M1.0 earthquake 4 km E of Saint Helena, California, United States
A magnitude 1.0 earthquake struck 4 km E of Saint Helena, California, United States at 03:03 local time (10:03 UTC) on 23 Sept 2026, at a depth of 7 km, according to USGS.
Data: USGS · updated 2026-09-23 18:12 UTC · reviewed
- Magnitude
- 1.0micro
duration magnitude (md) - Depth
- 6.9 km
- Status
- Reviewed
- Origin time
- 2026-09-23 10:03:52 UTC
- Local: 23 Sept 2026, 03:03:52 (America/Los Angeles)
- Epicentre
- 38.509° N, 122.423° W
Circles show this event and others within 30 km from 7 days before to 30 days after. Lines: plate boundaries and GEM active faults. Open on the live map
Nearby places
| Place | Distance | Est. shaking |
|---|---|---|
| Saint HelenaCalifornia, United States · pop. 5,814 | 4 km | INot felt |
| CalistogaCalifornia, United States · pop. 5,330 | 16 km | INot felt |
| Agua CalienteCalifornia, United States · pop. 27,090 | 22 km | INot felt |
| NapaCalifornia, United States · pop. 80,434 | 26 km | INot felt |
| Santa RosaCalifornia, United States · pop. 178,127 | 26 km | INot felt |
| Rohnert ParkCalifornia, United States · pop. 42,407 | 31 km | INot felt |
| OaklandCalifornia, United States · pop. 419,267 | 80 km | INot felt |
| SacramentoCalifornia, United States · pop. 524,943 | 82 km | INot felt |
Shaking is an estimate from the intensity prediction equation of Allen, Wald & Worden (2012), not an observation. Local ground and buildings can change it by a level or more.
Every agency's solution
Agencies locate earthquakes with their own stations and methods, so their numbers differ. We match reports of the same event by time, distance and magnitude and show them side by side. The headline uses USGS · NC, which has been reviewed by an analyst.
| Agency | Magnitude | Depth | Epicentre | Time offset | Status |
|---|---|---|---|---|---|
| USGS · NCheadlineNorthern California Seismic System via U.S. Geological Survey | 1.0 md | 6.9 km | 38.51° N, 122.42° W | – | Reviewed |
Advertisement
How big was it?
Usually not felt. Recorded by nearby seismometers; thousands occur worldwide every day.
It released roughly the energy of 461 g of TNT. Magnitude is logarithmic: each whole step means about 32 times more energy and 10 times larger waves on a seismogram. Depth matters too: at 7 km this was a shallow earthquake, which is felt more strongly at the surface than a deeper one of the same size.
M1.0 releases about 1× the energy of M1.0, and its waves are about 1× taller on a seismogram. Energy: 477 g of TNT vs 461 g of TNT.
Each square is the energy of one M1.0. Energy from log₁₀E = 1.5M + 4.8.
Aftershocks and nearby activity
2 later earthquakes within 30 km; the largest was M2.2 1 day ago. 2 in the 7 days before. Aftershock rates usually fall off roughly in proportion to 1/time.
Mainshock M1.0
2 later earthquakes nearby. Aftershock rates typically fall off roughly as 1/time (Omori's law).
| Time after mainshock (hours) | Magnitude | Depth (km) |
|---|---|---|
| 3.11 | 2.2 | 8 |
| 108.11 | 2.2 | 8 |
Did you feel it?
Reports from people help show where shaking was strong. They are anonymous and shown here in aggregate.
Reports on SeismoAtlas
No reports yet.
Event id nc75440292. First recorded here 28 Sept 2026, 17:32:11 UTC. Solutions: USGS · NC (usgs:nc75440292). Cite as: SeismoAtlas, “M1.0 earthquake 4 km E of Saint Helena, California, United States”, https://seismoatlas.com/earthquake/nc75440292-m1-0-earthquake-4-km-e-of-saint-helena-california-united-states, data from USGS.
JSON: /api/v1/events/nc75440292. SeismoAtlas is not an official warning service.