A landscape shaped by immense forces
The coast of Myanmar's Rakhine State is a direct consequence of immense tectonic forces. It sits at the northern end of the Sunda megathrust, a 5,500-kilometer-long fault where the Indian Plate subducts, or slides beneath, the Burma microplate. This slow, continuous motion of about 50 millimeters per year builds up enormous strain in the Earth's crust. Every few centuries, this strain releases in a violent earthquake.
The most recent major event was the Arakan earthquake on April 2, 1762. Lasting for about four minutes, this earthquake had an estimated magnitude of 8.5 to 8.8. In those few moments, the coastline was permanently and dramatically reshaped. Eyewitness accounts and geological evidence show that offshore islands like Cheduba and Ramree were thrust upwards by as much as 7 meters. This sudden uplift left former shorelines and coastal features stranded far inland. In other areas near Chittagong, the land subsided, submerging entire villages. The event was so powerful it generated a tsunami in the Bay of Bengal and was felt as far away as Kolkata.
This process is not unique to the 1762 event. The landscape itself records previous great earthquakes. A series of ancient, raised beach ridges and marine terraces can be seen along the coast, running parallel to the current shoreline. These terraces, found at distinct elevations of approximately 1-2 meters, 5 meters, and 10 meters above the present sea level, mark the locations of former beaches that were suddenly lifted out of the water by prehistoric earthquakes.
Reading the coastal archive
Scientists use these stranded coastlines as a natural archive of seismic activity. By studying the marine terraces, they can reconstruct the history of massive earthquakes in the region stretching back thousands of years. Radiocarbon dating of coral and shell fossils found within these raised landforms dates these sudden uplifts.
Geological investigations have identified evidence for at least three great earthquakes in the last 3,400 years. Radiocarbon dates from different terrace levels point to major uplift events occurring between 1395–740 BC, AD 805–1220, and again between AD 1585–1810, which corresponds with the historical 1762 quake. This data suggests a recurrence interval for magnitude 8+ earthquakes in this region of approximately 500 to 700 years.
The study of coral microatolls—small, flat coral colonies that grow up to a specific height limited by the low tide level—provides more precise measurements. On Saint Martin's Island, dead coral microatolls found 2 to 2.5 meters above the current living corals have been dated to the 1762 event, confirming the scale of the uplift. Analysis of the coral's annual growth bands also reveals that the land was slowly subsiding at a rate of 2.2 millimeters per year before the earthquake, showing the immense strain accumulating as the Burma plate was being dragged down by the subducting Indian plate. Today, GPS measurements and living corals show the island is slowly uplifting, beginning the cycle anew.