Seismic activity associated with the subducting motion of the Philippine Sea plate beneath the Kanto district, Japan

The Pacific plate and the Philippine Sea plate overlap and subduct underneath the Kanto region, central Japan, causing complex seismic activities in the upper mantle. In this research, we used a map selection tool with a graphic display to create a data set for earthquakes caused by the subducting motion of the Philippine Sea plate that are easily determined. As a result, we determined that there are at least four earthquake groups present in the upper mantle above the Pacific plate. Major seismic activity (Group 1) has been observed throughout the Kanto region and is considered to originate in the uppermost part of mantle in the subducted Philippine Sea plate, judging from the formation of the focal region and comparison with the 3D structure of seismic velocity. The focal mechanism of these earthquakes is characterized by the down-dip compression. A second earthquake layer characterized by down-dip extension (Group 2), below the earthquakes in this group, is also noted. The focal region for those earthquakes is considered to be located at the lower part of the slab mantle, and the Pacific plate located directly below is considered to influence the activity. Earthquakes located at the shallowest part (Group 3) form a few clusters distributed directly above the Group 1 focal region. Judging from the characteristics of later phases in these earthquakes and comparing against the 3D structure of seismic velocity, the focal regions for the earthquakes are considered to be located near the upper surface of the slab. Another earthquake group (Group 4) originates further below Group 2; it is difficult to consider these earthquakes within a single slab. The seismic activities representing the upper area of the Philippine Sea plate are Group 3. This paper proposes a slab geometry model that is substantially different from conventional models by strictly differentiating the groups.     

Geochemical and hydrological controls on biannual lamination of tufa deposits

Fluvial tufa deposits in southwest Japan commonly develop biannual lamination consisting of dense summer layers and porous winter layers, and the clearness of the laminae varies among the sites. The laminae have been largely attributed to a seasonally variable inorganic precipitation rate of calcite. This rate-controlled hypothesis was examined by using quantitative data for calcite packing-density (CPD) and the precipitation rate of calcite (PWP rate) calculated from water chemistry. The results for four tufa-depositing sites in SW Japan show that a positive correlation between CPD and PWP rate becomes less certain with increasing PWP rate. In the temperature realm of SW Japan, tufas develop regular distinct seasonal change in CPD when deposited in water containing Ca values less than 65 mg/l, which results in a relatively low precipitation rate. The CPD of tufa deposits rarely exceeds 65%, owing to pore space between fine-grained calcite crystals and to porosity derived from decomposed cyanobacteria and other microorganisms. By increasing the Ca content to more than 65 mg/l, the CPD often attains an upper limit and becomes insensitive to seasonal changes in the PWP rate. Therefore, seasonal variations in CPD at sites with a higher Ca content are unclear, as seen in two examples from tropical islands in southern Japan and in one locality in a temperate climate. The flow rate and microbial density on the tufa surface are subordinate factors with respect to the CPD. Seasonal changes in these two factors often enhance the porous/dense contrast of biannual lamination in SW Japan.     

Experimental and analytical studies on earthquake resisting behaviour of confined concrete block masonry structures

To improve the seismic performance of masonry structures, confined masonry that improves the seismic resistance of masonry structures by the confining effect of surrounding bond beams and tie columns is constructed. This study investigated the earthquake resisting behaviour of confined masonry structures that are being studied and constructed in China. The structural system consists of unreinforced block masonry walls with surrounding reinforced concrete bond beams and tie columns. The characteristics of the structure include: (1) damage to blocks is reduced and brittle failure is avoided by the comparatively lower strength of the joint mortar than that of the blocks, (2) the masonry walls and surrounding reinforced concrete bond beams and tie columns are securely jointed by the shear keys of the tie columns. In this study, wall specimens made of concrete blocks were tested under a cyclic lateral load and simulated by a rigid body spring model that models non‐linear behaviour by rigid bodies and boundary springs. The results of studies outline the resisting mechanism, indicating that a rigid body spring model is considered appropriate for analysing this type of structure. Copyright © 2006 John Wiley & Sons, Ltd.     

