Paleoseismology and Landslides

Abstract

Most moderate to large earthquakes trigger landslides (Fig. 1). In many environments, landslides preserved in the geologic record can be analyzed to determine the likelihood of seismic triggering. If evidence indicates that a seismic origin is likely for a landslide or group of landslides, and if the landslides can be dated, then a paleoearthquake can be inferred, and some of its characteristics can be estimated. Such paleoseismic landslide studies thus can help reconstruct the seismic shaking history of a site or region (Jibson 2009). Paleoseismic landslide studies differ fundamentally from paleoseismic fault studies (▶ Paleoseismic History of the Dead Sea Fault Zone,▶ Paleoseismic History of the North Anatolian Fault Zone, ▶ Paleoseismic History of the San Andreas Fault Zone). Whereas fault studies seek to characterize the movement history of a specific fault, landslide studies characterize the shaking history of a site or region irrespective of the earthquake source. In regions that contain multiple seismic sources and in regions where surface faulting (▶ Seismic Actions due to Near-Fault Ground Motion) is absent, paleoseismic ground-failure studies thus can be valuable tools in hazard and risk studies that are more concerned with shaking hazards than with interpretation of the movement histories of individual faults. In fact, paleoseismic studies in some parts of the world typically rely more on ground failure than on surface fault ruptures. The practical lower bound earthquake that can be interpreted from paleoseismic landslide investigations is about magnitude 5–6 (▶Earthquake Magnitude Estimation). This range is comparable or perhaps slightly lower than that for paleoseismic fault studies. Obviously, however, larger earthquakes tend to leave much more abundant and widespread evidence of landsliding than smaller earthquakes; thus, available evidence and confidence in interpretation increase with earthquake size. Paleoseismic landslide analysis involves three steps: (1) identify a feature as a landslide, (2) date the landslide, and (3) determine if the landslide was triggered by earthquake shaking. This article addresses each of these steps and discusses methods for interpreting the results of such studies. Only subaerial landslides are discussed here; submarine landslides are analyzed using different methods. In this article, landslide is used as a generic term to include all types of downslope movement of earth material, including types of movement that involve little or no true sliding. Thus, rock falls, debris flows, etc., are considered types of landslides. The classification system of Varnes (1978) is used, which categorizes landslides by the type of material involved (soil or rock) and by the type of movement (falls, topples, slides, slumps, flows, or spreads). Other modifiers commonly are used to indicate velocity of movement, degree of internal disruption, state of activity, and moisture content.