Abstract
We have analyzed the temporal variation in coda Q in the northeastern part of an inland high strain rate zone, the Niigata–Kobe Tectonic Zone (NKTZ), in central Japan, to investigate the response of coda Q to the modification of the strain field induced by the 2011 Tohoku earthquake (Mw 9.0). We observe no statistically significant temporal variations in the spatial distribution of coda Q as the whole analyzed area, implying that the crustal deformation induced by the 2011 Tohoku earthquake has provided no significant temporal variation in crustal heterogeneity as the whole analyzed area. For the middle frequency bands, before and after the 2011 Tohoku earthquake, we have commonly found a negative correlation between the spatial distributions of coda Q and the differential strain rate and a positive correlation between the spatial distributions of coda Q and the perturbation of S-wave velocity in the upper crust. These features, together with previous works, suggest that the ductile deformation with a high rate in the upper crust plays an important role in generating the high strain rate in the analyzed area not only before but also after the 2011 Tohoku earthquake. In other words, the existence of a persistent ductile deformation in the upper crust contributes essentially to the generation process of the high strain rate in the northeastern part of the NKTZ. It is important to note that the location of the persistent ductile deformation in the northeastern part of the NKTZ, mainly in the upper crust, differs from that in the central part of the NKTZ, mainly in the lower crust.
Highlights
Crustal stress and strain are fundamental factors to generate an inland earthquake
In the low frequency bands (1–2 and 2–4 Hz), low coda Q is distributed around the Itoigawa–Shizuoka Tectonic Line (ISTL) and some volcanoes and high coda Q around the Niigata plain
We use the result of our previous work to compare the distribution of coda Q before and after the 2011 Tohoku earthquake
Summary
Crustal stress and strain are fundamental factors to generate an inland earthquake. In recent decades, the progress in geodetic and seismological observations has revealed spatiotemporal variation in crustal activity, strain rate and seismicity, in detail. An inland high strain rate zone from Niigata to Kobe, called the Niigata–Kobe Tectonic Zone (NKTZ), in central Japan, revealed by a nation-wide dense GNSS observation network (Sagiya et al 2000), is an important field in understanding the Variation in stress and strain in the crust has been clarified by the investigation of seismic waves. Coda Q and shear wave splitting as well as seismic velocity are sensitive to variation in crustal stress/strain. A spatial correspondence between a low coda Q area at lower frequency bands and the NKTZ was first pointed out by Jin and Aki (2005). This correspondence has been confirmed by Hiramatsu et al (2010, 2013), Carcolé and Sato (2010), Tsuji et al (2014) and Dojo and Hiramatsu (2017). Hiramatsu et al (2010) reported that shear wave splitting and coda Q were a function of the strain rate in/around
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