JpGU-AGU Joint Meeting 2017

講演情報

[EE] ポスター発表

セッション記号 S (固体地球科学) » S-SS 地震学

[S-SS04] [EE] Subduction zone dynamics from regular earthquakes through slow earthquakes to creep

2017年5月24日(水) 15:30 〜 17:00 ポスター会場 (国際展示場 7ホール)

コンビーナ:金川 久一(千葉大学大学院理学研究科)、小原 一成(東京大学地震研究所)、Demian M Saffer(Pennsylvania State University)、Wallace Laura(University of Texas Institute for Geophysics)

[SSS04-P26] Precise hypocenter determination of deep low-frequency earthquakes in the Tokai area of the Nankai subduction zone using a 3D array and Hi-net

*鈴木 貞臣1大久保 慎人2今西 和俊3中島 淳一4 (1.(公財) 地震予知総合研究振興会 東濃地震科学研究所、2.高知大学、3.独立行政法人 産業技術総合研究所、4.東京工業大学)

キーワード:deep low-frequency earthquake, precise hypocenter determination, 3D array, plate boundary, P and S waves, Hi-net

We have developed a novel method that uses a 3D array to detect the P and S waves of deep low-frequency earthquakes (LFEs) that occur along the Nankai subduction zone of the Philippine Sea plate in southwest Japan. Obtaining accurate hypocenters of LFEs is very difficult because their seismic waves are characterized by low amplitude and the absence of sharp pulses. In particular, identifying P phase arrivals is not readily possible using only conventional seismic networks. To determine their hypocenters accurately not only their epicenters but also their depths we tried to find their P and S wave pairs and obtain S-P times by using a 3D array (6 km x 4 km area) with 14 seismic stations in the Tokai area including ones with deep (600 m at the deepest) borehole seismographs. We observed remarkable LFE activity occurring in the Tokai area over November 10-30, 2010. We calculated the semblance distributions for their seismic waves of 75 LFEs to identify P and S phases and obtained their propagation parameters (back azimuths and incident angles) . Using the back azimuths and incident angles of S waves, and S-P times with high quality result, we calculated the preliminary hypocenters of 15 LFEs by using a shooting method. Referring to those identified P and S phases we manually picked the arrival times of not only both P and S waves of the 3D array stations but also S waves and rarely P waves of Hi-net stations. Using the arrival times we relocated precise hypocenters of the 15 LFEs. Those hypocenters distribute in the depth range from 26 km to 34 km (red stars in Fig.1) approximately along the plate interface (Hirose et al., 2008) inclining in depth from 30 km to 32 km. The errors of those hypocenters estimated from residuals of the arrival times are 0.2-0.9 km in horizontal and 0.4-0.8 km in depth, respectively. By using the same procedure as we explained for LFEs, we also relocated precise hypocenters of 13 regular earthquakes occurring in the subducting Philippine Sea plate. Those regular earthquakes belong to the intra-slab events which are normal-fault or strike-slip type earthquakes, with the T–axis oriented in an E-W direction (e.g. Miyoshi and Obara, 2010). Those hypocenters distribute in the depth range from 35 km to 45 km (light blue squares in Fig.1). We found that the hypocenters of LFEs did not overlap with the regular earthquakes in depth. In Fig. 2 we compared the depth distributions of LFEs and the regular earthquakes with a simple model of inter-plate structure under Tokai area. From Fig.2 we may conclude that LFEs occur not only along a thin subduction interface (less than about 1(?) km in width) but also in the nearly whole layer of oceanic crust and several km width in wedge mantle. And the total depth range of the LFEs obtained from the standard deviation is about 5 km. This distribution of LFEs may support the undrained condition model (Nakajima and Hasegawa, 2016) of enhanced pore-fluid pressure for LFE activity.
(Acknowledgment) National Research Institute for Earth Science and Disaster Prevention (NIED) , Japan Meteorological Agency (JMA)
(References)
Hirose F, Nakajima J, Hasegawa A (2008) Three-dimensional seismic velocity structure and configuration of the Philippine Sea slab in southwestern Japan estimated by double-difference tomography, J. Geophys. Res., 113:B09315. doi:10.1029/2007JB005274.
Kato A, Iidaka T, Ikuta R, Yoshida Y, Katsumata K, Iwasaki T, Sakai S, Thurber C, Tsumura N, Yamaoka K, Watanabe T, Kunitomo T, Yamazaki F, Okubo M, Suzuki S, and Hirata N (2010) Variations of fluid pressure within the subducting oceanic crust and slow earthquakes, Geophys. Res. Lett., 37:L14310. doi:10.1029/2010GL043723.
Miyoshi T, Obara K (2010) Double seismic zone within the ridge-shaped slab beneath southwest Japan, Earth Planets Space, 62, 949-954.
Nakajima J, Hasegawa A (2016) Tremor activity inhabited by well-drained conditions above a megathrust, Nature Communications 7, doi:10.1038/ncomms13863.