The October 21, 1942 earthquake

M5.6near Niland, CA

A magnitude 5.6 earthquake 3.7 miles deep, centred in Imperial County. The 157th strongest in California’s record since 1900, and the strongest within 50 miles since a magnitude 6.6 on October 21, 1942. 7 aftershocks of magnitude 2.5 or more followed within 30 days.

The earthquake

Magnitude: 5.6; local magnitude (ml)
Depth: 3.7 mi; 6.0 km below the surface
Local time: Wednesday, October 21, 1942; 18:50 PWT; 01:50 UTC on Oct 22, 1942
Epicentre: 33.27°N, 115.57°W; In Imperial County
In California’s record: 157th; strongest of 97,065 since 1900
Within 50 miles: 3; counties; more on this date

USGS ANSS Comprehensive Earthquake Catalog, event ci3366107 (“6km WNW of Niland, CA”). The USGS event page carries the ShakeMap, felt reports and the official summary.

Aftershocks

Earthquakes of magnitude 2.5 or more within 20 miles of the epicentre in the 30 days after.

Aftershocks in 30 days

4.4

Largest aftershock, Oct 22, 1942

On the first day

In the 7 days before

The search radius is the surface rupture length expected for an earthquake of this magnitude (Wells and Coppersmith, 1994), at least 20 and at most 600 miles, so a great earthquake's sequence is counted along its whole fault.
Before the modern seismic networks of the 1970s many small aftershocks went unrecorded, so early sequences read short.

Counties and cities within 50 miles

Distance from the epicentre to each county boundary (zero when inside) and to each city centre.

Counties

Imperial CountyCA · inside
San Diego CountyCA · 31.6 mi
La Paz CountyAZ · 48.6 mi

Cities, largest first

IndioCA · 50.0 mi
CoachellaCA · 44.1 mi
El CentroCA · 33.2 mi
La QuintaCA · 48.1 mi
CalexicoCA · 40.4 mi
BrawleyCA · 20.0 mi
ImperialCA · 29.5 mi

Earthquakes around the epicentre

2,512 earthquakes of magnitude 3 or more within 50 miles of this epicentre since 1973.

Earlier earthquakes of magnitude 5 or more within 50 miles, strongest first

USGS ANSS Comprehensive Earthquake Catalog. Local time is the time zone of the county nearest the epicentre. Magnitudes are as the catalogue reports them.