Remote triggering of seismicity at Japanese volcanoes following the 2016 M7.3 Kumamoto earthquake

The M[JMA]7.3 Kumamoto earthquake occurred on April 16, 2016, in the western part of Kyushu, at a depth of 12 km, on an active strike-slip fault. Here, we report on a relatively widespread activation of small remote earthquakes, which occurred as far as Hokkaido, detected by analyzing the continuous waveform data recorded at seismic stations all over Japan. Such relatively widespread remote seismicity activation, following a large inland earthquake, has not been reported before for Japan. Our analysis demonstrates that the remote events were triggered dynamically, by the passage of the surface waves from the Kumamoto earthquake. Most of the remotely triggered events in the Tohoku and Hokkaido regions, as well as close to Izu Peninsula, occur at or close to volcanoes, which suggests that the excitation of crustal fluids, by the passage of Rayleigh waves, played an important triggering role. Nevertheless, remote activation in other regions, like Noto Peninsula, occurred away from volcanoes. The relatively large-amplitude Love waves, enhanced by a source directivity effect during the Kumamoto earthquake, may have triggered seismicity on local active faults. The dynamic stresses in the areas where remote activation has been observed range from several kPa to tens of kPa, the thresholds being lower than in previous dynamic triggering cases for Japan; this might relate to a change in the crustal conditions following the 2011 M9.0 Tohoku-oki earthquake, in particular at volcanoes in NE Japan

Nucleation process of the 2011 northern Nagano earthquake from nearby seismic observations

東北地方太平洋沖地震後に日本全国で発生した誘発地震のメカニズムを解析 --稠密地震観測網で捉えられた長野県北部の地震の前駆過程--. 京都大学プレスリリース. 2021-04-19.High-density seismic network detected inland earthquake precursors. 京都大学プレスリリース. 2021-05-10.The 2011 magnitude (M) 9.0 Tohoku-oki earthquake was followed by seismicity activation in inland areas throughout Japan. An outstanding case is the M6.2 Northern Nagano earthquake, central Japan, occurred 13-h after the megathrust event, approximately 400 km away from its epicenter. The physical processes relating the occurrence of megathrust earthquakes and subsequent activation of relatively large inland earthquakes are not well understood. Here we use waveform data of a dense local seismic network to reveal with an unprecedented resolution the complex mechanisms leading to the occurrence of the M6.2 earthquake. We show that previously undetected small earthquakes initiated along the Nagano earthquake source fault at relatively short times after the Tohoku-oki megathrust earthquake, and the local seismicity continued intermittently until the occurrence of the M6.2 event, being likely ‘modulated’ by the arrival of surface waves from large, remote aftershocks off-shore Tohoku. About 1-h before the Nagano earthquake, there was an acceleration of micro-seismicity migrating towards its hypocenter. Migration speeds indicate potential localized slow-slip, culminating with the occurrence of the large inland earthquake, with fluids playing a seismicity-activation role at a regional scale

Characteristics of foreshock activity inferred from the JMA earthquake catalog

We investigated the foreshock activity characteristics using the Japan Meteorological Agency Unified Earthquake Catalog for the last 20 years. Using the nearest-neighbor distance approach, we systematically and objectively classified the earthquakes into clustered and background seismicity. We further categorized the clustered events into foreshocks, mainshocks, and aftershocks and analyzed their statistical features such as the b-value of the frequency–magnitude distribution. We found that the b-values of the foreshocks are lower than those of the aftershocks. This b-value difference suggested that not only the stochastic cascade effect but also the stress changes/aseismic processes may contribute to the mainshock-triggering process. However, forecasting the mainshock based on b-value analysis may be difficult. In addition, the rate of foreshock occurrence in all clusters (with two or more events) was nearly constant (30–40%) over a wide magnitude range. The difference in the magnitude, time, and epicentral distance between the mainshock and largest foreshock followed a power law. We inferred that the distinctive characteristics of foreshocks can be better revealed using the improved catalog, which includes the micro-earthquake informatio

