When an earthquake struck near the Indonesian island of Sulawesi on September 28 last year, thousands of people lost their lives. Trapped to the sea, buried in the mud, were the victims of the most devastating earthquake of 2018.
Satellite data has now revealed that the grinding sections of the crust responsible for the 7.5-degree tremor collapse at dizzying speeds. The speed is surprising because of such a flaw, but could help explain the power of such prominent seismic events.
Two recently published studies on the 2018 earthquake provided strong evidence that the disruption behind the catastrophe shifted into what is described as superscalar speed.
All earthquakes start somewhere. Mounting pressure between titan crustal pieces can only accumulate until a weakness finally yields to the rupture of a planet moving along the fault, relieving each plate in new positions.
Energy pulses, called shear waves, scattered all over the crust, felt like the sound of an earthquake. But the adjustment of the plates propagates at a speed determined by the friction of the surrounding geology.
This puts a limit on most breaks of about 3 miles per second, a bit slower than the shear waves that normally move around 4 to 8 miles per second.
Supershear earthquakes support this trend by advancing at speeds that surpass the ripples to create what is effectively a sonic rumble stirring the ground.
Although not exactly common, they have been observed a handful of times in recent history, and are even thought to be behind the cataclysm that struck San Francisco in 1906.
If what struck Indonesia last year was an earthquake of super-rains, would be a long way to explain its intensity. There is only one problem – the fault line behind the earthquake is not what we would expect from supershear structures.
Sulawesi sits in the middle of a tectonic plate puzzle. The most active junction is the Palu-Koro fault, formed by plates sliding laterally against each other in opposite directions in a & quot; slip & quot; shape.
For sliding slip breaks to move at supershear speeds, the break should theoretically begin in a slightly rougher zone before increasing speed in a smooth line.
Zig-zags and structural complexities in the Palu-Koro fault would make it difficult for a break to increase speed, or so it was thought.
There were some initial signs that the earthquake of 2018 was rapid. Replica data and satellite images suggested that it took only 35 seconds to skip a distance of about 150 kilometers (about 93 miles) along the fault.
Thus, a team of researchers at the University of California, Los Angeles, used telescopic data and remote sensing of the earthquake to obtain detailed images of the bursting process, obtaining an accurate measurement of the speed of 4.1 kilometers per second. .
This fast pace is not up to speed with other supershear earthquakes. A magnitude 7.5 quake that struck Alaska in early 2013 is believed to reduce the gap by 5.5 to 6 kilometers per second.
But Palu-Koro's fault is not typical. Its unique mess of broken rocks left by a history of earthquakes could help explain why it did not break any record speed.
A separate study by researchers at the Université Savoie Mont Blanc in France adds additional details to the structure of the flaw.
Satellite images were used to map the major secondary and rupture structures associated with slippage. An image emerged from a previously unrecognized fault section that was highly complex.
From there, the glide extended southwards for a total distance of 180 kilometers, going through two great twists and descending to 30 kilometers by the capital Sulawesi, in Palu.
This relatively short and extremely smooth line seems to be to blame for the earthquake's superspear jump, causing it to take off at full speed, rather than running a little.
"Even in these complicated and difficult failures, it can be done in a supershear way and can be done immediately," UCLA seismologist Lingsen Meng told Paul Vondersofre. Science Journal.
If a catastrophe of this magnitude is to be repeated in Sulawesi's future, no one knows. While the tsunami seems to have been a case of bad luck, seismologists have a long way to go before they can accurately predict the size of the tremors.
Knowing that "sonic" tremors can occur in flaws such as Palu-Koro could at least help identify disaster potential more accurately in the future.
This research was published in Natural Geosciences here and here