Visualization System for Subsurface Structures
The Visualization System for Subsurface Structures helps to "view" subsurface structures beneath active faults in terms of three-dimensional distributions of physical properties (at the moment, elastic wave velocities only). It deals with large-scale structures that encompass both the crust and the upper mantle. You cannot dig holes to literally view the inside on such scales, so the term "viewing" (= visualization) here refers to the act of plotting subsurface structure images estimated by seismic tomography using records of natural earthquakes.
Seismic Tomography?CT Scanning of the Earth
The word "tomography" may sound unfamiliar to non-experts, but it would not be too far off the mark to think of CT scanning used in hospitals (in fact, CT is short for computerized tomography). In the CT scanning in medical practice, X-rays are irradiated from multiple directions around your body, and receiving them at a multitude of sensors allows you to learn about your bodily structures. Seismic waves are used in place of X-rays in the tomography of the Earth. The X-ray sources correspond to seismic sources, and the X-ray receivers correspond to seismic stations.
Dense networks of seismic stations got to be installed all over Japan after the mid-1990s. As data from those observation networks accumulated, scientists began to use seismic tomography to obtain detailed crustal structures and synoptic images of the subsurface that encompass the whole Japanese Islands.
Unavoidable Problems
In the CT scanning in medical practice, X-ray sources and sensors surround the affected part of the body on all sides, but in seismic tomography, seismic stations are located on only one side of the "affected part," namely on the ground surface. As a result, you can only get blurred images of subsurface structures, as if you wore thick eyeglasses. The resolution obtained in reality can be much lower than the resolution anticipated by the analyzer (on the order of the grid size used in the tomography analysis). In such situations, the images obtained differ slightly from one analyzer to another, depending on differences in the subsurface structures assumed prior to the tomography analysis (called the initial models) and in the analysis methods.
Purpose of This System
This system has been created to facilitate the visualization and browsing of a large number of seismic tomography analysis results that serve as basic information for the understanding of large-scale subsurface structures associated with the activities of active faults. For this objective, we have designed it so that it plots not only the analysis results from tomography but also seismic source data, geological information including topography, geology and gravity anomaly, and the distribution of active faults and volcanoes. We wish to facilitate research communication and development by having it used by a broad spectrum of people, including experts in Earth sciences and engineering, science teachers, students and other amateur scientists.