Abstract:

The May 12th, 2008 Wenchuan earthquake (Mw 7.9) in Sichuan Province led to severe casualties and a huge economic loss. Field investigations after the earthquake have obtained a lot of coseismic deformation data. However, most of them are strike and vertical offset; the horizontal shortening which is as important as the strike and vertical offset was rarely reported. In order to estimate the horizontal shortening, we suggested a new method: drawing reference geometric features in QuickBird images first; then calculating the space between adjacent features before and after earthquake, respectively; finally obtaining the coseismic horizontal shortening based on the contrast of the spaces before and after earthquake. The Baishahe rupture section was selected as the sample place to test this new method for estimating coseismic horizontal shortening because there are two roads, one on the northern side of the rupture and another on the southern with 300 m to 900 m spacing. By contrasting the space between two roads before and after earthquake, we obtained a series of horizontal shortening amounts; the average, maximum and minimum of those are 6.8 m, 13.2 m and 1.1 m, respectively, and the error is 2.33 m. Checking the field measurement of the two roads after earthquake, we inferred that about 1.18 m may have been overrated using this new method. Moreover, our estimated results using this new method are larger than the calculated results based on restoration of deformed constructions and strata near the surface rupture, suggesting that the coseismic horizontal shortening should have been distributed in a wider area, including the immeasurable permeated deformation far from the rupture and the measurable near the rupture.

Key words: Wenchuan earthquake, surface rupture, co-seismic horizontal shortening, image contrast