The Borah Peak Earthquake
By, Steven W. Moore, Geologist and R. David Hovland, Geologist

 Earthquake damage to the Custer Hotel in Mackay 

ALSO READ Idaho Geologic Survey's GeoNote 05

On Friday, October 28,1983 east central Idaho awoke to a crisp autumn morning. Suddenly, the peaceful mountain morning was shattered as powerful forces within the earth's crust were unleashed. Normal morning activities were interrupted as windowpanes vibrated and alarmed people ran out of their houses. These were the first hand effects of a major earthquake.

The authors were in the Salmon area returning from a geologic project when the earthquake struck. After the initial radio reports of the location of the earthquake, they traveled to the Mackay-Challis area and were among the first geologists to witness firsthand the effects of the earthquake.

The earthquake occurred at 8:06 a.m. (MDT) with the epicenter located in a sparsely populated mountainous area between the small rural communities of Challis and Mackay. These two cities lie in a northwest-trending valley on the west side of the Lost River Range. Updated calculations indicate that the Borah Peak earthquake was a 7.3 magnitude on the Richter scale. The epicenter was located about 19 miles northwest of Mackay, at the south margin of the Thousand Springs Valley just west of the 12,662-foot Borah Peak, at latitude 44.05 degrees N., longitude 113.88 degrees W. The hypocenter, or depth in the crust at which the fault rupture began, was about 9 miles. The earthquake caused two deaths in Challis and an estimated $15 million damage in the affected region.

Some of the primary effects of the Borah Peak earthquake included ground shaking and surface rupture directly related to fault movement along the western flank of the Lost River Range. Ground shaking was most intense near the epicenter between Challis and Mackay, but the earthquake was also felt over most of the northwestern United States and in parts of Canada. The initial shock was followed by numerous aftershocks. In the 10 months following the main earthquake, there were at least 20 aftershocks with a magnitude of 4.0 or greater.

Secondary effects produced as a consequence of the earthquake included seismically induced landslides, ground cracking, and modification of the hydrologic system. Damage to roads, buildings, and other structures occurred in the area between Mackay and Challis.

Ground Rupture Produced by Fault Displacement

Fault displacement that produced this intense earthquake was expressed in spectacular surface ground rupture along a northwest-trending, 22 mile-long zone on the western flank of the Lost River Range. A west-northwest-trending section of faulting that branches off of the main surface fault trace west of Dickey Peak gives the surface faulting pattern a Y-shape. Much of the zone of surface rupture follows the Holocene and upper Pleistocene fault scarps of the Lost River fault. Fault scarps, the most common features along the zone of surface faulting, look like small steps or cliffs. Fault scarps are produced when adjacent blocks of the earth's crust move relative to each other and are displaced along a fault plane.

Detailed study of the surface faulting and focal mechanisms reveals that the dominant fault movement associated with the Borah Peak earthquake was dip slip, or vertical. That is, the Lost River Range was uplifted vertically along the fault relative to the adjacent valley. Maximum throw (vertical displacement) measured along the west flank of Borah Peak is about 9 feet. A more minor component of leftslip is also evident, indicating that that the Lost River Range block also moved laterally northward, relative to the valley to the west, as well as upward. The high relief and linear northward trending mountain ranges in this region and elsewhere in the Basin and Range Province have been produced by similar repeated fault movements over geologic time.

The surface fault trace is complex and comprised of multiple, gently curved, subparallel fault scarps extending along a trend of approximately N15'W to N25'W. The zone of extensive ground rupture is more than 300 feet wide in some locations. The complex nature the fault rupture can be clearly observed in the area immediately north of Birch Springs. Along this section of the mountain front, two to four en echelon scarps are visible (parallel structural features that are offset like the edges of shingles on a roof when viewed from the side). The maximum scarp height north of Birch Springs is as much as 16 feet. The newly created fault scarps dip from 70 degrees to near vertical and face westward toward the valley. However, within a few days after the earthquake, many fault scarps, particularly those in well-sorted stream sand and gravel deposits, were rapidly degraded to angle of repose dips of about 30 degrees. At the base of the mountain-front slope, a small down-dropped block or graben approximately 75 to 100 feet also formed.

Although most of the fault scarps face west representing the actual fault movement, other minor faults referred to as antithetic faults, faults whose scarps face the opposite direction of the main scarp, also occur as a result of the tensional stresses in the crust. In the case of minor grabens along the fault zone, antithetic faults form the western edges of the grabens and their scarps face the east, opposing the main fault scarps.

