from California Geology, March 1981, Vol. 34, No. 3.

GORDA BASIN EARTHQUAKE, NORTHWESTERN CALIFORNIA

November 8, 1980

By

RICHARD T. KILBOURNE and GEORGE J. SAUCEDO, Geologists

California Division of Mines and Geology

On November 8, 1980 at 2:27 a.m. Pacific Standard Time, residents of northwestern California and southern Oregon were awakened by the rumbling shock of a magnitude 7.0 earthquake. This was the largest earthquake to affect California in more than 28 years (since the 7.7 magnitude Arvin-Tehachapi earthquake in 1952). The center of this shock was located offshore (Latitude 41.1° N, Longitude 125.1° W) about 65 km west of Trinidad and Patricks Point (preliminary magnitude and epicenter location from Seismology Laboratory at University of California, Berkeley, January 20, 1981). Aftershocks appear to be spread out along a northeast-southwest trend, suggesting that the causative fault has the same strike (R. C. McPherson, TERA Corp., Berkeley, California, personal communication).

This fault trend is different from the more commonly studied northwest trending faults of the San Andreas system.

TECTONIC SETTING

The tectonic setting of this region is quite different from that of the portion of California below Latitude 40° N (figure 1). In this region three

plates of the earth's crust bump and grind against each other to produce earthquakes. The seismic history of this area indicates that this is one of the most active earthquake regions within the state.

SEE FIGURE 1

Figure 1. Generalized regional tectonic setting of northwestern California. From various sources.

The major seismic zones are located along a series of spreading ocean ridges connected by transform faults (a strike-slip fault characteristic of midoceanic ridges and along which the ridges are offset). Earthquake epicenters in the region are concentrated along the Blanco fracture zone, the Gorda (spreading) ridge, and the Mendocino fracture zone (east and south of the Gorda Ridge). Earthquake epicenters are concentrated along these major plate boundary faults, and there are a smaller number of earthquake epicenters spread out across the Gorda plate (Atwater, 1970) and the adjacent portions of the North American plate.

The distribution of epicenters is the result of the collision between the Gorda and North American plates (figures 1, 2). Movement within these plates is the cause of the November 8, 1980 tremor. Active spreading at the Gorda ridge is in part responsible for the eastward motion of the Gorda plate. This collision is taking place at about 2-4 centimeters per year (Riddihough, 1978), very slowly compared to other plate collisions around the world. The collision zone forms a subduction zone (one plate sliding under another plate), but it is not a deep oceanic trench such as the Aleutian trench in Alaska. The earthquake producing portion of the Gorda plate is about 10 km thick and it dips under the North American plate at an angle of about 15° (Silver, 1971; and figure 3).

SEE FIGURE 2

Figure 2.•Block diagram of lithosphere plates in region of the November 8, 1980 earthquake. Line with barbs indicates the subduction zone, where the Gorda plate is sliding beneath the North American plate: the barbs are on the overriding plate. Strike-slip faults which trend northeast-southwest are shown in the Gorda Basin area. Adapted from various sources.

Many earthquakes also occur in the Gorda plate, which is stressed due to the collision and appears to be fragmented along northeast trending, vertically dipping, left-lateral faults. These earthquakes occur both in the Gorda basin and beneath the North American plate, and under the continental shelf and coastal regions of California.

As the Gorda plate is subducted, friction with the overlying North American plate causes more earthquakes in the Gorda plate than in the North American plate (figure 3). Earthquakes that occur in the North American plate are typically shallow quakes along northwest trending thrust faults.

HISTORIC SEISMICITY

Earthquakes are not a new phenomenon to residents of north coastal California, where an earthquake of this intensity of shaking and damage occurs more often than once a decade. Damaging quakes that have occurred in this region since 1871 are listed in table 1.

