Aftershocks Explained: Why They Happen and How Long They Last

When the magnitude-7.8 earthquake struck off Sarangani in June 2026, the shaking was only the beginning. In the days that followed, PHIVOLCS recorded more than a thousand aftershocks, some strong enough to send people running back outdoors. It's the question on everyone's mind after any big quake: Are more coming? How big? For how long? Here's what the science says.

First, the vocabulary

Seismologists group the shocks in a sequence by their size and timing:

Here's the catch: you never know in real time whether you've felt the mainshock or just a foreshock. Occasionally a quake everyone assumed was "the big one" turns out to have been a foreshock to something larger hours or days later. That uncertainty is exactly why caution after a quake matters.

Why aftershocks happen

A large earthquake doesn't relieve all the stress in the crust — it redistributes it. When one section of a fault slips, it loads extra stress onto neighbouring patches of rock that hadn't yet broken. Those patches then rupture in turn, adjusting to the new arrangement. Aftershocks are the crust settling into its new equilibrium around the rupture zone.

How big do aftershocks get?

There's a useful rule of thumb called Båth's Law: the largest aftershock is typically about one magnitude smaller than the mainshock. So a magnitude-7.8 earthquake would be expected to produce a largest aftershock in the region of magnitude 6.7 — itself a strong, potentially damaging quake. The 2026 Sarangani sequence fit this pattern closely, with aftershocks reaching into the high 6s.

It's a statistical tendency, not a guarantee. Sometimes the "aftershock" is nearly as large as the mainshock — as in the October 2025 Davao Oriental doublet, where a magnitude-7.4 quake was followed hours later by a roughly magnitude-6.8 event.

How long do they last?

Aftershocks follow Omori's Law: their frequency drops off roughly in proportion to time since the mainshock. In practice that means they come thick and fast at first, then taper:

• Day 1: possibly hundreds of aftershocks, the most numerous and largest.
• First week: still frequent, decreasing daily.
• Weeks to months: a handful per day, then occasional.

For very large earthquakes (magnitude 8+), the sequence can technically continue for years before it fades into the normal background rate. But the greatest danger is concentrated in the first hours and days, when aftershocks are both frequent and strong.

Can aftershocks be predicted?

Not precisely. Scientists cannot say "a magnitude 6 will strike tomorrow at noon." What they can do is give probabilities — for example, the likelihood of a strong aftershock within the next week — which decrease as time passes. PHIVOLCS issues this kind of guidance after major quakes. Treat it as a reason to stay prepared, not a countdown.

What to do during an aftershock sequence

• Drop, Cover, and Hold On for every aftershock, just as you would the mainshock.
• Stay out of visibly damaged buildings. Don't go back in for belongings until they're declared safe.
• Expect them. Knowing aftershocks are normal helps you stay calm and act correctly.
• Keep your go-bag handy and follow PHIVOLCS advisories.

Related: magnitude vs. intensity explained, the strongest quakes since 2020, and the full earthquake safety guide.

Sources: USGS Earthquake Hazards Program (aftershock forecasting); Omori's Law (1894) and Båth's Law on aftershock decay and magnitude; PHIVOLCS earthquake bulletins. Aftershock behaviour is statistical — individual sequences vary.