Tree-ring dated landslide movements and their relationship to seismic events in southwestern Montana, USA

To determine periods of incremental landslide movement and their possible relationship to regional seismic events, the tree-ring records of 32 tilted and damaged conifers at three sites on landslides in the Gravelly Range of southwestern Montana were examined. Several signs of disturbance in the tree-ring record indicating landslide movement were observed. Commonly, the tree-ring record displayed a marked reduction in annual ring width and/or the reaction wood formation. The tree-ring records from the three landslide sites indicate multiple periods of movement during the 20th century. Many of the periods of movement indicated by the strongest signals (most trees) at the sites occurred the year following significant earthquakes in the region. Those seismic events for which evidence in the tree-ring record was found at one or more of the three sites are the 1983 Borah Peak, 1959 Hebgen Lake, 1935 Helena, 1925 Clarkson, and 1908 Virginia City earthquakes. This study suggests that many of the landslide movements were triggered by, or are coincident with, earthquakes as much as 200 km from the study area.