User:Earthquake Maestro/2017 Baldy Mountain Earthquake

2017 Baldy Mountain Earthquake

2017 Baldy Mountain Earthquake

Salmon Kalispell Helena Havre Butte Sheridan
UTC time	2017-07-06 06:30:17
ISC event	610766354
USGS-ANSS	ComCat
Local date	6 July 2017 (2017-07-06)
Local time	00:30
Magnitude	Mw 5.8
Depth	7.6 mi (12.2 km)
Epicenter	46.88°N 112.58°W / 46.88; -112.58
Type	Oblique strike-slip
Areas affected	Western Montana United States
Total damage	less than $1 million
Max. intensity	VII (Very Strong)
Peak acceleration	.58g
Landslides	Yes
Aftershocks	Yes

The 2017 Baldy Mountain Earthquake occurred on July 6, 2017, at 00:30:17 local time in west-central Montana, with a moment magnitude of 5.8 and a maximum perceived intensity of VII (very strong). This strike-slip earthquake was widely felt, but no casualties were reported, and only minor damage occurred near the epicenter. The event occurred 11 km (6.8 mi) southeast of Lincoln, Montana in the General Robert E. Lee Range and had a depth of 12.2 km (7.6 mi). The epicenter was on a previously unmapped faultline along the axis of the Intermountain Seismic Belt, which runs through the northern Rocky Mountains from British Columbia to Utah and includes the infamous Teton and Wasatch Faults.

Earthquake

The tremors were felt throughout Idaho and Montana, as well as parts of Wyoming and Alberta]]. Minor shaking was felt as far as Casper, Wyoming and Spokane, Washington.

Aftershocks

Hundreds of aftershocks occurred in the weeks following the main event, with 26 being at least magnitude 3. On July 7, a magnitude 4.9 earthquake was reported, later upgraded to 5.0. This was the largest aftershock in the sequence. Over 300 aftershocks were reported in the first week alone.

Damage

The earthquake produced unusually little damage in the region. No casualties or infrastructure damage was reported. A power outage occurred in the town of Lincoln and the surrounding area, affecting 1,350 homes. The substation responsible had no damage reported to it, and the outage lasted about 45 minutes. There was however, significant damage in the Lincoln area, as moderate shaking was expected there. In Lincoln and its surrounding communities, some structural damage was reported, merchandise was thrown from shelves in stores and many buildings had cracked or fallen plaster and ceiling tile. In towns further from the epicenter, some minor damage was reported. A two-story garage was damaged in Winston, and in Butte, a brick parapet fell from the top of the Napton Apartments, plunging through the windshield of a parked car. In Helena, the earthquake snapped a tree in half and felled it across a highway. A gas leak was also reported in Helena. Roads and bridges in western Montana were inspected but officials found no damage. The Selis Ksanke Qlispe Dam, 5 miles northwest of Polson, Montana was initially feared to be at risk of collapse due to its proximity to the epicenter, but upon inspection, no damage was reported.

Background

The earthquake, at magnitude 5.8, is the 8th largest earthquake to strike Montana in recorded history, nosing out the 1952 magnitude 5.7 quake near Bigfork, and the 2005 magnitude 5.6 quake near Dillon. Although many were surprised by the occurrence of this earthquake, it is historically common for large earthquakes to strike Montana. The 1959 magnitude 7.5 Hebgen Lake Earthquake is the largest on record in Montana, striking near the state's southwest corner and killing at least 28 people. In October 1935, a large earthquake sequence struck near Helena, the largest a magnitude 6.4, devastating the city, killing 4 and causing approximately $4 million dollars in damages. In June 1925, a large earthquake hit Lombard, destroying multiple local schools.

Tectonic Setting

The geology behind the earthquake is largely unknown. The faultline in question was unmapped before the earthquake, and the earthquake was too small and deep to leave a surface trace. The earthquake occurred in the Intermountain Seismic Belt, a common area of earthquake activity stretching from British Columbia in the north, to southern Utah in the south. It is responsible for earthquakes such as the 1983 Borah Peak Earthquake in Idaho, the 2020 Salt Lake City earthquake, and the 2020 Central Idaho earthquake. The region's mountainous landscape provides the root cause for the earthquake, forming around 60 million years ago during the Laramide Orogeny, when separate and discrete crustal sheets were shoved eastward over one another. The region in western Montana in particular that seems responsible for the earthquake is host to the Lewis and Clark Lineament, a prominent fault system of primarily right-lateral faulting, with some normal faulting and thrust faulting components, stretching from far northeastern Washington and northern Idaho southeast into central Montana. It is unclear, however, how exactly these faults are being reactivated to produce large earthquakes. One possibility is the stress that the Pacific Plate is placing on western Canada and the United States, providing a means for movement along these faults.