Tectonic Line Active Fault System Research Articles

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

Newly-found Surface Faulting Event on the North-central Portion of the Itoigawa-Shizuoka Tectonic Line Active Fault System

The 150 km long Itoigawa-Shizuoka Tectonic Line Active Fault System (ISTL) in central Japan is one of the most active fault systems in Japan. Paleoseismologcal studies 1980s have revealed that the most recent event and the average recurence interval of the ISTL. The approximately 7 km long portion of the fault system between Matsumoto and Okaya has been regarded as a gap without any active fault trace. The gap namely the “Shiojiri Pass Gap” has long been taken as a segment boundary owing to the geometric discontinuity. Recent geomorphological analyses of the gap have demonstrated a through-going left-lateral slip assocaited with recent earthquakes in this area, based on aerial photograph interpretation and excavation studies. Excavation study on this portion revealed that the latest faulting event occurred between 1,700 cal. B.P. to 1,310 cal. B.P. (255 A.D. -645 A.D.). The timing of the last faulting event at this study area coincides with the timing in the Gofukuji fault and Okaya fault. The active faults extending from the Matsumoto basin as far as the northwestern margin of the Suwa basin display the evidence for its recent reactivation at the same time.

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.

糸魚川－静岡構造線活断層帯中南部，茅野～富士見～白州の変動地形の再検討と写真測量システムを利用した詳細平均変位速度解明

糸魚川－静岡構造線活断層帯中南部，茅野～富士見～白州の変動地形の再検討と写真測量システムを利用した詳細平均変位速度解明

A fault scarp in an urban area identified by LiDAR survey: A Case study on the Itoigawa–Shizuoka Tectonic Line, central Japan

A light detection and ranging (LiDAR) survey was conducted in a densely built-up area to generate a high-resolution digital elevation model (DEM) to look for active faults. The urban district of Matsumoto City in central Japan is located in a 3-km 2 basin along the Itoigawa–Shizuoka Tectonic Line active fault system, one of Japanese onshore fault systems with the highest earthquake probability. A high-resolution DEM at a 0.5-m-grid interval was obtained after removing the effects of laser returns from buildings, clouds and vegetation. It revealed a continuous scarp, up to ~ 2 m in height. Borehole data and archaeological studies indicate the scarp was formed during the most recent faulting event associated with historical earthquakes. In addition, the fault scarp strongly supports that the urban district is in a pull-apart basin related to a fault step-over between two left-lateral strike-slip faults. Consequently, accurate interpretation of fault geometry is crucial to provide estimates of future surface deformation and to allow modeling of basin structure and strong ground motion. Thus, the LiDAR mapping survey in urban districts is effective for detailed active fault mapping in order to constrain basin structure and to forecast the exact location of surface rupturing associated with large earthquakes.

糸魚川‐静岡構造線活断層系・松本盆地東縁断層南部に沿う左横ずれ変位地形

The East Matsumoto Basin faults (EMBF) consist of the northern segment of the Itoigawa-Shizuoka Tectonic Line active fault system (ISTL), central Japan, and extend for ca. 30 km along the eastern margin of the Matsumoto Basin. The faults have been considered to be eastdipping reverse faults, which were re-activated sometime after the tectonic inversion event during the middle Miocene in central Japan. However, we found geomorphic evidence of strikeslip movement on the EMBF, such as systematic left-lateral offsets of abandoned channels and scissoring vertical offsets on the fan surface, which were probably formed during the Nara era. These offsets were measured as left-lateral displacements of 6 to 7 m with a slight vertical component, and were probably formed in association with the most recent faulting event of EMBF. In this case, the style and the amount of slip per event are comparable with those of the neighboring Gofukuji fault, which has one of the highest probabilities of seismic risk calculated among active faults on land in Japan. These refined geomorphic features indicate that the principal slip component on the southern portion of EMBF is not merely dip-slip but left-lateral slip, taking account of the cumulative vertical displacement inferred from uplifted hills since Miocene to Plio-Pleistocene at the eastern side of the fault traces. These facts and presumptions are consistent with the deformation pattern around the ISTL revealed by recent continuous GPS observations. In addition, these new findings allow us to understand that the subsidence in the Matsumoto urban district can be explained as a pull-apart basin related to a 1.5-km-wide left-step between the EMBF and Gofukuji faults. Su8rface geometry, style, and amount of faulting of both faults suggest that this left-step does not seem to be a significant enough segment boundary of the ISTL to arrest rupture propagation. Thus, the portion between the southern EMBF and Gofukuji fault is appropriately a single fault segment.

