Yasuko Yamagishi

Cell pattern of thermal convection induced by internal heating

A numerical simulation of thermal convection induced by internal heating in a three-dimensional thin flat box is performed. The upper boundary is held at a constant temperature and the lower boundary is thermally insulated with a uniform internal heating in a fluid layer. We observe hexagonal cells with downwelling at their center in moderate Rayleigh number (RaH≲30RaHc). As the Rayleigh number increases, the cell size increases. We also observe a pattern transition: sheet-like downwellings gradually develop around the columnar downwelling with increasing Rayleigh number. At ∼30RaHc, hexagonal cells with sheet downwelling at their center (spoke pattern) is observed. These sheets can be seen only near the upper boundary. At 30RaHc∼70RaHc, the cells experience successive transitions to the pattern which are dominated by sinking sheet.

Visualization of geochemical data for rocks and sediments in Google Earth: Development of a data converter application for geochemical and isotopic data sets in database systems

We developed a Keyhole Markup Language (KML) generator for converting geochemical and isotopic data sets for rocks and sediments stored in database systems into KML. Our program allows users to visualize geochemical or isotopic data easily in Google Earth. The generator accepts data files produced by the database systems PetDB, SedDB, GEOROC, and GANSEKI. The data are plotted three-dimensionally as a bar graph on the surface of the virtual Earth at the sampling site. This type of visual presentation, including information on sample localities, directly shows the distribution of isotopic or compositional anomalies of specific samples on the Earths surface. We provide a Web application for the generator, so anyone can set the parameters for visualization over the Internet. With other KML generators we developed earlier, geochemical data can be overlain on a seismic tomographic model. This overlay image can provide information on the origin of samples in the tomographic model.

Tectonic history of Europa : Coupling between internal evolution and surface stresses

A stress history in the ice shell of Europa is presented. Europa’s surface is ubiquitous in extensional tectonic features such as banded terrains. These surface features suggest that the surface may have been fractured and extended due to tensional stress, and various origins for such stresses have been proposed. We have focused on the solidification of the liquid water layer and the accompanying volume change as one of the dominant sources for such stresses. To estimate the stress state in the ice shell, we first performed numerical simulations of the thermal history. Based on the resulting structural evolution, we calculated stresses in the viscoelastic ice shell due to the solidification of the liquid layer. Europa’s liquid layer solidifies slowly and may partially survive at present, and its solidification induces sufficient tensional stress to drive extensional tectonic activity. Consequently, we propose the tectonic scenario that the volume change due to phase change develops the basic amplitude of the stress, while tidal forces work as a trigger to fracture the surface.

Development of a database and visualization system integrating various models of seismic velocity structure and subducting plate geometry around Japan

AbstractTo estimate strong ground motions caused by future earthquakes in Japan and to more accurately predict seismic hazards and tsunamis, it is necessary to accurately model the geometry of the subducting plate and the seismic velocity structure around Japan, particularly in offshore areas. Although various seismic velocity structure and plate boundary models have been proposed around Japan, they are all managed individually and differ in extent, data type, and format. Ensuring consistency among those models requires knowledge of their spatial distribution around the subduction zones of Japan. Here, we describe a database system to store and serve various velocity structure and plate geometry datasets from around Japan. Seismic structure models in this database include 3D seismic velocity models obtained by seismic tomography, 3D plate geometry models, 2D seismic velocity structure models, 2D plate geometry models obtained by offshore seismic surveys, and hypocenter distributions determined by offshore observations and the Japan Meteorological Agency. Using this database (currently available only in Japanese), users can obtain data from several structural models at once in the form of the original model data, equal-interval gridded data in a text file, and Keyhole Markup Language (KML) data. Users can grasp the distributions of all available seismic models and hypocenters using a web-based interface, simultaneously view various models and hypocenters as KML output files in Google Earth, and easily and freely handle the structural models in a selected area of interest using the gridded text-file output data. This system will be useful in creating more accurate models of the geometries of the subducting plate and the seismic velocity structure around Japan.