Intraslab earthquake shakes (half of) Java, Indonesia, again

A magnitude-6.6 earthquake rattled Java in mid-January, causing shaking, but no tsunami.
 

By Anne Sirait, Ph.D., Department of Geophysics, University of Indonesia
 

Citation: Sirait, A., 2022, Intraslab earthquake shakes (half of) Java, Indonesia again, Temblor, http://doi.org/10.32858/temblor.237
 

This article is also available in Indonesian.
 

On January 14, 2022, I was enjoying my afternoon coffee with a snack as my baby slept peacefully — a dream come true for a new mother like me. Suddenly, my chair began to shake. Baby toys rattled. My mind started racing. I immediately picked up the baby to get him out of the house.

Turns out, it was an earthquake.

A magnitude-6.6 earthquake originated offshore of Ujung Kulon, West Java, Indonesia, shaking much of the western part of the island of Java. The temblor struck at 4:05 p.m. local time. According to the Badan Meteorologi, Klimatologi, dan Geofisika (BMKG), the national seismic agency in Indonesia, the earthquake was located at 40 kilometers depth, which is considered a relatively shallow event. Fortunately, a tsunami warning was unnecessary because earthquakes of less than magnitude-7.0 do not typically generate tsunamis. Additionally, buoys that monitor sea level did not record any changes expected when an earthquake triggers a tsunami.

Additional analysis of seismic data by BMKG located this earthquake within the Indo-Australian tectonic plate that plunges beneath western Java. Called an intraslab earthquake, these types of events occur within the subducting slab of the oceanic lithosphere, and the rupture remains wholly contained. In other words, the earthquake — though somewhat shallow — did not break the surface.
 

 

Surprising shakes

Ground shaking was felt at nearby locations, including the Cikeusik and Panimbang districts located about 132 kilometers from the epicenter. People also felt shaking in the capital city of Jakarta, about 186 kilometers from the epicenter. These populations felt movement ranging from small vibrations to intense shakes. The shaking lasted for almost 30 seconds in some places. People were surprised and many immediately ran outside.

About 45 minutes after the mainshock, an aftershock of magnitude-5.6 followed. This aftershock was 10 kilometers deeper than the mainshock. During the next 24 hours, a total of 33 aftershocks occurred, ranging from magnitude-2.5 to -5.6. The number of aftershocks is considered small for a mainshock of this size. A small number of aftershocks tend to characterize intraslab earthquakes, supporting BMKG’s conclusion that the mainshock was, indeed, an intraslab earthquake.

Damage caused by the earthquake was reported from nearby locations like the Munjul and Cimanggu districts, up to about 100 kilometers from the epicenter. Houses, schools, hospitals, and public facilities all sustained varying levels of damage. A total of 1,378 houses were affected ranging from light to heavy damage. The shaking was felt strongly in Jakarta due to the local site effect, a thick-soft ground which amplified the shaking and due to long period vibration from strong earthquake located relatively far.
 

History of intraslab earthquakes in western Java

Historically, few intraslab earthquakes have occurred within the region. Yet, in these relatively rare events, ground shaking caused widespread damage at locations near their epicenters.

For earthquakes that struck prior to modern monitoring methods, seismologists can model historical earthquakes based on documented damage, shaking and the presence or absence of a tsunami (Griffin et al., 2018). With this type of information, they can extract estimates of magnitude, location and type of earthquake.

For example, on January 5, 1699, an instraslab earthquake struck near Bantam (Banten today), with shaking felt as far as Batavia (Jakarta today), and as far as Lampong (Lampung today) in the south of Sumatra. In three places, houses, barns, buildings collapsed. A total of 128 lives were lost. The shaking also triggered numerous landslides around Mt. Salak, Bogor which disrupted the flow of the Ciliwung River, affecting the river transportation and access to clean water.

On June 10, 1867, another likely intraslab event was felt from Bantam (Banten today) in western Java to Negara in western Bali. Central Java and the western part of East Java experienced the greatest damage. A total of more than 1,000 buildings, factories, and historical places were damaged, and more than 700 lives were lost.

More recently, on September 2, 2009, a magnitude-6.8 earthquake occurred at about 49 kilometers depth, rupturing about 200 kilometers north of the Sunda Trench, close to populated coastal cities in Java. This earthquake did not generate a tsunami but induced landslides in coastal areas. The shaking was felt toward the western part of Java, as far as Jakarta. This earthquake caused more than 80 causalities. Another 188,000 people were displaced from their homes.

