Stress Loading in the Northern Ibaraki Prefecture, Northeast Japan, Inferred from Geodetic and Seismicity Data

The 2011 Tohoku-oki earthquake impacted seismicity not only on and around the plate interface, but also in the continental crust of Japan Islands. In Fukushima Hamadori and northern Ibaraki prefecture areas in northeastern Japan, seismicity rate had been quite low before 2011, but became very high after the Tohoku-oki earthquake. In northern Ibaraki prefecture area, two M-6 class earthquakes occurred just 8 minutes after the origin time of the Tohoku-oki earthquake (Event 2011a) and 8 days after (Event 2011b). At the end of 2016, just 6 years after, another M 6 event struck the same area (Event 2016).

These events have similarities and differences. An InSAR analysis indicated almost identical surface ruptures (Nakai et al., 2017). Our relocated earthquake catalog implied that major faults ruptured by Event 2016 was already ruptured by Events 2011a and 2011b. On the other hand, the seismograms from those events were clearly different. Our finite-fault slip models inferred from strong-motion data suggested that the slip area of Event 2016 overlapped those of Events 2011a and 2011b, whereas the locations of the peak slip did not agree with each other.

To understand the driving force of Event 2016, we investigated the stress history at the epicenters of Event 2016 (IBR) and, for reference, the 2011 Iwaki earthquake (FKS). A surface strain analysis using GNSS data of GEONET, GSI revealed a sudden increase mainly in east-west extensional component (e_xx) due to the Tohoku-oki earthquake. e_xx logarithmically increased up to 1000 days from the Tohoku-oki earthquake in IBR and 500 days in FKS. Afterward, e_xx kept increasing in IBR, whereas decreased in FKS. This reflects the different behavior in the seismicity rate.

A seismicity analysis based on the ETAS model (Ogata, 1988) implied the stress rate inversely proportional to the time since the Tohoku-oki earthquake. This indicates the logarithmic change in the stress, consistent with the geodetic analysis.

The logarithmic stress change is probably due to the afterslip of the Tohoku-oki earthquake, whereas afterward other mechanisms such as the viscoelastic deformation of the crust and mantle may have worked. The consistency between the implications from geodetic and seismicity data ensures the reliability of seismicity data for estimating the stress state at seismogenic depths.