Just as Filipinos around the world were celebrating Alex Eala, the 19-year-old Filipina sensation who has captivated the tennis world on Friday, March 28, Myanmar was struck by a magnitude 7.7 earthquake so vicious that its effects reached Bangkok, Thailand some 1,000 kilometers away!
The earthquake hit near the city of Mandalay in east-central Myanmar at around 2:20 pm of Friday Manila time. Global seismological observatories reported varying magnitudes ranging from 7.7 to 7.9, with a shallow depth of focus of only 10 kilometers.
The culprit: Sagaing Fault
Earthquake parameters measured by global seismological observatories suggest the earthquake was generated by the Sagaing Fault, the geologic structure that defines the boundary between the Indian Plate and the Eurasian Plate.
The Indian Plate is a continental crust that has been travelling from the south pole since the Paleozoic Era, around 225 million years ago. Moving northwards, it has collided since about 65 million years ago, right about the time the dinosaurs became extinct, with another continental but much larger and stationary crust, the Eurasian Plate to the north. This collision has produced the famous Himalayan Mountain Range that hosts the tallest mountains in the world, Mt. Everest being at the roof.
When tectonic plates of similar nature (continental in the case of both India and Eurasia) collide, they produce a bulge at the point of contact (collision zone), elongated perpendicular to the direction of convergence. Imagine a moving car colliding head on with another car at rest – the crumpled and now melded front hoods would represent the mountain range, with some peaks higher than others.
The eastern side of the Indian Plate however, instead of colliding, is sliding against portions of the Eurasian Plate. This sliding face is manifested as a large fault that runs for more than 1,000 kilometers from near the Myanmar- Tibet boundary in the north to Yangoon the Burmese capital, in the south. This fault is the Sagaing Fault, named after the city west of Mandalay, which the fault directly traverses.
The Sagaing Fault is a strike-slip fault, the sliding face between two tectonic blocks that simply slide horizontally along each other’s sides. Furthemore, the Sagaing Fault is said to be a dextral (or right-lateral) fault — which means that the sense of movement allows one block to move to the right with respect to the other. Try standing on one side of a road, assume the white center line as a strike-slip fault, and allow the other side of the road to virtually move parallel to the center line to your right – you have just generated motion on a virtual dextral fault!
Major strike-slip faults (about 1,000 kilometers in length) are notorious generators of large magnitude earthquakes, especially when the focal depth is shallow. When they traverse on land, they can cause heavy infrastructure damage that can translate to heavy death tolls. Such was what happened when the East Anatolian Fault in Turkey ruptured in February 2023.
Other major strike-slip faults include the San Andreas Fault in California, the Great Sumatran Fault in Indonesia, the Alpine Fault in New Zealand, the Denali Fault in Alaska, and the Philippine Fault which generated a magnitude 7.8 earthquake on July 16, 1990.
Even Bangkok 1,000 kilometers away shook!
The epicenter of the magnitude 7.8 Luzon earthquake of July 1990 was located near Cabanatuan City in Nueva Ecija, but the extent of damage reached as far as Baguio City and Dagupan City located more than 100 kilometers away. The infrastructure damage in these two cities was largely attributed to the elevated intensity of ground shaking in those cities, further amplified by altitude in mountainous Baguio, while causing widespread liquefaction in coastal Dagupan.
A similar but different phenomenon was observed during the Myanmar earthquake, when a high-rise building under-construction in Bangkok, Thailand collapsed. Similar because Bangkok is located away from Mandalay, but different because the Bangkok-Mandalay distance of 1,000 kilometers is ten times farther than the 100-kilometer Cabanatuan-Baguio-Dagupan distance during the 1990 Luzon earthquake. Bangkok is built largely on very thick sequences of soils and sands deposited by the great Chao Phraya River and the Gulf of Thailand.
During earthquakes, this ground condition is prone to the amplification of seismic waves that promote more intense ground shaking, manifested in Bangkok in the swaying of high-rise buildings spilling swimming pool water from their penthouse decks. It also promotes liquefaction, a process that allows saturated sandy material to behave like liquid when shaken by an earthquake. Buildings, despite their internal integrity, would sink, tilt and topple when built over a liquefiable foundation.
Another observation is the extent of aftershock distribution which has already reached, as of this writing, more than 400 kilometers. Theoretically, a magnitude 7.7 earthquake may rupture only a fault segment not longer than 200 kilometers. This may suggest that other segments may have been triggered by events along the main segment, a phenomenon called stress triggering.
Lessons
The Bangkok high-rise building that collapsed highlights the importance of strictly adhering to provisions of building and structural codes relating to earthquake safety, not only in the completed as-built design, but more importantly DURING the construction stage.
The collapse of a building located 1,000 kilometers away from the epicenter is proof that an earthquake of magnitude 7.7 or greater with an epicenter that, for example, is located in Davao City, can inflict damage to a building being constructed some 1,000 kilometers away say in Manila, on Bangkok-like soil foundation.
Earthquakes generated by movement along a segment of a major fault (like the Philippine Fault), may trigger movement along another segment of the same fault, or other faults, producing a wider-than-empirically expected distribution of aftershocks, than can translated to larger-than-expected extent of damage and casualties. – Rappler.com
Mario A. Aurelio, PhD is a professor of geology at the National Institute of Geological Sciences – University of the Philippines, and head faculty of the Structural Geology and Tectonics Laboratory of UP NIGS. Among his fields of interest is the study of earthquakes. He is part of a team based in UP Diliman that maintains a seismological network consisting of seismometers operated by citizen scientists.