The Effect of Interstitial Fluid Pressure on Earthquake Generation

Abstract

The essential role of interstitial fluid pressure (pore pressure) in earthquake occurrence is strongly suggested by the observational fact that seismic activity was induced by water injection into a deep well and water impounding of an artificial reservoir. Elaborate studies related to water injection revealed following important characteristics of the induced seismicity : (1) Earthquakes are triggered to occur when the interstitial fluid pressure of the basement rock exceeds some threshold level ; (2) Focal mechanisms of induced earthquakes are in good harmony with the regional stress field ; and (3) Seismic activity propagates as far as several kilometers from the injection well. Based on those observational facts, we conclude that the injected water raising the interstitial fluid pressure releases the tectonic stress naturally accumulated. The effect of pressure change on the fracture strength of rock S is formulated in terms of the effective stress hypothesis as
S=τ₀+ (σ_n-P₀) tanφ
where σ_n and P₀ are normal stress and interstitial fluid pressure, and τ₀ and φ are the coefficient of cohesive strength and internal friction angle, respectively. This criterion of rock fracture may also be applied to account for the reservoir induced seismicity. For the case of reservoir induced seismicity, however, the loading effect of water mass should be taken into consideration since some of the artificial reservoirs exhibit induced aseismicity.
The effect of interstitial fluid pressure seems to be essential even for occurrence of natural earthquakes. Detailed studies on the Matsushiro earthquake swarm revealed that the earthquake swarm was brought about by the “water eruption”; high pressure water supplied at the depth beneath Matsushiro erupted to the ground surface accompanying a large number of small earthquakes which were generated by the increase of interstitial fluid pressure. Remarkable upheaval of the focal area, gushing out of large amount of water, and other associated phenomena are consistently interpreted by the water eruption model. It will be of great value for deeply understanding the nature of swarm activity to investigate whether such a mechanism is common or not to other earthquake swarms.
The dilatancy-diffusion model of earthquake occurrence is briefly discussed from the viewpoint of the effect of interstitial fluid pressure. The model is a fascinating product of the effective stress hypothesis and results of laboratory experiments of rock fracture. It, however, is still dubious if the results obtained in the laboratory can directly be applied to the earthquake phenomenon occurring in the natural environment. We, therefore, emphasize the especial importance of advanced studies on induced earthquakes which occur under the half-natural and half-controlled circumstances.