Reviewed by USGSCalifornia42y ago
M4.5 earthquake 251 km W of Ferndale, California
Thursday, September 20, 1984, 19:04:35 UTC · 40.276°N, 127.205°W
A magnitude 4.5 earthquake struck 251 km W of Ferndale, California on Thursday, September 20, 1984, 19:04:35 UTC, at a depth of 10.0 km (shallow). It is the strongest earthquake within 300 km since an M4.8 on September 20, 1984.
- Magnitude
- 4.5
- Light · mb
- Depth
- 10 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
85 tons of TNT
That is about 1/178 of the Hiroshima bomb. A magnitude 5.0 releases about 6 times more energy.
Tectonic setting
17 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
10 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 2 M2.5+ earthquakes within 50 km in the following 30 days; the largest was M4.0 on September 26, 1984. Most nearby activity right after a large earthquake is aftershocks, which usually become less frequent over days and weeks.
Regional history
Past M5+ earthquakes within 300 km
USGS lists 135 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 · 169 km NE of this epicenter
- 7.2The 1980 Eureka, California EarthquakeNovember 8, 1980 · 266 km E of this epicenter
- 7.1The 1923 Humboldt County, California EarthquakeJanuary 22, 1923 · 185 km E of this epicenter
- 6.8123 km W of Ferndale, CaliforniaNovember 25, 1954 · 128 km E of this epicenter
- 6.884 km W of Ferndale, CaliforniaFebruary 9, 1941 · 170 km E of this epicenter
- 6.8off the coast of Northern CaliforniaJuly 6, 1934 · 167 km NE of this epicenter
- 6.7217 km W of Ferndale, CaliforniaSeptember 10, 1984 · 41 km NE of this epicenter
- 6.552 km WNW of Big Lagoon, CaliforniaJuly 15, 1918 · 239 km NE of this epicenter
History · NOAA
Deadliest earthquakes in this area
NOAA’s significant-earthquake database lists 9 damaging, deadly or tsunami-generating earthquakes within 300 km of this epicenter since 1700, which together caused about 2 recorded deaths. The most severe:
- 1954M6.6
Humboldt County
21 Dec 1954 · 287 km away
1 death$2 million damage
- 1932M6.5
Eureka
6 Jun 1932 · 252 km away · intensity VIII
1 deathModerate damage ($1–5M)
- 1980M7.2
North Coast
8 Nov 1980 · 266 km away · intensity VII
$2 million damage
- 1865
Fort Humboldt, Eureka
1 Oct 1865 · 242 km away · intensity IX
Moderate damage ($1–5M)
- 1922M7.6
Northern
31 Jan 1922 · 165 km away
Limited damage (under $1M)
- 1923M7.1
Northern
22 Jan 1923 · 185 km away · intensity VIII
Limited damage (under $1M)tsunami
NOAA NCEI Global Significant Earthquake Database. Historical figures are estimates; older records are incomplete.