M3.9 earthquake 76km W of Petrolia, CA

A magnitude 3.9 earthquake struck 76km W of Petrolia, CA on Thu, Aug 5, 2021, 8:21 AM PDT (15:21 UTC), 8 km (5 mi) below the surface.

A larger quake followed: M4.5 10km SW of Petrolia, CA. This one was a foreshock.

Map

Where this earthquake was located, with the others nearby. Select one for its page.

Different scales measure different things and differ by a few tenths. The USGS marks one as preferred, and it is the one this page uses.

Mag.	Scale	Computed by	Notes
3.88	Mw, moment	Northern California Seismic System	Preferred · Final · 4 stations
3.9	Mwr, regional moment	USGS National Earthquake Information Center	Reviewed · 4 stations

The USGS ShakeMap: shaking estimated from seismic stations and the quake’s size, on the Modified Mercalli intensity scale.

2 people reported to the USGS what they felt; the strongest reported shaking was III, weak shaking.

How the rock slipped, from the moment tensor, and the USGS estimate of landslides and liquefaction.

Reviewed by a seismologist.

Distance and direction from each place to the epicenter, with its forecast.

Within 100 km, magnitude 1.5 and up: 60 in the 60 days after this one and 35 in the 30 days before it.

From the USGS Earthquake Hazards Program and its partner networks, event nc73606111 (also filed as us6000f2hq). Read Mon, Oct 5, 2026, 7:38 AM UTC.