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Center for Mathematical Science and Advanced Technology (MAT)

Seminar Schedule

[MAT Seminar]

Date:
2019/08/28 (Wednesday) 13:00 - 15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Ryoichiro Agata (FEAT)
Title:
Introduction of covariance components in slip inversion of land and seafloor geodetic data and application to slip deficit rate estimation in the Nankai Trough subduction zone
Abstract:
When spatial distribution of observation stations has bias in geodetic slip inversion, modeling errors in the inversion scheme may result in significant unnatural short-wave components in estimated slip distribution, which overfit to data. Combined use of both land and seafloor geodetic data in slip inversion often leads to such situations. To avoid overfitting, I proposed a method to incorporate proper covariance components in the data covariance matrix originated by modeling errors in the geodetic slip inversion. Because linearity of the inversion problem is retained, widely-known approaches to introduce prior constraints to the inversion, which assumes linear inversion, are easily applied to the proposed method. Synthetic tests showed that the introduction of covariance components allowed for the estimation of slip distributions closer to the true one, avoiding overfitting to geodetic data in biasedly distributed observation stations, compared to a conventional approach that does not introduce covariance components. I applied the proposed method to the estimation of the slip deficit rate in the Nankai Trough subduction zone, using geodetic data of displacement rates provided by land GNSS stations and seafloor GNSS-Acoustic stations. The proposed method estimated a reasonably smooth distribution of slip deficit rate compared to the conventional approach. Spatial distribution of residuals in the displacement rates suggested that the proposed method successfully avoids overfitting to the block motion at the south of the Median Tectonic Line, which the physical model I assumed here cannot describe.

Upcoming Seminar schedule
09/04 (Wed) 13:00- Yoji Kawamura (MAT)
09/11 (Wed) 13:00- Manabu Morishige(MAT)
09/18 (Wed) 13:00- Yasuko Yamagishi(MAT)
※title and abstract of each seminar will be informed when they are decided.

[MAT Seminar]

Date:
2019/08/21 (Wednesday) 13:00 - 15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Muneo Hori (MAT)
Title:
Most suitable coordinate system for numerical analysis of wave equation
Abstract:
Development of better numerical analysis of wave equation is a fundamental issue of continuum mechanics. Most of numerical analysis methods employ Cartesian coordinate system, mainly because physics laws from which the wave equation is derived do not depend on coordinate system and Cartesian coordinate system with vertical axis and horizontal axes is a natural choice. There is a possibility of the presence of the coordinate system that is most suitable for numerical analysis. Starting particle discretization scheme, I explain recent trail of using a particular coordinate system to solve wave equation. I make an excuse that using a particular coordinate system is practically useful in identifying errors in input parameters of a numerical analysis model.

[MAT Seminar]

Date:
2019/08/07 (Wednesday) 13:00 - 15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Dmitry Kolomenskiy (CEIST)
Title:
A wavelet-based data compression method for three-dimensional numerical simulations on regular grids
Abstract:
High-performance computing can produce large volumes of output data. Even though data reduction may be performed during the course of computation in order to store only the quantities of interest such as statistical moments, it is often necessary to store full three-dimensional fields for purposes including simulation restart, time-resolved visualization and exploratory analyses. In this talk, I will present a wavelet-based method for compression of numerical simulation data on regular structured grids. It is inspired by image compression, and it consists of discrete wavelet transform, quantization adapted for floating-point data, and entropy coding. I will discuss different aspects of these numerical techniques, present an open-source software WaveRange, and show example numerical tests, ranging from idealized configurations to realistic global weather simulation data.

