1852 Banda Sea earthquake

A publication in 2020 led by Phil Cummins suggests the event did not occur on a megathrust fault, but rather, a shallow normal fault in the Banda Sea. The normal fault was determined as being the Banda Detachment, a shallow-dipping structure that accommodates extension of the Banda Sea. It is thought to be the only major potentially seismogenic structure to cause heavy shaking and a large tsunami. The earthquake potential and history of this structure is still poorly understood, and have never been considered a tsunami and seismic hazard to the region.[9] The Weber Deep, a 7.2-km deep forearc basin between the megathrust front and Banda Arc volcanic chain is a massive fault scarp which formed when extension along the Banda Detachment started.[10] However, there have been no instrumentally recorded earthquakes on this fault, therefore, it is suggested that the structure is creeping aseismically, or that earthquakes on this fault have occurred before the birth of modern seismic instruments.[5]

The research presented a much smaller earthquake moment magnitude of only 7.5 with a rupture close to the Banda Islands. An earthquake anywhere on the megathrust is unlikely to result in violent ground motions as described in the historical accounts because it is too far from the islands.[5] The study dismissed all reports of shaking felt in Surabaya, Java as being a separate, local earthquake which timing coincided with the one in the Banda Sea.[5] This event in Java, named the Grati earthquake, is thought to have a magnitude of M_w  5.7–6.0 and occurred on the Pasuruan Fault.[8]

Explicit descriptions of the tsunami in the Banda Islands described a rise in sea level, followed by a drop. This meant the earthquake could not be sourced from a subduction zone megathrust event as such a tsunami would be observed as a drawback of the water first in the same location. The Banda Islands situated on the overriding plate would experience a negative wave first before a large tsunami could hit, thus a drawback would be observed. Furthermore, a megathrust earthquake-sourced tsunami would take much longer for it to hit the islands, while historic descriptions stated that the tsunami arrived 20 minutes after the five minutes of shaking.[5]

The research paper presented two nearby plausible tsunami sources; the western seafloor expression of the Banda Detachment at 100 km south southeast of Banda Neira, or a slump on the eastern edge of the Weber Deep. Modeling the tsunami sourced by a slump fits the well-documented accounts of the tsunami better. The associated tsunami therefore was caused by submarine slumping triggered by the M_w 7.5 earthquake.[5]

Cross section of the Banda Arc

In a 2016 study by Fisher and Harris, they concluded that the earthquake and tsunami was the result of thrust faulting on a subduction zone. A series of reconstructions with varying locations and maximum slip yielded moment magnitudes of 8.4–8.8. Using the models, they concluded that the earthquake was a thrust faulting, megathrust event which ruptured a subduction zone along the Tanimbar Trough. They estimate at least 10–15 m (33–49 ft) of slip had been cumulated, which would suggest a minimum moment magnitude of 8.4.[11]

A 2021 paper led by Hayden Ringer and others suggested a larger moment magnitude of 8.8, southeast of Seram Island.[12] The authors noted Phil Cummins' conclusion (normal fault theory) that the resulting tsunami was by an earthquake-triggered slump of low possibility because the 1938 Banda Sea earthquake did not trigger a large tsunami from a major submarine landslide.[12]

Modeling of their earthquake showed that the 1852 event had a magnitude of approximately 8.8 M_w . The earthquake epicenter coordinate is at 4.5°S 131.5°E / -4.5; 131.5, southeast of Seram Island. A tsunami model for rupture along the Seram Trough fits well with arrival time and run-up data. They also noted the tsunami reached distant locations with heights greater than what would be expected for a landslide-induced tsunami.[12] Had the tsunami been triggered by a submarine landslide, the waves would have weakened significantly at a faster rate than if triggered by faulting.[12]

Cummins counter-argued the megathrust claim by Fisher and Harris on the absence of a drawback along the coast which was not described in observational accounts but reflected in their model.[11] However, the accounts of survivors recalled a surge in seawater, followed by the tsunami waves.[5] Ringer's paper stated that the non-existent record of a drawback associated with the negative wave does not rule out the plausibility of a megathrust earthquake. His team also distinguished the day and period of the event, which was during the spring tide, meaning tide levels were at extreme lows. This meant even a negative wave would not cause a significant difference in sea level. It is therefore unlikely that Dutch officials would document a barely noticeable change in tide level.[12]