Investigating the role of the Itoigawa-Shizuoka tectonic line towards the evolution of the Northern Fossa Magna rift basin

AbstractThe Itoigawa-Shizuoka tectonic line (ISTL) fault system is considered to have one of the highest probabilities for a major inland earthquake occurrence in the whole of Japan. It is a complex fault system with the dip directions of the local fault segments changing from north to south between an east-dipping low-angle thrust fault, a strike slip fault and a west-dipping thrust fault. The tectonic relations between the different parts of the fault system and the surrounding geological units are yet to be fully explained. This study aims to reveal the juncture of the northern and central parts of the ISTL and investigate its contribution towards the shaping of the Northern Fossa Magna rift basin. We conducted 3 deployments of 1 or 2 linear arrays of seismic stations across the central and northern ISTL regions and observed local micro-earthquakes for a period of 3 years. Each deployment recorded continuous waveform data for approximately 3 months. Using arrival times of 1193 local earthquakes, we jointly determined earthquake locations and a 3D velocity model, applying the tomography method. We were able to image the regional crustal structures from the surface to a depth of 20km with a spatial resolution of 5km. Subsequently, we used the obtained 3D velocity model to relocate the background local seismicity from 2003 to 2009. The juncture of the northern and central parts of the ISTL was well constrained by our results. The depth extension of the northern parts of the ISTL fault segments follows the bottom of the Miocene Northern Fossa Magna rift basin (NFM) and forms an east-dipping low-angle fault. In contrast, the central parts of the ISTL fault segments are estimated to lie along the eastern boundary of the Matsumoto basin forming an oblique strike slip fault (Fig. 1)

応力テンソル逆解析による本州 : 千島弧会合部の沈み込む太平洋プレート内の応力場

This study addresses the space distribution of the stress field in the Wadati-Benioff zone of Northeastern Japan and southernmost Kurile area based on homogeneous data of earthquake focal mechanisms and the inverse technique by Gephart and Forsyth (1984). The data set used consists of 785 JMA focal mechanism solutions (FMS) and 97 FMS listed in Kosuga et al. (1996) for shallow and intermediate depth earthquakes. The detailed analysis of the space distribution of orientation of P (compression) and T (extension) axes of FMS allowed the outlining of the following WBZ subvolumes for which we applied the stress inversion: three planar structures in North Honshu (NH) and the Hokkaido corner (HC) WBZ (Plane1, Plane 2, Plane 3), and upper and lower subvolumes in the Hokkaido Island (HI) WBZ. The stress field parameters are evaluated along the northeastern Japan and southern Kurile arcs for these WBZ subvolumes. The stress field in Plane 1, mainly low-angle thrust faults, is characterized by shallow dipping and close to strike normal maximum compression s1 and down dipping minimum compression σ3. Plane 2, the upper surface of the WBZ below 60-70km in NH and HC, is under slab parallel s1 and close to slab normal s3 all along NH, while in HC the minimum compression rotates counterclockwise about 30° relative to the slab normal. Plane 3, the lower surface of the WBZ, is characterized by close to slab normal s1 and close to slab parallel σ3. The stress regime in Plane 1 is of general compression everywhere but in segment HT, beneath the Hokkaido corner, where it is of general extension (Guiraud, 1989). The stress regime in Plane 2 is of general compression, and in Plane 33 of pure extension. A characteristic feature of the two WBZ subvolumes outlined beneath Hokkaido is that the upper subvolume overlies the lower one everywhere but in the southern part of the island. The orientations of the maximum and minimum compressive stresses in the upper subvolume of the Hokkaido WBZ, considered relatively to the local slab geometry, are similar to these in Plane 1 of NH and HC WBZ, i.e. close to strike normal s1 and down dipping σ3. However, the dip of the principle stresses is different 3 here s1 is steeper and s3 is shallower. The stress field in the lower WBZ subvolume beneath Hokkaido is characterized by strike aligned s3 dipping north at about 50°, s1 trends SE being strike normal beneath the southern part of the island and slab normal beneath its northern part. The orientations of P and T in the upper WBZ subvolume in central Hokkaido differ significantly from these in the upper WBZ volumes to the south and to the north but are similar to those in the lower subvolume here. The stress inversion results indicate homogeneous stress field in the upper and lower WBZ subvolumes beneath central Hokkaido. The orientation of the minimum compression here (strike aligned, trending north) is close to the orientations of s3 in the southern and northern lower parts of the HI WBZ, while the s1 is dipping steeply WSW. These stress directions, if considered kinematically, indicate that the preferred faulting occurs at plane that is almost vertical and perpendicular to the strike of the slab (the strike of the trench) with the northern wall moving down and the southern one moving up. The stress regime is of general extension in all the considered subvolumes in the HI WBZ. The results of this study clearly indicate 3-planar distribution of stresses in the WBZ beneath North Honshu and the Hokkaido corner. We outlined two subvolumes (upper and lower) in the WBZ beneath Hokkaido, which are characterized by different orientations of the principle stresses. The stress field in the upper WBZ subvolume is perturbed by a deformation zone (DZ), located beneath central Hokkaido. This DZ is perpendicular to the slab’s strike and is cutting through the slab, the stresses in the upper and lower subvolumes of it are of similar orientation. The directions of the best-fit stress model in the DZ suggest that its northern wall moves down while its southern wall moves up. One plausible explanation is that this deformation zone represents a crack or a tear cutting through the entire slab

