A magnitude 5.0 earthquake struck in the Pacific about 111 kilometers (69 miles) west of Petrolia, California, at 8:56 p.m. Pacific time on Nov. 4, which was 04:56 UTC on Nov. 5. The U.S. Geological Survey reviewed the event and counted 142 felt reports by late Thursday, with a green alert level that signals little expected damage. No foreshocks or aftershocks of magnitude 3.5 or larger appeared in the surrounding offshore box.
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USGS record: 4.96 Mw, 10 kilometers deep
The record for event nc75462637, contributed by the Northern California Seismic System network, lists a preferred moment magnitude of 4.96 that the USGS rounds to M 5.0 in the event name. The epicenter sits at 40.407 degrees north, 125.585 degrees west, and the depth is 10 kilometers, a value the record marks as operator-assigned rather than solved from the waveforms. The solution used 136 phases from 134 stations, and its moment tensor shows about 95 percent double-couple motion.
That mix points to a slip on a fault, not a volcanic or explosive source. The event status is reviewed, meaning a seismologist has checked the automatic solution. The magnitude rests on 39 stations, and the azimuthal gap of 246 degrees means the recording stations cover only part of the compass around the epicenter, an expected limitation for a quake this far offshore where every seismometer sits on land to the east. The epicenter lies at nearly the same latitude as Cape Mendocino, well out to sea from the Humboldt County community of Petrolia, which is why the USGS names the nearest town rather than any offshore landmark.
142 felt reports and a green alert level
The felt count has moved. An early pull of the USGS event feed showed 140 felt reports, and the event record had risen to 142 by the time it was last updated at 00:02 UTC on Nov. 6, which was 4:02 p.m. Pacific time on Nov. 5. The USGS event page compiles those through its DYFI community felt-report system, in which residents describe what they noticed, and the community intensity product shows a maximum reported intensity of 3.8, which falls in the weak-shaking range.
The model-based rating agrees with the survey of residents. The USGS PAGER estimate for the quake is green, and the calculated instrumental intensity at the strongest point is about 2.8, with a significance score of 432. The feed summary does not break the 142 reports down by town, so which Humboldt and Mendocino county communities reported the shaking is not stated in the record.
Where three plates meet off Cape Mendocino
The epicenter lies in water west of the Mendocino Triple Junction, where the Pacific, North American and Gorda plates meet. The Berkeley Seismological Laboratory’s explainer, Temblors Where Three Plates Meet, describes the Gorda Plate as a small cousin of the Pacific Plate that dives beneath North America in the Cascadia subduction zone. The Gorda Plate is squeezed from two sides, the North American Plate from the east and the Pacific Plate pushing north, and that stress has cracked it into a network of faults.
The same page notes that the Mendocino fault, a transform running roughly east to west into the Pacific, carries the Gorda Plate east relative to the Pacific Plate at about 2 inches a year. Earthquakes along it show right-lateral motion, while quakes inside the Gorda Plate show left-lateral motion on southwest-to-northeast faults. The USGS record for this event does not name the fault that slipped, so the setting is regional context rather than a finding.
Against that backdrop, a magnitude 5.0 is a routine event. The Berkeley page lists the offshore network of faults among the sources of the 1994 magnitude 7.0 earthquake, about 60 miles west of Cape Mendocino, and of a magnitude 6.5 shock on Dec. 8, 2016, which Humboldt County residents felt without serious damage or injuries. The Nov. 4 quake released far less energy than either, which fits its weak reported intensities of 3.8 or lower and a PAGER rating of green.
The tsunami flag and the aftershock check
The USGS record carries a tsunami flag of 1, which the agency uses to indicate that a National Tsunami Warning Center information statement is linked to the event. The flag is not a warning by itself. The Berkeley explainer says mostly horizontal motion on these structures does not pose a significant tsunami risk, and nothing in the USGS record reports a wave.
A second query for magnitude 3.5 and larger events between Oct. 29 and Nov. 6 across the north coast and offshore box returned only this earthquake, so no foreshock or aftershock reached that threshold. USGS also lists aftershock forecast products for the event, so a probability table for further shaking is available to anyone on the north coast who wants one. Whether the felt count climbs further as more residents file reports is the open number to watch.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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