Intensity and Damage

The damage to man-made structures was in some cases spectacular, but overall, relatively light for the magnitude (7.0) of the November 8, 1980 earthquake. Damage from earthquakes is usually measured regionally with the Modified Mercalli Intensity Scale (table 2). Initial assessments indicate that this earthquake caused Intensity VII damage at Fields Landing, Loleta, King Salmon, and Big Lagoon. Chimney and glass window failures were common (photo 1); a portion of the Highway 101 Tompkins Hill overpass bridge failed (photos 2, 3); and surface cracks were formed by slumping and liquefaction in water saturated sediments. In the larger communities of Fortuna, Ferndale, Trinidad, Arcata, and Eureka damage was generally limited to a few cracked windows and broken dishes, and grocery store items which fell from shelves. In addition, the earthquake shaking awakened nearly all residents. These observations would warrant an assignment of Intensity VI.

Ground surface failures were common especially in the area of Intensity VII damage. These included numerous small landslides (photos 4, 5, 6), liquefaction---caused slumping along the Eel River and along the sand spit at Big Lagoon (Gary Carver, Humboldt State University, personal communication, 1980), and cracks in roads and parking lots, especially in water saturated alluvial materials. Ground surface failures and areas of high intensity seem to be very repetitive in occurrence for all of the region's earthquakes (Kilbourne and others, 1980, in press).

SEE FIGURE 3

Figure 3. Cross section near Eureka showing distribution of epicenters, January 1974 --- February 1977 (see figure 1 for location). After, Smith and Knapp, 1980.

OTHER EFFECTS

Of special interest in this earthquake are the aquatic and submarine effects. Several people reported seeing the ocean light up, or seeing a glow on the horizon during the quake. Reports of lightning, arching of power lines, and release of luminous marsh gas are common for M > 7 earthquakes on land. As the light was described repeatedly as a glow, and power lines and marsh gas would not occur over the ocean, the explanation favored by the authors is that the vibration induced the billions of phosphorescent plankton found in these north coastal waters to light up. Another interesting submarine phenomenon is a three-meter change (deepening) in the bathymetry in an area several tens of kilometers west of the Klamath River. This deepening was reported by fisherman who cross this area daily watching their sonic depth finders in search of fish. Michael Field of the U. S. Geological Survey is currently charting this scarp-like feature with a sonic profiler aboard a USGS research ship (Kenneth Lajoie, USGS, personal communication).

Table 1. Earthquakes of north coastal California, M > 6.0

(Real and others. 1978).

There was no tsunami associated with this earthquake. This is not unusual for a strike-slip mechanism event, as tsunamis are more often generated by thrust faulting along a subduction zone. The Tsunami Warning Center in Hawaii cautiously issued an earthquake advisory for the Pacific Basin minutes after the quake (as they do for any large earthquake along the margins of the Pacific Ocean). No tsunami WATCH or WARNING was ever issued and the advisory was canceled at 3:53 a.m. after it was determined that no wave had been generated.

Table 2. Modified Mercalli Intensity Scale of 1931.

The first scale to reflect earthquake intensities was developed by de Rossi of Italy, and Forel of Switzerland, in the 1880s. This scale, with values from I to X was used for about two decades. A need for a more refined scale increased with the advancement of the science of seismology, and in 1902 the Italian seismologist, Mercalli, devised a new scale on a I to XII range. The Mercalli Scale was modified in 1931 by American seismologists Harry O. Wood and Frank Neumann to take into account modern structural features:

I Not felt except by a very few under especially favorable circumstances.

II Felt only by a few persons at rest, especially on upper

floors of buildings. Delicately suspended objects may swing.

III Felt quite noticeably indoors, especially on upper floors

of buildings, but many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration like passing of truck. Duration estimated.

IV During the day felt indoors by many, outdoors by few.

At night some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V Felt by nearly everyone, many awakened. Some dishes, windows,

etc., broken; a few instances of cracked plaster; unstable objects overturned. Disturbances of trees, poles and other tall objects sometimes noticed. Pendulum clocks may stop.

VI Felt by all, many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight.

VII Everybody runs outdoors. Damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motor cars.

VIII Damage slight in specially designed structures; considerable

in ordinary substantial buildings, with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving motor cars disturbed.

IX Damage considerable in specially designed structures;

well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken.

X Some well-built wooden structures destroyed; most masonry and

frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks.

XI Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.

XII Damage total. Practically all works of construction are

damaged greatly or destroyed. Waves seen on ground surface. Lines of sight and level are distorted. Objects are thrown upward into the air.

