A rare earthquake in the upper mantle of subcontinental lithospheric mantle of North America

Earthquakes occurring in stable continental mantle lithosphere are extremely rare, with a small number of cases restricted to northern Australia, the India-Asia collision zone, and western North America. Here, we use a combination of regional seismic waveform modelling and teleseismic array data to confirm the location of a small (Mw 4.1) earthquake from September 2025 under northern Utah, whose depth places it firmly below the local Moho, and into the continental mantle. This joins other reported small-magnitude seismicity under the eastern Rocky Mountains, principally in Utah and Wyoming, who’s causative mechanism remains enigmatic. The focal mechanism of this earthquake is consistent with those of other reported mantle seismicity, but contrasts with the shallower crustal stress field, indicating that seismicity in the mantle in this region may not reflect the shallow crustal geodynamics of the region, and is unlikely to be playing a role in supporting long-term stresses.