Coseismic velocity change in and around the focal region of the 2008 Iwate‐Miyagi Nairiku earthquake
Tóm tắt
The 2008 M7.2 Iwate‐Miyagi Nairiku earthquake occurred in NE Japan where dense seismic networks exist. Using the data from these networks, we determined the coseismic seismic velocity change associated with this earthquake by two different methods: ambient noise interferometry and vertical array interferometry of coda wave. The purpose of this article is to reveal the spatial distribution and the cause of the velocity change by integrating these two approaches. Ambient noise interferometry revealed a coseismic velocity decrease of Rayleigh wave by 0.1–0.5% at 0.25–0.5 Hz. We also estimated the spatial distribution of the velocity change by a tomographic inversion. The velocity decrease was distributed in and around the focal area. In the second method, we applied cross‐correlation analysis to coda waves observed by the KiK‐net vertical array. We detected a shear velocity decrease of approximately 5% in shallow layers up to a few hundred meters depth. Quantitative comparison of the two results reveals that the 5% shear velocity decrease in shallow layers can explain the 0.1–0.5% decrease of Rayleigh wave velocity. The distribution of the velocity decrease is similar to that of the strong ground motion and the static stress change. However, a significant velocity increase is not observed in the compression area, where a velocity increase is expected. Based on the observations, we consider that the primary factor affecting the velocity change is damage in shallow layers due to strong motion. The effect of the static stress change might be masked by the larger effect of the strong motion.
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Tài liệu tham khảo
Chin B. H., 1991, Simultaneous study of the source, path, and site effects on strong ground motion during the 1989 Loma Prieta earthquake: A preliminary result on pervasive nonlinear site effects, Bull. Seismol. Soc. Am., 81, 1859
Dunkin J. W., 1965, Computation of model solutions in layered, elastic media at high frequencies, Bull. Seismol. Soc. Am., 55, 335, 10.1785/BSSA0550020335
Fujiwara H. et al. (2009) A study on subsurface structure model for deep sedimentary layers of Japan for strong‐motion evaluation (in Japanese) Tech. Note 337 Natl. Res. Inst. for Earth Sci. and Disaster Prev. Tsukuba Japan.
Jenkins G. M., 1968, Spectral Analysis and Its Applications
Okada T., 2012, Group for the aftershock observations of the Iwate‐Miyagi Nairiku Earthquake in 2008, Hypocenter distribution and heterogeneous seismic velocity structure in and around the focal area of the 2008 Iwate‐Miyagi Nairiku Earthquake, NE Japan–Possible seismological evidence for a fluid driven compressional inversion earthquake, Earth Planets Space
Saito M., 1988, Seismological Algorithms: Computational Methods and Computer Programs, 293
Shiomi K., 2003, Estimation on the azimuth of the Hi‐net and KiK‐net borehole seismometers (in japanese), J. Seismol. Soc. Jpn., 56, 99