Six aftershocks have already hit New Jersey, and more could be on the way, according to USGS, after a strong 4.8 earthquake struck Tewksbury Township earlier today. USGS has circled a zone around today’s primary earthquake where aftershocks could continue to occur for the next week. The aftershocks are from the strong earthquake that struck northern New Jersey earlier today at 10:23 am.

According to USGS, “foreshock” and “aftershock” are relative terms. Foreshocks are earthquakes that precede larger earthquakes in the same location. An earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs. It is not yet known if the earthquake in March was a foreshock to today’s. Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a larger event or “mainshock.” They occur within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the mainshock. The frequency of these aftershocks decreases with time. Historically, deep earthquakes (>30 km) are much less likely to be followed by aftershocks than shallow earthquakes.

USGS warns, “Damaging earthquakes can occur in the future, so remember to: Drop, Cover, and Hold on.” According to USGS, more earthquakes than usual (called aftershocks) will continue to occur near the mainshock. The mainshock is the largest earthquake in a sequence , which is a series of earthquakes related to each other. When there are more earthquakes, the chance of a large earthquake is greater which means that the chance of damage is greater. But USGS also cautions, “No one can predict the exact time or place of any earthquake, including aftershocks. Our aftershock forecasts give us an understanding of the chances of having more earthquakes within a given time period in the affected area.”

The USGS estimates the chance of more aftershocks within the next week until April 12, 2024 18:30 (UTC) as follows:

- There is a 38% chance of magnitude 3 and above aftershocks within the next week, and it is most likely that 0 to 10 of these will occur.
- There is a 9% chance of magnitude 4 and above aftershocks within the next week, and it is most likely that 0 to 1 of these will occur.
- There is a 2% chance that this earthquake becomes a foreshock to a larger earthquake (magnitude 4.8 or greater) in the next week, such an earthquake is possible but with a low probability.
- There is a 1% chance of magnitude 5 and above aftershocks within the next week, such an earthquake is possible but with a low probability.
- There is a 1 in 800 chance (0.1%) of magnitude 6 and above aftershocks within the next week, such an earthquake is possible but with a low probability.
- There is a 1 in 8,000 chance (0.01%) of magnitude 7 and above aftershocks within the next week, such an earthquake is possible but with a very low probability.

Most earthquakes with a magnitude of 2.0 or greater can be felt.