Aftershocks: what the next 72 hours look like

Båth’s law in plain English

A useful rule of thumb when a major earthquake has just happened and you are wondering what comes next: the largest aftershock is statistically about 1.2 magnitude units smaller than the mainshock. This is Båth’s law, derived empirically from decades of catalogued sequences. A M7.5 mainshock will typically produce a largest aftershock around M6.3. A M6.0 mainshock will typically produce a largest aftershock around M4.8.

The law has two practical implications:

• The largest aftershock is itself a damaging earthquake. After a mainshock that brought buildings down, an aftershock around 1.2 units smaller is still a serious shake that can collapse already-weakened structures. The 22 February 2011 Christchurch earthquake was technically an aftershock of the 4 September 2010 Darfield mainshock; it killed 185 people in buildings that had survived 2010 with damage.
• The largest aftershock does not necessarily come first. The major aftershock can arrive hours to days after the mainshock, sometimes a week or more later. The 2011 Christchurch event arrived more than 5 months after Darfield; the 2023 Türkiye-Syria sequence produced its second devastating event (M7.5 Elbistan) just 9 hours after the M7.8 Pazarcık mainshock.

The decay curve

Aftershock frequency follows Omori’s law: the rate decreases roughly inversely with time since the mainshock. In plain terms:

• First hour: highest aftershock density. Multiple felt aftershocks per hour are normal for a M7 mainshock.
• First 24 hours: roughly half of the total aftershocks in the first month occur in this window.
• First 72 hours: the operational decision window for travellers. By the end of 72 hours, the frequency is dropping but the largest aftershock has often still not arrived.
• First week: the period covered by most national emergency-response protocols.
• First month: residual elevated rate but back to a few felt events per day for most sequences.

The rate is approximate but reliable enough that emergency managers use it operationally. The IRIS and USGS aftershock forecasts published within 24 hours of major events use Omori-law extensions plus Reasenberg-Jones statistics to give probability ranges.

Doublets and triggered events

Two phenomena complicate the simple Båth-Omori picture:

• Doublets: two mainshocks of similar magnitude separated by hours to days, on adjacent fault segments. The 6 February 2023 Türkiye sequence (M7.8 followed 9 hours later by M7.5) is the recent extreme case. The 2016 Kumamoto sequence in Japan (M6.5 foreshock April 14, M7.0 mainshock April 16) is another. Doublets defeat the “1.2 unit smaller” rule of thumb because the second event is structurally distinct, not a true aftershock.
• Triggered events: stress transfer from the mainshock can load adjacent faults to failure within days to years. The 1992 Landers M7.3 triggered the 1999 Hector Mine M7.1 seven years later. For traveller purposes, the relevant triggered-event horizon is days to weeks.

The practical takeaway: do not assume that a strong felt aftershock means “that was the big one, we’re done.” Watch the national seismic agency’s forecast updates for at least 72 hours.

The building re-entry decision

After a major earthquake, the question that determines whether you survive the next 72 hours is: is this building safe to re-enter, sleep in, and remain in during the aftershock sequence? The professional answer requires ATC-20 structural inspection by a qualified engineer. The traveller answer requires recognising the warning signs that tell you to leave even without a formal inspection.

The categories used by professional inspectors (ATC-20 and equivalent international standards) translate roughly to:

• Green (safe to use): no structural damage visible; no cracking in load-bearing walls or columns; no shift in floors or ceilings; no leaning. Most modern reinforced-concrete and steel-framed buildings in earthquake- engineered jurisdictions fall here even after strong shaking.
• Yellow (restricted use): some damage but structurally stable in the short term. You can retrieve belongings but should not sleep there until aftershock sequence settles.
• Red (unsafe): visible structural damage; do not enter. Standard signs: tilted columns, large diagonal cracks in load-bearing walls, sagging or partially-collapsed floors, foundation shift.

As a traveller without engineering training, you cannot make the green-yellow distinction reliably. You can usually recognise red. The bias should be conservative: a building that looks fine on the outside but has internal warning signs (covered below) is yellow-or-worse and you should not sleep there.

