Earthquake · reviewed by a seismologist
M 1.6md
Earthquake near Alamo, CA
No earthquake was likely felt in Alamo in the 24 hours to 3:30 am PST (11:30 UTC). The last one was M 2.1, 11 km (7 mi) NNW, at 4:05 am PST on 8 November 2026, estimated intensity II (weak) in town; 3 felt reports to USGS. Every earthquake near Alamo
This page: a magnitude 1.6 earthquake was recorded in Alamo, CA, at 2:48 pm PDT on 23 October 2021 (21:48 UTC), 8.8 km deep.
USGS: 1km SW of Alamo, CA
18 km (11 mi) ENE of Oakland, CA · 7 km (4 mi) S of Walnut Creek, CA
- When
- 2:48 pm PDT on 23 October 2021 ()
- Where
- in Alamo, CA · earthquakes near Alamo
- Magnitude
- M 1.6 (md)
- Depth
- 8.8 km below the surface
- Felt
- no reports yet
- Nearest
- Contra Costa County, 12 km NE
- Nearest mapped fault
- Calaveras fault, 2 km from its surface trace — near is not the same as on
- Status
- Reviewed by a seismologist
USGS · last changed by USGS · we first saw it 1835 days ago
Earthquake alerts for Alamo, CA
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Alerts are on for .
Not a warning. An alert comes after USGS publishes a quake, usually minutes after the shaking. For early warning use ShakeAlert, through your phone's Wireless Emergency Alerts or the MyShake app.
This was a minor earthquake. Quakes this small are rarely felt and do not cause damage.
At this magnitude and depth, people at rest would likely notice it out to about 6 km (4 mi) from the epicentre. Estimated from magnitude, depth and distance with the intensity equation USGS Did You Feel It? uses; felt reports, where there are any, are the measurement.
Likely felt in: Alamo in town, intensity II · Danville 4 km, intensity II. Estimated for each town centre; each link is that town's own record.
How big was it?
This earthquake released about 6.4 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 2.5 would have released about 32 times as much.
- Mineral, Virginia 2011 M 5.8 2,400,000× the energy of this one
- Magna, Utah 2020 M 5.7 1,700,000× the energy of this one
- Around the size people start to feel M 3.0 150× the energy of this one
- This earthquake M 1.6 about 6.4 lb of TNT
Energy from the Gutenberg–Richter relation (log10E = 1.5M + 4.8), rounded to two figures; bar length is magnitude, which is already a logarithm of energy. Magnitudes of past earthquakes from the USGS catalogue.
For this area
The largest earthquake within 50 km since 2016 was M 4.5 on 2019-10-15 — about 23,000 times the energy of this one.
Before 2016, the USGS record lists 6 earthquakes of M5 or more within 50 km; the five largest:
- 7.9 The 1906 San Francisco, California Earthquake · 46 km WSW of this epicentre
- 6.0 The 2014 South Napa, California Earthquake · 48 km NNW of this epicentre
- 5.8 The 1980 Livermore, California Earthquake · 24 km E of this epicentre
- 5.4 9 km NE of Livermore, California · 31 km ESE of this epicentre
- 5.3 The 1957 San Francisco, California Earthquake · 43 km WSW of this epicentre
Since 2016: the QuakeIndex catalogue (US, M1.5+), built from the USGS FDSN event service. Before 2016: USGS ComCat, M5+ from 1900. Distances are between epicentres. This is the record, not a forecast.
Where
Epicentre 37.8435, -122.0423. Map © OpenStreetMap contributors.
Nearby places
| Place | Distance | Direction from the epicentre | |
|---|---|---|---|
| Contra Costa County, CA | 12 km (7 mi) | NE | earthquakes near Contra Costa County |
| Alameda County, CA | 25 km (15 mi) | SSE | earthquakes near Alameda County |
| Solano County, CA | 48 km (30 mi) | N | earthquakes near Solano County |
| San Mateo County, CA | 56 km (35 mi) | SSW | earthquakes near San Mateo County |
| Marin County, CA | 66 km (41 mi) | WNW | earthquakes near Marin County |
| San Joaquin County, CA | 68 km (42 mi) | E | earthquakes near San Joaquin County |
Places and coordinates from the US Census Bureau Gazetteer (2024). Distances are from the epicentre.
Aftershocks so far
55 earthquakes have been recorded within 50 km since this one.
- 1.7 11km NE of Milpitas, CA
- 2.6 2km SW of Colma, CA
- 1.5 10km ENE of Milpitas, CA
- 1.5 2km S of Alamo, CA
- 2.3 4km NE of Fairview, CA
- 1.6 2km NE of San Ramon, CA
- 1.9 1km NW of Danville, CA
- 1.7 1km WNW of Danville, CA
- 2.0 1km N of Pacifica, CA
- 1.6 6km E of Vallejo, CA
- 2.3 2km SW of Discovery Bay, CA
- 1.9 6km E of Vallejo, CA
- 1.6 3km SE of Concord, CA
- 2.2 2km E of Pacheco, CA
- 1.6 6km WSW of Colma, CA
Counted from the QuakeIndex database, which ingests the USGS feed every 30 seconds · as of . QuakeIndex does not forecast aftershocks. When USGS issues an operational aftershock forecast it appears on the USGS event page.
On USGS
- The official USGS event page — the source of record for every value above
- Did you feel it? Report it to USGS — reports go to USGS, not to us
- ShakeMap for this event, when USGS has produced one
QuakeIndex does not issue warnings. For earthquake early warning see ShakeAlert.