On Thursday, December 5th, a major 7.0 magnitude earthquake rocked Northern California, prompting tsunami warnings across California and Oregon’s coastlines and evacuations in certain areas affected. The quake occurred about 100km southwest of Ferndale, CA at 10:44am PDT.

The Event

This earthquake happened in an area known as the Mendocino Fracture Zone, where the Pacific and Juan de Fuca/Gorda tectonic plates meet. Here, the Gorda plate moves eastward relative to the Pacific plate, meeting as a right-lateral strike-slip fault. This region is no stranger to seismic activity: five events of magnitude 7 or higher have occurred here in the last 124 years.

The earthquake was preceded by multiple relatively minor foreshocks, signaling that an event could potentially take place here. The 7.0 earthquake is considered the mainshock in the sequence, with many aftershocks expected to follow, some of which could be upwards of magnitude 5.0.

Immediately after the earthquake rocked Northern California, sending seismic waves that were felt as far south as Phoenix, Arizona, tsunami warnings were issued for parts of the California and Oregon coast. However, due to its nature as a strike-slip fault, the earthquake only produced a very minor tsunami, and thus the warnings were cancelled. Strike-slip earthquakes, even those undersea, tend not to produce tsunamis when they rupture.

What about the San Andreas/Cascadia subduction zone?

While this quake stirred up many longstanding fears about major earthquakes occurring along the subduction zone to the north and the major fault line to the south, there has been little evidence pointing to this specific event triggering a rupture along either. While it’s true that a major rupture along Cascadia could trigger the San Andreas, this quake along the Mendocino Fracture is unlikely to do so. However, all three fault zones still present a major hazard and should be watched closely. Remember, if you live in ANY of these areas, or really any earthquake-prone area, it’s important to keep a plan in place and know your procedures!