Abstract

The spatial and temporal variations of coda attenuation (forumla) were studied in the source region of the 2011 Off the Pacific Coast of Tohoku, Japan (Mw 9) earthquake. The forumla values were determined from the amplitude decay rate of the S‐wave coda in narrower overlapping frequency bands in the range f=1.0–24  Hz, based on a single isotropic scattering model for more than 400 earthquakes (MJMA 3∼6.5) in the region recorded in a period from January 2005 to August 2011, including pre‐ and postseismic period. Our estimates of the spatially averaged forumla value, forumla in f=1.0–24  Hz, is almost stable with small variations (<10%) during the preseismic period, which is in good agreement with that estimated by former studies in the area of northeast Japan. We found that the value of forumla increases by about 10%–16% after the 2011 Tohoku event in f=1.25–3.5  Hz in some stations of northern Japan, which is confirmed by a statistical t‐test at 99.9% confidence level. The change in forumla is spatially limited to the rupture zone, while other paths remain nearly unaffected, suggesting local changes of scattering properties in the vicinity of the Tohoku‐Oki source volume. This change may be attributed to an increase in the density of open microcracks in the mainshock source volume (such as due to increase in stress induced by the 2011 event) and probably the fluid content in fractures in the rocks. A model of heterogeneity due to coseismically opened cracks (dominant scale length of a=0.6–1.8  km in f=1.25–3.5  Hz) enhanced by increased stress change possibly controls the increased forumla after the 2011 event in the source rupture region.

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