As rescue and recovery efforts continue to ramp-up in earthquake-ravaged Myanmar, new details about how the geologic setting amplified the disaster are beginning to emerge.
The March 28 magnitude 7.7 earthquake that rocked through Southeast Asia collapsed buildings, dams and bridges, and killed at least 2,700 people. The rupture occurred along several hundred kilometers of a roughly 1,400-kilometer-long fault known as the Sagaing Fault. The epicenter of the event was just 10 kilometers beneath Earth’s surface and occurred near the Myanmar city of Mandalay.
The shallowness of the rupture meant that all of the waves of seismic energy it generated arrived at Earth’s surface at nearly the same time, producing powerful, catastrophic shaking, says Susan Hough, a geophysicist with the U.S. Geological Survey’s Earthquake Hazards Program in Pasadena, Calif. Science News talked with Hough about the quake and how a phenomenon known as liquefaction added to its damaging effects.
SN: What is liquefaction?
Hough: We sometimes explain the amplification of [ground] shaking as a bowl of Jell-O. But if the sediments are sandy and wet enough, the material is shaking so hard it stops acting like a solid. It’s more like quicksand.
SN: How does ground shaking turn a pile of sediment into quicksand?
Hough: When the sediment is loosely packed and lubricated with a lot of water, shaking can disrupt that packing, pushing the grains of sand closer together. As a result, the water pressure in the pore spaces between the grains goes up, and the soil’s effective stress, or shear strength, goes down to almost zero. The pore water pressure is now bearing the load.
And that can cause the ground to slump, along with any structures on top of it. The footings of buildings are undermined because the ground just isn’t solid anymore.
SN: So it’s the combination of thick wet sediments with seismic activity that’s dangerous?
Hough: Yes. River deltas, which have a lot of thick sediments, and seismic activity combine badly. And many people tend to live in delta regions, because they’re close to waterways, so that increases the potential danger. There are cities around the world that are in places that can be quite hazardous.
SN: Including Mandalay?
Hough: Right. The Irrawaddy River is the main river system that runs north to south [through Myanmar]. It’s a huge drainage system, and there’s a huge river delta there. They get monsoonal rains, they have issues with flooding, tropical storms. There’s a lot of water coming down that river.
Mandalay [in central Myanmar, closest to the origin of the quake] is in a central basin that’s surrounded by mountains. There are a lot of people living in the basin and in proximity to the river. There are very fertile plains and it’s a transportation artery. It’s a setting that is ripe for liquefaction.
The city of Yangon [about 200 kilometers south of the fault break] wasn’t badly impacted by this earthquake. But it’s also sitting on the river delta, and that’s a concern [for future quakes]. If you put a magnitude 7-plus earthquake closer to Yangon, that’s a nightmare scenario. It’s a much bigger population center, and it’s sitting on a massive pile of sediments.
SN: Are there other recent examples where liquefaction greatly increased the damages? I’m thinking of the 1985 Mexico City quake.
Hough: There are many. Some other examples are the 1886 earthquake in Charleston; the whole Atlantic coast has thick piles of sediments. The 1811 to 1812 earthquakes in New Madrid, [Missouri], in the central United States, which is sitting on Mississippi River valley sediments. And San Francisco in 1906.
SN: What more have we learned about the origins of the Myanmar quake in the last few days?
Hough: The ground surveys are just getting started. The length of the rupture, of the fault break, has come into better view, thanks to remote sensing imagery of the region before and after. It was as much as 400 kilometers long, that’s better constrained at this point. There’s not a lot of local seismic data coming out of the country right now.
Falling into a slump
Strong ground shaking during an earthquake can cause large deposits of saturated, loose soil to behave like a liquid rather than a solid, called liquefaction. At left, before the shaking, the grains are packed together in the soil, forming a firm scaffold and bearing the weight of the structures on the surface. At right, shaking increases the water pressure in the spaces between grains, causing them to slip and slide past one another, liquidlike, and destabilizing the overlying buildings.
SN: Does Myanmar have a seismic network?
Hough: It does. Back in 2012, when Myanmar was taking steps toward democratization, I was involved in a project in which the U.S. Geological Survey partnered with Myanmar’s Department of Meteorology and Hydrology, which ran a seismic network.
It was pretty rudimentary as of 2012. Our top priority was to upgrade the seismic network, and that culminated with an upgraded network in 2016, with modern telecommunications and five core stations. One of the reasons seismology was limited is that telecommunications was very strongly controlled [by the government]; the stations could record the data but had to transmit it to a network hub. If you don’t have internet, you end up using the cell phone network, which was just then rolling out and becoming accessible.
We built the hub and the five core stations, and other groups began adding stations. Once you have the infrastructure it’s easier to install. It was up to a dozen or so stations. But so far as I can tell, none of those have reported data from this most recent quake.
Two of the network stations [we installed] in Yangon have reported in, and those are the only two in the country [from which we have data on this quake so far]. There is a station in Mandalay and one in Naypyitaw, but we’ve gotten no data from them yet.
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. 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.
SN: Will anyone from the USGS go to Myanmar?
Hough: I don’t expect the USGS would go over. There’s quite a bit we can do, we have established partnerships and contacts, but it’s probably going to be virtual.
We’ve been in touch remotely and calculated the aftershock forecasts. I was able to get in touch with Myanmar’s Department of Meteorology and Hydrology and make sure they had the aftershock forecast at least an hour before it was live [on the USGS website] so they could communicate it. I’ve also been in touch with a Myanmar team that’s going to do a survey to assess humanitarian needs.
SN: What is the estimated aftershock risk from the Myanmar earthquake?
Hough: It’s still quite high, although the more time goes by, the more the odds drop. Aftershock sequences are front-loaded. Charles Richter [the U.S. seismologist who developed his namesake earthquake magnitude scale in 1935] said that when you get a lot of earthquakes, you get a lot of earthquakes. The odds are higher now than they were last Thursday, [the day before the earthquake].
Currently there’s about an 80 percent chance of a greater than magnitude 5 aftershock over the next week, and a 17 percent chance of a quake greater than magnitude 6. There’s a 2 percent chance of one greater than 7 — which is low, but if you think about it, it’s 1 in 50. And there’s particular concern for aftershocks at the southern end of the fault, because that’s closer to Yangon.