The September 30, 1971 earthquake

M5.0near Westmorland, CA

A magnitude 5.0 earthquake 3.7 miles deep, centred in Imperial County. The 445th strongest in California’s record since 1900, and the strongest within 50 miles since a magnitude 5.5 on April 28, 1969. 15 aftershocks of magnitude 2.5 or more followed within 30 days.

Watch the sequence unfold

Magnitude 5.0 · September 30, 1971

How the ground moved

Every recorded earthquake in the sequence, in order, through the month after.

About this replay

Local time (PDT)—Historical replay — logged times

Main shock—

Aftershocks—of 15 in 30 days

Largest so far—

Sequence milestones

EarthquakesSized by magnitude; each flares as it strikes, then fades

Main shockMagnitude 5.0 at 3.7 miles deep

Shaking frontsThe faster and slower seismic waves, drawn at typical speeds of 6 and 3.5 km per second, not measured ones

Sequence areaEarthquakes within 20 miles are counted, the length of fault a magnitude 5.0 typically breaks

Source: U.S. Geological Survey ComCat catalogue, every event it holds within 20 miles of the epicentre from seven days before to 30 days after; before the 1970s only the larger aftershocks were recorded. Times as logged, shown in local time (PDT); the clock runs faster as the sequence slows. Towns are today’s Census places.

The earthquake

Magnitude: 5.0; local magnitude (ml)
Depth: 3.7 mi; 6.0 km below the surface
Local time: Thursday, September 30, 1971; 15:46 PDT; 22:46 UTC
Epicentre: 32.98°N, 115.80°W; In Imperial County
In California’s record: 445th; strongest of 97,065 since 1900
Within 50 miles: 2; counties; more on this date

USGS ANSS Comprehensive Earthquake Catalog, event ci3321788 (“18km WSW of Westmorland, 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

3.7

Largest aftershock, Oct 6, 1971

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 · 17.3 mi

Cities, largest first

El CentroCA · 19.6 mi
CalexicoCA · 27.3 mi
BrawleyCA · 16.0 mi
ImperialCA · 16.7 mi

Earthquakes around the epicentre

2,943 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.