A magnitude 6.6 earthquake rocked Java in mid-January, causing tremors but no tsunami.

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

Quote: 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 having my afternoon coffee with a snack while my baby slept peacefully – a dream come true for a new mother like me. Suddenly my chair started shaking. Baby’s 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 has occurred off Ujung Kulon, West Java, Indonesia, shaking much of the western part of the island of Java. The quake hit at 4:05 p.m. local time. According to Badan Meteorologi, Klimatologi, dan Geofisika (BMKG), Indonesia’s national seismic agency, the earthquake was located 40 kilometers deep, which is considered a relatively shallow event. Fortunately, a tsunami warning was not necessary because earthquakes of magnitude less than 7.0 generally do not generate tsunamis. Additionally, buoys that monitor sea level have not recorded 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 dips beneath West Java. Called intraslab earthquakes, these types of events occur within the subduction slab of oceanic lithosphere, and the rupture remains fully contained. In other words, the earthquake – although shallow – did not break the surface.

Surprising shakes

Earthquakes were felt nearby, especially in the districts of Cikeusik and Panimbang located about 132 kilometers from the epicenter. People also felt tremors in the capital, Jakarta, about 186 kilometers from the epicenter. These populations felt movements ranging from small vibrations to intense jolts. The shaking lasted nearly 30 seconds in some places. People were surprised and many immediately ran outside.

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

Earthquake damage was reported in nearby places like Munjul and Cimanggu districts, up to about 100 kilometers from the epicentre. Homes, schools, hospitals and public facilities all sustained varying degrees of damage. A total of 1,378 homes were affected, ranging from light to heavy damage. The shaking was felt strongly in Jakarta due to local site effect, thick soft ground which amplified the shaking and due to long period vibrations from a strong earthquake located relatively far away.

History of intraslab earthquakes in West Java

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

For earthquakes that struck before modern monitoring methods, seismologists can model historic 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 the magnitude, location, and type of earthquake.

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

On June 10, 1867, another probable intraslab event was felt from Bantam (Banten today) in West Java to Negara in West Bali. Central Java and western part of East Java suffered the greatest damage. In total, more than 1,000 buildings, factories and historic sites were damaged and more than 700 lives were lost.

More recently, on September 2, 2009, a magnitude 6.8 earthquake occurred about 49 kilometers deep, breaking about 200 kilometers north of the Sunda Trench, near populated coastal towns of Java. This earthquake did not generate a tsunami but caused landslides in coastal areas. The tremor was felt towards the western part of Java, as far as Jakarta. This earthquake caused more than 80 victims. Another 188,000 people have been displaced from their homes.

On December 15, 2017, a magnitude 6.5 earthquake 90 kilometers deep struck about 11 kilometers from Tasikmalaya in West Java. The quake was felt in West Java, Central Java, Banten and Yogyakarta. Many buildings, including a hospital, were damaged or collapsed. The collapsed houses cause panic and many victims.

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 from the epicenter . However, more than 160,000 people outside the 60 kilometer radius also reported feeling varying levels of tremor.

Implications

The great impact of the latest intraslab earthquake on the people of Java forced immediate responses from government and seismological institutions, while simultaneously reminding the people of Java to prepare for potentially upcoming earthquakes. Preparedness from a government perspective includes ongoing seismic hazard research to better predict future events, mitigation procedures that are easy for the public to understand and implement, and enforcement of existing building codes. that help structures adapt to ground shaking produced by an earthquake.

For those of us who reside on the island of Java, we must be prepared for the earthquakes to come. Before an earthquake happens, we need to understand the structure and the surrounding conditions where we live, work and play. We should reassess buildings and renovate, if necessary, to cope with the tremors of an earthquake. We need to secure the furniture so that it does not move easily during an earthquake. Other important guidelines include keeping flammable materials in a safe place and always turning off water, gas and electricity when not in use. Since a large portion of Java’s population resides in overcrowded regions, every person must find an escape route with easy access to open outdoor space. This is especially important if a disabled person lives with you.

During an earthquake, the official BMKG advice 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 you are already outdoors, avoid structures such as buildings, utility poles, trees, or anything else that could fall. Pay attention to your surroundings; determine if you are near a fissure, a site prone to liquefaction, or a potential landslide hazard location.

Because Indonesia is an archipelago of islands nestled in the ocean, avoid coastal areas prone to potential tsunamis. After an earthquake, examine yourself and others for injury and administer first aid. Check the surroundings for any fire, gas leaks, electrical shorts, broken pipes, or fallen debris. If you are in a safe place, stay out for possible aftershocks. Also, only listen to and amplify earthquake information from reliable sources to avoid spreading hoaxes.

The 2017 event was the first earthquake to shake my world. The last was my second earthquake, and my family and I are well prepared for a third, which is almost certain to come.

The references

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

Further reading

Alif, SM, Fattah, EI, Kholil, M. (2021). Source of the 2019 Mw6.9 Banten Intraslab earthquake modeled 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, GI, Sunarti, E., Ida, R., Gusman, AR (2019). Fault source of the September 2, 2009 Tasikmalaya Mw 6.8 intraslab earthquake, Indonesia: Analysis from GPS data inversion, tsunami height simulation, and stress transfer. Physics of Earth and Planetary Interiors, 291, 54-61, https://doi.org/10.1016/j.pepi.2019.04.004.

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