Hydraulic properties of Chelungpu, Shuangtung and Shuilikeng fault zones and their implication for fault motion during 1999 Chi-Chi earthquake

We report laboratory measurements of permeability and specific storage for natural fault gouge and breccia samples, both from drill core and surface outcrops, from the three main thrust faults (Chelungpu, Shuangtung and Shuilikeng faults) that are developed in the focal area of 1999 Taiwan Chi-Chi earthquake. The stratigraphic cross-sections and the vitrinite reflectance data suggest that deeper exhumed fault structures might be observed from Chelungpu, to Shuangtung, and to Shuiliuken fault zones. Permeability structure of fault zone varied greatly among three faults. Permeability of the Chelungpu fault gouge was the highest with (10-15 m2 to 10-17 m2), and permeability of the Shuangtung fault gouge showed the lowest values of (10-17 m2 to 10-19 m2). On the other hand, there are small variations in specific storage, (10-9 to 10-10 Pa-1), among three fault gouges. Those measured hydraulic properties were incorporated in the thermal pressurization analysis. Pore pressure evolution during rupture propagation varied greatly among these three faults because of large variation of permeability structure. Thermal pressurization is least effective for Chelungpu fault zone with slip-weakening distance, Dc, of 1-10 m at a depth of 2-8 km, and is most effective for Shangtung fault zone with Dc of 0.3-1 m at a depth of 2-8 km. If Shangtung fault zone we see at surface outcrops represent deeper hydraulic properties of Chelungpu fault zone, one may argue that thermal pressurization would be effective at deeper parts of Chelungpu fault. However, numerical basin analysis, which was evaluated using laboratory derived hydraulic parameters for sedimentary rocks and fault rocks, suggests that large amount of overpressure due to mechanical compaction, dehydration of clay minerals, influx from the depth and fault sealing would be maintained at depths deeper than about 4 km in the focal area of Chi-Chi earthquake. Our predicted overpressure trend is also consistent with the published data on abnormal pore pressure in the gas field to the north of focal area. This regional overpressure will increase the permeability of fault zone and may suppress thermal pressurization at depths. Our preliminary analysis of thermal pressurization suggest that Dc is quite large unless other weakening mechanisms exists. If Dc is large, a puzzling question is how such fault motion (3 to 5 m/s) was possible at the northern part of Chelungpu fault during Chi-Chi earthquake. Key words: Taiwan Chelungpu-fault Drilling Project (TCDP), 1999 Chi-Chi earthquake, Chelungpu fault, Shuangtung fault, Shuilikeng fault, thermal pressurization, permeability, porosity, high-velocity friction