Median Tectonic Line Active Fault Research Articles

1916年8月6日の中央構造線断層帯の地震について

1916年8月6日の中央構造線断層帯の地震について

The Destructive Earthquake on 1916 Aug. 6<sup>th</sup> near Ishizuchi Fault of Median Tectonic Line Active Fault Zone in Ehime Prefecture

The Destructive Earthquake on 1916 Aug. 6<sup>th</sup> near Ishizuchi Fault of Median Tectonic Line Active Fault Zone in Ehime Prefecture

Late quaternary slip rates and vectors on the Median Tectonic Line active fault zone in eastern Shikoku, southwest Japan

The Median Tectonic Line active fault zone (MTLAFZ) extends for about 190 km through Shikoku, southwest Japan. Though the MTLAFZ is the most significant onshore active tectonic feature in southwest Japan, its late Quaternary slip rate has been estimated at only a few locations with reasonable references and ages. Better information on this feature's recent slip rates is critical to understanding the ongoing tectonic processes in the region and evaluating the seismic risk of this fault zone. In this paper, new estimates of the late Quaternary slip rate are reported from the Ikeda and Chichio faults in the central portion of the MTLAFZ. The author mapped late Pleistocene fluvial terrace surfaces and used tephrochronology and radiocarbon dating to constrain the age, and measured offset of terrace risers. The slip vectors of both faults are similar, as derived from piercing points on the bottoms of the terrace risers. The vertical component of displacement is 2–6% of the horizontal component. Long-term slip rates during the late Quaternary were calculated at 7.8–9.1 mm/yr, which is more precise than those in the previous studies and represents the highest slip rate in the MTLAFZ. This rate is also much faster than previously reported shorter-term slip rates of geodetic study. The long-term seismic risk of large earthquakes (>M8) along the Ikeda and Chichio faults in the next 30 years are evaluated to be more than 0.4–1.9% and 0.1–3%, respectively, much higher than a previous estimate of 0–0.3%.

Rediscovery of the Hakuchi thrust associated with the Median Tectonic Line active fault system in Awa-Ikeda town, eastern Shikoku, southwest Japan

Rediscovery of the Hakuchi thrust associated with the Median Tectonic Line active fault system in Awa-Ikeda town, eastern Shikoku, southwest Japan

中央構造線活断層帯（四国）における最近の研究成果

中央構造線活断層帯（四国）における最近の研究成果

Arrayed-boring survey near the eastern end of the Kawakami fault of the Median Tectonic Line Active Fault Zone in central Shikoku, southwest Japan

地形情報を基に推定された断層線の地質学的な証拠を得るため，西条市横黒地点において4本（北からBr.A～D）の群列ボーリング調査を実施した．Br.A～Br.Cコアは4つのユニットで構成されるが，Br.Dには最下位ユニットが観察されない．それぞれの対比層準は南側低下の撓曲構造を示唆しており，対比層準の高度差はBr.BとBr.D間で累積性を示す．さらに，Br.Dの孔内水位が他の孔と大きく異なることから，Br.CとBr.D間に南側低下の断層の存在が示唆される．また，K-Ahの降下層準がBr.CとBr.D間およびBr.BとBr.D間でそれぞれ2.24 mおよび4.03 mの高度差がある．このことから，少なくとも約7300年前以降では，本地点の川上断層の縦ずれ方向の変位速度は0.3 mm/yr（Br.BとBr.D間の撓曲構造を考慮した場合0.55 mm/yr）程度である．

中央構造線活断層帯・重信断層の 歴史時代の断層活動

中央構造線活断層帯・重信断層の 歴史時代の断層活動

Restoration of fault slips revealed by aerial photogrammetry along the Chichio fault of Median Tectonic Line Active Fault Zone in the Eastern Shikoku, Southwest Japan

Restoration of fault slips revealed by aerial photogrammetry along the Chichio fault of Median Tectonic Line Active Fault Zone in the Eastern Shikoku, Southwest Japan

中央構造線活断層系根来断層の性状と最新活動--和歌山市今滝(仁王谷)でのトレンチ調査

中央構造線活断層系根来断層の性状と最新活動--和歌山市今滝(仁王谷)でのトレンチ調査

Holocene faulting history of the Iyo Fault, part of the Median Tectonic Line active fault system in northwest Shikoku, southwest Japan

Holocene faulting history of the Iyo Fault, part of the Median Tectonic Line active fault system in northwest Shikoku, southwest Japan

