Terrestrial phenomenon disasters/Earthquakes/Urban structure damage. Urban disasters/Urban structure damage.
Disaster name
The Great Hanshin-Awaji Earthquake Disaster (SAR Image)
Author of WEB conversion
Masashi Matsuoka

Case Study

No. 16

1. Analysis objective

Understand regions where buildings collapsed or land was deserted

2. Analysis procedure Analysis flow chart

When changes in the earth's surface result from a disaster such as an earthquake, the altered region can be sampled by comparing SAR images before and after the earthquake because the strength and phase of backscatter will change.

Before an earthquake, the backscatter strength of microwaves irradiated by satellite are larger because of multiple reflections between buildings and roads (cardinal effect). The components of microwaves irradiated from a satellite into a damaged area after an earthquake and returned to the satellite, however, will be smaller because of multidirectional scattering from collapsed buildings. In other words it is projected that the spatial distribution of backscatter characteristics will differ before and after an earthquake, and the change can be expressed by the difference or correlation of the backscatter strength.

Using data obtained by the ERS satellite on October 12, 1994 and May 23, 1995 as images before and after the earthquake, we calculated the difference in strength (after the earthquake - before the earthquake) and the correlation values. When we compared the results with the damage data, we could see that in the images of those regions that suffered extensive damage such as collapses, or in regions where the surface had changed following the earthquake, the strength had decreased compared to the strength before the earthquake. The correlation values were also seen to be low.

Based on this tendency, we sampled the damage regions through linear discriminate analysis, using the difference value and correlation value as the explanatory variables.

3. Analysis results

The results from sampling the damage regions using discriminate analysis based on the SAR images before and after the earthquake roughly corresponded to the regions in which damage was actually concentrated.

Despite the fact there were disparities on average in the difference and correlation by damaged region, these varied widely. It will be necessary to clarify the cause of these differences in the future study, and construct a technique for extracting earthquake damaged regions with a higher degree of precision.

4. Results from using the analysis results

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.

5. Sources

Aoki H., Matsuoka M. and Yamazaki F.: Characteristics of the Area Damaged by the Hyogoken-Nanbu Earthquake based on Satellite SAR Images, Proceedings of the Fourth Symposium on the Mitigation of Urban Disasters caused by Near-Field Earthquake, pp. 617-620, 1999 (in Japanses).