Hardened foliated fault gouge from the Nojima Fault zone at Hirabayashi Evidence for earthquake lightning accompanying the 1995 Kobe earthquake?

Two anomalous features were found in the Nojima Fault zone at Hirabayashi i n Awaji Island, south-west Japan: (i) hard foliated gouge between weathered granitic fault breccia and weakly consolidated mudstone of the Osaka Group ; and (ii) mudstone near the gouge showing anomalous magnetization behavior . Roots of herbaceous vegetation near the foliated gouge were extraordinari ly charred. In order to understand the nature of the gouge, shallow drillin gs were made to a depth of 3-14 m across the fault zone. Various physicoche mical measurements of the gouge at depths and charred roots of herbaceous v egetation were conducted. The main results were: (i) Using electron spin re sonance (ESR) analysis, the carbon radical peak (g = 2.006) of the charred roots was found to be 25 times larger than that of the non-charred roots of the same vegetation taken near the fault, indicating that the charred root s were subjected to baking; (ii) the hard foliated gouge clearly showed a l amellar structure consisting alternately of gray and black layers; (iii) th e black layers in most of the foliated gouge showed flow structures almost parallel to the fault, but the gray layers rarely showed flow patterns; (iv ) natural remanent magnetization (NRM) of the foliated gouge was 430 times greater than that of the granitic fault breccia and approximately 70 times greater than that of the mudstone; (v) the NRM intensity of the mudstone ne ar the fault was highest near the ground level and decreased as the depth i ncreased, although the magnetic susceptibility of the mudstone was almost c onstant and independent of depth; (vi) the high-coercivity magnetization co mponent vectors of both the mudstone and the foliated gouge in a Schmidt eq ual-area projection was quite different from that of the present direction of the Earth's field; and (vii) using a magnetic force microscope, intense magnetic force lines were found in the black parts of the foliated gouge. I t is suggested that these anomalies were possibly caused by earthquake ligh tning that accompanied the 1995 Kobe earthquake. In a spark plasma sinterin g test, which was conducted to simulate the possibility of earthquake light ning-induced sintering of the gouge, weakly altered gouge was successfully sintered within 10 s. The hardness of sintered sample was comparable to tha t of the hard foliated gouge.