You live in Earthquake Country
Most earthquakes occur at or near plate boundaries. The motion of these plates stress faults through Northern California.
In the past 150 years, nearly 40 earthquakes of magnitude 6 or larger have affected Northern California. Most of these earthquakes were centered on faults nearby. But very large earthquakes located elsewhere in the Pacific basin, like the 1964 magnitude 9.2 Alaska earthquake can generate tsunamis that threaten our coast.
All areas of Northern California have experienced earthquakes in the past and will do so again in the future. Since 1900, nearly 40 earthquakes of M 6 or larger have occurred in California north of Santa Rosa and in the adjacent offshore areas.
Plate Motions Load the Faults
Three plates meet on California’s North Coast at the Mendocino triple junction. To the north of the triple junction, the Gorda plate is pulled to the northeast beneath the North American plate at a few inches per year. To the south of the triple junction the Pacific plate grinds to the northwest past the North American plate at a similar speed.
California’s most damaging earthquakes of the past 150 years, such as the 1906 “San Francisco” earthquake, have occurred on faults in the San Andreas fault system. While we are at risk of future San Andreas tremors, there are many other seismic zones, some capable of producing earthquakes as large or larger as the one in 1906. North of the Mendocino triple junction lies the 700 mile long Cascadia subduction zone, believed capable of producing magnitude (M) 9 earthquakes. Faults in northeastern California show evidence of past earthquakes in the M 7 range.
Types of 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 13 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.
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 earthquake that set off the 2004 Indian Ocean tsunami. 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.
Recent Earthquakes Don’t Tell the Whole Story
The first seismographs were installed in California just over 100 years ago and written records only go back as far as the mid-1800s. For evidence of earlier earthquakes, scientists look for geologic clues such as surface fault rupture and tsunami deposits, oral history of native peoples and written records from distant areas that were affected by a Northern California-generated tsunami. The study of ancient earthquakes is called paleoseismology.