Bridging great earthquake doublets through silent slip: On‐ and off‐fault aftershocks of the 2006 Kuril Island subduction earthquake toggled by a slow slip on the outer rise normal fault of the 2007 great earthquake

Abstract

The November 2006 great thrust earthquake of moment magnitude (Mw) 8.3 on the plate boundary in the Kuril (Chishima) Islands triggered as many vigorous normal‐faulting aftershocks in the outer rise region seaward of the trench as on‐fault aftershocks. Here we show that the aftershocks for a 40 day period preceding the January 2007 Mw = 8.1 event in the outer rise were less frequently observed than the rate expected by both the Omori‐Utsu formula and the epidemic‐type aftershock sequence (ETAS) model while at the same time were more frequent along the plate boundary. We discuss whether such simultaneous quiescence and activation in the pervasive 2006 aftershocks might have been initiated by the onset of aseismic slip on the 2007 rupture plane. The stable fault sliding in an elastic half‐space demonstrates progressive stress shadowing in the outer rise and stress loading along the plate boundary. Furthermore, the rate‐ and state‐dependent friction of Dieterich quantitatively simulates temporal changes in seismic behavior. The same trends of the anomalous activities last in both zones after the 2007 rupture, which suggests that the slips continuously take place in the similar region on the fault throughout the period before and after the rupture. The Kuril case implies that temporal seismic quiescence in an aftershock sequence is useful in evaluating the possibility of the subsequent triggered event, thus suggesting itself as a potential earthquake‐forecasting model together with pervasive earthquake‐clustering models.