[MAT Seminar]

Date:
2019/07/31 (Wednesday) 13:00 - 15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Gen Li (MAT)
Title:
New understanding on the optimising strategies in fish swimming from CFD and engineering perspectives
Abstract:
(Attention Please: this talk substantially overlaps my presentation in the JAMSTEC international day 2019/07/16 )
In this talk, I would like to introduce two recent CFD-robotic hybrid projects: stingray-inspired round-plate robot and magnetic field driven flexible swimmer, as well as an development of fluid-structure-interaction (FSI) solver assorted with the magnetic swimmer.
Also, I would like to introduce my discovery of the optimising strategies in fish swimming. Drag is a key factor to explain behaviours and predict strategies in fish, but extremely difficult to be measured in experiment. By utilizing computational approach, interesting characteristics of drag in a swimming fish were found: the (scaling) trend of drag follows that of a three-dimensional object, while the magnitude of drag depends on the kinematics of the undulatory wave, much exceeding that of a rigid fish gliding at the same speed. The duality in swimming fish determines its swimming strategy. In continuous swimming at a specific speed, fish need to wisely choose its frequency and amplitude to prevent excessive energy consumption due to drag. Meanwhile, since gliding drag is much less than undulatory drag at the same speed, properly switching between undulatory propulsion and gliding (burst-and-coast) may become a feasible strategy for fish to save energy.

[MAT Seminar]

Date:
2019/07/24(Wednesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Ettore Barbieri (MAT)
Title:
A Numerical Method for the Mechanics of Folds in Plates
Abstract:
Folds occur in a variety of physical phenomena and engineering applications: from chevron folds and mountain ridges in geology to the ribbed shells of marine crustaceans to deployable antennas for satellites. Even in the arts, famous examples of folding are origami sculptures. Interestingly, the folding problem shares a mathematical trait with the fracture problem. Mathematically speaking, fractures are discontinuities on the displacements. Folds, instead, are discontinuities in the displacements gradients. This presentation will show how to treat such discontinuities in numerical simulations of folds for thick and thin plates, drawing from the author's experience in computational fracture mechanics. Furthermore, this talk will present newly obtained analytical solutions for the nonlinear folding problem in 1D, necessary for the verification of the method.

REFERENCES:
Curvature tuning in folded strips through hyperstatic applied rotations, E Barbieri (2019), Frontiers in Materials 6, 41

A meshless method for the nonlinear von Kármán plate with multiple folds of complex shape, E Barbieri, L Ventura, D Grignoli, E Bilotti, (2019) Computational Mechanics, 1-19

[MAT Seminar]

Date:
2019/07/17(Wednesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Richard Spitz (University of Lausanne)
Title:
3D numerical modelling of nappe emplacement during orogenic wedge formation
Abstract:
Fold-and-thrust belts are common to orogens like the Canadian Rocky Mountains or the Swiss Alps. One prominent km-scale structural feature of such fold-and-thrust belts are so-called fold-and-thrust nappes. The emplacement of nappes is an ongoing research topic since the late 19th century in geology. Therefore there is a manifold of studies utilizing different analysis tools like geological cross-sections, geometrical, theoretical, analog, and numerical models. These models often rely on individual two-dimensional (2D) cross sections. Series of cross sections separated by 10-100 km often reveal significant structural variations. These variations likely indicate lateral changes in the geometry, material properties, and physical conditions before deformation (i.e., initial conditions). In this study, we investigate the control of initial lateral geometrical variation on the evolution of geological structures. We perform 2D and 3D numerical simulations of the emplacement of tectonic nappes. Results show that cross-sections from the 3D simulations differ significantly from 2D simulations with the same initial cross-sectional geometry. We systematically test different geometrical transition between two cross-sections, and we compare the results with natural examples from the Helvetic nappe system in the Western Swiss Alps.

