The October 12, 1935 earthquake
M5.9near Helena, Montana
A magnitude 5.9 earthquake 3.1 miles deep, centred in Lewis and Clark County. The 9th strongest in Montana’s record since 1900, and the strongest within 50 miles since a magnitude 6.4 on June 27, 1925. 2 aftershocks of magnitude 2.5 or more followed within 30 days.
The earthquake
- Magnitude
- 5.9
- as reported (fa)
- Depth
- 3.1 mi
- 5.0 km below the surface
- Local time
- Saturday, October 12, 1935
- 00:50 MST; 07:50 UTC
- In Montana’s record
- 9th
- strongest of 1,459 since 1900
USGS ANSS Comprehensive Earthquake Catalog, event ushis1109 (“2 km ENE of Helena, 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.
2
Aftershocks in 30 days
6.1
Largest aftershock, Oct 19, 1935 · its page
0
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
- Lewis and Clark CountyMT · inside
- Jefferson CountyMT · 2.6 mi
- Broadwater CountyMT · 10.3 mi
- Powell CountyMT · 13.9 mi
- Meagher CountyMT · 23.4 mi
- Cascade CountyMT · 23.8 mi
- Deer Lodge CountyMT · 35.2 mi
- Silver Bow CountyMT · 37.5 mi
- Gallatin CountyMT · 42.2 mi
Cities, largest first
- HelenaMT · 0.9 mi
Earthquakes around the epicentre
142 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
- June 27, 1925M6.6
- June 27, 1925M6.4
- June 27, 1925M5.5
- June 27, 1925M5.5
- July 10, 1925M5.5
- April 19, 1910M5.4
USGS ANSS Comprehensive Earthquake Catalog. Local time is the time zone of the county nearest the epicentre. Magnitudes are as the catalogue reports them.