The July 21, 1986 earthquake

M6.4Chalfant Valley, California

A magnitude 6.4 earthquake 5.5 miles deep, centred in Mono County. The 33rd strongest in California’s record since 1900, and the strongest within 50 miles in the catalogue, which runs from 1900. 602 aftershocks of magnitude 2.5 or more followed within 30 days.

Watch the sequence unfold

Magnitude 6.4 · July 21, 1986

How the ground moved

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

About this replay

Local time (PDT)—Historical replay — logged times

Main shock—

Aftershocks—of 602 in 30 days

Largest so far—

Sequence milestones

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

Main shockMagnitude 6.4 at 5.5 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 6.4 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. 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: 6.4; local magnitude (ml)
Depth: 5.5 mi; 8.8 km below the surface
Local time: Monday, July 21, 1986; 07:42 PDT; 14:42 UTC
Epicentre: 37.54°N, 118.44°W; In Mono County
In California’s record: 33rd; strongest of 97,065 since 1900
Within 50 miles: 7; counties; more on this date

USGS ANSS Comprehensive Earthquake Catalog, event nc10085763 (“The 1986 Chalfant Valley, 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.

602

Aftershocks in 30 days

5.8

Largest aftershock, Jul 31, 1986 · its page

271

On the first day

101

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.

Counties and cities within 50 miles

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

Counties

Mono CountyCA · inside
Fresno CountyCA · 18.4 mi
Inyo CountyCA · 18.8 mi
Esmeralda CountyNV · 24.8 mi
Mineral CountyNV · 24.8 mi
Madera CountyCA · 31.9 mi
Tuolumne CountyCA · 45.2 mi

Earthquakes around the epicentre

3,992 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.