Earthquake Aftershock Survival, Structural Tag Inspection, and Safe Re-Entry¶

The end of mainshock ground shaking is only the beginning of an earthquake emergency. Every moderate-to-great earthquake is followed by dozens to thousands of aftershocks, which are secondary fault ruptures triggered as stress redistributes along the ruptured fault plane and adjacent crustal faults. While aftershock frequency decays over time according to Omori's Law (\(n(t) = K / (t + c)^p\)), individual aftershocks frequently reach a magnitude only 1.1 to 1.2 units smaller than the mainshock (Båth's Law), meaning a Magnitude 7.2 mainshock commonly generates a Magnitude 6.0 to 6.1 aftershock minutes, hours, or days later, and roughly 5 to 6 percent of major earthquakes turn out to be foreshocks to an even larger mainshock within 72 hours. Because the initial earthquake has already cracked shear walls, yielded steel welds, spalled concrete columns, and loosened brick parapets (cumulative cyclic degradation), a moderate Magnitude 5.0 to 6.0 aftershock routinely collapses buildings that remained standing through the mainshock.

According to the United States Geological Survey (USGS), the Federal Emergency Management Agency (FEMA), the Applied Technology Council (ATC-20 Postearthquake Safety Evaluation of Buildings standard used across North America, Europe, Japan, and New Zealand), and international Urban Search and Rescue (USAR / INSARAG) teams, a large proportion of post-earthquake fatalities and severe injuries occur after the mainshock, when residents rush back into structurally compromised buildings to retrieve pets, valuables, or documents, or when someone shuts off or turns back on a natural gas meter improperly. Knowing how to brace during an aftershock inside a damaged structure, how to read official Green, Yellow, and Red ATC-20 building safety placards, how to perform an exterior-to-interior structural walkaround before stepping through a doorway, and when to turn your gas service valve a quarter-turn off saves lives in the hours and days following a quake.

Do this now¶

1. Drop, Cover, and Hold On immediately whenever an aftershock strikes, and move AWAY from damaged masonry walls, chimneys, and glass facades. Whenever you feel secondary shaking, drop to your hands and knees before the jolt knocks you down, crawl under a sturdy desk or table (or against an interior load-bearing wall if no furniture is nearby), and hold on with one hand while protecting the back of your head and neck with the other. If you are outdoors after the mainshock, stay in open space at least 1.5 times the building height away from unreinforced brick buildings, stone churches, overhangs, and utility poles.
2. Evacuate to open ground immediately after the mainshock stops if your building shows severe structural distress, gas leaks, or fire. Put on hard-soled shoes and work gloves before stepping out of bed or off a couch (floors are covered in shattered window glass and drywall nails), grab your go-bag, and exit via the stairs (never use an elevator after an earthquake, as counterweights derail from their guide rails and trap passengers between floors).
3. Check for a natural gas leak using smell, sound, and the meter test dials, and shut off the main gas valve ONLY if a leak is indicated. Go to your exterior natural gas meter:
4. If you smell the rotten-egg odor of mercaptan, hear gas hissing, see the small \(1/2\text{-foot}\) or \(2\text{-foot}\) test dial moving while all appliances are off, or see structural shifting around the meter: Use a 30-centimeter (12-inch) adjustable crescent wrench or earthquake gas shutoff tool to turn the rectangular brass lug on the supply pipe (inlet side) one-quarter turn (90 degrees) in either direction until the lug sits perpendicular (crosswise) to the pipe.
5. If there is NO smell, NO hissing, and NO dial movement: Leave the gas ON so you retain heating and hot water; utility companies can take weeks to relight thousands of unnecessarily shut-off meters.
6. Perform a 360-degree exterior structural walkaround from a safe distance before attempting to re-enter any home or workplace. Before walking up to your front porch, inspect all four sides of the building from at least 5 to 10 meters (16 to 33 feet) back: look for a house shifted off its concrete foundation sill plate, a leaning ground-floor garage (soft-story racking), a cracked brick chimney leaning over the roof, or downed overhead power lines draped across the eaves or metal fence.
7. Obey official ATC-20 / municipal building inspection placards (Green, Yellow, Red) posted on your door. Never remove, cover, or ignore a building safety placard:
8. GREEN ("INSPECTED"): Lawful occupancy permitted; no apparent structural hazard found, though minor non-structural repairs may be needed.
9. YELLOW ("RESTRICTED USE"): Specific hazards identified; entry is restricted to brief, supervised property retrieval or specific safe rooms only, with no overnight sleeping unless expressly stated on the tag.
10. RED ("UNSAFE"): Extreme collapse or life-safety hazard; entry is strictly prohibited by law until structural engineers shore or demolish the building.
11. If a family member or neighbor is trapped under heavy concrete slabs or collapsed floor joists, call emergency services and DO NOT pull out load-bearing debris. Call your local emergency number (911 in the United States and Canada, 112 across the European Union, 999 in the United Kingdom, or see emergency numbers) and alert Urban Search and Rescue (USAR) or CERT teams. Only lift light surface rubble if you can crib and wedge every centimeter of lift with solid wood timbers so an aftershock cannot drop the beam back onto the victim.

