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2017年の臨時地震観測による鹿児島湾・喜入沖の震源分布と発震機構 : 鹿児島地溝形成に伴う断層に沿った顕著な地震活動

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  • Hypocenter distribution and focal mechanism of earthquakes off the coast of Kiire, Kagoshima Bay obtained from seismic observation in 2017 : Remarkable seismic activity along the faults that associated with the formation of Kagoshima Rift
  • 2017ネン ノ リンジ ジシン カンソク ニヨル カゴシマワン キイレオキ ノ シンゲン ブンプ ト ハッシン キコウ : カゴシマ チコウ ケイセイ ニ トモナウ ダンソウ ニ ソッタ ケンチョナ ジシン カツドウ

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Abstract

On July 11, 2017, a magnitude (M) 5.3 earthquake occurred off the coast of Kiire, Kagoshima which was the largest crustal earthquake in and around Kagoshima Bay in the last 100 years. Kagoshima Bay is included in Kagoshima Rift as formed by normal faults in its both eastern and western margins. The structure and tectonic process of the rift have been considered to relate with the seismic phenomena in this region. Because major calderas and active volcanoes, such as Sakurajima, also locate within Kagoshima Bay, local tectonics behind geophysical activity is quite complicated. Therefore, to investigate the detailed hypocenter distribution and focal mechanism of the remarkable M5-class event and its sequence is essential for understanding the crustal background stress and faulting process.Since 2005, Nansei-Toko Observatory for Earthquakes and Volcanoes (NOEV), Kagoshima University have installed five onshore offline seismic stations around Kagoshima Bay, which consist of 2-Hz velocity sensors and data loggers. The M5.3 earthquake (hereafter, the mainshock) occurred in the immediate the vicinity of the last installed station which had just started observation from March, 2017. We merged both five offline and eight realtime seismic station data from 03/29/2017 to 12/31/2017 and then finally determined 776 hypocenters before and after the mainshock. Location method of this study is based on manual picking of arrival times of P- and S-phases and the event list is obtained from NOEV routine earthquake catalog. In order to minimize travel time residuals in the hypocenter determination, the station correction terms are applied to the procedure. Out of the final 776 events, we obtained the focal mechanisms for the mainshock and 157 aftershocks in the period 07/11/2017 to 09/30/2017 using P-wave first-motion polarities.For the period before the mainshock, NOEV catalog data show that seismicity off Kiire region have started to be active from 11/20/2015. The beginning of the active phase might be associated with the static stress change triggered by the M7.1 earthquake off the west of Satsuma Peninsula on 11/14/2015. In a series of the preceding active episodes of seismicity, the mainshock occurred in July, 2017. For the period including the mainshock-aftershock sequence, the final hypocenter distribution of this study clearly indicates that most aftershocks are located along two separate east-dipping planes aligned parallel to each other in the east-west direction. Both seismic planes have a horizontal distance of approximately 1.7 km and dip angles of 53°. Because the up-dip extensions of the two seismic planes correspond to the respective topographic features of Kagoshima Rift, both seismic planes can be explained as the faults formed by Kagoshima Rift. On the other hand, approximately 90% of the focal mechanisms for the mainshock and aftershocks determined by this study show strike-slip faulting with a NW-SE tension axis, which is not the faulting type expected from the east-dipping fault planes but are in good agreement with the direction of principal stress estimated from shallow crustal earthquakes in southern Kyushu. Focusing on the day of the mainshock, the subsequent aftershocks distributed nearly vertical at depths of about 9–11 km that corresponds to the dip angle (86°) of the assumed fault plane of the mainshock. After two weeks from the mainshock, the aftershock areas gradually expanded in the up-dip directions along both fault planes with the dip angles of 53°. This aftershock migration might be caused by supply of fluids from the deep crust to the faults as preferential flow paths. In conclusion, the occurrences of the mainshock and the subsequent aftershocks are constrained by the regional stress field and the migration of the crustal fluids along the faults formed by rifting process of Kagoshima Rift, respectively.

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