SHIPS - Serpentinized Upper Mantle Beneath Puget Lowland
SHIPS Maps Serpentinized (Hydrated) Upper Mantle Beneath Puget Lowland
SHIPS has mapped an region beneath Puget Lowland where the upper mantle has unusually low seismic velocities. The region having low velocities is also associated with an unusually large magnetic anomaly. In a recent paper by Blakely, Wells, and Brocher (2005), these unusually anomalies were explained by the serpentinization of the upper mantle: serpentinite has low seismic velocities but the magnetite associated with serpentinite makes it very magnetic.
Because the upper mantle has such a low seismic velocity, the boundary between the crust and the upper mantle makes a poor reflector of seismic waves. Seismologists call the reflection from this boundary PmP, and Brocher et al. (2003) showed that there is a narrow swath along Washington and Oregon where this reflection is very weak or even absent.
The attached map shows the location of the magnetic anomaly high along Washington and Oregon, with the locations of the weak upper mantle reflection superimposed. The existence of these anomalies confirms an earlier proposal that the dewatering of the subducting oceanic slab would hydrate (serpentinize) the upper mantle. So far so good, but the scientists had also proposed that the dewatering of the subducting slab should also produce earthquakes such as the 2001 Nisqually earthquake.
Such earthquakes do occur beneath Puget Sound, but not in Central Oregon, where the dewatering of the slab is thought to occur. Thus, scientists now think that the down going slab must undergo additional stresses, such as from additional bending or stretching, to produce these earthquakes.