Seismic frame design via inverse mode design of frame-ground systems

A two-step stiffness design procedure is developed for a moment-resisting planar frame supported by a prescribed two-dimensional finite-element ground-pile system. In the first step, a hybrid inverse eigenmode problem is formulated and its solution is derived in an analytical form. A difficulty resulting from the existence of multiple interface nodes is overcome by incorporating a deformation constraint into a set of linear equations for finding the lowest-mode displacements at the interface nodes and in the ground. In the second step, the fundamental natural frequency of the combined system and the lowest mode-strain ratios in the frame specified in the first step are regarded as the parameters for adjusting the mean peak seismic member-end strains to their specified values. If the fundamental natural frequency of the frame with a fixed-base happens to be close to that of the ground, a difficulty arises in the two-step stiffness design procedure because of an irregular response amplification and of the non-predominance of the lowest-mode components. A new practical design procedure of rapid convergence is proposed such that an initial design is found for a stiff ground and that a sequence of stiffness designs is generated with respect to a ground stiffness parameter without any differential coefficient of series expansion. The accuracy of the model utilized in this paper and the validity of the present stiffness design procedure are verified through time-history response analysis.     

Mobility-dependent response of aquatic animal species richness to a wetland network in an agricultural landscape

Management of wetland connectivity is important for biodiversity conservation. In the modern agricultural landscape, the natural connections between floodplain wetlands have been greatly altered. Agricultural ditches and channelized streams are widely distributed in floodplains, which may contribute to the maintenance of wetland connectivity and biodiversity. To determine how these watercourse networks affect wetland biodiversity, we examined the relationship between the species richness of aquatic animals and wetland connectivity, with a special focus on species mobility. From July to August 2011, fish and aquatic insects were collected from 24 wetlands in northern Japan. To determine the degree of wetland connectivity, we assessed the relative importance of individual wetlands in maintaining the entire wetland network using two connectivity indices: hydrologic connectivity via watercourses and spatial connectivity defined as Euclidian distances between wetlands using graph theory. We found that only high mobility groups of both taxa could enhance species richness in either a hydrologic (fish) or spatial (insect) wetland network. The species richness of insects with high-flying ability was found to increase as spatial connectivity increased. Furthermore, the species richness of fish with high-swimming ability was positively influenced by hydrologic connectivity, most likely because highly mobile species were able to reach suitable habitats and migrate from source populations in a wetland network owing to their good mobility. Our findings indicate that hydrologic network is important for maintaining biodiversity as well as spatial connectivity. It is important to focus conservation efforts on key wetlands with high hydrologic and spatial connectivity in future wetland management.     

Substorm observations in the early morning sector with Equator-S and Geotail

Data from Equator-S and Geotail are used to study the dynamics of the plasma sheet observed during a substorm with multiple intensifications on 25 April 1998, when both spacecraft were located in the early morning sector (03–04 MLT) at a radial distance of 10–11 R_E. In association with the onset of a poleward expansion of the aurora and the westward electrojet in the premidnight and midnight sector, both satellites in the morning sector observed plasma sheet thinning and changes toward a more tail-like field configuration. During the subsequent poleward expansion in a wider local time sector (20−04 MLT), on the other hand, the magnetic field configuration at both satellites changed into a more dipolar configuration and both satellites encountered again the hot plasma sheet. High-speed plasma flows with velocities of up to 600 km/s and lasting 2–5 min were observed in the plasma sheet and near its boundary during this plasma sheet expansion. These high-speed flows included significant dawn-dusk flows and had a shear structure. They may have been produced by an induced electric field at the local dipolarization region and/or by an enhanced pressure gradient associated with the injection in the midnight plasma sheet.     

Regionality of deep low-frequency earthquakes associated with subduction of the Philippine Sea plate along the Nankai Trough, southwest Japan

