DEFORMATION FABRICS, MINERALOGY AND NANOMETRIC CHARACTERISTICS IN THE DRILLED CORES OF 1999 RUPTURED CHELUNGPU FAULT, TAIWAN

The 1999 Chi-Chi earthquake (Mw 7.6) in central Taiwan produced about 90 km surface ruptures along the well-mapped north-south trending, west-vergent Chelungpu fault, with displacements up to 12 m at surface. Teleseismic data revealed very different seismic slips along the fault during the earthquake. In the north, the fault movement was marked by high velocity (3-5 m/s), large displacement (over 10 m), and normal ground acceleration; whereas it was of a typical inland earthquake in the south, with velocities on the order of 1 m/s, displacements of a few meters, and accelerations of exceeding 1 G.. To understand first-order difference in deformation mechanisms that dominate the contrasting amount of slip between the northern and southern segments, two shallow boreholes drilled during 2000 to 2001 were attempted by collecting cores and basic measurements.

Here, we report on the results of analysis on the core samples from a 450 m long inclined borehole through the northern portion of the Chelungpu fault zone near Fengyuan city. Near the drilling site surface rupture lies in the vicinity of the boundary between the Chinshui Shale and Kuechulin Formation, which is composed of fine-grained sandstone to siltstone and mudstone. Two major fault zones that possibly slip during the Chi-Chi earthquake were identified among fault zones characterized by breccia and fault gouge. Both meso- and microscopic deformation mechanisms show that breccia zones grow in thickness with increasing the degree of deformation. Fault gouge developed from non-foliated in both matrix and pods of broken fragments to zones of rich in clay-size particles and S-C foliations as the degree of deformation and strain localization increase.

Primary minerals including quartz, plagioclase, calcite, siderite, and clay minerals are common both in the protolith and fault zones. The grain-morphology of the clay-size particles was revealed by using an atomic force microscope (AFM). Nanoscopic investigation revealed that fractional heating might have reached 1200oC, that caused most of the siderite in the fault gouge to evaporate and resulted in a large number of nano-size siderite grains of mean diameter 20+-5 nm. These nano grains might have acted as lubricants that reduced the dynamic frictional resistance during sliding, hence gave rose to large but smooth slips in the north.