Characteristics of earthquake-triggered landslides and post-earthquake debris flows in Beichuan County

Field investigations and aerial photography after the earthquake of May 12, 2008 show a large number of geo-hazards in the zone of extreme earthquake effects. In particular, landslides and debris flows, the geo-hazards that most threaten post-disaster reconstruction, are widely distributed. We describe the characteristics of these geo-hazards in Beichuan County using high-resolution remote sensing of landslide distribution, and the relationships between the area and volume of landslides and the peak-discharges of debris flows both pre- and post-earthquake. The results show: 1) The concentration (defined as the number of landslide sources per unit area: Lc) of earthquaketriggered landslides is inversely correlated with distance from the earthquake (D-F) fault. The relationship is described by the following equation: Lc = 3.2264exp(-0.0831D(F)) (R-2 = 0.9246); 2) 87 % of the earthquake-triggered landslides were less than 15x10(4) m(2) in area, and these accounted only for 50% of the total area; 84% of the landslide volumes were less than 60x10(4) m(3), and these accounted only for 50% of the total volume. The probability densities of the area and volume distributions are correlated: landslide abundance increases with landslide area and volume up to maximum values of 5x10(4) m(2) and 30x10(4) m(3), respectively, and then decreases exponentially. 3) The area (AL) and volume (VL) of earthquake-triggered landslides are correlated as described with the following equation: VL=6.5138AL(1.0227) (R-2 = 0.9131); 4) Characteristics of the debris flows changed after the earthquake because of the large amount of landslide material deposited in the gullies. Consequently, debris flow peak-discharge increased following the earthquake as described with the following equation: Vpost = 0.8421Vpre(1.0972) (R-2 = 0.9821) (Vpre is the peak discharge of pre-earthquake flows and the Vpost is the peak discharge of post-earthquake flows). We obtained the distribution of the landslides based on the above analyses, as well as the magnitude of both the landslides and the post-earthquake debris flows. The results can be useful for guiding post-disaster reconstruction and recovery efforts, and for the future mitigation of these geo-hazards. However, the equations presented are not recommended for use in site-specific designs. Rather, we recommend their use for mapping regional seismic landslide hazards or for the preliminary, rapid screening of sites.