Trees as indicators of past movements on the San Andreas Fault
Trees are sources of information about fault movements that have occurred before the earliest historical reports. This kind of evidence can be used to improve estimates of when earthquakes will recur on faults known to be seismically active and to identify active faults that have no record of movement during recent history. The approach is not new. Robert Page of the U.S Geological Survey described the effects of the 1958 earthquake on trees along the Fairweather fault in Alaska. He showed that tree rings methods could have been used to identify and to closely date this event. We undertook a similar study of the northern part of the San Andreas fault, in part because there are no historic records prior to 1906 along this segment. Earthquakes and surface rupture along faults affect trees in several different ways. Direct effects include fracturing, twisting, and tilting of trees that grown on the surface of the break. In a much wider zone along the fault, trees may be felled or topped as a result of ground motion. Among the indirect effects are tilting, felling, or burial of trees in earthquake-triggered landslides. Long-term effects may include changes in growth rate due to local hydrologic and topographic changes as well as to biological effects such as the death of neighboring trees. Under favoralbe circumstances these can be dated by tree ring methods.