provides a unique opportunity to investigate the faulting mechanism of the Wenchuan earthquake that struck the Longmenshan fault zone in 2008. The following conclusions have been obtained by analyzing the cores from the 1201 m-deep WFSD-1 borehole and interpreting the logging data.

  • (1)

    Three types of fault-related rocks are present in the WFSD-1 cores: fault gouge, cataclasite, and fault breccia. The core profile contains at least 12

Acknowledgments

The authors thank Shiyou Hu, Wei Zhang, Jun Jia and Lasheng Fan of the Wenchuan Earthquake Fault Scientific Drilling Centre for their help. The authors also thank the field drilling workers and well logging by the 403 and 405 geological survey teams of the Sichuan Geology & Mineral Resources Bureau, respectively. We thank T. Shimamoto, Lu Yao and Yu Wang for helping us with reexamination of our fault-zone data. This work was supported by the “Wenchuan Earthquake Fault Scientific Drilling” of

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