Direct, underground observation of hypocenters

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As it is very hard to reach the hypocenters of natural earthquakes, direct information on the environment and property of hypocenters is very valuable. In South African gold mines, seismic faults of earthquakes (2<M<5) are often well exhumed at tunnels and stopes intersecting the faults. So, it is possible to directly investigate the environment and property of hypocenters. We planned to investigate the fault underground and carry out stress measurements at the proximity of the fault of the events (2<M<5).In a 3-year period from 2002 to 2004, the hypocenters of events with 2<M<5 were investigaed underground at five gold mines in South Africa. At a hypocentral area 〜3 km deep, actually with M2 events during our investigation period, we observed rock mass behaviour a few times a week for couples of months. The rock mass quickly yielded within couples of days once it started yielding. Although we attempted an overcoring stress measurement, the associated serious borehole breakout and tunnel deformation did not allow us to make the overcoring. We also attemped the ASR and DSA stress measurement with the drilled core. Little ASR was observed and the stress-strain profile was almost straight. This suggested that the intact rock mass was almost elastic.Fault rock was investigated at other two mines where two M4 events caused the fault slip of several cm. However, no evidence of melting was found. It is also confirmed that the fault rock that the mine geological enginners and rock engineers refer to as "pseudotachilyte" was the solidified cataclasite.