Aftershocks Explained: How USGS Aftershock Forecasts Work
Aftershocks are smaller earthquakes that follow a larger earthquake, the mainshock, in the same area. USGS says their rate decreases with time, and the larger the mainshock, the larger and more numerous the aftershocks and the longer they continue. After significant US earthquakes, USGS publishes aftershock forecasts that give the chance of aftershocks above set magnitudes over the next day, week, month and year.
A forecast is a probability, not a prediction. USGS states that neither it nor any other scientists have ever predicted a major earthquake.
What is the difference between foreshocks, mainshocks and aftershocks?
USGS uses these terms as relative labels. The mainshock is the largest earthquake in a sequence. Aftershocks are the smaller earthquakes that follow it. A foreshock can only be identified after a larger earthquake happens in the same area.
The label can change. If a later earthquake is larger, it becomes the new mainshock, and the earlier ones become foreshocks. The 2019 Ridgecrest, California, sequence is a USGS example: a magnitude 6.4 earthquake on July 4 was followed by a magnitude 7.1 on July 5, which made the first one a foreshock.
That outcome is uncommon. USGS says that around the world, the chance that an earthquake will be followed by a larger earthquake nearby within a week is about 5 percent. The USGS foreshock FAQ explains the figure.
How long do aftershocks last?
USGS says most aftershocks happen within a few hours to days of the mainshock. The larger the mainshock, the longer the aftershocks last, and for the largest earthquakes a sequence can go on for years to decades.
The rate of aftershocks generally decreases with time. USGS notes that it can temporarily increase after a larger aftershock, which is one reason forecasts are updated.
Depth matters as well. USGS says earthquakes deeper than 30 km (19 miles) are much less likely to be followed by aftershocks than shallow earthquakes. Our guide to earthquake depth covers the depth classes.
A swarm is different. USGS describes a swarm as a sequence without one clear mainshock, often lasting days, weeks or sometimes months, with the largest earthquake often coming later in the sequence. USGS links most swarms to geothermal activity.
When does USGS issue an aftershock forecast?
According to the USGS aftershock forecast overview, USGS posts forecasts for:
- earthquakes of magnitude 4 and larger in the contiguous United States;
- earthquakes of magnitude 5 and larger elsewhere in US states and territories;
- some smaller earthquakes of special public interest, such as those in densely populated areas;
- occasionally, significant earthquakes outside the United States.
Forecasts are usually not computed for aftershocks of a larger earthquake or for earthquakes during known volcanic activity. USGS waits at least 20 minutes after an earthquake before issuing the first forecast, so the preferred magnitude of the mainshock can settle. Forecasts are updated regularly, and each one lists its release time and the time of the next planned update.
How do you read the aftershock forecast table?
The forecast appears on the USGS event page under Aftershock Forecast. The main table has rows by magnitude and columns by time window.
| Part of the table | What it means |
|---|---|
| Rows: M3+, M4+, M5+, M6+, M7+ | Aftershocks at or above that magnitude |
| Columns: day, week, month, year | The time window, starting from the forecast’s release time |
| A percentage | The chance of at least one aftershock of that size or larger in that window |
| A number or range | The expected count, shown when more than one aftershock of that size is likely |
Longer windows always show higher numbers. USGS explains that each time interval contains the one before, so the year includes the month, the week and the day.
The USGS Hawaiian Volcano Observatory describes magnitude 5 and larger aftershocks as large enough to potentially do damage. That makes the M5+ row the one to read first. To see recent activity near a mainshock, open the state pages, such as recent earthquakes in California or recent earthquakes in Alaska.
What is the forecast based on?
USGS says its forecast is based on a statistical model of past aftershock sequences in similar tectonic settings. The first forecasts use general values from those past sequences. As aftershocks are recorded, USGS blends in values fitted to the current sequence. The models USGS names are Reasenberg-Jones and ETAS.
The numbers describe chances over an area near the mainshock. They do not give the time or place of any single aftershock. The USGS FAQ on earthquake prediction explains why USGS calculates probabilities instead.
Our guide on how to read a USGS earthquake report shows where the forecast sits among the other products on the event page.
What do officials advise during an aftershock sequence?
Ready.gov, the federal preparedness site, says to expect aftershocks to follow the main shock of an earthquake and to be ready to Drop, Cover and Hold On if you feel one. It advises against entering damaged buildings, and FEMA materials note that aftershocks can cause additional damage to unstable buildings. People using a walker or wheelchair are advised to Lock, Cover and Hold On.
The full guidance is on Ready.gov’s earthquake page. After a damaging earthquake, follow instructions from your local emergency officials.
Bottom line
Aftershocks are a normal part of an earthquake sequence and usually become less frequent with time. The USGS forecast turns that pattern into percentages and expected counts for the next day, week, month and year. Read the M5+ row first, check the release time and next update, and remember that about 5 percent of earthquakes worldwide are followed by a larger one nearby within a week.
FAQ
Can an aftershock be bigger than the mainshock?
If a later earthquake is bigger, USGS relabels it as the mainshock and the earlier one becomes a foreshock. USGS puts the worldwide chance of a larger earthquake nearby within a week at about 5 percent.
Where do I find the USGS aftershock forecast?
Open the mainshock’s USGS event page and select Aftershock Forecast. It appears at least 20 minutes after the earthquake for qualifying events.
Why does the forecast change?
USGS updates forecasts on a schedule and blends in data from the current sequence. A larger aftershock can temporarily raise the rate.
Do deep earthquakes have aftershocks?
Less often. USGS says earthquakes deeper than 30 km (19 miles) are much less likely to be followed by aftershocks.