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

Abstract

[1] 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 (M w 6.9), 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.Citation: Kumazawa, T., Y. Ogata, and S. Toda (2010), Precursory seismic anomalies and transient crustal deformation prior to the 2008 M w = 6.9 Iwate-Miyagi Nairiku, Japan, earthquake,

Pre-seismic anomalies in seismicity and crustal deformation: case studies of the 2007 Noto Hanto earthquake of M6.9 and the 2007 Chuetsu-oki earthquake of M6.8 after the 2004 Chuetsu earthquake of M6.8

“…Also, the synchronous seismicity changes in northern Honshu, Japan, are observed preceding the 2003 great Tokachi-oki earthquake of M8.0 (Ogata 2005b). However, in this study, the seismicity change-points vary widely, which is also seen in Ogata (2007) and Kumazawa et al (2010). Another feature seen from these studies is that geodetic change-points of anomalies appear to precede the seismic ones.…”

“…Also, near-field CFS pattern is not very reliable, while far-field one does not change much with the location of the slip. For example, it is notable that, regarding the case of the M7.2 Iwate-Miyagi Nairiku earthquake in 2008, the analysis of baseline distances alone using the permanent GPS stations is too restrictive to make the inversion of the pre-slips (Kumazawa et al 2010), but the transient dense GPS stations around the same source (Iinuma et al 2009) made it possible to invert the post-seismic slips in space and time.…”

“…In fact, to validate the significance of the change-point, one has to compare AIC 0 with AIC 1 + AIC 2 + 2q(N), where q(N) is the penalty for the change-point time t c , because one seeks t c that minimizes AIC 1 + AIC 2 . Here the penalty q(N) monotonically varies from 3.5 to 5.5 depending on the number of events in the interval (S, T); see Kumazawa et al (2010) for the calculation of q(N) as a function of the number of earthquakes N in the period. The criteria for this comparison take into account the overfitting bias due to the greater complexity of two-stage models and also the freedom in searching for a change-point (Ogata 1992).…”

Statistical monitoring of aftershock sequences: a case study of the 2015 Mw7.8 Gorkha, Nepal, earthquake

“…As discussed in Ogata (2005a, b, c, 2006a, b, 2007, 2010b, 2011), Ogata et al (2003, Kumazawa et al (2010), and Kumazawa and Ogata (2013), we should consider and demonstrate the relationship of seismicity lowering (seismicity shadow) in the space-time aftershock occurrence patterns together with slow slips on the plate boundary using geodetic studies for the present Gorkha case (e.g., Avouac et al 2015;Zhang et al 2015;Fielding et al 2015;Grandin 2015;Ingleby et al 2015). Specifically, scenarios explaining the relative quiescence can rely on the seismicity rate change based on the rate-and-state friction law of Dieterich (1994) as the quantitative basis of the triggering (Ogata et al 2003;Ogata , 2010aOgata and Toda 2010).…”

Measuring seismicity diversity and anomalies using point process models: case studies before and after the 2016 Kumamoto earthquakes in Kyushu, Japan

“…If we hypothesize a fixed change-point time, this model is compared to the single baseline ETAS model using the Akaike information criterion (AIC; Akaike 1973Akaike , 1992. However, if the changepoint time is estimated, the goodness-of-fit relative to the baseline ETAS model is compared using a modified version of the AIC (Ogata 1992;Kumazawa et al 2010) because the maximum likelihood estimate (MLE) of the change-point does not satisfy ordinary large-sample theory (Ogata 1978).…”