380 research outputs found
Deep Investigations of Outer-Rise Tsunami Characteristics Using Well-Mapped Normal Faults Along the Japan Trench
To assess the risk of tsunamis from outer-rise earthquakes, we carried out tsunami simulations using 33 simple rectangular fault models with 60° dip angles based on marine seismic observations and surveys of the Japan Trench. The largest tsunami resulting from these models, produced by a Mw 8.7 normal-faulting event on a fault 332 km long, had a maximum height of 27.0 m. We tested variations of the predictions due to the uncertainties in the assumed parameters. Because the actual dip angles of the Japan Trench outer-rise faults range from 45° to 75°, we calculated tsunamis from earthquakes on fault models with 45°, 60°, and 75° dip angles. We also tested a compound fault model with 75° dip in the upper half and 45° dip in the lower half. Rake angles were varied by ±15°. We also tested models consisting of small subfaults with dimensions of about 60 km, models using other earthquake scaling laws, models with heterogeneous slips, and models incorporating dispersive tsunami effects. Predicted tsunami heights changed by 10â15% for heterogeneous slips, up to 10% for varying dip angles, about 5â10% from considering tsunami dispersion, about 2% from varying rake angles, and about 1% from using the model with small subfaults. The use of different earthquake scaling laws changed predicted tsunami heights by about 50% on average for the 33 fault models. We emphasize that the earthquake scaling law used in tsunami predictions for outer-rise earthquakes should be chosen with great care
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Occurrences of large earthquakes with estimated magnitudes as large as 8 along the Nankai Trough have been well documented in historical materials, and the oldest documented earthquake dates back to A.D. 684. The recurrence periods are fairly constant, and are approximately 200 years for the earthquakes occurring before 1361, and about 100 years for those after 1361. However, the number of smaller earthquakes observed by the on-land seismic stations is very small. Both seismic and tectonic couplings along the interface between the overriding and subducting plates are estimated to be nearly 100% from seismic and geodetic data. Therefore, it has been considered that relatively simple physics govern the generation of the historical large earthquakes along the Nankai Trough. With the availability of abundant information on large earthquakes and modern data sets from dense seismic and geodetic networks, the Nankai Trough is one of the best-studied seismogenic zones. Although improvements have been seen in estimating the fault-plane parameters for the historical earthquakes, there still remain important unanswered questions, such as if there have been unidentified earthquakes that fill in the 200-year recurrence period. Estimates of precise fault-plane parameters are being demanded to have a better understanding of earthquake generation. One of the most important topics is where the updip limit of the seismogenic zone is located. The location had not been resolved by on-land seismic observations. Seismic observations using ocean bottom seismometers have recently been conducted in both western and eastern regions along the Nankai Trough. The observations in the western region confirm that the updip limit coincides well with both the updip limit of the estimated fault plane for the 1946 Nankaido earthquake and the estimated 150â isotherm along the interplate interface. Earthquake activitiy has been proved to be very low in both regions
Geometry of the Philippine Sea plate subducting beneath the westernmost Nankai Trough
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Great interplate earthquakes have repeatedly occurred in pairs along the Nankai Trough. In order to reduce a great deal of damage to coastal area from both strong ground motion and tsunami generation, it is necessary to understand rupture synchronization and segmentation of the Nankai megathrust earthquake. For a precise estimate of the rupture zone of the Nankai megathrust event based on the knowledge of realistic earthquake cycles and variations of magnitude, it is important to know the geometry and property of the plate boundary of the subduction seismogenic zone. To improve a physical model of the Nankai Trough seismogenic zone, the large-scale high-resolution wide-angle and reflection (MCS) seismic studies, and long-term observation have been conducted since 2008. Marine active source seismic data have been acquired along grid two-dimensional profiles having the total length of ~800km per year. A three-dimensional seismic tomography using active and passive seismic data observed both land and ocean bottom stations have been also performed. 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. The seismic survey was conducted off the Tokai area including the onshore survey across the eastern Kii Peninsula in 2012, the final year of this project. Compiling those studies provides a three-dimensional plate geometry and velocity structure models of the western Nankai Trough at the moment. Although their reliability and resolution should be evaluated, these models can be applied to a numerical simulation to examine if the observed rupture zone of the historical event can be reproduced. We will also try to construct more fine-scale model for the entire Nankai Trough area.SSS31-P15ãã¹ã¿ãŒèŠæš / æ¥æ¬å°çææç§åŠé£å2013幎倧äŒïŒ2013幎5æ19æ¥ïœ5æ24æ¥, å¹åŒµã¡ãã»åœéäŒè°å ŽïŒ / æ¥æ¬ææç§åŠé£åã®èš±è«Ÿã«åºã¥ãæ¬æãã¡ã€ã«ãæ²èŒhttp://www.godac.jamstec.go.jp/darwin/cruise/kairei/kr10-11/ehttp://www.godac.jamstec.go.jp/darwin/cruise/kairei/kr11-09/