Tectonic model and fault segmentation of the Median Tectonic Line active fault system on Shikoku, Japan

Fault segmentation models have been developed by using various types of information about faults, including both geological and geophysical data, in order to predict the size of future cascade earthquakes. In these models, however, stress conditions within the fault systems have not been considered in sufficient detail. Major fault systems generally exhibit heterogeneous stress conditions. The Median Tectonic Line active fault system (MTLAFS) is the longest and most active arc‐parallel, right‐lateral, strike‐slip fault system in Japan. The stress conditions along the MTLAFS change from transpression on eastern Shikoku Island to transtension on Kyushu Island. The change of stress conditions along the fault system is caused by the counterclockwise rotation of the Nankai fore‐arc sliver in response to the relative motion of the Philippine Sea and Eurasian tectonic plates and back‐arc spreading in the Okinawa trough. Here, we propose “stress condition segmentation” as a concept that deals with the diversity of stress conditions and can be applied in seismic hazard assessment. According to this concept, the MTLAFS is divided into three segments: the East Shikoku, West Shikoku, and Kyushu segments. The East Shikoku and Kyushu segments are under compressional and tensional stress conditions, respectively. Stress in the West Shikoku segment, which is between the other two segments, is transitional from compressional to tensional conditions. In particular, change in the magnitude of fault‐normal stresses along strike is one of the key factors that govern the geometry of fault discontinuities, rupture propagation and termination, and, consequently, fault segmentation. Application of our stress condition segmentation concept to the MTLAFS suggests that future earthquakes in the West Shikoku and Kyushu segments will be of lower energy than those in the East Shikoku segment.

Geomorpholgy, geology and subsurface structure of the Median Tectonic Line active fault zone in the central part of Shikoku

中央構造線活断層帯は日本列島で最長の活断層であり，変位地形の規模も大きく明瞭である．断層露頭も見事である．1995年兵庫県南部地震（M7.3）の発生以降も，多くの調査機関や大学（研究者）が各種の活断層調査を実施してきた．とくに，ボーリング・反射法地震探査・トレンチ掘削調査などが各所で行われ，中央構造線活断層帯の性質・活動履歴・地下構造などもかなり詳しく判明してきた．こうした成果に基づいて，地震調査委員会（2003）から長期評価も公表された．四国中央部における中央構造線活断層帯の代表的な活断層地形や断層露頭などを見学するとともに，地下構造調査の成果も紹介し，活断層に関する総合的な考察を行い，残された課題や問題点などについて検討する．

四国の中央構造線断層帯の最新活動に伴う横ずれ変位量分布

Recent paleoseismological studies revealed that the entire length of the right-lateral Median Tectonic Line active fault zone in Shikoku ruptured after the 16th century A.D. Based on aerial photograph interpretation and field observations, we identified geomorphic and artificial features offset along the fault zone that record the amount of right-lateral slip associated with the most recent and previous earthquakes. The amount of offset during the most recent earthquake varies considerably among the active faults that comprise the fault zone: about 7m along the Chichio and Ikeda faults in eastern Shikoku and 2-3m along the Shigenobu and Iyo faults in western Shikoku. In general, surface slip associated with the latest event is greater than 5m between the Zunden and Okamura faults, and decreases gradually to the east and west. There is a correlation between horizontal slip rates of the individual faults and surface slip during the most recent event. This is consistent with the paleoseismic observation that the recurrence intervals of the individual faults do not vary considerably along the fault zone and are in a range of 1000-2000 years. Based on the surface offsets and segmentation model by Goto and Nakata (2000b), we are able to calculate moment magnitudes for scenario earthquakes from the fault zone. Separate ruptures of the western Shikoku, Okamura, and eastern and east-central Shikoku segments would produce earthquakes with magnitudes of Mw7.3-7.4, Mw7.2-7.4, and Mw7.7-7.8, respectively. These ruptures would be as large as or greater than the largest recorded historical earthquake, the 1891 Mw7.4 Nobi earthquake.

