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 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][3][4] The official USGS report in their historical earthquakes list stated a magnitude of Mw5.2 and an intensity of VIII ("Severe").

[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.

[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] On 1 October, the second major aftershock destroyed many of the damaged buildings, including a paint store in Elsinore.

[1][3] More large boulders were dislodged and additional landslides were caused in various canyons near the affected towns.