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Uchide T.,Kyoto University | Uchide T.,Geological Survey of Japan
Geophysical Research Letters | Year: 2013

The 2011 Tohoku earthquake produced tens of meters of fault slip near the Japan Trench, which generated devastating tsunami. The rupture process before the huge slip is still unclear due to a lack of resolution. Here I perform a multiscale slip inversion analysis to examine the first 10 and 20 s of the rupture process and the whole rupture process at different scales. The result shows that 4 s after the initiation, this earthquake started with a relatively high-speed rupture that had a peak slip rate faster than 1 m/s and a rupture velocity comparable to 3 km/s. Fourteen seconds after the initiation, the rupture propagation direction changed from northward to westward, near the edge of the M 7.3 foreshock coseismic slip area. The stress release by the foreshock may contribute to the complex small-scale rupture propagation, which may appear to be slow rupture propagation when only looking at long-period data. Key Points A multiscale slip inversion for the initial process of the Tohoku earthquake The Tohoku earthquake started with a dynamic rupture, not a slow rupture A complex rupture propagation pattern in the first 20 s ©2013. American Geophysical Union. All Rights Reserved.

Namegaya Y.,Geological Survey of Japan | Satake K.,University of Tokyo
Geophysical Research Letters | Year: 2014

The rupture parameters and magnitude of the A.D. 869 Jogan earthquake, a predecessor of the 2011 Tohoku earthquake, were previously estimated by matching tsunami deposit distributions with simulated inundation areas the tsunami inundation associated with the 2011 Tohoku earthquake, however, extended farther inland than the sandy tsunami deposits. Numerical simulation of the 2011 tsunami indicated that flow depths and velocities were approximately 1 m and 0.6 m/s, respectively, at the most inland sand deposit sites on the Ishinomaki and Sendai plains. While these values depend on the assumed bottom roughness, we used these values to compare tsunami deposits and inundation simulation of the 869 Jogan earthquake from both uniform-slip and 2011-type variable-slip fault models the results showed that the rupture length of the 869 Jogan earthquake was at least 200†‰km and its minimum moment magnitude was 8.6. Key Points The 2011 flow depth was at least 1 m at the most landward tsunami deposit The revised estimate of the AD 869 Jogan earthquake magnitude is at least Mw 8.6 Paleoearthquake size can be estimated from tsunami deposits and flow depth © 2014. American Geophysical Union. All Rights Reserved.

Yamazaki T.,Geological Survey of Japan | Ikehara M.,Kochi University
Paleoceanography | Year: 2012

In the Southern Ocean, magnetic mineral concentration increases in glacial periods. The variation pattern closely resembles eolian dust flux records from Antarctic ice cores, but the cause of the linkage remains unclear, as the dust flux is too small for the source of terrigenous materials in the Southern Ocean. We have conducted an environmental magnetic study of late Pleistocene sediments from the south Indian Ocean to investigate the origin of the magnetic concentration changes. Biogenic magnetites can be detected using the characteristics of almost no magnetostatic interactions and narrow coercivity distribution, reflecting occurrence of single-domain magnetites in a chain. We interpret that a non-interacting component on first-order reversal curve diagrams and low-coercivity components with small dispersion from isothermal remanent magnetization (IRM) component analyses represent biogenic magnetites, and that the interacting and middle-coercivity components represent terrigenous maghemites. The ratio of anhysteretic remanent magnetization susceptibility to saturation IRM reflects relative abundance of the biogenic and terrigenous components. It was revealed that biogenic magnetites are a dominant constituent of the magnetic minerals. In glacials, the abundance of both biogenic and terrigenous components increased with increased proportions of the latter. Increased ocean productivity in glacials is suggested from increased proportions of biogenic magnetites with elongated morphologies, indicative of less-oxic conditions, and increased sedimentation rates. These observations suggest that the increased magnetic concentration in glacials in the Southern Ocean may be explained by iron fertilization; the production of biogenic magnetites was enhanced associated with increased ocean productivity, which was fueled by increased eolian dust flux. © 2012 by the American Geophysical Union.

