The
left-lateral strike-slip earthquake was the largest to occur in California since the
1952 Kern County earthquake. The mainshock (which was described as a multiple-rupture, with four subevents in the initial 80 seconds) and its aftershocks occurred on a northeast-trending
fault that extended from near the Mendocino Fault to a point northwest of
Eureka. Movement along the fault is due to a north–south compressional regime and the resulting
intraplate deformation of the Gorda plate. Previous events in this area were the January 1922 7.3 and the January 1923 7.2 shocks. The earthquake occurred offshore (to the northwest of Eureka, about west of Patrick's Point) and had a maximum Mercalli intensity of VII (
Very strong). Some people reported intense shaking that lasted for 15 to 30 seconds. In the epicentral area, items were knocked off shelves and furniture was displaced, but damage was considered light (the smaller 5.2 event that occurred in June 1975 caused more damage).
Damage While most of the damage from the Gorda Basin earthquake was considered light, there were some exceptions. In Eureka, effects included broken windows and dishes, fallen chimneys, and merchandise that fell from store shelves. In Fields Landing, King Salmon, Loleta, and Big Lagoon areas, the strongest effects included surface cracks on the ground,
soil liquefaction, small landslides and
rockfalls, and numerous
slumps along the
Eel River. Similar effects occurred along the Old Coast Highway near
Trinidad and
Moonstone, where the roadway was reduced to one lane of travel in some areas. Several homes were knocked off their foundations and a highway overpass collapsed in Fields Landing.
Seismic wave amplification, poor design, or inadequate construction style may have contributed to losses there.
Tompkins Hill Road overpass The Tompkins Hill Road overpass is situated just south of Fields Landing and was built in the late 1960s. It suffered slight damage during the 1975 earthquake and was scheduled for a retrofit in 1981. The overpass was constructed with cement
abutments on earthen ramps on either end and a series of concrete support columns in the middle. Eight reinforced concrete spans accommodated northbound and southbound lanes of traffic, with no
anchors connecting the spans with each other or to the abutments. At the time of the shock, two of the southbound spans came off their support (a ledge) and six people were injured when a
Volkswagen Beetle and a small
pickup truck plummeted off the bridge.
Intensity With an area of perceptibility of more than 97,000 mi2, it was felt from the
San Francisco Bay Area and the
Central Valley to
western Nevada, and as far as
Salem in northern Oregon. Intensity VI (
Strong) effects were limited to a narrow coastal zone that included
Myers Flat in the south and
Brookings, Oregon in the north. Damage at this intensity level included broken plate glass windows, cracked walls, foundation and chimney damage, and significant loss of merchandise. In Fields Landing, two homes that came off their foundations, the fallen overpass, and broken gas, water, and sewer lines were associated with intensity VII (
Very strong) shaking.
Strong motion Pacific Gas and Electric Company initially reported that peak ground accelerations in the range of 0.16–0.4
g were recorded on the floor of the refueling building on the plant's strong motion instruments (
accelerometers). Low voltage from a faulty power supply left the instruments in a condition that was functional, but the records were not considered reliable. Three TERA Technology film recorders were also in use as a backup system, but these instruments also did not produce usable records because lack of maintenance had allowed dirt and grit to get inside. Only records from one instrument (an Engdhal peak shock recorder) was believed to be operating correctly, and with a close examination of the energy dispersed at various frequencies, an estimate of 0.15–0.25
g was given for the event at that location. ==See also==