[MAT Seminar]

Date:
2019/07/10(Wednesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Ryo Onishi (CEIST)
Title:
Super-Resolution Simulation for Real-Time Prediction of Urban Micrometeorology
Abstract:
We propose a super-resolution (SR) simulation system that consists of a physics-based meteorological simulation and an SR method based on a deep convolutional neural network (CNN). The CNN is trained using pairs of high-resolution (HR) and low-resolution (LR) images created from meteorological simulation results for different resolutions so that it can map LR simulation images to HR ones. The proposed SR simulation system, which performs LR simulations, can provide HR prediction results in much shorter operating cycles than those required for corresponding HR simulation prediction system. We apply the SR simulation system to urban micrometeorology, which is strongly affected by buildings and human activity. Urban micrometeorology simulations that need to resolve urban buildings are computationally costly and thus cannot be used for operational real-time predictions even when run on supercomputers. We performed HR micrometeorology simulations on a supercomputer to obtain datasets for training the CNN in the SR method. It is shown that the proposed SR method can be used with a spatial scaling factor of 4 and that it outperforms conventional interpolation methods by a large margin. It is also shown that the proposed SR simulation system has the potential to be used for operational urban micrometeorology predictions.

[MAT Seminar]

Date:
2019/07/03(Wednesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Ippei Oshima (MAT)
Title:
Mechanism of Liquid Sheet Atomization Induced by Air Flow
Abstract:
I have joined MAT (Center for Mathematical Science and Advanced Technology) in June, 2019. So, in this seminar, I’ll introduce my previous activities of research and development and future work in JAMSTEC.
My research interest includes fluid mechanics, multiphase flow and atomization. I have researched air-blasted atomization of a liquid sheet induced by air flow for gas turbine engines. To clarify the mechanism of the liquid sheet atomization by air flow and develop the atomization model based on the mechanism, I have performed visualization and optical experiments and numerical simulation and theoretical analysis. These experiences and techniques will be useful for JAMSTEC’s work.
In the research topic, I’ll introduce overview of atomization and the background of my research. I also explain how droplets are produced through the atomization process in gas turbines. Based on that, in this presentation, I’ll briefly explain following two small topics;
1) Is the liquid sheet deformation injected from the injector governed by pure Kelvin-Helmholtz instability?
2) What kind of correlation is proposed on the longitudinal wavelength of the liquid sheet deformation?
In the development topic, I’ll introduce one of missions from my current hobby work of developing a small satellite.
Finally, work plan in JAMSTEC which is concerned with geothermal power plant will be introduced.

[MAT Seminar]

Date:
2019/06/26(Wednesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Osamu Kuwano (MAT)
Title:
Evolution of size and shape distributions of comminuted particles under shear deformation
Abstract:
A faults zone contains fine rock powders called gouge that have been ground up by past fault motions.
Particle size distribution and particle shape of gouge particles affect the frictional properties of the fault and reflect the comminution process by the past fault motions. It is well known that particle size distribution of fault gouge show power-law distributions. Exponent of this power law is considered to reflect the style and degree of deformation. We did several shear experiments using a rotary shear apparatus with the shear displacement ranges from 10mm to 1m. In this seminar, we will discuss about the relationship between the particle size distribution and the degree of comminution of model particles by particle image analysis.

[MAT Seminar]

Date:
2019/06/19(Wednesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Natsuki Hosono (MAT)
Title:
SPH Simulations on AI-specific processors
Abstract:
The Smoothed Particle Hydrodynamics method is a widely accepted particle-based numerical simulation method.
Since it is a particle-based method, a straightforward implementation of SPH has O(N^2) computational cost.
Due to this expensive computational cost, in order to carry out large scale numerical simulations, we need to apply some technique to accelerate calculations.
One of the standard techniques is to employ many-core devices.

Recently, AI-specific processors, such as NVIDIA's V100 and Google's TPU, are a hot topic. They often are many-core devices with low precision arithmetics.
AI-specific processors can be attractive candidates to carry out SPH simulations.
However, to obtain high efficiency of AI-specific processors, we need to use low precision.
In other words, there is a tradeoff between the precision and calculation speed.

However, it is not well known that low precision can be used for SPH simulations.
Hence, as the first step to effectively use AI-specific processors on SPH, we aimed at investigating the dependence of simulation accuracy on the precision.
We chose the Couette flow as a benchmark test and quantified its simulation accuracy by L2-error.
In this seminar, we will report the on-going status of the above work.