Visible structural warning signs

The checklist a traveller can run without engineering training. Any one of these means the building is yellow or red, get out and stay out until an inspector clears it:

• Visible lean of the building or any wallpast the natural plumb line.
• Diagonal X-pattern cracks in concrete columns or shear walls (these mean the wall has shed load cyclically; structural capacity is permanently reduced).
• Cracks at the interface between a structural column and the floor or ceiling slab (the column is separating from the diaphragm; load path failing).
• Spalled concrete with exposed rebar at column bases or beam-column joints.
• Floors visibly out of level; doors that previously closed now jam or swing freely; gaps appearing between floor and walls.
• Foundation cracks visible from outside; gaps between the building and the surrounding ground.
• Loud creaking, popping, or settling soundsin the hours after the mainshock, especially during aftershocks.
• Gas smell, water-line breaks, or electrical arcing: utility failure can cause secondary disasters; evacuate even if the structure itself looks sound.
• Falling debris: facade tiles, false ceilings, parapet stones detaching during aftershocks. This kills people standing near buildings that the buildings themselves survive.

Sleeping protocol nights 1 to 3

The practical rules:

• Night 1: if your accommodation has any visible damage, sleep outside (vehicle, designated evacuation centre, open ground in fair weather) rather than indoors. Most aftershock fatalities are people who returned to apparently-stable buildings that collapsed in a subsequent aftershock.
• Nights 2 and 3: if a structural inspection has cleared the building, you can re-enter. If no inspection has happened, follow the local emergency-management guidance.
• Sleep with shoes within reach for the first 72 hours regardless. Going barefoot through broken glass and debris during a panicked aftershock evacuation produces avoidable injuries.
• Have your go-bag at the bedroom door: passport, phone with battery pack, water, ID copies. Practiced exit time matters.
• Bedroom layout matters: head of bed away from windows and heavy wall-hangings; clear path to doorway.

Hotel and accommodation logic

After a major event, the hotel’s own structural assessment matters and is sometimes available to guests on request. The questions to ask:

• Has the building been inspected by a qualified engineer since the mainshock?
• What is the assessment colour (green/yellow/red or equivalent local standard)?
• Where is the designated outdoor assembly point for staff and guests?
• What is the backup-power and water status?
• Are there reinforced-concrete rooms designated as shelter-in-place areas?

If the answers are unsatisfactory or the staff is themselves evacuating, relocate. Most consular agencies will help locate replacement accommodation after major events; the relevant embassy emergency line is usually answered 24/7 in earthquake-affected countries.

Communications through aftershock sequence

• SMS over voice: text messages get through saturated cell networks far more reliably than voice calls in the immediate aftermath.
• Satellite messengers: Garmin inReach, Apple Emergency SOS via satellite (iPhone 14+), Android satellite SOS work when terrestrial networks fail.
• USGS Earthquake Notification Service: sign up before travel for push alerts on any earthquake above a chosen magnitude near a chosen location. JMA Bosai app and BMKG app provide same for Japan and Indonesia.
• Battery management: power outages of 24 to 72 hours are routine after major events. Carry a 10,000+ mAh battery pack; charge to full at first opportunity.
• Family check-in cadence: agree before any earthquake-risk trip how often you will check in and what the no-news-means-act window is.

Country brief

• Japan: JMA aftershock forecasts world-leading; building re-entry standards applied at extreme speed; Tōhoku 2011 reference event.
• Indonesia: BMKG aftershock monitoring covers the Sunda Arc; 2018 Sulawesi (Palu) and 2018 Lombok aftershock sequences are recent references.
• Chile: SHOA and the CSN seismic-monitoring network; 2010 Maule M8.8 mainshock followed by a major aftershock sequence including several M7+ events.
• Türkiye: 2023 Kahramanmaraş doublet (M7.8 and M7.5 nine hours apart) is the defining recent event; AFAD and Kandilli Observatory monitor.
• Morocco: September 2023 Al Haouz M6.9 aftershock sequence; Moroccan monitoring expanded post-event.
• Mexico: SSN (Servicio Sismológico Nacional) monitoring; multiple major events with extended aftershock sequences (1985, 2017 Puebla, 2017 Chiapas).
• United States: USGS and state networks; ShakeAlert covers the West Coast for early warning of aftershocks too.
• Peru: IGP monitoring; 1970 Ancash (Yungay) and 2007 Pisco references.

One more time

Aftershocks are statistically predictable in rate and maximum size but not in timing. The largest aftershock is typically about 1.2 magnitude units smaller than the mainshock (Båth’s law) and often arrives hours to days after, not in the first few minutes. The traveller decision is the building re-entry decision: any visible structural warning sign means yellow-or-red and you do not sleep there. Night 1 outside if there is any doubt; shoes by the bed; go-bag at the door; SMS over voice; battery pack at the ready. The Field Manual’s earthquake guide covers the during-shaking response that precedes everything here; the tsunami evacuation guide covers the coast-specific transition.