Types of Earthquakes
Offshore earthquakes
MORE THAN TWO-THIRDS of our large historic earthquakes have been located offshore on faults within the Gorda plate or along the Mendocino fault. Fortunately many of these earthquakes have been too far offshore to cause damage. However, more than 20 were close enough to the coast to knock down chimneys and damage buildings. For offshore earthquakes of M 7 or larger, tsunami warnings may be issued.
Onshore earthquakes
The most damaging Northern California earthquakes in the past century were caused by faults onshore. Earthquakes as small as M 5 can cause damage if they are close to populated areas. There are many faults throughout the region that are capable of producing earthquakes in the M 7 range.
The Big One – the Cascadia subduction zone
The world’s largest faults are associated with subduction zones and have produced earthquakes in the M 9 range! The last great earthquake on the Cascadia subduction zone occurred in 1700, a little over 300 years ago. Geologists have found evidence for at least 13 great Cascadia earthquakes during the past 7,000 years—and estimate they occur irregularly at intervals anywhere between 200 and 800 years. The next Cascadia earthquake may be similar to the M9 2011 Japan earthquake. It could cause strong ground shaking from Northern California to Southern Canada lasting for five minutes or longer. It will also produce a tsunami that could affect not only our coast, but other countries throughout the Pacific basin.