石村 大輔

Long sedimentary successions extracted for palaeoclimate research regularly preserve volcanic ash (tephra) fall from explosive eruptions and are increasingly used to elucidate the timing and scale of past events. This study investigates the non-visible tephra (cryptotephra) layers preserved in the annually laminated and intensively 14C dated sediments of Lake Suigetsu (SG14 core), Japan. The cryptotephra investigations reported here focus on the Late-glacial to early Holocene sediments that were deposited between two visible tephra layers, the Ulleungdo (U)-Oki (10.2 ka) and the Sambe ‘Sakate’ (19.6 ka), and consequently span an interval of abrupt climate change making any newly identified cryptotephra layers invaluable chrono-stratigraphic markers. Using major and trace element volcanic glass compositions the cryptotephra are used to assign provenance to chrono-stratigraphically relevant eruption units. Five new cryptotephra layers are identified within this time interval. Three cryptotephra layers are from Kyushu volcanoes (SG14-1337 and SG14-1554 [Sakurajima]; and SG14-1806 [Kirishima]), all of which offer important chronological constraints on archaeological (Jomon) cultural transitions in southern Japan during the last termination. Another cryptotephra (SG14-1579), is assigned to activity on Niijima Island providing the first known distal occurrence and age of the eruption. Finally, the SG14-1798 cryptotephra precisely dated at 16,619 ± 74 IntCal20 yrs BP (2σ) is linked to Asama (As) volcano and more precisely the later phases of the As-YKU eruption. This discovery greatly expands the distribution of ash fall from this multi-phased eruption at Asama volcano, which affected an area in the region of 120,000 km2. Refining the timing of the eruption and the distribution of As-YKU ash fall is important as it offers an excellent chrono- and climato-stratigraphic marker suitable for assessing spatial variability in environmental response to past climate change during the termination of the last glacial.

The Mw7.0 2016 Kumamoto earthquake occurred on the previously mapped Futagawa-Hinagu fault causing significant strong ground motions. A similar to 30-km-long dextral surface rupture appeared on the major fault zone and dextral slip was up to 2-3 m. However, the surface ruptures were also broadly and remotely distributed approximately 10 km away from the primary rupture zone. These numerous distributed secondary surface slips with vertical displacement of less than a few tens of centimeters were detected by the interferometric synthetic aperture radar (InSAR) technology in previous studies. Such displacements occurred not only on previously mapped faults but also on unknown traces. Here, we addressed the following fundamental issues: whether the broadly distributed faults were involved in the past major earthquakes in the neighborhood, and how the fault topography of such secondary faults develops, seismically or aseismically. To find clues for understanding these issues, we show the results of field measurements of surface slips and paleoseismic trenching on distributed secondary faults called the Miyaji faults inside the Aso caldera, 10 km away from the eastern end of the primary rupture zone. Field observations revealed small but well-defined dextral slip surface ruptures that were consistent with vertical and dextral offsets derived from InSAR. On the trench walls, the penultimate event with vertical displacements almost similar to the 2016 event was identified. The timing of the penultimate event was around 2 ka, which was consistent with that of the primary fault and archeological information of the caldera. Considering the paleo-slip event and fault models of the Miyaji faults, they were presumed not to be source faults, and slip on these faults have been triggered by large earthquakes along major adjacent active faults. The results provide important insights into the seismic hazard assessment of low-slip-rate active faults and fault topography development due to triggered displacement along secondary faults.

The 16 April 2016 Mw 7.0 Kumamoto earthquake caused prominent fault displacements and crustal deformation, not only around the main rupture faults but also around numerous secondary-ruptured faults. The physics and characteristics of such secondary faulting have not yet been studied in detail. We investigated a set of two secondary faults that appeared at the timing of the Mw 7.0 quake in the Aso Caldera by mainly using synthetic aperture radar interferometry and fault slip modeling. The two faults were found to be associated with surface displacement offsets of several centimeters or more, in the oblique sense of right-lateral and vertical motion. Fault slip inversions found that the slip was dominantly in normal sense with smaller contribution from the right-lateral component. The deeper limit of the slips was estimated to be around 1.3 km, which may coincide with the boundary between the superficial sediment layer and the basement rock. The shallowness of the slip and the difference in the dip angles of the main secondary fault and the Mw 7.0 seismogenic fault suggest separation of the two fault systems, although the fault strike and sense of motions were similar. The amount of slip on the two secondary faults was larger than that expected from the scaling law derived from seismogenic faults, which may indicate the difference in the physics of seismogenic and secondary faultings.

