Coseismic landslides triggered by the 2022 Guanshan-Chihshang earthquake sequence, eastern Taiwan

Coseismic landslides, or earthquake-triggered landslides, are a major type of hazard during seismic events and often lead to considerable casualties. The systematic establishment of an accurate landslide inventory that includes both landslide location and area is crucial for understanding the distribution characteristics and mechanisms of coseismic landslides. Furthermore, correlating the inventory with factors such as topographic, geologic, and seismic parameters can help delineate potential landslide zones, thereby enhancing disaster response and improving early warning capabilities and hazard mitigation.

A major earthquake sequence occurred in Taitung, Taiwan, on 17 and 18 September, 2022, with a M_w 6.5 foreshock in Guanshan followed by a M_w 6.9 mainshock in Chihshang. The Guanshan-Chihshang earthquake sequence triggered many coseismic landslides in eastern Taiwan. Since previous landslide inventory studies of this earthquake are mostly incomplete, this study aims to establish a complete, accurate, and detailed landslide inventory for this event.

High-resolution remote-sensing data, including 50-cm resolution Pléiades satellite images and 25-cm resolution aerial photos, were utilized to identify and map the coseismic landslides. The Normalized Difference Vegetation Index (NDVI) was initially employed for rapid detection of landslide areas, followed by visual inspection and mapping. By searching all regions that experienced a seismic intensity of 5-lower or higher from the Central Weather Administration during both earthquakes, this study identified a total of 876 landslides, with a combined area of ​​approximately 1.9×10⁶ m².

A subsequent quantitative analysis was conducted on landslide controlling factors, including distance to the epicenter, distance to surface rupture, seismic intensity, lithology, slope, slope aspect, elevation, and distance to rivers. The results reveal a distinct correlation between landslide frequency and four factors, including seismic intensity, epicentral distance, distance to surface rupture, and distance to rivers. Although the largest number of landslides occurred in sedimentary rocks, the total landslide area was higher in regions with metamorphic and igneous rocks. However, after normalizing for total area, the igneous rock region has the highest landslide area density. Spatially, the majority of landslides exhibit a preferential orientation on south- and southeast-facing slopes, which may be associated with directivity effect of seismic wave propagation. Additionally, the largest number of landslides was observed on slopes of 30˚–45˚ and at elevations of 250–500 meters.

This study successfully established a comprehensive landslide inventory for the 2022 Guanshan-Chihshang earthquake sequence and provides a quantitative analysis of landslide contributing factors. We hope these results will serve as important references for subsequent research on coseismic landslide susceptibility assessment and related geohazard mitigation.