Infiltration of Meteoric and Sea Water into Deep Fault Zones during Episodes of Coseismic Events : A Case Study of the Nojima Fault, Japan

Abstract

Fluid infiltration into active faults and shear zones is usually studied in the middle to upper crustal environment. Circulating fluids deposit clay and carbonate material into cracks within the fault zone. Such crack-fill clay, calcite veins, and oxidized/weathered open cracks are well observed in the drill cores, from near-surface to a depth of 1,800 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 clay veins are mainly composed of siderite, calcite, laumontate, and fine-grained clasts of granitic rock. Isotopic analyses of carbonate material within the clay and calcite veins reveal variable δ18O (SMOW) values ranging from 20.1‰ to 27.7‰, and δ13C (PDB) values of -4.3‰ to -18.5‰, which are comparable to those of typical meteoric and sea water. 14C dates of 10 clay and calcite vein samples range from 35.0 kyr B.P. to 58.4 kyr B.P. Geological, petrological, stable isotopic, and 14C data suggest that these crack-fill clay and calcite veins and brown-colored open cracks developed due to the repeated infiltration of O2- and CO2-bearing meteoric and sea water 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 a rapid potential change during episodes of seismic faulting