IS SEISMICITY IN WESTERN MONTANA AND CENTRAL IDAHO RELATED TO UPPER MANTLE STRUCTURES?

Intraplate earthquakes in western Montana and central Idaho form a parabolic pattern of seismic zone with its vertex near Bozeman. Spatial comparisons between the seismicity and various geophysical models, such as surface heat flow, magnetotellurics, and seismic tomography, are presented. Recent tomographic models using EarthScope’s USArray Transportable Array indicate a strong S-wave low velocity anomaly beneath the Snake River Plain (SRP) volcanic province possibly resulting from mantle flow at the tip and around an edge of a fragmented Farallon slab at depth below the SRP. The elongated low-velocity zone coincides with the axis of the Yellowstone seismic parabola. A similar elongate East-West trending low-velocity zone at the depth of the lithosphere-asthenosphere boundary coincides with the axis of the Montana-Idaho seismic parabola. This low-velocity strip may represent a tip-and-edge flow from another advancing remnant of the Farallon slab fragment—this one lying below the Idaho batholith. A lateral thermal contrast between the low-velocity zone and its surroundings may be responsible for the Montana-Idaho seismicity.