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An undocumented earthquake found that moved a river – the state of the planet

An undocumented earthquake found that moved a river – the state of the planet
An undocumented earthquake found that moved a river – the state of the planet

 



Sand dam exposure and research team in the Ganges River floodplain of Bangladesh. Photo: Liz Chamberlain

Liz Chamberlain and Steve Goodbread, sedimentologists from Vanderbilt University, were surfing around the coast of Bangladesh in March 2018 when they saw sand dikes. Chamberlain and Goodbread came to Bangladesh to investigate how quickly rivers meander or shift in the coastal part of the Ganges-Brahmaputra delta.

Their research team, which also consists of Goodbread graduate student Rachel Payne, Abdullah Al Nahyan, and Mahfuzur Rahman of Dhaka University, has been sampling sediment from loops in the numerous rivers that cross the lower delta plain. They will use these samples to date the sediments left by the rivers.

Halfway through the journey, they came across a large pond that had just been dug to store fresh water. On the pond side, there were vertical sand dykes up to 30-40 cm wide, cutting through the horizontal layers of sediment. The team already knew that earthquakes often leave behind injections of sand through sedimentary layers. The vibration caused by the earthquake separates the sand grains and increases the pressure until they explode like a sand volcano, creating an “earthquake.” But these quakes were far from the tectonically active parts of Bangladesh and India that could have been the source of the quake, and the size of the levees so far from the epicenter suggests it was a large earthquake.

Sedimentologist Liz Chamberlain collects a clay sample from an abandoned Ganges River channel for dating using optically stimulated luminescence. In the background, a pump delivers groundwater to irrigate rice that is often grown in relatively low elevation areas of the Ganges River floodplain, in Bangladesh. Photography: Mahfooz Rahman

Chamberlain and Goodbread were drawn to the local area by a wide, disused river channel that was clearly visible in the digital elevation model as a low-rise curve. The abandoned canal is about 1.5 kilometers wide and is used for rice cultivation. They spent the morning under the hot sun surveying the abandoned canal, and it was late in the day when the team discovered earthquakes in the walls of the freshly excavated pool. With the help of Al Nahyan and Rahman as translators, Goodbread asked the pool owner not to fill it with water overnight so they could continue the investigation.

When the team realized how important this discovery was and the need to document it immediately before the pond was flooded, they contacted us (Lamont-Doherty Earth Observatory geophysicist Michael S. Stickler W. As experts in geophysics, structural geology, and modern tectonics — the study of contemporary movements and deformations of the Earth's crust — we have more experience In the tectonics of the region and the development conditions of sand dams, it is not easy to distinguish between other sand dams, for example, the distinction between sand dams can also be caused by varying degrees of overpressure or excess pore water pressure at a certain depth.

Abdullah Al Nahyan and Foreign Minister Arivor Rahman, co-authors of the study, drilled a well to examine shallow sediments in the Ganges River floodplain in Bangladesh. Manual drilling to depths of up to 5.8 m was used for this study. Photo: Liz Chamberlain

To find out whether these formations could actually be seismic, it was necessary to document the structure as much as possible. The first task was to immediately improvise the basic research design. The next day, the field team returned to the pond and documented all the aspects that could help understand both the fluvial (river-related) and seismic history of the site – sediment texture, dike dimensions and orientation, optically stimulated scintillation samples to date the sediment and more.

We tested the hypothesis that these were indeed earthquakes from all sides, checking the orientation of the sand veins and any external topographic features that could have played a role. Only when all other possible explanations proved insufficient were we confirmed that it was seismic. After completing the fieldwork, we considered possible sources of the earthquake, the stress field in the delta plain and empirical relationships from previous research on the width of the dyke, distance to the origin, and magnitude of the earthquake.

Steve Goodbread removes sediment from the muddy wall of a freshly excavated pond in the Ganges floodplain of Bangladesh, revealing intrusive sand dykes. Photo: Liz Chamberlain

We found that the potential sources of the earthquake were more than 180 km away. By studying the size of the sand embankments with the distance to the earthquake, we concluded that the likely magnitude of the earthquake was magnitude 7-8.

The ages of samples in the sand dikes and abandoned river channel, based on optically stimulated luminescence, showed that channel abandonment and dike formation occurred at the same time, about 2,500 years ago. The large size of the canal, coupled with the finding of a similar abandoned canal at the same time about 85 kilometers downstream, suggests that this was a major shift, known as the splitting of the Ganges. Rivers in deltas deposit sediments that increase in height over time. Eventually, they move to a new lower altitude path. For example, the Mississippi River capsized seven times during the Holocene. The coincidence of the timing of the Ganges overturn and the earthquake suggests that an earthquake occurred.

The discovery is the first confirmation that earthquakes can trigger massive river floods in the delta region, potentially creating a cascade hazard in vulnerable locations.

Laboratory technician Erna van den Hengel Vosskoelen prepares sediments for optically stimulated luminescence dating at the Dutch Center for Luminescence Dating at Wageningen University. Photo: Jay Ackermans

This article is adapted from the “Beyond Paper” publication on the Springer Nature website.

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