Professor Gordon Lister from the ANU Research School of Earth Sciences shares his perspective, as a tectonicist and structural geologist, on the earthquakes in Bali.

The recent Lombok Earthquakes once again bring home the message that our planet is not always a stable, happy place. In a moment, a loaded fault may let go, shaking the ground, wreaking destruction, claiming life and creating misfortune.

Many of us stare in the face of such disaster, and feel helpless. Learning what actually happens during an earthquake could help you feel better prepared.

There are some simple things you can do if you wake in the night and realise the ground is shaking. Do not panic. Instead, take a breath and begin counting. Move to a place of relative safety. For example, brace yourself beneath a strong doorframe, and get clear of anything that might fall on you, but keep counting.  One thing that we do know about earthquakes is how fast a fault rupture moves, which is typically between two or three kilometres a second. If you have been counting for less than ten seconds, and the ground did not roar, relax. You have survived, it was not a really big one, and you appear to been far enough away not to have been badly shaken. But now you must still take precautions, assess your immediate environment, and if necessary, move to a safer location.

Geology of the Lombok Earthquakes

This map (Figure 1) and a cross-section (Figure 2) show the subducting slab and the Flores back-thrust. The 2018 Lombok Earthquakes occurred on the Flores Backthrust, where Lombok is being pushed to the north over a newly-forming subduction zone.

Two of my colleagues from the ANU Research School of Earth Sciences (Achraf Koulali and Simon McClusky) spent years collecting GPS data to work out the background movement that was taking place. This data extrapolated into deep time strongly suggested the backthrusting began only a few million years in the past, and was caused by New Guinea and Australia pushing against the Sunda subduction zone (Figure 2). In turn this has pushed Lombok north, but not so far north as to cut off the flux of fluid and magma that feeds the Rinjani volcano. The aftershocks (black spots in Figure 1) decorate the ruptured back-thrust, so now we know that these movements have pushed the Rinjani volcano over the newly-forming slab. You might have heard reference to the “ring of fire” but that has nothing to do with earthquakes. Earthquakes are caused by the huge megathrust faults associated with subduction zones . Sure, subduction also causes volcanoes, and these give rise to the epithet. But if you want to understand the earthquake geology, you need to look at earthquake maps (for example, using the United States Geological Survey site) and identify the structures causing those earthquakes.

Quaking for less than one or two seconds  

A magnitude five quake can be bad enough, especially in buildings not designed for it. But such quakes do not usually spell widespread disaster unless in built-up areas with sadly lacking building codes.

Quaking longer than ten seconds

Take a deep breath, you’re still alive, but you need remain calm and make sensible decisions. The second Lombok quake broke the Flores back-thrust pretty well under the entire areal extent of northern Lombok, over a distance of about 50 km east-west, and about 25 km north-south, with the rupture moving rapidly west and south from where what we call catastrophic failure first initiated. It took just under 15 seconds to do this.

The epicenter is nothing more than the place where (in the ground beneath) the propagating rupture began causing movement fast enough to begin radiating elastic waves, so the earth begins to quake. Anyone talking about how “We’ve got to get to the epicenter!” is sadly misled by the habit the media has in locating the earthquake epicenter (where the earthquake starts) with a nice blue radiating circle. A quake with moment magnitude almost seven (for example, the second big quake beneath northern Lombok) may run 15 seconds and rupture 30 to 45 km of the backthrust. You counted, so you know that all of northern Lombok is likely to have been devastated. You can make people aware of what is happening, with continual quaking to be expected as the earth adjusts, faulting the already formed rupture over and over again. There are two options: one is to get off the island; the second is to move southward from the ruptured area. No point discussing how and why, but hanging around on the beach may not be such a good idea, particularly if the earthquakes so far are precursors to an even greater release of energy, still to come.

Earthquake science is not yet at the point that predictions can be made. But slowly science is moving in this direction. For now, the only thing that can safely be said is that there is always a risk that one earthquake will be followed by another, in particular in megathrust environments. The first quake (moment magnitude Mw 6.4) broke ground moving east from the epicenter (Figure 1). The next big quake was even larger (releasing four times as much energy, with moment magnitude Mw 6.9). It started in almost the same place, perhaps on a different splay of the back-thrust, and ruptured rapidly west, but also beneath the ground ruptured in the first quake to the east. You did not know that the first event was going to be followed by a second, but this eventuality is not uncommon, so plan for it.

When subsequent quakes get larger, in hindsight we realise that the early events were precursors. In these circumstances it would be wise to have recognised this possibility, and to have moved away from the afflicted zone. Sure, maybe nothing will happen, but better safe than sorry. We do not fully understand how the Earth releases its stored strain energy, and funding is tight, so we progress slowly. There’s no point in being alarmist but also no point in ignoring the fact that precursor sequences have in the past heralded the onset of Great Earthquakes (with moment magnitude Mw = 8 or 9 and above).

Quaking longer than one minute 

It’s grim, you are in a really major disaster zone. The Lombok quakes were trivial in comparison (15 seconds for the Mw 6.9 event). In other words, it could have been far worse. Bottom line - before you go somewhere (next time), try to assess the geological hazards. In any disaster, use your knowledge to spread wisdom, and help maintain calm.

Earthquake geology is a remarkable science. If you want to know more, come see us at ANU.