A powerful earthquake struck southern Peru on Thursday, triggering alarms across several regions. The US Geological Survey put the magnitude at 6.7. Peru’s own Geophysical Institute reported 7.2. The quake occurred at 1pm local time at an intermediate depth of 108 kilometres, and authorities urged vigilance nationwide.

Why two agencies give two numbers

A gap of half a magnitude point looks like someone has made a mistake. Usually nobody has.

Magnitude is calculated from seismic waves recorded at stations, and the answer depends on which stations, which type of wave, and which formula. A national agency draws mainly on instruments inside its own country — close to the epicentre, dense, but all on one side of it. A global agency uses a worldwide network, which gives better geometry and worse local detail. The two produce different estimates from the same event, and both are legitimate.

Early figures also get revised. The first automatic estimate is produced within minutes by software; analysts refine it over the following hours as more data arrives. It is normal for a magnitude to move by a couple of tenths, and occasionally more.

One thing worth holding on to: the scale is logarithmic. A 7.2 releases roughly five times more energy than a 6.7, so this is not a trivial disagreement — it is a disagreement about something genuinely uncertain in the first hours.

The number that matters more

The depth is the detail most likely to determine the outcome, and it is the one least likely to be in the headline.

At 108 kilometres, this was an intermediate-depth earthquake. Seismic energy weakens as it travels, so a quake this deep shakes the surface considerably less than one of the same magnitude at ten or twenty kilometres. That is why an event that sounds alarming on the scale can produce alarms and rattled buildings across a wide area rather than concentrated destruction in one place.

Depth cuts the other way in one respect: deep quakes are felt over a much larger area, because the energy spreads outward from further down. That is consistent with reports of alarms triggered across multiple regions.

Why Peru

Peru sits on one of the most active plate boundaries on Earth. The Nazca plate is being forced beneath the South American plate along the coast, a process that has built the Andes and generates earthquakes continuously.

That subduction geometry is also why intermediate-depth events happen there at all. The descending slab of oceanic crust carries stress far below the surface, and quakes occur along it at depths that would be impossible in a region without subduction. A 108-kilometre-deep earthquake in southern Peru is a normal expression of how that boundary works.

The other emergency

The earthquake arrives while parts of Peru are already dealing with heavy rains that have triggered floods and landslides.

The combination is more dangerous than either alone, for a specific and well-documented reason: saturated ground is unstable ground. Slopes that have absorbed days of rain fail more readily when shaken, and landslides triggered by an earthquake in already-soaked terrain have caused more deaths in mountainous regions than the shaking itself. Emergency services also end up dividing resources between two events at once.

No casualty figures were available at the time of writing. In an intermediate-depth event across a wide area, the picture typically takes a day or more to assemble as reports come in from dispersed communities, and early silence from remote districts is not evidence that they are unaffected.

Sources

• Anadolu Agency — morning briefing, 21 August 2026, on the southern Peru earthquake