Post-earthquake re-entry, gas shutoff, and aftershock survival decision tree¶

Use this field decision flowchart immediately after the mainshock stops to decide whether to shut off utilities, evacuate, or safely inspect your home.

flowchart TD
    Start["Mainshock Shaking Stops:<br/>Put On Shoes & Gloves First"] --> ImmediateCheck{"Do you smell gas, see smoke/fire,<br/>or hear the building creaking/shifting?"}

    ImmediateCheck -->|"YES: Gas Smell, Fire, or<br/>Active Structural Cracking"| EvacNow["IMMEDIATE EVACUATION:<br/>1. Exit via STAIRS only (Never use elevators)<br/>2. Watch overhead for falling brick parapets, chimneys & glass<br/>3. Shut off exterior main gas valve (90° crosswise to pipe)<br/>4. Move to open space 1.5x building height away from structures"]

    ImmediateCheck -->|"NO Immediate Odor or Collapse"| ExteriorScan["360° EXTERIOR STRUCTURAL WALKAROUND:<br/>Inspect all 4 sides from 5 to 10 m (16 to 33 ft) back:<br/>- Is the ground floor leaning or garage door jammed?<br/>- Did the house slide off its concrete foundation?<br/>- Are concrete columns X-cracked or rebar buckled?<br/>- Is the brick chimney cracked or separated from the wall?"]

    ExteriorScan -->|"ANY Red-Flag Structural Damage<br/>OR Official RED Placard"| DoNotEnter["DO NOT RE-ENTER BUILDING:<br/>1. Treat as imminent collapse hazard during next aftershock<br/>2. Shut off main gas valve and main electrical breaker (if outside)<br/>3. Cordon off fall zone around chimneys and masonry walls<br/>4. Shelter in open park, vehicle in open lot, or Red Cross shelter"]

    ExteriorScan -->|"YELLOW Placard<br/>(Restricted Use)"| YellowEntry["SUPERVISED BRIEF ENTRY ONLY (If Permitted by Tag):<br/>1. Wear hard hat, shoes, gloves, N95 mask, and headlamp<br/>2. Post an outside spotter with a whistle at the street<br/>3. Prop all doors open so an aftershock cannot jam you inside<br/>4. Grab meds, pets, and documents in <10 mins and exit"]

    ExteriorScan -->|"No Visible Structural Damage<br/>(or GREEN Inspected Tag)"| SafeInside["CAUTIOUS INTERIOR CHECK:<br/>1. Check gas water heater straps & flue vent pipe on top<br/>2. Check for water pipe leaks and sewage backups<br/>3. Open cabinets carefully (contents shift and fall)<br/>4. Keep shoes and flashlight next to bed for night aftershocks"]

Official ATC-20 post-earthquake building safety placards explained¶

Following a damaging earthquake, municipal building departments and certified civil/structural engineers conduct Rapid and Detailed Evaluations under the ATC-20 (Applied Technology Council) standard and post color-coded placards at the main entrance of every inspected structure.

