ANALYSIS OF INDUCED SEISMICITY FOR STRESS-FIELD DETERMINATION AND PORE PRESSURE MAPPING

The focal mechanisms of some one hundred microseismic events induced b y various water injections have been determined. Within the same depth interval, numerous stress measurements have been conducted with the H TPF method. When inverted simultaneously, the HTPF data and the focal plane solutions help determine the complete stress field in a fairly l arge volume of rock (about 15 x 10(6) m(3)). These results demonstrate that hydraulically conductive fault zones are associated with local s tress heterogeneities. Some of these stress heterogeneities correspond to local stress concentrations with principal stress magnitudes much larger than those of the regional stress field. They preclude the dete rmination of the regional stress field from the sole inversion of foca l mechanisms. In addition to determining the regional stress field, th e integrated inversion of focal mechanisms and HTPF data help identify the fault plane for each of the focal mechanisms. These slip motions have been demonstrated to be consistent with Terzaghi's effective stre ss principle and a Coulomb friction law with a friction coefficient ra nging from 0.65 to 0.9. This has been used for mapping the pore pressu re in the rock mass. This mapping shows that induced seismicity does n ot outline zones of high flow rate but only zones of high pore pressur e. For one fault zone where no significant flow has been observed, the local pore pressure has been found to be larger than the regional min imum principal stress but no hydraulic fracturing has been detected th ere.