A strong earthquake struck the north coast late Tuesday evening, January 21. The earthquake, at 11:17 PM PST, was located just off the coast in the Cape Mendocino region, about 1 mile NW of Punta Gorda or 6 miles SW of Petrolia. The magnitude was initially estimated by the USGS to be 4.9 based on preliminary information. The magnitude value was revised by the Berkeley Seismographic Station and Harvard and are now estimated in the 5.6 to 5.7 range. The earthquake was located 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. Some objects were reported toppled over or knocked from shelves in the Petrolia area and a market in Scotia reported several broken bottles. The earthquake was also felt widely throughout the Humboldt Bay region and by some as far north as Trinidad. No significant damage has been reported although we have had a few reports of minor cracks. The Berkeley Seismographic Station's analysis of the earthquake showed that it was rich in low frequencies (long period waves) which helps to explain the discrepancies between the initial magnitude estimates which are based on shorter period waves. It also helps collaborates the reports of many residents who described the earthquake as "long, slow, rolling motion". The Humboldt Earthquake Education Center is currently conducting a study of the pattern of ground shaking produced by this earthquake. Persons interested in contributing should fill out our Felt Report form.
You can see the distribution of shaking and damage on this Isoseismal map of the Punta Gorda earthquake.
The Punta Gorda earthquake was followed by a number of aftershocks. The largest to date were a magnitude 5.2 earthquake which occurred on Saturday, January 25 at 10:23 PM, and a magnitude 4.6 on Wednesday January 22 at 12 AM. It was preceded by a magnitude 4.2 foreshock on January 13.
The Punta Gorda Sequence to date (magnitude 2.5 or larger events):
1 Jan. 13 6:45 AM mag. 4.2 Foreshock 2 Jan. 21 11:17 PM mag. 5.6 Main shock 3 Jan. 21 11:25 PM mag. 3.0 Aftershock 4 Jan. 22 12:00 AM mag. 4.6 " 5 Jan. 22 12:20 AM mag. 2.8 " 6 Jan. 22 12:57 AM mag. 2.8 " 7 Jan. 22 10:21 AM mag. 3.1 " 8 Jan. 22 5:13 PM mag. 2.6 " 9 Jan. 23 6:23 AM mag. 2.9 " 10 Jan. 25 10:23 PM mag. 5.2 " 11 Jan. 26 1:33 PM mag. 2.9 "
The Punta Gorda earthquake was the largest earthquake to occur on the north coast since a magnitude 6.0 earthquake on July 24, 1996. That earthquake was located about 115 miles west of Crescent City in the Gorda plate and was too far away from the coast to be felt. The last widely felt earthquake was a magnitude 6.6 earthquake on February 18, 1995 which was located offshore about 80 miles west of Cape Mendocino. The current level of seismicity on the north coast is not at all unusual. In an "average" year, the coastal and offshore areas of Humboldt and Del Norte Counties CA are likely to experience on the order of 100 magnitude 3 or larger earthquakes, and nearly 20 of magnitude 4 or larger. 1996 was an unusually quiet year for this region with only 76 magnitude 3 or larger earthquakes recorded. The Punta Gorda sequence is located in one of the most seismically active regions in the United States and does not in itself suggest increased probabilities for future earthquakes. However, large earthquakes are a fact of life on the north coast and residents should take actions to prepare their families, homes, schools and workplaces for the next inevitable quake.
For further information on earthquake preparedness, contact your local county Office of Emergency Services (look in the Government Section of your telephone directory), the Governor's Office of Emergency Services Coastal Region at (510) 286-0895, or the Humboldt Earthquake Education Center at (707) 826-6019.
Lori Dengler
Last updated: 13 February 1997
