M5.4 - NORTHERN CALIFORNIA

Tectonic Summary

The magnitude (M) 5.4 earthquake that occurred on Wednesday, April 30, 2008 at 03:03:06 UTC (Tuesday, April 29, 2008 at 08:03:06 PM PDT) located at a depth of 28.5 km (17.7 miles). Both the moment tensor and first-motion fault plane solution indicate that the earthquake had a "normal" mechanism. By comparison of this quake's depth with the depth of background seismicity recorded since 1974 (map and cross-section), it is apparent that the quake occurs below the Gorda - North America plate interface.

Earthquakes that occur on the plate interface would likely have a "reverse" mechanism, such as observed in the April 25, 1992 M7.2 Cape Mendocino earthquake near Petrolia. The depth and mechanism of the April 30 quake indicate that it occurred within the Gorda plate and not on the plate boundary. The mechanism of the April 30 quake is also similar to those of previous, smaller quakes east of the Mendocino Triple Junction which also show normal mechanisms. Normal mechanisms likely reflect bending stresses within the subducting Gorda plate.

Historically, the closest M>5 earthquake to the April 30 earthquake occurred on 1954/12/21 19:56:29.00 UTC with a M=6.5. The quake locates 32 km to the WSW of the April 30 earthquake. Seismic instrumentation in 1954 was quite sparse, and therefore its depth is poorly determined. However, relocations of the quake indicate that it likely occurred at shallower depth that the April 30 quake.

Earthquake Information for California