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Chapter 2 -- Description of the Smith River watershed (continued)

Seismology

Because of continuing activity on the nearby Cascadia subduction zone, the Smith River watershed and all of northwestern California have a high potential for earthquakes. On the North Coast, the estimated return interval for an earthquake having a magnitude of 8.0 on the Richter Scale is 303 years. Since European American settlement, no devastating earthquakes have occurred in this area. However, evidence from radiocarbon, Native American oral history, and Japanese tsunami records indicate that earthquakes of estimated 8.5 magnitude have occurred 296, 750, 1100, 1350, and 1650 years before the present. (Jennings 1994, Carver personal communication 1996).

The Klamath Mountains are traversed by many faults including two major thrust faults that extend roughly north and south and dip to the east. Fault lines are used to define boundaries between ecological subsections in the Smith River watershed. Numerous additional faults are located offshore within 14 miles of the coast. The Smith River area may also be affected by earthquakes generated from active faults farther to the south including the Trinidad Fault and the Little Salmon Fault (Burke 1996, California 1994b).

Degree of shaking caused by an earthquake is influenced not only by distance from the fault plane but also by geology of the site. Ground motion is amplified where unconsolidated sediments are present. In the Smith River watershed, amplified ground motion is expected in areas underlain by sand dunes or alluvial materials including large areas of the Crescent City Plain. Other areas of intensified shaking include alluvial stream bottoms along Mill Creek, Rowdy Creek, and the Smith River below the mouth of the South Fork. The mountains of the Smith River watershed are susceptible to rock falls generated by earthquakes. Landslides can also be triggered especially following heavy rains when soils are saturated (Toppozada et al. 1995). Given the estimated recurrence interval for strong earthquakes and the estimated frequency of saturated soils, an earthquake triggering massive and disastrous landsliding might occur every five thousand to ten thousand years (Burke 1996).

The potential for liquefaction is high in areas northwest of Lake Earl which are underlain by sand dunes. There is a low potential for liquefaction in areas near the lower Smith River which are underlain by alluvial fan deposits and terrace materials (Toppozada et al. 1995). In other areas of the watershed, liquefaction is unlikely.

During major earthquakes, coastal areas may drop in elevation by one or two meters, which abruptly alters shoreline and estuary ecosystems. Coastal forests may suddenly perish when inundated. Over centuries, subsided areas gradually rise back up as pressure rebuilds on the fault. Eventually, another major earthquake causes another subsidence event (Carver 1996). In addition to shaking, liquefaction, and subsidence, coastal areas may also be devastated by tsunamis.

 

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