The Modified Mercalli intensity scale measures the intensity of an earthquake's effects in a given locality, and is perhaps much more meaningful to the layman because it is based on actual observations of earthquake effects at specific places. It should be noted that because the data used for assigning intensities can be obtained only from direct firsthand reports, considerable time-weeks or months --- is sometimes needed before an intensity map can be assembled for a particular earthquake. On the Modified Mercalli intensity scale, values range from I to XII. The most commonly used adaptation covers the range of intensity from the conditions of "I-not felt except by very few, favorably situated," to "XII-damage total, lines of sight disturbed, objects thrown into the air." While an earthquake has only one magnitude, it can have many intensities, which decrease with distance from the epicenter.

However, unusual waves associated with the earthquake were observed in the Sacramento-San Joaquin Delta region. People sleeping aboard boats in parts of the Delta were shaken awake by a small, but stomach twisting "seiche" wave that swept through the region (Eastbay Today, November 12, 1980). Seiches are waves that occur in enclosed or semi-enclosed basins and are analogous to the sloshing of water in a bathtub or ice tray. Often seiches are induced by earthquakes, but they also can be caused by landslides, passing ships, and even rapidly moving low pressure weather fronts. Sometimes seiches occur hundreds or even thousands of kilometers from the epicenter of an earthquake. Seiches occurred in New Orleans following the 1964 (M 8.4) Alaskan earthquake and in Puget Sound during the 1906 (M 7.9) San Francisco earthquake (Robert Nason, U. S. Geological Survey, Menlo Park, personal communication, 1980).

INJURIES

There were no fatalities attributable directly to the earthquake. However, earthquake damage at the Tompkins Hill overpass on Highway 101 caused accidents. A Volkswagen, with five occupants, plummeted off the damaged overpass, followed shortly thereafter by a pick-up truck with one occupant. The drivers of both vehicles had no forewarning that the overpass was damaged. All victims of the accidents are recovering.

Two persons were treated for heart attack symptoms possibly due to fright from the earthquake and an unidentified man was treated at the hospital in Fortuna for cuts on his hand caused "when he jumped out of his window in panic" during the earthquake (Eureka Times-Standard, November 9, 1980).

SEE PHOTOS 1, 2, 3, 4, 5, 6

Photo 1. Chimney damage at Big Lagoon, November 8, 1980.

Photo 2. Six people were injured when two vehicles became airborne at this point where Tompkins Hill Road overpass crosses southbound section of Highway 101.

Photo 3. Cal Trans engineers examine Tompkins Hill Road overpass failure on Highway 101.

Photo 4. A small landslide, caused by the earthquake, blocked a portion of Petrolia Road south of Ferndale.

Photo 5. A State park ranger examines a beach-access walkway which was broken by a landslide near Trinidad.

Photo 6. Ground shaking from the November 8, 1980 earthquake caused redwood logs to roll down a roadcut slope onto Highway 101, north of Redcrest.

REFERENCES

Atwater, T., 1970, Implications of plate tectonics for the Cenozoic tectonics of western North America; Geological Society of America Bulletin, v. 80, p. 3513-3536.

Eastbay Today, November 12, 1980.

Eureka Times-Standard, November 9, 1980.

Kilbourne, R. T., Mualchin, L., and Saucedo, G. L., 1980, Geology for planning, Eureka and Fields Landing 7˝ quadrangles, Humboldt County, California; California Division of Mines and Geology, Open-File Report 80-9 SF, in press.

Real, C. R., Toppozada, T. R., and Parke, D. L., 1978, Earthquake catalog of California, January 1, 1900-December 31, 1974; California Division of Mines and Geology, Special Publication 52.

Riddihough, R. P., 1978, The Juan De Fuca plate; Transactions, American Geophysical Union, v. 59, no. 9, p. 836-842.

Silver, E. A., 1971, Tectonics of the Mendocino triple junction; Geological Society of America Bulletin, v. 82, p. 2965-2978.

Smith, S. W., and Knapp, J. S., 1980, The northern termination of the San Andreas fault; California Division of Mines and Geology, Special Report 140, p, 153-164.