四国北西部中央構造線活断層系, 米湊断層および本郡断層の地下構造と活動履歴

四国北西部中央構造線活断層系, 米湊断層および本郡断層の地下構造と活動履歴

Active fault Research on the Median Tectonic Line Active Fault Zone

Active fault Research on the Median Tectonic Line Active Fault Zone

Identification of multiple faulting events of the Median Tectonic Line active fault system in the Tokushima Plain, Japan, based on close-interval radiocarbon dating

Close-interval radiocarbon dating of terrestrial macrofossils from sediment cores across a fault is an instructive tool for identifying timing of multiple faulting events. We performed accelerator mass spectrometric (AMS) 14C measurements and high-resolution sedimentological analysis. At least six paleoseismic events of the Median Tectonic Line active fault system in the Tokushima Plain, Japan, are detected during the last 16000 years: around 850 cal BP, 2850 cal BP, 5300 cal BP, 7500 cal BP, 9000 cal BP, 16000 cal BP.

Identification of multiple faulting events of the Median Tectonic Line active fault system in the Tokushima Plain, Japan, based on close-interval radiocarbon dating

Close-interval radiocarbon dating of terrestrial macrofossils from sediment cores across a fault is an instructive tool for identifying timing of multiple faulting events. We performed accelerator mass spectrometric (AMS) 14 C measurements and high-resolution sedimentological analysis. At least six paleoseismic events of the Median Tectonic Line active fault system in the Tokushima Plain, Japan, are detected during the last 16 000 years: around 850 cal BP, 2850 cal BP, 5300 cal BP, 7500 cal BP, 9000 cal BP, 16 000 cal BP.

Pictorial 4 : Timing and Displacement of the Most Recent Surface Faulting on the Hatano Fault of the Median Tectonic Line Active Fault System, Southwest Japan

The Median Tectonic Line active fault system (MTL), with an average slip rate as high as 5-10 mm/yr, is one of the most active inland faults in Japan. However, the long-term seismic risk of the MTL has been poorly known, because of insufficient paleoseismological data, especially timing and displacement associated with the most recent surface faulting.We carried out a trench excavation survey across the Hatano fault in Doi town, Ehime Prefecture, and were able to precisely determine the timing of the latest faulting event. The survey site is located between range-facing fault scarps level 0.8 m high on an alluvial fan. We first excavated two trenches across the fault to precisely locate fault traces. Faults cutting Holocene sediment were exposed on both walls of each trench. The sense of apparent displacement across the fault zone is down to the south, which is consistent with fault scarps around the trench site. Then we excavated two trenches parallel to the fault zone to expose stratigraphic evidence of horizontal displacement associated with past earthquakes.The sediment exposed on the trench walls contains evidence of two faulting events in the past 3500 years B.P. The most recent surface faulting along the Hatano fault occurred between 1520 cal A.D and 1660 cal A.D. This is the first paleoseismological data that precisely constrain the timing of the most recent faulting event of the MTL. We have estimated 2.5 ± 0.5 m right-lateral displacement and 0.3-0.5 m vertical displacement up to the north during the most recent faulting event based on an offset of paleo-channel deposit.

Timing and Displacement of the Most Recent Surface Faulting on the Hatano Fault of the Median Tectonic Line Active Fault System, Southwest Japan

The Median Tectonic Line active fault system (MTL), with an average slip rate as high as 5-10 mm/yr, is one of the most active inland faults in Japan. However, the long-term seismic risk of the MTL has been poorly known, because of insufficient paleoseismological data, especially timing and displacement associated with the most recent surface faulting.We carried out a trench excavation survey across the Hatano fault in Doi town, Ehime Prefecture, and were able to precisely determine the timing of the latest faulting event. The survey site is located between range-facing fault scarps level 0.8 m high on an alluvial fan. We first excavated two trenches across the fault to precisely locate fault traces. Faults cutting Holocene sediment were exposed on both walls of each trench. The sense of apparent displacement across the fault zone is down to the south, which is consistent with fault scarps around the trench site. Then we excavated two trenches parallel to the fault zone to expose stratigraphic evidence of horizontal displacement associated with past earthquakes.The sediment exposed on the trench walls contains evidence of two faulting events in the past 3500 years B.P. The most recent surface faulting along the Hatano fault occurred between 1520 cal A.D and 1660 cal A.D. This is the first paleoseismological data that precisely constrain the timing of the most recent faulting event of the MTL. We have estimated 2.5 ± 0.5 m right-lateral displacement and 0.3-0.5 m vertical displacement up to the north during the most recent faulting event based on an offset of paleo-channel deposit.

四国北西部地域の中央構造線活断層系の地下構造とセグメンテーション

四国北西部地域の中央構造線活断層系の地下構造とセグメンテーション