SINGAPORE: Unprecedented and unforeseen, Lombok was struck by a cascading series of powerful earthquakes in a span of three weeks.
The damage and devastation that resulted from these strong quakes, as well as the aftershocks that numbered well into the thousands, left the people of Lombok wondering what was going on underneath their city and when the powerful shakings might stop.
All of the four large events measured greater than 6.0 in magnitude and, according to the Indonesian earthquake monitoring institution, Agency for Meteorology, Climatology, and Geophysics, more than 1,500 of the aftershocks had magnitudes stronger than 2.0.
Fortunately, in spite of the immense power unreleased by these earthquakes, no tsunamis got triggered because the earthquakes had produced only relatively small vertical deformations to the sea floor on the Flores thrust fault.
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Only minor tsunami waves of less than 20cm in height were observed on the coast of Lombok Island. (A major tsunami typically requires the sea floor to be deformed vertically by more than 1m.)
Because Lombok is located on the seismically-active Ring of Fire, it is not surprising that the city has a long historical record of getting struck by earthquakes and tsunamis.
Three such disaster events in the years 1976, 1979, and 1992 were particularly devastating, with the 1992 tsunami-earthquake event resulting in the deaths of more than 2,500 people – the largest number of fatalities from an earthquake-related disaster to ever be recorded on the Nusa Tenggara islands.
What is surprising this time is the cascading pattern from the 2018 Lombok earthquake sequence, which scientists say is unique and rarely seen. The last time this happened in Asia was in April 2016 when the Kumamoto earthquake sequence struck Japan.
Usually, a main shock is followed by aftershocks of smaller magnitudes and the number of aftershocks decays exponentially over time.
This most recent Lombok earthquake sequence, which had shocks of magnitude 6.4, 6.9, 6.3 and 6.9, makes it hard to distinguish the main shock from the aftershocks and foreshocks.
Scientists at the Earth Observatory of Singapore (EOS) believe that this sequence was unlike most others.
Rather than having several fault segments of different geometry all rupture in a single event, the fault segments here ruptured at different times – that is, each earthquake likely triggered the next following quake.
The seismicity (aftershocks) distribution indicates the earthquake energy was released towards the northern part of Lombok Island.
This brings us to two critical questions: Has the sequence of earthquakes in Lombok come to an end, and what is the potential of future earthquakes happening in the Java subduction zone?
HARDER TO PREDICT NEXT EARTHQUAKE
Earthquake scientists can calculate the amount of change in stress to the surrounding faults resulting from this earthquake sequence. However, the pre-stress conditions of these faults are not known and, therefore, a precise prediction of the next earthquake is extremely difficult to make.
To be able to make such predictions, the spatial and temporal migration of seismicity needs to be carefully monitored with precision.
As a step towards achieving this goal, the Bandung Institute of Technology (ITB), with support from EOS, installed 13 seismometers around the Lombok Island just one week after the occurrence of the first, 6.4-magnitude main shock.
Thousands of earthquakes have since been recorded on this temporary seismic monitoring network, and this dataset will undoubtedly be extremely valuable to the study of the Lombok earthquake sequence.
And with the knowledge, we can help the people of Lombok recover from the impact of this tragic earthquake disaster and, more importantly, be better prepared for any future earthquakes to hit the city.
With Indonesia being in a zone of such extreme seismic activity, preparation is key.
Dr Muzli is a research fellow at NTU’s Earth Observatory of Singapore.