<p>2011年東北地方太平洋沖地震以前は，三陸海岸の長期間にわたる津波履歴は十分に理解されていなかった．2011年以降，従来は古津波堆積物調査に適した地域ではないと考えられてきた三陸海岸において，2011年津波の記録や徹底した掘削調査により，古津波堆積物の数や分布が明らかにされてきた．特に歴史記録にある1611年慶長三陸津波や869年貞観津波によると考えられるイベント堆積物は三陸海岸の広い範囲で認められ，その被害の様子が捉えられてきた．加えて，より古い数千年間のイベント堆積物が津波起源と推定され，三陸海岸ではある年代に古津波堆積物の年代が集中することがわかってきた．今後，各地点における古津波堆積物の対比方法の確立や詳細な古津波堆積物研究が，過去に起きた津波の規模・波源の理解に繋がるものと考えられる．</p>

<p>マグニチュード（M）7前後以上の内陸地殻内地震は必ずしも既知の活断層から発生しない．伏在活断層や短い活断層によっても生じ，その評価が課題となっている．2016年熊本地震では，震源となった日奈久断層北部，布田川断層だけではなく，その他の約200個所以上で小変位が検出された．既知の活断層も多数含まれる．これらは熊本地震の静的応力変化や地震動によって誘発されたと考えられる．また，余震を起こし地震発生層まで達した変位と表層付近だけのものなど，地震性・非地震性の観点からも多様である．同様の事例は干渉SARなどによって国内外で報告されている．これらのことから，短い活断層は必ずしも独自の大地震を起こすわけではなく受動的に変位することがわかる．これにより，地震動を生成させる短い活断層の総数は減るが，小規模ながら頻繁に地表変位を繰り返すという意味では断層変位ハザードは上昇する．</p>

<p> We conducted drilling survey to re-examine near surface geometry and a rate of the vertical deformation on the Kamishiro fault, northern part of the Itoigawa-Shizuoka Tectonic Line active fault system (ISTL), central Japan. 40- and 45-m-long core samples, extracted from the hanging wall of the Kamishiro fault, consist of fluvial sediments (alternation of sand-mud and sand-gravel layers). In the two cores, bedding plane is almost horizontal at uppermost part, gently sloped at upper to middle part, upstanding at lower part, and mildly sloped or horizontal at lowest part, respectively. The core samples enable us to estimate horizon of penetration of the Kamishiro fault on the basis of change with sharp boundary from upstanding strata older than 30,000 cal BP to underlying mildly sloped or horizontally laminated strata younger than 30,000 cal BP. Geologic cross-section on the basis of correlation of both the two cores with previously known stratigraphy, indicates that estimated fault is a reverse fault with dip of about 30 degrees. Altitude of facies boundaries and over fifty radiocarbon ages show cumulative vertical displacements of 14-20 m during the past 10,000 years, indicating average vertical displacement rate of 1.4-2.0 mm/yr. And, the net slip rate is estimated to be 2.8-4.0 mm/yr by using 30 degree dip of fault. The average vertical displacement rate in this study is smaller than that in the previous study where data of lithofacies and radiocarbon age is insufficient on the upthrown side. Because the Kamishiro fault is associated with drag folding near the surface, previously reported net slip rate during the past 10,000 years is considered to be underestimated. The largeness of net slip rate in this study relative to previously estimated rate is consistent with the forecast that the previous estimation is underestimated. The lowness of Holocene average vertical displacement rate, compared with previously estimated rate during the past 28,000 years, suggests decrease in activity of the Kamishiro fault during the past 10,000 years. And, the Holocene average vertical displacement rate shows that average recurrence interval is 150-360 years assuming that faulting with vertical displacement of 0.3 to 0.5 m as same as that at 2014 Nagano-ken-hokubu earthquake has repeated. This interval is shorter than that obtained from previous paleoseismic study, implying unknown paleoseismic event with small vertical displacement can be detected through reconsideration of paleoseismic survey.</p><p></p>