[MAT Seminar]

Date:
2019/06/12(Wednesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Shun Nomura(MAT)
Title:
Sedimentation process of experimentally produced turbidity current
Abstract:
Turbidity current is a particle-laden current driven by density difference due to the presence of suspended sediment particles. Since it moves down along submarine channels with large amount of sediment for a long distance, it is one of the most important factors to understand the submarine systems. To clarify its sediment transport systems, the sedimentation process of an experimentally produced turbidity current in an inclined flume was investigated by the electrical resistance based depositmeter.
After introducing the measurement technique of the depositmeter, the experimental results performed in three different initial fluid densities are introduced. Then the semi empirical model for spatio-temporal sedimentation process is explained and its applicability is discussed.

[MAT Seminar]

Date:
2019/06/05(Wednesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Masaru Nakano (FEAT)
Title:
Discrimination of seismic signals from earthquakes and tectonic tremor by applying a convolutional neural network to running spectral images (SRSpec-CNN)
Abstract:
The recent development of dense high-sensitivity geophysical observation networks has led to many observations of seismic and geodetic signals known as the slow-slip earthquakes. Monitoring the slow deformation at plate boundaries as the Nankai trough would provide insights into deformation processes in the source regions of megathrust earthquakes. Tectonic tremor is one of signals radiated from such slow deformations and recorded by seismometers. Their signals occur dominantly in the 2–8 Hz frequency band and can last for tens of seconds to several minutes, in contrast to typical earthquakes that produce seismic signals at frequencies up to several tens of hertz and last less than a minute. Because tremor is caused by stochastic processes, the resultant waveforms are represented by a stochastic function and construction of deterministic measures to discriminate tremor signals from earthquakes is very difficult. In this study, we used a convolutional neural network (CNN) to discriminate the signals of tectonic tremor from those of local earthquakes in running spectral images of these signals. Ordinary methods of image recognitions are designed insensitive to the location where the objects appear in the target image. In the discriminations of earthquake and tremor signal, on the other hand, the absolute frequency of signal appearance is important which reflects the physical properties of the signal source. Therefore, we developed a method (SRSpec-CNN) that is sensitive to the absolute frequency of signal appearance, but is insensitive to the time of signal onset. Based on simultaneous analyses of the frequency contents and durations of the signals, we achieved 99.5% accuracy for our identifications of signals from tectonic tremor, local earthquakes, and noise. Because running spectra clearly differentiate the characteristic features of these signals, we were able to achieve this high accuracy by using a CNN of simple architecture.

[MAT Seminar]

Date:
2019/05/22(Wednesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Daisuke Nishiura (MAT)
Title:
SPH simulation of wave channel -Wave impact edition featuring Davide-
Abstract:
Dr. Davide Wüthrich has experimentally studied about tsunami-like wave hydrodynamics by using a wave channel at Laboratory of Hydraulic Constructions (LCH) of Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland. On the other hand, the MAT has developed the large scale SPH simulation for the tsunami study. However, we need to validate the accuracy and reproducibility of our SPH simulation to apply the numerical model for actual engineering problems. So, there are many good benefits for us to collaborate with each other. Previously, fundamental wave characteristics such as a wave height and velocity profiles without buildings were validated by comparing with many experimental data. This time, we investigated the wave impact forces acting on the buildings with and without openings. The reproducibility of our SPH simulation was validated in the case of buildings with different geometries (opening size and ratio) and then some issues for the computational accuracy of our SPH simulation were pointed out.

[MAT Seminar]

Date:
2019/05/15(Wednesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Takehiro Miyagoshi (MAT)
Title:
Experiments of horizontal convection
Abstract:
We have studied horizontal thermal convection by experiments with fluids. Horizontal convection is interesting in terms of dynamics in the interior of the planet. At the core-mantle boundary, stagnant slabs produce temperature difference in horizontal direction. We want to know whether those temperature differences can drive vigor core convection and dynamo. In massive super-Earths, it is known that the tidal lock often occurs. Then, the temperature difference at the surface between the noon side and night side becomes very large. The temperature difference may drive the horizontal convection in the mantle of super-Earths. The experimental vessel is rectangular box with the aspect ratio of 4:1:1. We use water with glycerin or pure water for working fluid. We have started this experimental study from December in the last year, and we would like to introduce our initial results in the MAT seminar.

