Repeated coseismic infiltration of meteoric and seawater into deep fault zones: a case study of the Nojima fault zone, Japan

Abstract

Fluid infiltration into active faults and shear zones is usually studied in middle to upper crustal environment. Circulating fluids deposit fine-grained sediments like clay and carbonate material into cracks within the fault zone. Such crack-fill fine-grained material, calcite veins, and oxidized/weathered open cracks are well observed in the drill cores, from near-surface to a depth of 1800 m, in the Nojima fault zone, Japan, which triggered the 1995 M7.2 Kobe earthquake. Powder X-ray diffraction analysis indicates that the crack-fill fine-grained materials are mainly composed of siderite, calcite, laumontite, and fine-grained clasts of granitic rock. Isotopic analyses of carbonate material within the fine-grained materials and calcite veins reveal variable δ13C (SMOW) values ranging from 19.3‰ to 27.7‰, and δ13C (PDB) values of −4.6‰ to −18.5‰, which are consistent with those of typical meteoric and seawater. 14C dates of 10 calcite vein samples range from 35.0 to 58.4 kyr B.P.Geological, petrological, stable isotopic, and 14C data suggest that these crack-fill fine-grained materials and calcite veins and brown open cracks were developed by the repeated infiltration of O2- and CO2-bearing meteoric and seawater downward into the deep Nojima fault zone during the last 35–60 kyr. We propose a seismic fault suction-pumping model to interpret the infiltration of subsurface waters being carried down into the deep fault zone by rapid potential change during episodes of seismic faulting.