[SPA]PM-3

[S09P-03] Stress field in the eastern margin of the Japan Sea and its relationship with faults of recent earthquakes(2)

*Ayaka Tagami¹, Tomomi Okada¹, Shin'ichi Sakai², Mako Ozono^2,3, Kei Katsumata³, Masahiro Kosuga⁴, Yoshiko Yamanaka⁵, Katao Hiroshi⁶, Takeshi Matsushima⁷, Hiroshi Yakiwara⁸, Takashi Nakayama¹, Satoshi Hirahara¹, Toshio Kono¹, Shu'ichiro Hori¹, Toru Matsuzawa¹, . Group for the aftershock observations of the 2011 off the Pacific coast of Tohoku Earthquake (1. Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, 2. Earthquake Research Institute, University of Tokyo, 3. Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, 4. Graduate School of Science and Technology, Hirosaki University, 5. Graduate School of Encironmental Studies, University of NAGOYA, 6. Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, 7. Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, 8. Nansei-toko Observatory for Earthquakes and Volcanoes, Kagoshima University)

1. はじめに
　日本海東縁変動帯(EMJS)では多数の活断層や褶曲構造がおよそ海岸に並行して存在し，多数の大地震が発生している(Okamura, 2010)．EMJSおよび東北日本では概ね東西方向の最大圧縮軸を持つ逆断層型の応力場により逆断層運動が生じている(e.g., Terakawa and Matsu’ura, 2010 )．本研究では，EMJS内における7つの規模の大きな地震(表1・2)に着目し，応力場との関係性について調査を行った．
2. データと方法
　本研究では応力場を推定する際にNIED(国立研究開発法人防災科学技術研究所)とOkada et al. (SSJ meeting, 2019)のメカニズム解データを使用した．応力場は，応力テンソルインバージョンにより推定し，SATSI (Spatial And Temporal Stress Inversion)法(Hardebeck and Michael, 2006)を用いた．断層面との関係についてはSlip Tendency analysis (Morris et al.,1996)を用いた．断層面の情報についてはNIEDのメカニズム解データまたは各地震について先行研究で推定されたメカニズム解のデータや断層モデルを使用した．
3. 結果
・断層面の傾斜方向・傾斜角の検討
各地震メカニズム解(表1)の節面に対してSlip Tendency(ST)値を算出した．全体的に低傾斜角の節面が高いST値を持つ傾向となった．この結果は，EMJSにおける大規模地震は日本海生成時の引張応力場により生成した高角正断層の再活動ではなく，現在の圧縮応力場によって生成した低角逆断層が活動したものであるとする考えに整合する．
・複数面モデルの検討
3つの地震(表２)について先行研究において推定された断層モデルを使用し、ST値を算出した．庄内沖地震と日本海中部地震については先行研究で最適とされる断層モデルが高いST値を示した．北海道南西沖地震については5つの断層面で構成されるTanioka et al. (1995)のモデルを使用しST値を算出した．結果，北部の3つの面では高いST値，南部の２つの面は低いST値を示した．南部の面は応力場に対しすべりにくいにも関わらず活動したと示唆される．


Introduction
The eastern margin of the Japan Sea (EMJS) is known as many earthquakes occurred (Okamura, 2010). In this study, we focus on the seven large to moderate-sized earthquakes (Table 1, 2).
Data and methods
We use focal mechanism (centroid moment tensor) data from the National Institute for Earth Science and Disaster Prevention (NIED) of Japan and focal mechanism by P-wave initial motion from Okada et al. (SSJ meeting, 2019). For estimating regional stress field, we deploy the SATSI (Spatial and Temporal Stress Inversion) stress tensor inversion method (Hardebeck and Michael, 2006). For estimating the likelihood of slip, we use the Slip Tendency analysis (Morris et al., 1996).
Results
We focused on the six earthquakes to estimate the relationship between fault planes and stress field, and calculated slip tendency to each focal mechanism based on Slip Tendency (ST) analysis. We used focal mechanisms estimated in several previous studies as fault plane model.
As a result, the nodal plane with shallow dip angle showed a high ST value in each focal mechanism. From these results, it is considered that these earthquakes are not occurred by reactivation of the high-angle normal fault which generated by the extensional stress filed during the Japan Sea formation but occurred by reverse faults with low dip angle which generated by the current compressive stress field.

Next, we focused on the three earthquakes (Table 2), which were estimated as multiple-faults models. We used fault model estimated in previous study for calculate ST value.
As a result, Shonai-Oki and Nihonkai-Chubu models showed high ST value. Hokkaido Nansei-Oki model (Tanioka et al., 1995) consist of 5 planes. Three planes in north showed high ST value but southern two planes showed low ST value.

References
Hardebeck, J. L., and A. J. Michael, 2006, J. Geophys. Res., v. 111(B11), B11310.

Morris, A., Ferril, D.A., Henderson, D.B., 1996, Geology 24, 275-278.

Okamura, Y., 2010, Jour. Geol. Soc. Japan, 116, 582-59.

Terakawa, T., and M. Matsu’ura, 2010, Tectonics, 29(6).