How a Tsunami Forms: the Flores, Indonesia Earthquake

In the early hours of August 15, 2026, a magnitude 7.7 earthquake shook the sea off the island of Flores, in eastern Indonesia, and triggered a tsunami alert that led to the evacuation of coastal areas. Beyond the shaking, the quake left a question worth understanding calmly: why do some earthquakes generate tsunamis and others don't? The answer has a lot to do with two things — where and at what depth they happen.

What happened, with the data available

According to the US Geological Survey (USGS), the earthquake had a magnitude of 7.7, with its epicentre about 68 km north-northwest of the city of Ende, in East Nusa Tenggara province, at a depth of 10 kilometres. In the following half hour, several strong aftershocks were recorded (magnitudes 5.9, 5.6 and 6.1).

Indonesia's meteorology and geophysics agency, BMKG, issued a tsunami alert across 22 zones in several provinces, and hundreds of people evacuated. The tsunami waves that were ultimately measured were modest — between about 0.19 and 0.36 metres at different coastal locations — but the alert was fully justified by the quake's characteristics.

Sadly, at least two fatalities were confirmed. Damage assessment was still under way at the time of writing.

How a tsunami forms (it isn't one giant wave)

The image of a colossal wall of water is misleading. A tsunami is not born as a giant wave: it is born from a vertical shove of the whole ocean.

When an undersea earthquake suddenly lifts or drops a portion of the seafloor, it doesn't just move the floor: it pushes up (or lets fall) the entire column of water above it — kilometres of water, all at once. That disturbance spreads outward in every direction, like the ripples when you throw a stone into a pond, but on a colossal scale.

That's why it matters so much that the quake be vertical (that it raises or lowers the floor, not just slides it sideways) and shallow: if the focus is only 10 km down, the energy reaches the seafloor with almost its full force and can displace the water. A very deep earthquake, by contrast, dissipates much of that energy before it ever reaches the seabed.

Why a tsunami is so fast (and why it grows near the coast)

Here is the physics that surprises people, and it explains why alerts give only a few minutes' margin. A tsunami's speed depends on the water depth: the deeper the ocean, the faster it travels. Move the widget.

The numbers are striking: in the open ocean, over 4,000 metres of depth, a tsunami travels at about 700 km/h — the speed of a passenger jet — but with a wave so low that a ship barely notices it. As it approaches the coast and enters shallow water, it slows to tens of km/h… and all that energy it carried compresses, making the wave grow in height. A tsunami is dangerous near the shore not because it is fast, but because of the enormous mass of water it pushes inland.

Why Indonesia: the Pacific Ring of Fire

It is no coincidence that this earthquake struck Indonesia. The country is an archipelago of more than 17,000 islands sitting on the Pacific Ring of Fire: a giant horseshoe bordering the Pacific Ocean where several tectonic plates of the Earth's crust collide.

At those boundaries, some plates slide beneath others in a slow but relentless process, and that friction builds up stress that is released all at once as earthquakes. The Ring of Fire concentrates the vast majority of the planet's quakes and volcanoes, and Indonesia, right in the middle, is one of the most active zones of all. For its inhabitants, tremors are part of life — which is why the country has one of the most developed tsunami warning systems in the world.

Why the alerts are taken so seriously

It is worth understanding the historical context, with respect. In 2004, a magnitude 9.1 earthquake off the coast of Sumatra — also in Indonesia — generated a tsunami that claimed more than 220,000 lives across several countries around the Indian Ocean. It was one of the deadliest natural disasters in recent history, and it completely transformed how the world monitors and warns about tsunamis.

Since then, networks of sensors, evacuation protocols and early-warning systems have been built. That is why, when a powerful, shallow, undersea quake like the one off Flores occurs, agencies issue the alert immediately, even if the tsunami ultimately turns out to be small. The cost of an alert that doesn't materialise is low; the cost of not alerting can be enormous. It is prudence, not alarmism.

To put the magnitude in context

One figure to grasp the scale: a magnitude 7.7 earthquake like the Flores one releases on the order of 32 times more energy than a magnitude 6.7 — remembering that the scale is logarithmic, where each whole point multiplies the energy by about 32. And even so, the 2004 Sumatra quake (9.1) released more than 100 times the energy of the Flores one. That is why a difference that looks small in the number is enormous in reality.

The takeaway

The Flores earthquake was a reminder of how geology shapes life on the Ring of Fire, and of why the depth and location of a quake matter as much as its magnitude. An undersea, shallow, vertical tremor can displace the ocean and unleash a tsunami that crosses the sea at the speed of a jet; that is why, when the alert sounds, every minute counts. That the waves were small this time does not invalidate the caution — it validates it.

If you want to see how much more energy one earthquake releases than another based on its magnitude — to compare Flores with other regions or with the great quakes of history — an earthquake magnitude comparator makes it clear in seconds.

Information on the August 14–15, 2026 event is based on reports from the US Geological Survey (USGS), Indonesia's BMKG agency and news agencies, verified as of that date and subject to official revision. Tsunami speed figures were calculated with the standard long-wave physics formula. For up-to-date data, consult the official sources.