[MAT Seminar]

Date:
2019/05/08(Wednesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Mikito Furuichi (MAT)
Title:
Progress of numerical modeling of the riser pipe flow for slurry transportation
Abstract:
For the efficient design of the mining system for deep-sea resources, prediction of the performance of pipe flow is important. Then, we develop a numerical model for evaluating the relationship between the flux by the slurry-pumps, property of mud (size, fraction and non-Newtonian), injection of mud at the bottom and so on. By using the model, we discuss the risk of stuck the pipe and efficiency of the energy cost of mining system. The obtained data should be also useful for the optimization of the operation.

[MAT Seminar]

Date:
2019/04/24(Wednesday) 10:00-11:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Takatoshi Yanagisawa (VERC)
Title:
Structural change from roll to large-scale cell in turbulent thermal convection of a liquid metal layer
Abstract:
To understand the expected nature of turbulent flow in liquid outer core of the Earth, we performed laboratory experiments on Rayleigh-Benard convection with a liquid metal in a square box geometry having a moderate aspect ratio. Horizontal velocity profiles of flow were measured at several lines by using ultrasonic velocity profiling. By combining the information from profiles, we can reconstruct organized flow structures.
Systematic variation of the structure was detected with increasing the Rayleigh number; a quasi-two-dimensional roll changes to a cell having a relatively larger horizontal scale. We found that the flow structure, whether it is roll or cell, show quasi-periodic oscillation whose representative period is approximately same as the circulation time of the flow.
We also performed numerical simulations of convection with the same geometry as the experiments by setting a small Prandtl number like a liquid metal. Quantitative comparison on the velocity profiles between experiments and simulations provided quite satisfactory agreement. By integrating these results, we propose a scaling low on the Rayleigh number dependence of horizontal size of flow structure.

[MAT Seminar]

Date:
2019/04/17(Wednesday) 13:00-14:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Yuzuru Yamamoto (MAT)

[MAT Seminar]

Date:
2019/04/01(Monday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Stephan Rudykh (University of Wisconsin Madison)
Title:
Instability-induced Microstructure Transformations for Switchable Soft Metamaterials
Abstract:
Nature actively uses sophisticated designs of microstructures to achieve astonishing material properties and functionalities. Thus, microstructures give rise to the incredible toughness of mother-of-pearl. Another example is an octopus, an amazingly effective soft machine created by the nature. The creature can squeeze its whole body through an extremely narrow space while preserving a large variety of functionalities. The nature created soft machine comprises highly deformable composites that are characterized by different dynamically tunable microstructures and phase properties, depending on the required functionalities. Indeed, such materials are highly desirable for human-interactive soft robotics, and novel actuators and sensors, and biomedical applications.

In this presentation, I will specifically focus on the role of microstructures in the performance of deformable multifunctional composites. We will consider how large deformations and elastic instabilities can be used to trigger dramatic pattern transformations, and to control a large variety of functionalities; in particular, the design of switchable acoustic metamaterials will be discussed. Recently discovered new type of instability-induced domain formations in soft composites will be presented.

Analytical and numerical findings, as well as experimental results of 3D-printed soft composites will illustrate the ideas.

Upcoming Seminar schedule
※title and abstract of each seminar will be informed when they are decided.

[MAT Seminar]

Date:
2019/03/19(Tuesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Jian Chen
Title:
Mechanics of soils - discrete and continuum approaches and a trial to bridge the two approaches
Abstract:
From a “microscopic” viewpoint, soils consist of granular particles of varying sizes and shapes. Micro-mechanics-based numerical methods, especially discrete element method (DEM), are popular for studying the mechanics/dynamics of such granular assemblies. A simple home-brew experiment would show that a heap can be made on a smooth mirror by pouring a pack of non-spherical particles, while it is extremely difficult using spherical particles. The geometry of individual particle plays an important role on the properties of granular assemblies, at least at the scales of sand heaps or laboratory experiments on soil samples (based on which constitutive laws derived). In this talk, we will introduce briefly a DEM approach which applies non-spherical particles for studying mechanics of soils.

