Earthquakes

Earthquakes occur primarily along plate boundaries — divergent (mid-ocean ridges), convergent (subduction zones, continental collisions), and transform (transcurrent faults like the San Andreas). Intraplate earthquakes also occur, often along ancient suture zones; the New Madrid sequence of 1811–1812 is the canonical US example.

Three main types of faulting:

• Strike-slip: blocks slide horizontally past one another (San Andreas Fault).
• Reverse / thrust: compressional, one block thrust over another (Cascadia, Wasatch, Andean subduction zones).
• Normal: extensional, one block drops relative to the other (Basin and Range, parts of the Wasatch).

The largest earthquakes are produced by megathrust faulting at subduction zones, where one tectonic plate is forced beneath another. The 2011 Tohoku-Oki M9.1, 2004 Sumatra-Andaman M9.1–9.3, and 1964 Alaska M9.2 events all originated at subduction-zone megathrusts.

The US Geological Survey (USGS) operates the Advanced National Seismic System and the ShakeAlert earthquake early warning system, which delivers seconds-to-tens-of-seconds of warning to areas in California, Oregon, and Washington before strong shaking arrives. Early warning does not predict earthquakes — no scientifically validated method does — but it detects them as they begin and uses the faster P-waves to alert before the slower, damaging S-waves arrive.

Induced seismicity, primarily from deep wastewater injection associated with oil and gas operations, has increased earthquake counts in Oklahoma, Texas, Kansas, and parts of Ohio over the past two decades. The largest induced events have reached M5.8 (Pawnee, OK, 2016). USGS hazard models now include induced-seismicity contributions for the central US.

Earthquakes have no seasonality. Statistical analyses of large catalogs find no significant correlation with time of year, lunar cycles, weather, or atmospheric pressure for tectonic earthquakes. Planning should be year-round and triggered by location, not calendar.

What does vary over time is forecast probability. The USGS publishes time-dependent earthquake forecasts for California (UCERF) and aftershock forecasts for any large mainshock. After a M6+ event in an urban area, aftershock forecasts are updated daily for weeks; residents in affected regions should follow the USGS guidance on aftershock preparedness.

Earthquake preparedness has three components: structural (the building itself), non-structural (contents inside the building), and operational (supplies and plans).

Structural preparedness

• Older single-family homes in seismic-prone regions often lack foundation bolts and cripple-wall bracing. Retrofit kits and grant programs are available through the California Earthquake Authority's Brace + Bolt program, the FEMA Building Resilient Infrastructure and Communities (BRIC) program, and many county programs in Oregon, Washington, and Utah.
• Soft-story buildings (apartments with parking on the ground floor and units above) collapsed at high rates in the 1989 Loma Prieta and 1994 Northridge earthquakes. Many California cities now require mandatory soft-story retrofits; verify your building's status with your local building department.
• Mobile homes should be secured with engineered tie-downs and earthquake-resistant pier systems.

Non-structural preparedness

• Secure tall furniture (bookcases, dressers, water heaters) to wall studs with strapping.
• Install latches on kitchen cabinets to prevent dishware ejection.
• Move heavy items off high shelves.
• Replace rigid gas connectors with flexible connectors and verify the location of your gas shutoff valve.

Operational preparedness

• Stock the standard 72-hour kit (water, food, medications, flashlight, NOAA Weather Radio, battery banks), and add a sturdy pair of closed-toe shoes near the bed for stepping on broken glass.
• Make an out-of-state contact who can serve as a communication hub for family members.
• Identify a meeting place outside your home and a second meeting place outside your neighborhood in case roads are blocked.
• Buy earthquake insurance if you live in a high-hazard region. Standard homeowners insurance does not cover earthquake damage. The California Earthquake Authority is the primary US residential earthquake insurer; other states have private markets.
• Enroll in ShakeAlert via the MyShake app (California, Oregon, Washington) or your local public-alert system.

Earthquakes begin without warning, although ShakeAlert may deliver seconds of warning before strong shaking arrives.

Drop, Cover, and Hold On is the consensus US safety guidance from the USGS, FEMA, the American Red Cross, and the Earthquake Country Alliance.

• Drop to your hands and knees. This prevents the earthquake from knocking you down.
• Cover your head and neck with your arms. If a sturdy table or desk is within easy reach, take cover under it.
• Hold on to your shelter (or your head and neck) until shaking stops.

Do not run outside during shaking. Most earthquake injuries result from being struck by falling objects or hit by failing exterior facades; the safest place is generally where you already are, low and covered.

Do not stand in a doorway. Modern doorways are no stronger than the rest of the house, and the door itself can swing and injure you.

If you are in bed, stay there, cover your head and neck with a pillow, and hold on. If you are driving, pull to the side of the road, stop, and stay in the vehicle until shaking stops, avoiding overpasses and power lines.

Tsunami evacuation: any coastal resident who feels strong shaking that lasts 20 seconds or longer, or sees rapid sea-level recession, should move to high ground immediately on foot. Official tsunami warnings may not arrive in time for locally generated tsunamis from nearby subduction zones, especially in the Pacific Northwest.

Once shaking stops:

1. Check yourself and others for injuries. Help injured people if you can do so safely.
2. Check for hazards: fire, gas leaks (turn off only if you smell gas or hear hissing), structural damage, downed power lines.
3. Expect aftershocks. The largest aftershocks typically occur within hours to days of the mainshock; the M6.6 Loma Prieta aftershock sequence produced damaging aftershocks for weeks.
4. Document damage with date-stamped photos and video before any cleanup.
5. File your earthquake insurance claim. Earthquake claims are paid separately from any homeowners claim; the deductibles are typically 5–25% of the dwelling limit.
6. If federally declared, apply for FEMA Individual Assistance at DisasterAssistance.gov.
7. Have your home inspected by a licensed structural engineer before assuming it is safe to occupy long-term. The American Society of Civil Engineers' ATC-20 protocol guides post-earthquake structural assessment.
8. The SAMHSA Disaster Distress Helpline (1-800-985-5990) is available 24/7 for survivors processing the psychological aftermath of major earthquakes.