The location of the epicenters of the earthquakes described below and their general geologic setting are shown on the North Coast Epicenter Map.
1/21/97 - 11:17 pm PST, magnitude 5.6 (Berkeley moment magnitude), located just off the coast in the Cape Mendocino region, about 1 mile NW of Punta Gorda or 6 miles SW of Petrolia. The focal mechanism was consistent with right-lateral slip on the Mendocino fault very near the Mendocino triple junction. It was centered at a depth of about 15 miles beneath the earth's surface. The earthquake was felt strongly in the Cape Mendocino region and southern Humboldt County. It was also felt widely in the Humboldt Bay region and by some as far north as Trinidad. No damage has been reported to date. A mag 4.6 aftershock occurred at 43 minutes later. This aftershock was located 16 miles WNW of Punta Gorda. The Humboldt Earthquake Eudcation Center is collecting information on the pattern of ground shaking produced by this earthquake. Your observations will help us to better understand both the effects of this earthquake and how this area will respond to future earthquakes. If you were anywhere in Humboldt or northern Mendocino Counties, please fill out the survey questionnaire. Even if you did not feel the earthquake, but were in this region, please respond!
You can see the distribution of shaking and damage on this Isoseismal map of thePunta Gorda earthquake.
2/18/95 - 8:03 pm PST, magnitude 6.4 - 6.6, located offshore 80 miles west southwest of Eureka, depth 12 miles. The earthquake was widely felt in coastal Del Norte, Humboldt and northern Mendocino Counties,CA and felt by a few persons as far north as Brookings, Oregon, as far south as the San Francisco Bay Area and in Shasta and Siskiyou Counties to the east. Even in the coastal communities of Petrolia, Ferndale and Eureka, however, shaking was generally described as "light", "mild" or "weak". There were reports of a few items knocked off shelves in Shelter Cove and in Patrick's Point north of Trinidad. There were no reports of damage. Our previous studies of North Coast earthquakes in the magnitude 6.5 to 7.1 range show that communities must be within 50 miles of the epicenter before significant damage occurs. In contrast, damage produced by the much smaller Dec. 26, 1994 Eureka earthquake (moment magnitude 5.4) located only 11 miles from Eureka, has topped $5 million in losses.
Fifteen aftershocks in the magnitude 3 or larger range have been recorded to date, most within the first eight hours after the earthquake. The largest aftershock was a magnitude 4.6 at 2:55 am on March 1. A number of small earthquakes located much closer to shore may have been triggered by the Feb. 18 earthquake. These included 4 small events located near the coast in the Cape Mendocino area, 3 others located onshore near Petrolia and 2 small earthquakes located along the northern portion of the San Andreas fault north of Fort Bragg.
The Feb. 18 earthquake was centered just north of the epicenter of the Sept. 1, 1994 Mendocino fault earthquake (moment magnitude 6.9) which had a very similar pattern of effects: a large felt area with no damage. The Sept. 1 earthquake was an interplate event rupturing about a 40 mile long segment of the Mendocino fault, the right-lateral east-west trending strike-slip structure marking the boundary between the Pacific and Gorda plates. The close proximity of the Feb. 18 epicenter suggests that it is an aftershock of that event. However, both the Berkeley and Harvard moment tensor solutions (an analysis of seismic waveforms which shows the type of faulting and possible orientation of fault planes) indicate that the Feb. 18 earthquake was not located on the same fault. It was an intraplate event located within the southern portion of the Gorda plate. Strike-slip faulting occurred along a nearly vertical fault oriented either to the northeast or northwest. The location of aftershocks is not currently sufficient to delineate the fault trend; however, the majority of recent Gorda earthquakes offshore of the coast have shown left-lateral slip along northeast trending faults and it is likely the the Feb. 18 earthquake is similar. This same type and orientation of faulting was responsible for both the 1980 Trinidad and the 1994 Eureka earthquakes.
2/8/95 - 1:36 am PST, magnitude 4.2 - 4.4, located offshore 11 miles west of Eureka, depth 11 -13 miles. This earthquake was an aftershock of the December 26, 1994 Eureka earthquake. The USGS has recorded 18 aftershocks since the December earthquake, only 4 of which have been magnitude 3.0 or larger. The 2/8 event is the largest magnitude aftershock to date. The earthquake awakened many persons in the Humboldt Bay region and was felt from Loleta, south of Eureka, to the Patrick's Point area north of Trinidad. The earthquake likely triggered a large landslide at Centerville Beach near Flenner Creek. There was no damage reported.