Though simulations based on micro-mechanics are useful tools to investigate mechanics of soil samples, it is usually not applicable for engineering problems of much larger scales, e.g., assessing the occurrence of soil liquefaction caused by earthquakes. For engineering problems, soils are generally treated (homogenized) as continuum media and the corresponding partial differential equations (PDEs) are solved for proper initial and boundary conditions. Physically, soil liquefaction is an unstable phenomenon caused by an excessive increase of pore water pressure. Mathematically, it should correspond to some unstable solutions of the (linearized) partial differential equations for soil dynamics, giving that the constitutive law and governing equation are adequate for describing the behaviors of soils of interest. In this talk, we will present briefly a simple stability analysis for the solutions of the PDES derived from a continuum approach for soil mechanics.

In discrete approaches, the main material parameters are essentially the spring constants and the basic math tool is ordinary differential equation (ODE). In continuum approaches, the material parameters are the constitutive tensors and the basic math tool is PDE. In this talk, we will also discuss briefly how to drive a continuumnized governing equations (and the corresponding material tensors), which is a set of PDEs, from a system of ODEs for individual particles, considering the translations and rotations.

[MAT Seminar]

Date:
2019/03/12(Tuesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Takane Hori (CEAT)
Title:
Trial to revise governing equations of earthquake generation cycle simulations
Abstract:
We have simulated earthquake generation cycles for decades basically based on the governing equations proposed by Rice (1993) (e.g., Hori et al., 2004). We have modified the algorithms to solve the equations, for example, to demonstrate much larger scale simulations introducing matrix approximation as Hierarchical matrix (Ohtani et al, 2011), to apply curved fault plane introducing triangular sub-faults (Hyodo et al, 2013), to include the effects of heterogeneous medium property combining with FEM (Agata et al., in press), and so on. However, we have never changed the basic concept and equations. Recently, we have realized that the governing equations, especially the fault constitutive law, have too non-linear and unstable characteristics for numerical calculations and too much parameters to be defined although we do not have enough data to constrain them. These become intrinsic problems when we use the equations to monitor and forecast earthquake generation cycle processes. In the seminar, I will explain above and introduce a concept how to revise the governing equations to overcome these difficulties.

[MAT Seminar]

Date:
2019/03/05(Tuesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Arthur Bauville (MAT)
Title:
Control of fault weakening on the structural styles of underthrusting-dominated non-cohesive tectonic wedges

