Only Very Strong Shaking can Break a Tree
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Susan Hough
USGS
- Date & Time
- Location
- Online-only seminar via Microsoft Teams
- Summary
Trees are shaken but rarely broken by earthquakes. A number of credible accounts in recent and historic times do, however, describe standing trees being snapped by shaking. These accounts have never been analyzed in detail. Apart from landslides and tsunamis, such accounts are uncommon, suggesting that trees only break where ground acceleration is extreme, or trees are especially vulnerable. I first consider the question: what ground acceleration is required to break a standing tree? I present simplified calculations from first principles, focusing on tall, thin trees that can be approximated as uniform vertical cantilevers. The results suggest that, possibly excepting tall trees with heavy canopies, standing trees can only be broken by shaking approaching or exceeding 1 g, anticipated by the rarity of documented accounts of snapped trees. Tall trees moreover potentially provide unique constraints on long-period energy that will not necessarily damage small historical structures. I consider several documented instances of tree damage during strong earthquakes, drawing inferences about local ground motions. Examples include an account of dead trees broken by the 1886 Charleston, South Carolina earthquake, and accounts of trees broken in a number of locations by the 1906 San Francisco earthquake. The former suggests that the most severe shaking in 1886 occurred near the upper edge of the rupture model proposed by Bilham and Hough (2025). In 1906, tree breakage occurred primarily in three areas: near Loma Prieta, near Fort Ross, and along a ~125 km swath near Cape Mendocino. The northernmost swath is well inland, 25-40 km inland of the mapped San Andreas Fault, through a region that was not surveyed by contemporary geologists, and for which early triangulation data was especially limited. The high intensities can be explained by wave propagation effects (directivity, Moho reflections, and topographic amplification), but it might be fruitful to reconsider the original triangulation data that constrains the northernmost rupture.