Reviewed by USGS35y ago
M4.0 earthquake off the coast of Northern California
Monday, October 28, 1991, 05:32:09 UTC · 40.379°N, 125.079°W
A magnitude 4.0 earthquake struck off the coast of Northern California on Monday, October 28, 1991, 05:32:09 UTC, at a depth of 4.1 km (shallow).
- Magnitude
- 4.0
- Light · ml
- Depth
- 4 km
- Shallow
- Felt reports
- 0
- Did You Feel It?
- Est. shaking
- —
- No ShakeMap
- PAGER alert
- None
- Not issued
- Tsunami
- No flag
- USGS feed
Outlined circle: this earthquakeDashed circle: 50 km aftershock search area
In context
How big was it, and why here?
Energy released
15 tons of TNT
That is about 1/998 of the Hiroshima bomb. A magnitude 5.0 releases about 32 times more energy.
Tectonic setting
5 km
from the nearest mapped plate boundary, between the Pacific and Juan de Fuca plates. Earthquakes cluster along boundaries like this one, where plates collide, pull apart or slide past each other.
Depth: shallow
4 km
Shallow earthquakes (under 70 km) produce the strongest surface shaking for their size, so they are the ones most likely to cause damage near the epicenter.
Did You Feel It?
Where it was felt
No “Did You Feel It?” reports have been published for this event yet. If you felt it, you can add your report on the USGS website.
I felt it — report to USGS ↗USGS PAGER estimate
Who was exposed to shaking
USGS publishes PAGER impact estimates for significant earthquakes, usually within an hour. No PAGER estimate is available for this event.
After the main event
Aftershocks within 50 km
USGS lists 5 M2.5+ earthquakes within 50 km in the following 30 days; the largest was M4.0 on November 26, 1991. Most nearby activity right after a large earthquake is aftershocks, which usually become less frequent over days and weeks.
- 2.631 km SW of Ferndale, California35y ago · 47 km E of this epicenter
- 2.762 km WSW of Ferndale, California35y ago · 10 km E of this epicenter
- 4.059 km WSW of Ferndale, California35y ago · 15 km NE of this epicenter
- 3.267 km WSW of Ferndale, California35y ago · 6 km NE of this epicenter
- 2.664 km WSW of Ferndale, California35y ago · 15 km SE of this epicenter
Regional history
Past M5+ earthquakes within 300 km
USGS lists 155 M5.0+ earthquakes within 300 km before this one (catalog from 1900; older records are less complete). The strongest:
- 7.4123 km W of Big Lagoon, CaliforniaJanuary 31, 1922 · 100 km NW of this epicenter
- 7.2The 1980 Eureka, California EarthquakeNovember 8, 1980 · 108 km NE of this epicenter
- 7.1The 1923 Humboldt County, California EarthquakeJanuary 22, 1923 · 12 km N of this epicenter
- 7.0136 km WSW of Brookings, OregonAugust 17, 1991 · 159 km NW of this epicenter
- 6.9103 km W of Pistol River, OregonJuly 13, 1991 · 206 km N of this epicenter
- 6.8123 km W of Ferndale, CaliforniaNovember 25, 1954 · 54 km W of this epicenter
- 6.884 km W of Ferndale, CaliforniaFebruary 9, 1941 · 34 km NW of this epicenter
- 6.8off the coast of Northern CaliforniaJuly 6, 1934 · 96 km NW of this epicenter
History · NOAA
Deadliest earthquakes in this area
NOAA’s significant-earthquake database lists 14 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700, which together caused about 3 recorded deaths. The most severe:
- 1969M4.8
Santa Rosa
2 Oct 1969 · 295 km away · intensity VIII
1 death$8 million damage
- 1954M6.6
Humboldt County
21 Dec 1954 · 113 km away
1 death$2 million damage
- 1932M6.5
Eureka
6 Jun 1932 · 77 km away · intensity VIII
1 deathModerate damage ($1–5M)
- 1980M7.2
North Coast
8 Nov 1980 · 108 km away · intensity VII
$2 million damage
- 1991M6.2
Honeydew, Whitethorn, Petrolia
17 Aug 1991 · 64 km away · intensity VII
Moderate damage ($1–5M)
- 1865
Fort Humboldt, Eureka
1 Oct 1865 · 85 km away · intensity IX
Moderate damage ($1–5M)
NOAA NCEI Global Significant Earthquake Database. Historical figures are estimates; older records are incomplete.