The foreshock sequence of large earthquakes: slow slip or cascade triggering?

Abstract:

Large earthquakes such as the 2011 M_w 9.0 Tohoku-Oki earthquake and the 2014 M_w 8.1 Iquique earthquake are often preceded by foreshock sequences migrating toward the hypocenters of mainshocks. Understanding the underlying physical processes is crucial for imminent seismic hazard assessment. Some of these foreshock sequences are accompanied by repeating earthquakes, which are thought to be a manifestation of a large-scale background slow slip transient. The alternative interpretation is that the migrating seismicity is simply produced by the cascade triggering of mainshock-aftershock sequences following Omori’s Law. In this case the repeating earthquakes are driven by the afterslip of the moderate to large foreshocks instead of an independent slow slip event. As an initial effort to discriminate these two hypotheses, we made a detailed analysis of the repeating earthquakes among the foreshock sequences of the 2014 M_w 8.1 Iquique earthquake. We observed that some significant foreshocks (M >= 5.5) are followed by the rapid occurrences of local repeaters, suggesting the contribution of afterslip. However the repeaters are distributed in a wide area (~40*80 km), which are difficult to be driven by only a few moderate to large foreshocks. Furthermore, the estimated repeater-inferred aseismic moment during the foreshock period is at least 3.041e19 Nm (5*5 km grid), which is of the same order with the total amount of seismic moment of all foreshocks (2.251e19 Nm). This comparison again supports the slow-slip model since the ratio of post-seismic to coseismic moment is small in most earthquakes. To estimate the contributions of the transient slow slip and cascade triggering in the initiation of large earthquakes, we propose to systematically search and analyze repeating earthquakes in all foreshock sequences preceding large earthquakes. The next effort will be made to the long precursory phase of large interplate earthquakes such as the 1999 M_w 7.6 Izimit earthquake and the 2003 M_w 8.3 Tokachi-Oki earthquake.