DETECTION OF PRECURSORY RELATIVE QUIESCENCE BEFORE GREAT EARTHQUAKES THROUGH A STATISTICAL-MODEL

Relative quiescence is defined as a significant decrease of earthquakes compared with the occurrence rate expected from a point-process model for ordinary seismic activity. This definition is linked with the idea that aftershocks, as a useful signal for the investigation of seismicity, should not be removed from original occurrence data. The Epidemic Type Aftershock-Sequences (ETAS) model is proposed to identify patterns in an earthquake sequence in an area, using occurrence time and magnitude data. Even in an active stage of seismicity, a decrease from the expected level can take place, and the significance and size of such a quiescence can be graphically shown using time-changed occurrence data which are transformed based on the estimated ETAS model. This procedure permits detection of a clear, relatively quiet stage before great earthquakes in Japan and elsewhere in the world. Such relative quiescence lasts a number of years prior to the occurrence of great earthquakes, which occasionally take place in the recovering stage. No significant relative quiescences were seen in the current seismicity of a few wide regions, in and around Japan, including several source regions of expected great earthquakes.