It's the crust's fault

Gov. C.L. “Butch” Otter signed a proclamation Oct. 1, 2009, making October “Earthquake Awareness Month” in Idaho. Oct. 28 is the 27th anniversary of the magnitude 6.9 Borah Peak Earthquake. The 1983 earthquake still ranks as one of the strongest quakes in the lower 48 states during the past 50 years.

Ada County is outside of the Seismic Zone where this event was centered, but residents of the county could feel the earth tremble. The question that remains is, could Ada County experience such a powerful earthquake?

The Earth’s crust is made up of separate moving landmasses called tectonic plates. These plates grind and slide passed one another as a normal part of their movement. The edges of these plates are not smooth. As two adjacent plates move, the jagged edges of the plates will get stuck and pressure will build up along these portions of the two plates.

When the pressure becomes too great, the two plates will suddenly slip past one another. The surface where the slip takes place is called a fault or fault plane. The sudden movement or slip is an earthquake.

Squaw Creek Fault
The Squaw Creek Fault is located near Emmet, about 25 miles north of Boise. Geologic evidence suggests that it has moved as recently as 7,600 years ago. It is believed that this fault in a worst-case scenario could produce a Magnitude 7.1 earthquake. Computer modeling of this event estimates most of Ada County would receive some damage from such an earthquake. It appears that the northwest sections of the county could experience the heaviest damages from such an event.

Water Tank Fault
The Water Tank Fault is located roughly 13 miles from Grand View, about 57 miles southeast of Boise. There is evidence of six prehistoric earthquakes along this fault at intervals ranging from 2,000-9,000 years apart. Those quakes are believed to have had magnitudes between 6.7 and 7.3. The fault is in a lightly populated area, but if the fault produced a magnitude 7.0 or greater earthquake it is estimated that there would be damage to the populated areas including parts of Ada County.

The Proverbial Pebble in the Pond
Drop a pebble on to the smooth surface of a pond and watch what happens. Ripples suddenly appear, radiating out in all directions from the source, affecting anything that happens to be floating on the surface in the nearby area. That is much like an earthquake.

You don’t have to be anywhere near the epicenter to be affected. Computer estimates on the likely damage from an earthquake are exactly that, estimates. It is unknown what would actually happen to the developed areas of Ada County. Even if the buildings survive an earthquake with minimal damage that does not mean life will be unchanged.

Utilities, phone service, fuel, sewer and fresh water conveyance systems all could be damaged. Roads and bridges might be temporarily unuseable. Dams and canals could be damaged, adding flooding to the list of issues associated with the quake. Communications systems could go down making it extremely difficult to relay public needs to the first responders. Private citizens could be faced with temporary isolation and lack of normal services.

“Your future depends on many things, but mostly it depends on you.” - Frank Tyger

Earthquakes can cause injuries and isolate people from food and hygiene supplies, everyday amenities (fresh water, restrooms) and from one another. To overcome these difficulties one must plan and prepare ahead of time. There are seven steps to earthquake safety. They can be found inside the publication “Putting Down Roots In Earthquake Country.” To view the booklet go to: To view ACCEM's monthly bulletin visit the website.

Published 10-8-2010