The 6 structural failure modes that cause buildings to collapse during an aftershock¶

Before you step inside any house, apartment building, or store after an earthquake, check for these six classic structural failure patterns identified by FEMA and the Earthquake Engineering Research Institute (EERI). If you see any of these six signs, treat the building as Red-Tagged ("UNSAFE") even if an official inspector has not arrived yet.

1. "Soft-Story" Ground-Floor Racking (Apartment Buildings Over Tucked-Under Parking)¶

Many 2-to-5-story wood-frame or concrete apartment buildings are built with wide, open ground-floor parking garages, tuck-under carports, or large glass retail storefronts on the first floor, supporting heavy framing and dozens of apartments on the floors above. - Because the ground floor lacks solid interior shear walls, horizontal earthquake waves cause the entire building to sway sideways on the slender ground-floor columns like a heavy box balancing on toothpicks. - What to look for: The first-floor walls or garage columns are visibly leaning out of plumb (even by \(2\text{ to }5\text{ degrees}\)); garage doors or ground-floor entry doors are jammed crooked in their frames; stucco or drywall at the top of the first-floor columns is crushed and flaking off. One moderate aftershock will drop the upper floors straight down onto the ground-floor garages.

2. Foundation Cripple-Wall Buckling and Sill-Plate Sliding (Older Wood-Frame Homes)¶

Older single-family wood-frame houses (especially pre-1980 homes) often sit on short \(0.5\text{ to }1.2\text{-meter}\) (\(1.5\text{ to }4\text{-foot}\)) wood stud walls called cripple walls between the concrete foundation footing and the first-floor joists, or lack anchor bolts securing the wooden mudsill to the concrete. - What to look for: The house appears dropped down by 30 to 90 centimeters (1 to 3 feet) or tilted to one side; exterior siding or stucco at ground level is buckled outward like an accordion; front porch steps have pulled completely away from the front door; or you can see bare concrete foundation sticking out on one side of the house while the opposite wall overhangs the concrete. - Immediate danger: When a house slides off its foundation or collapses its cripple wall, it shears off the rigid steel natural gas pipe and water lines underneath the crawlspace, creating an extreme sub-floor gas explosion trap.

3. Diagonal "X-Cracking" and Spalling in Concrete Columns and Shear Walls¶

In reinforced concrete office buildings, parking structures, schools, and high-rises, vertical columns and concrete shear walls resist both the gravity weight of the floors above and the back-and-forth horizontal shear of earthquake waves. - What to look for: - Diagonal X-shaped cracks (crossing at \(45\text{-degree}\) angles) cutting all the way through the thickness of a concrete column or structural wall between windows. - Concrete spalling and "bird-caging" rebar: Chunks of concrete have popped off the column, exposing the steel reinforcing bars (rebar) inside, and the vertical steel bars are bent outward like a birdcage because the horizontal steel ties snapped. - Punching shear around flat-slab columns: In concrete parking garages or flat-slab buildings, look at where the ceiling slab meets the top of each column; if the column appears to be punching upward through a ring of cracked concrete in the ceiling, the entire floor slab is about to pancake downward.

4. Unreinforced Masonry (URM) Brick Parapets, Chimneys, and Wall Separation¶

Unreinforced brick, stone, adobe, and hollow clay-tile buildings (common in historic downtowns and pre-1950 neighborhoods worldwide) have brittle mortar joints with no internal steel rebar. - The Deadly Brick Chimney & Parapet Trap: Brick chimneys and decorative rooftop parapet walls whip back and forth violently during an earthquake. Even if the chimney does not fall during the mainshock, it frequently cracks clean through at the roofline or attic level. During the very next Magnitude 4.5 aftershock, tons of bricks crash either outward onto the driveway/patio or inward straight through the living room ceiling. - Ledger-Board Separation: Look where the roof or second floor meets the exterior brick wall. If you see daylight between the brick wall and the wooden roof rafters, or if the brick wall is bowing outward in the middle, the wall has detached from the floors and will peel away and collapse during the next aftershock.

