What Is the San Andreas Fault and Where Does It Run?

The San Andreas Fault is an 800-mile-long transform fault that marks the boundary between the Pacific Plate and the North American Plate. The Pacific Plate moves northwest relative to the North American Plate at roughly 1.5 to 2 inches per year — about the rate your fingernails grow. This constant motion builds stress that releases periodically as earthquakes.

The fault runs through California from Cape Mendocino in the far north to the Salton Sea in the south, passing through or near several of the state's most densely populated areas. It is not a single clean crack but a zone of fractures up to a mile wide in places. Geologists divide it into three major segments:

• Northern segment: Runs through the San Francisco Bay Area and Point Reyes Peninsula. This is the segment that ruptured in the catastrophic 1906 San Francisco earthquake, estimated at magnitude 7.9. Cities at elevated risk include San Francisco, Daly City, and Santa Cruz.
• Central creeping segment: Stretches roughly from Hollister to Parkfield. This segment moves slowly and continuously at about 1.3 inches per year near Parkfield, releasing stress gradually rather than in large ruptures. Hollister, California, has buildings visibly deformed by this continuous creep.
• Southern segment: Runs from Parkfield through the Coachella Valley to the Salton Sea. This segment has not had a major rupture since the Fort Tejon earthquake of 1857, estimated at magnitude 7.9. Over 160 years of accumulated stress make it one of the highest-risk sections in the United States.

You can view the fault's exact location using the USGS Quaternary Fault and Fold Database, which lets you enter your address and see fault proximity in detail.

Which Cities and Counties Face the Highest Risk?

Distance from the fault matters, but the type of ground beneath your home matters just as much. Soft sediment — bay mud, river alluvium, loosely packed fill — amplifies earthquake shaking significantly compared to solid bedrock. The same earthquake can feel two to three times more intense on bay mud than on granite a few miles away.

High-risk areas include:

• San Bernardino: The city sits almost directly on the fault trace. Cajon Pass, where the fault crosses Interstate 15, is a critical infrastructure chokepoint — a major rupture would likely cut road and rail connections between Southern California and the rest of the country.
• Coachella Valley (Palm Springs, Indio, Desert Hot Springs): These cities sit within a few miles of the southern San Andreas, the segment most overdue for a major rupture. The USGS ShakeOut scenario projects that a magnitude 7.8 rupture here would cause widespread building damage across Southern California.
• Los Angeles metro: Downtown L.A. is about 30 miles from the main San Andreas trace, but the metro area sits above several other active faults including the Puente Hills Fault running under the civic center and the Hollywood Fault running under the Hollywood Hills.
• East San Francisco Bay Area: The Hayward Fault, a major branch of the San Andreas system, runs directly under Oakland, Berkeley, Fremont, and Hayward. Many seismologists consider it overdue for a major rupture. Berkeley's UC campus and several hospitals sit directly on this fault.

To check your specific address, use the California MyHazards tool from Cal OES, which maps your proximity to fault lines, flood zones, and tsunami inundation areas in a single lookup.

How to Build a 72-Hour Emergency Supply Kit

FEMA and the California Governor's Office of Emergency Services both recommend maintaining at least 72 hours — three days — of supplies at home. After a major earthquake, first responders will be overwhelmed, roads may be impassable, water mains ruptured, and power out for days. A two-week supply is the extended goal, but three days is the non-negotiable baseline.

Water

Store one gallon per person per day. A family of four needs a minimum of 12 gallons for a three-day supply. Use commercially sealed water bottles (easiest to rotate) or food-grade plastic containers filled from the tap. Mark containers with the fill date and replace every six months.