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

Tectonic geomorphology related to activity of the Itoigawa-Shizuoka Tectonic Line active fault system in the Saigawa Hills, northern Fossa Magna

Tectonic geomorphology related to activity of the Itoigawa-Shizuoka Tectonic Line active fault system in the Saigawa Hills, northern Fossa Magna

Crustal structure in the northern Fossa Magna region, central Japan, modeled from refraction/wide-angle reflection data

The northern Fossa Magna (NFM) is a back-arc rift basin filled with thick Tertiary sediments, which show strong NW-SE shortening deformation. In the NFM, there exist two major active fault systems, the Itoigawa-Shizuoka Tectonic Line active fault system (ISTL) and the Western Nagano Basin active fault system (WNB), both of which have great potentials to cause destructive earthquakes. By reanalyzing five sets of refraction/wide-angle reflection data, we successfully obtained detailed and consistent models of the crustal structure in the NFM region. It was a very effective modeling procedure to incorporate vicinal seismic reflection data and geologic information. The geometries of the active faults in the NFM region were revealed. The ISTL is east dipping, and the WNB is northwest dipping. The Tertiary sedimentary layer (<4.0 km/sec) west and adjacent to the ISTL extends to a depth of 4–5 km. The basement rocks below the Central Uplift Belt (CUB) form a wedge structure, which suggests the westward movement of the CUB basement rocks.

The slip-rate along the northern Itoigawa-Shizuoka tectonic line active fault system, central Japan

The slip-rates on the northern extent of Itoigawa-Shizuoka tectonic line (ISTL) are estimated based on seismic reflection profiles, drill core data and analysis of tectonic geomorphology. The ISTL is a major tectonic line that passes through the Honshu Island of Japan, and its northern and central segments form an active fault system characterized by high slip-rates. In the Kamishiro basin, near the northern end of the ISTL active fault system, the rate of net slip is estimated to be 4.4–5.4 m/kyr over the last 28 ka, with a vertical-separation-rate of 2.2–2.7 m/kyr. In the Omachi area, south of the Kamishiro basin, the Quaternary slip-rate is estimated to be at least 2.9 m/kyr based on the balanced cross-section derived from reflection profiles and surface geology. The dip angle of 30° determined from the Omachi seismic profile suggests a vertical-separation-rate of at least 1.5 m/kyr. Based on compiled evidence from the available geomorphological and paleo-seismological data, vertical-slip-rates of 1.0–2.9 m/kyr are inferred for the region between Hakuba and Toyoshina over the past 3 ka. The northern ISTL exhibits dip-slip-rate of at least 2.9 m/kyr, with a constant average slip-rate of 2.0–5.8 m/kyr since the Early Quaternary. A paleoseismological data and long-term slip-rate along the northern ISTL has potential for a large earthquake.

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

ボーリング試料高密度連続分析に基づく徳島平野における中央構造線活断層系の活動

High-resolution sedimentological analysis of core samples from the Tokushima Plain enable us to identify evidence for multiple faulting of the Median Tectonic Line active fault system (MTL) during the Holocene period. Continuous and close-interval analyses of lithology (0.5 cm-order) and magnetic susceptibility (2 cm-interval) were carried out on three core samples from the hanging wall and footwall sides of the Naruto-minami fault, which is part of the MTL. In addition, analyses of grain size (10 cm-interval) and composition of very fine sand fraction (20 cm-interval) were carried out on core samples obtained at both ends of the fault deformation zone. Based on these sedimentological analyses, we correlated 34 horizons across the bore holes and divided the strata into 8 sedimentary units. We also identified 6 intervals that are thicker at the footwall side of the fault. These intervals appear to be related to movements of the MTL and the age of these paleoseismic events are estimated on the basis of radiocarbon ages. At least 6 paleoseismic events are detected for the last 15000 years : 15151700 cal BP, 28003200 cal BP, 34004200 cal BP, 73007600 cal BP, 970010100cal BP, 1300015000 cal BP. The average dip slip rate of the Naruto-minami fault is estimated at 1.2 mm/year.

糸魚川-静岡構造線活断層系市之瀬断層群の最近の断層活動

Co-seismic faulting activity during the last three hundred thousands years of the Ichinose fault group, in the southern part of the 150km long Itoigawa-Shizuoka Tectonic Line active fault system, central Japan, was appraised. The Ichinose fault group consists of the mainly two distinct arcuate reverse fault traces bulged toward the east (frontal) and west (behind) directions. We conducted a trench excavation at the Nakano site across the behind fault, where the detailed geological and chronological information for the last ten thousands years was absent. The excavation resulted in evidences for the latest, at least two surface-rupturing events. The events were present sometimes between 665 and 1275 cal.y.B.P. and between 3355 and 4810 cal.y.B.P. The timing of the penultimate event is coincident with the previously estimated latest event on the frontal fault (3, 990-6, 270 cal.y.B.P.). The recurrence time and slip rate between these two events is approximately estimated as 3, 200 years and 0.9-1.8mm/yr in dip-slip, respectively. In comparison with a slip rate estimated from the geomorphological characteristics, the dip-slip rate from our excavation result is coincident well.A reappraisal of geomorphological reference deposits and a result of the seismic investigation study suggest that the total deformation rate of the Ichinose fault group is 3.5-3.7mm/yr (possibly up to 5.0mm/yr) and it becomes to increase in recent several ten thousands years. While the deformation rate suggests that the co-seismic faulting activity at the Ichinose fault group as a whole is ranked as the one of most active faults in Japan, however, it is smaller than that of 6.3-8.3mm/yr estimated previously from the gravimetric and numerical analyses.

