To understand areas of building damage such as destruction by fire and collapse, and areas of geotechnical disasters such as sand boils caused by liquefaction.

With satellites circling the globe, it is possible in many cases to obtain both images after a disaster occurs as well as pre-disaster images. Therefore, basically we consider it to be possible to extract changes (damage zones) on the earth's surface by using the contrasts between images before and after an earthquake. This is accomplished by calculating the difference in each pixel after making a highly accurate positional match of images taken at two different times (registration).

The TM sensor on-board the U.S. LANDSAT satellite observed the earthquake disaster area of the Great Hanshin-Awaji Earthquake Disaster, on January 24, 1995 and obtained an excellent image. An image taken on August 17, 1994 was used for data before the earthquake. When we calculated the finite difference in the brightness values before and after the earthquake (value after the earthquake - value before the earthquake), in areas where sand boils were seen to accompany liquefaction, the brightness value after the earthquake increased, while the brightness value fell in areas destroyed by fire. In addition, in areas where the soil under walls and roof tiles came to light with the collapse of old wooden houses, the brightness value increased in the near and middle infrared wavelength regions. Based on the above-mentioned tendency, we classified the damage areas using the maximum likelihood method.

When we classified the damage areas using the maximum likelihood method, we classified bare ground in mountainous areas as boiled sand by liquefaction. Although areas that in fact suffered only slight damage and no damage were frequently mistakenly classified as areas destroyed by fire or extensively damaged, in general the result was roughly correspond to the results of damage surveys.

The importance of damage information in the initial stage following a disaster occurrence was strongly felt after the Great Hanshin-Awaji Earthquake Disaster in 1995. Given the time required to assess the damage when an earthquake occurs directly beneath an urban area and results in widespread damage, for example, active efforts are now underway to develop and introduce a real-time earthquake disaster mitigation system that will quickly estimate damage during the initial stages, based upon seismic monitoring using a seismograph network and a database prepared using the Geographical Information System (GIS). One example of an early stage damage estimation system at the national level is the Disaster Information System (DIS) for earthquakes in the Cabinet Office (Formerly, National Land Agency). The Early Estimation System (EES) for earthquake damage is part of this system and covers all of Japan using a 1km grid. Prepared with ground and buildings data, this system utilizes information from measurement seismographs the Japan Meteorological Agency is developing throughout Japan and can automatically estimate and total the scale of earthquake damage approximately 30 minutes after a disaster occurs, based on empirical equations. The goal of this system is to support national emergency response activities. In the end, however, the damage result estimates of such a system are still only estimates, and discrepancies can be quite large.

Accordingly, a broad understanding of actual damage must be carried out at the earliest stage possible. Remote sensing may be said to be one powerful technology for this purpose. The fact is, however, that there are limits to the ability to obtain images at the necessary times and in the required areas from remote sensing using earth observation satellites, because the satellites' recurrent periods are fixed, which presents problems when trying to use such satellites to assess damage during the early stages of a disaster.

Optical sensors, moreover, cannot be used when the target region is covered by clouds. Because of the inverse relationship between the degree of resolution and the number of recurrent days of a satellite returns to the same orbit, the use of high-resolution satellites is particularly limited.

In situations where a broad-range of damage conditions must be understood, however, even if the necessary information in the initial stages is not in detail but is merely general information, it can serve a useful purpose for decision-making at the national level in particular. From this point of view, there will also be situations in which satellites having current capabilities of resolution will be adequate.

In any event, by using multiple satellites, including satellite equipped with SAR that have been launched or are in the planning stages for launch by various countries in recent years, the problems of recurrent period and resolution have been greatly reduced.

Matsuoka M. and Yamazaki F.: Extraction of the Area Damaged by the 1995 Hyogoken-nanbu Earthquake using Satellite Optical Images, Proceedings of the Third Symposium on the Mitigation of Urban Disasters caused by Near-Field Earthquake, pp. 431-434, 1998