Direct, underground observation of hypocenters

由于天然地震的震源区很难到达，因此直接了解震源区的环境和性质是非常有价值的。在南非金矿中，地震断层（2<M<5）在与断层相交的巷道和采场中经常被很好地掘出。因此，可以直接调查震源的环境和性质。2002 ~ 2004年3年间，我们在南非5个金矿进行了2 <M<5地震震源的地下调查，并在2<M<5地震断层附近进行了应力测量。在一个深约1.3公里的震源区，实际上在我们的调查期间发生了M2事件，我们在几个月内每周观察几次岩体行为。岩体一旦开始屈服，几天之内就迅速屈服。虽然我们尝试了套芯应力测量，但相关的严重钻孔崩落和隧道变形不允许我们进行套芯。利用钻孔岩心进行了ASR和DSA应力测量。观察到很少的ASR，应力-应变曲线几乎是直的。在另外两个矿山对断层岩进行了调查，两次M4地震造成断层滑动数厘米。同时也证实了被矿山地质工程师和岩石工程师称为“假玄武质岩”的断层岩是凝固碎裂岩。

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.