Stress relaxation arrested the mainshock rupture of the 2016 Central Tottori earthquake

地震の破壊はなぜ止まるのか? --2016年鳥取県中部地震の断層サイズを決めたもの--. 京都大学プレスリリース. 2021-08-12.After a large earthquake, many small earthquakes, called aftershocks, ensue. Additional large earthquakes typically do not occur, despite the fact that the large static stress near the edges of the fault is expected to trigger further large earthquakes at these locations. Here we analyse ~10, 000 highly accurate focal mechanism solutions of aftershocks of the 2016 Mw 6.2 Central Tottori earthquake in Japan. We determine the location of the horizontal edges of the mainshock fault relative to the aftershock hypocentres, with an accuracy of approximately 200 m. We find that aftershocks rarely occur near the horizontal edges and extensions of the fault. We propose that the mainshock rupture was arrested within areas characterised by substantial stress relaxation prior to the main earthquake. This stress relaxation along fault edges could explain why mainshocks are rarely followed by further large earthquakes

Case report: Laparoscopic gastrojejunostomy for duodenal atresia with situs inversus and preduodenal portal vein: a report of two cases

Congenital duodenal atresia with situs inversus is occasionally accompanied by a preduodenal portal vein (PDPV), which is incidentally diagnosed during surgery. Duodenoduodenostomy is the most common and effective treatment. However, some patients require other anastomoses. Here, we present two cases of laparoscopic gastrojejunostomy for congenital duodenal atresia with situs inversus and PDPV and describe the reason for selecting gastrojejunostomy. The optimal surgical strategy is patient specific and should be determined based on the patient's general and physical condition

Prominent reflector beneath around the segmentation boundary between Tonankai-Nankai earthquake area

In the Nankai Trough subduction seismogenic zone, the Nankai and Tonankai earthquakes had often occurred simultaneously, and caused a great event. In most cases, first break of such large events of Nankai Trough usually begins from southwest off the Kii Peninsula so far. The idea of split Philippine Sea plate between the Kii Peninsula and the Shikoku Island, which explains seismicity, tectonic background, receiver function image and historical plate motion, was previously suggested. Moreover, between the Kii Peninsula and the Shikoku Island, there is a gap of deep low-frequency events observed in the belt-like zone along the strike of the subducting Philippine Sea plate. In 2010 and 2011, we conducted the large-scale high-resolution wide-angle and reflection (MCS) seismic study, and long-term observation from off Shikoku and Kii Peninsula. Marine active source seismic data have been acquired along grid two-dimensional profiles having the total length of ~800km/year. A three-dimensional seismic tomography using active and passive seismic data observed both land and ocean bottom stations have been also performed. From those data, we found a possible prominent reflector imaged in the offshore side in the Kii channel at the depth of ~18km. The velocity just beneath the reflector cannot be determined due to the lack of ray paths. Based of the amplitude information, we interpret the reflector as the forearc Moho based on the velocity gap (from ~6.4km/s to ~7.4km/s). However, the reflector is shallower than the forearc Moho of other area along the Nankai Trough. Similar reflectors are recognized along other seismic profiles around the Kii channel. In this presentation, we will show the result of structure analysis to understand the peculiar structure including the prominent reflector around the Kii channel. Relation between the structure and the existence of the segmentation of the Nankai megathrust earthquake or seismic gap of the deep low-frequency events will be also discussed. This study is part of 'Research concerning Interaction Between the Tokai, Tonankai and Nankai Earthquakes' funded by Ministry of Education, Culture, Sports, Science and Technology, Japan.Poster abstract T43C-2670 presented at 2013 Fall Meeting, AGU, San Francisco, Calif., 9-13 Dec