In this study, the distribution and characteristics of the tsunami deposits by the 2011 off the Pacific coast of Tohoku and A.D. 869 Jogan earthquakes were identified in Hirota Bay, Sanriku Rias through high-resolution seismic profiles and core samples. The seabed sediments of Hirota Bay are composed of sand layer and silt layer, and separated by a well-defined boundary. The tsunami deposit by the 2011 Tohoku-oki earthquake on the Hirota Bay seafloor exhibit the characteristics of: (1) the bed separated from lower layer by a clear distinct boundary, (2) coarser grain size than ordinary bay-floor sediment, (3) composed by more than two sand layers, (4) parallel lamination, and (5) broad distribution within Hirota Bay. The volume of the 2011 tsunami deposit is 2.6×106 m3, about four times larger than the coeval onshore tsunami deposit in the Rikuzentakata Plain. The total volume of these onshore and offshore tsunami deposits is greater than the total volume of coastal erosion ranging from the beach ridge to shallow-marine. Hence, the sediment sources of the 2011 Tohoku-oki tsunami deposit may also include the materials from the coasts (east and west sides of bay) and deeper seafloor of Hirota Bay, which are not included in the calculation for the volume of erosion. Another event layer occurs beneath the 2011 Tohoku-oki tsunami deposit in Hirota Bay. This layer is interpreted as A.D. 869 Jogan tsunami deposit based on their distribution, sediment facies and radiocarbon-ages.
This paper reviews the basic concept, workflow and methodological variation of geostatistical sediment body modeling. Geostatistics was originally developed in the mining industry during 1960’ and 1970’ for the purpose of mathematical estimation of high-quality lode distributions, and since then, has widely been used for multiple purposes, including petroleum exploration and production. Geostatistical sediment body modeling is a quantitative and stochastic modeling method of sediment body basically using geostatistics. The basic data for the geostatistical sediment body modeling include well petrophysics as hard data and 2D/3D seismic attribute data as soft data, and they are integrated using various geostatistical algorithms to conduct the geostatistical stochastic simulation. To obtain realistic modeling results, geological and sedimentological deterministic/qualitative model information should be provided as the constraints for the geostatistical stochastic simulation. Various geostatistical simulation methods have been developed in response to the demand for the geological/sedimentological information input, as the geological/sedimentological factors are crucial for simulating the precise facies and property distributions. In addition to the past standard method of two-point pixel-based geostatistical simulation, new methodology has been proposed after the mid 1990’, including object-based modeling, multiple-point pixel-based modeling, process-aided modeling, surface-based modeling and process model-based modeling. The object-based modeling is a stochastic simulation based on a sediment body object such as a lobe and meandering channel, which size, dimension and orientation trends are set using known information. The multiple-point pixel-based modeling is a stochastic simulation using a pixel-based training image of a sediment body, and utilizes data points as hard data. The process-aided modeling is a modeling using sedimentary process algorithm and experimental results. Surface-based modeling is a simulation based on a depositional surface, which dimension is determined on the basis of known information. The depositional process-based modeling is a simulation based on the forward process model using reasonable algorithm matching actual sedimentary processes.
In addition to the new methodology developments, selection of appropriate modeling workflow and methodology is crucial for successful modeling, in consideration of the purpose of the modeling, depositional system of the target, and data condition in density, quality and distribution.
I have developed a method for easily displaying reflection cross sections, geological cross sections, and columnar charts in three dimensions, and created a program to realize them. A three-dimensional display was realized by automatically converting location information into polygon data and pasting prepared images as a texture. The procedures to visualize in three-dimensional display are summarizing location information in text, preparing images, and executing the program. The data can be viewed on Google Earth, which is available free of charge. The program is published on the web free of charge (https://staff.aist.go.jp/tomoyuki-sato/), and there are few restrictions on viewing, so please use it to organize data, share with persons concerned and for outreach.