On November 21 2022 an earthquake close to the Indonesian metropolis of Cianjur in West Java brought on not less than 268 deaths and damaged 22,000 buildings.
At magnitude 5.6, this earthquake was a lot smaller than many other earthquakes which have brought on dying and destruction in Indonesia over the previous few a long time.
Why is that this one so completely different? One of many principal causes the Cianjur earthquake was so harmful was its shallow depth of 10km.
This occasion ought to function a wake-up name to enhance constructing practices in Indonesia, as a result of we all know from the previous that a lot bigger shallow occasions can happen in Java; it’s not a query of if however when.
The position of earthquake depth
Two of crucial components that decide the depth of floor shaking brought on by an earthquake are its magnitude and distance.
Giant earthquakes of higher than 50km depth can and do trigger widespread injury, however the depth of shaking is diminished as a result of the seismic waves journey not less than 50km earlier than they attain folks.
Such earthquakes not often trigger large fatalities – the magnitude 6.5 Tasikmalaya, Java earthquake in 2017 occurred at 90km depth and killed only four people and damaged 4,826 homes.
The current Cianjur earthquake was a lot smaller – at magnitude 5.6, its power was eight instances smaller than the Tasikmalaya earthquake, nevertheless it did a lot higher injury.
The Cianjur earthquake had a higher affect as a result of it ruptured inside a couple of kilometres of town of Cianjur, the place the shaking was categorised as “extreme” (Modified Mercalli Intensity 8).
An identical comparability might be made with big subduction zone earthquakes that happen offshore. Whereas these might be far higher in measurement than this week’s Java earthquake, they’re usually 100km or extra distant from inhabitants centres, in order that they kill fewer folks by constructing collapse.
Rare hazard
There may be another excuse inland shallow earthquakes might be so devastating, significantly in Java: they happen occasionally, so most individuals are oblivious to the hazard.
The inhabitants of Java elevated by a factor of four by the 20th century, and through this time there was just one shallow earthquake in 1924 that brought on almost 800 deaths, and one other 4 that brought on between 10 and 100 deaths.
It wasn’t till 2006 {that a} actually main occasion occurred: the 2006 Yogyakarta earthquake, magnitude 6.3, which killed killed 5,749 folks.
Elsewhere in Java there isn’t a lived expertise of a significant earthquake, usually stretching again a number of generations.
As a consequence, little consideration is paid to the earthquake resilience of residential development, so when an earthquake does happen lots of the weak buildings will collapse.
A starkly completely different colonial previous
Java’s earthquake historical past through the colonial period paints a starkly completely different image. Our recent study reveals many damaging earthquakes have occurred in Java because the seventeenth century. At the very least 9 earthquakes since 1865 have brought on shaking so extreme they have been virtually definitely shallow occasions.
These embody two earthquakes close to Wonosobo in central Java in 1924 that brought on catastrophic mudslides that killed almost 900 folks.
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In our recent study we additionally documented extraordinarily violent shaking brought on by the October 25 1875 earthquake close to Kunningan in West Java. An eyewitness described being thrown off a chair and noticed a herd of cows being knocked off their toes.
Cirebon additionally skilled a dangerous earthquake on 16 November 1847 which is assumed to have brought on a river offset of 5m, suggesting a magnitude of seven or higher.
Cianjur, the location of this week’s earthquake, has skilled not less than one damaging earthquake, on March 28 1879, which brought on the collapse of a number of buildings in Cianjur with some lack of life.
A reality of life
Geologists perceive nicely that earthquakes are a reality of life in Java. In work over the previous twenty years they’ve identified many faults – cracks or joints in Earth’s crust – in Java which can be prone to be energetic, however solely a handful of those have been studied intimately.
Learn extra:
Why do people in Indonesia still live in disaster-prone areas?
The Lembang Fault on the outskirts of Bandung, Indonesia’s fourth-largest metropolis (inhabitants 8.8 million, versus Cianjur’s 170,000), is among the few for which geologic evidence of prehistoric earthquake exercise has been established. This fault is considered able to producing a magnitude 6.5–7.0 earthquake each 170–670 years.
Different energetic faults are identified to threaten the cities of Jakarta, Surabaya and Semarang, along with Yogyakarta. And these are simply those we find out about.
Making ready for the subsequent quake
Shallow earthquakes might happen which can be a lot bigger than the Cianjur earthquake, subsequent to cities which can be a lot bigger than Cianjur. What can Indonesia do to keep away from large fatalities in such an occasion?
The everyday reply is to enhance – and implement – constructing codes, forcing any new development to be extra earthquake-resilient.
Indonesia does have a constructing code based mostly on a contemporary seismic hazard map, however it’s only utilized to buildings of eight flooring or larger. Given the excessive poverty stage in Indonesia, common enforcement of the constructing code is considered impractical.
Learn extra:
We may never be able to predict earthquakes – but we can already know enough to be prepared
An alternate stands out as the adoption of straightforward, minimal requirements for concrete energy, high quality of reinforcement and different facets of constructing follow that will not conform to the constructing code, however not less than afford the next stage of safety than present follow.
Any change in constructing follow requires a change in tradition: folks need to anticipate extra of builders, and be prepared to pay for it.
