The western half of Lewis and Clark County is in a high hazard seismic zone, which means that an earthquake can cause major damage. This area includes Helena, East Helena and Lincoln.
Click here for a fault map of the Helena Valley.
The worst earthquakes to hit the Helena area occurred in October, 1935, when 6.3 and 6.0 earthquakes struck on October 18 and 31, respectively. Four people were killed and property damage exceeded $4 million. About sixty per cent of the buildings in Helena were damaged to some degree. Swarms of earthquakes hit the area, with over 1800 temblors recorded from October 4, 1935 to April 30, 1936.
Click here for a summary of the 1935 Helena earthquakes.
A computer simulation of a 6.3 earthquake in Helena today reveals that property damage would be about $500 Million. Fatalities and injuries would depend upon the time of day that the earthquake would occur. The next major seismic event for the area cannot be predicted, but preparedness and mitigation steps taken now will reduce the loss of lives and property when the next earthquake strikes.
Much of the Helena valley has a high potential for liquefaction, which is the result when an earthquake causes soil to become unstable and semi-liquid, like heavy oil. Susceptibility is determined by alluvial (loosely compacted deposits) soil and a high water table. Liquefaction magnifies ground motion and causes the soil to become unstable. Click here for a liquefaction susceptibility map for the Helena Valley.
For more information go to Montana Bureau of Mines and Geology.
Earthquake Preparedness Tips
Before an Earthquake
1. Remove heavy objects from high places or anchor them down to prevent them from becoming flying missiles when the ground shakes. Sticky back Velcro works will secure computers and other office machinery to desk tops and angle brackets can be used to anchor book cases to walls.
2. Strap the water heater to the wall with plumber's tape to prevent a gas line break when the shaking starts.
3. Learn how to shut off gas, water and electrical at main switches and valves. Do not turn off utilities unless they are damaged.
4. For more information on earthquake preparedness, click here.
During an Earthquake
1. Stay calm and stay where you are. Do not run outside of buildings, where you become a target for falling debris. Get under sturdy furniture and away from windows. If you are outside, stay there, and get away from buildings.
2. If you are in a vehicle, pull over to a safe spot of of the road and stay in your vehicle. Only continue to drive if you are on, or beneath, an overpass.
After an Earthquake
1. Once the earthquake is over, which is usually less than one minute, check for injuries.
2. Shut off utilities only if there is a leak or electrical lines are shorting out. If you smell gas, shut off the valve at the meter, open doors and windows and report the leak to authorities.
3. Leave your building after the shaking stops. Go to a safe spot away from buildings and utility lines. Prepare for after shocks, which will probably be smaller than the main shock. But after shocks can cause further damage to buildings already damaged from the first earthquake.
4. Stay away from damaged areas and tune in to local radio and TV stations for the latest information.
Paul N. Spengler, Coordinator
Law Enforcement Center
221 Breckenridge Avenue
Helena, MT 59601
Hours of Operation M-F, 8-5