The August 18, 1959 earthquake
M6.5near Madison Junction, Wyoming
A magnitude 6.5 earthquake 6.2 miles deep, centred in Park County. The strongest in Wyoming’s record since 1900, and the strongest within 50 miles since a magnitude 7.2 on August 17, 1959. 5 aftershocks of magnitude 2.5 or more followed within 30 days.
The earthquake
- Magnitude
- 6.5
- as reported (uk)
- Depth
- 6.2 mi
- 10.0 km below the surface
- Local time
- Tuesday, August 18, 1959
- 00:56 MST; 07:56 UTC
- In Wyoming’s record
- 1st
- strongest of 1,557 since 1900
USGS ANSS Comprehensive Earthquake Catalog, event official19590818075616490 (“14 km N of Madison Junction, Wyoming”). 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.
5
Aftershocks in 30 days
6.3
Largest aftershock, Aug 18, 1959 · its page
3
On the first day
1
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
- Park CountyWY · inside
- Teton CountyWY · 10.6 mi
- Park CountyMT · 17.9 mi
- Gallatin CountyMT · 18.1 mi
- Fremont CountyID · 19.0 mi
- Madison CountyMT · 23.9 mi
- Beaverhead CountyMT · 29.0 mi
- Clark CountyID · 38.7 mi
- Sweet Grass CountyMT · 42.8 mi
- Stillwater CountyMT · 49.1 mi
Earthquakes around the epicentre
413 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
- August 17, 1959M7.2
- 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.