The March 28 earthquake that occurred in Myanmar (Burma) is a very large -scale disaster. It is a very large earthquake-magnitude 7.7 is rarely affected by a continental earthquake-of the Turkish earthquake order in February 2023, New Zealand in November 2016, Sichuan in May 2008 or Alaska in November 2002. The main shock was followed twelve minutes later by a first replica.
The toll is most likely very underestimated for all kinds of reasons: difficulties of access, countries at war … and could, according to my experience and the American Institute of Geology, largely reaching several tens of thousands of victims.
The reasons for such a assessment are multiple: the earthquake itself is very violent because of a high magnitude, on a very long fault and with a perhaps very fast break. In addition, the fault runs in a sedimentary valley, that of the Irrawady river, where the floors are not very consolidated, which gives rise to phenomena of “liquefaction”, fatal to constructions, during which the soil is completely hidden under the foundations of buildings. The constructions themselves are of fairly low quality (little armed concretes, with little cement, badly chained, etc.). Finally, the emergency services are not very organized and slow, while many injured need rapid care.
Myanmar is based on a loaded tectonic system
The earthquake occurred on the sagaing flaw that crosses the myanmar from north to south over more than 1000 kilometers long. This flaw results from plates tectonics in the region: the Indian plaque “goes up” to the north at almost 4 centimeters per year. In front of her, the Himalayas. On both sides, to the west and east, two fault systems accommodate the shift between the Indian plate and the Eurasian plate. To the east, it is the sagaing flaw, named after a big city in the country.
GPS measures made in Myanmar at the end of the 1990s by our team produced a lot of results: we first observed that the fault was well blocked. This implies that the continuous displacement of the Indian and Eurasian tectonic plates causes “elastic deformation” in the plates, and that this deformation should be released all or later in the form of earthquake, when the accumulation exceeds the threshold of resistance of the friction on the flaw.
But we had also made a slightly disconcerting discovery: the sagaing flaw only accommodated little less than 2 centimeters per year of deformation (exactly 1.8), the rest of the 4 centimeters per year imposed by the movement of Indian and Eurasian plates to be accommodated elsewhere … But where? Mystery.
The following studies suggested that this missing deformation occurs further west, on the so-called “Rakhine-Bangladesh” subduction.
Many earthquakes in Burmese history
There have been many earthquakes along the story in Myanmar. Archaeological studies carried out in the early 2000s on the Imperial City of Pegu, in southern Myanmar, revealed that the walls had been frequently experienced by earthquakes (seven since the end of the 16the century), but also offbeat, because the city was built exactly on the fault. The measurement of the total gap between two pieces of walls (6 meters in 450 years) gives an average speed over this period of 1.4 centimeters per year.
Further north, the imperial city of the city of Mandalay is also marked by earthquakes: massive statues have been shears by the seismic waves of past earthquakes.
Better understand the seismic cycle, or how the constraints accumulate before being released during an earthquake
Thanks to these studies, we today have a better vision of the tectonic situation in Myanmar.
The flaw is cut in more or less long segment, 50 to 250 kilometers in length. Each of these segments breaks more or less irregularly, every 50-200 years, producing earthquakes of magnitude ranging from 6 to almost 8.
The longest segment is that known as Meiktila. It is about 250 kilometers between Mandalay and Naycyidaw. He broke for the last time in 1839, with an estimated magnitude earthquake between 7.6 and 8.1. The calculation is therefore ultimately quite simple: here, the deformation accumulates around the fault at the rate of 1.8 centimeter per year and the last earthquake occurred 184 years ago: the next earthquake will therefore have to release 3.3 meters, before the accumulation resumed.
However, a displacement of 3.3 meters on a flaw 250 kilometers long and approximately 15 kilometers deep corresponds well to an earthquake of magnitude 7.7 – like the one that has just struck.
Finally, the very first analyzes by satellite imaging seem to indicate that the rupture would have spread widely in the south of the new capital Naycyidaw, almost 500 kilometers long in total. It would therefore have broken, simultaneously or successively several segments of the fault.
Seismic forecast: we can anticipate the maximum magnitude of a next earthquake, but not its date
Based on previous considerations (blocked flaw, deformation accumulation speed and time elapsed from the last earthquake), it is quite easy to establish a forecast: an earthquake is inevitable since the fault is blocked while the plates, they move well. The magnitude that this next earthquake can reach is estimable and corresponds to the size of the blocked area multiplied by the accumulated deformation (admitting that the previous earthquake has “cleaned” the fault of all the accumulated constraints).
The difficulty remains precisely the date on which this earthquake will occur: earlier it will be smaller, later it will be larger. It is therefore the resistance of friction on the fault that will control the day of the trigger of the earthquake. But it can vary depending on time due to external parameters. For example, one can imagine that a flaw that has not broken for a long time is well “stuck” and has greater resistance, unlike a fault which has recently broken and which is weakened.
Thus, rather than similar earthquakes occurring at regular time intervals, we can also have earthquakes of different sizes occurring at more variable time intervals. However, the resistance of the fault cannot exceed a certain limit and after a certain time, the earthquake becomes inevitable. It is therefore logical to be able to evoke the high probability of an imminent earthquake on a given fault segment, and a magnitude corresponding to the accumulated deformation available. The magnitude of 7.7 in the case of the recent earthquake on the sagaing flaw corresponds exactly to the seismic cycle calculations.
On the other hand, the determination of the date of the trigger of the earthquake to the nearest day remains impossible. Indeed, if the deformation increases a few centimeters per year, it increases only a few hundredths of millimeters each day, a very small quantity in terms of constraints.
Is the seismic crisis over?
There are always replicas after a great earthquake, but they are smaller.
Today there are in Myanmar enough seismological stations, and the smallest replicas (up to magnitude 3) are therefore not recorded. The replicas of magnitude between 3 and 4.5 are generally perceived by the Thai network, but only the magnitude replicas greater than 4.5 are recorded and located by the global network.
However, it seems that there are quite few aftershocks in the central part of the rupture, which could be an indication of a “super-shear” break because they would tend to leave behind a very well broken and very “clean” flaw.
