Post-seismic Effects of an M 7.2 Earthquake and Microseismicity in an Abandoned, Flooded, Deep Mine

摘要

— An M 7.2 earthquake took place 67 km southeast of an abandoned mine containing flooded, vertical ore veins at depth of 1 km. Multidisciplinary monitoring of the unweathered country rock within the mine was carried out for a distance of about one kilometer. The M 7.2 earthquake was followed by significant post-seismic changes in strain and tilt of ∼10−6, with a self-potential of ∼10 mV, and an increase in the water level in the mine of about 10 cm/day. These phenomena continued for several months before returning to pre-earthquake levels. Coseismic elastic deformations were too small to account for the observed post-seismic changes, and have different senses in strain and tilt from the observed post-seismic changes. The contractions in strain and the changes in the electric self-potential and water-elevation rate strongly suggest an increase in pore pressure. An increase in microseismic activity (M < 0) was also observed around the flooded, vertical ore veins about 1–2 km from our multidisciplinary monitoring network, when pore-pressure reached a maximum following the M 7.2 earthquake.