Microseismicity Induced by Heavy Rainfall Around Flooded Vertical Ore Veins

Abstract

 — Microseismicity (M < 0) induced by heavy rainfall was investigated around the flooded, vertically dipping Tertiary ore veins with dimensions of about 1 km × 1 km in the Ikuno mine, Japan. The ore veins had rock bursts (M < 3) before the mine was closed in 1973, as well as seismic events (M < 3) during flooding after it was closed down. The stress state is therefore critical to failure, at least within one stress drop of a seismic event. Since 1987, when the veins had become mostly flooded, 56 mine tremors (M < 0) were observed over a 5-year period. Several times during this five-year period the mine sustained heavy rainfall of several tens of millimeters per day, and the water table flooding over the ore veins was elevated by several meters. Significant changes in strain larger than 10⁻⁶ were also monitored at a crustal movement monitoring station located several hundred meters from the veins. It was found that the opening of the vertical ore veins primary led to significant strain and tilt, but not to seismicity, because the delay and the longer duration of the seismicity were significant. Most seismic events involve thrusting mechanisms that are consistent with the present stress state of E-W-oriented tectonic compression, but are not consistent with the opening of the deepest ore vein. Interstingly, all the events within a few months of the heavy rainfall occurred near the faults that offset the deepest ore veins, wheareas all those events located away from the deepest ore veins occurred many months after the heavy rainfall. Consequently, the delayed diffusion of water appears to have played a dominant role in reducing rock strength, which led to seismicity in the Ikuno mine.