Temporal and Spatial Variations of Seismicity during the 1998 Hida Mountain Earthiuake Swarms,Central Honshu,Japan-Preliminary Results-

1998年飛騨山脈群発地震のマグニチュード頻度分布の係数, b値について, 京大および気象のデータについて時空間分布を調べた。その結果，b地が深さ4kmから6kmにかけて，通常地の0.9から約1.5-1.7に増加することがわかった。このことはb地が火山地域における｢異常領域｣の検出に有効であることを示している。これらはマグマの存在に関連していると思われる。We analyze the spatio-temporal distribution of b-value in the frequency-magnitude relation of earthquakes, for the 1998-1999 Hida Mountain earthquake swarms. We found a b-value that varies from 0.8 to 1.5-1.7 when depth is increasing from 4 to 6km. The high b-value is located in a crustal region that is characterized, according to some previous studies, by low-velocity and low-density. The results suggest that the b-value can be a useful toolfor mapping such "anomalous" areas, possibly associated with magma movements. Our study confirms other similar investigations in volcanic areas.1998年飛騨山脈群発地震のマグニチュード頻度分布の係数,b値について,京大および気象のデータについて時空間分布を調べた。その結果，b地が深さ4kmから6kmにかけて，通常地の0.9から約1.5-1.7に増加することがわかった。このことはb地が火山地域における｢異常領域｣の検出に有効であることを示している。これらはマグマの存在に関連していると思われる。We analyze the spatio-temporal distribution of b-value in the frequency-magnitude relation of earthquakes, for the 1998-1999 Hida Mountain earthquake swarms. We found a b-value that varies from 0.8 to 1.5-1.7 when depth is increasing from 4 to 6km. The high b-value is located in a crustal region that is characterized, according to some previous studies,by low-velocity and low-density. The results suggest that the b-value can be a useful toolfor mapping such "anomalous" areas, possibly associated with magma movements. Our study confirms other similar investigations in volcanic areas

ニホン オヨビ ルーマニア ニ オケル ジシン ケイレツ ノ カイセキ ニ ヨル ジシン カツドウ ノ ジクウカン ヘンカ パターン ト ソノ チカク コウゾウ オヨビ ジシン ノ ブツリ カテイ トノ カンレン

京都大学0048新制・課程博士博士(理学)甲第10627号理博第2769号新制||理||1403(附属図書館)UT51-2004-G474京都大学大学院理学研究科地球惑星科学専攻(主査)教授 伊藤 潔, 教授 梅田 康弘, 教授 竹本 修三学位規則第4条第1項該当Doctor of ScienceKyoto UniversityDA

Unusual low-angle normal fault earthquakes after the 2011 Tohoku-oki megathrust earthquake

A few low-angle normal fault earthquakes at approximately the depth of the plate interface, with a strike nearly parallel to the trench axis, were detected immediately after the 2011 Tohoku-oki earthquake. After that, however, no such normal fault events have been observed until the occurrence of the 2014 M W 6.6 Fukushima-oki earthquake. Here we analyze the teleseismic body waveforms of the 2014 Fukushima-oki earthquake. We first compare the observed teleseismic body waves of the 2014 Fukushima-oki earthquake with those of the largest previous low-angle normal fault aftershock (M W 6.6), which occurred on 12 March 2011, and then estimate the centroid depth and moment tensor solution of the 2014 Fukushima-oki earthquake. The teleseismic body waves and moment tensor solution of the 2014 Fukushima-oki earthquake are similar to those of the 2011 normal fault aftershock, which suggests that the 2014 Fukushima-oki earthquake occurred at a similar depth and had a similar mechanism to that of the 2011 aftershock. We detected five low-angle normal fault aftershocks at approximately the depth of the plate interface, with a strike nearly parallel to the trench axis, and confirmed that all of them except for the 2014 Fukushima-oki earthquake occurred within 17 days after the mainshock. The occurrence of these low-angle normal fault events is likely to reflect the reversal of shear stress due to overshooting of slip during the 2011 Tohoku-oki earthquake. We speculate that a fast but heterogeneous recovery of stress state at the plate interface may explain why these events preferentially occurred immediately after the megathrust event, while one of them occurred with a significant delay. In order to better understand the characteristics of stress state in the crust, we have to carefully observe the ongoing seismic activity around this region

