Aftershocks typically last from several weeks to months, and in some cases, they can continue for years or even decades after the main earthquake. The frequency and magnitude of aftershocks generally decrease over time, following a pattern described by Omori's law, where the rate of aftershocks is roughly inversely proportional to the time since the mainshock
. Key points about aftershock duration and behavior include:
- Aftershocks are smaller earthquakes occurring in the same fault system as the mainshock, often numbering in the hundreds
- The largest aftershock is usually about one magnitude less than the mainshock and often occurs within days to about 10 days after the main event, depending on whether the earthquake was inland or offshore
- The number of aftershocks decreases rapidly initially—for example, to about one-tenth within the first 10 days and to one-hundredth within 100 days—but the decrease slows over time, which can make aftershocks feel like they last a long time
- Some aftershock sequences, especially following very large earthquakes, can persist for decades or even centuries, as seen in the New Madrid earthquakes of 1811–12
- The exact duration and intensity of aftershocks depend on the earthquake's size, location, and fault characteristics, and predicting their precise behavior remains an active area of research
In summary, while most aftershocks occur within weeks to months after the mainshock, smaller aftershocks can continue for years, with their frequency and magnitude diminishing over time but never completely disappearing immediately
