We investigated structural, mineralogical and geochemical characteristics of the fault rocks from the active Atera fault, Gifu Prefecture, Japan. We recognized three dominant structural zones at the Tase outcrop: light gray fault gouge, brown fault gouge and black fault breccia. The light gray fault gouge is mainly composed of clay minerals, and exhibited intense shear foliation. The brown fault gouge shows the clearest linear continuity at the outcrop, cutting other structural zones, but the degree of shearing in this zone is relatively weak. The black fault breccia is composed of rock fragments, originated from surrounding host rocks, and black matrix. Fault rocks in these structural zones show fluid-mobile trace element compositions essentially consistent with those of host rocks, indicating that no fluid–rock interaction at high temperatures above 250 ºC took place in these fault zones.
Based on these features of each zone and their cross-cutting relationship at the outcrop, we inferred the structural evolution of the fault zone. The light grey fault gouge and the black fault breccia formed since the Atera fault became active. Under oxidative condition, water might react with minerals in both zones to form smectite and halloysite. The brown fault gouge formed most recently, possibly by the 1586 Tensho earthquake. Mineralogical and geochemical analyses of the fault rocks used in this study could be useful for understanding the structural and chemical evolution of the active faults.
The Japan arc system is one of the typical island arc systems in the globe. The framework of plate tectonics, however, is still disputable. The author tries to elucidate the tectonic framework of this island arc system from the view point of slip rate and evolution history of active faults.
In past few decades, we have accumulated knowledge on the active tectonics. Especially slip rate and deformation mode of active faults during the late Quaternary have been revealed by intensive works. The author integrates the mean slip rate and strain rate distribution and evolution history of active structures in the Japan arc system. The results show the followings.
1) Horizontal shortening rate around central Hokkaido is much lesser than around the Fossa Magna region.
2) Major horizontal strain occurs along the Japan Sea coastal area of the Northeastern Honshu, in the inner zone from the mid Honshu to the Kinki district (Niigata-Kobe Tectonic Zone) and along the Median Tectonic Line in the Shikoku Island.
3) The east-west shortening rate of the Niigata-Kobe Tectonic Zone is 5〜15mm/yr, and it is comparable to the dextral slip rate of the Median Tectonic Line in the Shikoku Island and convergence rate of the Amur Plate and Okhotsk (Northeastern Japan) Plate.
4) The extension and deepening of the Bikal rift zone which locates northwestern margin of the Amur Plate has occurred since the late Pleistocene, and it accelerated since the late Early Pleistocene. This development history is concurrent with the east-west shortening along the Niigata-Kobe Tectonic Zone and along the Median Tectonic Line.
5) Above mentioned facts indicate that the eastern margin of the Japan Sea area and the Niigata-Kobe Tectonic Zone is a broad plate boundary between the Amur Plate and the Northeast Japan and Southwest Japan Plates, has 200km-width, and is made up by numerous discontinuous active faults and tectonic basins. The southeastern boundary of Amur Plate would continue from the Median Tectonic Line in the Shikoku Island to the East China Sea via the mid-Kyushu Island. But its location in the Kyushu Island and East China Sea is still unidentified.
6) The subsidence dominant, reverse fault-related basins occur along the Japan Sea Coast in the northeastern Honshu Island and along the Niigata-Kobe Tectonic Zone. But the length of each basin and fault is less than 100 km. These phenomena are thought to be characters accompanied with incipient arc-arc collision.
Paleoseismic trenching at Ikeda Kemi-kita site, Ikeda Town, Nagano Prefecture, reveals the mid-to-late Holocene surface-rupturing history of the Matsumoto-bonchi-toen fault (MBTF), a constituent fault of the eastdipping reverse-faulting dominant northern section of the Itoigawa-Shizuoka Tectonic Line active fault system(ISTL). The trench exposed shallowly eastward dipping thrust faults that displace mid-to-late Holocene fluvial terrace and alluvial fan sediments. The sediments contain evidence for two surface rupturing events with an about 1 m vertical offset individually in the past 6,000 years. Plenty of 14C dates allow to place bounds on timing of each event; AD 690 to AD 1170 (possibly constrained between AD 900 and AD 1120) for event 1 (the latest event) and 2280 BC to 1770 BC for event 2. 14C wiggle matching applied to a buried tree that is interpreted to be toppled down in association with event 2 estimates the date of outermost ring to be between 2330 BC and 2250 BC. The age range for the latest event overlaps with those previously reported in the north section and the Gofukuji fault to the south, which constitutes the left-lateral strike-slip-faulting dominant middle section.The latest event may be correlated with either historically documented AD 762 or AD 841 earthquake, or a historically unknown large earthquake, depending on choice of the 14C dating materials. Timing of two events suggests the possibility that the rupture on MBTF was less frequent compared to the Gofukuji fault since the middle Holocene.
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.
