Crustal structure in and around the Onikobe geothermal area, northeastern Honshu, Japan, inferred from the spatial variation of coda decay

The Onikobe area is an active geothermal area situated in the Ou backbone range of northeastern Honshu, Japan. It is home to calderas from the Tertiary to Quaternary eras and active volcanoes. A systematic spatial variation of Q_c has been found in this area: Q_c values are lower at stations in and around calderas than at other stations. The amplitude of coda waves with high Q_cs decreases more slowly after a lapse time of around 7-10s than that with low Q_cs. In the present study, to determine causes for these coda decay variations, coda envelopes were synthesized in a structure model in which high attenuation zones existed beneath the Onikobe and Sanzugawa calderas and where scattering coefficients were higher in the lower crust than in the upper crust. Using hypocenters shallower than 10km, envelopes were calculated for 256 station-hypocenter pairs with epicentral distances of less than 10km. It was assumed that the coda waves were composed of S-S scattered waves, and that the scattering was single and isotropic. The observed features of the Q_c distribution were reproduced in the synthesis, and synthesized envelopes were found to mostly coincide with observed decay curves. The top of high attenuation zones was thus estimated as being deeper than 7.5km. The structure assumed for the synthesis was consistent with that of previous studies. We consider that the structure model used was appropriate, and that high attenuation zones beneath calderas and the reflective lower crust caused the spatial variation of the Q_c and decay curves in the Onikobe area. We also consider that studies using coda decay would be beneficial in detecting high attenuation zones and the reflective lower crust.