M4.0 earthquake 9 km WSW of Cobb, California

A magnitude 4.0 earthquake struck 9 km WSW of Cobb, California on Sat, Sep 22, 1984, 11:52 AM PDT (18:52 UTC), 2 km (1 mi) below the surface.

A larger quake followed: M4.3 11 km ENE of Clearlake, California. 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
4.0	ML, local (Richter)	Northern California Seismic System	Preferred · Final · 0 stations
4.2	mb, body-wave	USGS National Earthquake Information Center	Reviewed · 1 stations

The USGS’s account of the effects of this earthquake.

Felt (V) at Cobb, (IV) at Loch Lomond and Geyserville and (III) at Navarro and Cloverdale.

From the USGS event page for nc28542. U.S. Geological Survey text and figures are in the public domain.

No one has reported feeling this quake to the USGS.

Reviewed by a seismologist.

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

Within 100 km, magnitude 1.5 and up: 167 in the 60 days after this one and 89 in the 30 days before it.

The first 60 of 167.

From the USGS Earthquake Hazards Program and its partner networks, event nc28542 (also filed as usp00027m3). Read Sun, Oct 4, 2026, 8:56 PM UTC.