The 1921 Sevier Valley earthquake was a series of three earthquakes. The primary quake was a magnitude Mw6.3 earthquake that occurred on Thursday, 29 September 1921 at approximately 7:12 AM MT in Elsinore, Utah, United States. The first aftershock occurred in the evening on the same day, and a second aftershock occurred two days later on 1 October. No people were killed in the quake or in the subsequent aftershocks.
Earthquake
The primary earthquake struck on Thursday 29 September 1921 at approximately 7:12 AM MT in Elsinore, Utah, United States, lasting for 7–10 seconds.[1][2][3] This quake was preceded by several weeks of smaller quakes.[3] A major aftershock occurred on the same day at approximately 7:30 PM MT, and a second major aftershock occurred on Saturday, 1 October at approximately 8:32 AM MT.[3]
Magnitude and intensity
The initial quake was a Mw6.3 and an intensity of VIII ("Severe").[1][3][4] The official USGS report in their historical earthquakes list stated a magnitude of Mw5.2 and an intensity of VIII ("Severe").[2][5][6] An official USGS report published in 1988 assigned the modified magnitude of Mw6.3.[3] It was felt in an area of between 2,500 square kilometers (970 sq mi) and 5,100 square kilometers (2,000 sq mi), which is considered a small area for a quake of this size.[2][3] The farthest distance from the epicenter for the felt area was 25 miles (40 km). The quake lasted several seconds.[7]
The first major aftershock was a Mw5.7 and an intensity of VII ("Very Strong").[3] The epicenter was at the same location as the initial quake, and the felt area was similar.[3] The second major aftershock was the same magnitude and intensity as the initial quake, was in the same location, and felt in a similarly sized area.[3] This quake was reported as "quick and short as the detonation of a cannon".[3] The quakes were felt as far north as Salina and as far south as Marysvale.[1]
Destruction
Damage to chimneys and brick walls from the initial quake was "considerable".[3] Over US$100,000 in damage were caused (over US$1.44 million in 2020) by the primary quake.[8][9]
The town of Elsinore, reported sunken foundations, damaged roofs from collapsing chimneys and brick walls, and brick, adobe, and stone buildings were significantly damaged.[4] More than half of the residences in the town were seriously damaged, a dozen were damaged enough to be condemned, and most residences received some cracking and plaster damage.[3] The newly built schoolhouse had walls collapse and a damaged roof, causing it to have to be rebuilt.[7][8] Damage in Monroe included collapsed pipe trenches, cracked buildings, and the hot springs at the town turned "blood red" from iron oxide.[1][3] Large boulders were dislodged and some landslides were reported in canyons near Monroe.[1]
The first major aftershock—in the evening on the same day—did further damage to structures, especially those already affected by the primary quake.[1][3] Structures in Monroe received significantly more damage from this aftershock.[3] Damage was also reported in Richfield.[1]
On 1 October, the second major aftershock destroyed many of the damaged buildings, including a paint store in Elsinore. It also caused new damage to undamaged buildings, and caused previously-damaged buildings to be completely destroyed.[1][3] More large boulders were dislodged and additional landslides were caused in various canyons near the affected towns.[1] Monroe was especially hard hit, with almost all chimneys being destroyed and many buildings being damaged beyond repair.[1] A man seated high on a bank of a river was thrown down to the edge of the river, the hot springs were discolored by iron oxide again, and the Monroe City Hall was "shattered".[3]
No people were killed in the quake, though at least two were injured by falling bricks or plaster.[3]
