Thermal anomaly around the Nojima Fault as detected by fission-track analysis of Ogura 500 m borehole samples

To better understand heat generation and transfer along earthquake faults, this paper presents preliminary zircon fission-track (FT) length data from the Nojima Fault, Awaji Island, Japan, which was activated during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake). Samples were collected of Cre taceous granitic rocks from the Ogura 500 m borehole as well as at outcrops adjacent to the borehole site. The Nojima Fault plane was drilled at a dep th of 389.4 m (borehole apparent depth). Fission-track lengths in zircons f rom localities > 60 m distance from the fault plane, as well as those from outcrops, are characterized by the mean values of approximate to 10-11 mum and unimodal distributions with positive skewness, which show no signs of a n appreciable reduction in FT length. In contrast, those from nearby the fa ult at depths show significantly reduced mean track lengths of approximate to6-8 mum and distributions having a peak around 6-7 mum with rather negati ve skewness. In conjunction with other geological constraints, these result s are best interpreted by a recent thermal anomaly around the fault, which is attributable to heat transfer via focused fluids from the deep interior of the crust and/or heat dispersion via fluids associated with frictional h eating by fault motion.