The October 18, 1935 earthquake
M6.1The 1935 Helena, Montana Earthquakes (Mainshock)
A magnitude 6.1 earthquake 9.3 miles deep, centred in Lewis and Clark County. The 4th strongest in Montana’s record since 1900, and the strongest within 50 miles since a magnitude 6.4 on June 27, 1925. 1 aftershock of magnitude 2.5 or more followed within 30 days.
The earthquake
- Magnitude
- 6.1
- moment magnitude (mw)
- Depth
- 9.3 mi
- 15.0 km below the surface
- Local time
- Friday, October 18, 1935
- 21:48 MST; 04:48 UTC on Oct 19, 1935
- In Montana’s record
- 4th
- strongest of 1,459 since 1900
USGS ANSS Comprehensive Earthquake Catalog, event iscgem904601 (“The 1935 Helena, Montana Earthquakes (Mainshock)”). 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.
1
Aftershocks in 30 days
5.9
Largest aftershock, Oct 31, 1935 · its page
0
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
- Lewis and Clark CountyMT · inside
- Broadwater CountyMT · 1.7 mi
- Jefferson CountyMT · 6.7 mi
- Meagher CountyMT · 11.4 mi
- Cascade CountyMT · 22.9 mi
- Powell CountyMT · 30.5 mi
- Gallatin CountyMT · 31.4 mi
- Deer Lodge CountyMT · 48.7 mi
- Silver Bow CountyMT · 49.5 mi
- Judith Basin CountyMT · 49.6 mi
- Park CountyMT · 49.6 mi
Cities, largest first
- HelenaMT · 17.5 mi
Earthquakes around the epicentre
129 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
- October 12, 1935M5.9
- June 27, 1925M5.5
- June 27, 1925M5.5
- July 10, 1925M5.5
- December 12, 1926M5.4
USGS ANSS Comprehensive Earthquake Catalog. Local time is the time zone of the county nearest the epicentre. Magnitudes are as the catalogue reports them.