Arthur Bauville, Mikito Furuichi, and Muriel Gerbault

Abstract:
Underthrusting is a typical process at compressive margins responsible for nappe stacking and sediment subduction. Despite being essential for orogenesis, and the subduction cycle, the mechanics of underthrusting is still incompletely understood. The critical taper theory predicts that underthrusting occurs only in the limit where μ ′ = μb′ , with μ ′ and μb′ the inner and basal effective coefficient of friction, respectively. Paradoxically, in nature, underthrusting is associated with weak faults. For instance, at erosive subduction margins, sediments are dragged in subduction, while normal faulting in the middle prism suggests the plate boundary is weak. Wang et al., (2010) have proposed that dynamic fault strength variations during the earthquake cycle can reconcile these observations. In their model, underthrusting occurs during earthquakes when the fault is strong and normal faulting occurs in the prism during the interseismic period when the fault is weak. However, recent evidence (Gao and Wang, 2016) suggests that plate boundaries are relatively weak also during earthquakes.
In this contribution, we investigate the control of permanent fault weakening on the dynamics of a strong-based (μ′ = μb′ ) non-cohesive tectonic wedge. We control the wedge material strength by a spatially constant fluid overpressure factor (λov), and fault strength by a strain weakening factor (χ). We investigated the problem in two steps. First, we used the critical taper theory to determine a mechanical mode diagram that predicts structural styles. This diagram is deduced from the function ∆α(β,χ,λov) which is the difference in taper angle at a given β, between the compressively critical taper of a strong-based wedge and the extensionally critical taper of a weak-based wedge (μb′ = (1 − χ)μ′). Then, we performed numerical simulations of accretionary wedge formation to establish the characteristics of these structural styles regarding wedge and fault geometry. We determined there is a continuum of structural styles with three end-members occurring at the theoretically determined mechanical mode transitions.
Style 1 is characterized by thin tectonic slices and little to no underthrusting. Surface angles are bimodal with a peak corresponding to the strong-based wedge stability, and another corresponding to the compressively critical taper of a weak-based wedge. Style 2 is characterized by thick slices and nappe stacking. Surface angles are multimodal and vary cyclically within the stability of the weak-based wedge; with part of the wedge being asymptotic to the strong-based stability line. Style 3 is characterized by the complete underthrusting of the incoming sediments. Sediments are exhumed when they reach the backstop to form a weak upper wedge whose base is the paleosurface of the incoming sediments. Surface angles are asymptotic to the stability line of a fully weakened wedge (μ′ =μb′ =(1−χ)μ0′, with μ0′ a reference friction coefficient).
The model gives a mechanical explanation that reconciles underthrusting with weak faults. In agreement with observations, the model predicts the occurrence of style 2 in strong-based sandbox experiment (μ = 0.6, λov = 0, χ = 0.1 − 0.2). Finally, we argue that style 3 is triggered by any mechanism, such as material weakening, erosion or sedimentation, that limits the surface angle such that the incoming material is always significantly under-critical.

[MAT Seminar]

Date:
2019/02/26(Tuesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Takashi Minoshima (MAT)
Title:
A divergence-free shock-capturing scheme for magnetohydrodynamics
Abstract:
The magnetohydrodynamic (MHD) modeling has been extensively applied to various macroscopic dynamics of magnetized gas and fluid, for example, the planetary magnetosphere, the Sun, an accretion disk around a protostar, and so on. The MHD simulation is an indispensable tool to study these nonlinear phenomena. Since a supersonic flow is frequently observed in these circumstances, the numerical simulation needs to solve fully compressible MHD equations.
The so-called upwind-type shock-capturing method developed for compressible hydrodynamic simulations has been extended to compressible MHD simulations. However, the multidimensional MHD simulation based on the shock-capturing method should take special care of the divergence-free condition for the magnetic field in order to avoid an unphysical solution. The one-dimensional shock-capturing method does not necessarily satisfy the divergence-free condition, and the simulation without any special divergence control could be easily crashed. Therefore, numerous studies have been devoted to develop novel methods so as to satisfy the shock-capturing property and the divergence-free condition. Here, we propose a new numerical scheme for MHD that satisfies the following criteria: (1) the scheme is based on a one-dimensional shock-capturing method, (2) the multidimensional scheme is divergence-free within a machine precision and reduces to the base one-dimensional scheme for one-dimensional problems, (3) the order of accuracy of the scheme can be improved to an arbitrary level, and (4) the scheme is easy to implement to existing codes. We present details of the proposed scheme and its application to ideal MHD simulations, and then discuss its extension to Hall-MHD simulations that is a minimal model to take into account the kinetic effect.

[MAT Seminar]

Date:
2019/02/05(Tuesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Shigenobu Hirose (MAT)
Title:
Code Development for Calculating Chemical Equilibrium of Ideal Gas Mixture and its Opacity
Abstract:
Hydrodynamics of multi-species gas mixture with radiative heat transport is relevant in many research areas including atmospheric science, engineering and astrophysics. To perform simulations of such hydrodynamics accurately, reliable databases of chemical equilibrium and opacity are required. However, public databases are available only in limited cases and researchers usually have to make databases by themselves for their own purposes. To reduce such tasks, I'm going to develop, as a project in the next mid-term, a public code that calculates chemical equilibrium and opacity for wide ranges of pressure and temperature (T = 10 ~ 10^6 K) as well as for many kinds of chemical species. In this talk, I'll briefly explain the basic concept of developing such numerical codes.

