San Andreas Fault

The San Andreas fault has had several great earthquakes in historical times and presents a serious hazard to the populations of nearby areas. Old trees can record earthquakes not observed and recorded by humans. A study of old trees along the fault revealed that they had been greatly disturbed and rings were very narrow or even missing for several or more years starting in 1813.

The anatomy of the rings indicated a normal ring for 1812 but suppressed growth in 1813, thus limiting the disturbance to the non-growing season of 1812-13. Only trees close to the fault were disturbed. Other trees in the area showed no unusual growth at this time.

A search of old coastal mission records revealed information about an earthquake in December of 1812 but there was uncertainty about the location. Evidence of disturbance was present in nine trees along 12 kilometers of the San Andreas fault. The conclusion was that the trees were disturbed by the 1812 earthquake.

Therefore, this segment of the fault had experienced two earthquakes only 45 years apart; the 1812 event and the documented 1857 event. Thus the trees provided an important addition to the record of seismicity in the area.

Examples of disturbed trees are two trees that grow directly on the San Andreas Fault and had their tops broken off and root systems damaged by the accelerations and displacement of the 1812 earthquake. Energies of the trees went toward restoring the photosynthetic crown and roots. The annual rings are missing or very narrow for nearly two decades after the event. The site chronology from the area shows a narrow ring for 1812 (arrow on graph below), most likely due to a drier year. However, these trees were unaffected by the earthquake because of their distance from the fault and show normal to fast growth through 1840.

The arrowhead on the graph indicates the year 1812.

Two trees that show evidence of reduced growth caused by the earthquake are

the Lone Pine Tree,

and the Pool Tree.