Satsumasendai, Japan
Satsumasendai, Japan

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Imanishi K.,Geological Survey of Japan | Kuwahara Y.,Geological Survey of Japan | Takeda T.,Japan National Research Institute for Earth Science and Disaster Prevention | Mizuno T.,Geological Survey of Japan | And 8 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2011

Focal mechanisms have been determined from P wave polarity data as well as body wave amplitudes for 154 microearthquakes that occurred around the Atotsugawa fault in central Japan between 2002 and 2004. While we found many microearthquakes with a pure strike-slip mechanism that is similar to the faulting style of the Atotsugawa fault, a considerable number of microearthquakes with reverse faulting components are also occurring. Most of the P axes are horizontal and oriented in the WNW-ESE direction, which conforms to the general tectonic trend in this area. In contrast, the T axes have a wide range of plunge, suggesting that reverse-faulting-type earthquakes as well as strike-slip ones are occurring. The most conspicuous feature in the focal mechanism distribution is the depth dependence, where shallow earthquakes are primarily reverse faulting and strike-slip earthquakes become predominant with depth. A stress tensor inversion reveals that the shallower part is characterized by a mixture of reverse and strike-slip faulting regimes and that a pure strike-slip faulting regime appears only around the bottom of the seismogenic zone. Together with other geophysical observational evidence for the fault, we suggest that the existence of a localized aseismic deformation below the Atotsugawa fault is the simplest scenario that can explain the observed stress fields. This scenario provides a stress accumulation mechanism of disastrous shallow inland earthquakes, in which the localized aseismic deformation accumulates stress onto the fault plane in the seismogenic zone during an earthquake cycle and the main shock would occur when the failure stress is reached on the fault. Copyright 2011 by the American Geophysical Union.

Tajikara M.,Association for the Development of Earthquake Prediction | Yasue K.,Japan Atomic Energy Agency | Yanagida M.,Hanshin Consultants Co. | Furusawa A.,Geological Survey of Japan | And 3 more authors.
Geographical Review of Japan | Year: 2011

Many papers reported that Quaternary climate and sea-level fluctuations controlled riverbed elevation in the river basins in northeastern Japan. However, in the southern part of the Chubu region of Japan, such climatic-controlled riverbed fluctuations have not been reported, except in a few papers based on uncertain chronological data. In this research, we investigated fluvial terraces along the Tokigawa (Shonaigawa) River that flows through the low-relief mountainous areas in the northernmost part of the Mikawa Highlands, in the southern part of the Chubu region of Japan, and examined whether riverbed fluctuations similar to those in rivers in northeastern Japan occurred in the Tokigawa River basin. We mapped fluvial terraces based on air photo analysis and inferred the age and climate at the time of formation of these terraces based on 14C dating, tephra analysis, and pollen analysis. Fluvial terraces in the Tokigawa River basin were classified into 10 treads (H1-4 surfaces, M1-3 surfaces, L1-3 surfaces). We estimated that the L2 terrace was a depositional terrace formed during marine oxygen isotope stage (MIS) 2, based on 14C age, marker tephra (Kikai Akahoya tephra) in the soil layer covering terrace deposits, distribution pattern and morphology of terrace surfaces, and thickness of terrace deposits. The M2 terrace was correlated to MIS 6 based on the relationship between the marker tephra (Aso-4 tephra, Kikai Tozurahara tephra) and terrace deposits, existence of red weathered soil, and distribution pattern of the M2 terrace. Based on these results, we concluded that the fluvial terraces in the Tokigawa River basin were formed as a consequence of riverbed fluctuations linked to climate change. We estimated uplift rates in the Tokigawa River basin at 0.11-0.16 mm/year based on the elevation difference between M2 and L1 terrace surfaces.

Fukuchi A.,Hanshin Consultants Co. | Kawai K.,Koyo Junior High School
Paleontological Research | Year: 2011

Abstract. A reinvestigation of Chilotherium from the Mizunami Group revealed that they do not belong to the genus and comprise two species, Brachypotherium? pugnator and Plesiaceratherium sp. It was found that B.? pugnator is more closely related to the Early Miocene Asian Brachypotherium (B. fatehjangense and B. shanwangensis) than to other species; this is inferred on the basis of the dental characteristics of strongly constricted protocones and prominent antecrochets in upper molars. Plesiaceratherium sp. resembles Pl. gracile rather than other European species in terms of the weak rugosities on the labial walls of the P 2-3. The assemblage of rhinoceros fossils of the Mizunami Group is very similar to that of the Early Miocene Shanwang Fauna in China. © 2011 by the Palaeontological Society of Japan.

Kurashimo E.,University of Tokyo | Iwasaki T.,University of Tokyo | Iidaka T.,University of Tokyo | Kato A.,University of Tokyo | And 7 more authors.
Geophysical Research Letters | Year: 2013

Active and passive seismic experiments in the Kii Peninsula, southwest Japan, revealed prominent structural features around the segment boundary of a megathrust earthquake associated with the subduction of the Philippine Sea Plate (PHS). A distinct reflection band in the uppermost part of the PHS shows significant lateral variation along its strike. The thicker reflection band, which corresponds to the deeper extension of the fault area of the 1944 Tonankai Earthquake, is interpreted to be a zone of high pore fluid pressure, causing a state of conditionally stable slip on the plate boundary by the reduction in effective normal stress. A thinner reflective band corresponds to the deepest part of the rupture area of the 1946 Nankai earthquake, where plate coupling is stronger due to less effect of fluids. This along-strike structural variation controls the differences in frictional properties and the lateral limit of rupturing along the plate boundary. © 2013. American Geophysical Union. All Rights Reserved.

Enescu B.,Japan National Research Institute for Earth Science and Disaster Prevention | Enescu D.,National Research Institute for Earth Physics NIEP | Ito K.,Hanshin Consultants Co.
Romanian Reports of Physics | Year: 2011

This work reviews some results obtained for the variations of the seismicity parameters b and p in different seismogenic and tectonic regions in Japan. We bring as well new evidence that the time and space changes in seismicity parameters are correlating well with the crustal structure and/or some parameters of the earthquake process. Moreover, we also analyze the variation of b-value as a function of depth for the Vrancea (Romania) region, of intermediate-depth seismicity. In the first part of the paper we show that several seismicity precursors (clear b-value changes, quiescence and clustering) occurred about two years before the 1995 Kobe earthquake and they correlate well with other geophysical premonitory phenomena of the major event. The precursory phenomena occurred in a relatively large area, which corresponds probably with the preparation zone of the future event. In the second part, we analyze the b and p value spatial and temporal distribution for the aftershocks of the 2000 Tottori earthquake. The results indicate significant correlations between the spatio-temporal pattern of b and p and the stress distribution after the main shock, as well as the crustal structure. The swarm-like seismic sequences occurred in 1989, 1990 and 1997 showed significant precursory b and p values. In the third part of the paper we analyze the seismicity during the 1998 Hida Mountain earthquake swarm. The double-difference-relocated events are analyzed for their frequency magnitude distribution and stress changes. While again the b-value is significantly different in south comparing with the north part of the epicentral area, the physical interpretation is difficult and complex. The changes in the Coulomb failure stress (ΔCFF) can explain the b-value distribution features, but the crustal structure may be also important. The seismicity distribution and migration, in relation with ΔCFF is also discussed. In the last part of the paper, we find that the b-value is higher in the upper part (60 km - 120 km) of the Vrancea sub ducting slab and decreases in the lower part (130 km - 220 km). We discuss this change in relation to stress variations within the sub ducting slab. We refer as well to other world-wide studies.

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