Why earthquakes struck Colombia and Peru almost simultaneously on Saturday

• Peru's Instituto Geofísico del Perú recorded a magnitude-3.8 earthquake off the coast near San Pedro de Lloc in La Libertad at 4.06 a.m. on September 5. The shallow, 15-kilometre-deep tremor caused no reported damage or injuries.
• About two hours later, Colombia's Servicio Geológico Colombiano recorded a magnitude-3.4 tremor beneath Los Santos in Santander, at a depth of 152 kilometres. No damage was reported.
• Officials have not linked either event to Colombia's catastrophic magnitude-7.4 earthquake near San José del Palmar in Chocó on August 10. The confirmed death toll from that disaster had risen to 331 people in the government's latest count.

A pre-dawn tremor off Peru's northern coast

Most of La Libertad was still asleep when the ground began to move. At 4.06 a.m. on Saturday, Peru's geophysical monitoring service located the epicentre 91 kilometres out in the Pacific, west of the fishing town of San Pedro de Lloc in Pacasmayo province. The tremor was shallow, at 15 kilometres below the surface, but far enough offshore to register only light intensity — II to III on the Mercalli scale — in the nearest town. That is roughly the level at which people may notice dishes rattling, rather than suffer structural damage.

The earthquake was not an isolated event. A day earlier, a stronger magnitude-3.9 tremor struck near San Vicente de Cañete in the Lima region, at a depth of 45 kilometres. A separate magnitude-3.7 earthquake was also recorded that morning near Atico in Arequipa's Caravelí province. None of the three events has been linked to the others. Peru's position, where the Nazca oceanic plate is continuously subducting beneath the South American continental plate, means its entire coastline experiences frequent, mostly unnoticed seismic activity.

Two hours later, a familiar tremor in Santander

By 6.11 a.m., attention had shifted to Colombia. The country's geological survey located a magnitude-3.4 tremor beneath Los Santos, a municipality in Santander that appears in seismic bulletins almost daily. The earthquake occurred much deeper than the one in Peru — 152 kilometres below the surface — a depth that generally reduces how strongly a tremor is felt at ground level, even when its magnitude is similar.

Los Santos lies above the Bucaramanga seismic nest, a concentrated source of deep, moderate earthquakes. According to the Servicio Geológico Colombiano's own account, the area generates nearly 45 per cent of all seismic activity recorded across the country. It is one of only three such "nests" identified anywhere in the world. Officials have repeatedly said that its constant background activity does not, by itself, indicate that a larger earthquake is building.

Not an aftershock, but a separate geological system

That reassurance carries particular weight this year. Colombia is still dealing with the aftermath of the magnitude-7.4 earthquake that destroyed parts of San José del Palmar in Chocó on August 10. It was the country's deadliest earthquake in decades, with the government's latest update, issued on September 3, putting the confirmed toll at 331 dead and more than 4,500 injured. Because Los Santos lies in a geologically distinct system from the subduction zone that produced the Chocó disaster, authorities have consistently declined to describe tremors such as Saturday's as aftershocks. They made the same distinction when a comparable earthquake in Santander drew attention in late August.

Saturday's tremor was also not an isolated occurrence. Two days earlier, on September 3, a magnitude-4.2 earthquake struck the same municipality at a nearly identical depth of 146 kilometres. Officials describe such smaller tremors as part of the zone's steady background activity, rather than signs of a larger event developing.

Why similar magnitudes can feel so different

The two events illustrate a point seismologists often stress: magnitude alone does not determine how dangerous, or even how noticeable, an earthquake will be. Depth is crucial — the closer an earthquake's source is to the surface, the more strongly it shakes the ground above. Distance from population centres and the type of soil on which a city is built also matter.

That is what separated Saturday's two tremors from August's catastrophe. The magnitude-3.8 and magnitude-3.4 earthquakes, at depths of 15 and 152 kilometres respectively, caused no more than a scare in the pre-dawn darkness. The magnitude-7.4 earthquake, which occurred at a much shallower depth and closer to densely populated areas, triggered a national emergency from which Colombia is still recovering three weeks later.

What officials are monitoring

Peru's Centro Sismológico Nacional and Colombia's geological survey continue to issue updated bulletins as new readings become available. Civil defence agencies in both countries repeated their standard advice after Saturday's tremors: keep an emergency kit ready, identify safe areas in the home and avoid tying up telephone lines immediately after an earthquake so emergency calls can get through. Neither agency has indicated any change in the underlying pattern of seismic activity in either region. For now, Saturday's tremors are being treated as two unrelated and unremarkable events in what has already been an unusually active year for earthquakes across South America.

Originally published on Latin Times