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Earthquakes at The Sea Ranch

Most Association members are aware of the fact that their lots lie at most one-half mile from one of the largest transform fault zones in the Western Hemisphere, the San Andreas Fault Zone (SAFZ). It is the boundary between two tectonic plates, the Pacific and North American, which are slowly grinding past one another on average at the rate of 1-3 cm per year. This has been going on for at least 25 and perhaps as long as 45 million years. It has resulted in the relative displacement, of one plate with respect to the other, of hundreds of miles. The sedimentary and volcanic rocks that make up The Sea Ranch bluffs were initially laid down in ocean basins far to the south. Periodically, large-scale movements are recorded on the SAFZ; the most recent in Northern California (1989) was associated with the Loma Prieta earthquake.

The SAFZ is a broad zone, perhaps 0.5- to I-mile wide, in Northern California. It consists of a family of quasi-parallel north-south trending faults including the most recent major breaks, such as occurred in 1906. Related is a group of similar trending faults lying offshore 1-2 miles to the west of the coastline. In the great 1906 San Francisco earthquake, the San Andreas broke over a large part of its 1,000-mile length. At The Sea Ranch, the surface break was seen on the western slopes leading down to the Gualala River. Traces of the old fractures can still be seen in some road cuts in the area. However, the relatively rapid erosion of the unconsolidated, ancient landslide deposits leading down to the river have obscured the breaks within the Ranch, e.g., above the "Hot Spot." Because the area was largely inaccessible in 1906, the exact displacement along the break or breaks was not recorded but, elsewhere in the Fort Ross-Alder Creek area, displacements of 10-15 feet are a matter of record.

Frequent questions in the minds of Sea Ranchers are: When can we expect the next big earthquake? Will we have any warning? What is the history of earthquake activity in this are? If I feel an earthquake, how can I find out where it originated? The answer to each of these questions deserves more space than available here. Nonetheless, a few words may be appropriate.

Predicting earthquakes, whether big or small, is not possible although some information concerning probability of occurrence is available. Historical records are also useful. For example, in 1994, in conjunction with some archaeological excavations in the vicinity of Fort Ross, trenching by geologists revealed breaks and offsets indicating that six large ancient earthquakes within the SAFZ preceded the 1906 quake. Each had a displacement of 3.5-4 meters and was separated by 300-350 years.

One approach to predicting earthquakes is to determine where there are "seismic gaps" or quiet zones where unrelieved strain may be building up. The U.S. Geological Survey has done this for the length of the San Andreas and has recognized the North Coast area including The Sea Ranch as within a long seismic gap extending from just south of Cape Mendocino to San Francisco. There have been no major or moderate earthquakes along this segment since 1906. Nevertheless, tremors frequently jolt the North Coast. The bulk of those tremors originates along the offshore northwest-southeast trending Mendocino Fracture Zone, another plate boundary beyond which the SAFZ cannot be recognized. This fracture zone, west of Petrolia, is the most seismically active region in the conterminous U.S. Earthquakes of Richter magnitudes of 5-6 are common in the course of a year, and The Sea Ranch feels the larger of them. In addition, we feel small local tremors frequently on offshore faults, such as the swarm of 13 small events from August 19 through September 1994 that were located 2 miles northwest of the Sea Ranch/Gualala area. There are also occasional tremors within the central SAFZ along the Fort Ross-Point Arena segment; however, they are typically of low magnitude and the precise location of their epicenters is not always possible.

The general consensus is that there will be some minor seismic activity or foreshocks proceeding any large- scale movement on the San Andreas perhaps years in advance. On this account, it is a consolation that almost any seismic activity on the northern segment of the SAFZ attracts the attention of the whole seismic community.

For more detail concerning history of earthquakes at Sea Ranch, see: I. Borg, "The San Andreas Fault at The Sea Ranch," The Sea Ranch Soundings, Fall 1992, no. 34,pp. 6-7.

Also walk the San Andreas Interpretative Trail, just uphill from the Hot Spot. A pamphlet is available at the head of the trail that allows you to locate signs of the last big quake.

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