Once the shaking had subsided, questions regarding earthquakes in our area and region began to surface.
Naturally, people in Boundary County wanted to know:
• Have there ever been any earthquakes recorded as originating in Boundary County?
• What is the risk for earthquakes in our area?
• What major earthquakes have happened in Idaho?
We hope to provide some valuable information on these questions.
Earthquakes in Boundary County
We are talking here not about earthquakes that may have originated in other nearby areas whose shaking and quaking were felt in Boundary County. We are talking about earthquakes whose epicenters are located within the borders of Boundary County.
What is an epicenter? Earthquakes originate at some point inside the earth, below the earth's surface. The epicenter of an earthquake is the spot on the earth's surface that lies directly above the earthquake's actual point of origin inside the earth. The depth of an earthquake is how far the earthquake's point of origin is below its epicenter.
According to data on the website of the U.S. Geologic Survey, three earthquakes have been recorded that originated within Boundary County:
1. About 30 years ago, on November 27, 1984: At 5:07 a.m., an earthquake of magnitude 3.2 was detected, with its epicenter located in Boundary County, Idaho, at a point about two miles directly west of the junction of U.S. Highway 95 with Idaho Highway 1 (the Mt. Hall junction).This earthquake's point of origin was about 3 miles below the earth's surface.
2. Last fall, on November 14, 2014 at 10:35 p.m., while Boundary County residents were enjoying the last few hours of an autumn Friday evening, a magnitude 1.8 earthquake jolted from only 656 feet below the earth's surface. The epicenter of this earthquake was located about three miles east of Priest Lake, just barely across the Boundary County line in the southern end of the county.
3. Only 10 days later, on November 24, 2014, at 10:52 in the morning, a magnitude 1.7 earthquake hit Boundary County with its epicenter located about a mile and a half north of Brush Lake. This earthquake hit at a depth about two miles beneath the earth's surface.
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These are the only Boundary County earthquakes listed on querying the Advanced National Seismic System Comprehensive Catalog database, asking for earthquakes recorded in our area's location coordinates dating back to the year 1900. Certainly other earthquakes may have occurred during that time period before the development or deployment of sensitive measuring devices, but these are the only ones listed in the database.
There have been other earthquakes with epicenters fairly close to Boundary County, whose shakes may have been felt within our county, but that weren't actually located within the county. These would include, for example, the recent Sandpoint area earthquakes from last week, also a couple of quakes recorded east of Boundary County just across the state line into Montana—a magnitude 3.1 earthquake in 1997, and a magnitude 2.5 earthquake just this past November.
Also, very likely several of the larger Idaho earthquakes described below in this article were felt by residents in Boundary County.
What is our risk of a serious earthquake here in Boundary County?
Before a simple answer can be given to this question, a few points about earthquake risk measuring need to be clarified:
First, simply knowing where the nearest geologic fault is located does not give much indication of the risk for an earthquake in an area, since an area can be affected by shaking from an earthquake many miles distant.
Experts have calculated the seismic hazard for regions throughout the United States, and have created seismic hazard maps which show, for a given location, the probability of different "peak ground accelerations" that can be generated by earthquakes. This "peak ground acceleration" may roughly be thought of as a measure of how hard the earth shakes in an earthquake. It is this shaking that can cause the damage created by earthquakes.
"Peak ground acceleration" in an earthquake is measured as a comparison to the acceleration of earth's gravitational force. How much "peak ground acceleration" does an earthquake need to have to start causing problems? Just to give an idea, in an earthquake, people can feel shaking at a "peak ground acceleration" of about 0.1% of gravity. A "peak ground acceleration" equal to 2% of earth's gravitational acceleration can cause people to lose their balance.
On this 2014 Seismic Hazard Map for Idaho, one can see that Boundary County has two color codes. Most of Boundary County is coded blue on the map, while a small slice of southern Boundary County is coded green.
The blue-coded area of Boundary County means that within the next 50 years, there is a 2% chance that earthquake shaking from a "peak ground acceleration" greater than 10%-14% of gravity will occur.
In Boundary County's southern green slice on this map, risk is a little higher. There is a 2% chance that "peak ground acceleration" earthquake shaking of 14%-20% of gravity will occur within the next 50 years.
(Note that at a "peak ground acceleration" of 50% gravity, structurally strong buildings can survive if the earthquake is of short duration).
All this sounds like a complicated way to measure earthquake risk, but from a scientific basis, that's how they do it. There isn't really an accurate parameter that says "if you live in Boundary County, you have an "X" percent chance of having an earthquake." The experts say that grading the risk is simply too complicated and involves too many factors to be measured that simply.
Perhaps it helps to say that in comparing seismic hazard maps, Boundary County has a lower risk for damaging shaking from an earthquake than most other areas of the western United States.
What major earthquakes have happened in Idaho?
According to the Earthquake Information Bulletin, a USGS publication, the first earthquake to be recorded in Idaho's history that caused significant damage occurred in November of 1884, when an earthquake with epicenter in northern Utah caused considerable damage to houses in areas of southern Idaho.
This publication lists other significant Idaho earthquakes, including:
An earthquake causing shaking in the Shoshone, Idaho area in 1905, and in the Boise area in 1916. Both of these earthquakes caused structural damages.
An earthquake shock occurred in November 1942 near Sandpoint, Idaho, felt over 25,000 square miles.
In July 1944, a high intensity earthquake occurred in central Idaho which caused fissures to open in the ground, and people reported being unable to walk due to the shaking. The shock from this earthquake was felt over 70,000 square miles, extending into surrounding states.
A February 1945 earthquake near Clayton, Idaho broke dishes and cracked plaster in central Idaho.
A December 1957 earthquake shock near Wallace, Idaho damaged the Galena Silver Mine and frightened miners working underground at the mine.
A magnitude 7.1 earthquake in 1959 in Montana caused millions of dollars in damage, and killed 28 people. Idaho experienced damage from this earthquake in the Big Springs, Henry's Lake, and Island Park areas.
A 1962 earthquake in northern Utah caused large landslides in the Fairview, Idaho area. Building damages occurred in the southern Idaho towns of Franklin and Preston. Shaking from this quake was felt over a 65,000 square mile area in five states.
An April 1969 shock of magnitude 4.9 caused some damage in the Ketchum area.
Idaho's largest earthquake
According to Seismicity of the United States, 1568-1989, a USGS publication authored by Stover and Coffman, the largest known earthquake in Idaho happened near Mt. Borah, Idaho's highest peak, in October 1983. The earthquake's 6.9 magnitude is the highest ever recorded in Idaho. This earthquake resulted in two deaths in the Challis area, and an estimated $12.5 million in damage in the Challis-Mackay area, where hundreds of buildings were damaged, including many severely damaged structures. The Challis High School had extensive damage. The earthquake caused significant ground breakage, rockfalls, and landslides.
Shaking from this earthquake was felt over several states and up into Canada. Aftershocks continued through the remainder of 1983.