Magnitude 6.2 Earthquake off the west coast of Sumatra.

The Indonesian Meteorological, Climatological, and Geophysical Agency recorded a Magnitude 6.2 Earthquake at a depth of about 29 km, off the West coast of Aceh Province, Sumatra, Indonesia slightly after 6.00 pm Western Indonesian Time (slightly after 11.00 am GMT) on Tuesday 31 January 2025. There have been no reports of any damage or casualties following this event, but people have reported feeling  tremors  across Aceh Province and parts of North Sumatra.

The approximate location of the 31 January 2025 Aceh Province Earthquake. USGS.

The Indo-Australian Plate, which underlies the Indian Ocean to the west of Sumatra, is being subducted beneath the Sunda Plate, a breakaway part of the Eurasian Plate which underlies Sumatra and neighbouring Java, along the Sunda Trench, passing under Sumatra, where friction between the two plates can cause Earthquakes. As the Indo-Australian Plate sinks further into the Earth it is partially melted and some of the melted material rises through the overlying Sunda Plate as magma, fueling the volcanoes of Sumatra.

The Subduction zone beneath Sumatra. NASA/Earth Observatory.

This does not happen at a 90° angle, as occurs in the subduction zones along the western margins of North and South America, but at a steeply oblique angle. This means that as well as the subduction of the Indo-Australian plate beneath the Sunda, the two plates are also moving past one-another. This causes rifting within the plates, as parts of each plate become stuck to the other, and are dragged along in the opposing plate's direction. The most obvious example of this is the Sumatran Fault, which runs the length of Sumatra, with the two halves of the island moving independently of one-another. This fault is the cause of most of the quakes on the island, and most of the island's volcanoes lie on it.

The movement of the tectonic plates around Sumatra. NASA/Earth Observatory.

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Eleven confirmed dead and twelve still missing following eruption on Mount Marapi, Sumatra.

Eleven people have been confirmed dead and another twelve are still missing following an eruption on Mount Marapi, a 2885 m high volcano in West Sumatra Province, Indonesia on Sunday 3 December 2023. The volcano erupted with little warning, producing a 3 km high column of ash as well as ash falls on villages more than 5 km from the summit of the volcano. Seventy five people, including a group of 18 students, were on the volcano at the time of the eruption. Forty nine people of the hikers were able to escape from the mountain promptly, leaving 26 unaccounted for. Over 160 rescue workers were deployed to the mountain, and were able to rescue three of the missing hikers alive, all of who were suffering from burns and one from a broken leg. They also recovered eleven bodies, but were eventually forced to break of the search before accounting for all of the missing persons, due to the ongoing eruption.

A survivor of the 3 December 2023 Mount Marapi eruption being carried off the mountain by rescue workers. Antara Foto/Reuters.

The Indo-Australian Plate, which underlies the Indian Ocean to the west of Sumatra, is being subducted beneath the Sunda Plate, a breakaway part of the Eurasian Plate which underlies Sumatra and neighbouring Java, along the Sunda Trench, passing under Sumatra, where friction between the two plates can cause Earthquakes. As the Indo-Australian Plate sinks further into the Earth it is partially melted and some of the melted material rises through the overlying Sunda Plate as magma, fuelling the volcanoes of Sumatra.

The Subduction zone beneath Sumatra. NASA/Earth Observatory.

The two plates are not directly impacting one-another, as occurs in the subduction zones along the western margins of North and South America, but at a steeply oblique angle. This means that as well as the subduction of the Indo-Australian plate beneath the Sunda, the two plates are also moving past one-another. This causes rifting within the plates, as parts of each plate become stuck to the other, and are dragged along in the opposing plate's direction. The most obvious example of this is the Sumatran Fault, which runs the length of Sumatra, with the two halves of the island moving independently of one-another. This fault is the cause of most of the quakes on the island, and most of the island's volcanoes lie on it.

The movement of the tectonic plates around Sumatra. NASA/Earth Observatory.

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Magnitude 5.6 Earthquake in West Java kills at least 162 people

The Baden Meteorologi, Klimatologi, dan Geofisika recorded a Magnitude 5.6 Earthquake at a depth of 10 km in the Cianjur Districs of West Java Province, Indonesia, slightly after 1.20 pm local time (slightly after 6.20 am GMT) on Monday 21 November 2022. The event triggered a series of landslides, as well as causing a large number of building collapses, and is now known to have killed at least 162 people. 

The approximate location of the 21 November 2022 West Java Earthquake. United States Geological Survey.

Many of those who have died are reported to have been children, with at least one school having been severely damaged by the Earthquake, which struck with very little warning. Many homes have also been destroyed and damaged, a hospital in Cianjur District damaged. Much of the area has been left without power, and many roads have been damaged, making communication with more remote areas very difficult. Nurses from the Indonesian Red Cross are reported to be trying to reach several remote villages on motorbikes.

Damage to a school in the Cianjur District of West Java, Indonesia, following an Earthquake on 21 November 2022. ABC News.

The Indo-Australian Plate, which underlies the Indian Ocean to the south of Java, Bali and Lombok, is being subducted beneath the Sunda Plate, a breakaway part of the Eurasian Plate which underlies the islands and neighbouring Sumatra, along the Sunda Trench, passing under the islands, where friction between the two plates can cause Earthquakes. As the Indo-Australian Plate sinks further into the Earth it is partially melted and some of the melted material rises through the overlying Sunda Plate as magma, fuelling the volcanoes of Java and neighbouring islands.

Subduction along the Sunda Trench beneath Java, Bali and Lombok. Earth Observatory of Singapore.

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Eruption on Anak Krakatoa produces column of ash 3 km high.

