Why are shallow earthquakes more destructive? The Java disaster is a devastating example.


A man reacts as he surveys the damage from Monday’s earthquake in Cianjur, West Java, Indonesia. Tatan Syuflana/AP

Phil R Cummins, Australian National University; Mudrik Rahmawan Daryono, Badan Riset dan Inovasi National (BRIN)Y Stacey Servito Martin, Australian National University

On November 21, 2022, an earthquake near the Indonesian city of Cianjur in West Java caused at least 268 dead and 22,000 buildings damaged.

With a magnitude of 5.6, this quake was much smaller than many other earthquakes that have caused death and destruction in Indonesia for the past decades.

Why is this one so different? One of the main reasons the Cianjur earthquake was so destructive was its shallow depth of 10 km.

This event should serve as a wake-up call to improve construction practices in Indonesia, because we know from the past that much larger shallow events can happen in Java; It is not a question of if, but of when.

The role of earthquake depth

Two of the most important factors that determine the intensity of ground movement caused by an earthquake are its magnitude and distance.

Large earthquakes deeper than 50 km can and do cause widespread damage, but the intensity of tremors is reduced because seismic waves travel at least 50 km before reaching people.

Such earthquakes rarely cause mass fatalities: the magnitude 6.5 Tasikmalaya, Java earthquake in 2017 occurred 90 km deep and killed only four people and damaged 4,826 homes.

The recent Cianjur quake was much smaller: at magnitude 5.6, its energy was eight times less than the Tasikmalaya quake, but it did much more damage.

The Cianjur earthquake had a greater impact because it struck a few kilometers from the city of Cianjur, where the tremor was rated as “severe” (Modified Mercalli intensity 8).

A similar comparison could be made with giant subduction zone earthquakes that occur offshore. While these may be much larger in size than this week’s Java quake, they are generally 100 km or more from population centers, so they kill fewer people due to collapsing buildings.

rare danger

There’s another reason why inland shallow earthquakes can be so devastating, particularly in Java: They happen infrequently, so most people don’t realize the danger.

The population of Java increased by a factor of four through the 20th centuryand during this time there was only one shallow earthquake in 1924 that caused almost 800 deaths, and four others that caused between 10 and 100 deaths.

It wasn’t until 2006 that a really major event occurred: the 2006 Yogyakarta earthquake, magnitude 6.3which killed 5,749 people.

In other parts of Java, there is no lived experience of a significant earthquake, often going back several generations.

As a consequence, little attention is paid to the seismic resilience of residential construction, so when an earthquake occurs, many of the weak buildings will collapse.

A completely different colonial past

The history of earthquakes in Java during the colonial era paints a completely different picture. Our recent study shows that many damaging earthquakes have occurred in Java since the 17th century. At least nine earthquakes since 1865 have caused tremors so severe that they were almost certainly shallow events.

These include two earthquakes near Wonosobo in central Java in 1924 that caused catastrophic landslides that killed nearly 900 people.

A sepia photo showing a small building completely collapsed.
Earthquake damage at Cianjur, Jawa Barat, in March 1879. Digital collections of the Leiden University Libraries, CC BY

In our recent study, we also document extremely violent tremors caused by the October 25, 1875 earthquake near Kunningan in West Java. An eyewitness described being thrown from a chair and seeing a herd of cows knocked down.

Cirebon also experienced a damaging earthquake on November 16, 1847 which is believed to have caused a Displacement of the river of 5 m.suggesting a magnitude of 7 or greater.

Cianjur, the site of this week’s earthquake, experienced at least one devastating earthquake, on March 28, 1879, which caused several buildings in Cianjur to collapse with some loss of life.

a fact of life

Geologists well understand that earthquakes are a fact of life in Java. At work for the past two decades they have identified many flaws – cracks or joints in the earth’s crust – in Java that are probably active, but only a few have been studied in detail.

The Lembang fault on the outskirts of Bandung, Indonesia’s fourth-largest city (population 8.8 million, compared to Cianjur’s 170,000), is one of the few where geological evidence of prehistoric seismic activity has been established. This fault is believed to be capable of producing a magnitude 6.5 to 7.0 earthquake every 170 to 670 years.

Other active faults are known to threaten the cities of Jakarta, Surabaya and Semarang, in addition to Yogyakarta. And these are just the ones we know about.

Preparing for the next earthquake

Shallow earthquakes that are much larger than the Cianjur earthquake could occur next to cities that are much larger than Cianjur. What can Indonesia do to avoid mass deaths in such an event?

The typical response is to improve, and enforce, building codes, forcing any new construction to be more earthquake resistant.

Indonesia has a building code based on a modern seismic hazard map, but it only applies to buildings eight stories or higher. Given the high level of poverty in Indonesia, universal application of the building code is considered impractical.

An alternative may be the adoption of simple minimum standards for concrete strength, reinforcement quality, and other aspects of construction practice that may not meet building code, but that at least provide a higher level of protection than current practice.

Any change in construction practice requires a change in culture: people must expect more from builders and be willing to pay for it.

Phil R CumminsProfessor, Australian National University; Mudrik Rahmawan Daryonosenior Research Scientist, Badan Riset dan Inovasi National (BRIN)Y Stacey Servito MartinPhD Candidate, Earth Sciences, Australian National University

This article is republished from The conversation under a Creative Commons license. Read the Original article.


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