Stress relaxation arrested the mainshock rupture of the 2016 Central Tottori earthquake

地震の破壊はなぜ止まるのか? --2016年鳥取県中部地震の断層サイズを決めたもの--. 京都大学プレスリリース. 2021-08-12.After a large earthquake, many small earthquakes, called aftershocks, ensue. Additional large earthquakes typically do not occur, despite the fact that the large static stress near the edges of the fault is expected to trigger further large earthquakes at these locations. Here we analyse ~10, 000 highly accurate focal mechanism solutions of aftershocks of the 2016 Mw 6.2 Central Tottori earthquake in Japan. We determine the location of the horizontal edges of the mainshock fault relative to the aftershock hypocentres, with an accuracy of approximately 200 m. We find that aftershocks rarely occur near the horizontal edges and extensions of the fault. We propose that the mainshock rupture was arrested within areas characterised by substantial stress relaxation prior to the main earthquake. This stress relaxation along fault edges could explain why mainshocks are rarely followed by further large earthquakes

昭和南海地震震源域-四国沖～紀伊半島沖-の構造変化

BE12-54講演要旨 / ブルーアース2012（2012年3月22日～23日, 東京海洋大学品川キャンパス

Structural variation from off Shikoku to the Kii Peninsula related to various earhthquake phenomena

南海トラフで繰り返し発生する巨大地震の中には、東海・東南海・南海地震が連動して発生する超巨大地震のケースがあることが指摘されている。このような超巨大地震について、どのような場合に連動し、あるいは非連動性巨大地震となるのかを明らかにすることが必要である。連動型地震の滑り域の範囲がどこまで広がるかを見積もることが重要な課題であり、そのためには南海トラフから沈み込むフィリピン海プレートの形状およびプレート境界周辺の構造、地震活動に関する詳細かつ高精度な情報が必要である。また、南海地震単独で発生した場合についても、深部および浅部低周波地震発生域まで含めた正確な破壊の広がりの把握、複雑な破壊分布の原因を明らかにするために南海地震破壊域とその縁辺での地殻構造や地震活動は重要な情報となる。平成20年度は日向灘、平成21年度は四国沖で調査を実施し、平成22年度は調査海域を東方の紀伊半島沖まで拡大し、沈み込み帯の地殻構造、巨大地震の発生、地震活動の相互関係の解明を目的として、南海・地震破壊域における沈み込みに関する詳細な構造のイメージングおよび地震のアスペリティに関する構造を明らかにするためのデータの取得、および西南日本の付加体先端部付近で発生している低周波地震や微動を含む自然地震観測を実施した。 　本講演では、四国沖?紀伊半島沖の平成21~22年度の調査結果について述べる。平成21年10月、および平成22年10~11月、（独）海洋研究開発機構の海洋調査船「かいれい」によって短周期海底地震計各々180台と大容量チューンドエアガン（7800cu. in.)を用いた屈折法・広角反射法探査を実施した（図１）。海底地震計設置期間中に自然地震観測も実施した。四国沖では21観測点、紀伊半島沖では20観測点による約9ヶ月間の長期地震観測も実施した（一部実施中）。なお、本調査は文部科学省からの受託研究「東海・東南海・南海地震の連動性評価のための 調査観測・研究」の個別研究テーマ「南海トラフ域海域地震探査・地震観測」（平成20年度から受託）の一環として実施した。 　一部の調査測線の解析の結果、足摺岬沖から日向灘に向かって約6km/sの古い付加体を示す岩体の分布が海側に張り出していること、また、SK05の構造モデルによると、SK03とSK02の中間付近からSK01付近までの付加堆積物が極端に薄いことなど、トラフ平行方向に構造変化があることがわかり、破壊様式の違いに関係する構造ではないかと考えられる。また、測線延長上のHi-net陸上観測点のデータを加えた海陸統合解析を実施しており、これにより深部低周波地震現象と構造との関連性が明らかになると期待されるC11-10発表要旨, 日本地震学会2011年度秋季大会（2011年10月12日～15日, 静岡県静岡市

Common Variants in MAGI2 Gene Are Associated with Increased Risk for Cognitive Impairment in Schizophrenic Patients

Schizophrenia is a complex psychiatric disorder characterized by positive symptoms, negative symptoms, and cognitive impairment. MAGI2, a relatively large gene (∼1.5 Mbps) that maps to chromosome 7q21, is involved in recruitment of neurotransmitter receptors such as AMPA- and NMDA-type glutamate receptors. A genetic association study designed to evaluate the association between MAGI2 and cognitive performance or schizophrenia has not been conducted. In this case-control study, we examined the relationship of single nucleotide polymorphism (SNP) variations in MAGI2 and risk for schizophrenia in a large Japanese sample and explored the potential relationships between variations in MAGI2 and aspects of human cognitive function related to glutamate activity. Based on the result of first schizophrenia genome-wide association study in a Japanese population (JGWAS), we selected four independent SNPs and performed an association study using a large independent Japanese sample set (cases 1624, controls 1621). Wisconsin Card Sorting Test (WCST) was used to evaluate executive function in 114 cases and 91 controls. We found suggestive evidence for genetic association of common SNPs within MAGI2 locus and schizophrenia in Japanese population. Furthermore in terms of association between MAGI2 and cognitive performance, we observed that genotype effect of rs2190665 on WCST score was significant (p = 0.034) and rs4729938 trended toward significance (p = 0.08). In conclusion, although we could not detect strong genetic evidence for association of common variants in MAGI2 and increased schizophrenia risk in a Japanese population, these SNPs may increase risk of cognitive impairment in schizophrenic patients