Outcrop of shear zone along the Kawakami fault, the Median Tectonic Line active fault system at Yuyaguchi, Tambara Town, Ehime Pref., Western Shikoku

愛媛県丹原町湯谷口の中山川河床で, 中央構造線活断層系川上断層の大規模な露頭が現れた. 2001年梅雨時期の増水により, 河床堆積物が洗い流され, 断層露頭が露出したと考えられる. 露頭の広がりは長さ約110mで, 幅約20mに及ぶ. 露頭は市ノ川～砥部時階の中央構造線断層露頭(岸ほか, 1996)より約100m下流に位置しており(第1図, 第2図), 岡田(1972)が報告した川上断層露頭の東方延長にあたる. 断層の走向はN75°Eで, 傾斜はほぼ鉛直である. 鮮新統～中期更新統の岡村層相当層の礫層および砂層は, 断層近傍では地層はほぼ直立している. また, 礫層を構成する礫の長軸は断層に沿ってほぼ水平に配列している(第4図). 和泉層群の破砕帯の幅は約20mである. 岡村層相当層の礫層と接するところでは, 幅約1mの厚さで礫層と粘土化した和泉層群が混在している(第5図). その南側は約1mの幅で著しく粘土化しており, 黒色, 灰白色, 褐色, 灰緑色を呈する粘土化帯が縞状の構造を示している. 縞状構造はほぼ鉛直で, 断層面にほぼ平行である(第5図, 第6図). さらに, その南側は破砕岩片がやや多く含まれる粘土化帯よりなる.

中央構造線活断層系三野断層の最新活動時期

The Median Tectonic Line (MTL) active fault system is one of the most active intraplate faults in Japan. The fault system, which is more than 300 km long, is a right-lateral strikeslip fault with an average slip rate of 5-10 mm/y in east Shikoku. The 13.5-km-long Mino fault of the MTL active fault system is located at the western part of Tokushima Prefecture in east Shikoku. We carried out trench excavation surveys of the Mino fault at Ueno in Mino Town and Ikenoura in Mima Town. Both sites are situated at fault depressions formed on the middle and lower terrace surfaces by the activity of the Mino fault. Fault depression deposits consist of younger and finer grained layers with abundant 14C dating samples. We inferred the dates of faulting events from upward fault terminations on the trench walls.At Ueno, it is recognized that the latest rupture event occurred between 1, 295-1, 390 cal A.D. and 1, 660-1, 950 cal A.D. (190 ± 50 years B.P.). Multiple faulting events are also suggested after K-Ah ash fall (about 5, 200 B.C.), because the ash layer steepens to near vertical. At Ikenoura, the latest rupture event is estimated to have occurred after 1, 525-1, 660 cal A.D.Taking account of the previous reports on the eastern extension at Chichio and Zunden faults, we conclude that the MTL active fault system in east Shikoku ruptured in the 16th century A.D. or later. The liquefactions, which occurred between the latter half of the 16th century A.D. and the beginning of the 17th century A.D., are recognized at archaeological sites at Maruyama and Ogaki in the western part of Tokushima Prefecture. These sites are located immediately south of the Mino fault and the Ikeda fault to the west. This fact suggests that the liquefactions were caused by faulting of the MTL active fault system in east Shikoku.

中央構造線活断層系 (四国) の最新活動時期からみた活断層系の活動集中期

The Median Tectonic Line active fault system (MTL) is one of the most active and longest active faults on land in Japan, and has a potential source of destructive large earthquakes. However, paleoseismological data along the MTL are sparse in Shikoku. The history of surface-rupturing earthquakes, particularly the timing of the latest events, on each segments is key data for examining the interaction of neighboring faults and evaluating the long-term seismic risk.To date the latest surface-faulting events on the MTL active fault system in Shikoku, we excavated mini-trenches on the Iyo fault and extracted vertical thin sections of unconsolidated soil layers by the Geoslicer on the Okamura and Ikeda faults. In this paper, we discuss the characteristics of active faulting along the MTL based on the results at the three sites and previously published data.The latest event occurred after the 14th century at Ichiba on the Iyo fault. The slip associated with the event was 2.1-2.3m based on a series of right-laterally offset rice paddy dikes. Geoslicer studies at Kishinoshita on the Okamura fault and at Iyo-Mishima on the Ikeda fault suggest that the latest event occured after the 16th century and after the 13th century, respectively.The youngest age of the latest event is not well defined by the geological studies. Since historical documents during the 18th century do not record any activity of the MTL on land, the timing of the most recent events on the fault is confined to between the 13th and 17th centuries. Therefore, timing of the latest events in Shikoku is concentrated within the past several hundred years. This suggests a temporal clustering of active faulting on the MTL, similar to that observed from other major fault worldwide.