Beneath Sumatra, a massive block of Earth's crust is being squeezed out sideways like a watermelon seed, escaping northwest at 20-30 millimeters per year.
Mikenorton, CC BY-SA 4.0, via Wikimedia Commons
An angled collision
The island of Sumatra is the product of an immense, slow-motion collision between two of Earth's tectonic plates. The Indo-Australian Plate is diving beneath the Sunda Plate (a part of the larger Eurasian Plate) at a rate of 50 to 70 millimeters per year. This process, called subduction, does not happen head-on. The plates meet at an angle, a phenomenon known as oblique subduction.
This angled crash partitions the tectonic strain into two distinct motions. One component is the downward plunge of the Indo-Australian plate, which occurs at the Sunda Trench offshore. The other is a sideways, northwestward slip that runs parallel to the island. This sideways motion is accommodated by a long, thin piece of crust caught between the trench and the main volcanic arc. This block, known as a fore-arc sliver, is being pushed northwest along the length of Sumatra. This sliver plate includes the offshore fore-arc islands and the part of Sumatra west of the Great Sumatran Fault.
A tale of two faults
Two massive fault systems define the edges of this escaping sliver. Offshore, to the west, is the Sunda Megathrust. This is the direct interface where the Indo-Australian Plate grinds beneath the sliver plate. Movement on this fault is responsible for some of the most powerful earthquakes ever recorded. On December 26, 2004, a section of this megathrust ruptured over a length of 1,300 kilometers, causing the magnitude 9.1–9.3 earthquake that generated the devastating Indian Ocean tsunami. The seafloor rose by several meters, displacing an estimated 30 cubic kilometers of water.
Onshore, forming the sliver's eastern boundary, is the Great Sumatran Fault (GSF). This 1,900-kilometer-long right-lateral strike-slip fault runs down the spine of the island, often following the Barisan Mountains. The GSF absorbs the northwestward motion of the fore-arc sliver. Its slip rate varies, from around 6 millimeters per year in the south to as much as 27 millimeters per year near Toba Lake in the north. This fault is highly segmented and is the source of frequent, damaging earthquakes on the island itself, with recorded events reaching magnitudes of 7.5 and 7.6.
The constant movement of this fore-arc sliver, driven by the angled collision of continents and defined by these two colossal faults, shapes the very terrain and hazards of Sumatra. The process fuels the island's chain of volcanoes, including the site of the Toba supereruption approximately 74,000 years ago, and drives seismic activity today.
💡Fun Facts
The Great Sumatran Fault is a major geological feature that carves a distinct valley system visible from space.
The fore-arc sliver is sometimes considered a microplate in its own right, often called the Burma Plate.
The 1883 eruption of Krakatoa, one of the most famous volcanoes on the Sunda Arc, produced the loudest sound in recorded history, heard up to 5,000 kilometers away.
The energy released by the 2004 Sumatra-Andaman earthquake caused the entire planet to vibrate by as much as 10 millimeters.
The geological features of Sumatra are part of the natural landscape and are always accessible.
Admission
Free.
Accessibility
The Great Sumatran Fault runs through the mountainous Barisan Range. While major roads cross the region, the terrain is rugged. Specific geological outcrops may require hiking.