The mélanges of the Jurassic accretionary complexes of Japan have very complicated structures which were formed by the progressive disruption and mixing of Ocean Plate Stratigraphy (OPS) during various stages of accretionary processes along the ancient arc-trench system. The OPS is composed of Permian basalt-limestone-chert, Permian-Triassic boundary claystone, Early Triassic-Early Jurassic chert, Early Jurassic-earliest Cretaceous siliceous shale and Jurassic to early Cretaceous turbidite, in ascending order.The OPS was detached from the oceanic plate along a decollement, which developed within the P-T boundary claystone during the offscraping and underplating processes. Disruption and mixing first occurred along the decollement, then along successively developed out-of-sequence thrusts. Neighboring rocks along the faults are mixed to become various sized clasts and matrices of mélanges. Mappable slabs in the mélanges were survivors from tectonic disruption of the accreted OPS. On the surface of accretionary wedge, a part of the OPS was collapse by submarine sliding and mud diaprisim to form chaotic sediments on the top of the OPS. Thus, the full extent of accretionary complex was analyzed by age-stage development of mélange formation. In the mélanges, some of the large tectonic slabs in the mélanges kept the original order of the OPS even though they were deformed in various degrees. This suggests that the OPS was tectonically stacked during the offscraping process, and was gradually disrupted along the decollement and out-of-sequence thrusts during the accretionary processes that form the mélange structures. The mélanges structures mentioned above are shown in the geological maps at the scale of 1:50,000 recently published by the Geological Survey of Japan. These geological maps clearly show the mélange formation processes during ocean plate subduction along the Mesozoic Asian continental margin. © 2011 Elsevier B.V.

Shinohara H.,Geological Survey of Japan
Journal of Volcanology and Geothermal Research | Year: 2013

Global volatile fluxes from subaerial volcanoes at subduction zones were estimated based on a compilation of fluxes from various sources, including persistent degassing, hot and cold springs, soil degassing, and eruptions. Because worldwide comprehensive datasets are not available, especially for diffuse volatile discharges, volatile fluxes from Japan arcs were estimated based on detailed datasets, and the regional fluxes were extrapolated to the global flux with consideration of the regional characteristics of volcanic volatile compositions, which were estimated based on volcanic gas compositions of persistent degassing. The estimated global fluxes indicate that persistent degassing is the major source of volatiles, especially for S with a contribution of 80%. Diffuse discharges and persistent degassing are similarly important sources of H2O, CO2, and Cl, but the contribution of explosive eruptions is less than 15% for all the volatiles. The estimates of diffuse degassing fluxes include large errors due to limited data. However, the potential impact of these sources on the global flux indicates the importance of further studies to quantify these fluxes. The volatile budget of subduction zone volcanism was evaluated by comparing the estimated volatile fluxes, the volatile contents in the crust, and the primitive magma volatile contents. The contribution of volatiles remaining in the crust are not significant; however, consideration of lower crust foundering significantly alters the volatile budget estimate because the primitive magma supply rate should be significantly increased to account for the lower crust foundering. © 2013 Elsevier B.V.

Magnetic properties are increasingly used for paleoclimatic and paleoceanographic studies. Utilizing recently developed proxies, an environmental magnetic study was conducted on the uppermost 12 m sediments of Integrated Ocean Drilling Program Site U1337 in the eastern equatorial Pacific Ocean. This interval is above the Fe-redox boundary, and covers the past ~800 k.y. The site is located near the present southern boundary of the Intertropical Convergence Zone (ITCZ), and is thus expected to be sensitive to variations of the ITCZ position. The ratio of anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization (kARM/SIRM), first-order reversal curve diagrams, and IRM acquisition curves indicate that the magnetic mineral assemblage consists of a dominant biogenic component and a minor terrigenous component. Two groups, the biogenic soft (BS) and hard (BH), are identified for the biogenic component, and probably correspond to different magnetofossil morphology. The BH component, probably carried by elongated magnetofossils, increases in sediments of glacial periods, which are probably in less oxic conditions due to increased ocean productivity. This demonstrates that magnetofossil morphology, which can be discriminated by the rock magnetic technique, is a sensitive indicator of slight oxic-suboxic environmental fluctuations in sediments. Temporal variations of the terrigenous component, most likely transported as eolian dust, were estimated from the kARM/SIRM ratio and S ratio (ratio of a moderate field IRM to SIRM, representing relative contribution of lower- and higher-coercivity magnetic minerals); significant glacial-interglacial variations occurred at marine isotope stage (MIS) 10 and before, but not after. In addition, coeval upcore increases in sedimentation rates and the BH component were observed, suggesting increased productivity. These observations may indicate that the position of the ITCZ was more southward than today ca. 250 ka and before. © 2012 Geological Society of America.

