This whole coastal area experiences greater damage from earthquakes and tsunamis than anywhere else on the Californian coast because of the influence of the Gorda offshore tectonic plate. There was widespread damage of buildings but no casualties.
A 7.2 magnitude earthquake struck California off the coast of Humboldt County on January 22, 1923. Houses were severely damaged at Ferndale, Petrolia, and Upper Mattole. Many chimneys were downed and water lines were broken. At Pepperwood, one house was shaken from its foundation and split apart, and another was twisted from its base. Chimneys also were knocked over at a number of places. Several landslides occurred. The impact of the earthquake was felt all the way from Siskiyou County on the Oregon border south to San Francisco, three hundred miles away, and eastward to Nevada County. It was also observed on several ships at sea. Many aftershocks occurred in the Petrolia and Upper Mattole regions. A small tsunami was recorded.
The North Coast of California and adjacent offshore area is the most active seismic region in the continental United States. The size, location, and frequency of past earthquakes give an indication of what to expect in the future. The instrumental record of earthquakes on this coast extends back only into the early 1900s. However, it is possible to learn about older earthquakes from written accounts in newspapers, church logs, and diaries.
From these accounts, seismologists can reconstruct the pattern of ground shaking and estimate the location and magnitude of early earth- quakes. Since 1853, more than sixty earthquakes have caused some damage to North Coast communities. The majority of these earthquakes have been centered offshore in the southeastern portion of the Gorda Plate offshore from Crescent City and Eureka.
These earthquakes recur frequently, causing some damage to communities, particularly in the Cape Mendocino area, almost every two years. Since the early 1990s there has been the highest level of regional earthquake activity of the twentieth century. Nine earthquakes of magnitude 6 or larger have struck the coastal and offshore areas. Seven earthquakes were close enough to coastal communities to cause at least moderate damage. Damaging earthquake activity was not restricted to the coast.
Klamath Falls, Oregon, experienced its strongest earthquake in historic times. West of Eureka an earthquake with a magnitude of 7.3 occurred in 1922. This potentially damaging earthquake was felt from Eugene, Oregon, all the way to San Francisco. In 1994, the area around Eureka experienced its largest earthquake since 1932. It caused an estimated $5 million in property losses. In that same year there was the Mendocino Fault Earthquake, the largest ever recorded along this fault. It was felt as extensively as the Eureka earthquake of 1922.
A major reason for the recurrence of earthquakes in north coastal California right up to the present time is found in the behavior of the Gorda Plate, offshore from the stretch of coast between Crescent City and Eureka. This smaller tectonic plate is gradually breaking apart under the stresses it is experiencing from much larger plates. In 2005, for example, an earthquake of magnitude 7 was recorded in this coastal area and, because of its size, it was feared that tsunamis would be triggered.
It turned out that the quake was caused by undersea pieces of the Gorda Plate jerking violently past one another in a sideways motion known as strike-slip, not the kind of motion that would create big tsunamis. A subduction motion of the Gorda Plate beneath the North American Plate would be a much more dangerous event. The fundamental action that is compressing the Gorda Plate is the northward movement of the Pacific Plate as it slides parallel to the San Andreas Fault. A strike-slip movement close to the coast could give rise to a tsunami and places such as Humboldt Bay and Crescent City would be the victims of the waves within half an hour.