Anak Krakatoa, a volcanic island located in the Sunda Strait between Sumatra and Java, underwent a major eruption on Sunday 24 April 2022, producing a column of ash about 3 km high. The volcano had been erupting intermitantly for several weeks, but this event was significantly larger than previous eruptions in this cycle, and 24 hours after the initial eruption the Anak Krakatoa was still erupting intermitently, producing ash columns between 500 m and 3 km high. The Badan Nasional Penanggulangan Bencana has advised that people living close to the volcano wear dust masks when going outside until the volcano subsides.

 

An ash column over Anak Krakatoa (at the right of the image) on 24 April 2024. Dziki Oktomauliyadi/AFP.

Anak Krakatau is a volcanic island located in the Sunda Strait between Sumatra and Java. The volcano (then known simply as Krakatoa) famously exploded in 1883, killing over 36 000 people (possibly over 120 000), largely through a series of tsunamis. This explosion more-or-less completely destroyed the island, but since then a new volcano, Anak Krakatau (meaning the 'son of Krakatoa'), has grown in its place. Anak Krakatau is almost never completely quiet, but goes through periods of greater and lesser activity. An eruption on 22 December 2018 triggered a tsunami which tsunami caused over 430 fatalities, injured 14 000 people, and displaced 33 000 more along the Sunda Strait. The tsunami risk of this area is particularly high as the coast is very popular with both locals and tourists and is home to over 20 million people within a 100 km distance from the volcano.

Anak Krakatau is located to the north of the Sunda Trench, along which the Australian Plate is being subducted beneath the Sunda Plate, on which the island sits. As the Australian Plate is subducted it is partially melted by the friction and the heat of the planet's interior. Some of this melted material then rises through the overlying Sunda Plate, fuelling Krakatau and the volcanoes of Sumatra and Java.

 

The Subduction zone beneath Sumatra. NASA/Earth Observatory.

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Evacuations following series of eruptions on Mount Merapi, Java.

Slightly over 250 people were evacuated from their homes temporarily following a series of eruptions on Mount Merapi, a 2970 m stratovolcano (cone-shaped volcano made up of layers of ash and lava) in Central Java, considered to be one of Indonesia's most active, that began slightly before midnight local time on Wednesday 9 March 2022. The volcano underwent a series of major eruptions between midnight and 2.00 am on Thursday 10 March, producing a series of lava rivers and pyroclastic flows. This prompted the Badan Nasional Penanggulangan Bencana to extend the exclusion zone around the volcano from 3 km to 7 km on a temporary basis, prompting the evacuations. The eruptions persisted till Friday 11 March, although they decreased in severity over time.

 

A lava flow on the flank of Mount Merapi. Volcano Discovery.

Mount Merapi lies in a densely populated area of Java, on the borders of Central Java and Yogyakarta Provinces, only 28 km to the north of Yogyakarta city. It has been erupting more-or-less continuously since 1548, and has been responsible for numerous fatalities, most recently in 1994 when a pyroclastic flow (avalanche of hot gas and ash) killed 27 people, mostly in the town of Muntilan, to the west of the volcano. Since then Merapi has undergone tow major eruptive episodes, in 2006 and 2010, without any further loss of life, largely due to prompt evacuations by Indonesian authorities.

 

The approximate location of Mount Merapi. Google Maps.

The Indo-Australian Plate, which underlies the Indian Ocean to the south of Java, is being subducted beneath the Sunda Plate, a breakaway part of the Eurasian Plate which underlies Java and neighbouring Sumatra, along the Sunda Trench, passing under Java, where friction between the two plates can cause Earthquakes. As the Indo-Australian Plate sinks further into the Earth it is partially melted and some of the melted material rises through the overlying Sunda Plate as magma, fuelling the volcanoes of Java and Sumatra.

 

Subduction along the Sunda Trench. Earth Observatory of Singapore.

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Magnitude 6.2 Earthquake beneath West Sumatra, killing at least seven.

The Indonesian Meteorological, Climatological, and Geophysical Agency recorded a Magnitude 6.2 Earthquake at a depth of about 10 km, beneath West Sumatra Province, Indonesia slightly before 8.40 am Western Indonesian Time (slightly before 1.40 am GMT) on Friday 25 February 2022. There have been seven reported fatalities following this incident, including two children, with another 85 injured and damage to about 410 buildings. The event was felt across West Sumatra and Riau provinces, as well as much of Peninsula Malaysia and Singapore, with about 6000 people being temporarily evacuated from their homes in Sumatra and Malaysia.

 

The approximate location of the 25 February 2022 West Sumatra Earthquake. USGS.

The Indo-Australian Plate, which underlies the Indian Ocean to the west of Sumatra, is being subducted beneath the Sunda Plate, a breakaway part of the Eurasian Plate which underlies Sumatra and neighbouring Java, along the Sunda Trench, passing under Sumatra, where friction between the two plates can cause Earthquakes. As the Indo-Australian Plate sinks further into the Earth it is partially melted and some of the melted material rises through the overlying Sunda Plate as magma, fuelling the volcanoes of Sumatra.

 

The Subduction zone beneath Sumatra. NASA/Earth Observatory.

This does not happen at a 90° angle, as occurs in the subduction zones along the western margins of North and South America, but at a steeply oblique angle. This means that as well as the subduction of the Indo-Australian plate beneath the Sunda, the two plates are also moving past one-another. This causes rifting within the plates, as parts of each plate become stuck to the other, and are dragged along in the opposing plate's direction. The most obvious example of this is the Sumatran Fault, which runs the length of Sumatra, with the two halves of the island moving independently of one-another. This fault is the cause of most of the quakes on the island, and most of the island's volcanoes lie on it.

 

The movement of the tectonic plates around Sumatra. NASA/Earth Observatory.

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