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Spatial variation in the response of tree rings to normal faulting during the Hebgen Lake Earthquake, Southwestern Montana, USA





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Tree rings have frequently been used to identify the effects of earthquakes on forests, but little is known about spatial variation in the response of trees to intraplate normal faulting. This paper documents and describes the effects of tree location (distance from and position above or below the fault scarp), size and age on the response of tree rings to the 1959 magnitude 7.5 Hebgen Lake earthquake, which occurred along a normal fault in the Gallatin National Forest in southwestern Montana. Core samples from 88 trees were collected along nine 100-m transects straddling the Hebgen scarp, and from 28 additional large-diameter trees near the scarp. The most common tree-ring response to the earthquake was a suppression in growth, usually lasting for several years. Among samples from the transects, suppressions were significantly more common below vs. above the scarp, but this pattern was not found among the large tree samples. Distance of trees (within 58 m) from the fault scarp had little effect on tree-ring responses. These results illustrate the importance of interactions between tree location and tree size/age in identifying tree-ring responses to earthquakes. Smaller, younger trees required the direct movement of the downthrown block below the scarp to incur sufficient damage to record a suppression, whereas larger, older trees were damaged even on the stationary slope above the scarp. The small effect of distance from the scarp on suppressions suggests that event-response trees may be found further from a fault than previously thought. r 2004 Elsevier GmbH. All rights reserved.