The phreatomagmatic eruption at Aso volcano on October 8, 2016, released PDC and fallout up to 320 km from the vent, providing an opportunity to study the dispersal and sedimentation of tephra ...bearing wet and dry aggregates. We investigated fresh air-fall tephra and conducted both dry- and wet-sieving to assess the size distribution of both aggregate-bearing deposits on the ground and particles in the plume prior to aggregation. The tephra was asymmetrically distributed in the ENE direction, with a relatively wide distribution and small maximum grain sizes north of the depositional axis, reflecting different wind directions at each altitude. The secondary accumulation maxima at 4–8 km NE from the vent was characterized by the segregation of lapilli followed by accretionary pellets that were rapidly created by aggregation in the wet plume, thereby enhancing the removal of airborne ash. Coalesced aggregates possibly arose from salt precipitation processes. For dry aggregates fallen in medial-to-distal regions, observed deposition duration and fall velocity calculations concluded that fine ash was deposited not as individual particles but as aggregates. Most grain-size distributions along the dispersal axis based on the two sieving methods used were multimodal, with the coarser mode fining according to distance from the vent and the finer mode stabilizing at around 4
ϕ
; this is interpreted as resulting from the mixture of grain-size distributions with different origins, i.e., the normal settling of individual clasts and particle aggregation. The wet-sieved total grain-size distribution included 20.4 wt% fine ash compared with only 2.4 wt% fine ash in the dry-sieved distribution, suggesting that aggregated particles accounted for 18 wt% of the deposit. Total grain-size distributions based on wet-sieving showed extremely poor sorting that could be explained by the following causes: the release of coarse-grained ejecta originating from the host rock of the crater, fine particles due to fragmentation from a phreatomagmatic eruption, the supply of water and salt from the crater to the plume, and aggregation enhancement of the fine ash on the ground, reflecting the effect of the crater lake as external water source.
Pores and cracks have an important role in the evolution of fault rocks because they strongly influence the behavior of the fluids that promote rock alteration and trigger the mechanical instability ...of faults. We used rock physics model inversion of measured elastic wave velocity and porosity to estimate the grain elastic moduli and crack aspect ratios of a range of fault rocks (intact rocks, fractured rocks, transition rocks, and fault gouge) from the Median Tectonic Line in southwest Japan. Our results show distinct gaps in the evolutionary trends of crack aspect ratios and grain elastic moduli from intact rocks to fault rocks. Crack aspect ratios show a nonlinear trend from intact rock to fault gouge, and then these values in fault gouge were considerably higher than in fractured rock and transition rock. In contrast, grain elastic moduli decreased as fracture evolved with the development and subsequent extinction of shear planes and then increased markedly with the formation of fault gouge. Our results show that crack aspect ratios and grain elastic moduli are clearly related to the evolution of shear fabrics in faults. Therefore, they might be useful indicators of fault activity and maturity.
•Elastic wave velocity and porosity are closely related to fault rock evolution.•There is a strong relationship between crack aspect ratio and shear fabric.•Crack aspect ratios and grain elastic moduli are indicators of fault activity and maturity.
The identification of widespread tephras is important, not only for understanding volcanic processes but also for age estimation and establishing stratigraphic relationships. Many widespread tephras ...have been erupted from six active and buried caldera volcanoes in the SW Japan-Ryukyu volcanic arcs. To reconstruct high-resolution tephrostratigraphy, we analyzed core UT from the Uwa basin on western Shikoku Island and correlated the tephras in the core with the proximal source deposits that we sampled. This 120 m-long core composed of fluvio-lacustrine deposits, spanning the middle Pleistocene to Holocene. We recognized and characterized 52 visible tephra and tephric sediment layers in the core on the basis of their stratigraphy, the petrographic features and the major-element composition of glass shards. We successfully correlated 15 tephras with the following previously identified widespread tephras, mainly from volcanoes on Kyushu: Kikai-Akahoya (7.3 ka), Aira-Tn (29 ka), Aso-4 (87 ka), Kikai-Tozurahara (95 ka), Aso-ABCD (100 ka), Ata (105-110 ka), Aso-3 (112.7 ka, calculated in this study), Aso-2 (140 ka), Ata-Toihama (240 ka), Aso-1 (250-270 ka), Tky-Ng1 (290-300 ka), Kakuto (330-340 ka), Oda (420-450 ka), Hiwaki (570-580 ka), and Yufugawa (600 ka). Above 50 m depth, all known widespread tephras from Kyushu younger than 350 ky were included in the sequence that has a fairly constant sedimentation rate of about 0.14 mm/yr. This high-resolution tephrostratigraphy was correlated with those of regional long-core records from the Lake Biwa in the central Japan and offshore of Shikoku that contributes to establish onshore-onshore and onshore-offshore correlations in southwest Japan. In the sediments, each of the following tephras from caldera-forming eruptions, Aso-4, Aso-ABCD, Ata, Aso-3, Aso-2 and Aso-1 occurred above multiple, compositionally similar tephra layers erupted from the same volcano over a long time period. This high-resolution record is therefore an important contribution to the correlation of regional stratigraphy and reconstruction of both the long-term eruptive history and the magma evolution of volcanoes in the SW Japan and -Ryukyu volcanic arcs.
