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

Abstract

Evolutionary 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.

Multiple damage zone structure of an exhumed seismogenic megasplay fault in a subduction zone - a study from the Nobeoka Thrust Drilling Project

“…Multiple fault strands surrounded by damage zones, individually up to several meters thick, are documented to be localized within single lithologies (less than approximately 4 km depth) at the San Andreas Fault (Zoback et al 2010), Wenchuan Fault (Li et al 2013), Chelungpu Fault Depth, fracture total number distribution, fracture density (number of fractures per 1 m), neutron porosity, resistivity, density, P-wave velocity, natural gamma ray values, and Archie's cementation exponent m for the intact zones, surrounding damage zones, and the brecciated zones in the footwall are presented. (Song et al 2007), and the Nojima Fault (Tanaka et al 2007). These strike-slip and reverse faults occur within sedimentary lithologies and exhibit fault zone thicknesses ranging from several to approximately 135-m-thick damage zones.…”

Geometry of the Nojima Fault at Nojima-Hirabayashi, Japan – II. Microstructures and their Implications for Permeability and Strength

“…MURAKAMI and TAGAMI (2004) reported a cooling age of 56 Ma for pseudotachylyte from surface exposures of the fault (OTSUKI et al, 2003), based on fission track thermochronologic analysis of zircons. They concluded that the pseudotachylyte was generated at depths of about 8-12 km (see also TAGAMI and MURAKAMI, 2007;TANAKA et al, 2007). Deformation textures in older, foliated cataclasites estimated to have formed between 5 and 10 km depth indicate right-lateral offset with some vertical component , the same as observed on the fault today.…”

“…Most of the samples are biotite-hornblende granodiorites, and a few are tonalites ( Fig. 1; see also IKEDA, 2001 andTANAKA et al, 2007). One K-feldspar rich sample from the deep NIED fault strand has a granitic composition; a second deep NIED sample not included in Figure 1 probably also is a granite.…”

Rapid Response Fault Drilling Past, Present, and Future

“…Distinct periods of seismic activity were recognized that were accompanied by intense hydrothermal alteration (Ohtani et al, 2000;Boullier et al, 2004;Tanaka et al, 2007). Ultracataclasites and pseudotachylytes that formed at 10-km depth or more were related to previous M6 to M7 earthquakes (Boullier et al, 2001;Murakami and Tagami, 2004).…”