Earthquake · reviewed by a seismologist
M 2.1md
Earthquake near Alamo, CA
No earthquake was likely felt in Alamo in the 24 hours to 10:09 am PST (18:09 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 2.1 earthquake was recorded 4 km (2 mi) west-southwest of Alamo, CA, at 5:32 pm PDT on 7 October 2021 (00:32 UTC), 15 km deep. 3 people have reported feeling it to USGS.
USGS: 4km WSW of Alamo, CA
16 km (10 mi) ENE of Oakland, CA · 8 km (5 mi) SSW of Walnut Creek, CA
- When
- 5:32 pm PDT on 7 October 2021 ()
- Where
- 4 km (2 mi) west-southwest of Alamo, CA · earthquakes near Alamo
- Magnitude
- M 2.1 (md)
- Depth
- 15 km below the surface
- Felt
- 3 reports · strongest reported intensity II
- Nearest
- Contra Costa County, 14 km NE
- Nearest mapped fault
- Calaveras fault, 3 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 1861 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 9 km (6 mi) from the epicentre. In Alamo, 4 km away, the estimate is intensity II (weak). 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 4 km, intensity II · Danville 6 km, intensity II · Lafayette 7 km, intensity II · Walnut Creek 8 km, intensity II. Estimated for each town centre; each link is that town's own record.
How big was it?
This earthquake released about 49 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 3.1 would have released about 32 times as much.
- Mineral, Virginia 2011 M 5.8 310,000× the energy of this one
- Magna, Utah 2020 M 5.7 220,000× the energy of this one
- Around the size people start to feel M 3.0 19× the energy of this one
- This earthquake M 2.1 about 49 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 3,000 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 · 43 km WSW of this epicentre
- 6.0 The 2014 South Napa, California Earthquake · 47 km NNW of this epicentre
- 5.8 The 1980 Livermore, California Earthquake · 26 km E of this epicentre
- 5.4 9 km NE of Livermore, California · 33 km ESE of this epicentre
- 5.3 The 1957 San Francisco, California Earthquake · 41 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 37.8340, -122.0682. Map © OpenStreetMap contributors.
Nearby places
| Place | Distance | Direction from the epicentre | |
|---|---|---|---|
| Contra Costa County, CA | 14 km (9 mi) | NE | earthquakes near Contra Costa County |
| Alameda County, CA | 25 km (15 mi) | SSE | earthquakes near Alameda County |
| Solano County, CA | 49 km (31 mi) | NNE | earthquakes near Solano County |
| San Mateo County, CA | 54 km (33 mi) | SSW | earthquakes near San Mateo County |
| Marin County, CA | 64 km (40 mi) | WNW | earthquakes near Marin County |
| San Joaquin County, CA | 71 km (44 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
57 earthquakes have been recorded within 50 km since this one.
- 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
- 2.5 4km N of San Leandro, CA
- 1.8 1km NE of San Ramon, CA
- 2.0 1km W of Kensington, CA
- 2.5 1km W of Kensington, CA
- 1.7 5km ENE of Vallejo, CA
- 1.9 7km ESE of Bethel Island, CA
- 1.9 8km SSW of Brentwood, CA
- 1.7 5km E of Fremont, CA
- 1.7 8km WSW of Mountain House, CA
- 1.6 8km E of Vallejo, CA
- 1.9 1km NE of San Ramon, 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.