Caribou County has many earthquake-prone areas

While working at trying to find local sources to talk about earthquake risk with, I happened across this article in the 2020 volume of the Sun.  Editor Mark Steele reprinted it in full from its July 1, 1971 issue, and I am doing the same, with the credit to Mark for editing and presenting it in 2020, and Ralph Reeves for writing the original and always timely article in 1971.

NOTE: The following is a brief geologic history of the county which seems appropriate to recall in celebrating our Centennial
Anniversary.

Awakened by sudden shaking of the earth, canned goods spilled from the shelves of a quiet country store in mid-afternoon, a new ranch house temporarily abandoned because of repeated earthquakes which also formed a crater nearby…these are examples of the earthquakes felt in Caribou County as the restless unstable rock masses shift along giant cracks in the earth called faults.  

Southeastern Idaho and California are both located in “Seismic Area III,” the most dangerous earthquake rating, according to a map published by the U.S. Geologic Survey.  Early settlers recorded earthquakes in Caribou County, and earthquake tremors have been noted from time-to-time, with an occasional stronger shock such as in 1959 when the large earthquake shook the Hebgen Lake area near West Yellowstone.  “Swarms” of microshocks have been recorded near Soda Springs by sensitive seismographs.  

SPECTACULAR FAULTS LOCATED IN THE AREA

Some of the worlds most spectacular faults are located bear Soda Springs.  The Ledge Creek Fault creates a spectacular fault scarp cliff of broken basaltic lava rock just northeast of town, visible from the East Side Store to the east of Highway 34.  This fault may be seen to the east of Highway 34 as one travels north toward Monsato’s phosphorus plant.  The Conda Fault crosses the Conda road one-half mile east of highway 34 and crosses highway 34 one-half mile north of the Conda road junction near the Torgesen ranch buildings, and then continues to the northwest to merge with the northward extension of the Ledge Creek Fault to form a spectacular 100 foot high fault scarp of broken basalt as it trends northwestward toward China Hat, which is a thousand foot high volcanic conce composed of rhyolite.

FAULTS FORMED LOCAL MOUNTAINS AND VALLEYS

The Scenic Bear River Valley and Bear Lake Valley, extending 60 miles from the Soda Springs area south southeast along the Bear River and past Bear Lake into Utah was formed by repeated movements along two faults on either side of the valley floor to its present elevation below the surrounding mountains.  Geologists call this sort of valley a “graben,” a valley formed by two faults.  Grabens abound in Caribou County, being most numerous in the China Hat area, though on a much smaller scale than the Bear River Valley graben.  

In the vicinity of China Hat are a succession of faults which create “horsts” (hills between two faults) and grabens.  Crag Lake lies in one of these grabens and is pictures in at least one textbook to illustrate a typical graben.  Just east of Crag Lake is Dike Lake, a lake formed by water leaking past a dike constructed to reduce seepage southward from this southernmost arm of Blackfoot River Reservoir.  Water seeps southward from Dike Lake along failts to form swampy Five-Mile Meadows a few miles north of Hooper Springs, east of the oiled road.  Wheat and barley are raised on the horsts and in the grabens with the broken rock of the fault scarps forming boundaries between the fields.  The area lies at an elevation of about 6200 feet, where spring arrives late and the snowdrifts linger until Memorial Day in the sheltered areas.

MOVEMENT ALONG FAULTS

Faults have a variety of causes.  As the rock layers buckle in the process of forming mountains, faults develop and the rock layers move on each side of the fault,  Some faults are nearly horizontal and the rocks move horizontally.  An example of this type of fault (called a thrust fault) occurs in the Soda Springs-Montpelier area where the mountains have moved southwest.  This is an old fault and hasn’t moved much in recent geologic times and consequently is largely obscured by erosion and alluvial deposits.  