5. Hillside Stilt Homes and Liquefaction Foundation Tilting¶

• Hillside homes on tall poles or stepped foundations: Check the uphill and downhill soil around the footings for tension cracks ( crescent-shaped soil fissures indicating an incipient landslide) and check that diagonal steel cross-bracing cables beneath the house are taut and unbroken.
• Soil Liquefaction: On coastal fill, river deltas, or sandy waterlogged soils, earthquake vibrations turn wet sand into quicksand (liquefaction), causing buildings to sink unevenly or tilt backward intact while sand boils erupt through cracks in the basement floor or street.

6. Jammed Doors and Popped Drywall Nails (The Warning Sign of Frame Racking)¶

Why do structural engineers pay close attention to whether your interior doors open and close smoothly after an earthquake? - If a wood-frame or steel-frame wall is still square (at a \(90\text{-degree}\) right angle), the rectangular door fits evenly inside its frame. - If the building frame has permanently racked (tilted into a parallelogram), doors jam tight inside their frames, windows shatter from diagonal corner compression, and rows of drywall screws or nails pop through the paint along the ceiling line, signaling that the wall's structural sheathing has torn loose from the studs.

How and when to shut off natural gas, propane, electricity, and water¶

Improper utility handling after an earthquake causes two opposite disasters: leaving a ruptured gas line open ignites post-earthquake urban conflagrations, whereas shutting off intact gas meters across an entire city leaves hundreds of thousands of residents without heat or hot water for weeks because utility workers must physically inspect and relight every single home one by one.

   HOW TO READ AND SHUT OFF YOUR NATURAL GAS SERVICE VALVE

   [1] GAS IS ON (Normal Flow)          [2] GAS IS OFF (Shut Off)
       Brass lug is PARALLEL                Brass lug is PERPENDICULAR
       (in-line) with the pipe              (crosswise / 90°) to the pipe

           |   |                                |   |
           | | |  <-- Lug vertical              |===|  <-- Turn 1/4 turn (90°)
           | | |      (In-line)                 |   |      either way until holes
           |   |                                |   |      align for padlock
      =====[Meter]=====                    =====[Meter]=====

1. Natural Gas Meter Inspection and Shutoff Protocol¶

1. Locate your gas meter and the correct shutoff valve BEFORE an earthquake occurs:
2. The homeowner shutoff valve (the service valve) is located on the inlet pipe coming up out of the ground before the gas enters the regulator and meter body (usually \(15\text{ to }30\text{ centimeters}\) [\(6\text{ to }12\text{ inches}\)] below or beside the meter).
3. It has a flat rectangular brass or iron tang (lug) with a circular hole through it.
4. Keep a dedicated non-sparking gas shutoff wrench or a 30-centimeter (12-inch) adjustable crescent wrench strapped or hanging right next to the gas meter so you never have to hunt through a dark, collapsed garage for a tool.
5. Perform the 3-Point Gas Leak Check after the quake:
6. Smell: Sniff around the meter, crawlspace vents, water heater closet, and kitchen stove for the distinctive rotten-egg / sulfur odor of mercaptan odorant added to natural gas and propane (see gas leaks).
7. Listen: Listen for a high-pitched hissing, whistling, or roaring sound near the meter or walls.
8. Watch the Test Dial: With all gas appliances in the house turned off, watch the smallest fractional test dial on the front face of the gas meter (usually labeled \(1/2\text{ Cubic Foot}\) or \(2\text{ Cubic Feet}\)) for 3 to 5 full minutes. If that needle is visibly rotating while no stove or furnace is running, gas is escaping through a broken pipe inside the house.
9. If ANY of the 3 checks is positive (or if the house has shifted on its foundation): SHUT IT OFF.
10. Fit your wrench over the rectangular brass lug on the inlet pipe and turn it 90 degrees (one-quarter turn) in either direction until the lug sits crosswise (perpendicular) to the pipe and the two padlock tabs align.
11. NEVER turn the gas valve back ON yourself. Turning the gas back on allows unburned gas to pour out of extinguished pilot lights or hidden pipe cracks inside walls, causing a lethal explosion the next time a refrigerator compressor or light switch clicks on. Only a qualified gas utility technician or licensed plumber may pressure-test the system and restore gas service.

