The June 27, 1925 earthquake
M6.6Clarkston Valley, Montana
A magnitude 6.6 earthquake, centred in Broadwater County. The 2nd strongest in Montana’s record since 1900, and the strongest within 50 miles in the catalogue, which runs from 1900. 4 aftershocks of magnitude 2.5 or more followed within 30 days.
The earthquake
- Magnitude
- 6.6
- moment magnitude (mw)
- Depth
- —
- Not recorded
- Local time
- Saturday, June 27, 1925
- 18:21 MST; 01:21 UTC on Jun 28, 1925
- In Montana’s record
- 2nd
- strongest of 1,459 since 1900
USGS ANSS Comprehensive Earthquake Catalog, event official19250628012105000 (“The 1925 Clarkston Valley, Montana 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.
4
Aftershocks in 30 days
6.4
Largest aftershock, Jun 28, 1925 · its page
3
On the first day
0
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
- Broadwater CountyMT · inside
- Gallatin CountyMT · 4.3 mi
- Jefferson CountyMT · 7.5 mi
- Meagher CountyMT · 22.7 mi
- Madison CountyMT · 23.4 mi
- Lewis and Clark CountyMT · 34.3 mi
- Park CountyMT · 35.6 mi
- Silver Bow CountyMT · 39.9 mi
- Powell CountyMT · 44.9 mi
Earthquakes around the epicentre
151 earthquakes of magnitude 3 or more within 50 miles of this epicentre since 1973.
USGS ANSS Comprehensive Earthquake Catalog. Local time is the time zone of the county nearest the epicentre. Magnitudes are as the catalogue reports them.