<p> During the 16 April Kumamoto earthquake (Mj7.3), ～30-km-long surface ruptures with right-lateral slip appeared along the previously mapped Futagawa and Hinagu faults. In Mashiki Town, surface ruptures also appeared north of the Futagawa fault across the alluvial plain of the Kiyama River where no tectonic geomorphic features were identified. In order to reveal shallow subsurface structure and past movements of the 2016 surface ruptures north of the Futagawa fault, we conducted a trenching survey at Jichu, Mashiki Town. Fluvial deposits derived from the Kiyama River and Aso-4 pyroclastic flow deposits were deformed by moderately to steeply south-dipping reverse faults. While the vertical offset during the 2016 earthquake was ～30 cm up on the south, older strata exposed on the trench walls were offset more than 2 m, suggesting that reverse faulting events occurred repeatedly in the late Quaternary. Based on the deformational features of the exposed strata, we identified three surface-rupturing events, including the 2016 earthquake. Radiocarbon dating of the strata suggests that at least two faulting events occurred after ～9,000 yBP. Since there were no strata deposited after ～8,500 yBP, it is possible that there were more seismic events than we identified during the middle to late Holocene. We suggest that the small vertical displacement associated with individual events, sedimentation/erosion by the Kiyama River, and artificial modification contributed to no tectonic geomorphic features along the surface rupture that appeared across the alluvial plain of the Kiyama River.</p>

After the 2011 Tohoku-old earthquake that occurred off the Pacific coast, significant problems arose concerning the Pacific coastal area (Sanriku Coast) such as the lack of the historical and paleo-tsunami records and the paradox between long-term and short-term vertical crustal movements. The data necessary to solve the issues are also essential for understanding the mechanisms of tsunami generation and earthquakes and assessing the risk of large, low-frequency disasters. Thus, we conducted excavation and coring surveys and reconstructed the paleoenvironment at Onuma on the southern Sanriku Coast, based on sedimentary facies, age data (radiocarbon dating and tephra), fossils (diatoms and mollusks), and topography. First, we discussed the preservation potential of tsunami deposits considering the geomorphologic and geologic evolution of the study area. Subsequently, we identified tsunami deposits and estimated their occurrence ages and inundation height. We discovered six tsunami beds deposited during ca. 5-1 ka when the preservation potential of tsunami deposits was high in the study site. We divided them into two types of deposits sand and gravel. These deposits reflect different sources and flow processes of paleo-tsunamis. Therefore, we estimated the heights of the paleo-tsunamis that is 5-10 m for sand tsunami deposits (S1-S3 deposits) and >10 m for gravel tsunami deposits (G1-G3 deposits). The tsunami deposits were dated at 690-1040 BP (S1), 2240-3310 BP (G1), 3320-3630 BP (G2), 3620-4060 BP (G3), 4030-4400 BP (S2), and 4440-5570 BP (S3). This approach and the setting of the study site enabled us to evaluate not only the ages of paleo-tsunami deposits but also the paleo-tsunami heights. Moreover, we estimated the paleo-sea level between 8 and 7 ka based on the sedimentary fades and paleoenvironment deduced from the fossil shells, and calculated the vertical crustal movement during the Holocene at 0.4 to 1.0 mm/yr by comparing with the theoretical relative sea-level curve. This information is significant for reconstructing the tectonic evolution along the subduction zone and for paleogeography to estimate the paleo-tsunami magnitudes. (C) 2017 Elsevier B.V. All rights reserved.

An ENE-trending similar to 30-km-long surface rupture emerged during the Mw = 7.0 16 April 2016 Kumamoto earthquake along the previously mapped Futagawa and northern Hinagu faults. This included a previously unknown 5-km-long fault within the Aso Caldera, central Kyushu. The rupture zone is mostly composed of right-lateral slip sections, with a maximum of 2-m coseismic slip. One of the noteworthy features we observed in the field are similar to 10-km-long segmented normal fault scarps, dipping to the northwest, along the previously mapped Idenokuchi fault, 1.2-2.0 km south of and subparallel to the Futagawa fault. The maximum amount of coseismic throw on the Idenokuchi fault is similar to 2 m, which is nearly equivalent to the maximum slip on the strike-slip rupture. The locations and slip motions of the 2016 rupture are also manifested as interferogram fringe offsets in InSAR images. Together with geodetic and seismic inversions of subsurface fault slip, we present a schematic structural model where oblique motion occurred on a northwest-dipping subsurface fault and the slip is partitioned at the surface into strike-slip and normal fault scarps. Our simple dislocation model demonstrates that this bifurcation into pure strike-slip and normal faults likely occurs for optimally oriented failure near the surface. The Kumamoto case, with detailed geological observations and geophysical models, would be the second significant slip-partitioned earthquake around the globe. It provides an important insight into scale-and depth-dependent stress heterogeneity and an implication to a proper estimate of seismic hazard in complex and broad multiple fault strands.