Prominent reflector beneath around the segmentation boundary between Tonankai-Nankai earthquake area

In the Nankai Trough subduction seismogenic zone, the Nankai and Tonankai earthquakes had often occurred simultaneously, and caused a great event. In most cases, first break of such large events of Nankai Trough usually begins from southwest off the Kii Peninsula so far. The idea of split Philippine Sea plate between the Kii Peninsula and the Shikoku Island, which explains seismicity, tectonic background, receiver function image and historical plate motion, was previously suggested. Moreover, between the Kii Peninsula and the Shikoku Island, there is a gap of deep low-frequency events observed in the belt-like zone along the strike of the subducting Philippine Sea plate. In 2010 and 2011, we conducted the large-scale high-resolution wide-angle and reflection (MCS) seismic study, and long-term observation from off Shikoku and Kii Peninsula. Marine active source seismic data have been acquired along grid two-dimensional profiles having the total length of ~800km/year. A three-dimensional seismic tomography using active and passive seismic data observed both land and ocean bottom stations have been also performed. From those data, we found a possible prominent reflector imaged in the offshore side in the Kii channel at the depth of ~18km. The velocity just beneath the reflector cannot be determined due to the lack of ray paths. Based of the amplitude information, we interpret the reflector as the forearc Moho based on the velocity gap (from ~6.4km/s to ~7.4km/s). However, the reflector is shallower than the forearc Moho of other area along the Nankai Trough. Similar reflectors are recognized along other seismic profiles around the Kii channel. In this presentation, we will show the result of structure analysis to understand the peculiar structure including the prominent reflector around the Kii channel. Relation between the structure and the existence of the segmentation of the Nankai megathrust earthquake or seismic gap of the deep low-frequency events will be also discussed. This study is part of 'Research concerning Interaction Between the Tokai, Tonankai and Nankai Earthquakes' funded by Ministry of Education, Culture, Sports, Science and Technology, Japan.Poster abstract T43C-2670 presented at 2013 Fall Meeting, AGU, San Francisco, Calif., 9-13 Dec

The Subcrustal Discontinuity with a Molten Material at the East Coast off the Izu Peninsula

To determine the scale and depth of a molten material near the volcanic front and at active volcanoes, the reflection phases on microearthquake seismograms have been studied. At the east coast off the Izu peninsula many phenomena, such as earthquake swarms and unusual up rifts of the ground, have occurred. Many interpretations have been presented regarding these phenomena. It became clear that these phenomena were caused by the molten material after the submarine eruption which occurred on 13, July, 1989. In this paper we discuss the location of a molten material beneath the area of earthquake swarms.火山前線付近,または活火山周辺に存在すると考えられる溶融体の深さとその規模を明らかにするために,これらの地域で発生する群発地震の地震波形記録に見られる溶融体からの反射波の解析が試みられてきた.伊豆半島東岸地域では,群発地震が頻発し,異常隆起等の特異な現象が見られており,これまでこれらの現象を説明するために様々な仮設が唱えられてきた.その後, 1989年7月13日に伊東沖で海底火山の噴火が起こったことからマグマが関与している可能性が高いことが示された.この論文ではこの地域において群発地震の震源域直下における溶融体の存在について考察する. 1987年5月, 1988年7～8月, 1989年7月に発生した各々震源域の異なる伊豆半島東方沖の群発地震の地震波形記録において, P波初動の3～4.5秒, 6～7秒後に顕著に後続波が観測された.これらの2つの後続波は走時解析を行った結果,深さ16～20kmに存在する不連続面からのSxP反射波, SxS反射波であることが明らかとなった.これらの反射波の大きな反射係数は発震機構を考慮してもコンラッド面では説明できず,不連続面の下の層のS波速度が0km/sに近い物質,つまり溶融体を伴う不連続面からの反射波である可能性があることがわかった.また,この不連続面の広がりを見るために反射点分布を求めたところ,群発地震の震源域の下に広範囲に存在することがわかった.これらの結果から伊豆半島東岸の群発地震及び噴火現象は,本研究によって求められた溶融体を伴う不連続面から上昇してきたマグマによって引き起こされた群発地震,海底火山噴火である可能性があると考えられる

Seismic Structure Beneath the Tibetan Plateau From Iterative Finite‐Frequency Tomography Based on ChinArray: New Insights Into the Indo‐Asian Collision

International audienceIndo‐Asian continental collision has contributed to the growth of the Tibetan Plateau, which is one of the most prominent uplifts worldwide since Cenozoic. The crustal and upper‐mantle structures are key factors in understanding the evolutionary process as well as lateral growth of the plateau. We present a new 3‐D seismic model beneath the Tibetan Plateau and its surrounding areas, which uses a large‐scale dense array from iterative finite‐frequency tomography. The new tomographic images show obvious east‐west geometrical change of the northward Indian lithosphere by high‐velocity anomalies beneath Tibet. The high‐velocity anomaly extends to Pamir in the western side, but it only reaches north Lhasa in central and eastern Tibet. A convective removal of the thickened Tibetan lithosphere is observed as a high‐velocity anomaly, followed by a subvertical subduction of the Indian mantle lithosphere beneath the Bangong‐Nujiang Suture in central Tibet. We speculate its link to the widespread magmatism and rapid uplift of central Tibet in late Miocene. A high‐velocity gap between 100 and 250 km underneath the Longmenshan fault indicates that lithospheric delamination may play an important role in surface evolution between eastern Tibet and surrounding rigid blocks. Our detailed seismological model will give an insight into how the continents collide