The Kokura-higashi fault located in the northern Kyushu Island is an active fault extending in NNESSW direction with west-side-up and right-lateral displacement. The Earthquake Research Committee of the Headquarters for Earthquake Research promotion evaluated that the probability of the earthquake occurrence in the future on the Kokura-higashi fault is unknown because of the luck of paleoseismological data. After such situation, we carried out a trench excavation survey on this fault. A trench was excavated on the fault trace of the Kokura-higashi fault estimated from the continuation of the fault scarplet to reveal the past activity of this fault.
On the trench wall, a steeply dipping fault cutting bedrocks and overlying sediments was cropped out. The lower part of the sediments includes some humic soil layers with many wood fragments. The fault cuts until the top of the sediments just below artificial soil. Radiocarbon ages ranging from 15 ka to 40 ka in calendar year were measured for samples from humic soil layers on the trench wall and the borehole cores dug up before and after trenching. One faulting event recognized on the trench wall estimated to occur in 19 to 20 ka and one liquefied event estimated to occur in 29 to 35 ka. The recurrence interval including the last two events estimated by a former trenching study is calculated to about 9,000 years.
Inland earthquake (Mj 6.7) occurred around Northern Nagano Prefecture in November 22, 2014. Surface ruptures emerged over 9 kilometers or more length intermittently along Kamishiro fault derived from this earthquake. Geospatial Information Authority of Japan (GSI of Japan) implemented the interferometric SAR (InSAR) analysis by using PALSAR-2/ALOS-2 data in the earthquake area after the earthquake. In Addition, GSI of Japan elucidated the crustal deformation induced by the main shock and aftershock activity of the earthquake (GSI of Japan, 2015a). There were some discontinuities of interference phase in the InSAR imageries. We performed a field survey in northern Hakuba Village and Otari Village while referring to the discontinuities of interference phase in the InSAR imageries. At that time, surface displacement wasnʼt reported in Otari Village.
As a result, the appearance position of surface ruptures matched the discontinuities of interference phase on the InSAR imagery in the northern Hakuba Village. Then, we discovered some surface displacements with relative east side uplifting and E-W shortening at the Dorosaki section and west side of JR Chikuni Station in Otari Village. These surface displacements appeared along the discontinuities of interference phase in the InSAR imageries. It is probable that these surface displacements were caused by activity of Himekawa fault. However, it is not clear whether there are surface rupture. These results indicate that InSAR imagery by using PALSAR-2/ALOS-2 data is able to detect surface displacement like surface rupture induced by earthquake. Moreover, InSAR imagery is useful to survey surface displacement widely.
The Nagano-ken-hokubu earthquake (Mw 6.2) occurred on 22 November 2014. A 9.2-km-long surface rupture appeared in association with the earthquake along the pre-existing trace of an active fault (the Kamishiro fault), which constitutes the northern part of the Itoigawa-Shizuoka Tectonic Line (ISTL) active fault zone. The ISTL bounds the western margin of the Northern Fossa Magna Basin, which is the southern extension of the Uestu depositional basin developing on the back-arc side of the Northeast Japan (NEJ) arc. In the back-arc region of the NEJ arc, normal faults, that originally formed in conjunction with the Japan Sea opening in the early and middle Miocene, were reactivated as reverse faults under compressive stress since the Pliocene. In order to understand the present-day tectonics of the Uetsu-Northern Fossa Magna basin, we should take into account the Miocene extension and the subsequent tectonic inversion. In this paper, we review the crustal evolution of the Uetsu-Northern Fossa Magna rift basin by retrogressively restoring the contractive structures since Pliocene time and extensional structures of Miocene time along a deep-seated detachment fault, and then discuss the geometry, behavior, and loading process of the source fault that generated the 2014 earthquake on the basis of the aftershock distribution, results of SAR interferometry, and GPS horizontal velocity data. We found that: (1) coseismic slip occurred only on the listric reverse fault, which merges down-dip onto the deep-seated detachment fault, and (2) before the 2014 earthquake, aseismic slip was likely to have occurred on the detachment fault
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.
An Mj 6.7 earthquake struck the northern part of the Nagano Prefecture, central Japan, at 22:08 JST on November 22, 2014. The earthquake was named 2014 Naganoken-hokubu earthquake, and its coseismic surface rupture has been identified along the northern part of the Kamishiro fault in the northernmost part of the Itoigawa-Shizuoka Tectonic Line active fault system. We have conducted 1) aerial-photograph interpretation, 2) field explorations, 3) photogrammetric analysis, and 4) drilling and pit excavation surveys on the northern part of the Kamishiro fault since 2005, in order to better estimate slip rates and timing of faulting on the fault. At the Shinden site, the Aira-Tanzawa volcanic ash (ca. 30 ka) was identified in the L1 terrace-surface deposit. In addition, the age of L2 terrace surface was estimated at ca. 5 ka. Because the amounts of vertical offset of the L2 terrace surface are 8.0 m and 6.5-7.0 m at the Shinden and Oide sites, vertical slip rates can be calculated to be 1.6 mm/yr and 1.3-1.4 mm/yr, respectively. We also revealed subsurface stratigraphy in the down-thrown side of the Kamishiro fault at the Oide site, and identified three possible paleoseismic events that occurred ca. 1,000-1,300 years ago, 500-600 years ago, and after 300 years ago. Further investigations are needed to better understand paleoseismic activity and long-term seismic risk of the Kamishiro fault.
