Evaluation of Effects on Trees of Past Movements on the San Andreas Fault, Northern California
Fault movement accompanied by surface rupture can fracture, twist, or tilt trees that grow on the surface break of the fault. Trees growing near, but not on, the fault may be felled or topped as a result of ground motion. Changes in growth rate of trees may relate to hydrologic and topographic changes or from “release effects” caused by death of neighboring trees. Many other causes such as landslides, wind, and fire can produce similar effects and must be considered when using trees to date past movements on the San Andreas fault. The effects of tilting of trees growing on surface ruptures seem to provide the least ambiguous evidence of past movement on the fault, but few ancient trees that record multiple movements in the past were found. A redwood tree near Fort Ross was tilted between 1400 A.D. and 1650 A.D.; tilting is tentatively attributed to movement on the San Andreas fault. The same tree also records two other tilts, and in 1906 the base of the tree was fractured by faulting. In contrast, a redwood tree possibly 500 to 600 yrs old in Woodside grows only 10 ft (3 m) from the 1906 surface rupture and yet no effects in growth rate or tilt record the event.