Shinohara H.,Geological Survey of Japan
Earth, Planets and Space | Year: 2013

Volcanic gas compositions of Shinmoedake, Kirishima volcano, Japan were measured by Multi-GAS during the persistent degassing period with repeating Vulcanian eruptions from March 2011 to March 2012. In order to avoid risks due to eruptions, the measurements were performed with the Unmanned Aerial Vehicles (UAV) that fly through the plume with the Multi-GAS and by an automatic Multi-GAS monitoring station installed 5 km away from the summit. Based on the UAV measurements on May 18, 2011, most of the major volcanic gas components were quantified as CO2/SO2 = 8, SO2/H2S = 0.8, H2O/C02 = 70 and H2/SO2 = 0.03 (mol ratio), and the SO2/H2S ratio of the plume was quantified as 8 on March 15, 2011. The Multi-GAS monitoring station occasionally detected a dilute plume with an SO2/H2S ratio ranging from 0.8 to 3.3 from April 2011 to March 2012. The decrease of the SO 2/H2S ratio from March 15, 2011, to May 18, 2011, is interpreted as the result of a ten times increase of the degassing pressure. Based on the S02 fluxes and the gas compositions, the conduit magma convection is considered to be the gas supply mechanism at the Shinmoedake, and the degassing pressure changes are attributed to the change of depth of the convecting magma column top.

Tamura T.,Geological Survey of Japan
Earth-Science Reviews | Year: 2012

Beach ridges are landforms commonly developed on prograded coasts with beach shorelines. A sequence of beach ridges, coupled with their subsurface deposits, can be regarded as a time series of coastal evolution. Methodological advances in field surveying and chronology applicable to beach ridges have led to detailed palaeoenvironmental reconstructions to be derived from such sequences. This paper reconsiders the basic aspects of beach ridges and deposits, which need to be properly understood for their comprehensive interpretation in a palaeo-environmental context. It also reviews case studies in which beach-ridge sequences have been used to unveil past sea-level history, catastrophic events, and climate changes.Proposed formative processes of beach ridges include: 1) progradation of sandy beach and berm formations in relation to fairweather waves, coupled with aeolian foredune accumulation; 2) building of gravel ridges by storm waves; 3) welding of longshore bars. Beach-ridge formation through sea-level oscillation is thought to be questionable and caution is suggested for this process when undertaking palaeoenvironmental reconstruction. Beach deposit stratification is known to dip either landwards or seawards, but landward dips are uncommon. Seaward dipping stratification is formed in relation to beachface progradation, and is usually dissected in places by erosion surfaces resulting from episodic beach retreat. The boundary between the foreshore and the underlying shoreface is well defined only in the case that longshore bars lead to complex bedding structure relative to that of the foreshore. Reliable chronology of beach ridges can be determined by radiocarbon and optically-stimulated luminescence (OSL) dating. Radiocarbon dating of articulated shells, which are considered not to be extensively reworked, provides robust results, but OSL dating is more useful as it enables direct dating of sediment grains. It is noted that there are restrictions in chronological resolution and continuity inherent to beach ridge and beach deposits. The plan-view geomorphic expression of beach ridges typically consists of ridge sets with multi-decadal intervals, whereas their internal sedimentary structures define shorter time scales. Records of beach sedimentation and erosion are likely to be reworked by episodic high-magnitude beach retreat, and the resultant record of the net progradation is likely to be sporadic and discontinuous.The height of sandy beach ridges is often variable due to differing degrees of aeolian sand accumulation, and they are thus not used as sea-level indicators unless purely wave-built. Gravel ridge height is a relatively reliable indicator of sea level, but can vary in response to storminess fluctuations. Subsurface sediment facies boundaries are preferred as sea-level indicators, and those proposed include: boundaries of aeolian/beach, foreshore/shoreface, and upper/lower shorefaces.Catastrophic events are expressed in both erosional and depositional records. Erosion surfaces, or scarp imprints, revealed in a cross section of beach deposits, indicate storm or tsunami events. However, erosional events are likely to rework previous records of sedimentation and even other erosional events, and thus the apparent history decoded from the resultant deposits tends to be biased. Several attempts for estimating the frequency and intensity of prehistoric cyclones rely on assumed relationships between the level of coarse sand beach ridges and cyclone inundation. The formative process of coarse sand ridges remains uncertain and needs to be clarified, as it constitutes the fundamental basis of these attempts.The growth rates of beach-ridge systems are expected to reflect fluctuations in river sediment discharge to the coast and in aeolian sand flux due to onshore winds, both of which are affected by climate change. Assessment of the growth rate is potentially improved by ground-penetrating radar survey of subsurface structure and by detailed chronology. Orientation of beach ridges reflects long-term trends in wave direction. Inferred relationships between beach ridges and cyclic fluctuations of sea level and climate rely on weak assumptions and are not substantiated by rigid chronological evidence, and thus remain highly questionable. Correlation between the inherently decadal signal of beach-ridge intervals and possible climate cycles, such as sunspot activity, is probably coincidental as it lacks causal explanations. © 2012 Elsevier B.V.

