Progress in Understanding the Physics of Large Induced Seismicity at Basel, Switzerland

Abstract

We have been studying for investigation of the physics behind the large induced sesimicity occurred during/after hydraulic stimulation at Basel, Switzerland in 2006. In this paper, we summarize our development in understanding on the physics behind the large events at Basel. We investitgated the characteristics of the large events at first, then we went on the study for trigger mechanism of the shear slip which brought the large events. Our recent study has been in the final phase of identification of the controlling factor of the event magnitude. We reveled fundamental characteristics of the large events which means hypocenter location, occurrence time, time series occurrence, waveform similarity to the other events, fault plane solutions, seismic source parameters, and stress drop. This revealed that the large events occurred from deep part of the reservoir and those from shallow part have different characteristics in their occurrence time and similarity in waveforms, suggesting they occurred from different rupture process. Based on the fundamental characteristics of the large events, we estimated candidates of trigger mechanism of shear slip: increase in pore pressure, diffusion pressure and static stress change caused by shear slip of preceding events and we investigated relathishiop between these parameters and occurrence of the large events. It is found that increase in pore pressure that has been recognized as main trigger mechanism can not explain totally the occurrence of the large events, even though pore pressure is still the most reasonable trigger mechanism considering small change in diffusion pressure and static stress change. These results indicated the dynamic behavior of the pore pressure or thermal effect on the stress should be investigated. Investigation on the control factor of the event magnitude is in progress. We have so far found that the large events were likely to occur from the fault plane where large shear stress working on. We concluded shear stress is one of the trigger mechanism of the magnitude, however it is supposed to exist further dominant control factor.