On December 15, 2017, a magnitude-6.5 earthquake located at 90 kilometers depth struck about 11 kilometers from Tasikmalaya, West Java. The shaking was felt across West Java, Central Java, Banten, and Yogyakarta. Numerous buildings, including a hospital, were damaged or collapsed. The collapsed houses lead to panic and many casualties.

On August 2, 2019, a magnitude-6.9 earthquake occurred about 100 kilometers northeast of the Sunda Trench, with moderate to severe shaking felt up to 60 kilometers away from the epicenter. However, more than 160,000 people located outside the 60-kilometer radius also reported feeling different levels of shaking.
 

Impacts

The great impact of the latest intraslab earthquake on the Java population forced immediate responses from the government and seismological institutions, while simultaneously reminding Java’s population to be prepared for potentially upcoming earthquakes. Preparation from the government’s perspective includes continued research into seismic hazards to better forecast upcoming events, mitigation procedures that are easily understood and accomplished by the populace, and enforcing existing building codes that help structures accommodate ground shaking produced by an earthquake.

For those of us who reside on Java Island, we need to prepare for upcoming earthquakes. Before an earthquake happens, we need to understand the structure and surrounding conditions where we live, work, and play. We should reevaluate buildings and renovate, if necessary, to accommodate shaking from an earthquake. We should secure furniture so it will not easily move during an earthquake. Other important guidelines include keeping flammable material in a safe place and always turning off water, gas, and electricity when not in use. Because much of Java’s population resides in crowded regions, each person should find an escape route with easy access to an outdoor open space. This is especially important if a disabled person lives with you.

During an earthquake, the official advice from BMKG is to run outside if possible, as I did last week. If it is not possible to get out, take cover under a table to protect your head and body from debris. Find a safe place away from debris and shaking. If already outside, avoid structures such as buildings, electric poles, trees, or anything else that can fall. Pay attention to your surroundings; consider whether you’re near a crack, a site prone to liquefaction or a potential place at risk of a landslide.
 

 

Because Indonesia is an archipelago of islands nestled into the ocean, avoid coastal areas prone to potential tsunamis. After an earthquake, check yourself and others for any injuries and do first aid. Check surroundings for any fire, gas leaks, electric shorts, pipe bursts, or any fallen debris. If you’re in a safe place, stay outside for any aftershocks. Additionally, only listen to, and amplify, earthquake information from trusted sources to avoid propagating hoaxes.

The 2017 event was the first earthquake to shake my world. The latest one was my second earthquake, and my family and I are well-prepared for a third, which is almost certainly coming.
 

References

Griffin, J., Nguyen, N., Cummins, P., Cipta, A., (2018). Historical Earthquakes of the Eastern Sunda Arc: Source Mechanisms and Intensity‐Based Testing of Indonesia’s National Seismic Hazard Assessment. Bulletin of the Seismological Society of America 2018, 109 (1), 43–65. doi: https://doi.org/10.1785/0120180085
 

Further Reading

Alif, S. M., Fattah, E.I., Kholil, M. (2021). Source of the 2019 Mw6.9 Banten Intraslab earthquake modelled with GPS data inversion. Geodesy and Geodynamics, 12 (4), 308 – 314, https://doi.org/10.1016/j.geog.2021.06.001

Gunawan, E., Widiyantoro, S., Marliyani, G. I., Sunarti, E., Ida, R., Gusman, A. R. (2019). Fault source of the 2 September 2009 Mw 6.8 Tasikmalaya intraslab earthquake, Indonesia: Analysis from GPS data inversion, tsunami height simulation, and stress transfer. Physics of the Earth and Planetary Interiors, 291, 54-61, https://doi.org/10.1016/j.pepi.2019.04.004.

Sirait, A. M. M., Meltzer, A. S., Waldhauser, F., Stachnik, J. C., Daryono, D., Fatchurochman, I., Jatnika, J., Sembiring, A. S., (2020). Analysis of the 15 December 2017 Mw 6.5 and the 23 January 2018 Mw 5.9 Java Earthquakes, Bulletin of Seismological Society of America, 20, 1–14. doi: 10.1785/0120200046