SEISMIC STATE
A look at why California gets so many earthquakes
California sits on one of the most active seismic zones in the world. Annually, the state records about 25,000 earthquakes.
California's ground is always moving.
Most of the time, that movement is too small to notice. But the state sits on one of the most active seismic zones in the world, where constant shifts deep underground regularly trigger earthquakes — from barely detectable tremors to stronger quakes that can be felt across wide areas.
That background level of activity is what makes sequences like the one that unfolded early June 3 off the Northern California coast a routine part of the state's seismic picture.
Near the Mendocino Triple Junction — a complex offshore meeting point of tectonic plates — a series of moderate earthquakes struck within hours of each other, including a magnitude 5.7 quake about 120 miles from McKinleyville, a magnitude 5.1 west of Petrolia in Humboldt County and a magnitude 4.5 aftershock minutes later, according to the U.S. Geological Survey.
How many earthquakes does California get daily?
A commonly cited estimate from the Berkeley Seismology Lab is that California records and analyzes about 70 earthquakes per day on average. Most are very small, not felt by people, and detected only by instruments.
Annually, California records about 25,000 earthquakes. The vast majority fall in the magnitude 1 to 3 range and go unnoticed.
Stronger earthquakes are far less frequent. The state averages two to three earthquakes each year of magnitude 5.5 or greater — large enough to cause moderate damage depending on depth, location and proximity to population centers.
What causes earthquakes?
The Earth has four layers: the inner core, outer core, mantle and crust. The crust and top of the mantle make up another area called the "lithosphere," which acts like a skin surrounding the Earth's surface.
The lithosphere, however, is not in one piece and exists like a puzzle or series of fragments called tectonic plates, according to the USGS. These parts of the lithosphere are not stationary and move slowly.
As the tectonic plates move and shift past one another, they occasionally bump or collide. This places stress on the plates' edges. When the stress becomes too great, it creates cracks called "faults." The point where these faults move against each other is called the "fault line."
When there is too much friction between the fault lines, energy is released suddenly, triggering seismic waves that lead to an earthquake.
Why are earthquakes so common in California?
California sits along the boundary between two massive tectonic plates — the Pacific Plate and the North American Plate. These plates are constantly moving, sliding past one another at different speeds.
The state also is home to more than 500 active faults, according to geologists, making it one of the most earthquake-prone regions in the United States. The most famous is the San Andreas Fault, a roughly 800-mile system that runs through much of the state and forms the primary boundary between the two plates. Other major fault systems include the Hayward Fault in the Bay Area, the Calaveras Fault in Central California, and the San Jacinto and Elsinore faults in Southern California.
But earthquakes are not limited to land. Many of them occur offshore, including near the Mendocino Triple Junction, where the Pacific, North American and Gorda plates all meet. That junction creates one of the most seismically active offshore regions along the West Coast.
Can earthquakes be predicted?
Earthquakes are not predicted. Neither the U.S. Geological Survey nor scientists have predicted a major earthquake, and there's no expectation for that to change in the near future.
What to do during an earthquake
■ Drop to your hands and knees.
■ Cover your head and neck under sturdy furniture if possible.
■ Hold on until shaking stops.
■ Stay indoors unless you are near a known coastal tsunami risk zone.
■ Stay informed. Get weather alerts via text.
