地震 第2輯
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
最新号
選択された号の論文の5件中1~5を表示しています
論説
  • 野末 陽平, 深畑 幸俊
    2022 年 75 巻 p. 1-12
    発行日: 2022/03/22
    公開日: 2022/05/27
    ジャーナル 認証あり

    Present day crustal displacement rates are accurately observed by global navigation satellite system (GNSS). In estimating continuous displacement fields from spatially discrete GNSS data, the method of basis function expansion is a powerful tool. As basis functions, the boxcar function is the easiest to be implemented, while the cubic B-spline function has good nature in estimating continuous and smooth fields. In this study, we quantitatively compare the performance of the boxcars with that of the cubic B-splines, changing the basis-function interval L in estimating velocity fields from observed GNSS data in Japan. The result shows that the performance of the cubic B-splines with L=50, 60, and 80 km is nearly the same or better than that of the boxcars with L=20, 25, and 30 km, respectively. In other words, to achieve similar performance, at least about 2.52=6.25 times as many boxcars as cubic B-splines are needed. In the method of basis function expansion, the computation of inverse matrices is the most time-consuming process. Because its computational cost is usually proportional to the cube of the matrix size (the number of basis functions), computing inverse matrices for the boxcars takes about 250 times as much time as that for the cubic B-splines, although the difference of actual computational costs in this study was about 100 times or more. In addition, strain rates can be easily obtained by analytically differentiating the velocity field when we use the cubic B-splines. On the other hand, a main disadvantage in using the cubic B-splines is complicated computation to obtain the explicit expression of the smoothness constraint for the inversion analysis. To mitigate this problem, we show the computation results in Appendix.

技術報告
  • 三浦 亮, 野 徹雄, 小平 秀一, 前川 拓也, 寺田 育正, 柴田 英紀, 岡部 圭二
    2022 年 75 巻 p. 13-28
    発行日: 2022/04/05
    公開日: 2022/05/27
    ジャーナル 認証あり

    Obtaining marine geophysical data, particularly from seismic surveys and observations, is difficult when using ocean-bottom seismographs (OBSs) in coastal areas without causing a conflict with fishery activities. To minimize conflict, we have designed and operated anchor-recovery-type OBSs in coastal areas shallower than 200 m. The OBSs shallower than 650 m were required to recover with their anchors to minimize such conflict during our seismic survey cruise off Yamagata Prefecture by the research vessel (R/V) Kairei in August 2019. Therefore, we improved the OBS anchor recovery system, which can operate from the continental shelf to the upper continental slope at depths shallower than 650 m. Two operation trials of the improved anchor-recovery-type OBS were conducted using a multi-purpose experiment tank at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Yokosuka Headquarters, to evaluate its performance before the cruise. In addition, we prepared a sweep-line with grapnel anchors in case unexpected problems occurred during the OBS recovery operation. The seismic survey cruise used a tuned air-gun array system and 39 OBSs (including eight anchor-recovery-type OBSs). The OBS recovery operation was initiated following the air-gun shooting. To prevent the propellers of R/V Kairei from tangling in the recovery rope, a two-ship operation was planned for the recovery of the anchor-recovery-type OBSs. The two-ship operation was performed as follows: First, the R/V Kairei sent an acoustic release command to the OBS. Second, the surfaced OBS was recovered by the ocean tugboat, motor vessel (M/V) Hirokai. Finally, the anchor recovery rope was wound using the winch on M/V Hirokai to recover the anchor. We successfully recovered seven OBSs (Sites 1-7) and their anchors; however, one OBS (Site 8) did not respond to acoustic communication, including the release command. We attempted to recover this unreleased OBS using the prepared sweep-line with grapnel anchors. The sweep-line was deployed from the stern of the R/V Kairei and towed around the OBS position with a radius of approximately 100 m. The OBS and its anchor were caught by the sweep-line and successfully recovered. Our improved anchor-recovery-type OBS can reduce conflict with fishery activities and enable to conduct marine seismic surveys in coastal areas from the continental shelf to the upper continental slope.

論説
  • 柳田 浩嗣, 仲谷 幸浩, 八木原 寛, 平野 舟一郎, 小林 励司, 山下 裕亮, 松島 健, 清水 洋, 内田 和也, 馬越 孝道, 八 ...
    2022 年 75 巻 p. 29-41
    発行日: 2022/04/27
    公開日: 2022/05/27
    ジャーナル 認証あり

    A magnitude 7.1 earthquake occurred west off Satsuma Peninsula, Japan in 2015, which was the largest earthquake ever recorded in the northern Okinawa Trough (OT). The northern OT is assumed to be in a beginning stage of a back-arc rifting that drives crustal extension, and the occurrence of the 2015 mainshock-aftershock sequence might be associated with such tectonics. In order to understand the rifting process that controls the seismicity, we precisely determined hypocenters and focal mechanisms of the aftershocks listed in the Japan Meteorological Agency catalog using both offshore and onshore seismic data. Initial hypocenters were determined using manual picks of P and S phases and a 1-D velocity structure. We then applied station correction terms to the P- and S-arrival times so as to consider low-velocity sedimentary layers beneath each station, especially beneath the offshore stations. Finally, the relative hypocenters were refined by the double-difference location method. The average root-mean-square residuals of the double differences decreased from 489 ms to 39 ms. The relocated hypocenter distribution clearly shows three linear alignments: one in the N95°E direction, consisting of left-lateral strike-slip fault events, and two in the N15-20°E direction, consisting of normal fault events. The tension axes for almost all the solutions lie in a common direction (NW-SE). The well-determined aftershocks mainly located in the upper crust with focal depths of 0-15 km. The results from our ocean-bottom seismic observation indicate that the two alignments consisting of normal fault events might be caused by crustal extension related to the back-arc rifting in the northern OT because the direction of the hypocenter distribution is consistent with the strike of the existing normal faults imaged by the multichannel seismic reflection survey. The left-lateral strike-slip fault alignment could be explained by along-axis variations in the back-arc spreading rate and presence of a transcurrent fault.

寄書
feedback
Top