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Evidence of Past Earthquakes Preserved in Coast Redwood Trees Along the Northern San Andreas Fault

Description: 

Prior to the 1906 San Francisco earthquake, the rupture history of the North Coast section of the San Andreas Fault is not well constrained. Current age bounds for the penultimate event are broad, generally spanning over a century (e.g., 1660–1812), based on radiocarbon dating of trench exposures and turbidites. Dendrochronology has the potential to provide much tighter constraints by crossdating growth rings associated with direct and indirect impacts to trees growing on or adjacent to surface ruptures, but past attempts to utilize long-lived coast redwoods (Sequoia sempervirens) failed due to difficulties in crossdating this species. Here, we leverage recently developed regional reference chronologies for a new dendroseismological investigation of Sequoia sempervirens trees and stumps along the San Andreas Fault near Gualala and Fort Ross, California. We developed the novel technique of using rope climbing to collect samples in the vicinity of external indicators of potential seismic damage on living trees, such as reiterated trunks that formed after top breaks, sympodial growth (expressed as trunk kinks), and rightening of trunk leans. We collected increment cores (up to 56 m above ground) from nine living trees, as well as sections from 23 stumps of historically logged trees that may extend the record further. These records are being analyzed for growth changes and anatomical indicators. We have found tree ring indicators of the 1906 earthquake including likely initiation dates for reiterated trunks, wood anatomical indicators, and locally absent growth rings. We show within-tree variation in the expression of dendroseismological indicators. We detail several disturbance events that may represent the penultimate earthquake, with the caveat that non-seismic causes cannot yet be confidently excluded because these trees grow in a disturbance-prone environment. As part of ongoing research, we plan for additional sampling of damaged trees as well as continuing our efforts to fully crossdate all samples in order to confidently identify the penultimate earthquake by its contemporaneous effects in many trees.

Session: From Faults to Fjords: Earthquake Evidence in Terrestrial and Subaqueous Environments [Poster Session]

Type: Poster

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Belle

Student Presenter: No

Invited Presentation: 

Authors