Rollback of ‘Banda Detachment’ Unexpected Historical Earthquakes in the Banda Islands

Adapted from article entitled “Earthquakes and tsunamis caused by low-angle normal faulting in the Banda Sea, Indonesia”

Phil R. Cummins, Ignatius R. Pranantyo, Jonathan M. Pownall, Jonathan D. Griffin, Irwan Meilano, and Siyuan Zhao

Published in Nature Geoscience

Volume 13, Issue 4, Pages 312 – 318, 1 April 2020

Although under the Banda Sea lies one of the most striking subduction, a group of scientist, including Irwan Meilano from Geodesy Research Group Faculty of Earth Science and Technology, Institut Teknologi Bandung, found that Banda earthquake and tsunami of 26 November 1852 caused by previously unidentified, low-angle normal fault system we name the ‘Banda Detachment’. This new study highlight two potential locations where the tsunami could have originated; the Banda detachment and submarine slump on the Weber Deep’s eastern side. Furthermore, the researchers model the two tsunamis and show that the tsunami waveforms generated by the submarine slump matches the historical records.

The paper on the results of this international research collaboration was published in Nature Geoscience under the title Earthquakes and tsunamis caused by low-angle normal faulting in the Banda Sea, Indonesia. Phil R. Cummins, a researcher at the Research School of Earth Sciences, Australian National University led the writing of the paper, along with the collaborator; I. R. Pranantyo from Research School of Earth Sciences, Australian National University; J. M. Pownall from Department of Geography, Geology, and Environment University of Hull, UK; J. D. Griffin from Community Safety Branch Geoscience Australia and Department of Geology University of Otago, New Zealand; I. Meilano from Geodesy Research Group Faculty of Earth Science and Technology, Institut Teknologi Bandung, Indonesia; and Siyuan Zhao from Research School of Earth Sciences, Australian National University.

Indonesia is a country that is prone to earthquakes. Throughout 2022, the Indonesian Agency for Meteorological, Climatological and Geophysics recorded that there were 10,729 earthquakes. This natural disaster is related to the existence of an earthquake source that is formed due to plate interaction in Indonesia.

Located in the southeastern part of the Asian continent, Indonesia is at the confluence of three Earth’s largest plates, the Eurasian, Indo-Australian and Pacific plates. The movement of these three plates produces two subduction zones, Sunda and Banda subductions.

Meanwhile, in eastern Indonesia, under the Banda Sea lies one of the most striking subduction zones in the world with a 180˚ arc bend. In the Banda arc, the Eurasian plate is the upper plate on both the Indo-Australian plate in the south and the Pacific plate in the north. The Banda arc has a tight 300 km radius of curvature.

Although over the past decade the western part of the Sunda subduction has experienced several megathrust earthquakes, the subduction zones of the eastern Sunda, Banda arc, and eastern Indonesia have remained relatively calm over the last 150 years. What could have caused destructive earthquakes in the Banda Islands is a question that has puzzled scientists. Thanks to the discovery of a fault in the Banda Sea, researchers finally have the answer to this question.

“It would at first appear likely that large earthquakes and tsunamis that devastated the Banda Islands in the historical past, as well as the potential threat of future events, should be attributed to a megathrust along this Banda outer arc. However, since the Banda arc is a zone of arc–continent collision, it no longer features an oceanic trench and, therefore, no megathrust. Hence, it is imperative that the mechanism for destructive Banda Sea earthquake and tsunami generation is re-evaluated,” said the study.

Recently, there has been a proposal related to the configuration of the Banda subduction zone, which at the same time proved to explain the most intriguing physiographic features of the Banda Sea; Weber deep. “Here we propose an eastward subduction rollback that drives forearc extension,” says Jonathan M. Pownall, a researcher from the University of Hull, UK.

The rollback causes the lower crust and subcontinental lithospheric mantle rises to fill the gap opened above the rolling-back slab. This rollback also drives the extreme lithospheric extension which is accommodated by a major, previously unidentified, low-angle normal fault system we name the ‘Banda Detachment’.

Researchers consider whether historical accounts of destructive earthquakes in the Banda Sea can be explained by large but infrequent earthquakes on the Banda detachment and how these might generate tsunamis. They were focus on the earthquake of 26 November 1852 because it has the most extensive and detailed accounts of the devastated the Banda Islands and of the subsequent tsunami.

Source of the earthquake & tsunami

The 1852 earthquake generated its strongest felt intensity in the Banda Islands and intensity decreased northward at Ternate. Researchers wrote, “The only major fault identified so near the Banda Islands is the Banda detachment, and we therefore consider whether an earthquake on this fault, just east of the Banda Islands could give rise to the observed seismic intensities.”

Researchers consider an earthquake located along the surface of the Banda detachment that has a fault dip that can break during an earthquake. To produce rapid fall-off of the intensity observed from the 1852 earthquake, even the rupture in the Tanimbar Trench generates insufficient intensity. “nstead, the observed intensities favor a smaller earthquake near the Banda detachment.

The four tsunami observations of the 1852 event all clearly show that the source could not have been a megathrust event in the outer arc. The 1852 Banda sea tsunami has a particularly clear description of its arrival time relative to the earthquake. “This 20 minute delay between the occurrence of the earthquake and the arrival of the tsunami is an important constraint on the locus of tsunami generation.”

Therefore, researchers highlight two potential locations where the tsunami could have originated; the Banda detachment and submarine slump on the Weber Deep’s eastern side. Furthermore, the researchers model the two tsunamis and show that the tsunami waveforms generated by the submarine slump matches the historical records.

It is not certain that the mechanism behind the 1852 earthquake and tsunami also applies to other historical earthquakes in the Banda Sea. However, researchers suggest that the other historical major tsunamis that have affected the Banda Islands were caused by an earthquake-triggered submarine slump.

“Regardless of which fault caused the Banda Sea earthquakes of 1629–1852, it would be a mistake to assume the Banda detachment cannot rupture in a future earthquake simply because it lacks recorded seismicity,” the researchers conclude.*