2/7/95 - 6:13 pm PST, magnitude 3.8 - 4.2, located onshore, 23 miles west of Weaverville or 43 miles east of Eureka, depth 13 to 18 miles beneath the earth's surface. The earthquake, about 50 miles east of the 2/8 earthquake, occurred along a deeply buried normal fault oriented nearly north-south most likely within the subducted portion of the Gorda plate. Previous studies (McPherson, 1989) have shown normal faulting to be typical of earthquakes within the subducted portion of the Gorda plate east of the coast where gravity exerts tension on the subducted plate. This earthquake was felt in Weaverville, Hyampom, Willow Creek and Trinidad. The earthquake was followed by a magnitude 2.4 earthquake at 6:54 pm in nearly the same location.
1/11/95 - 5:53 am PST, magnitude 4.2 - 4.4, located offshore, 27 miles west of Petrolia at a depth of 9 miles. This location puts the earthquake very close to the Mendocino fault although it is not clear whether this fault or an adjacent fault within the southernmost part of the Gorda plate was its source. The 1/11 earthquake is not an aftershock of the 12/26 Eureka earthquake which was located about 35 miles to the northeast. The earthquake was sharply felt in southern Humboldt County including the communities of Petrolia, Honeydew, Redway and Garberville. A few items were reportedly knocked off shelves in Ettersburg, about 30 miles southeast of Cape Mendocino. The earthquake was lightly felt in Eureka.
1/7/95 - 5:00 pm PST, magnitude 4.5, located far offshore, 165 miles west of Cape Mendocino along the Gorda ridge, the spreading center which separates the Gorda and Pacific plates. The earthquake was too far offshore to be felt. The Gorda ridge is seismically active but produces mainly very small magnitude earthquakes and this was one of the larger events which has been recorded.
12/26/94 - The Eureka Earthquake: 6:10 am PST, magnitude 5.3-5.4, located 12 miles WSW of Eureka at a depth of about 11 miles beneath the earth's surface. The location of the hypocenter (earthquake focus) puts it within the Gorda plate, on the order of 3 miles beneath the Cascadia subduction zone interface between the Gorda and North American plates. Slip occurred along a nearly vertical fault located entirely within the Gorda plate (intraplate earthquake). The Berkeley moment tensor "fault plane solution" and the location of aftershocks indicates that the fault was oriented NE - SW with the side to the north moving southwest relative to the rocks on the south side of the fault (left-lateral strike-slip motion). This orientation and sense of movement is similar to the 1980 Trinidad earthquake (see below). A slightly smaller intraplate Gorda earthquake (magnitude 4.7) occurred on December 16, located about 60 miles further west of the 12/26 event. but was too far away to be considered a foreshock. The largest aftershock to date was a magnitude 4.2 on February 8 (see above).
The Eureka earthquake was the largest earthquake to occur within a 15 mile radius of Eureka since 1932 and it produced the most damage in the Humboldt Bay region (which includes the cities of Arcata and Eureka) since the 1954 magnitude 6.5 earthquake which was centered about 12 miles northeast of Arcata. Historically, near-shore intraplate Gorda earthquakes have been the most common source of damage to communities in the Humboldt Bay region. Large damaging earthquakes in 1980 and 1932 were of this type and moderate magnitude earthquakes in 1975 near Fortuna and 1979 offshore of Arcata produced localized damage in the epicentral region similar to the December 26 earthquake.
You can see the distribution of shaking and damage on this Isoseismal map of the Eureka Earthquake.
Current damage estimates from the Eureka earthquake are in excess of $5,000,000. A State of Emergency was declared by the Governor. Most of the damages were within the city limits of Eureka, although a fire which destroyed a commercial building in Arcata 5 days after the earthquake has been linked to a gas leak exacerbated by the earthquake. Injuries were minor, consisting mainly of lacerations and bruises. Preliminary intensity data suggest Modified Mercalli values of VII in Eureka and at least VI in Arcata, Loleta and Fortuna. Preliminary analysis of strong motion instruments from the Myrtle area of Eureka show peak horizontal accelerations of 56% of gravity, 46% in the Eureka downtown area and 33% in Arcata. Duration of strong motion was exceedingly short - on the order of one to two seconds in Eureka.
9/27/94 - 4:25 am PDT, magnitude 3.6 located on shore, 11 miles SSE of Eureka at a depth of about 18 miles beneath the earth's surface. This was a mild event by North Coast standards. It was sharply felt in the Eureka area but caused no damage. It is interesting because it was an intraplate Gorda earthquake located to the east of the Mendocino triple junction. The Berkeley moment tensor for the 9/27 event shows NS oriented normal faulting. McPherson's 1989 Masters thesis on the seismicity of the North Coast region found this type of faulting typical of the Gorda plate east of Mendocino triple junction and the edge of the Pacific plate. Historically earthquakes in this area have been small in magnitude and relatively infrequent compared to offshore Gorda events.
