Slip zone and energetics of a large earthquake from the Taiwan Chelungpu-fault Drilling Project

Kuo-Fong Ma, Hidemi Tanaka, Sheng-Rong Song, Chien-Ying Wang, Jih-Hao Hung, Yi-Ben Tsai, Jim Mori, Yen-Fang Song, Eh-Chao Yeh, Wonn Soh, Hiroki Sone, Li-Wei Kuo (+1 others)
<span title="2006-11-23">2006</span> <i title="Springer Nature"> <a target="_blank" rel="noopener" href="https://fatcat.wiki/container/drfdii35rzaibj3aml5uhvr5xm" style="color: black;">Nature</a> </i> &nbsp;
Determining the seismic fracture energy during an earthquake and understanding the associated creation and development of a fault zone requires a combination of both seismological and geological field data 1 . The actual thickness of the zone that slips during the rupture of a large earthquake is not known and is a key seismological parameter in understanding energy dissipation, rupture processes and seismic efficiency. The 1999 magnitude-7.7 earthquake in Chi-Chi, Taiwan, produced large slip
more &raquo; ... to 10 metres) at or near the surface 2 , which is accessible to borehole drilling and provides a rare opportunity to sample a fault that had large slip in a recent earthquake. Here we present the retrieved cores from the Taiwan Chelungpu-fault Drilling Project and identify the main slip zone associated with the Chi-Chi earthquake. The surface fracture energy estimated from grain sizes in the gouge zone of the fault sample was directly compared to the seismic fracture energy determined from near-field seismic data 3,4 . From the comparison, the contribution of gouge surface energy to the earthquake breakdown work is quantified to be 6 per cent. The North-South-trending Chelungpu fault is a major 90-km structure that dips shallowly to the east (30u), and principally slips within, and parallel to, bedding of the Pliocene Chinshui shale 5 . Taiwan Chelungpu-fault Drilling Project (TCDP) drilled two vertical holes 40 m apart (hole A to a depth of 2 km, and hole B to a depth of 1.3 km), and a side-track from hole B (hole C) at the depth of 950 m to 1,200 m about 2 km east of the surface rupture, near the town of DaKeng (Fig. 1a) . The subsurface location of the Chinshui shale was known from high-resolution seismic reflection profiles 6,7 at a depth of about 1,000 m under the DaKeng site. The spatial slip distribution for the earthquake was well constrained from close strong motion stations and Global Positioning System (GPS) data 3,4 and shows a slip of 8.3 m on the fault near the drill site. The drilling carried out continuous coring for depths of 500-2,000 m for hole A, 950-1,300 m for hole B and 950-1,200 m for hole C, respectively. Geophysical well logs were carried out in hole A to collect seismic velocities, densities and digital images. From the hole-A core, the Chelungpu fault zone is seen within the Chinsui shale as a damaged zone at depths of about 1,105 to 1,115 m, consisting of fault breccia and fault gouge (Fig. 1b) . The degree of fracturing increases from the top to the bottom of the zone. Near the bottom of the broad zone of deformation, a 12-cm-thick primary slip zone (PSZ) can be identified based on the presence of ultra-finegrained fault gouge and increased fracture density at depths of 1,111.23 to 1,111.35 m. A corresponding feature was also found in the hole-B core at depths of 1,136.50 to 1,136.62 m, confirming the fault dip of 30u E. The geophysical logging measurements of low
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