What Is an Aftershock?
An aftershock is a smaller earthquake that follows a larger one, the mainshock, in the same area. Aftershocks happen within one or two fault lengths of the mainshock as the rock around the break adjusts.
Two related terms:
- A foreshock is an earthquake that comes before a larger one in the same place. It can only be called a foreshock after the larger earthquake happens.
- A swarm has no mainshock at all: it is a run of similar-sized earthquakes. See our guide to earthquake swarms.
Deep earthquakes, more than about 30 km (19 miles) down, are much less likely to have aftershocks. The Magna mainshock was about 12 km (7 miles) deep.
How Many Aftershocks Follow an Earthquake?
The bigger the mainshock, the more aftershocks it triggers, the larger they get and the longer they continue. Their pace falls off quickly: the rate is roughly inversely proportional to the time since the mainshock, so there are about 10 times as many aftershocks on the first day as on the tenth.
The 2020 Magna earthquake shows what that looks like in Utah. The University of Utah Seismograph Stations located 2,590 earthquakes in the Magna area in the first year, including the mainshock. Six of them reached magnitude 4 and 30 reached magnitude 3. In our count of the USGS catalog, 39 earthquakes of magnitude 3 or larger have struck within 20 km of the epicenter since March 18, 2020, the mainshock included.
| When | What Happened |
|---|---|
| Mar 18, 2020, 7:09 a.m. | Magnitude 5.7 main shock near Magna |
| Mar 18, 2020, 8:02 a.m. and 1:12 p.m. | The two largest aftershocks, both magnitude 4.6 |
| Apr 14 and Apr 16, 2020 | Two magnitude 4.2 aftershocks |
| Mar 18, 2020, to Feb 28, 2021 | 2,590 earthquakes located, including the main shock: 6 of magnitude 4 to 4.9 and 30 of magnitude 3 to 3.9 |
| Mar 2025, Five Years On | More than 2,800 aftershocks recorded, 90% of them in the first year |
| Feb 13, 2026 | A magnitude 3.5 with 13 aftershocks of its own, likely an aftershock of the 2020 earthquake |
How Big Can Aftershocks Get?
The largest aftershock is usually about a full magnitude smaller than the mainshock. Magna fit the rule: its two largest aftershocks, both magnitude 4.6, came about a magnitude below the 5.7 mainshock, within hours of it.
Smaller doesn't mean harmless. Aftershocks can still damage buildings already weakened by the mainshock, which is why emergency managers say to stay out of damaged buildings until they have been checked.
Can an Aftershock Be Bigger Than the First Earthquake?
By definition, no: if a later earthquake is bigger, the first one becomes a foreshock and the later one is the mainshock. That happens less often than people fear. Any earthquake has about a 5% chance of being followed by a larger one nearby within a week, according to the USGS, or roughly one in twenty in the University of Utah's words.
How Long Do Aftershocks Last?
Days, weeks or months, and years after a very large earthquake. Most come early, and the rest trickle in more and more slowly.
Magna is a good measure for Utah. Five years after the 2020 earthquake, the University of Utah had recorded more than 2,800 aftershocks, 90% of them in the first year. In the 12 months before March 2025 it still recorded 49 earthquakes within 15 km (9 miles) of the mainshock, all small. And on February 13, 2026, a magnitude 3.5 in the same area, followed by 13 aftershocks of its own, was judged likely to be an aftershock of the 2020 earthquake. As the University of Utah put it, it is not uncommon for aftershock sequences to last for years.
Aftershock Forecasts
After an earthquake of magnitude 4 or larger in the lower 48 states, and magnitude 5 or larger in Alaska, Hawaii and U.S. territories, the USGS posts an aftershock forecast. It waits at least 20 minutes for data to come in, then estimates how many aftershocks to expect over the next day, week, month and year. Forecasts are updated regularly as the sequence unfolds.
They are odds, not predictions: they tell you how busy the next few days are likely to be, not when the next earthquake will strike.
What to Do During Aftershocks
- Drop, cover and hold on every time the ground shakes, just as you did during the mainshock.
- Stay out of damaged buildings until they have been inspected.
- Follow each one on the live Utah earthquake map as it's published.
The full checklist is in our guide to what to do during an earthquake.