The April 8, 1968 earthquake

M6.6Borrego Mountain, California

A magnitude 6.6 earthquake 6.2 miles deep, centred in San Diego County. The 19th strongest in California’s record since 1900, and the strongest within 50 miles since a magnitude 6.6 on October 21, 1942. 119 aftershocks of magnitude 2.5 or more followed within 30 days.

Watch the sequence unfold

Magnitude 6.6 · April 8, 1968

How the ground moved

Every recorded earthquake in the sequence, in order, from the foreshocks to a month after.

About this replay

Local time (PST)—Historical replay — logged times

Main shock—

Aftershocks—of 119 in 30 days

Largest so far—

Sequence milestones

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

Main shockMagnitude 6.6 at 6.2 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 6.6 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 (PST); the clock runs faster as the sequence slows. Towns are today’s Census places.

The earthquake

Magnitude: 6.6; moment magnitude (mw)
Depth: 6.2 mi; 10.0 km below the surface
Local time: Monday, April 8, 1968; 18:28 PST; 02:28 UTC on Apr 9, 1968
Epicentre: 33.18°N, 116.10°W; In San Diego County
In California’s record: 19th; strongest of 97,065 since 1900
Within 50 miles: 2; counties; more on this date

USGS ANSS Comprehensive Earthquake Catalog, event ci3329122 (“The 1968 Borrego Mountain, California Earthquake”). 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.

119

Aftershocks in 30 days

4.5

Largest aftershock, Apr 9, 1968

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

San Diego CountyCA · inside
Imperial CountyCA · 7.3 mi

Cities, largest first

IndioCA · 38.9 mi
Palm DesertCA · 41.4 mi
Cathedral CityCA · 49.8 mi
CoachellaCA · 35.4 mi
Palm SpringsCA · 49.9 mi
El CentroCA · 41.6 mi
La QuintaCA · 33.4 mi
CalexicoCA · 49.1 mi
BrawleyCA · 36.0 mi
ImperialCA · 38.8 mi
Rancho MirageCA · 44.5 mi

Earthquakes around the epicentre

2,944 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.