California is widely recognized for its seismic activity, a result of its unique geological setting where massive sections of the Earth’s crust constantly interact. This frequent movement shapes the landscape and makes earthquakes a regular occurrence across the state.
California’s Geological Foundations
California’s earthquake activity stems from its location atop the boundary of two immense tectonic plates: the Pacific Plate and the North American Plate. These colossal segments of the Earth’s outer layer are continuously moving. The Pacific Plate, which underlies much of the Pacific Ocean and parts of California’s coast, grinds northwestward. Conversely, the North American Plate, carrying the rest of the continent, moves in a generally southwestward direction.
The primary interface between these two plates in California is the San Andreas Fault system, a right-lateral strike-slip transform fault. This fault extends for approximately 1,200 kilometers (750 miles) through California, from the Salton Sea in the south to Cape Mendocino in the north. Along this boundary, the plates slide horizontally past each other. This lateral motion averages about 33 to 37 millimeters (1.3 to 1.5 inches) per year across California.
Movement along the San Andreas Fault and its associated branch faults, like the Hayward Fault and Garlock Fault, is not smooth. Friction causes sections of these faults to become “locked,” preventing steady movement. As the plates continue to move, stress and elastic energy build up in the rocks along the fault lines.
When this accumulated stress exceeds the strength of the rocks, the fault suddenly ruptures, releasing stored energy as seismic waves. This sudden release is what people experience as an earthquake.
Earthquake Frequency and Strength
California experiences tens of thousands of earthquakes annually. While many of these are minor and go unnoticed by residents, advanced seismic instruments detect approximately 51,000 per year. The vast majority are small, often below magnitude 3.0, and are typically not felt by people.
Earthquake strength is measured using the Moment Magnitude Scale (MMS or Mw), which quantifies the energy released. This scale considers the rupture area, the amount of slip on the fault, and the force required to move it. It is a logarithmic scale, meaning each whole number increase represents about 32 times more energy released. For example, a magnitude 7.0 earthquake releases 1,000 times more energy than a magnitude 5.0 event.
While smaller tremors are constant, larger, more destructive earthquakes are less frequent. On average, California experiences about 48 earthquakes of magnitude 4 or higher per year. Earthquakes of magnitude 5 or higher occur about 4.5 times per year. Major earthquakes, those with a magnitude of 7 or higher, are much rarer, occurring about once every 5 to 10 years in California. Southern California has seen an average of 10 to 12 earthquakes of magnitude 4.0 or greater annually since 1932.
Living with Earthquakes
Living in California requires integrating earthquake preparedness into daily life, focusing on mitigating risks and enhancing safety. One measure is the implementation of stringent building codes and engineering practices. Newer buildings are designed to be more resilient to seismic activity, prioritizing the safety of occupants during an earthquake, though this does not guarantee structures will avoid all damage. Modern construction techniques allow buildings to sway or flex with ground motion, dissipating seismic energy rather than rigidly resisting it.
Personal preparedness is also important. Residents are encouraged to practice “Drop, Cover, and Hold On,” a protective action recommended during shaking. This involves dropping to the ground, taking cover under a sturdy piece of furniture like a desk or table, and holding on until the shaking ceases. If no sturdy cover is available, crouching against an interior wall and protecting the head and neck can offer some safety.
Developing a family emergency plan is another important step. This plan includes identifying an out-of-state contact person for family members to check in with if separated, as long-distance phone service may be restored faster than local service after a major event.
Preparing emergency kits, often called “Go-Bags,” is also advised. These kits should contain essential supplies for at least 72 hours, such as water, non-perishable food, a first-aid kit, medications, a flashlight, a portable radio, and copies of important documents. Securing heavy furniture and appliances in homes can prevent them from falling and causing injury during shaking. These measures help reduce the impact of seismic events.