[MAT Seminar]

Date:
2019/01/29(Tuesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Yasuko Yamagishi (MAT)
Title:
Improvement of database system for cruise information of research vessels
Abstract:
We have developed a database system for cruise information of research vessels belonging to JAMSTEC to provide useful information for planning future research cruises. The database is available now on LAN of JAMSTEC. We are also developing an application to predict the downtime of future research cruise using the data provided by the database. At present, we are improving the database system because on developing the application it is revealed that the current database system cannot provide the information necessary for data analysis. In this seminar we will report the current status of the database improvement and the application development. The data stored into the database system is based on the “vessel operation report” which is E-mail sent from the vessel to the ground management department for research curises. At present, we are making the data manually from the E-mail. We are also developing an application for automatic creation of vessel operation reports to produce the data to be stored into the database system. In addition, to know the situation of the vessel operation report creation, I boarded on the research vessel. We will talk about the developing the application and the first boarding on the research vessel.

[MAT Seminar]

Date:
2019/01/22(Tuesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Yoji Kawamura (MAT)
Title:
Phase reduction of oscillatory thermal convection with a spatially translational mode
Abstract:
We formulate a theory for the phase reduction of oscillatory thermal convection in a two-dimensional incompressible Navier-Stokes system with laterally periodic boundary conditions. This system possesses spatial translational symmetry in the lateral direction as well as temporal translational symmetry. Oscillatory thermal convection in this system is described by a limit-torus solution that possesses two phase modes; one is a spatial phase and the other is a temporal phase. The spatial and temporal phases indicate the position and oscillation of the thermal convection, respectively. The theory formulated in this study can be considered as a phase reduction method for limit-torus solutions to partial differential algebraic equations representing oscillatory thermal convection with a spatially translational mode. The theory can also be considered as a generalization of our earlier work [Kawamura and Nakao, Physica D 295-296, 11-29 (2015)], in which we studied oscillatory cylindrical-Hele-Shaw convection. In this talk, we derive the phase sensitivity functions for spatial and temporal phases; these functions quantify the spatiotemporal phase responses of oscillatory thermal convection to weak perturbations applied at each point and at each time. Using the phase sensitivity functions, we analyze the spatiotemporal phase synchronization between weakly coupled systems of oscillatory thermal convection.

[MAT Seminar]

Date:
2019/01/15(Tuesday) 13:00-15:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Ryoichiro Agata (CEAT)
Title:
Segmentation of earthquake rupture caused by three-dimensional material heterogeneity
Abstract:
Subduction zone earthquakes are known to occur repeatedly in an identical portion of the entire focal region. This phenomenon is referred to as segmentation of earthquake rupture zone. The Nankai region in the southwest Japan hosts typical earthquake segmentations characterized by well-documented earthquakes in the past several hundred years. Estimated elastic properties of rocks around the segmentation boundary of this region implies that local heterogeneity of elastic structure in the vicinity of the earthquake fault plane contributes to the earthquake segmentation (Kodaira et al. 2006). In this study, we examine how such local heterogeneous elastic structures affects earthquake generation patterns using an idealized subduction zone model by directly considering such heterogeneity in large-scale numerical simulations of earthquake generation, which has not been done before due in part to the associated computation cost. We show that under certain conditions, earthquake segmentation is seen in the simulation results when a local stiff dome-like structure, which resembles one found by Kodaira et al (2006) off Kii-peninsula, is located in the overriding plate on the fault plane.

[MAT Seminar]

Date:
2019/01/08(Tuesday) 11:00-12:00
Place:
MAT theater on the 5th floor, IT Building, Yokohama Institute
Language:
English
Speaker:
Muneo Hori (MAT)
Title:
REBEL WITH CAUSE - CORRECTION OF ELASTO-PLASTICITY THEORY