The deadly Sulawesi earthquake occurred on September 28 last year, killing more than 2,500 people on the Indonesian island. The magnitude 7.5 quake was the most destructive of 2018 and dragged people out to sea, buried them in mud and damaged more than 70,000 houses. The main shock was followed by a series of aftershocks, with 14 of magnitude five or higher in the first 24 hours.
Since then, a total of 150 aftershocks have struck the region.
Four and a half months later, satellite data has revealed the sections of crust responsible for the devastating tremor unravelled at unusual high speeds.
The earthquake was felt over a wide area and the high tempo in which the quake had is extremely rare for a fault of this nature.
Two recently published studies of the Sulawesi earthquake have now provided strong evidence the rupture behind the disaster moved at what’s described as a “supershear” velocity.
A supershear earthquake is a quake in which the propagation of the rupture along the fault surface occurs at speeds in excess of the seismic shear wave (S-wave) velocity.
Supershear earthquakes will shoot forward at speeds overtaking the ripples to create what is effectively a ground-shaking sonic boom.
A sonic boom is the sound associated with the shock waves created whenever an object travelling through the air travels faster than the speed of sound.
Pulses of energy called shear waves reportedly spread out through the crust in all directions, but the adjustment of the plates themselves propagates at a speed set by the friction of surrounding geology.
Normally, this would put a limit on most ruptures of approximately 1.8 miles per second (3km per second).
This is slightly slower than shear waves which typically move at around 2.5 to 3.1 miles per second (4km to 8km per second).
Supershear quakes are extremely rare, but have been observed a handful of times in recent history.
They are even thought to be behind the cataclysm that struck San Francisco in 1906.
If the deadly tremor that hit Indonesia last year was a supershear earthquake, it would go a long way to explain its intensity.
But there is one problem with the theory – the fault line behind the quake is not what’s expected of supershear structures.
According to Science Alert, Sulawesi sits in the “midst of a jigsaw puzzle of tectonic plates.”
Science Alert explained: “The most active junction is the Palu-Koro fault, comprised of plates that slide laterally against one another in opposing directions in a ‘strike-slip’ fashion.
“For strike-slip ruptures to move at supershear velocities, the rupture should theoretically begin in a slightly rougher zone before building up speed down a smooth straight.”
The discovery has led to a team of researchers from the University of California, Los Angeles, using teleseismic data and remote sensing of the earthquake to come up with detailed imagery of the rupture process.
They received a precise measurement of the velocity as 2.5 miles per second (4.1 kilometres per second.)
A separate study by researchers from the Université Savoie Mont Blanc in France adds additional detail to the fault’s structure.
The researchers used satellite imagery to map the main rupture and secondary structures associated with the slip.
The study led to a picture of a previously undescribed section of fault that was highly complex.
This relatively short, smooth straight appears to be the culprit for the tremor’s supershear jump, causing it to blast off at top speed rather than taking a run-up.
UCLA seismologist Lingsen Meng told Paul Voosen at Science Magazine: “Even in these complicated and rough faults, it can go supershear and it can go supershear right away.”