Journal Article

Upper Mantle Earthquakes Along the Edge of the Wyoming Craton

Burlacu, Relu, Koper, Keith D., Zeng, Qicheng, Lin, Fan‐Chi, Zandt, George, Hutchings, Sean J.

2025

Overview

Earthquakes in continental regions overwhelmingly occur in the crust where low pressure and temperature promote brittle failure in response to tectonic stress. In rare cases, primarily in the thickened lithosphere near the Himalayas and Tibet, continental earthquakes occur in the uppermost mantle, perhaps implying an abnormally deep brittle‐ductile transition zone created by relatively low temperatures (≲600°C) and the increased strength of olivine‐rich mantle rocks. Here we present evidence for nine mantle earthquakes—only four of which were previously recognized—along the edge of the Wyoming Craton in the western U.S. Eight of the nine earthquakes occurred >15 km beneath the Moho where temperatures are likely above 700°C. We infer a mixture of brittle and ductile (thermal runaway) source processes facilitated by elevated strain rates from regional or edge‐driven mantle convection, which is thought to be a primary force behind crustal seismicity in the Intermountain West. Plain Language Summary Continental earthquakes typically occur in the uppermost 10 km of the crust. In rare cases, they can occur deeper in the uppermost mantle. The Intermountain West of the U.S. provides convincing evidence of intraplate upper mantle seismicity on the edge of the ancient Wyoming Craton in Utah and Wyoming, a portion of the stable interior of the North American continent. We present the most up to date map of upper mantle seismicity—nine confirmed events in total—beneath the Wyoming Craton through verification of earthquake depths and comparisons to crustal thickness. We find that these earthquakes are likely occurring in ductile mantle material at temperatures exceeding 700°C and are located in areas exhibiting rapid changes in lithospheric thickness. These earthquakes are likely facilitated by regional or localized mantle convective forces interacting with complex lithospheric structure, which is suspected to be a leading cause of crustal seismicity in the Intermountain West. Key Points We document the occurrence of nine small earthquakes in the upper mantle beneath Wyoming and Utah between 1979 and 2023 The earthquakes occurred along the western edge of the Wyoming Craton at relatively high temperatures (>700°C) Brittle and ductile (thermal runaway) source processes are likely facilitated by relatively high strain rates from mantle convection

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Brittleness / Convection / Cratons / Ductile-brittle transition / Earthquakes / Lithosphere / Low pressure / Low temperature / Mantle convection / Moho / Olivine / Seismic activity / Seismicity / Tectonics / Thermal runaway / Transition zone / Upper mantle