The August 17, 1959 earthquake

M7.2Hebgen Lake, Montana

A magnitude 7.2 earthquake 7.5 miles deep, centred in Gallatin County. The strongest in Montana’s record since 1900, and the strongest within 50 miles in the catalogue, which runs from 1900. 10 aftershocks of magnitude 2.5 or more followed within 30 days.

Watch the sequence unfold

Magnitude 7.2 · August 17, 1959

How the ground moved

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

About this replay

Local time (MST)—Historical replay — logged times

Main shock—

Aftershocks—of 10 in 30 days

Largest so far—

Sequence milestones

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

Main shockMagnitude 7.2 at 7.5 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 35 miles are counted, the length of fault a magnitude 7.2 typically breaks

Source: U.S. Geological Survey ComCat catalogue, every event it holds within 35 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 (MST); the clock runs faster as the sequence slows. Towns are today’s Census places.

The earthquake

Magnitude: 7.2; moment magnitude (mw)
Depth: 7.5 mi; 12.0 km below the surface
Local time: Monday, August 17, 1959; 23:37 MST; 06:37 UTC on Aug 18, 1959
Epicentre: 44.79°N, 111.20°W; In Gallatin County
In Montana’s record: 1st; strongest of 1,459 since 1900
Within 50 miles: 8; counties; more on this date

USGS ANSS Comprehensive Earthquake Catalog, event official19590818063714000_12 (“The 1959 Hebgen Lake, Montana Earthquake”). The USGS event page carries the ShakeMap, felt reports and the official summary.

Aftershocks

Earthquakes of magnitude 2.5 or more within 35 miles of the epicentre in the 30 days after.

Aftershocks in 30 days

6.5

Largest aftershock, Aug 18, 1959 · its page

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

Gallatin CountyMT · inside
Fremont CountyID · 7.5 mi
Madison CountyMT · 9.1 mi
Teton CountyWY · 11.3 mi
Park CountyWY · 11.3 mi
Beaverhead CountyMT · 14.7 mi
Park CountyMT · 16.3 mi
Clark CountyID · 26.5 mi

Earthquakes around the epicentre

421 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

November 23, 1947M6.1

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