M6.5 - OFFSHORE NORTHERN CALIFORNIA

Tectonic Summary

This earthquake occurred approximately 35 km WNW of Ferndale, CA in a deformation zone of the southernmost Juan de Fuca plate that is commonly referred to as the Gorda plate. The earthquake's epicenter is northwest of the Mendocino Triple Junction, which is formed by the intersection of the Mendocino fracture zone, the San Andreas fault and the Cascadia subduction zone. The Gorda plate is subducting beneath the North America plate at about 2.5-3 cm/year in the direction N50E. The Gorda plate is also subjected to intense compressive stresses by oblique-convergence of the northwestward migrating Pacific Plate as well as localized eastward spreading at the Gorda Ridge. The resulting internal deformation of the Gorda plate is manifested primarily by intraplate strike-slip events on vertical NE-oriented faults.

Preliminary analysis of the earthquake indicates that it results from slip on a near vertical, left-lateral fault oriented about N47E. Large strike-slip earthquakes like this one are common in the interior of the Gorda plate. There are no reports of this earthquake causing a tsunami. Strike-slip earthquakes are less likely to produce large tsunamis because they cause relatively little vertical ground displacement. Shaking was strongest near the coast line between Petrolia and Eureka, CA, although felt reports for this event extend from as far south and north as Capitola, CA and Eugene, OR, respectively, and as far east as Reno, NV. The maximum recorded shaking was observed in Eureka (33%g), which is sufficient to cause moderate damage.

This is the largest quake to occur in this region since the April 25, 1992 M7.2 Petrolia and the June 15, 2005 M7.2 Gorda plate earthquakes.

The probability of a strong and possibly damaging aftershock (M>5) in the 7 days following the earthquake is approximately 78%. Most likely, the mainshock will be the largest in the sequence. However, there is a small chance (~5-10%) of an earthquake equal to or larger than this mainshock in the next 7 days. In addition, numerous M3-5 aftershocks are expected to occur in the same 7-day period, but most are unlikely to be felt due to the distance from land.