Terrace profiles along rivers based on accurate correlations of terraces and geochronological examination are needed to reveal regional crustal deformation from the distribution of river terrace ...surfaces. Tephra beds, which are intercalated or cover terrace sediments, are used as beneficial information to correlate terrace surfaces accurately. On the other hand, in cases lacking tephric sediments, the correlation of terrace surfaces is mainly based on geomorphic characteristics, such as degree of dissection, relative elevation, and geologic properties, such as facies of sediments and weathered degree of terrace gravels. A terrace surface correlation based on qualitative indicators may cause uncertainty or variability depending on the criteria defined in each study. In this research, weathered degree of gravels in terrace sediments is introduced as another quantitative indicator to validate or compensate for the terrace surface correlation. The examination process is as follows. The research target is the Hiji-kawa River in western Shikoku, along which many terrace surfaces (P, HH, H, M, and L) are distributed. Few tephric sediments exist in terrace sediments. The 0.6 Ma tephra is intercalated at the upper part of P Surface deposits. 0.3 Ma tephra is intercalated at the upper part of Hf1 Surface deposits. An analysis of volcanic ash grains suggests that M surfaces are covered with Aso-4 tephra (85 to 90 ka). First, the ages of other non-aged-terrace surfaces are estimated based on height differences from aged-terraces. Second, real density values are measured for several gravels collected from 15 stages of terrace sediments. Finally, measured real density values are normalized by measured values of current riverbed gravels. Relative density values (RDV) show a negative correlation with ages of terrace surfaces. The research results suggest that RDV is very useful for estimating the ages of terrace surfaces without tephric sediments. The average uplift rates of the middle stream and downstream along the Hiji-kawa River are calculated to be 0.17-0.18 mm/yr. There are no nick points in the current riverbed slope and the spatial distribution of each terrace surface. These results indicate that there is no local topographic deformation due to faulting and folding around the Hiji-kawa River valley.
We carried out a magnetotelluric (MT) survey along a profile crossing the Median Tectonic Line (MTL) in western Shikoku, Japan. The MTL is a terrane boundary that formed during the Cretaceous between ...the Sanbagawa belt, consisting of high pressure/temperature (P/T) metamorphic rocks, and the Ryoke belt of granites and low P/T metamorphic rocks. The MT image shows a zone of remarkably low resistivity, dipping northward at 40°, at the surface coincides with the surface trace of the MTL. The low-resistivity zone probably corresponds to a fluid-filled damaged zone of porous media composed of clay minerals and cracked rocks, formed by repeated faulting of the MTL since Late Cretaceous time. The calculated maximum porosity of the damaged zone is 7.1%, which is clearly higher than that of the non-damaged crystalline rocks.
•We conducted a magnetotelluric survey of the Median Tectonic Line (MTL) in Japan.•The MTL dips northward at 40 degrees and extends to a depth of 20km.•The MTL has a wide low-resistivity zone along the fault plane.•The maximum porosity value of the MTL damaged zone is estimated to be 7.1%.