b値とp値，その変化と地震の物理過程との関連

最近の日本における3つの地震群に対して解析したb値およびp値についてのレビューを行い，さらに，新しい解析結果を付け加えて，それらのパラメ―タと地下構造および震源過程との関連について調べた。1995年兵庫県南部地震については，約2～3年前からb値が変化したが，この変化は他の観測地の変化と対応している。また，この前兆的なb値の変化は震源域だけでなく，広い範囲で観測された。2000年鳥取県西部地震については，余震のb値とp値は本震付近で大きく，余震域の北部で小さい。この変化は地域的な構造の相違か応力変化のいずれかだと思われる。また，1989，1990，1997年に起きた前駆的地震活動ではb値は0.6程度で小さかった。これらのb値はp値とともに時間とともに増加した。1998年飛騨山地の群発地震については，相対的震源決定を行い，深さを含めて地域的なb値の変化を調べた。さらに，その結果，b値は山脈の北部で小さく南部で大きい。また，18個の大きな地震についてクーロン応力を計算したが，地震の80%は応力が増加した地域で発生している。B値の変化は応力変化と対応するように思われる。This work reviews some results obtained already for the variations of the seismicity parametersb and p in different seismogenic and tectonic regions in Japan. We bring as well newevidence that the time and space changes in seismicity parameters are correlating well with thecrustal structure and/or some parameters of the earthquake process. In the first part of the paperwe show that several seismicity precursors (clear b-value changes, quiescence and clustering)occurred about two years before the 1995 Kobe earthquake and they correlate well with othergeophysical premonitory phenomena of the major event. The precursory phenomena occurredin a relatively large area, which corresponds probably with the preparation zone of the futureevent. In the second part, we analyze the b and p value spatial and temporal distribution for theaftershocks of the 2000 Tottori earthquake. The results indicate significant correlations betweenthe spatio-temporal pattern of b and p and the stress distribution after the main shock, aswell as the crustal structure. The swarm-like seismic sequences occurred in 1989, 1990 and1997 showed significant precursory b and p values. In the third part of the paper we analyzethe seismicity during the 1998 Hida Mountain earthquake swarm. The double-difference-relocatedevents are analyzed for their frequency-magnitude distribution and stress changes. While again the b-value is significantly different in south comparing with the north part of the epicentral area, the physical interpretation is difficult and complex. The changes in the Coulomb failure stress (?CFF) can explain the b-value distribution features, but the crustal structure may be also important. The seismicity distribution and migration, in relation with ?CFF is also discussed. We refer as well to other world-wide studies.最近の日本における3つの地震群に対して解析したb値およびp値についてのレビューを行い，さらに，新しい解析結果を付け加えて，それらのパラメ―タと地下構造および震源過程との関連について調べた。1995年兵庫県南部地震については，約2～3年前からb値が変化したが，この変化は他の観測地の変化と対応している。また，この前兆的なb値の変化は震源域だけでなく，広い範囲で観測された。2000年鳥取県西部地震については，余震のb値とp値は本震付近で大きく，余震域の北部で小さい。この変化は地域的な構造の相違か応力変化のいずれかだと思われる。また，1989，1990，1997年に起きた前駆的地震活動ではb値は0.6程度で小さかった。これらのb値はp値とともに時間とともに増加した。1998年飛騨山地の群発地震については，相対的震源決定を行い，深さを含めて地域的なb値の変化を調べた。さらに，その結果，b値は山脈の北部で小さく南部で大きい。また，18個の大きな地震についてクーロン応力を計算したが，地震の80%は応力が増加した地域で発生している。B値の変化は応力変化と対応するように思われる。This work reviews some results obtained already for the variations of the seismicity parametersb and p in different seismogenic and tectonic regions in Japan. We bring as well newevidence that the time and space changes in seismicity parameters are correlating well with thecrustal structure and/or some parameters of the earthquake process. In the first part of the paperwe show that several seismicity precursors (clear b-value changes, quiescence and clustering)occurred about two years before the 1995 Kobe earthquake and they correlate well with othergeophysical premonitory phenomena of the major event. The precursory phenomena occurredin a relatively large area, which corresponds probably with the preparation zone of the futureevent. In the second part, we analyze the b and p value spatial and temporal distribution for theaftershocks of the 2000 Tottori earthquake. The results indicate significant correlations betweenthe spatio-temporal pattern of b and p and the stress distribution after the main shock, aswell as the crustal structure. The swarm-like seismic sequences occurred in 1989, 1990 and1997 showed significant precursory b and p values. In the third part of the paper we analyzethe seismicity during the 1998 Hida Mountain earthquake swarm. The double-difference-relocatedevents are analyzed for their frequency-magnitude distribution and stress changes. While again the b-value is significantly different in south comparing with the north part of the epicentral area, the physical interpretation is difficult and complex. The changes in the Coulomb failure stress (?CFF) can explain the b-value distribution features, but the crustal structure may be also important. The seismicity distribution and migration, in relation with ?CFF is also discussed. We refer as well to other world-wide studies