Other faults dip steeply into the earth and are called “normal” or “reverse” faults by geologists, depending upon certain aspects of the movement along the fault.  These faults are the spectacular, visible faults of Caribou County, these are the faults along which much movement has occurred in recent geologic times, causing the earthquakes of modern times.  Movement along these faults may be vertical, as along the Ledge Creek fault, or horizontal as along the Chubb Springs fault north of Henry, or a combination of vertical and horizontal movements.

MANY SPRINGS ARE FAULT RELATED

Other fault scarps lie ahead as one proceeds north of China Har on Highway 34 toward Henry.  The highway curves abruptly as it crosses a fault scarp a few miles south of Henry.  This fault scarp plunges into Blackfoot River Reservoir where a fault-related spring—a spring caused where movement of the rocks along this fault cut off the flow of water along a porous layer of lava cinders or other rocks ], or created a pathway of broken rocks along which water could flow—bubbles up beneath the water of the reservoir.  Most of the springs in Caribou County are caused by faults, which often conduct the water to depths of as much as a few thousand feet below the surface, where it is heated by the higher temperatures found everywhere as the crust of the earth is penetrated.  One such spring located in the Corral Creek area west of Blackfoot River Reservoir is sometimes visited by snowmobilers in the winter who have been known to enjoy its warmth in mid-winter.

HOT SPRINGS DEPOSIT MINERALS

To the southeast of Henry is yet another fault scarp, and at Henry are travertine terraces from a formerly large, but now much smaller spring which even now carries dissolved limestone from the depths of the faults, bringing it to the surface and depositing this limestone as travertine when the waters of the springs lose carbon dioxide to the air thereby rendering the dissolved limestone in soluble, depositing it in thick layers, a process which continues slowly today.  Travertine (locally called “Formation” after the extensive travertine deposits of Formation Springs a few miles northeast of Soda Springs on Trail Canyon road) is widely deposited by Hot Springs in Caribou County.

WESTERN PHOSPHATE FIELD

To the east of Henry lies Monsanto’s Henry Mine, in the heart of the “Western Phosphate Field,” the world’s largest reserve of rock phosphate.  Numerous faults break the rock layers of the mine, and are revealed as the rock is stripped away to reveal the rich phosphates  which were deposited in the Phosphoria Formation by a shallow ocean 270 million years ago…phosphate deposits since raised to 7,000 feet above sea level by processes of the earth which have resulted in faults and earthquakes as the area was an continues to be slowly raised from below sea level to its present location near the mountain tops.

HIGHWAY THROUGH MEADOW CREEK GRABEN

More fault scarps are located north of Henry.  The highway winds through the easy path of Meadow Creek graben’s flat, swampy meadows, with the spectacular, broken rock of Pelican Fault along the east side of the highway.  At the north end of the graben the highway crosses Chubb Springs Fault, and passes by Chubb Springs, a fault-related spring where the constant temperature of the water creates ideal conditions for the growth of water cress, a delicacy gathered by residents of the area.  In the 40 degrees below zero temperature of mid-winter a few hard golden eye and mallard ducks may be seen on the open waters of Chubb Springs and other similar springs in the area.

A LAKE IN OLD AGE

Beyond Chubb Springs highway 34 turns east and soon crosses along the south side of Grays Lake National Wildlife Refuge where ducks, geese, sandhill cranes, and other waterfowl nest.  Grays Lake is a lake in Old Age, the final stage in the life of every lake, a stage which results in its destruction by filling with peat, the black or brown residue of the plants of the swamp.

Grays Lake, like Lake Tahoe, Crag Lake, and many others, was formed in part by the movement of the land along faults around the periphery of the lake, forming a basin which in this case was closed off at the north end by basalt flows from volcanoes.  The faults around Grays Lake are old and invisible to the untrained eye—but the earthquakes which accompany movements along these faults have shaken canned goods from the shelves of the store at Wayan, and are felt even more often by residents near the north end of the lake.  Coast and Geodetic Survey Reports indicate that the earthquake epicenters are located on and around Caribou Mountain, and to the east in Wyoming near the Cokeville Trench, a deep, long valley extending from Cokeville Wyoming north past Afton, and then curving westward toward Rexburg.  The Cokeville Trench was formed in a manner similar to Bear river Valley, and continues to deeper as the land of the valley sinks from time to time and the nearby mountains rise, along faults located at each side of the valley, each small movement is accompanied by an earthquake.