The Fukuoka District Meteorological Observatory recently logged three possible deep low-frequency earthquakes (LFEs) beneath eastern Kyushu, Japan, a region in which LFEs and low-frequency tremors have never before been identified. To assess these data, we analyzed band-pass filtered velocity seismograms and relocated LFEs and regular earthquakes using the double-difference method. The results strongly suggest that the three events were authentic LFEs, each at a depth of about 50 km. We also performed relocation analysis on LFEs recorded beneath the Kii Peninsula and found that these LFEs occurred near the northwest-dipping plate interface at depths of approximately 29–38 km. These results indicate that LFEs in southwest Japan occur near the upper surface of the subducting Philippine Sea (PHS) plate. To investigate the origin of regional differences in the occurrence frequency of LFEs in western Shikoku, the Kii Peninsula, and eastern Kyushu, we calculated temperature distributions associated with PHS plate subduction. Then, using the calculated thermal structures and a phase diagram of water dehydration for oceanic basalt, the water dehydration rate (wt.%/km), which was newly defined in this study, was determined to be 0.19, 0.12, and 0.08 in western Shikoku, the Kii Peninsula, and eastern Kyushu, respectively; that is, the region beneath eastern Kyushu has the lowest water dehydration rate value. Considering that the Kyushu–Palau Ridge that is subducting beneath eastern Kyushu is composed of tonalite, which is low in hydrous minerals, this finding suggests that the regionality may be related to the amount of water dehydration associated with subduction of the PHS plate and/or differences in LFE depths. Notable dehydration reactions take place beneath western Shikoku and the Kii Peninsula, where the depth ranges for dehydration estimated by thermal modeling agree well with those for the relocated LFEs. The temperature range in which LFEs occur in these regions is estimated to be 400–500 °C.     

Hydrothermal activity on the southern Mid-Atlantic Ridge: Tectonically- and volcanically-controlled venting at 4–5°S

We report results from an investigation of the geologic processes controlling hydrothermal activity along the previously-unstudied southern Mid-Atlantic Ridge (3–7°S). Our study employed the NOC (UK) deep-tow sidescan sonar instrument, TOBI, in concert with the WHOI (USA) autonomous underwater vehicle, ABE, to collect information concerning hydrothermal plume distributions in the water column co-registered with geologic investigations of the underlying seafloor. Two areas of high-temperature hydrothermal venting were identified. The first was situated in a non-transform discontinuity (NTD) between two adjacent second-order ridge-segments near 4°02′S, distant from any neovolcanic activity. This geologic setting is very similar to that of the ultramafic-hosted and tectonically-controlled Rainbow vent-site on the northern Mid-Atlantic Ridge. The second site was located at 4°48′S at the axial-summit centre of a second-order ridge-segment. There, high-temperature venting is hosted in an  18 km² area of young lava flows which in some cases are observed to have flowed over and engulfed pre-existing chemosynthetic vent-fauna. In both appearance and extent, these lava flows are directly reminiscent of those emplaced in Winter 2005−06 at the East Pacific Rise, 9°50′N and reference to global seismic catalogues reveals that a swarm of large (M 4.6−5.6) seismic events was centred on the 5°S segment over a  24 h period in late June 2002, perhaps indicating the precise timing of this volcanic eruptive episode. Temperature measurements at one of the vents found directly adjacent to the fresh lava flows at 5°S MAR (Turtle Pits) have subsequently revealed vent-fluids that are actively phase separating under conditions very close to the Critical Point for seawater, at  3000 m depth and 407 °C: the hottest vent-fluids yet reported from anywhere along the global ridge crest.     

Diffusive dehydration and bubble resorption during open-system degassing of rhyolitic melts

Degassing through open paths such as bubble and/or fracture networks is considered to be the principal mode of degassing in silicic magmas. However, its detailed mechanisms remain unclear. To investigate the behavior of bubbles in a hypothetical open-system condition, we performed a series of vesiculation experiments on natural rhyolitic obsidian using a newly designed semipermeable cell, which artificially maintains a pressure difference between its inside and outside. The thick-wall cell maintains a constant volume within the sample chamber, while allowing water vapor to escape the cell during the experimental runs. The cells containing obsidian cores with ca. 0.66 wt.% initial water content were externally heated to 1000 °C for a period of 1–288 h. The run charges generally showed a zonal structure composed of two contrasting regions: a central region within which the bubbles were concentrated (bubble-rich core, BC) and a bubble-free melt region surrounding the BC (bubble-free margin, BFM). With increased heating duration, the thickness of the BFM increased via dissolution of the outermost bubbles in the BC. The water content was nearly uniform throughout the BC, whereas an outward-decreasing gradient was observed in the BFM. We found that diffusive dehydration occurred from the sample surface, and the bubbles were resorbed into the melt. Thus, the BFM–BC boundary moved inwards. These processes were modelled numerically, and the calculation results were in good agreement with the experimental data. If a “lifetime” of open paths is approximated as the relaxation time of a melt in a shallow volcanic environment, then the paths have to be pinched off quickly (1.2 h at maximum pressure difference between the open path and the melt) and thus the thickness of the bubble-free layer reaches at most ~ 0.1 mm. For the formation of bubble-free obsidian layers with a width of a few millimeters, which are often observed in natural obsidian flows, open paths should be maintained for at least a few hundred hours.     

