BORAH PEAK FAULT SCRAP
ARCO - IDAHO, USA
AUG 28TH 2020
The Snake River Plain has been shaped by the past eruptions of the Yellowstone Supervolcano and its north-east moving hotspot, along with the East-West movement of the Basin and Range. Active Volcanoes still exist within the area to this day, the most well-known being Craters Of The Moon National Monument and its rifting eruptions. Most of the area south of the Lost River Range is located in a Pleistocene Basalt lava plain, along with Pleistocene rhyolite domes that have pierced the thin sedimentary layers which have been deposited over 1000s of years.
Borah peak's west side is bordered by the Thousand Springs Segment of the Lost River Fault which ruptured in 1983. Borah peak itself is located on the east limb of a syncline and is also in the hanging wall of a small thrust fault.
The magnitude 6.9 Earthquake happened at 8:06 a.m on October 28th1983, the Earthquake dropped the Lost River Valley 8 feet (2.4 meters) as the Lost River Range rose1.8 feet (0.5 meters). At its highest point, the fault scrap is 10 feet (3 meters) high, Horizontal displacement also occurred, this shifted the eastern side of the fault to the north. The earthquake itself occurred in the centre of the Lost River Fault closer to Challis, its focus was 9 miles (14.5 km) deep and Its epicentre was the Lost River Valley. Before the 1983 rupture, the fault produced a major earthquake 6,000 to 8,000 years ago other than that the fault has remained silent for 15,000 years.
The earthquake was felt as far south as Salt Lake City and as far North as the Canadian Border, aftershocks continued months after the initial event. $12 million in property damage was recorded, as commercial buildings in Challis and Mackay experienced structural failures along with around 200 homes. Rock slides, landslides and slope failure occurred due to shaking within the Lost River Valley.
The earthquake increased groundwater pressure which resulted in bursts of sandy water towards the base of Chilly Buttes, the groundwater eruptions which were ejected 20 feet (6 meters) into the air creating small craters in its place and continued for 48 hours. The craters were about 20 feet (6 meters) wide and 7 feet (2 meters) deep. A mine shaft in the Clayton Silver Mine north of Borah Peak flooded with 250 feet (76 meters) of water, this begun 3 hours after the earthquake as the water pumps were overwhelmed which in turn shut mine operations down for 2 months.
The fault scrap left behind extends along the base of the Lost River Range, scraps like this one are only visible with surface ruptures that are as shallow as 6.0 km and end up being more damaging than deeper focused earthquakes as the seismic energy isn't lost with distance.
Basin and Range
Idaho is certainly no stranger to earthquakes as it ranks 6th out of all of Americas 50 states, the reason for this is because the southern part of Idaho sits within the Basin and Range.
The Basin and Range was given the name for its topographic standing as it contains basins and mountain ranges. The reason it exists is because of the east-west stretching of the crust, why this is occurring is still unknown. it is known that the crust began stretching around 17 million years ago and continues to this day.
Faults are very common within the Basin and Range Province as the earth's crust is stretching and thinning out, this forms mountains and deep valleys with altering patterns visible in the topography. Currently, the Rocky Mountains and Sierra Nevada ranges are moving away from each other at a rate of 17mm per year. Because of the movement of these mountain ranges, a set of Normal Faults has formed. With repeated earthquakes within the Basin and Range over the progression of millions of years, the landscape has produced a trademark type of topography which is high mountains separated by intervening valleys.
Blue: Columbia River Flood Basalts
Green: Snake River Plain Volcanic Province
Red: Northern Section, Basin & Range (Great Basin)
Rough map of the basins and plains of the west
Lost River Fault
The Lost River Fault is 130-km long and runs alongside the southwestern portion of the Lost River Range, and is southwest facing. The fault is split into 7 sections according to the USGS, from (A-F) that make up the main fault. The sections are based off mapping, morphological study, dating, and trenching of the fault scarp along with the offset surfaces.
The 7th section of the fault is much more complex as many of the scraps are discontinuous, this links the Lost River Fault to the smaller Lone Pine Normal Fault to the west. The part of the Fault that ruptured in 1983 was the thousand springs section. During the 1983 earthquake, the Lost River Fault folded the Borah Peak horst and internally thrusted the area affected. Those rocks were Neoproterozoic to Mississippian in age which is broken down basin-fill sediment located at the base of the mountains.
The USGS Hazards program has put up a scenario event that has not happened but it is an outline of what could happen. The Lost river fault is predicted to possibly produce a magnitude of 7.0 at a depth of 9.0 kilometres according to the USGS Scenario program.
Geology Underfoot in Southen Idaho
Shawn Willsey Page 141 - 151 (The Borah Peak Earthquake)
Mountain Press Publishing Company (2017)
Department of Geology, Idaho State University, Department of Geology, Utah State University, Link, P. K., & Janecke, S. U. (1999). Geology of East-Central Idaho: Geologic Roadlogs for the Big and Little Lost River, Lemhi, and Salmon River Valleys.
Idaho Geological Survey, United States Geological Survey (USGS),
Haller, K. M. & Wheeler, R. L. (2010). Lost River fault, Thousand Springs section (Class A) No. 601c. Quaternary Fault and Fold Database of the United States.
USGS National Earthquake Information Center & United States Geological Survey (USGS). (n.d.). M 7.0 Scenario Earthquake - Lost River fault. USGS.
USGS Geologic Hazards Science Center Golden Colorado