Food

• Canned beans, tuna, sardines, and chicken (no refrigeration, high protein)
• Peanut butter, crackers, granola bars, dried fruit, and nuts
• A manual can opener — not an electric one
• A three-day supply of any prescription medications
• Infant formula or pet food if applicable to your household

First Aid and Safety

• Standard first-aid kit: bandages, antiseptic wipes, gauze, medical tape, pain reliever, antihistamine, and tweezers
• Flashlight with extra batteries, or a hand-crank flashlight that needs no batteries
• Battery-powered or hand-crank radio for emergency broadcasts when cell networks and internet are down
• Whistle to signal for help if trapped under debris
• N95 dust masks — post-quake debris creates significant particulate hazards for several days
• Wrench or pliers to shut off your gas and water lines at the meter

Documents and Cash

Store photocopies of your ID, insurance policies, bank account records, prescriptions, and mortgage or lease documents in a waterproof bag in your kit. Upload scans to cloud storage as a backup. Keep small bills — ATMs and card readers may be offline for days after a major event.

A go-bag that you can grab quickly should also include a change of clothes, sturdy closed-toe shoes for walking over broken glass, a warm layer, and a phone charger with a portable battery bank charged to full.

Securing Your Home Against Earthquake Damage

The most common sources of earthquake injury inside a home are not structural collapse — they are objects tipping, sliding, and falling. Structural collapses kill people, but falling bookshelves, water heaters, and refrigerators injure far more. Securing your home is a one-weekend project that costs under $100 in hardware.

Water Heater

Strap your water heater to wall studs using a metal plumber's strapping kit, available at any hardware store for about $15. Install two straps: one in the upper third of the heater and one in the lower third. An unrestrained water heater can tip during shaking, rupture the gas supply line, and ignite a fire — the leading cause of earthquake-related structure fires.

Tall Furniture

• Bookshelves and wardrobes: Attach L-brackets (at least 3 inches per side) into wall studs near the top of each unit. A stud finder costs about $20 and makes the job straightforward. Angle brackets designed specifically for earthquake strapping are sold at home centers.
• Refrigerators: Use an appliance anti-tip strap or a refrigerator restraint kit. These attach to the back of the appliance and anchor to the wall stud behind it.
• Flat-screen TVs: Mount to the wall with a proper wall mount rated for your screen size. If you prefer not to mount, use hook-and-loop anti-tip straps that tether the base to the furniture beneath it.

Cabinets

Install childproof cabinet latches on kitchen and bathroom cabinets so heavy items — cast iron pans, full jars, bottles — do not spill out onto people below. Move the heaviest items to the lowest shelves. Place rubber shelf liners under items on open shelves to reduce sliding.

Windows

Apply safety window film to large windows and glass doors. It costs roughly $30 to $50 per window and does not prevent glass from breaking, but it holds the shards together in a sheet, dramatically reducing laceration injuries during and after shaking.

Know Your Utility Shutoffs

Locate your gas shutoff valve — usually on the outside of the house next to the meter — and keep a pipe wrench attached nearby. Locate your main water shutoff as well. Only shut off gas if you actively smell it or hear a hissing sound; once shut off, a gas utility must physically inspect and restore service, which can take days to weeks after a major disaster.

What to Do During an Earthquake: Drop, Cover, Hold On

The correct response in most situations is Drop, Cover, and Hold On. This is the protocol endorsed by FEMA, the USGS, and the American Red Cross, based on decades of post-earthquake injury analysis.

1. Drop to your hands and knees immediately. This position prevents you from being knocked to the floor by violent shaking and lowers your center of gravity.
2. Cover your head and neck with one arm and hand. Move under a sturdy desk or table if one is within a step or two. If not, move next to an interior wall away from windows and crouch low, covering your head.
3. Hold On to your shelter and be prepared to move with it. Stay in place until the shaking completely stops — even brief pauses in shaking are often followed by renewed motion.

What Not to Do

• Do not run outside during shaking. Most earthquake injuries occur in the first moments when people try to move or flee buildings. Falling debris, shattered glass, and unstable facades cause far more casualties than building collapses in typical earthquakes.
• Do not stand in a doorway. This is an outdated myth based on old adobe construction. In modern wood-frame or concrete buildings, doorframes are no stronger than any other part of the structure, and you have no protection from flying objects.
• Do not use elevators after shaking stops — they may be damaged or lose power during the aftershocks that follow a major quake.

