Service Areas
Representative Experience - Seismic Hazard Assessment
Depth of earthquake activity in the northern California along latitude 40°N
Earthquake focal depths are used to determine the thickness of the seismogenic crust in a region. From a seismic hazard point of view this provides a lower limit to the area of brittle failure and helps constrains the maximum size of potential earthquakes. In the figure above two distinct distributions of seismic activity can be seen under the northern Coast Ranges of California. The earthquakes in the upper crust are associated with the system of right-lateral faults that accommodate motion between the Pacific plate and the Sierran microplate. The deeper activity is related to deformation in the subducted Gorda plate as it continues its downward trajectory along the southern Cascadia subduction zone. Note that the upper crustal seismicity deepens from west to east. This is due to the relatively high heat flow in the western Coast Ranges, which has elevated the brittle-ductile transition in the crust (Eberhart-Phillips and Oppenheimer, 1984; Castillo and Ellsworth, 1993). It has been postulated that as the Mendocino triple junction and the Gorda plate have migrated north along the coast of California, a subcrustal 'window' or 'slab gap' has developed that allows upwelling asthenospheric mantle to come in contact with the lithosphere of the Sierran microplate (Dickinson and Synder, 1979; Lachenbruch and Sass, 1980).