Timing and development of sedimentation of the Cleaverville Formation and a post‐accretion pull‐apart system in the Cleaverville area,coastal Pilbara Terrane,Pilbara, Western Australia

In the Cleaverville area of Western Australia, the Regal, Dixon Island, and Cleaverville Formations preserve a Mesoarchean lower‐greenschist‐facies volcano‐sedimentary succession in the coastal Pilbara Terrane. These formations are distributed in a rhomboidal‐shaped area and are unconformably overlain by two narrowly distributed shallow‐marine sedimentary sequences: the Sixty‐Six Hill and Forty‐Four Hill Members of the Lizard Hills Formation. The former member is preserved within the core of the Cleaverville Syncline and the latter formed along the northeast‐trending Eighty‐Seven Fault. Based on the metamorphic grade and structures, two deformation events are recognized: D₁ resulted in folding caused by a collisional event, and D₂ resulted in regional sinistral strike‐slip deformation. A previous study reported that the Cleaverville Formation was deposited at 3020 Ma, after the Prinsep Orogeny (3070–3050 Ma). Our SHRIMP U–Pb zircon ages show that: (i) graded volcaniclastic–felsic tuff within the black shale sequence below the banded iron formation in the Cleaverville Formation yields an age of (3 114 ±14) Ma; (ii) the youngest zircons in sandstones of the Sixty‐Six Hill Member, which unconformably overlies pillow basalt of the Regal Formation, yield ages of 3090–3060 Ma; and (iii) zircons in sandstones of the Forty‐Four Hill Member show two age peaks at 3270 Ma and 3020 Ma. In this way, the Cleaverville Formation was deposited at 3114–3060 Ma and was deformed at 3070–3050 Ma (D₁). Depositional age of the Cleaverville Formation is at least 40–90 Myr older than that proposed in previous studies and pre‐dates the Prinsep Orogeny (3070–3050 Ma). After 3020 Ma, D₂ resulted in the formation of a regional strike‐slip pull‐apart basin in the Cleaverville area. The lower‐greenschist‐facies volcano‐sedimentary rocks are distributed only within this basin structure. This strike‐slip deformation was synchronous with crustal‐scale sinistral shear deformation (3000–2930 Ma) in the Pilbara region.     

Physical parameters identification for full-scale ten-story reinforced concrete building with degrading tri-linear model by modal iterative error correction method

To assess the post-earthquake seismic safety of buildings, it is crucial to predict seismic response, and it is necessary to set the appropriate physical parameters of the response analysis model. Numerous methods have been proposed to identify physical parameters. However, most of them are limited to linear systems, and previous researches on nonlinear systems have difficulties in practical applications. In this paper, a nonlinear response analysis model is identified for a full-scale ten-story reinforced concrete building with the degrading tri-linear stiffness model by the modal iterative error correction (MIEC) method, and the accuracy of this technique is discussed by comparing with the shaking table test.     

利用断层应力积累探讨大地震发生的地点——以日本2011年东北9级大地震为例

大地震在哪里发生是地震预报首先要解决的问题.利用反演GNSS观测数据得到的2011年日本东北9级大地震前7年（2004—2010年）断层上的应力变化,我们发现了这次地震断层的孕震区.为了进一步研究该孕震区的演化过程,本文继续反演这次大地震在1997—2003年间的断层应力变化过程.通过这两期的反演工作,我们看到,在这14年中,断层应力的年变化图案的主要特征基本是稳定的,并存在明显的应力增加区和降低区.前者与地震的破裂区吻合,后者与前震、重复小地震和无震滑动的区域一致.显著的剪切应力增加区不但与主震,而且还与大余震的破裂区相符合.我们发现断层面上高应力积累区的零剪应力和零正应力变化的等值线不重合,前者在断层面上的深度大于后者,这意味着在剪应力增加区存在着正应力降低区或剪切强度降低区（由于剪切强度与正应力成正比）.断层初始破裂点似乎更偏好零正应力等值线附近的位置,这是因为该处不但靠近剪切强度降低区,而且位于剪应力积累最显著的地方.研究结果表明,正应力变化对大地震的初始破裂有影响;本文所使用的断层应力变化反演方法,可以用来作为预测大地震发生位置的一种手段.