Precursory Phenomena of Seismicity in the Vrancea Region, Romania

ルーマニアにおけるカルパチア山地の屈曲付近のブランチャ地域では, 孤立した地震群が見られる。この地震活動は中深発地震で深さ60km以深200kmまで, ほぼ垂直な分布をしている。この地域ではM7以上の地震が時々発生し, 大きな被害をもたらしてきた。本研究では, この地域の地震カタログの人工的な原因による擾乱をチェックし, 実際の地震活動の変化を調査した。そのために, 1978-1998年の間の完全に観測されている地震のマグニチュードの下限(Mc)の時間的変化をを調べた。その結果, Mcは全体的には3.2であるが, 3.0から3.8と期間ごとに変化することが分かった。さらに, Mcを3.7とすることによって, 1986年(M7.1)及び1990年(M6.9)の大地震前に, 地震活動の低下とb値の変化があることが分かった。The Vrancea seismic region contains an isolated cluster of events beneath the Carpathian Arc Bend in Romania, dipping down to about 200 km depth. Seismic activity mainly belongs to intermediate depths (h≧60 km). This paper aims at identifying the so called man-made changes in seismicity, included in the studied catalogue, to reveal possible real changes of seismicity. The magnitude of completeness of the catalog is found to vary significantly from 3 to 3.8, for the time period studied (1978-1998). However, taking the minimum magnitude as 3.7, the two big earthquakes which occurred in this period show possible premonitory patterns of quiescence and b-value changes.ルーマニアにおけるカルパチア山地の屈曲付近のブランチャ地域では,孤立した地震群が見られる。この地震活動は中深発地震で深さ60km以深200kmまで,ほぼ垂直な分布をしている。この地域ではM7以上の地震が時々発生し,大きな被害をもたらしてきた。本研究では,この地域の地震カタログの人工的な原因による擾乱をチェックし,実際の地震活動の変化を調査した。そのために,1978-1998年の間の完全に観測されている地震のマグニチュードの下限(Mc)の時間的変化をを調べた。その結果,Mcは全体的には3.2であるが,3.0から3.8と期間ごとに変化することが分かった。さらに,Mcを3.7とすることによって,1986年(M7.1)及び1990年(M6.9)の大地震前に,地震活動の低下とb値の変化があることが分かった。The Vrancea seismic region contains an isolated cluster of events beneath the Carpathian Arc Bend in Romania, dipping down to about 200 km depth. Seismic activity mainly belongs to intermediate depths (h≧60 km). This paper aims at identifying the so called man-made changes in seismicity, included in the studied catalogue, to reveal possible real changes of seismicity. The magnitude of completeness of the catalog is found to vary significantly from 3 to 3.8, for the time period studied (1978-1998). However, taking the minimum magnitude as 3.7, the two big earthquakes which occurred in this period show possible premonitory patterns of quiescence and b-value changes