EARTHQUAKES OCCUR IN MANY AREAS OF SE IDAHO

Epicenters of other recent earthquakes have been located one and one half miles southwest of Bancroft at the base of the mountains, and in the vicinity of Swan Lake five miles east of Soda Springs.  Earthquakes are the result of vertical or horizontal movements along faults or large cracks in the earth, some of which extend through the crust of the earth to the plastic mantle below, a depth of perhaps 30 miles.  These faults occur in profusion in Caribou County, the obvious ones ranging from a few hundred yards apart to a few miles apart.  Like faults everywhere, the faults of Caribou County are mostly “invisible,” being disguised as a change in the slope of the mountain, or being hidden by the passage of a stream or gully down the broken rocks of a fault.  Tourists who stand on the carbon dioxide geyser a block northwest of the courthouse will observe a crack extending north and south through the mineral deposits of the geyser.  This crack (fault) extends toward the office building to the south, and extends to the north where knolls and humps of travertine mark the path of the fault.  When the geyser is turned on, mineral springs at the Torgesen Ranch, located at the base of the hills to the north, decrease in flow.  Beyond the Torgesen Ranch, the fault travels north through the saddle just west of the “S” on the “S” hill and passes about two hundred yards west of Hooper Springs, another of the many fault-related springs of Caribou County.

OTHER FAULT-RELATED SPRINGS

Swan Lake, located one and a half miles NE of Highway 30N five miles east of Soda Springs is the most spectacular of the fault-related springs of the county.  Swan Lake and the several related lakes of the area, is located along the not-very-visible fault which is responsible for the abrupt rise of the mountains along the northeast side of Bear River Valley.  Swan Lake proper is 200 feet across and is ringed by 20 feet high deposits of travertine.  It is located along the crest of a spectacular travertine terrace 100 feet or more high, which is the summit of several square miles of travertine deposits which cover much of the valley floor from west of the golf course to a few  miles southeast of Swan Lake, and the deposits of Formation Springs are on the same fault.

Six miles west of Soda Springs, east of the junction of Highway 34 and 30N is another of the spectacular fault scarps of the county.  North of the Union Pacific tracks the scarp reaches its max height of around 100 feet.  The fault is a series of parallel cracks in the earth, one large and several small, extending north and south along the dace of the mountains, which were formed by a series of small movements along this fault, accompanied by earthquakes.  North of the highway toward Chesterfield are two large travertine terraces.

Many streams run out of the mountains into the broken rocks of the faults, disappearing, carrying silt and nearby soil with them to reappear as the fault-related springs seen at lower elevations.  In other areas the faults, concealed by soil, pose a hazard to wheat farmers, threatening to swallow tractors and combines.  Still other faults extend radically from the many cinder cones of the valley, wallowing soil and runoff from the melting snows and rain.  

The broken rocks of the fault scarps provide deep, secure homes for marmots, animals which eat the tender green wheat and barley of spring and summer.  

MOUNTAIN BUILDING CONTINES IN COUNTY

The lands of Caribou County are active areas where mountain building is still underway, as it is over much of the west, with the earthquakes which result when the rocks of the earth occasionally move along faults.  The youngest mountains on earth, the Grand Tetons, are located just 75 miles north of Caribou County.  Hot Springs and travertine deposits attest to the disturbance of the water pathways by faults, and to the movement of water deep into the warmer depths of the earth along the broken rocks of the faults.  Movement along the faults, in addition to building mountains, have formed large lakes, such as Grays Lake and Bear Lake, adding to the spectacular beauty of the area.  Earthquakes, small and occasionally large, may be expected as mountain-building continues in the area.