Today's report from the USGS, regarding the 6.8 Ferndale Earthquake
Tectonic Summary
The March 10, 2014 Mw6.9 earthquake off the coast of northern California occurred as the result of the oblique strike slip motion on a fault approximately 80 km offshore of Eureka, California. The preliminary location places the earthquake within the Juan de Fuca plate (or Gorda subplate), which subducts beneath northern California, Oregon, and Washington at a rate of ~23 mm/yr. This location is outboard of the trench in the oceanic crust. The earthquake was widely felt along the coast of northern California and southern Oregon, particularly in the city of Eureka.
The general tectonics of this region are characterized by transitions between oceanic subduction of the Juan de Fuca plate beneath the Pacific northwest region and the continuation of the San Andreas Fault offshore. The intersection of the Juan de Fuca, North America, and Pacific plates forms the Mendocino Triple Junction off the west coast of California, with the subduction zone extending to the north and the San Andreas Fault diverging to the west offshore and continuing to the south. The offshore extension of the San Andreas Fault and southern extent of the Juan de Fuca plate are defined by the easternmost exposure of the Mendocino Fracture Zone. Several large earthquakes have occurred in this region since 1900 within 100 km of the March 2014 event, including events of M7.2 in 1922, M7.1 in 1923, M7.3 in 1980, M7.0 in 1994, M7.2 in 2005, as well as several events near the coast or inland of California, including the 1992 M7.2 Petrolia earthquake with its M6.6 and M6.4 aftershocks. Most recently, an earthquake of M6.5 in January 2010 with a similar faulting mechanism to the March 2014 event occurred.
The area discussed above is known as the "Mendocino triple junction." Fascinating geology.
