Los Angeles is highly susceptible to earthquakes due to its location directly atop a complex and active network of fault lines. The region sits at the boundary of the Pacific and North American tectonic plates, which are constantly grinding past one another. This constant geological movement means that seismic activity is a reality for everyone living in or visiting the Los Angeles area.
The Major Fault Systems Underlying Los Angeles
The reason for Los Angeles’s seismic activity is its proximity to the boundary where the Pacific Plate moves northwest relative to the North American Plate. This plate movement is primarily accommodated by the San Andreas Fault, which acts as a major transform, or strike-slip, boundary. The San Andreas Fault is located about 35 miles northeast of Los Angeles and is capable of generating an earthquake up to magnitude 8.1.
The southern segment of the San Andreas has not experienced a major rupture in over 160 years, meaning it is currently storing tremendous energy. The Los Angeles area also faces threats from numerous closer, secondary faults that accommodate the region’s overall compression. This compression results in thrust faults, where one section of the crust is pushed up and over another.
One of the most dangerous local structures is the Puente Hills Thrust Fault, a “blind” fault that does not reach the surface and runs directly beneath the densely populated Los Angeles Basin, including downtown. A rupture on this fault could generate a magnitude 7.5 earthquake. Due to its location directly under the city’s infrastructure, a magnitude 7.5 on the Puente Hills fault could cause damage comparable to an 8.0 on the more distant San Andreas.
Another significant local threat is the Newport-Inglewood Fault Zone, a right-lateral strike-slip system running for over 47 miles from Culver City through Long Beach and offshore. This fault was responsible for the devastating 1933 Long Beach earthquake, a magnitude 6.4 event. Modern studies suggest that the Newport-Inglewood fault could rupture in a magnitude 7.3 or 7.4 event, posing a severe hazard to coastal and western Los Angeles.
Frequency and Typical Seismic Activity
Southern California experiences constant, low-level seismic activity, with an estimated 10,000 earthquakes per year. The vast majority of these tremors are below magnitude 3.0 and are too small to be felt by people. This constant background noise is simply the earth’s crust adjusting to tectonic pressures.
The region experiences a noticeable number of moderate earthquakes, with an average of about 50 events of magnitude 4.0 or greater occurring in California each year. The scale of an earthquake is measured by its magnitude, which reflects the energy released at the source. The intensity of shaking felt on the surface is measured by scales like the Modified Mercalli Intensity, which depends on factors like distance from the epicenter and local geology.
The most significant future risk is the “Big One,” a major rupture on the southern San Andreas Fault, which last experienced a large earthquake in 1857. Geologists estimate there is a 60% chance of a magnitude 6.7 or greater earthquake striking the Los Angeles area within the next 30 years. Seismologists use a vast network of sensors, including the statewide early warning system, to monitor for foreshocks and measure ground motion in real-time.
Essential Earthquake Safety and Preparedness
The most immediate safety action during earthquake shaking is to follow the “Drop, Cover, and Hold On” protocol. This means dropping immediately to the ground, taking cover under a sturdy desk or table, and holding onto it until the shaking completely stops. It is important to stay away from windows and heavy furniture that could fall, and not to run outside during the shaking.
Preparing your home environment minimizes potential injury and damage. Heavy items like televisions, bookcases, and water heaters should be secured to the wall studs to prevent them from tipping over. Moving tall or heavy objects away from beds and sitting areas is a simple way to increase safety.
Residents should assemble and maintain an emergency “Go-Bag” with supplies for self-sufficiency, as emergency services may be overwhelmed for at least 72 hours. This kit should contain:
- Non-perishable food
- Water
- A first aid kit
- Essential medications
- A battery-operated radio
- Cash
Families should also establish a communication plan, including an out-of-state contact, and designate an outdoor meeting place in case they are separated during the event.