Why does Indonesia experience frequent earthquakes? Causes and megathrust explained

Jakarta (ANTARA) - Earthquakes have once again drawn attention after several regions of Indonesia were shaken by tremors in recent times. In Flores, East Nusa Tenggara (NTT), a magnitude 7.7 earthquake on 15 August 2026 was one of the strongest and most impactful. BMKG recorded that aftershock activity continued, reaching 4,258 events with magnitudes ranging from 1.1 to 6.2 by Friday (21/8) at 05.00 WIB. As many as 118 aftershocks were felt by the public, with 31 of them exceeding magnitude 5.0.

In nearly the same period, Central Sulawesi was also shaken by a magnitude 6.2 earthquake in the southwestern region of Parigi Moutong on 15 August 2026. BMKG explained that the earthquake was related to rock deformation within the plate and did not pose a tsunami risk. Meanwhile, earthquake activity was still recorded in several NTT regions until 21 August, including quakes centred around Manggarai and Mbay-Nagekeo.

These phenomena are not isolated events. In late 2025, Aceh also experienced a series of earthquakes, including a magnitude 6.3 quake south of Simeulue Island on 27 November 2025. BMKG classified it as a shallow earthquake with no tsunami potential. On 31 December 2025, BMKG’s Aceh Besar Geophysics Station also recorded 11 earthquake events in the Aceh region from early morning until around 11.10 WIB.

This series of events has revived a long-standing question: why is Indonesia so frequently shaken by earthquakes, and how does this relate to the term ‘megathrust’ that has been widely discussed in recent years? To understand this, we need to look at Indonesia’s position on the world tectonic map.

One of the main reasons Indonesia is prone to earthquakes is its location at the meeting point of several active tectonic plates.

Indonesia lies within the Ring of Fire, or Pacific Ring of Fire. In this region, plate dynamics take the form of subduction and collision zones. These movements make some parts of Indonesia home to highly active earthquake sources.

In simple terms, a tectonic plate is a large section of the Earth’s crust that is constantly moving, albeit very slowly. When two plates meet, a collision can occur, or one plate may plunge beneath the other.

This plunging process is known as subduction. When the pressure and energy stored in the zone are suddenly released, an earthquake occurs.

Indonesia has numerous subduction zones

Indonesia’s vulnerability does not stem from a single earthquake source. BMKG explains that there are several subduction zones in Indonesia, including the Sunda zone, Banda zone, northern Sulawesi, the Maluku Sea, and northern Papua. These zones serve as earthquake sources and can also trigger activity on active faults.

Because the sources are spread across various regions, earthquakes can occur in Sumatra, Java, Nusa Tenggara, Sulawesi, Maluku, and Papua.

This is why earthquakes in Aceh, Flores, Central Sulawesi, and other regions do not always share the same geological cause. Each event must be analysed based on its epicentre location, depth, source mechanism, and surrounding tectonic structure.

The term megathrust often arises when discussing the potential for major earthquakes in Indonesia. Simply put, a megathrust is the shallow part of a subduction zone with a relatively gentle dip angle. This zone lies around plate boundaries and can be a source of large earthquakes.

One frequently discussed example lies to the west of Sumatra. In that region, the Indo-Australian Plate is subducting beneath the Eurasian Plate. BMKG, in several earthquake analyses for Sumatra, explains that this subduction activity is related to the megathrust zone in the Indian Ocean west of Sumatra.

Megathrust zones also exist in other regions. Therefore, the term does not refer to a specific earthquake but rather to an earthquake source comprising part of a subduction zone.

Does every large earthquake mean a megathrust earthquake?

The answer is no.

The Flores earthquake on 15 August 2026, for example, was indeed large in magnitude, but it would be inaccurate to label every earthquake in that region as a megathrust earthquake.

The same applies to the magnitude 6.2 earthquake in Tomini Bay, Central Sulawesi. BMKG stated that the earthquake resulted from rock deformation within the plate with an oblique thrust fault mechanism. This means that determining an earthquake’s source requires geological and seismological analysis, not merely looking at its magnitude.

This is important because the term megathrust often creates the impression that every earthquake is a sign that a giant earthquake will soon follow. In reality, the timing of earthquakes cannot be determined solely based on the occurrence of previous quakes.

Aceh is one of the regions with a strong seismic history because it lies near the Sumatra subduction zone and several active tectonic structures.

On 27 November 2025, for instance, a magnitude 6.3 earthquake shook the southern region of Simeulue Island. BMKG stated that its epicentre was approximately 1 kilometre south of Simeulue at a depth of 14 kilometres and classified it as a shallow earthquake.

The earthquake was followed by further seismic activity in Aceh. By the end of December 2025, BMKG had recorded 11 earthquakes across the entire Aceh region in just the period from early morning until around midday on 31 December.

This series demonstrates that tectonic activity in Indonesia is indeed dynamic. However, an increase in the number of earthquakes over a given period cannot be used as a basis for concluding that a particular megathrust earthquake will soon occur.

Why can earthquakes occur repeatedly?

Earthquakes do not always arrive as a single event. After a sufficiently large mainshock, the rocks around the earthquake source may undergo adjustment, producing aftershocks.

This condition was clearly visible after the M7.7 earthquake in Flores, where thousands of aftershocks continued to be recorded for days.