Direct, underground observation of hypocenters

由于很难到达自然地震的震源，直接获取震源的环境和性质信息是非常有价值的。在南非金矿中，在与地震断层相交的巷道和采场中，经常很好地挖掘出地震断层（2<M<5）。因此，直接研究震源的环境和性质是可能的。我们计划在地下调查断层，并在事件断层附近（2<M<5）进行应力测量。在2002 ~ 2004年的3年间，对南非5个金矿的2<M<5级地震震源进行了地下调查。在一个约3公里深的震源区域，实际上在我们的调查期间有M2事件，我们每周观察几次岩体的行为，持续了几个月。岩体一旦开始屈服，几天内就会迅速屈服。虽然我们尝试了覆盖应力测量，但相关的严重钻孔破裂和隧道变形使我们无法进行覆盖。我们还尝试了钻孔岩心的ASR和DSA应力测量。ASR很小，应力-应变曲线几乎为直线。这表明完整的岩体几乎是有弹性的。在另外两个矿山，两次M4事件导致断层滑动数厘米，对断层岩石进行了调查，但没有发现熔化的证据。也证实了被矿山地质工程师和岩石工程师称为“伪断层岩”的断裂岩是凝固的碎裂岩。

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.