Precursory seismic anomalies and transient crustal deformation prior to the 2008 Mw = 6.9 Iwate‐Miyagi Nairiku, Japan, earthquake

Abstract

The epidemic‐type aftershock sequence (ETAS) model has been widely used for detecting seismicity anomalies, such as quiescence and activation during aftershock sequences of large earthquakes and background levels of seismicity. The causes of such anomalies are thought to be associated with spatiotemporal changes in stress, even tiny perturbation of local stress. Here we analyze the seismic activity during the decade prior to the 2008 Iwate‐Miyagi inland earthquake of M7.2 (Mw6.9), to fit the ETAS model to the seismicity from various regions around the source over northern Honshu, Japan. From the viewpoint of the analyses with the ETAS model, we find northern Honshu is divided into three areas of distinctive behaviors, increased seismicity, decreased seismicity, and normal seismicity relative to the ETAS prediction. As in other previously published papers, here we hypothesize that Coulomb stress changes caused by a few years of precursory slip of the Iwate‐Miyagi earthquake resulted in the seismicity changes in and around the 2008 source region. The confirmed significant seismic anomalies in respective regions are consistent with the increments of the Coulomb failure stress of the corresponding regions that are calculated by the assumed slow slip on the southern part of the faults of the main shock. The local crustal deformations observed from a dense Global Positioning System network, including a station right above the focal fault, supports that slow slip on the fault had been taking place for about 5 years prior to the occurrence of the focal earthquake and suggests that the slip terminated or migrated to down‐dip extension of the fault around 1 year before the rupture.