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Coseismic rupture area of the great interplate earthquake concerned about its occurrence along the Nankai Trough presumed by government of Japan is now wider to the west, north and south than the former assumption. Although the new estimation is based on seafloor topography, source area of the past largest megathrust event, present seismic activity and so on, structural information has not always been enough reflected yet. In order to estimate precise coseismic rupture area of the Nankai megathrust earthquake, it is necessary to improve a physical model of the Nankai Trough seismogenic zone based on the geometry of the subducting plate and velocity structure model. Japan Agency for Marine-Earth Science and Technology had conducted the large-scale high-resolution wide-angle and reflection seismic survey and long-term observation from off Kyushu to Tokai between 2008 and 2012. Layered velocity structure models are now obtained along grid two-dimensional seismic profiles from the Hyuga-nada to the Kii channel area. A three-dimensional seismic tomography using active and passive seismic data observed both land and ocean bottom stations had been also performed for the western Nankai Trough. In this study, we constructed a three-dimensional velocity model of the Nankai Trough with the procedure as follows; 1) Sampling the velocity structural information along each seismic profile with interval of ~1km in horizontal, and ~100m in vertical directions 2) Preparing the geometry model of each interface included in layered models, e.g., basement, plate boundary, Moho, etc. 3) Setting minimum and maximum velocities of each layer based on the velocity models along two-dimensional seismic profiles 4) Interpolating sampled velocity information considering layered structure (Landmark DecisionSpaceDesktop is used for constructing 3-D modeling) Previously published layered models are also used to make up for insufficient structural information for the eastern Nankai Trough. Reliability of the three-dimensional model was confirmed by comparing calculated travel-times with observed travel-times along each seismic profile. We will also try to evaluate the reliability of the model by comparing the hypocenter distribution using three-dimensional velocity model obtained in this study with that determined by three-dimensional seismic tomography using active and passive source data. We will plan to revise our 3D model with additional structural information and construct more precise and detailed model for the entire Nankai Trough area so that the model can be applied to more realistic numerical simulation. This study is part of 'Research concerning Interaction Between the Tokai, Tonankai and Nankai Earthquakes (FY2008-2012)' funded by Ministry of Education, Culture, Sports, Science and Technology, Japan.SSS30-P16ãã¹ã¿ãŒèŠæš / æ¥æ¬å°çææç§åŠé£å2014幎倧äŒïŒ2014幎4æ28æ¥ïœ5æ2æ¥, ãã·ãã£ã³æšªæµïŒ / æ¥æ¬ææç§åŠé£åã®èš±è«Ÿã«åºã¥ãæ¬æãã¡ã€ã«ãæ²
Development of a Super-deep-sea Self Pop-up Ocean Bottom Seismometer using a Ceramic Pressure-tight Housing
We have developed a Super-deep-sea self-popup Ocean Bottom Seismometer (SDOBS) that can be deployed to the ocean floor up to 9,000 m depth. Because the maximum applicable water depth of a conventional self-popup Ocean Bottom Seismometers (OBS) is 6,000 m, some areas have remained inaccessible to seismic surveys, such as the deep part of Japan Trench, where the Great East Japan Earthquake occurred in 2011. Using a ceramic pressure-tight sphere, we were able to develop a SDOBS that has almost identical size, weight, and buoyancy to those of a conventional self-popup OBS using a glass sphere. Regarding the acoustic transponder, which is a key device for the development of SDOBSes. We heighten the transmitting acoustic level of an existing acoustic transponder to raise the positioning accuracy. Detailed results of sea tests conducted to evaluate the acoustic transponder performance are described herein. We used the same built-in seismometers, recorders, batteries, and other equipment as those used for conventional OBSes. We also report that by improving the test procedures, we were able to heighten the measurement accuracy of the uniaxial compressive strength of ceramics, which are important parameters to determine the applicable hydraulic pressure. We have developed seven SDOBSes to date, and have deployed 18 SDOBSes cumulatively for seismic surveys in the Japan Trench and Ryukyu Trench
Construction of the three-dimensional velocity model for Nankai Trough seismogenic zone
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