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

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

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

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

This event should be a wake-up call for better construction practices in Indonesia, because we know from the past that much larger shallow events can occur in Java; It is not a matter of if but when.

The role of earthquake depth

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

Large earthquakes greater than 50 km deep can cause widespread damage, but the intensity of the shaking decreases because seismic waves travel at least 50 km before reaching people.

Such earthquakes rarely cause massive fatalities – a 6.5-magnitude earthquake struck Tasikmalaya, Java in 2017 at a depth of 90 kilometers and killed just four people and damaged 4,826 homes.

The recent Cianjur earthquake was much smaller – with a magnitude of 5.6, its energy was eight times smaller than the Tasikmalaya earthquake, but it caused much more damage.

The Cianjur earthquake had a greater impact because it ripped apart within a few kilometers of the city of Cianjur, where the shaking is rated “intense” (Modified Mercalli intensity 8).

A similar comparison can be made with giant subduction zone earthquakes that occur offshore. While their magnitude may be much larger than this week’s Java earthquake, they are generally located 100 km or more from population centers, so they kill fewer people by collapsing buildings.

Rare risk

There is another reason shallow, inland earthquakes can be so destructive, particularly in Java: they occur infrequently, so most people are oblivious to the danger.

Java’s population quadrupled during the 20th century, and during this time there was only one shallow earthquake in 1924 that caused nearly 800 deaths, and four others that caused between 10 and 100 deaths.

It wasn’t until 2006 that a really big event happened: the 2006 Yogyakarta earthquake, with a magnitude of 6.3, which killed 5,749 people.

Elsewhere in Java, there is no vivid experience of a major earthquake that often spans several generations.

As a result, not much attention is paid to the earthquake resistance of residential construction, so when an earthquake occurs, many weak buildings will collapse.

A very different colonial past

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

These include two earthquakes near Wonsobu in Central Java in 1924 that triggered catastrophic mudslides that killed nearly 900 people.

Earthquake damage at Cianjur, Jawa Barat, in March 1879. Leiden University Libraries, Digital Collections, CC BY

In our latest study, we also documented very violent shaking caused by the October 25, 1875 earthquake near Kaningan in West Java. An eyewitness described how he was thrown from a chair and saw a herd of cows being knocked off their feet.

Cirebon was also hit by a devastating earthquake on November 16, 1847 which is believed to have displaced the river by 5 metres, indicating a magnitude of 7 or more.

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

A fact of life

Geologists understand very well that earthquakes are a fact of life in Java. In working over the past two decades, they’ve identified several faults — cracks or joints in the Earth’s crust — in Java that are likely to be active, but few have been studied in detail.

Read more: Why do people in Indonesia still live in disaster-prone areas?

The Limbang Fault on the outskirts of Bandung, Indonesia’s fourth largest city (population 8.8 million, vs Cianjur 170,000), is one of the few for which geological evidence of prehistoric earthquake activity has been demonstrated. It is believed that this fault is capable of causing an earthquake of magnitude 6.5-7.0 every 170-670 years.

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

Prepare for the next earthquake

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

The typical answer is to improve—and enforce—building codes, and to force any new construction to be more earthquake-resistant.

Indonesia has a building code based on a recent earthquake risk map, but it is only applied to buildings eight stories or higher. Due to Indonesia’s high level of poverty, universal enforcement of the building code is considered impractical.

Read more: We may never be able to predict earthquakes — but we can already know enough to prepare

An alternative might be to adopt simple minimum standards for concrete strength, quality of reinforcement and other aspects of building practices that may not comply with the building code, but at least provide a higher level of protection than current practice.

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

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