Monitoring of the spatio-temporal change in the interplate coupling at northeastern Japan subduction zone based on the spatial gradients of surface velocity field

Abstract

S U M M A R YA monitoring method to grasp the spatio-temporal change in the interplate coupling in a subduction zone based on the spatial gradients of surface displacement rate fields is proposed. I estimated the spatio-temporal change in the interplate coupling along the plate boundary in northeastern (NE) Japan by applying the proposed method to the surface displacement rates based on global positioning system observations. The gradient of the surface velocities is calculated in each swath configured along the direction normal to the Japan Trench for time windows such as 0.5, 1, 2, 3 and 5 yr being shifted by one week during the period of 1997-2016. The gradient of the horizontal velocities is negative and has a large magnitude when the interplate coupling at the shallow part (less than approximately 50 km in depth) beneath the profile is strong, and the sign of the gradient of the vertical velocity is sensitive to the existence of the coupling at the deep part (greater than approximately 50 km in depth). The trench-parallel variation of the spatial gradients of a displacement rate field clearly corresponds to the trenchparallel variation of the amplitude of the interplate coupling on the plate interface, as well as the rupture areas of previous interplate earthquakes. Temporal changes in the trench-parallel variation of the spatial gradient of the displacement rate correspond to the strengthening or weakening of the interplate coupling. We can monitor the temporal change in the interplate coupling state by calculating the spatial gradients of the surface displacement rate field to some extent without performing inversion analyses with applying certain constraint conditions that sometimes cause over-and/or underestimation at areas of limited spatial resolution far from the observation network. The results of the calculation confirm known interplate events in the NE Japan subduction zone, such as the post-seismic slip of the 2003 M8.0 Tokachi-oki and 2005 M7.2 Miyagi-oki earthquakes and the recovery of the interplate coupling around the rupture area of the 1994 M7.6 Sanriku-Haruka-oki earthquake. The results also indicate the semi-periodic occurrence of slow slip events and the expansion of the area of slow slip events before the 2011 Tohoku-oki earthquake (M9.0) approaching the hypocentre of the Tohoku-oki earthquake.

Stress change before and after the 2011 M9 Tohoku-oki earthquake

“…A similar change of stress before the M9 is seen to the SW of this particular bin (Figure 4). Such a pre-M9 stress field modification might relate to the change in geodetically inferred deformation state of the overriding plate close to the M9 as analyzed by Ozawa et al (2012) and Mavrommatis et al (2014), and recently explored in terms of temporal evolution of coupling by Iinuma (2018). Another contribution to the stress field change as seen in Figure 6 Figure 5b), large jump due to the co-seismic effect of the M9 (cf.…”

Megathrust Modeling Workshop Report

“…Slow slip transients and foreshock sequences have either been observed or inferred prior to several significant events, including the 2011 Mw 9.1 Tohoku-Oki earthquake (e.g., Kato et al, 2012;Ito et al, 2013) and 2014 Mw 8.1 Iquique, Chile, earthquake (e.g., Ruiz et al, 2014;Brodsky & Lay, 2014;Soquet et al, 2017). Apparent changes in coupling have also been observed during the interseismic period (e.g., Mavrommatis et al, 2014;Yokota & Koketsu, 2015;Socquet et al, 2017;Iinuma, 2018;Materna et al, 2019).…”

Stress change before and after the 2011 M9 Tohoku-oki earthquake

“…A similar change of stress before the M9 is seen to the SW of this particular bin (Figure 4). Such a pre-M9 stress field modification might relate to the change in geodetically inferred deformation state of the overriding plate close to the M9 as analyzed by Ozawa et al (2012) and Mavrommatis et al (2014), and recently explored in terms of temporal evolution of coupling by Iinuma (2018). Another contribution to the stress field change as seen in Figure 6 Figure 5: Spatially-averaged, normalized tensor dot product ( θ , eq.…”

“…Simons et al, 2011). Analysis of geodetic time-series for GPS stations indicate that this earthquake was preceded by a remarkable modification of the effective plate boundary deformation starting ∼ 9 years before the event (Suito et al, 2011;Ozawa et al, 2012;Mavrommatis et al, 2014;Yokota and Koketsu, 2015;Johnson et al, 2016;Loveless and Meade, 2016;Iinuma, 2018). This pre-M9 modification of the plate boundary system can alternatively be interpreted as an exceptionally long slow-slip event or perhaps a preparatory process related to the M9.…”