California’s dramatic landscapes are a direct result of the immense geological forces constantly at work beneath the surface. The state is situated on the boundary of two major tectonic plates, making seismic activity a regular part of life. When people ask about the last “big” earthquake, they are usually referring to an event of magnitude 6.5 or higher, or one that caused significant, widespread damage. Understanding the most recent major event provides context for the ongoing seismic risk that shapes California’s infrastructure and preparedness efforts.
Identifying the Last Major Earthquake
The most recent significant earthquake to strike California was the magnitude 7.1 mainshock that occurred on July 5, 2019, near the town of Ridgecrest. This powerful event was centered in a remote area of the Mojave Desert, near Searles Valley in Kern County, approximately 150 miles north of Los Angeles. The rupture occurred at a relatively shallow depth of about 10.5 miles, leading to intense ground shaking.
This mainshock followed a large foreshock, a magnitude 6.4 event, which had struck the day before on July 4, 2019. The two earthquakes were part of a rapid sequence, representing the most powerful seismic activity in the state in two decades. The M7.1 quake caused a surface rupture visible for dozens of miles.
Scope of Damage and Recovery
Although the epicenter was in a sparsely populated region, the 2019 Ridgecrest earthquake sequence caused substantial and costly damage to local infrastructure and military facilities. The most significant structural impact was felt in the small communities of Trona and Ridgecrest, where older homes, particularly mobile homes and unreinforced masonry buildings, suffered the worst damage. Several homes in Trona were knocked off their foundations, and multiple rockslides made roads to the town temporarily impassable.
The economic loss stemming from the earthquake sequence was estimated to be at least $1 billion. A major portion of the cost was attributed to the nearby Naval Air Weapons Station China Lake, a sprawling military base that experienced extensive infrastructure damage. Repair and replacement costs for the base exceeded $5 billion. In the immediate aftermath, state and federal emergency declarations were issued to mobilize resources, focusing on restoring power, repairing ruptured water lines, and clearing debris.
The Underlying Tectonic Forces in California
California’s intense seismic activity is a direct consequence of its position straddling the boundary between the Pacific Plate and the North American Plate. This boundary is defined by the San Andreas Fault system, which acts as a massive transform fault. The Pacific Plate is grinding slowly northwestward, while the North American Plate moves generally southeastward, leading to a constant accumulation of tectonic stress.
Friction causes sections of the fault to lock up for decades or centuries, leading to a build-up of strain energy. When the accumulated stress exceeds the strength of the rock, the fault ruptures, resulting in a sudden, violent release of energy known as an earthquake. The 2019 Ridgecrest events occurred on a previously unmapped network of faults within the Eastern California Shear Zone, a region that accommodates some of the plate boundary motion away from the main San Andreas Fault trace.
Personal and Community Preparedness
Given the constant threat of seismic activity, personal and community preparedness is paramount for all California residents. The most immediate safety protocol during shaking is “Drop, Cover, and Hold On,” which involves dropping to the ground, taking cover under a sturdy piece of furniture, and holding on until the shaking stops. This simple action significantly reduces the risk of injury from falling objects.
Preparing your home involves securing heavy furniture, such as bookcases and water heaters, to wall studs to prevent them from toppling. Every household should also assemble an emergency kit containing enough water, non-perishable food, and necessary medications to be self-sufficient for at least 72 hours. Furthermore, the state provides access to the ShakeAlert system, an earthquake early warning system that can deliver a few seconds of advance warning to mobile devices.