9/1/94 - The Mendocino fault earthquake: 8:15 am, PDT, magnitude 6.9-7.2, located offshore about 90 miles west of Cape Mendocino. This earthquake was a major plate boundary (interplate) event centered along the Mendocino fault, the boundary between the Pacific plate to the south and the Gorda plate to the north. This is a very active fault with frequent small to moderate magnitude earthquakes and occasional large ones and accounts for about 30% of the seismic energy released annually in the North Coast area. The Mendocino fault is vertical and oriented east-west. Motion is almost entirely strike-slip with the Gorda plate side moving to the east relative to the Pacific plate side of the fault. Even though this was the largest magnitude earthquake located within the United States in 1994, it produced no damage due to its far-offshore location. The September 1 earthquake is the largest magnitude earthquake in historic times which can be clearly associated with the Mendocino fault. This earthquake was felt from south of the San Francisco Bay Area to Roseburg Oregon. A 6 inch wave, interpreted as a tsunami, was recorded by the Crescent City tide gauge about 45 minutes after the earthquake.
You can see the distribution of shaking and damage on this Isoseismal map of the Mendocino Fault Earthquake.
4/25/92 - Cape Mendocino earthquake (also called the Petrolia, Ferndale or Lost Coast earthquake: 11:06 am PDT, magnitude 7.1, located onshore near Petrolia at a depth of about 7 miles beneath the earth's surface. This earthquake was located very near the inferred position of the Cascadia subduction zone boundary between the Gorda plate and the North American plate and has been interpreted as the first major historic earthquake on the subduction zone. Motion was along a north-south oriented fault plane dipping gently down to the east. The North American plate was thrust up and over the Gorda plate beneath it. It produced measurable coastal uplift in the vicinity of Cape Mendocino on the order of 4 to 5 feet and a tsunami which was recorded at tide gauges from Port Orford, Oregon to Port San Luis near San Luis Obispo. Maximum recorded wave heights were just under 2 feet in Crescent City.
The mainshock was followed by two magnitude 6.6 aftershocks within 18 hours (4/26, 12:41 am and 4:18 am PDT). The aftershocks were both intraplate Gorda earthquakes located about 15 miles offshore of Cape Mendocino at depths on the order of 13 miles. Both aftershocks were strike slip earthquakes along vertical faults and at least the second exhibited right-lateral slip along a NW oriented fault. The mainshock and large aftershocks caused on the order of $60 million in damages and resulted in a federal disaster declaration.
You can see the distribution of shaking and damage on this Isoseismal map of the Cape Mendocino Earthquake.
9/17/91 - The Honeydew earthquake: 12:29 pm PDT, magnitude 6.0 - 6.2, located onshore about 13 miles SSE of Cape Mendocino at a depth of 7 miles. This earthquake was located in the immediate vicinity of the Mendocino triple junction only about 7 miles SW of the April 92 earthquake. Like the Cape Mendocino earthquake, it involved thrust motion along a NS oriented fault. There is some controversy as to the fault orientation. Surface features, such as the pattern of damage, surface ground cracks and displaced boulders along with the geologic structure in the region suggest a west dipping fault plane. Others, noting its proximity to the Cape Mendocino earthquake and the similarity of the focal mechanisms, have interpreted this earthquake as a foreshock and prefer the east dipping fault plane. Peak Modified Mercalli intensities were VIII in Honeydew, where over 60% of residents reported structural damage.
11/8/80 - Trinidad earthquake: 2:27 am PST, magnitude 7.1, located offshore about 30 miles west of Trinidad at a depth of about 12 miles. This earthquake was located within the Gorda plate and was caused by left-lateral slip along a 60 mile long vertical fault oriented northeast - southwest. A freeway overpass south of Eureka collapsed and regional damages were $1.75 million.
-------------
This report was complied by Lori Dengler, Department of Geology, Humboldt State University and is based on the work of many researchers and institutions including McPherson, 1989; Oppenheimer and others, 1993; Berkeley Seismographic Stations, U.S. Geological Survey, California Strong Motion Instrumentation Program and Humboldt State University. Information on recent earthquakes is preliminary and may change as more reports become available.
| HSU Home Page | University's Geology Home Page | Geology Department's Home Page | Return to Earthquake Information Page | Return to top of page | Watershed Management Council |
