Detection of Anomalous Seismicity as a Stress Change Sensor

Seismic quiescence has been attracting the attention as one of the precursors to a large earthquake. Also, it has been getting more attention than ever that the stress changes due to a slip can explain the mechanism of triggering another event. Likewise, seismic quiescence due to stress lowering transferred from a rupture or silent slip elsewhere can be discussed as a general phenomenon. Specifically, the quiescence in an active focal area could be found in the shadow zones where Coulomb stress change would be negative from a seismic or aseismic slip nearby. The seismicity rate lowering could be well seen in the regions where the activity has been high, including aftershock activity itself. To see the seismicity change explicitly, however, we need to fit a statistical model of either the modified Omori formula or its extension (the ETAS model [Ogata, 1992]), to the seismic activity before the suspected triggering time. Then, the activation and lowering of the seismicity relative to the predicted one by the model are of great concern to explore matching the pattern of Coulomb's stress changes due to a rupture or silent slip. A number of such examples from the recent activities in Japan will be shown. These lead us to a summarized observation that even a small size of negative Δ CFF down to the order of millibars can trigger such lowering, which is also supported by the Dieterich's seismicity rate equation [Dieterich, 1994; Toda and Stein, 2002]. Thus, we expect that aftershock and general seismic activity relative to the modeled rates is sensitive enough to measure a slight negative stress change. An earthquake prediction scenario from the implication of the relative quiescence is based on the asperity hypothesis in the sense that precursory slip around asperities applies more shear stress to the asperities, which in turn promotes the rupture of the main fault. However, on the other hand, aseismic slips in some region is not necessarily a precursor to a large event. Indeed, we have observed a number of aseismic slips (silent earthquakes; sometimes repeated in the same region) with no subsequent large events in the last decades. This is consistent with some empirical results that (relative) quiescence was not always followed by a large event. Therefore, distinguishing whether or not an aseismic slip leads to the rupture of asperity remains a further difficult research theme in earthquake prediction. At present, this issue can only be described in terms of probabilistic prediction [Ogata, 2001]. References Dieterich, J., H., J. Geophys. Res., 99, 2601_E618, 1994. Ogata, Y., J. Geophys. Res. 97, 19845-19871,1992. Ogata, Y., J. Geophys. Res. 106, 8729-8744, 2001. Ogata, Y., Jones, L.M. and Toda, S., J. Geophys. Res., 108, 10.1029/2002JB002009, 2003. Toda, S. and Stein R.S., J. Geophys. Res., 107, 10.1029/2001JB000172, 2002