Strong ground motion prediction using macro–micro analysis method

The authors propose a new analysis method, called the macro–micro analysis method (MMAM) in a companion paper. (Earthquake Engng. Struct. Dyn., this issue) for strong motion prediction with higher resolution and accuracy. The MMAM takes advantage of the bounding medium theory to obtain optimistic and pessimistic estimates of the expected strong motion and the singular perturbation expansion that leads to an efficient multi‐scale analysis. The results of the numerical simulation with the MMAM are given as the sum of waves of low resolution covering the whole city and waves of high resolution for each part of the city. While the huge computation amount is reduced by the MMAM, the computation amount is huge still. For resolving this problem, this paper applies the finite element method with voxel element to numerical simulation tools after some numerical verification. To reproduce complicated material properties of surface soft deposits, fundamental hysteresis attenuation is implemented in the three‐dimensional simulation code. The proposed method is verified by carrying out the strong motion prediction with MMAM and comparing with measured data. In addition, the effect of three‐dimensional soil–structure and frequency component on the maximum velocity distribution, which is simulated by proposed method with high spatial resolution, is discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

On the divergence of the ionospheric electric field around the westward travelling surge

The pattern of the ionospheric electric field around the westward travelling surge (WTS) is theoretically studied. This is obtained by solving the current continuity equation at the ionospheric altitude for temporal and spatial development of the field-aligned current density modelled as the WTS phenomenon. The results show that the divergence of the ionospheric electric field is significantly changed depending on the dawn-to-dusk convection electric field E₀ because of non-uniformity in the ionospheric conductivity: the ionospheric electric field diverges in the upward current region (around the head of the WTS) when a westward electric field E₀ of 10 mV m⁻¹ is uniformly applied. On the other hand, the ionospheric electric field converges without E₀. From the observational inference that the ionospheric electric field converges around the head of the WTS, it is suggested that the WTS phenomenon may not be accounted for by the discharging process in the presence of the enhanced dawn-to-dusk convection electric field and non-uniform conductivity as was studied by previous authors.     

Trends in the seasonal length and opening dates of a winter road in the western James Bay region,Ontario, Canada

Theoretical and Applied Climatology - In northern Canada, winter roads are essential for communities. The duration of the winter road season depends on particular meteorological conditions. In this...     

Non-local memory effects of the electromotive force by fluid motion with helicity and two-dimensional periodicity

In mean-field dynamo theory, the electromotive force term 〈u′ × B′〉 due to small-scale fields connects the small-scale magnetic field with the large-scale field. This term is usually approximated as the α-effect, assumed to be instantaneous in time and local in space. However, the approximation is valid only when the magnetic Reynolds number Rm is much less than unity, and is inappropriate when Rm ? 1, which is the condition satisfied in the Earth's core or solar convection zone. We introduce a function φ _qr as a non-local and non-instantaneous generalization of the usual α-effect and examine its behaviour as a function of Rm in the range 1/64 ≤ Rm ≤ 10 for a kinematic dynamo model. We use the flow of G.O. Roberts 1972 Roberts, GO. 1972. Dynamo action of fluid motions with two-dimensional periodicity. Phil. Trans. Roy. Soc. London Ser. A, 271: 411–454. [Crossref], [Web of Science ®] , [Google Scholar] (Phil, Trans. Roy. Soc. London Ser. A, 1972, 271, 411–454), which is steady and has non-zero helicities and two-dimensional periodicity. As a result, we identify three regions in Rm space according to the behaviour of the function φ _qr : (i) Rm ? 1/4, where the function φ _qr is local and instantaneous and can be approximated by the traditional α and β effects, (ii) 1/4 ? Rm ? 4, where the deviation from the traditional α and β effects increases and non-localness and non-instantaneousness increase, and (iii) Rm ? 4, where boundary layers develop fully and non-localness and non-instantaneousness are prominent. We show that the non-local memory effect for Rm ? 4 strongly affects the dynamo action and explains an observed augmentation of the growth rate in the dispersion relation. The results imply that the non-local memory effect of the electromotive force should be important in the geodynamo or the solar dynamo.     

Macro–micro analysis method for wave propagation in stochastic media

This paper presents a new analysis method, called macro–micro analysis method (MMAM) for numerical simulation of wave propagation in stochastic media, which could be used to predict distribution of earthquake strong motion with high accuracy and spatial resolution. This MMAM takes advantage of the bounding medium theory (BMT) and the singular perturbation expansion (SPE). BMT can resolve uncertainty of soil and crust structures by obtaining optimistic and pessimistic estimates of expected strong motion distribution. SPE leads to efficient multi‐scale analysis for reducing a huge amount of computation. The MMAM solution is given as the sum of waves of low resolution covering a whole city and waves of high resolution for each city portion. This paper presents BMT and SPE along with the formulation of MMAM for wave propagation in three‐dimensional elastic media. Application examples are presented to verify the validity of the MMAM and demonstrate potential usefulness of this approach. In a companion paper (Earthquake Engng. Struct. Dyn., this issue) application examples of earthquake strong motion prediction are also presented. Copyright © 2005 John Wiley & Sons, Ltd.