<p>Research of tsunami deposits greatly increased after the 2011 off the Pacific coast of Tohoku earthquake and tsunami. However, historical or paleo-tsunami deposits are buried under the topsoil, and thus we generally conducted coring survey and correlate tsunami deposits between each other site based on lithology, age and so on. Correlation of each layer of tsunami deposits is very significant for assessment of tsunami frequency and risk. In this study, we carried out very close interval (2.5 m interval) array Handy Geoslicer survey to confirm continuity of tsunami deposits. Additionally, we changed the interval of coring sites and tried to compare the correlation of each tsunami deposits. Consequently, we correlated each tsunami deposits confidently in 2.5 m, 5m, and 10 m interval. In 20 m and 50 m interval, we can correlate some tsunami deposits, however accuracy of the correlation is much lower than those of 2.5 m and 5 m interval. Although such feature varies regionally, it is important to discuss about relation between coring interval and accuracy of information obtained from them. On the other hand, we observed that one tsunami deposits did not necessarily show same characteristics (thickness, composition, and sedimentary structure) along the survey line, and re-realize the difficulty and complexity of correlation of tsunami deposits. This study is one case study, thus we need to conduct similar discussion at other sites and examine the accuracy of correlation in the future.</p>

<p>The surface rupture associated with the 22 November 2014 Nagano-ken-hokubu earthquake of Mj = 6.7(Mw = 6.2) occurred on the previously mapped Kamishiro fault, the northernmost section of the ItoigawaShizuoka Tectonic Line active fault system (ISTL). This is the first surface-rupturing earthquake occurred on one of the ~110 major inland active faults intensively evaluated by the Headquarters for Earthquake Research Promotion. We mapped the locations of the surface breaks along the rupture zone immediately after the earthquake, using handy GPS equipment. We also measured vertical and horizontal displacements at these sites using a conventional tape, folding ruler, simple hand level, and handheld laser finder. As a result, we found a N-S trending 9.2-km-long surface rupture and ground deformations mostly along the pre-existing scarp of the Kamishiro fault. Most of the surface ruptures involved flexural and warped surface deformation associated with significant contraction near the fault tip and local extension on the bended hanging wall. Observed deformation suggests that dip of the reverse fault changes to low-angle at shallow depth and deform unconsolidated sediments in the basins. The rupture trace is not simple: there are several short subsidiary faults including three rupture traces involving back-thrust faulting in the northern part. These features and the mapped distribution indicate an east-dipping reverse fault (east side up), which is consistent with early aftershock distribution and a geodetically inferred source fault. However, the amount of displacement associated with the 2014 earthquake was much smaller than the ones expected from previously conducted geomorphological and paleo-seismological studies. To seek the reason why we overestimated the rupture dimension, we need more peleo-seismic data (event age and displacement) and perform further tectonic geomorphological analyses.</p>

Large tsunamis occurring throughout the past several hundred years along the Sanriku Coast on the Pacific coast of northeastern Japan have been documented and observed. However, the risk of large tsunamis like the tsunami generated by the 2011 off the Pacific coast of Tohoku earthquake could not be evaluated from previous studies, because these studies lacked evidence of historical and paleo-tsunami deposits on the coastline. Thus, we first identified event deposits, which are candidates for tsunami deposits, from excavating surveys conducted on the coastal marsh in Koyadori on the Sanriku Coast, northeastern Japan. Second, we determined the physicochemical sediment properties of the deposits (roundness of grains, color, wet and dry densities, and loss on ignition) and established their geochronology by radiocarbon dating and tephra analysis. Third, we identified event deposits as tsunami deposits, based on their sedimentary features and origin, sedimentary environment, paleo-shoreline, and landowner interviews. In this study, we report 11 tsunami deposits (E1-E11) during the past 4000 years, of which E1, E2, E3, and E4 were correlated with the 2011 Tohoku-oki tsunami, the 1896 Meiji Sanriku tsunami, the 1611 Keicho Sanriku tsunami, and the 869 Jogan tsunami, respectively. From age data and the number of tsunami deposits in the trench, we estimated that tsunamis larger than the 1896 Meiji Sanriku tsunami occur and hit the study area on average every 290-390 years. However, historical tsunami correlations revealed variable tsunami occurrence, indicating diverse tsunami generation and/or the combination of several types of large earthquakes from different sources around the Japan Trench.

