An Attempt to Monitor Stress/ Physical-property at Hypocenters, Using Tidal Response, Mponeng Gold Mine, South Africa

In 2002, at the Mponeng Mine, South Africa, the Research Group for the Semi-controlled Earthquake- generation Experiments in South African Deep Gold Mines started the observation aimed for monitoring the entire life span of an earthquake. Around our site, the Pretorius fault zone, a length of several km, offsets a gold reef. Within the fault zone, we found a distinct weak plane where an M3 event is expected. In May 2004, both seismicity and strain rate were very high in around our site, because the surrounding area of our site was being mined. The areal strain rate was 5*E-7 strain/day (contraction), equivalent to 0.5MPa/day (compression). If fast changes in mining layout cause these high seismicity and strain rate, the ratio of observed tidal strain amplitude to that theoretically estimated varies with time; also, onset of change in the ratio corresponds to that of each change in strain rate or seismicity. On the other hand, if inelastic deformation is dominant, the tidal amplitude ratio will not significantly vary with time. To analyze tidal strain amplitude, first, we decimated the original 25Hz data to every hour data as pre- processing. The data had a large number of co-seismic steps. So, we adjusted those offsets. Power failure or data communication problems caused irregular, abnormal values, and then we excluded them. Next, we decomposed strain data into trend, tide and irregular components, using BAYTAP-G (Ishiguro et. al. 1984; Tamura et. al. 1991) An example of the analysis, we mention the amplitude from 1st to 3rd of May, 2004(Sat, Sun and Mon), when mining influence on strain changes are little. Tidal amplitudes from BAYTAP-G were approximately 1.0E-7. We could check that they were in good agreement with theoretical amplitudes (21 tidal components) from GOTIC2 (Matsumoto et. al. 2001), regardless of fast, background rate of strain. We will investigate any temporal change in the tidal amplitude ratio. In addition, we will discuss if the temporal changes in tidal phase correspond to the change in seismicity.