Earthquake · reviewed by a seismologist
M 2.7md
Earthquake near Byron, CA
No earthquake was likely felt in Byron in the 24 hours to 3:39 am PST (11:39 UTC). The last one was M 3.5, 53 km (33 mi) SSE, at 7:21 am PDT on 27 September 2026, estimated intensity II (weak) in town; 8 felt reports to USGS. Every earthquake near Byron
This page: a magnitude 2.7 earthquake was recorded in Byron, CA, at 9:36 am PDT on 4 October 2021 (16:36 UTC), 4.4 km deep. 48 people have reported feeling it to USGS.
USGS: 0km N of Byron, CA
10 km (6 mi) SE of Brentwood, CA · 66 km (41 mi) NNE of San Jose, CA
- When
- 9:36 am PDT on 4 October 2021 ()
- Where
- in Byron, CA · earthquakes near Byron
- Magnitude
- M 2.7 (md)
- Depth
- 4.4 km below the surface
- Felt
- 48 reports · strongest reported intensity III
- Nearest
- Contra Costa County, 28 km W
- Nearest mapped fault
- Greenville fault, 16 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 1798 days ago
Earthquake alerts for Byron, CA
Get a notification in this browser when USGS records an earthquake near Byron, CA. No account or email needed.
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 small earthquake. Shaking like this is often felt indoors nearby and very rarely causes damage.
At this magnitude and depth, people at rest would likely notice it out to about 24 km (15 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: Discovery Bay 5 km, intensity III · Brentwood 8 km, intensity III · Oakley 15 km, intensity II · Antioch 21 km, intensity II · Tracy 24 km, intensity II · Livermore 24 km, intensity II. Estimated for each town centre; each link is that town's own record.
How big was it?
This earthquake released about 330 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 3.7 would have released about 32 times as much.
- Mineral, Virginia 2011 M 5.8 46,000× the energy of this one
- Magna, Utah 2020 M 5.7 33,000× the energy of this one
- Around the size people start to feel M 3.0 2.9× the energy of this one
- This earthquake M 2.7 about 330 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 450 times the energy of this one.
Before 2016, the USGS record lists 4 earthquakes of M5 or more within 50 km:
- 5.8 The 1980 Livermore, California Earthquake · 12 km WSW of this epicentre
- 5.7 15 km NE of East Foothills, California · 44 km S of this epicentre
- 5.4 9 km NE of Livermore, California · 15 km SSW of this epicentre
- 5.1 11 km E of Blackhawk, California · 14 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 37.8718, -121.6375. Map © OpenStreetMap contributors.
Nearby places
| Place | Distance | Direction from the epicentre | |
|---|---|---|---|
| Contra Costa County, CA | 28 km (17 mi) | W | earthquakes near Contra Costa County |
| San Joaquin County, CA | 33 km (20 mi) | ENE | earthquakes near San Joaquin County |
| Alameda County, CA | 35 km (22 mi) | SW | earthquakes near Alameda County |
| Solano County, CA | 51 km (32 mi) | NNW | earthquakes near Solano County |
| Stanislaus County, CA | 66 km (41 mi) | ESE | earthquakes near Stanislaus County |
| Sacramento County, CA | 69 km (43 mi) | NNE | earthquakes near Sacramento 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.
- 1.5 2km NE of Union City, CA
- 1.7 11km NE of Milpitas, 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
- 2.3 2km SW of Discovery Bay, CA
- 1.6 3km SE of Concord, CA
- 2.2 2km E of Pacheco, CA
- 2.5 4km N of San Leandro, CA
- 1.8 1km NE of San Ramon, CA
- 1.9 7km ESE of Bethel Island, CA
- 1.9 8km SSW of Brentwood, 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.