2. Strapped vs. Unstrapped Gas Water Heaters (And the Flue Vent Hazard)¶

Even if your gas water heater is strapped to the wall studs with two heavy-gauge metal seismic straps (top third and bottom third of the tank) and did not tip over: - Check the sheet-metal exhaust flue pipe on top of the water heater immediately. Strong shaking frequently jars the metal draft hood and exhaust pipe loose from the top of the tank. If the water heater continues firing with a disconnected exhaust flue, odorless, lethal carbon monoxide pours directly into your home. - Check for gas flex-connector kinks: Verify the corrugated metal gas line was not stretched or cracked during shaking.

3. Electricity and Water Main Shutoff Rules¶

• Electricity: Shut off the main circuit breaker at your electrical panel if you see frayed or sparking wires, smell burning insulation, have standing water on the floor from burst pipes, or are evacuating a structurally damaged building.
• CRITICAL EXCEPTION: If you smell natural gas or propane inside the house, DO NOT touch the electrical breaker panel, light switches, or appliance plugs inside the house; flipping a breaker or switch creates a tiny internal electrical arc that will detonate the gas-filled room. Evacuate first and shut off gas from the outside meter.
• Household Water Main: If indoor pipes have burst and are flooding the ceiling or floors, close the main house water shutoff valve (where the water pipe enters the house or basement) to prevent structural water damage and to preserve the 150 to 300 liters (40 to 80 gallons) of clean drinking water stored inside your hot water heater tank before street water mains lose pressure or become contaminated by broken sewer lines.
• Propane (LPG) Tanks (Rural and Off-Grid Homes): Unlike natural gas, which is lighter than air and rises toward the ceiling, liquefied petroleum gas (propane) is 1.5 times heavier than air and pools along the ground, flowing into basements and crawlspaces. After an earthquake, inspect your outdoor bulk propane tank to verify it did not slide off its concrete blocks and break the copper pigtail line; if you smell gas or see frost on the tank fitting, turn the round handwheel valve under the tank dome clockwise (to the right) until tight.

Safe brief re-entry checklist (ONLY for Green-Tagged or Permitted Yellow-Tagged Buildings)¶

If your building is Green-Tagged (or has a Yellow Tag that explicitly permits brief supervised entry to retrieve medicine or essentials) and passes your 360-degree exterior inspection:

• Never enter alone; post an "Outside Lookout" with a loud whistle: One adult must stay outdoors on the street clear of falling hazards, holding a pea-less rescue whistle. If the lookout feels an aftershock starting or sees the building shift, one long whistle blast signals the person inside to drop, cover, or exit immediately.
• Wear full protective gear: Put on a hard hat or bike/climbing helmet, thick-soled work boots, leather gloves, safety glasses, an N95 dust respirator (damaged plaster and older popcorn ceilings release silica and asbestos dust), and carry a bright battery headlamp (never use candles, matches, or lighters).
• Wedge every door wide open as you enter: Carry wooden door wedges or heavy objects and prop the front door and every interior hallway door wide open behind you so that if an aftershock racks the door frames while you are in a back bedroom, you will not be locked and trapped inside.
• Open overhead kitchen cabinets and closet doors by only 2 centimeters (1 inch) first: During the mainshock, heavy glass jars, ceramic plates, and cans slide forward and lean directly against the inside of closed cabinet doors; swinging a cabinet door open normally dumps heavy glass onto your face and hands.
• Sweep up broken glass and cap spilled household chemicals immediately: Check under kitchen and bathroom sinks where bottles of bleach and ammonia or drain cleaner may have tipped and mixed (see bleach mixed with ammonia toxic gas).