Displacement along the multiple scarps generally occurred in poorly sorted, unconsolidated, Quaternary gravels and other alluvial materials. Closely spaced, parallel tensional cracks are abundant over much of the 200-to 300-foot fault zone north of Birch Springs and other areas. The complexity of the fault zone and the variability of fault-scarp morphology is to a great degree a function of the material and the amount of water saturation.


Landslides Induced by Seismic Shaking

Landslides, rock falls, rock slides, and other ground failures were induced in an area of approximately 1600 square miles surrounding the epicenter as an immediate result of the intense seismic shaking. The steep and rugged terrain of the Lost River Range, and of the adjacent Salmon River, Boulder, and White Knob Mountains significantly contributed to the susceptibility to landsliding as a result of

earthquake shaking in the region. Earthquake-triggered slope failures occurred in a variety of materials including colluvium, glacial deposits, talus slopes, and in fractured bedrock areas.

The majority of the landslides were rock falls or rock slides. Particularly susceptible to rock falls and rock slides were steep and rocky slopes composed of Challis Volcanics. Open joints and fractures, typical of weathered Challis Volcanics, produced inherent slope instability in this unit allowing large blocks to be easily loosened with seismic shaking.

Seventy-five miles north of the epicenter along steep roadcuts and cliffs on Highway 93 near Salmon, the road was severely obstructed by rock falls and rock slides originating in outcrops of Challis Volcanics. Dozens of boulders of Challis Volcanics, some as large as 10 feet in diameter, tumbled down steep slopes of Challis damaging several houses. Some boulders rolled as much as 200 feet out into the valley.


Debris flows and Liquefaction in Water-Saturated Sediment

Other types of ground failures were related to the level of water saturation of sediment. At Birch Springs, seismic shaking produced ground failure and downslope movement of water-saturated colluvium that resulted in a large rotational slump-debris flow. Some of the most spectacular effects occurred at the southern end of Thousand Springs Valley near Chilly Buttes where severe liquefaction occurred in Holocene valley fill sediment. Liquefaction occurs when intense seismic shaking causes complete loss of cohesion of sediment by the transferal of load from the sediment particles to the pore fluid. More than 40 sediment boils (turbid upflows of water, silt, and sand) were produced adjacent to Chilly Buttes when seismic shaking released large quantities of ground water into alluvium. After "boiling" of the sediment ceased, a series of large craters with sediment aprons were left. There is evidence that old craters formed by an earlier liquefaction event were also reactivated into sediment boils during the seismic shaking. New craters produced by the liquefaction were as much as 75 feet in diameter and 16 feet deep.


Dramatic Changes in the Hydrology

Soon after the earthquake, dramatic changes occurred to the hydrologic systems in the Thousand Springs Valley and adjacent areas along the fault zone. Significant hydrologic changes also took place as far away as Yellowstone National Park where the eruptive cycles of many geysers, including Old Faithful, were modified. Early reports of hydrologic effects included increases, decreases, or stoppage of well and spring flows. In the epicenter area, ground water levels rose as much as 13 feet in a water well, then declined for several months before leveling off at about 5 feet above pre-earthquake levels. Reports also mentioned well water becoming murky or silty soon after the earthquake. The earthquake produced a tremendous outflow of water that amounted to as much as 0.25 cubic miles in excess of normal hydrologic output for the region.

At Chilly Buttes on the day of the earthquake, seismic shaking produced a new northwest-trending fissure about 200 feet long and a new cold spring formed on the east side of the northern butte. From the new spring, a tremendous flow of groundwater burst from newly opened fractures in the limestone buttes producing extensive flooding in the valley to the east around the site of Chilly. In the valley floor a few hundred feet to the north and east of Chilly Buttes, more than 40 sediment boils issued huge volumes of water that added to the flooding.

Also, immediately after the earthquake, water levels began to rise at the underground Clayton Silver Mine located 32 miles northwest of the epicenter area and about 15 miles west of the nearest known surface rupture. This flooding of underground levels in the mine necessitated the stoppage of mining operations and the subsequent use of high-capacity pumps to handle this increased flow.

Ingram's Warm Spring Creek, located south of Challis, experienced an interesting change in flow. After the earthquake, the warm spring ceased to flow and dried up leaving hundreds of fish on the dry creek bed. Eight days later, the spring began flowing at an increased flow rate and peaked 46 days later with a flow rate of 51 cubic feet per second, nine times the pre-earthquake flow. The major river and streams draining the earthquake-affected area flowed at above normal rates for a period of several months following the earthquake.

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