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 15×10⁴ m² in area, and these accounted only for 50% of the total area; 84% of the landslide volumes were less than 60×10⁴ m³, 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 5 × 10⁴ m² and 30 × 10⁴ m³, 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.