Frequently asked questions¶

Are earthquake aftershocks always smaller than the main earthquake, or can a bigger quake hit later?¶

While the majority of aftershocks are smaller than the mainshock (typically peaking about 1.1 to 1.2 magnitude units lower according to Båth's Law, meaning a Magnitude 7.0 quake normally produces a Magnitude 5.8 to 5.9 largest aftershock), seismological records from the USGS show that roughly 5 to 6 percent of significant earthquakes are actually foreshocks followed within hours or days by a larger earthquake. Even an aftershock that is 10 times smaller in energy than the mainshock can completely collapse a building whose concrete columns, brick walls, or foundation connections were already cracked and weakened by the first quake.

Why shouldn't I automatically turn off my natural gas meter right after every earthquake?¶

You should only turn off your natural gas service valve if you smell the rotten-egg odor of gas, hear a hissing leak, see the small test dial on the meter face turning while all gas appliances are off, or see that your house has shifted on its foundation. Once a gas valve is turned off, homeowners are legally and physically prohibited from turning it back on themselves because unburned gas can flow through extinguished pilot lights or hidden stress cracks and explode. When tens of thousands of residents turn off undamaged gas meters unnecessarily after a moderate quake, it can take utility crews several weeks of door-to-door pressure testing to restore heat and hot water to the community.

What do the Green, Yellow, and Red tags placed on buildings after an earthquake mean?¶

Under the international ATC-20 Postearthquake Safety Evaluation system, a GREEN ("INSPECTED") placard means no apparent structural hazard was found and lawful occupancy is permitted, though non-structural damage may exist. A YELLOW ("RESTRICTED USE") placard means specific structural or falling hazards were found; continuous occupancy or sleeping inside is prohibited unless explicitly noted, and entry is restricted to safe zones or brief supervised retrieval of essentials. A RED ("UNSAFE") placard means the building has suffered severe structural damage or is threatened by an adjacent collapse hazard, and entering the building for any reason is strictly prohibited and life-threatening.

Why is it dangerous to walk outside a building during shaking, and when is the right moment to evacuate?¶

Running out of a building while the ground is actively shaking puts you directly beneath the most lethal zone of an earthquake: the exterior facade, where unreinforced brick parapets, chimneys, roof tiles, air conditioners, and shattered upper-story window glass rain down onto sidewalks and porches. During active shaking (whether the mainshock or an aftershock), stay inside and Drop, Cover, and Hold On under sturdy furniture or against an interior wall. The right moment to evacuate is immediately AFTER the shaking stops, moving briskly down the stairs and watching overhead as you exit into an open area at least 1.5 times the building's height away from structures.

Sources checked¶

• Applied Technology Council (ATC, United States): ATC-20-1 Field Manual: Postearthquake Safety Evaluation of Buildings (Green Inspected, Yellow Restricted Use, and Red Unsafe Placard Criteria).
• United States Geological Survey (USGS): Aftershock Forecasts, Omori's Law and Båth's Law Aftershock Decay Physics, Foreshock Probabilities, and Post-Earthquake Hazard Communications.
• Federal Emergency Management Agency (FEMA) and Ready.gov (United States): Earthquake Safety Checklist (FEMA B-526), Identifying Soft-Story and Cripple-Wall Structural Vulnerabilities, and Utility Shutoff Guidelines.
• Pacific Gas and Electric (PG&E) and Southern California Gas Company (SoCalGas, United States): Post-Earthquake Natural Gas Meter Inspection, 90-Degree Service Valve Shutoff Procedures, and Safe Restoration Protocols.
• International Search and Rescue Advisory Group (INSARAG / United Nations OCHA) and CERT: Structural Triage, Light Search and Rescue Cribbing, and Damaged Building Re-Entry Safety Rules.

Related guides¶

• Earthquakes: Drop, Cover, and Hold On survival
• Structural damage and building collapse at home
• Gas leak: how to recognize and shut off natural gas
• Crush injury and compartment syndrome
• Tsunamis: natural warning signs and high-ground evacuation
• Go-bag and emergency evacuation pack