If You Are in a Car

Pull over away from overpasses, bridges, trees, power lines, and tall signs. Stay inside with your seatbelt on until the shaking stops. After motion ends, proceed carefully — roads may be cracked or heaved, and bridges should be treated as potentially unsafe until inspected.

If You Are Outdoors

Move away from buildings, utility wires, and trees. The greatest outdoor hazard is falling debris from building facades, parapets, and glass curtain walls. Drop to the ground and protect your head and neck with your arms until shaking stops.

What to Do Immediately After an Earthquake Stops

The 30 minutes after shaking stops are critical for assessing and preventing secondary hazards — particularly fires from gas leaks, which are the leading cause of earthquake-related deaths in urban areas.

1. Check yourself and those around you for injuries. Administer first aid before moving seriously injured people — spinal injuries can be worsened by premature movement. Call 911 only for life-threatening emergencies; for a major quake, systems will be overwhelmed and response delayed.
2. Check for gas leaks. If you smell gas or hear a hissing sound, do not operate any electrical switches — including light switches and the garage door opener. Open windows and doors, leave the building immediately, and call your gas utility from a safe distance outside.
3. Check for fires. Extinguish small, contained fires with a household fire extinguisher if it is safe to do so. If fire is spreading, evacuate immediately.
4. Inspect utilities. If you see sparking wires or damaged electrical panels, shut off electricity at the main breaker. Only shut off water if you see pipes are visibly broken and actively spraying.
5. Assess your building's structural condition. Look for large cracks in load-bearing walls, sagging rooflines, or leaning structures. Do not re-enter a building that shows any of these signs — a building that partially collapsed in the main shock is far more likely to collapse fully in an aftershock.
6. Expect aftershocks. A major earthquake is almost always followed by multiple aftershocks — some potentially powerful enough to bring down already-damaged structures. Treat each aftershock with the same Drop, Cover, Hold On response.
7. Monitor emergency broadcasts. Use your battery-powered or hand-crank radio to tune to local emergency management stations for shelter locations, road closures, and boil-water advisories. The FEMA app and your county's emergency notification system (look it up now, before a quake) provide location-based alerts.

Earthquake Insurance and Financial Preparedness

Standard homeowner's and renter's insurance policies in the United States do not cover earthquake damage. This surprises many people until the moment they need to file a claim. You need a separate earthquake insurance policy.

California Earthquake Authority

California residents can purchase residential earthquake insurance through the California Earthquake Authority, the largest provider of earthquake insurance in the country. CEA policies are sold through participating home insurance companies. Coverage options typically include dwelling coverage (the structure), personal property coverage (furniture, electronics, clothing), loss-of-use coverage for temporary living expenses while your home is repaired, and emergency repair coverage.

Deductibles are typically 5% to 25% of the dwelling coverage limit. On a $500,000 home with a 15% deductible, you pay the first $75,000 out of pocket before coverage kicks in. Compare deductible percentages carefully — a lower deductible costs more in premium but provides substantially better protection after a catastrophic event.

Document Your Possessions Now

Walk through your home today and record a video inventory of your belongings, narrating the make, model, and approximate value of significant items. Store the video in cloud storage — not only on a local hard drive that may be destroyed or buried. This documentation dramatically speeds up insurance claims after a disaster.

Check Your Home's Retrofit Eligibility

Older California homes built before 1980 on raised wood foundations are particularly vulnerable to cripple-wall collapse, where the short wooden stud walls between the foundation and the first floor buckle inward. The state-funded Earthquake Brace + Bolt program provides up to $3,000 toward seismic retrofitting costs for eligible homeowners. Check eligibility and apply at earthquakebraceandbolt.com — retrofits cost $3,000 to $7,000 total and significantly reduce the risk of the type of damage that makes homes temporarily uninhabitable.