A shallow crust earthquake struck West Java

A shallow, moderate-magnitude 5.6-magnitude earthquake struck West Java, Indonesia this week, causing shaking and loss.

Written by Anne Seawright, Ph.D., Department of Geophysics, University of Indonesia and member of the Seismology Department of the Association of Indonesian Geophysicists (HAGI).

Citation: Searite, A, 2022, Shallow crust earthquake affecting West Java, Templor, http://doi.org/10.32858/temblor.284

When: Monday, November 21, 2022, around 1 p.m

Where: Depok, West Java, Indonesia.

My mathematical physics class was preparing for an exam. Ten minutes into the test, as I watched my students try to answer the questions, the desks, screen projector, and curtain started to vibrate.

It’s an earthquake.

Fortunately, we remember all of our training when an earthquake occurs. We sit under the table and cover our heads so that the vibrations stop. Since we are on the fourth floor, we need to go down the stairwell to get out of the building. We managed to keep our calm and order. We wait about half an hour before we go back to class. The test continues after that, because we were at a safe distance from the epicenter and did not see any building collapse.

what happened?

The class shaking earthquake was 5.6 on the Richter scale and had its epicenter in Sukalarang Regency, West Java, Indonesia. This earthquake shook most of the western part of the island. The earthquake occurred at 1:21 pm local time. According to Badan Meteorologi, Klimatologi, dan Geofisika (BMKG), Indonesia’s national seismological agency, the quake occurred on land, at a depth of 11 kilometers. Because it occurred on land, no tsunami likely would have occurred. Within an hour of the main earthquake, 25 aftershocks were recorded, ranging in magnitude from 1.8 to 4.0 on the Richter scale.

A map showing the location of the November 21, 2022 earthquake that shook West Java, Indonesia.

Vibration varied across West Java. Near the epicenter, intense shaking of V on the modified Mercalli intensity scale (MMI scale) was felt by all the people located in the city of Cianjur (about 40 kilometers, or about 25 miles, from the epicenter). But people more than 100 kilometers (about 62 miles) from the epicenter also felt the quake, including my students and I in Depok, where we felt the shaking at about the second level on the scale. Other places that experienced shaking include Garut, Sukabumi (Scale IV MMI), Rangkasbitung, Bogor (Scale III MMI), and the capital, Jakarta (Scale II to IV MMI, likely variable because people in tall buildings can experience more shaking than on the ground ).

Map showing ground shaking felt by residents in different locations. The modified Mercalli Intensity (MMI) is shown as a color bar below the figure. Credit: BMKG

losses

Extensive damage was reported in Cianjur, a town near the epicenter, and surrounding areas. Homes, schools, public infrastructure and utilities have been severely damaged and/or collapsed. Several landslides occurred due to the shaking. The hardest hit area was Kujinang district, where a landslide buried a village. Over the past 24 hours, 268 casualties have been reported, 1,083 people have been injured, and 151 people are still missing. More than 22,000 homes were damaged, causing tens of thousands to be evacuated from their homes. The human losses are attributed to the collapse of houses and landslides. Not many homes and infrastructure are built to be earthquake-proof. Most of the brick houses were severely damaged, while the houses made of wood were not demolished.

More than 22,000 homes were destroyed or damaged in the 5.6-magnitude earthquake in West Java. Credit: ABSEE HOUSE / Shutterstock.com

Cimandiri error

Analysis by BMKG indicates that this earthquake may be related to activity in the Cimandiri fault, one of the major faults in West Java. Based on seismic data analysis, the source mechanism of this earthquake is strike slip, which corresponds to the Semandiri fault mechanism, which is a left lateral slip fault.

Based on Peta Sumber dan Bahaya Gempa Indonesia by Pusat Gempa Nasional (PUSGEN, 2017), Cimandiri consists of three sections: the Cimandiri sector in the south, the Nyalindung-Cibeber sector in the center, and the Rajamandala sector in the north. The epicenter of Monday’s earthquake is located near the Rajamandala section of the Simandiri fault. But this analysis is preliminary. Further study is needed to ascertain the role of any part of the Cimandiri fault.

The Cimandiri fault is moving at a rate of 4 millimeters per year (Safitri, 2016) and has the potential to generate a magnitude 7.2 event, about 100 times the energy that occurred on Monday. The fault has hosted several historical earthquakes: a magnitude 7 earthquake in 1900 (Weser, 1922), a magnitude 5.5 earthquake in 1982, and magnitude 5.4 and 5.1 events in 2000 (Supartoyo et al., 2008). Between 2009 and 2015, there were 10 earthquakes identified along the Cimandiri fault (PUSGEN, 2017).

Given the frequency with which earthquakes occur in the Cimandiri fault—about a 20-year period of time (1982, 2000, and 2022)—we need to prepare for another earthquake in the future, regardless of whether this earthquake is in the Cimandiri fault or not.

what are you expecting?

The Semandiri fault cuts through a densely populated area, including Pelabuhan Ratu, Sukabumi, Cianjur and Padalarang. Given the impact of Monday’s earthquake and its impact on the population, we are reminded of the need for risk education and mitigation.

Natural hazard education is needed to make people aware of the presence of active faults and areas subject to earthquakes that may lead to landslides in their vicinity. Damage to homes and infrastructure due to vibrations resulting in injuries is a reminder to be extra careful in implementing appropriate building code. Houses and infrastructure must be built to be earthquake resistant. The building code is there, but it is hardly being implemented. We need to encourage everyone to implement code to make buildings safer, and in turn reduce losses.

References

National Center for Seismological Studies (PUSGEN). (2017). Earthquake Source and Hazard Map of Indonesia in 2017, National Earthquake Study Center Team, Housing and Settlements Research and Development Center, Balitbang PUPR, Bandung, Indonesia, ISBN: 978-602-5489-01-3 (in Indonesian).

Savitri, A. & Milano, I, Gunawan, Indra & Abidin, Hasanuddin Z. and Effendi, G. and Kreswati, Estu. (2018). Strain variation along the Cimandiri Fault, West Java based on continuous campaign and global positioning system (GPS) monitoring from 2006 to 2016. IOP Conference Series: Earth and Environmental Sciences. 132. 012027. 10.1088/1755-1315/132/1/012027.

Supartoyo, ET Putranto, And Djadja (2005), Active Faults and Destrake Epicenter Distribution Map of Indonesia.

Visser, SW (1922), Inland and Submarine Earthquakes of Sumatra and Java, Javasche Boekhandel and Drukerij.

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