On the similarity of strong ground motions from a deep-focus earthquake (01/01/2012, Mj7.0) and the 2011 Tohoku earthquake - possible effects of specific propagation path
Abstract
On 1st January, 2012, there was a deep-focus earthquake (Mj7.0) at the depth of 397km close to the Izu-Ogasawara Trench, south of Japan. It is well known that, for such a deep-focus earthquake, anomalous seismic intensity distribution is obtained because of the propagation of seismic waves through the high-Q subducting plate (in this case, The Pacific plate). According to our analysis, such a specific propagation path may have significant effects, not only for intensity distribution but also for the characteristics of strong ground motions. For this earthquake, records of ground motions were obtained at permanent strong motion stations in a wide area. We focused on the records in the Kanto region, where largest ground motions were observed. We analyzed the Fourier phase characteristics of the records, using the method of substituting the Fourier phase characteristics of the records with those from other earthquakes at the same station [Nozu and Irikura, 2008; Wakai and Nozu, 2012]. It was surprising to find that the Fourier phase characteristics of the deep-focus event were similar to those of the 2011 Tohoku earthquake. In the past study, such similarities were observed for two or more closely-located earthquakes, because, for the closely-located earthquakes, the path and site effects are shared between the two events [Nozu and Irikura, 2008; Wakai and Nozu, 2012]. However, for the current earthquake pair, the earthquakes are not closely-located at all. Our interpretation is as follows. In this case, seismic waves radiated from the deep-focus event can propagate with little attenuation through the Pacific plate until they approach to the plate boundary close to the Kanto region. Then they can propagate through the landside plate, toward the observation sites, being affected by the existence of the sedimentary basin. By the way, the seismic waves from the Tohoku earthquake also propagate from the plate boundary to the observation sites, being affected by the sedimentary basin. The sedimentary basin has similar effects for the two events because the incident angles are similar. This should be the reason why strong ground motions from the two events are so similar. Thus, we concluded that the specific propagation path for the deep-focus event contributed to the similarity of strong ground motions of the two events. The permanent strong-motion stations (black triangles), the rupture starting point of the 2011 mainshock by JMA (red star), epicentres of a deep-focus events (blue star), the SPGAs of the 2011 main shock (orange cross and circle) [Nozu, 2011] .
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.S52C..06W
- Keywords:
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- 7212 SEISMOLOGY Earthquake ground motions and engineering seismology