Kanai Y.,Geological Survey of Japan
Journal of Environmental Radioactivity | Year: 2012

Artificial radionuclides were released into the atmosphere by the Fukushima Dai-ichi Nuclear Power Plant incident after a strong earthquake on 11 March 2011. Aerosol monitoring at the Geological Survey of Japan, Tsukuba, was started 20d after the incident. Radionuclides such as 99Mo/ 99mTc, 132Te/ 132I, 129mTe/ 129Te, 131I, 137Cs, 136Cs, 134Cs, 140Ba/ 140La, 110mAg, and 95Nb were observed and, with the exception of 137Cs and 134Cs, these radionuclides decreased to below the limit of detection in the middle of June. The activity ratio of atmospheric 134Cs/ 137Cs in aerosols decreased over time almost following physical decays. Therefore, the 134Cs/ 137Cs activity ratio in the averaged air mass in this study could be regarded as homogeneous although those of several reactors in the Nuclear Power Plant were not ascertained. A further research on the released 137Cs and 134Cs would be necessary for the sedimentology of lake sediment. © 2011 Elsevier Ltd.

Lei X.,Geological Survey of Japan
European Physical Journal: Special Topics | Year: 2012

In order to shed some lights to the "dragon-kings" concept, this paper re-examines experimental results on rock fracture tests in the laboratory, obtained from acoustic emission monitoring. The fracture of intact rocks as well as rocks containing natural structures (joints, faults, foliations) under constant stress rate loading or creep conditions is generally characterized by typical stages with different underlying physics. The primary phase reflects the initial rupture of pre-existing microcrack population in the sample or in the fault zone. Sub-critical growth dominates the secondary phase. The third phases termed nucleation phase corresponds to the initiation and accelerated growth of the ultimate fracture. The secondary and nucleation phases in both intact rock and faulted rock show power-law (of time-to-failure) increasing event rate and moment release. Samples containing planar structures such as foliations and faults demonstrate very similar features to natural earthquakes including: 1) small number of immediate foreshocks by which fault nucleation zones could be mapped; 2) the critical nucleation zone size is normally a fraction of the sample dimension; 3) a lot of aftershocks concentrated on the fault ruptured during the main event; 4) stress drop due to the main rupture is of the order from a few tens to a few hundreds MPa; 5) b-value drops during foreshocks and recovers during the aftershocks. All these results agree with the suggestion that laboratory measurements require no scaling but can be applied directly to the Earth to represent local fault behavior. The ultimate failure of the sample, or fracture of major asperities on the fault surface, normally lead to extreme events, i. e., dragon-kings, which has a magnitude significantly greater than that expected by the Gutenberg-Richter power-law relation in the magnitude-frequency distribution for either foreshocks or aftershocks. There are at least two mechanisms that may lead to dragon-kings: 1) The power-law increasing event rate and moment release; and 2) Hierarchical fracturing behavior resulting from hierarchical inhomogeneities in the sample. In the 1st mechanism, the final failure corresponds to the end point of the progressive occurrence of events and thus the resulted dragon-king event can be interpreted as a superposition of many small events. While for the 2nd mechanism an event of extreme size is the result of fracture growth stepping from a lower hierarchy into a higher hierarchy on fault surface having asperities characterized by hierarchical distribution (of size or strength) rather than simple fractal distribution. In both mechanisms the underlying physics is that fracture in rocks is hard to stop beyond certain threshold corresponding to the critical nucleation zone size. © 2012 EDP Sciences and Springer.

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