The August 20, 1999 earthquake
M5.1near Lima, Montana
A magnitude 5.1 earthquake 9.9 miles deep, centred in Beaverhead County. The 21st strongest in Montana’s record since 1900, and the strongest within 50 miles since a magnitude 5.1 on January 5, 1965. 9 aftershocks of magnitude 2.5 or more followed within 30 days.
The earthquake
- Magnitude
- 5.1
- body-wave magnitude (mb)
- Depth
- 9.9 mi
- 16.0 km below the surface
- Local time
- Friday, August 20, 1999
- 07:50 MDT; 13:50 UTC
- In Montana’s record
- 21st
- strongest of 1,459 since 1900
USGS ANSS Comprehensive Earthquake Catalog, event usp0009dcx (“22 km NW of Lima, Montana”). 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.
9
Aftershocks in 30 days
4.0
Largest aftershock, Aug 26, 1999
2
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.
Counties and cities within 50 miles
Distance from the epicentre to each county boundary (zero when inside) and to each city centre.
Counties
- Beaverhead CountyMT · inside
- Lemhi CountyID · 15.6 mi
- Clark CountyID · 19.7 mi
- Madison CountyMT · 26.5 mi
- Butte CountyID · 39.4 mi
- Custer CountyID · 39.8 mi
Earthquakes around the epicentre
201 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.