Tephras interbedded with Holocene sediments in coastal lowlands along the Sanriku Coast, northeast Japan are described, and well-known widespread and local tephras are correlated based on morphology, refractive index, and chemical composition of volcanic glass shards, stratigraphy, and radiocarbon age. As a result, tephras that are correlated with Towada-a (To-a), Towada-Chuseri (To-Cu), Kikai-Akahoya (K-Ah), Oguni pumice, Towada-Nambu (To-Nb), and Hijiori-Obanazawa (Hj-O) are identified. In particular, To-Cu is distributed throughout the study area and is recognized to be a useful key tephra. In addition, To-a and Oguni pumice were discovered for the first time on the Sanriku Coast. The results suggest that the morphologies and the refractive indexes of volcanic glasses are useful and important for distinguishing tephras and correlating with widespread and local tephras on the Sanriku Coast.

The process of terrace formation in Japan is discussed in the context of advances in tephrochronology. In particular, the ages of fluvial and marine terraces correlate with climate and glacio-eustatic sea-level changes. However, previous studies do not distinguish between the influences of climate changes and base-level changes (glacio-eustatic sea-level changes) for terrace formations, and mainly target northeast Japan due to advantages related to tephrochronology and terrace development. Therefore, this study focuses on the Ohmi Basin, Shiga Prefecture, southwest Japan, to reveal the process of terrace formation under conditions with no base-level changes, and compares them to those in northeast Japan. To achieve this aim, a drilling survey at terrace surfaces and a cryptotephra analysis are conducted. Widespread tephras, Kikai-Akahoya (K-Ah) tephra, Aira-Tn (AT) tephra, and Kikai-Tozurahara (K-Tz) tephra, are used to correlate fluvial terraces and establish the chronology of fluvial terraces in the eastern and western parts (Koto and Takashima regions, respectively) of the Ohmi Basin. Terrace correlation shows that terraces formed during Marine Isotope Stage (MIS) 2 are distributed in both regions under different tectonic settings. This indicates that climate change is the main factor of terrace formation in the Ohmi Basin. Therefore, river conditions during MIS 1, 2, and 5 are compared, and influences of climate changes and crustal movements for terrace formation are estimated. As a result, terrace formation in the Takashima region is explained by climate changes and fault movements. On the other hand, terrace formation in the Koto region is explained by climate changes and tectonic tilting. Consequently, these results suggest that the fluvial terraces in the Ohmi Basin are climatic terraces and that older to younger terrace steps could be the result of a combination of climate changes and crustal movements without base-level changes.

Fluvial and marine terraces have been used as geomorphic indicators to reconstruct the paleoenvironment and estimate crustal movements. This study focuses on the west coast of Ise Bay, where fluvial terraces are widely distributed and active faults have developed. The region is located far from active volcanoes; therefore, terrace chronology and processes of terrace formation during the late Quaternary have been poorly established. This study applies a tephra analysis of aeolian deposits covering terrace surfaces to identify invisible tephra horizons, and discusses the chronology and processes of terrace formation. Furthermore, it estimates crustal movements during the late Quaternary using altitudes of climatic terrace surfaces and data on subsurface geology.&lt;br&gt; The tephra analysis identifies widespread tephras such as Kikai-Akahoya tephra (K-Ah: 7.3 ka) and Aira-Tn tephra (AT: 26–29 ka) from aeolian deposits. Whether or not Kikai-Tozurahara tephra (K-Tz: 95 ka) covered terrace surfaces is assessed from the existence of β-quartz. Based on these tephras and geomorphic features, it is deduced that terrace formation in the study area corresponds to climate changes. In particular, L2 and Md1 terraces that formed during Marine Isotope Stage (MIS) 2 and MIS 5e are important landforms for discussing the processes of terrace formation in response to climate changes. The process of L2 terrace formation indicates that a decrease in precipitation is a major factor of terrace formation upstream during glacial periods.&lt;br&gt; Uplift and subsidence rates during the late Quaternary are estimated not only along active faults but also in areas remote from them. The distribution of uplift and subsidence is consistent with geomorphic and geologic features. This implies that movements of active faults greatly affected geomorphic and geologic development in the study area since they began to move. Based on uplift and subsidence rates across the faults, vertical slip rates of the Yoro, Kuwana, and Yokkaichi faults are estimated at &gt; 1.7 mm/yr, 1.0–1.2 mm/yr, and 0.5 mm/yr, respectively.

