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[J08-P-09] Plate boundary property of the Philippine Sea plate revealed from later phase analysis beneath southwestern Ibaraki and northwestern Chiba prefectures, central Japan
We found clear phases in vertical component of 1-4 Hz coherently in the MeSO-net (Metropolitan Seismic Observation network) line, between direct P and S arrivals (hereinafter, X phase). Traveltime difference between the X phase and direct P decreases for shallower earthquake. These features indicate that the X phase is converted from S to P above the hypocenter.
We compared observed traveltime difference and amplitude ratio between SP and direct P with synthetic values to cancel out surface structure effects, which were calculated using velocity structure and the upper boundary of the Philippine Sea plate (UPHS) determined by MeSO-net (Nakagawa et al., 2010; Hirata, 2012) with sedimentary layer (J-SHIS; Fujiwara et al., 2012). For amplitude calculation, radiation pattern expected from focal mechanism of NIED F-net or Hi-net, geometrical spreading, anelastic attenuation, and conversion or transmission coefficients on the UPHS were considered. Q of P and S waves were assumed to be 1000. Observed traveltimes were well explained. This indicates that the X phase is generated at the UPHS. On the other hand, small change in velocity structure was necessary to explain observed amplitude ratio. Estimated velocity exhibits large fluctuation, however, Vs was estimated to be 3 % slower on average just below the UPHS.
Since the target region is within the network of MeSO-net, reliability of the velocity structure and the UPHS are likely to be high. On the other hand, grid spacing is 5-10 km or larger, and hence structure smaller than this scale could not be imaged. Examination of amplitude ratio of later phase indicates small scale inhomogeneous structure around the UPHS.