Tatsuo Matsuda

Architectural evolution of the Nojima fault and identification of the activated slip layer by Kobe earthquake

[1]xa0Evolutionary history of Nojima Fault zone is clarified by comprehensive examinations of petrological, geophysical, and geochemical characterizations on a fault zone in deep-drilled core penetrating the Nojima Fault. On the basis of the results, we reconstruct a whole depth profile of the architecture of the Nojima Fault and identify the primal slip layer activated by 1995 Kobe earthquake. The deepest part (8- to 12-km depth) of the fault zone is composed of thin slip layers of pseudotachylite (5 to 10 mm thick each, 10 cm in total). Middle depth (4- to 8-km depth) of the fault zone is composed of fault core (6 to 10 m thick), surrounded by thick (100 m thick) damage zone, characterized by zeolite precipitation. The shallow part of the fault zone (1- to 4-km depth) is composed of distributed narrow shear zones, which are characterized by combination of thin (0.5 cm thick each, 10 cm in total) ultracataclasite layers at the core of shear zones, surrounded by thicker (1 to 3 m thick) damage zones associated with carbonate precipitation. An extremely thin ultracataclasite layer (7 mm thick), activated by the 1995 Kobe earthquake, is clearly identified from numerous past slip layers, overprinting one of the shear zones, as evidenced by conspicuous geological and geophysical anomalies. The Nojima Fault zone was 10 to 100 times thicker at middle depth than that of shallower and deeper depths. The thickening would be explained as a combination of physical and chemical effects as follows. (1) Thickening of “fault core” at middle depth would be attributed to normal stress dependence on thickness of the shear zone and (2) an extreme thickening of “damage zone” in middle depth of the crust would result from the weakening of the fault zone due to super hydrostatic fluid pressure at middle depths. The high fluid pressure would result from faster sealing with low-temperature carbonate at the shallower fault zone.

Carbon-forming reactions under a reducing atmosphere during seismic fault slip

Graphite is a well-known solid lubricant and can be as important as clay minerals in reducing the frictional strength of faults. Some natural fault zones contain carbonaceous material (CM) even where host rocks do not contain it, and seismic fault motion can promote the graphitization of low-grade CM. Thus, the origin of CM in fault zones is an important issue in fault mechanics. Previous high-velocity friction experiments have revealed various chemical reactions in fault zones during seismic fault motion, but most experiments have been conducted in an atmosphere under oxic conditions. Here we report experimental results on Carrara marble (free of CM), conducted under N2 or H2 atmospheres at a slip rate of 1.3 m/s and normal stresses of 2.0–3.1 MPa. A small amount of blackish material formed in generated gouge only under reducing conditions with the H2 atmosphere, and Raman spectroscopic analysis revealed the presence of CM (amorphous carbon) in the material. The CM is attributable to (1) the generation and pyrolitic dissociation of CH4, and/or (2) a reduction reaction of emitted CO2 due to calcite decomposition. We confirmed the formation of CH4 using gas chromatography. The CM produced in experiments resembles CM in the Nojima fault (Japan) gouge in terms of Raman spectra. The granitic host rock of this fault is free of CM, and calcite is precipitated close to the CM; therefore, the CM probably formed through processes similar to those simulated in our experiments. Future research should investigate the amount and origin of CM in natural fault zones.

Fission-track analysis of the Atotsugawa Fault (Hida Metamorphic Belt, central Japan): fault-related thermal anomaly and activation history

Abstract Fission-track (FT) thermochronology was applied to the Atotsugawa Fault in the Hida Metamorphic Belt, central Japan, to detect any ancient thermal anomaly associated with fault displacements. Apatite and zircon grains from gouges (c. 2 cm wide) and fractured rocks (c. 10 cm) at six fracture zones within a 15 m-wide fault zone were dated. Most of the zircon (120–150 Ma) and apatite (44–60 Ma) ages agree well with emplacement ages for the granites that intrude the Hida Belt. The discordance in zircon and apatite FT ages is interpreted to reflect cooling due to regional uplift and associated erosion. A thermal anomaly was identified at one of the gouge zones that showed an exceptionally young apatite age (c. 32 Ma) with an unimodal FT length distribution. It presumably indicates secondary heating induced by frictional slip during an earthquake, possibly giving a younger limit of the initiation of the activity in the Hida Belt.