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The 1978 Miyagi-oki earthquake
Abstract
On June 12, 1978, a large earthquake (M=7.4) occurred off the Pacific coast of Miyagi prefecture, northeastern Japan. The earthquake killed 28 persons and caused extensive damage in Miyagi prefecture. The previous large earthquake occurred in this region on November 3, 1936. Therefore, a next large earthquake is expected in this region in near future. It is important to estimate the slip distribution of both the 1978 and 1936 events in order to discuss a possible rupture area of the next event. In this paper, we estimate the slip distribution of the 1978 Miyagi-oki event using the tsunami waveforms observed at 12 tide gauge stations along the Pacific coast of northeastern Japan. Previously, the rupture process of the 1978 Miyagi-oki earthquake was estimated using the seismological data (eq. Seno (1980)). Seno (1980) show that the focal mechanism of the earthquake is a thrust type and the seismic moment is 3.1x10**20Nm. The event was separated into two subevents, one ruptured the eastern (trench-ward) part and the other ruptured the western (landward) part of the aftershock area. We numerically compute the tsunami waveforms using the finite-difference computation for the linear long-wave equations. The grid size is basically 20 sec of arc (about 600m), but finer grids (4 sec of arc) are nested near the tide gauge stations. Tsunami waveforms at 12 stations are computed for two subfaults (the western and eastern parts) with a unit amount of slip, and use as the Green's function for the inversion. The subfault size is 30km X 30km. The result of the inversion shows that the slip amounts of the western and eastern parts are about 1.3 m and 1.0 m, respectively. The total seismic moment is calculated as 1.4 X 10**20Nm (Mw=7.4) assuming that the rigidity is 5 X 10**10 N/m**2. This estimate is smaller than the seismic moment estimated from seismological data. The slip amount in the western subfault is larger than that in the eastern subfault. This slip pattern is consistent with the previous result. It will be interesting to estimate the slip pattern of the 1936 event and compare it with the slip pattern of the 1978 event in this study.
... The distances between the hypocenters and the plate interface are the same for all the cases. The assumed parameters of Case 1 are similar to those of the M = 7.1 Miyagi- Oki intraslab earthquake of May 26, 2003 (e.g., Okada and Hasegawa, 2003; Yamanaka and Kikuchi, 2003).Figure 4 shows static shear and normal stresses on the plate interface caused by the intraslab earthquakes inTable 1, where shear stress in slip direction and compression of normal stress are taken to be positive. These stresses are calculated by using analytical expressions for stresses due to dip slip (Rani and Singh, 1992). ...
The effect of static stress perturbations due to an intermediate depth intraslab earthquake on seismic cycles of large interplate earthquakes at a subduction zone is examined through a numerical simulation using a laboratory-derived friction law. The frictional stress on a plate interface in a two-dimensional uniform elastic half-space model is assumed to obey a rate-and state-dependent friction law, and the plate interface is loaded by a steady plate motion, resulting in recurrence of large interplate earthquakes in the model. Static stress perturbations on the plate interface due to an intermediate depth intraslab earthquake with reverse fault type is introduced to the model. This model is set up so as to simulate seismic cycles at the Miyagi-Oki subduction zone, northeastern Honshu, Japan, where a large interplate earthquake is expected to occur and an intraslab earthquake of M = 7.1 took place in May, 2003. The simulation result indicates that the static stress changes due to the intraslab earthquake promote slip just below the interplate seismogenic zone, accelerating aseismic sliding there. This aseismic sliding generates stress concentration at the bottom of the seismogenic zone, often advancing the occurrence time of the next interplate earthquake. The shear-stress increases at aseismic slip regions with velocity-strengthening frictional property cannot be held and then aseismic sliding should take place to relax the increased stress. The present simulation result suggests that the promotion of aseismic sliding around the source area is important for evaluating the triggering effects of earthquakes. The conventional Coulomb failure stress approaches to the evaluation of the effect of static stress changes on seismic activity may be insufficient when static stress changes influence aseismic slip rates.
This study investigates the effects of using different finite-fault source models in evaluating rupture distances for megathrust subduction earthquakes. The uncertainty of the calculated rupture distances affects interpretation of the recorded ground motions significantly. To demonstrate this from an empirical perspective, ground motion data and available finite-fault models for the 2011 M9.0 Tohoku, 2003 M8.3 Tokachi-oki, and 2005 M7.2 Miyagi-oki earthquakes are analyzed. The impact of different finite-fault models on the development of ground motion prediction equations for these large subduction events is significant. Importantly, the results suggest that comparison of observed ground motion data with existing ground motion prediction models is not straightforward; different conclusions may be reached regarding agreement/disagreement between empirical data and developed models, depending on the selected finite-fault model. These results are particularly relevant to the development of ground motion prediction equations for subduction regions.
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