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The Taiwan Chelungpu‐fault Drilling Project penetrated three fault zones as the Chelungpu fault system, which slipped during the 1999 Chi‐Chi earthquake, discovering disk‐shaped black material (BM disk) within the middle and lower fault zones in Hole B. The microscopic features of the BM disks indicated that they were pseudotachylytes, and they showed high magnetic susceptibility, possibly the result of intense shearing or high temperature conditions. Inorganic carbon content of the BM disks was low, possibly because of thermal decomposition of carbonate minerals. The high temperatures might be related to frictional heating during the earthquake, implying that the BM disks were produced under intense shearing with frictional heating that reached melting temperature. Because the disks, which provide the only evidence of melting, pre‐date the 1999 earthquake, we concluded that frictional melting did not occur during the earthquake.

Integrated Ocean Drilling Program Expedition 301 was preceded during 2000 and 2002 by three surveys that helped to delineate seafloor and basement relief, sediment thickness, and the nature of ridge-flank hydrothermal conditions and processes on the eastern flank of the Juan de Fuca Ridge. These surveys generated swath map, seismic, and thermal data used to select locations for primary and secondary drilling targets, building from several decades of earlier work. We show compilations and examples of data from several characteristic settings in and around the Expedition 301 work area and use these observations to evaluate sedimentation patterns and thermal conditions in basement. There remain important unanswered questions in this area concerning fluid circulation within the upper oceanic crust, the magnitude of lithospheric heat input, the quantitative significance of advective heat loss from the crust, and relations between basement relief, sedimentation, and sediment alteration. These questions may be resolved through collection of a modest amount of additional data focusing on a few critical locations.

[1] We measured permeability in sandstone and granite sheared at slip rates from 10 −4 to 1.3 m/s under low-normal stress at confining pressures up to 120 MPa. As the slip rate increased, the permeability of Berea sandstone decreased by an order of magnitude, whereas that of Indian sandstone and Aji granite increased by 3 orders of magnitude at high slip rates. A fine-grained gouge layer of thickness developed during slip, and the wear rate was increased abruptly at high slip rates. Microcracks and mesoscale fractures formed at slip rates above 0.13 m/s. Numerical modeling showed that the slip surface temperature increased by several hundred degrees for slip velocities above 0.13 m/s and exceeded the a-b phase transition temperature of quartz at 1.3 m/s. Both the temperature rise and the temperature gradient at the slip surface were high at fast slip rates. We attributed reduced permeability after slip in porous sandstone to the low-permeability gouge layer. An abrupt permeability increase in low-permeability rocks at high slip rates was caused by heat-induced cracks. An increase in the rate of wear of gouge with increasing slip velocity was caused by frictional heating that reduced the rock strength. The host-rock permeability that separated reductions and increases in permeability was about 10 −16 m 2 at 10 MPa effective pressure. Our results suggest that abrupt increases in shear stress during slip in a low-permeability fault zone caused by thermal cracking, which may decrease the total slip displacement. The abrupt permeability increase at high slip rates in low-permeability rocks agrees with hydrogeochemical phenomena observed after earthquakes.

[1] We measured transport properties at a confining pressure of 60 MPa in core samples from the fault zone in Hole B of the Taiwan Chelungpu Fault Drilling Project (TCDP). Permeability and specific storage of the fault gouge range from 3 Â 10 À15 to 1 Â 10 À17 m 2 and from 2 Â 10 À10 to 7 Â 10 À10 Pa À1 , respectively, and the measured hydraulic diffusivity was 6 Â 10 À5 m 2 /s, which is consistent with the data measured in situ. Numerical analysis of the thermal pressurization mechanism during the 1999 Chi-Chi earthquake using laboratory measured transport and frictional properties showed that pore pressure at the fault zone increased dramatically during slip, whereas temperature increased only moderately to 400°C at the end of slip. The results indicate that dehydration of interlayer water in smectite is plausibly caused by frictional heating, although such dehydration does not influence fault weakening. The decomposition reactions of other minerals are difficult to explain for only one slip event. A magnetic susceptibility anomaly observed in the fault zone is consistent with the modeling results, although low contents of inorganic carbon and clay minerals are not. We concluded that these inconsistencies can possibly be explained by the combined effects of enhancement of chemical reactions by mechanochemical influences and periodic movement on the Chelungpu fault. High-temperature water-rock interactions are also a possible explanation for inconsistencies.

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We presented the total and inorganic carbon contents of core samples recovered from the Taiwan Chelungpu fault system, which slipped at the 1999 Chi-Chi earthquake, and reported lower contents of inorganic carbon within the black gouge zone in FZB1136 (fault zone at depth 1136 m in Hole B) and in the black-material disks in FZB1194 and FZB1243.