Kinki district is a tectonically active area in southwest Japan. This area is located far from active volcanoes, and eolian deposits covering fluvial terrace deposits are not as thick as those in Kanto district. Thus, terrace chronology and processes of terrace formation previously were poorly established in Kinki district. Cryptotephra analysis identified invisible tephra horizons in eolian deposits covering fluvial terrace deposits in Takashima, Sekigahara and Inabe regions, including the Kikai-Akahoya tephra (K-Ah: 7.3 ka), Aira-Tn tephra (AT: 26-29 ka) and Kikai-Tozurahara tephra (K-Tz: 95 ka) horizons. Taisanjino 1 terrace in Takashima and Md1 terrace in Inabe were formed during Marine Isotope Stage (MIS) 5e, and Nakano terrace in Takashima, L2 terrace in Sekigahara and L2 terrace in Inabe were formed during MIS 2, based on their ages and geomorphic features. Aggradational terraces formed during MIS 2 exhibit similar geomorphic features among the three regions, although these regions have different base-levels of erosion and active fault movements. This means that the terrace formation during MIS 2 was mainly affected by climate changes, not by base-level changes and active fault movements. The main factor of terrace formation during glacial periods was the decrease in precipitation and water discharge due to climate changes, because the study area was not affected by glacial and periglacial processes during MIS 2. This indicates that aggradational terraces formed during glacial periods not only in glacial and periglacial areas but also in non-glacial and non-periglacial areas. From processes of terrace formation during MIS 5e, the tectonic base-level descent was important for terrace formation during interglacial periods, because terraces were formed during MIS 5e by active fault movements in the Takashima region despite small lake-level changes. Uplift rates of the Kamidera and Kuwana faults were 1.08 mm/y and &gt;0.5 mm/y, respectively, based on the height of terrace surfaces formed during MIS 5e. (C) 2011 Elsevier Ltd and INQUA. All rights reserved.

The Lake Mikata and Nakayama lowlands are situated along the central coast of Wakasa Bay, at the center of the Japan Sea side of Honshu Island, and fringed by the Mikata fault zone at the east side. The 1662 Kanbun Great Earthquake (estimated magnitude of 7.2 to 7.6) occurred there with vertical crustal movements up to 3 to 4 meters. Submerged topography with thick alluvial deposits in the lowlands might have been caused by Late Quaternary crustal movements of the Mikata fault zone. We analyzed the subsurface geology of the lowlands using many drilling cores recently obtained for scientific purpose such as the 60-m long MK09 core and highway construction (represented by NEXCO core). The main results are summarized below.&lt;br&gt; (1) Subsurface geology beneath the eastern basin and eastern coast of Lake Mikata is clarified by detailed investigations of many drilling cores up to 100 m deep including the NEXCO core. Many radiocarbon (&lt;sup&gt;14&lt;/sup&gt;C) dating and tephrochronological correlations have elucidated the standard chronostratigraphy during the Late Quaternary in this area. At least 100-m thick sediments have deposited continuously for about 130 kyr, recording climatic and seismic events etc. during the last glacial period.&lt;br&gt; (2) The MK09 core is composed of a repeated coarsening-upward sequence with rapid facies changes at unit boundaries. We infer that these units reflect the rapid rise of a relative level of a paleo-lake and subsequent progradation of alluvial fans. The mean interval of individual units is estimated to be about 7.7 kyr from the sedimentary age of each unit, and is considered to be an upper limit of the recurrence interval of the Mikata fault zone.&lt;br&gt; (3) The Mikata fault is recognized for observations of new outcrops and core inspections at the northeastern part of the Nakayama lowland. The vertical displacement across the Mikata fault is inferred to be 30-35 m, and probably reached further after formation of a higher terrace.&lt;br&gt; (4) The concealed Mikata fault extending N-S parallel to the Mikata fault on the east, is estimated from our drilling data. On the basis of the displacements of tephra horizons such as the Aira-Tn volcanic ash beds (AT) in deposits of the Nakayama lowland, it is possible that activity of this fault is equivalent to or larger than that of the Mikata fault.&lt;br&gt; Further detailed studies by deeper drilling cores and seismic refraction surveys are required to clarify the E-W direction underground structure across the Mikata fault zone. They will unravel major development histories of this lowland with many datable materials.

