Abstract
The 1999 Chi-Chi, Taiwan earthquake (Mw 7.6) produced surface ruptures for about 90 km along the north-south trending Chelungpu fault, with surface displacements of up to 12 m. Based on the combination of nanoscopic investigation and geochemistry analysis of core samples from a 450 m long inclined borehole drilled through the slip zone, we suggest the dynamical processes that likely occurred in the northern portion of the Chelungpu fault during the faulting. Our analysis revealed that the frictional heating could have reached 1200 °C, which would cause most of the siderite in the fault gouge to evaporate, resulting in a large amount of nano-size siderite grains with a mean diameter 20 nm. These nano grains could have acted as a mechanical lubricant to reduce the dynamic frictional resistance during sliding, giving rise to the large but smooth type of slipping seen in the north.
Highlights
The shaking and displacement of the ground seen during earthquakes are the result of a sudden release of energy in the earth’s lithosphere that creates seismic waves which propagate through the lithosphere
We focus on the important question of what dynamic processes might have occurred within the fault zone during the slipping that would give rise to the heterogeneous large slipping at the northern end of the Chi-Chi earthquake with a high slipping velocity and large ground displacement
Smectite dehydrates at ~250 °C and pore water vaporizes at ~180 °C, indicating that elastohydrodynamic lubrication would only work for a limited depth range and at the beginning of the faulting process, when the frictional heating was not yet severe
Summary
The shaking and displacement of the ground seen during earthquakes are the result of a sudden release of energy in the earth’s lithosphere that creates seismic waves which propagate through the lithosphere. Out of the thousands of earthquake that occurred in Taiwan in the 20th century, the 1999 Chi-Chi earthquake (Mw 7.6) along the Chelungpu fault was the largest in magnitude It appears that the fault movement along the northern segment was marked by a high velocity (3–5 m/s) and large displacement (over 10 m); whereas a typical inland earthquake, with moderate velocities on the order of 1 m/s and displacements of several meters, was recorded in the southern segment near the epicenter[4,5,6,7,8,9]. The vertical uplift ranges from tens of centimeters to 3.5 m at the southern end, and gradually increases northward to 4 m at Takeng, with a maximum of 7 m near Shihgang[10,11]
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