Fault segmentation models have been developed by using various types of information about faults, including both geological and geophysical data, in order to predict the size of future cascade ...earthquakes. In these models, however, stress conditions within the fault systems have not been considered in sufficient detail. Major fault systems generally exhibit heterogeneous stress conditions. The Median Tectonic Line active fault system (MTLAFS) is the longest and most active arc‐parallel, right‐lateral, strike‐slip fault system in Japan. The stress conditions along the MTLAFS change from transpression on eastern Shikoku Island to transtension on Kyushu Island. The change of stress conditions along the fault system is caused by the counterclockwise rotation of the Nankai fore‐arc sliver in response to the relative motion of the Philippine Sea and Eurasian tectonic plates and back‐arc spreading in the Okinawa trough. Here, we propose “stress condition segmentation” as a concept that deals with the diversity of stress conditions and can be applied in seismic hazard assessment. According to this concept, the MTLAFS is divided into three segments: the East Shikoku, West Shikoku, and Kyushu segments. The East Shikoku and Kyushu segments are under compressional and tensional stress conditions, respectively. Stress in the West Shikoku segment, which is between the other two segments, is transitional from compressional to tensional conditions. In particular, change in the magnitude of fault‐normal stresses along strike is one of the key factors that govern the geometry of fault discontinuities, rupture propagation and termination, and, consequently, fault segmentation. Application of our stress condition segmentation concept to the MTLAFS suggests that future earthquakes in the West Shikoku and Kyushu segments will be of lower energy than those in the East Shikoku segment.
The 2016 Kumamoto earthquake sequence in Kyushu included a foreshock (Mw 6.2) on 14 April and the mainshock (Mw 7.0) on 16 April, both of which were caused by fault ruptures at the intersection of ...the Futagawa and Hinagu fault zones. However, not all sections of the two fault zones were ruptured during the mainshock; in particular, although the northernmost (Takano–Shirahata) section of the Hinagu fault zone ruptured, the rupture did not propagate to southern sections of the fault zone. We examined fault geometry, geological structure, and seismicity around the fault zones, and conducted numerical Coulomb stress change and slip tendency analyses to investigate rupture conditions around the source faults. Fault geometry and slip tendencies indicated that before the foreshock, the source fault (the Futagawa section of the Futagawa fault zone) was favorable for rupture. Seismicity analysis showed that, before the foreshock, a remarkable low-seismicity zone existed where the rupture ultimately terminated. A material or structural boundary in the fault zones probably caused this anomalous seismicity and affected rupture propagation during the mainshock. Coulomb stress change analysis indicated a positive stress change in the Hinagu section just after the mainshock. In addition, the strikes of the optimum slip planes were parallel to that of the Hinagu section just after the mainshock, and slip tendencies in the section increased at that time. These results suggest that the Hinagu section was brought closer to failure just after the mainshock. In contrast, later in the post-mainshock period, the strikes of the optimum slip planes were oblique to that of the Hinagu section and the slip tendencies of the section were low. These results suggest that the favorability for a future rupture of the Hinagu section has declined since 2016 Kumamoto earthquake sequence.
•The Futagawa section was favorable for rupture before the foreshock.•A material or structural boundary curtailed southward rupture propagation.•Favorability for a delayed earthquake in the Hinagu fault zone has changed.
Fault displacement is an important hazard to be considered in the seismic design of structures, along with strong ground motion. In recent years, standards and safety guides for probabilistic fault ...displacement hazard analysis have been published, and social demand for probabilistic verification of fault displacement underneath critical facilities has increased. However, the practice of this method is very limited worldwide, and there are no reports of its full-scale implementation for a specific structure in Japan. In order to achieve wide application in the future, we should first practice probabilistic fault displacement hazard analysis based on a model with higher accuracy, and then understand the current issues properly. Here, we practice the probabilistic fault displacement hazard analysis using model of seismic source characteristics at the Ikata site based on guidelines for SSHAC Level 3, and report on the specific issues in its application at the practical level, referred to the knowledge obtained from the observation data of the 2016 Kumamoto earthquake.
The 2011 off the Pacific coast of Tohoku Earthquake and the 2016 Kumamoto Earthquake have increased the interest in the seismic hazard assessment. As new knowledge has been revealed with every new ...major earthquake, it is important to consider the range of the uncertainty of the parameters which are needed to the assessment and to understand the impact of each earthquake by probabilistic seismic hazard analysis, in advance. The Ikata SSHAC project is the first attempt in Japan to apply SSHAC Level 3 to the probabilistic seismic hazard analysis. In this paper, we report on the results of the modeling of various seismic sources, which include the Median Tectonic Line fault zone which is a long active fault and trench earthquakes occurring in the Nankai Trough. In addition, we discuss the effectiveness of guidelines for SSHAC Level 3, the range of the uncertainty and on the impact on the seismic hazard, because the gained knowledge and know-how of the scope would be useful for subsequent studies.
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