On March 28, the city of Mandalay, Myanmar, became the epicenter of a powerful earthquake that could be felt as far away as Bangkok, Thailand, and that is estimated to have killed over 3,000 people and injured thousands more. As rescue and relief efforts continue amid the collapsed buildings and wrecked roads, geologists are uncovering the details of what made this earthquake so devastating.
A “supershear” quake
Last week’s earthquake, which occurred along the Sagaing Fault, was not only super intense, but scientists now think that it also moved both incredibly fast and far. Writing on LinkedIn, Frederik Tilmann, a seismologist at the GFZ Helmholtz Centre for Geosciences, said that this suggests the seismic event was a “supershear earthquake”.
This is a type of earthquake where the fault ruptures faster than the seismic waves it produces can travel through the rock, which leads to the buildup and violent release of energy – and quickly too. Tilmann called it “the earthquake equivalent of a supersonic jet,” and it might go a way to explaining why the earthquake’s shakes were felt well over 1,000 kilometers (621 miles) away in Thailand’s capital city.
Supershear events are generally rare, but become more common with larger earthquakes. Looking at the data from the magnitude 7.7 Myanmar quake, Tilmann and graduate student Felipe Vera found that “the southward propagating rupture accelerates up to a speed of ~5 km/s which is much faster than the shear wave speed, indicating a so-called supershear [rupture].”
Tilman and Vera also found that the total rupture length of the fault was “well over 400 [kilometers (249 miles)]”, though more recent analysis from Max Van Wyk de Vries, an assistant professor in natural hazards at the University of Cambridge, suggests the fault might even have ruptured almost as far as 500 kilometers (311 miles).
A shallow epicenter
One feature that plays a role in the power of an earthquake is the depth of its epicenter. “The strength of shaking from an earthquake diminishes with increasing distance from the earthquake’s source,” the US Geological Survey (USGS) explains, “so the strength of shaking at the surface from an earthquake that occurs at 500 [kilometers] deep is considerably less than if the same earthquake had occurred at 20 [kilometers; 12 miles] depth.”
Data from the USGS shows that the epicenter of the Myanmar earthquake was particularly shallow, at just 10 kilometers (6.2 miles) below the surface. Together with the supershear nature of the quake, this helps to explain why the shaking was so intense, with buildings collapsing and roads buckling across the country and in Bangkok.
Unanswered questions
There’s still a lot left to find out about the March 28 quake, and those details are likely to emerge slowly. USGS geophysicist Susan Hough told Science News that while Myanmar does have a seismic network, only two of its five core stations have reported data. What’s happened to the other three is unclear.
“The stations could have been knocked offline by the quake. But they also could have been offline before the quake; we don’t know, because we haven’t been able to work there since 2019, following Myanmar’s military coup,” said Hough.
“But the fact that we got data from two stations tells me that the network hub is still alive, and there could be data we haven’t received yet. It’s the kind of data that would be critical both for Myanmar and the world community.”
Source Link: What Do We Know About The Geology Of Myanmar’s Devastating Magnitude 7.7 Earthquake?
