SHIPS - Subducting Slab Beneath Puget Lowland

SHIPS images subducting slab beneath Puget Lowland

SHIPS successfully imaged the slab subducting beneath Puget
Lowland to the depths where it produces large earthquakes,
such as the 2001 Nisqually earthquake.

SHIPS scientists were able to map the slab as it descends using
a very large seismic reflection produced by the contrast between
the crust of the down going slab and its upper mantle.

seismic profile

By mapping the down going slab so accurately, they were able to
determine the locations of the deep earthquakes relative to the down
going crust. Preston et al. (2003) report that as the slab descends the
earthquakes in the slab move up from the upper mantle to the
crust. At the depth where the 1949, 1965, and 2001 slab
earthquakes happened, the earthquakes lie within the crust and
the mantle.

This result implies that such earthquakes may rupture the upper
mantle, and thus can have larger magnitudes than those limited
strictly to the crust.