Episodic subsidence events at the east coast of Lake Mikata, Fukui Prefecture, suggesting fault activity in the Mikata fault zone, are recognized from two drilling cores, MK09 core (60-m long) and NEXCO core (100-m long).&lt;br&gt; Detailed lithology including widespread tephra horizons is described for the MK09 core, with 18 AMS-&lt;sup&gt;14&lt;/sup&gt;C dates. The radiocarbon ages and tephrochronology of the core sediments indicate the continuous sedimentary history of Lake Mikata back to about 130 ka. We recognize eight depositional units ranging in thickness from 5 to 10 m in the MK09 core, each of which shows a clear coarsening-upward sequence from clay to gravel beds. These units are also recognized in the NEXCO core drilled 50 m from the MK09 core site by the correlation of tephras and lithofacies.&lt;br&gt; Analysis of sedimentary features suggests that each unit and its boundary reflect a rapid rise of lake level and subsequent progradation of alluvial fans. The mean sedimentary interval of individual units is equivalent to about 10.6 kyr, which is meaningfully shorter than astronomical cycles of climate change (∼20, 41 and ∼100 kyrs). Thus, we interpret these cyclic units to be the results of recurrent co-seismic subsidence due to Mikata fault zone activity. According to this interpretation, we could identify at least seven subsidence events besides the 1662 Kanbun Earthquake. The mean recurrence interval of these events is estimated to be about 7.7 kyr.

境峠－神谷断層帯は，長さや累積変位量などの点で中部日本を代表する活断層帯のひとつである．断層帯の南部にあたる長野県木祖村薮原下川原において，最近認定された活断層線を横切るトレンチ掘削調査を実施した．その結果，明瞭な断層が確認され，最近2回の活動が800～1,520 cal BP, および3,840～4,860 cal BPに発生した可能性が高いことがわかった．今回認定された最新活動に相当するイベントは，既存研究では指摘されていない．また，既存研究による最新活動時期（2,370～4,870 cal BP）は，本研究による1回前の活動時期と調和的である．以上は，本断層帯から発生する地震の長期評価に再検討を迫る知見であり，今後の検証が期待される．

The Umi fault was first described by Ikeda et al. (2004) as an active fault'of about 9km in length. Fukuoka Prefectural Government organized a research committee for examination and investigation of the Umi fault in 2005. The present study is based on air-photograph analysis, geological and topographical field surveys, soundreflection surveys in the marine areas, drilling and trenching, and tephrochronological studies. Some characters of the Umi fault are clarified through this intensive study.<BR>The Umi fault extends father north and south than was previously known, but it does not extend as far north as Hakata Bay and the Genkainada Sea. The length of the fault is 17km at least. The average vertical slip rate in the late Quaternary is estimated to be 0.02-0.03m/kyr (Activity Rank C in Matsuda 1975). This slip rate of the Umi fault is lower than those of the neighboring Kego and Nishiyama faults. The last activity of the fault was confirmed to be younger than 4300y. B. P. by the trenching study at Yamaura.<BR>The average recurrence interval of the activity is estimated at 29 kyr. This estimation is based on the assumption that the observed slip at the Yamaura trench (0.6m) occurred in one earthquake event and that the slip at Uenohara (1.9m), which is observed on middle terrace surface, formed immediately after the Aso-4 pyroclastic flow, is a result of repeated earthquake events of the same size slip.