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Trembling Tales: How Mangroves Keep Earthquake Records

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It was a lazy and eventless morning after Christmas on the sunny beaches of North Andaman in 2004, and there was little expectation of the massive earthquake of magnitude 9 on the Richter scale (M>9.2) that occurred near Aceh, Sumatra. (Mw stands for torque meter). The earthquake displaced the sea floor vertically by 10-20 meters.

Geologists describe an earthquake of this magnitude as a massive earthquake. It pushed a large amount of sea water and caused a tsunami. No tsunami of this magnitude occurred in the Indian Ocean recently. Which is why the tsunami in 2004 was such a complete surprise to coastal communities and Earth scientists around the world.

The tsunami struck 11 countries from East Africa to South Asia and killed more than 200,000 people. The immediate question that came to my mind was – did such a huge earthquake first occur in the Indian Ocean?

A team of researchers from the Department of Geology at Presidency University in Calcutta, including myself, studied a rare occurrence of peat on the island of North Andaman.

The earthquake, which was hit by the 2004 earthquake, has awakened the residents of the small village of Kichoringar in the northwestern part of the northern island of Andaman. The villagers discovered that part of the beach – known locally as Talabghan Bank – had risen above sea level. The massive earthquake lifted the northwestern part of the North Andaman Island by about 1.3 meters above sea level. This, in turn, revealed a three-centimeter-thick layer of fine dark gray peat embedded within the beach sand at Talabghan.

Radiocarbon dating revealed that peat was buried in the sand 200 years ago.

Peat is the precursor to coal. It consists of partially decomposed remains of plant material and bears the imprint of the geological events responsible for its formation. Chemical and geological analyzes of peat fragments are often used to decipher the type of plant and the process that led to peat formation.

The geochemistry of the stable isotopes of peat indicated that the peat bed was formed from the locally abundant coastal mangrove vegetation. However, the occurrence of a layer of peat embedded in beach sand is obscure because beach sand is unlikely to support vegetation large enough to form a layer of peat several centimeters thick.

We, researchers from the Presidency, in collaboration with Japan’s Shinshu University, determined that peat was formed due to the catastrophic mass deaths of mangrove plants that thrived along the vast extensions of the North Andaman Island about 200 years ago.

An earthquake similar to the 2004 earthquake lifted large swaths of land on the northern island of Andaman that supported dense forests of mangroves and associated tidal vegetation. The plants were raised beyond the tides and cut off from the tidal water supply. They eventually perished due to drought and a gradual decrease in soil salinity.

The tides moved dead tree trunks and branches into the open sea. The sea waves pushed the plant debris into the high tide line, where the hardwood material accumulated over the years.

In the end, all of this was buried under the sand when the uplifted ground slid after a periodic geological process after the earthquake. The plant debris remained preserved as a separate peat layer within the sand.

Research published in Catena, an interdisciplinary journal on landscape evolution, explains all this. Provides a scientific explanation for the occurrence of peat bodies buried at shallow depths within the unconsolidated beach sand of Talabghan.

Similar ground uplift occurred on the northern island of Andaman after the massive 2004 earthquake.

Geological field observations made between 2005 and 2015, and satellite images taken between 2004 and 2019, provided a clear picture of the massive mass mortality processes of mangrove plants in the higher parts and the ongoing process of peat formation from dead vegetation. The periodic process of the descending motion of the land mass is revealed about three years after the earthquake. The remnants of the partially decomposed mangrove forests would eventually be buried in the sea and preserved as a layer of peat until the next massive earthquake brought them to tell future scientists the account of the 2004 disaster.

The presence of mangrove-derived peat on the North Andaman Island is of importance beyond its role as a geological record of past catastrophic events.

More than 50 percent of the mangrove vegetation in the Andaman Islands was destroyed following the massive 2004 earthquake and Indian Ocean tsunami. However, the mangroves acted as an effective barrier against the tsunami and protected the coastal villages from the massive tsunami waves. The good news is that mangrove forests show evidence of rapid regeneration.

The writer is Assistant Professor, Department of Geology, Presidency University, Calcutta

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