The October 31, 1935 earthquake
M5.9The 1935 Helena, Montana Earthquakes (Largest Aftershock)
A magnitude 5.9 earthquake 6.2 miles deep, centred in Jefferson County. The 8th strongest in Montana’s record since 1900, and the strongest within 50 miles since a magnitude 6.1 on October 18, 1935. 2 aftershocks of magnitude 2.5 or more followed within 30 days.
The earthquake
- Magnitude
- 5.9
- moment magnitude (mw)
- Depth
- 6.2 mi
- 10.0 km below the surface
- Local time
- Thursday, October 31, 1935
- 11:37 MST; 18:37 UTC
- In Montana’s record
- 8th
- strongest of 1,459 since 1900
USGS ANSS Comprehensive Earthquake Catalog, event iscgem904621 (“The 1935 Helena, Montana Earthquakes (Largest Aftershock)”). 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
5.0
Largest aftershock, Nov 28, 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
- Jefferson CountyMT · inside
- Broadwater CountyMT · 3.5 mi
- Lewis and Clark CountyMT · 8.8 mi
- Powell CountyMT · 21.1 mi
- Meagher CountyMT · 21.3 mi
- Cascade CountyMT · 25.8 mi
- Gallatin CountyMT · 34.2 mi
- Deer Lodge CountyMT · 38.8 mi
- Silver Bow CountyMT · 39.9 mi
- Madison CountyMT · 47.4 mi
Cities, largest first
- HelenaMT · 8.9 mi
Earthquakes around the epicentre
144 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 18, 1935M6.1
- October 12, 1935M5.9
- June 27, 1925M5.5
- July 10, 1925M5.5
- June 27, 1925M5.5
- April 19, 1910M5.4
- 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.