Details

Autor(en) / Beteiligte

• Chou, Yu‐Min
• Aubourg, Charles
• Yeh, En‐Chao
• Song, Sheng‐Rong
• Lin, Yi‐Kai
• Humbert, Fabien
• Jiang, Xiaodong
• Lee, Teh‐Quei

Titel

The Magnetic Fabric of Gouge Mimics the Coseismic Focal Mechanism of the Chi‐Chi Earthquake (1999, Mw 7.6)

Ist Teil von

• Geophysical research letters, 2020-11, Vol.47 (22), p.n/a

Ort / Verlag

Washington: John Wiley & Sons, Inc

Erscheinungsjahr

2020

Quelle

Wiley Online Library All Journals

Beschreibungen/Notizen

• We present a magnetic fabric investigation of small oriented rock fragments from gouges and wall rocks from the Chelungpu fault (Taiwan), where the Chi‐Chi earthquake occurred (Mw 7.6, 1999). Samples are obtained from surface outcrops and at a depth of ~1 km, including the 3‐mm‐thick Chi‐Chi principal slip zone. Wall rocks at depth show magnetic foliation roughly parallel to bedding, whereas they are oblique at the surface. Within the gouges, the mean shape of magnetic ellipsoid is oblate, and the degree of anisotropy is up to 12%. The magnetic foliation diverges of ~40° from the fault plane, compatible with a simple shear westward. The magnetic lineation is oblique, resulting probably from coseismic and interseismic records. The reconstructed focal mechanism from the magnetic fabric closely matches the focal mechanism solutions for the Chi‐Chi earthquake. Our work demonstrates that magnetic fabric of gouge fragments can bring valuable kinematic information on modern earthquakes. Plain Language Summary Fault gouges often provide important and valuable information that can help to understand the fault mechanisms and to assess the seismic hazard, but to get access to the shape preferred orientation of micrometric minerals is generally a complex, time consuming and expensive procedure. In this study, we show that the anisotropy of magnetic susceptibility, which is a fast and nondestructive technique, can be a proxy of the structural fabric of historical gouges. This technique has already been previously applied to gouge zones, but this is the first time that it is used in the gouge from one of the best documented historical large seismic events: the 1999 Chi‐Chi earthquake (Mw 7.6) along the Chelungpu fault (Taiwan). Samples within and around this fault were obtained from fresh surface outcrop and 1‐km‐deep borehole (Hole B, Taiwan Chelungpu‐fault Drilling Project). The 1‐km‐deep gouge hosts the 3‐mm‐thick principal slip zone of the Chi‐Chi earthquake. The results show that the reconstructed focal mechanism based on the magnetic fabric geometry matches closely the focal mechanism solutions for the Chi‐Chi earthquake estimated by several previous studies. Key Points For the first time, the bulk fabric of small fragments of historical gouge, including the 1999 Chi‐Chi earthquake (Mw 7.6) slip zone, has been analyzed successfully The bulk fabric of gouge is anisotropic, reflecting the grain orientation of billions of clays and magnetic particles The magnetic fabric of gouge mimics the focal mechanism solutions of historical earthquake We provide a new tool to elucidate earthquake properties from fault gouges