The Andaman Trench

Where the Trench Sits

• Location: the Andaman Trench lies in the northeastern Indian Ocean. It sits just west of the Andaman and Nicobar Islands, running roughly north to south.
• Part of a bigger system: it is the northern end of the much larger Sunda Trench system. That system stretches far south, past Indonesia.
• How it forms: it forms where the Indian Plate slides down, or subducts, under the Burma microplate. A microplate is a smaller tectonic plate wedged in between two bigger ones.
• Maximum depth: about 4,198 metres deep. Most mountains on land could sink into it and vanish.
• A sideways slide too: the Indian Plate doesn’t push straight into the Burma microplate here. It meets at a slanted angle instead, so the plates also grind sideways past each other, not just downward.

Worked example: picture two long conveyor belts meeting at a slight angle, not head-on. One belt dips down under the other. That’s the Andaman Trench: mostly one plate diving under the second, but with a real sideways grind mixed in too.

Six Landforms, West to East

• One full system: the trench is just one part of a full subduction-zone system. Geologists count six distinct landforms across it, laid out west to east.
• 1. The foredeep: the trench itself, where the two plates actually meet.
• 2. The accretionary prism: scraped-off seabed mud and rock pile up here, over millions of years.
• 3. The island arc: the visible ridge above sea level. The Andaman and Nicobar Islands are this ridge’s highest points. Most of the same ridge actually stays underwater.
• 4 and 5. The fore-arc basin and volcanic arc: further east sit a low basin, then the volcanic arc. Barren Island, India’s only active volcano, sits on this volcanic arc. Rising magma from the subducting plate feeds it.
• 6. The back-arc basin: the Andaman Sea itself. The same tectonic forces are slowly stretching its seafloor apart.

The Sagaing Fault and Myanmar’s Risk

• A wider fault family: the same slanted plate motion continues north into Myanmar. There, it becomes the Sagaing Fault, a major fault where land slides sideways rather than sinking.
• A real regional hazard: the Sagaing Fault runs directly through crowded Myanmar cities, including Mandalay and Yangon. It stays a genuine earthquake risk for that whole region, not just the Andaman Sea.

The 2004 Earthquake and Tsunami

• A massive rupture: on 26 December 2004, a magnitude 9.1 earthquake broke along this subduction zone. It remains one of the largest earthquakes ever recorded.
• 1,200 km of fault: the rupture ran for about 1,200 km, from off Sumatra up into the Andaman stretch. Very few earthquakes anywhere have broken such a long piece of fault at once.
• A sudden uplift: it pushed the seafloor up by as much as 10 metres in places, shoving huge volumes of water aside.
• A wave across an ocean: the resulting tsunami crossed the whole Indian Ocean, reaching coastlines in East Africa, thousands of kilometres away.
• Striking home: the same tsunami struck the Andaman and Nicobar Islands directly, since they sit right above this fault.
• Indira Point’s sinking: it also made the land around Indira Point, India’s southernmost point, sink by about 4.25 metres. Part of it stays underwater even today.

A Still-Active Fault System

• Not just one fault: beyond the main subduction zone, several other faults affect the islands too. These include the West Andaman Fault, and other faults running the length of the island chain.
• A long history: earlier earthquakes here include 1847 at Kondul, 1881 at Car Nicobar, and 1941 in West Andaman — all well before the 2004 megathrust quake.
• Genuinely active, not new: this nearly two-century-long earthquake record shows this subduction zone has always been active. The 2004 disaster didn’t suddenly create that activity.
• Ongoing smaller quakes: smaller earthquakes still happen regularly along this zone, a steady reminder of how active this boundary remains.

Life in the Deep Trench

• The hadal zone: waters below about 6,000 metres are called the hadal zone. The Andaman Trench itself sits just short of that true hadal depth.
• Extreme conditions: organisms living at these depths survive crushing pressure and near-total darkness, conditions harsh for most familiar sea life.
• Poorly studied: deep-sea trenches like this one stay poorly studied compared to shallower ocean zones, simply because reaching them is so technically hard.
• Similar trenches worldwide: the same basic subduction process also builds the Japan Trench and the Peru-Chile Trench. Both are major earthquake risk zones for their own countries.
• A useful comparison: the same basic physics governs subduction everywhere. That’s why studying one trench often helps predict hazards at a completely different one.

A New Highest Risk Zone (2025)

• The update: India revised its 2025 Earthquake Design Code. It placed the Andaman and Nicobar Islands in a brand-new Zone VI. This is now the highest seismic risk category on the national scale. (Source: The Wave Andaman, 1 December 2025)
• A new top tier: Zone V used to be the most severe category. It covered the Northeast, Kashmir, the Rann of Kutch, and these islands. Zone VI now sits above it.
• Why this zone: officials pointed to the islands’ position right along the active Andaman-Sumatra subduction zone, plus extra fault systems nearby, as the reason for the change.
• Why it matters here: this updates this article’s own subduction-zone geography with a real, practical result — stricter earthquake-resistant building rules for the islands.