TREMR

After a large earthquake strikes, the shaking does not simply stop and stay stopped. Hours later, the ground trembles again — sometimes almost as hard as the first time. Days later, smaller tremors continue. Weeks later, instruments still record activity along the fault. For anyone who has lived through a major earthquake, the aftershock sequence is often more psychologically wearing than the main event itself: each new tremor bringing the same spike of adrenaline, the same wondering whether this one is going to be bigger.

Aftershocks are not random bad luck. They follow predictable physical laws, and understanding those laws helps explain not just why they happen but how long they last, how large they can be, and why some earthquake sequences seem to go on far longer than others.

The Stress Transfer Problem

To understand aftershocks, you first need to understand what an earthquake actually does to the surrounding rock. When a fault ruptures, it doesn't simply release all the stress that had accumulated along it. It redistributes that stress. The section of fault that slipped has released its stored energy — but the rock around it, particularly near the ends of the rupture and on faults that branch off from the main one, has experienced a sudden change in the stress acting on it.

Some areas experience stress increases from the rupture. These are called regions of Coulomb stress increase or "stress shadows" — zones where the new stress field brought the surrounding rock closer to its own failure threshold. Faults in those zones, which may have been stable for years, are now more likely to slip. That slipping is what we experience as aftershocks.

The process cascades. Each aftershock redistributes stress again, which can trigger smaller aftershocks of its own. The result is not a single discrete event but a complex cloud of seismic activity that gradually dissipates over time as the stress field in the region settles into a new equilibrium.

Omori's Law

In 1894, Japanese seismologist Fusakichi Omori published observations on aftershock sequences from several Japanese earthquakes. He noticed a remarkably consistent pattern: the rate of aftershocks — measured as the number of earthquakes per unit of time — decreased in a specific mathematical way. Roughly speaking, the number of aftershocks per day was inversely proportional to the time elapsed since the main shock. Double the time, and the rate approximately halves. This relationship is now known as Omori's Law.

The practical implication of Omori's Law is that the aftershock rate is highest immediately after the main event and declines rapidly but never abruptly. After a major earthquake, you might expect hundreds of aftershocks in the first day, tens per day a week later, and a handful per month a year later. The sequence fades gradually rather than stopping on any particular day.

Magnitude Relationships

Aftershock sequences also follow a consistent pattern in terms of magnitude. Båth's Law, formulated by Swedish seismologist Markus Båth in 1965, states that the largest aftershock of a sequence is typically about 1.2 magnitude units smaller than the main shock. So a M7.0 earthquake would be expected to produce a largest aftershock around M5.8. A M9.0 event would likely produce a largest aftershock around M7.8 — itself a major earthquake by any standard.

The 2011 Tōhoku earthquake illustrates this dramatically. The main shock was M9.1. Three days later, a M7.7 aftershock struck offshore — an earthquake that, had it occurred independently, would have been a major disaster in its own right. The sequence also included numerous M6 and M7 events in the weeks that followed. For a region already devastated, these were not trivial aftershocks.

The Gutenberg-Richter relation adds another layer: for every increase of one magnitude unit, the number of earthquakes decreases by about a factor of 10. This means that for every M5 aftershock, you can expect roughly 10 M4s, 100 M3s, and so on. After a large earthquake, the smaller end of this distribution produces thousands of events — most too small to feel, but detectable by instruments.

How Long Do Aftershock Sequences Last?

There is no clean answer to when an aftershock sequence "ends," because the sequence fades continuously rather than stopping. Practically, seismologists consider a sequence over when the rate of activity has returned to the background level — the rate of earthquakes in that region before the main event. For a large earthquake, that can take months or years.

The 1992 Landers earthquake in California (M7.3) produced aftershocks that continued to exceed background rates for over a decade. Some sequences associated with truly great earthquakes, like the 1960 Valdivia (M9.5), showed elevated activity for years. In Japan, regions affected by the 2011 Tōhoku earthquake still recorded elevated seismicity years after the event.

Watching Aftershocks on Tremr

One of the most striking things you can see on Tremr in the hours and days after a large earthquake is the aftershock sequence developing in real time. When a significant event occurs, the area around it lights up with smaller dots — the aftershocks cascading outward from the rupture zone, gradually decreasing in frequency but continuing to accumulate.

Set Tremr to show the past 7 days after a major event and you can see the density of the aftershock cloud, how it clusters around the main fault trace, and how it gradually spreads as stress redistribution triggers secondary faults. The pattern is not chaos. It is a physical system finding its new balance — slowly, jerkily, over weeks and months of microadjustments that are seismically loud but ultimately routine. Each aftershock is the crust doing the same thing it always does: moving, settling, and releasing stress one small rupture at a time.