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Earthquake loss estimation using high resolution satellite imagery

Damage states are generally not recognizable by remote sensing, only buildings completely destroyed being detectable. However, a distinction was made between two levels of destructions: extensive damage and complete damage (Fig. 3 a, b and c). The extensive damage is considered when a building or an entire zone is damaged, but the building(s) is not totally collapsed (partial failure of the structure). The complete damage corresponds to a totally collapsed building (total failure of the structure).

FIGURE 3. a) A collapsed building  classified completely damaged;  b) An example of complete damage  (in red) vs. extensive damage (in yellow);  c) An entire zone considered with severe  damage.

Loss Estimation
A GIS was used in order to display the spatial distribution of damages (Fig. 4). An important concentration of destructions can be observed in the north side of the city, where the high vulnerability of the old town due to the traditional architectural style had amplified the losses .


FIGURE 4. The spatial distribution of losses for the urban area of Bhuj city.

A statistical approach based on engineering judgments were applied in order to obtain a fast estimation of losses. First, the surface associated to each polygon corresponding to a damaged zone was computed using a GIS tool. Over the entire urban zone covered by the image it was founded an area of 0.43 km 2 considered as damaged, with around 0.25 km 2 of extensive damage (ED) and around 0.18 km 2 of complete damage (CD). Comparing with the total area of Bhuj, around 4.3 km 2 , 10% of the city is considered with severe damages.

The population of Bhuj, estimated at 121.000 people, was divided by the total urban area of the city, in order to obtain a local density of population.

TABLE 1. Computation of population density

Total Population	121000
Surface (km2)	4.3
Density (people/km2)	28140

Simplified hypotheses were applied in order to estimate losses. There are hypotheses based on various statistics and engineering judgments, this theme being the subject of a work in progress. The following suppositions are proposed:

1. regarding the complete damage, given that the structure is completely destroyed, it was assumed that 80% of the occupants are dead, and 20% of the occupants are injured.
   
   
2. regarding the extensive damage, it was assumed that 5% of the occupants are dead, and 60% are injured.

These ratios were applied to the persons affected by a damage level. A rapid computation of losses was performed, detailed in the following table:

A simplified statistical approach allowed a fast evaluation of losses. For the urban area of Bhuj city, the results are, in terms of human casualties, a number of 4404 dead persons and 5234 injured persons.

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