Mako Ohzono

Coseismic fault model of the 2008 Iwate-Miyagi Nairiku earthquake deduced by a dense GPS network

A large earthquake of Mj 7.2 occurred on June 14, 2008, beneath the border between Iwate and Miyagi prefectures in northeastern Japan. We propose a simple rectangular fault model based on a dense GPS network, including continuous GPS sites run by four agencies, to describe the coseismic deformation. The coseismic displacements are estimated by kinematic PPP (precise point positioning) analysis. Near the hypocenter, colocated independent instruments (integrated accelerogram and kinematic PPP) measure the same large displacement caused by the mainshock. The fault model explains the observations well and reproduces the observed complex spatial pattern, especially around the northern part of the focal area, which is the focus of a debate on whether or not the coseismic slip occurred on the Dedana fault system. Our results show that no major slip on the Dedana fault system occurred. The estimated amount of moment release was equivalent to Mw 6.9, and the maximum slip reached 3.5 m on the southern sub-fault.

Geodetic evidence of viscoelastic relaxation after the 2008 Iwate-Miyagi Nairiku earthquake

Continuous GPS observations, for over two years, detected long-term postseismic deformation after the 2008 Iwate-Miyagi Nairiku earthquake (Mj 7.2). The displacement field exhibits ESE-WNW shortening and subsidence near the focal area. These features are attributed to a viscoelastic relaxation caused by the mainshock. A simple two-layered structural model, which consists of an elastic layer having a thickness of 19.0–23.5 km and an underlying Maxwell viscoelastic layer having a viscosity of 2.4–4.8 × 1018 Pa s, explains the far-field deformation pattern, which probably reflects the viscoelastic response exclusively. These estimated parameters are consistent with the deeper limit of the seismogenic layer in the upper crust and the previous rheological model in northeastern Japan. However, near-field deformation requires additional sources in order to reproduce the observed postseismic deformation, such as long-term afterslip and/or a complicated response due to the highly heterogeneous structure suggested by seismic tomography studies.

Strain anomalies induced by the 2011 Tohoku Earthquake (Mw 9.0) as observed by a dense GPS network in northeastern Japan

We have evaluated an anomalous crustal strain in the Tohoku region, northeastern Japan associated with a step-like stress change induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). Because the source area of the event was extremely large, the gradient of the observed eastward coseismic displacements that accompanied uniform stress change had a relatively uniform EW extension in northeastern Japan. Accordingly, the deformation anomaly, which is determined by subtracting the predicted displacement in a half-space elastic media from the observed displacement, should reflect the inhomogeneity of the rheology, or stiffness, of the crust. The difference of the EW extension anomaly between the forearc and backarc regions possibly indicates a dissimilarity of stiffness, depending on the crustal structure of the Tohoku region. The Ou-backbone range—a strain concentration zone in the interseismic period—shows an extension deficit compared with predictions. A low viscosity in the lower crust probably induced a relatively small extension. Meanwhile, the northern part of the Niigata-Kobe tectonic zone, another strain concentration zone, indicates an excess of extensional field. This is probably caused by a low elastic moduli of the thick sedimentation layer. The detection of strain anomalies in the coseismic period enables a new interpretation of the deformation process at strain concentration zones.

Modeling of coseismic crustal movements initiated by the May 24, 2013, Mw = 8.3 Okhotsk deep focus earthquake

The Okhotsk deep focus earthquake (Mw = 8.3), the largest in the history of instrumental seismology, occurred on May 24, 2013, at 05:45 UTC in the Sea of Okhotsk near the western coast of the Kamchatka Peninsula. For the first time we have succeeded in catching the field of horizontal and vertical coseismic offsets generated by a strong deep seismic event, and investigating its characteristics using continuous GPS measurements. Based on these data and taking into account the seismological information, we have developed a dislocation model of the Okhotsk deep focus earthquake.

Prestate of Stress and Fault Behavior During the 2016 Kumamoto Earthquake (M7.3)

Fault behavior during an earthquake is controlled by the state of stress on the fault. Complex coseismic fault slip on large earthquake faults has recently been observed by dense seismic networks, which complicates strong motion evaluations for potential faults. Here we show the three-dimensional prestress field related to the 2016 Kumamoto earthquake. The estimated stress field reveals a spatially variable state of stress that forced the fault to slip in a direction predicted by the “Wallace and Bott Hypothesis.” The stress field also exposes the pre-condition of pore fluid pressure on the fault. Large coseismic slip occurred in the low-pressure part of the fault. However, areas with highly pressured fluid also showed large displacement, indicating that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to improved seismic hazard evaluations. Plain Language Summary The three-dimensional prestress field around the 2016 Kumamoto earthquake controlled fault behavior of the earthquake. The estimated heterogeneous state of stress on the fault forced the fault to slip in the direction predicted. The stress field also exposed the precondition of pore fluid pressure on the fault. Large coseismic slip occurred not only at the low-pressure part of the fault but also highly pressured part. It indicates that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to upgrading seismic hazard evaluation.

Rheological Structure Beneath NE Japan Inferred from Coseismic Strain Anomalies Associated with the 2011 Tohoku-oki Earthquake (Mw9.0)

Strain concentration zones (SCZs), in which the E–W contraction under a slow tectonic loading is larger than the surrounding area, in NE Japan have been attributed to low viscosity anomalies (LVAs) in their lower crust. The 2011 Tohoku-oki earthquake (Mw9.0) induced a stepwise stress change over NE Japan. The coseismic E–W extension in the SCZ along the Ou backbone Range (OBR) was smaller than theoretical one, whereas it was larger in the forearc SCZ (FSCZ). This suggests variation in rheological structure beneath the SCZs. We numerically evaluated responses of variety of rheological models to the tectonic slow loading and the coseismic instantaneous unloading. A model with a viscoelastic upper crust, which is caused by high temperature relating to magmatic processes along volcanic front, below the OBR can reproduce the observed deformations in the OBR. The surface deformations in the FSCZ were explained as enhanced deformations of thick, compliant sediment. The viscoelasticity in the upper crust was not allowed beneath the FSCZ. LVA in the lower crust was not essential to reproduce the preseismic and the coseismic deformation anomalies in the FSCZ. However, the postseismic deformation was strongly affected by the existence of LVA in the lower crust. Precise observations of the postseismic deformations should provide key clues to elucidate the rheological structure beneath the FSCZ.