Abstract

The 1978 earthquake at Stephens Pass, northeastern California, dropped a series of grabens that average 4.5 m in width, extend up to 1 m in depth, and are found intermittently along a 2-km-long rupture zone. The formation of this graben series killed or otherwise affected many trees growing in or immediately adjacent to the rupture zone. Nine trees responded to the 1978 earthquake with anomalously narrow ring widths, beginning in 1979 and continuing for several years. One tree responded with anomalously wide latewood relative to total ring width. This example of tree-ring responses to a normal-fault earthquake complements other cases of tree-growth responses to earthquakes of thrust and strike-slip tectonic settings. The 1978 earthquake at Stephens Pass was unique in that it caused tree-ring responses even though it was only moderate (magnitude 4.6). This study serves as a specific calibration example for dendrochronologically studying prehistoric earthquakes, as well as eruptions, at the nearby Medicine Lake Highlands. Medicine Lake has been seismically and volcanically active during the past 1000 yr, and it supports a forest of several coniferous tree species that can be used for dendrochronologically studying geomorphological processes.

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