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Department of Mathematical science and Advanced Technology (MAT)

Seminar Schedule

[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.

Upcoming Seminar schedule
06/12 (Wed) 13:00- Shun Nomura
06/19 (Wed) 13:00- Natsuki Hosono
06/26 (Wed) 13:00- Osamu Kuwano
※title and abstract of each seminar will be informed when they are decided.

[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