Abstract
We measured the magnetic susceptibility of the core from the first borehole of the Wenchuan Earthquake (May 12, 2008, Mw 7.9) Fault Scientific Drilling Project (WFSD-1) at 1-cm intervals. The correlations between magnetic susceptibility anomalies and fault rock occurrence are shown by a few fault zones in the WFSD-1 core. The values for the mass and ferromagnetic material magnetic susceptibility for the sample at 589.25-m depth are higher than those for the other samples. All the thermomagnetic curves display a rapid increase in slope after 380°C, and a marked peak occurs at about 510°C in the heating curves. The cooling curves are clearly higher than the heating curves. The saturation magnetization (Ms) shows a significant peak at a depth of 589.25 m, as do the mass magnetic susceptibility and the ferrimagnetic magnetic susceptibility. The mechanism principally responsible for the high magnetic susceptibility at a depth of 589.25 m might be the production of new magnetite from iron-bearing silicates (e.g., chlorite) or clays caused by frictional heating during seismic slip. Therefore, we suggest that the presence of high magnetic susceptibility fault gouges in the same country rock can be considered as an indicator of earthquakes or seismic signatures.
Highlights
The magnetic properties of fault rocks can be used as tracers for physical and chemical alterations caused by frictional heating during earthquakes
We report the results of the magnetic susceptibility measurements for all core samples of the Wenchuan Earthquake Fault Scientific Drilling Project (WFSD)-1 borehole, present the magnetic susceptibility characteristics of the core samples, and discuss the difference between the core measurements and the logging data
Iron contamination In the WFSD-1 borehole, the high magnetic susceptibility values obtained from logging data near the primary slip zone, between the depths of 505 and 590 m and the depths of 675 and 695 m, seen on a larger scale, look similar to the patterns observed at a single fault zone on a small scale (Figure 5)
Summary
The magnetic properties of fault rocks can be used as tracers for physical and chemical alterations caused by frictional heating during earthquakes. Magnetic susceptibility and rock magnetism have commonly been used to understand the physical characteristics and chemical processes of fault slip zones (Enomoto and Zheng 1998; Nakamura and Nagahama 2001; Ferré et al 2005, 2012). Motivated by an interest in investigating the mechanism of the 2008 Wenchuan Earthquake, the Wenchuan Earthquake Fault Scientific Drilling Project (WFSD) was launched on November 4, 2008, only 178 days after the Wenchuan Earthquake (Xu and Li 2010). The first borehole (WFSD-1) of the project was drilled to investigate the mechanism of the southern segment of the Yingxiu-Beichuan Fault (Li et al 2012)
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