S wave attenuation structure on the western side of the Nankai subduction zone: Implications for fluid distribution and dynamics. Issue 10 (29th October 2014)

• Record Type: Journal Article
• Authors: Takahashi, Tsutomu; Obana, Koichiro; Yamamoto, Yojiro; Nakanishi, Ayako; Kodaira, Shuichi; Kaneda, Yoshiyuki
• Is Part Of: Journal of geophysical research. Volume 119:Issue 10(2014:Oct.)
• Page Start: 7805
• Page End: 7822
• Publication Date: 2014-10-29
• Subjects: Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1002/2014JB011103
• Languages: English
• ISSNs: 2169-9313

Abstract

<abstract abstract-type="main"> <title>Abstract</title> <p>We estimated the <italic>S</italic> wave attenuation structure in southwestern Japan and the western Nankai Trough by analyzing maximum <italic>S</italic> wave amplitudes at 4–8, 8–16, and 16–32 Hz with a correction term for apparent amplitude attenuation due to multiple forward scattering. Because the estimated attenuation (<italic>Q</italic><sup>−1</sup>) in our tomographic study was much larger than <italic>Q</italic><sup>−1</sup> due to wide‐angle scattering, our estimated <italic>Q</italic><sup>−1</sup> was composed mainly of intrinsic attenuation. High‐attenuation areas (<italic>Q</italic><sup>−1</sup> > 1/300 at 4–8 Hz) were imaged beneath Quaternary volcanoes and south off Shikoku. Low (<1/1500 at 4–8 Hz) or moderate <italic>Q</italic><sup>−1</sup> (1/500–1/1000 at 4–8 Hz) was imaged beneath Shikoku and nonvolcanic areas of Chugoku. High and moderate <italic>Q</italic><sup>−1</sup> in and around Shikoku are located near the top of subducting Philippine Sea Plate. This correspondence implies that these high and moderate <italic>Q</italic><sup>−1</sup> reflect fluid in the subducting slab. By applying a theoretical model of attenuation in water‐saturated porous random media, we examined wave‐induced fluid flow induced by lower frequency (<1 Hz) seismic waves that may be related with triggering of nonvolcanic tremor by surface waves. Even though <italic>Q</italic><sup>−1</sup> structure in this study cannot fully explain the tremor triggering by wave‐induced fluid flow, large uncertainties of <italic>Q</italic><sup>−1</sup> in tremor zone suggest that high resolution imaging of <italic>Q</italic><sup>−1</sup> and random inhomogeneities would give some constraints for the spatial variation of permeability and other medium properties.</p> </abstract>