The Mw = 6.2 2014 northern Nagano earthquake ruptured the northern and middle strands of the NNEtrending Kamishiro fault located in the northern part of the Itoigawa-Shizuoka tectonic line (ISTL) active fault system. The ISTL is located between the NE and SW Japan arcs, and it is one of the most major tectonic lines in Japan. Although the northern part of the ISTL active fault system had been assessed as a M = 7.5–8.5 seismic source to cause surface ruptures which heights are over several meters, the maximum vertical displacement on the 2014 surface ruptures was only 0.8–0.9 m which was observed at the Shiojima site in Hakuba village, the Nagano prefecture. To investigate shallow subsurface structures and cumulative deformation of the surface ruptures located along the northern part of the Kamishiro fault, we conducted ground-penetrating radar (GPR) profiling across the ruptures. The GPR survey lines were located at the Oide site, where about 0.5 m coseismic vertical displacement was observed, and Shiojima site. These line lengths were about 10–40 m. The GPR data were collected by common-offset modes using 100 MHz GPR system (pulseEKKO PRO made by Sensors and Software Inc.) We acquired also common mid-point (CMP) ensembles at the all survey lines to estimate the electromagnetic wave velocity. The depth-converted GPR sections after careful data processing show subsurface structures above a maximum depth of about 10 m. The shallow subsurface fault angle in the interpreted GPR section at the Oide site is lower than those at the Shiojima site. The dip-slip component of the coseismic displacement calculated from the fault angle at the Oide site was close to that calculated at Shiojima site. The interpreted GPR sections suggested also that the past several coseismic vertical displacements on the Kamishiro fault has been accumulated in the study area and that about 0.5–1.0 m vertical displacement was probably caused during each seismic event.
In order to infer the shallow subsurface structure, especially the dip angle of the subsurface rupture, of the earthquake fault induced by the Northern Nagano Prefecture earthquake in 2014, we implemented ground penetrating radar (GPR) survey at several points of the Hakuba Village. We identified clear deformation of the reflection structure in the cross sections at Hokujo Shiojima and Oide Districts of Hakuba Village. Estimated dip angle of the subsurface rupture from the result were ca. 25-60 degrees. The dip angle is low as a shallow zone.
We also observed the soil deformation derived from the slip of rupture on the wall of a trench for a drainage work at Hokujo Shiojima District. The depth and dip angle of this deformation correspond to those of deformation identified in the profile of GPR survey implemented nearby trench. Although we were not able to identify the clear deformation in the results of GPR survey implemented at Kamishiro Iida and Horinouchi Districts, previous studies indicate that the dip angle of the Kamishiro Fault in this area is assumed to be low. We conclude that the dip angle of the northern part of the subsurface rupture induced by this earthquake is relatively high. And the results have relation that the degree of damage to the buildings in the northern part of Hakuba Village was less severe than in the southern part, although larger vertical displacement of the surface rupture emerged in the northern part.
The 2014 M 6.7 Naganoken-hokubu earthquake was caused by movement of the Kamishiro fault located in the northernmost part of the Itoigawa-Shizuoka Tectonic Line (ISTL) active fault system, central Japan. We conducted a series of field research immediately after the earthquake to describe coseismic surface ruptures. Our description methods were: 1) field reconnaissance using pre- and post-earthquake airphotos; 2) quick measurement using staff; 3) topographic profiling using Auto Level and Total Station; and, 4) UAV and highpole SfM measurement. We identified 9-km-long coseismic surface rupture, most of which was located along the pre-existing surface trace of the Kamishiro fault. The maximum value of coseismic vertical offset was ca. 1 m or more, which was recorded at Oide in the northern part of the rupture. Based on comparison of the 2014 coseismic slip distribution with the long-term slip rate distribution, both 2014 slip amount and cumulative offset amounts of L2 and L3 terrace surfaces are larger in the northern end of the ruptures. This implies that the subsurface coseismic slip during pre-2014 earthquakes continued toward the north, similar to that during the 2014 earthquake. In addition, both coseismic slip and long-term slip rate becomes smaller toward the south, indicating that the Kamishiro area is one of the segment boundaries in the northern part of the ISTL active fault system. Further investigations of the 2014 earthquake and the Kamishiro fault are needed to understand formation of tectonic landforms, landscape development, or earthquake prediction model of active faults.