Why a major earthquake thousands of miles away can trigger another one

Rescue workers search for survivors in a damaged building in the Altamira neighborhood of Caracas, Venezuela, on Wednesday (local time). [EPA]

Two powerful earthquakes — magnitude 7.2 and 7.5 — struck Venezuela on June 24 within about 30 seconds of each other, with casualties still being tallied. The following day, a magnitude 7.2 quake hit off the coast of Iwate Prefecture in Japan.

The back-to-back major quakes, separated by thousands of kilometers and just one day, have prompted a familiar question: can a large earthquake trigger another one somewhere far away?

A study published in volume 12, issue 26 of the international journal Science Advances by Chao Huang and Jun Yang of the Department of Civil Engineering at the University of Hong Kong has identified a mechanism by which a major earthquake can destabilize faults thousands of kilometers away and set off another one.

A building collapsed by an earthquake. [Reuters]

Seismic waves are in fact extremely weak by the time they travel far

When an earthquake occurs, energy radiates outward in all directions as waves.

Near the epicenter, those waves carry enough force to collapse mountains — but after traveling thousands of kilometers, they weaken dramatically. By then, the force they exert on a distant fault is just one-hundredth to one-thousandth of the stress that fault normally bears. In theory, that is far too little to move it.

In practice, however, earthquakes do occur in sequence thousands of kilometers from the original epicenter.

The phenomenon — earthquakes following in the wake of strong seismic waves at distances of hundreds or thousands of kilometers — was first officially documented after the 1992 Landers earthquake in California. It has since been observed in a wide range of tectonic settings across South America, Asia and Africa.

A schematic diagram illustrating fault structure. The fine, clay-like powder known as "fault gouge" fills the spaces where rock has been ground away over long periods of time. [Science Advances, Vol. 12, No. 26]

Clay trapped in fault gaps amplifies pressure tenfold

The research team focused on the material that fills the interior of a fault.

A fault is the boundary where two rock masses meet. Over long periods, rock ground down along that boundary leaves behind a fine, clay-like powder known as "fault gouge."

Fault gouge retains water much like clay and has low permeability, meaning water passes through it slowly. When seismic waves repeatedly vibrate this material, the pore water pressure inside begins to rise. The lower the permeability, the harder it is for that pressure to escape.

According to the team's simulations, pressure built up inside the fault gouge to more than 10 times the direct force of the seismic waves themselves. The principle resembles what happens on a beach: sand feels firm when the sea is calm, but repeated wave action saturates it and turns it soft.

As pressure rises, the fault's resistance falls. Once it weakens enough, even a fault that would not normally slip begins to move.

The team modeled this process by applying Rayleigh waves — a type of surface seismic wave — to a normal fault inclined at 45 degrees at a depth of 4.25 kilometers, running the simulation for 200 seconds over 10 repeated cycles.

Yellower colors indicate higher pressure. (A) As strong seismic waves struck the fault's core, internal pressure spiked sharply; by the 200-second mark, pressure had reached its peak (yellow line). (B) After the waves passed, the high pressure concentrated at the fault's core gradually spread outward into the surrounding rock over time. [Science Advances, Vol. 12, No. 26]

As a result, pore water pressure inside the fault gouge climbed to 1.7 megapascals (MPa) — more than 10 times the 0.15 MPa of direct force the seismic waves applied to the fault.

The key driver of earthquakes thousands of kilometers away, the study concluded, is not the force of the seismic waves themselves but the pressure those vibrations build up inside the fault.

Rescue teams use heavy equipment to search for survivors at a collapsed building in Caracas, Venezuela, on Wednesday (local time). [AFP]

The danger persists long after seismic waves pass

The research team also shed light on why triggered earthquakes sometimes strike not immediately after the seismic waves arrive but minutes, hours, or even months later.

Because fault gouge has low permeability, elevated pore water pressure drains away only slowly.

In the simulations, internal fault pressure remained nearly unchanged for at least 30 minutes after the seismic waves stopped, then declined gradually over several hours — and even three hours later had not returned to pre-wave levels.

Delays of several months are also explained by permeability: the lower a fault gouge's permeability, the longer pressure takes to dissipate. A tenfold drop in permeability translates to a tenfold increase in drainage time.

However, the team noted that the study relies on numerical simulations and that experimental data from repeated-loading tests measuring the actual physical properties of fault gouge remain insufficient.

Reference

DOI: 10.1126/sciadv.aec4754

Chao Huang, Jun Yang, "What triggers seismicity thousands of kilometers away from a mainshock?" Science Advances 12, eaec4754 (2026).

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