Monotonic And Cyclic Pore Pressure Loading Of Sawcut Sandstone

Sawcut sandstones were subjected to pore pressure loading and cycling in a triaxial testing apparatus, at confining pressures between 100 and 550 bars. The law of effective stress gives a reasonable estimate of the pore pressure levels required to produce failure, but a more accurate estimate is obtained by considering several features of the loading history, especially a tendency for frictional strength to increase with repeated axial loading to failure. In pore pressure cycling tests, little stable sliding took place during cycles, and failure by fatigue did not occur. After pore pressure cycling there was a tendency for the next failure induced by pore pressure loading to be accompanied by a relatively large stress drop. INTRODUCTION Pore fluid pressure plays an important but incompletely understood role in shallow seismicity. In the cases of earthquakes induced by reservoir impounding and by fluid injection through wells1, it is almost certain that relatively small fluid pressure increases served as the triggers of seismogenic movement on critically stressed faults. Pore pressure increases may also be implicated in some natural seismicity. Hagiwara2 has suggested, on the basis of observed surface deformations, that a number of shallow earthquake swarms in Japan could have been caused by the influx of fluid at high pressure to a critically stressed area, presumably as an accompaniment to igneous intrusion. The fact that seismicity can be activated by filling a large reservoir or by injecting fluid into a deep well shows that artificially produced pore pressure variations can have significant effects on the behavior of shallow faults in critically stressed areas. Therefore it is conceivable that pore pressure might someday be used as a tool for the control of shallow seismicity.