Unraveling the Seismic Puzzle on the Northern Coast of Lombok Island

Indonesia is no stranger to seismic activity. This archipelago, often referred to as the Emerald of the Equator finds itself at the juncture of three dynamic tectonic plates: the Eurasian Plate, the Indo-Australian Plate, and the Pacific Plate. This positioning places Indonesia within the infamous Pacific Ring of Fire.

In the year 2022, the Meteorology, Climatology, and Geophysics Agency of Indonesia (BMKG) recorded a total of 10,792 earthquakes. Among these, 205 were of significant magnitude, measuring at M 5 or higher. The majority, comprising 10,587 earthquakes, fell below the M5 threshold. Interestingly, 807 of these could be felt by the population.

Among all the earthquake events in Indonesia, the 2018 Lombok earthquake was one of the rare significant earthquakes. Unlike other earthquakes, which are usually followed by aftershocks with lower magnitudes, the 2018 Lombok earthquake was followed by four aftershocks with magnitudes above 5.

The report from the National Disaster Mitigation Agency (BNPB) states that there were 460 fatalities, 7,733 people injured, and 417,529 displaced individuals. As reported by CNN Indonesia on August 6, 2018, the Head of Data, Information, and Public Relations Center of BNPB, Sutopo Purwo Nugroho, revealed that the Lombok earthquake originated from the Flores arch thrust fault activity at a depth of 15 km, located 18 km northwest of Lombok.

Lecturer of the Geography Department at Muhammadiyah University of Surakarta (UMS), Annisa Trisnia Sasmi, S.Si., M.T., suspects the formation of a new segmentation fault off the northern coast of Lombok Island. This is detailed in her research titled "Separation of Shear Wave in the Aftershock Area of the 2018 Lombok Earthquake, Indonesia". The research has been published in the Geoscience Letters journal, indexed in Scopus Q1.

Snapshot of Lombok Earthquake

The 2018 Lombok earthquake sparked a wave of curiosity among domestic seismology experts. This seismic event, which unfolded off the northern coast of Lombok Island, was a rare phenomenon in the annals of earthquakes in Indonesia.

Annisa stated that typically in a series of earthquakes, significant earthquakes only occur occasionally over a relatively long period of time, which are then followed by aftershocks.

The pattern of the Lombok earthquake is new. Usually, if earthquakes occur in succession, the pattern will not be significantly different from earthquakes that have occurred in the past, both in terms of their position and magnitude," explained Annisa.

During the Lombok earthquake event, the seismic activities unfolded on July 29, 2018, marked by a magnitude of 6.4. Seismology experts speculate that this was the mainshock. This is supported by a series of aftershocks with lower magnitudes around the epicenter of the earthquake.

The speculation was then disproven when larger earthquakes occurred on August 5 (M 7.0), August 9 (M 5.9), and August 19 (M 6.3 and M 6.9). Seismology experts subsequently revised their assessment, identifying the earthquakes on August 5 and 19 as the mainshocks of the Lombok earthquake sequence. BMKG recorded a staggering total of at least 5,500 aftershocks, all with magnitudes below 5, were recorded between August and October of 2018

"Such a significant earthquake, coupled with a massive distribution of aftershocks, has never occurred in the northern coastal area of Lombok Island. Usually, it occurs in areas near Bali or in the southern regions close to the subduction zone," stated the faculty member of UMS's Geography Department.

Suspected New Segmentation Fault

The research conducted by Annisa raises the suspicion of a series of new fault or fracture formation processes off the northern coast of Lombok Island. This four-year study utilized seismic tomography as its method. This technique aims to image the subsurface structure based on information gathered through seismograph equipment.

Rosette diagram distribution of ф direction parameter in each seismographic station, shown by circles with blue bars. The radius of the rosette diagram shows the number of ф recorded at each seismic station. The red lines show the average value of ф. The yellow stars represent the epicenter of significant earthquakes (Sasmi et al. 2020), and the yellow diamond shows the epicenter of M 6.4 July 29, 2018 earthquake (BMKG, 2019). The blue inverted triangles indicate the distribution of the seismographic station

"From the research we have conducted so far, there is a suspicion that the earthquakes that occurred in Lombok are part of a series of processes involving the formation of new faults in the northern region of Lombok Island," said Annisa.

Annisa stated that there is new deformation around the northern coast of Lombok Island. According to her, this fault is segmented and appears in several phases or stages. Intriguingly, she notes that this fault emerges at a relatively shallow depth of 20 kilometers.

The discovery of this new fault still requires stronger evidence. Annisa agrees that further in-depth research is needed to map the extent of the new fault area and assess the potential for future earthquakes.

"I plan to conduct further research to provide a clearer mapping of the distribution of fault zones based on the findings in my study," she expressed.

The distribution of spatially averaged ф in 10×10 km grid size. The blue bars show the average ф in each grid block

Annisa mentioned that this further research is necessary not only to map the fault zones but also to study the potential for earthquakes in that area in the future.

"To justify the potential occurrence of significant earthquakes in the future, further studies are needed to strengthen the shear wave splitting study results. This includes studies using other geophysical methods as well as a geodetic approach. As for the exact timing and location of future earthquakes, no research so far can answer these two aspects," explained Annisa.

Temporal variation in delay time at station LM05; b temporal variation in delay time at station LM06; c variation of earthquake magnitude with respect to recording time. The grey dots in Fig. 6a and 6b show the delay time variation before moving-average is applied. The green dots in Fig. 6a and 6b show the delay time variation after 10-window moving-average. Blue dots in Fig. 6c show the magnitude variation before the moving-average process is applied. Orange dots in Fig. 6c show the magnitude variation after generate the 10-window moving-average. Blue oval symbols depict the trend correlation with respect to August 9 (Mw 5.9) earthquake, and yellow oval symbols show the correlation towards August 19 earthquakes magnitude

Mitigation Procedures

While it showed potential for earthquakes exceeding magnitude 5 in 2018, Annisa urges Indonesians not to worry. She emphasizes the crucial need for community-level disaster mitigation efforts. This proactive approach is pivotal in ensuring preparedness, particularly in vulnerable areas like Lombok.

"Being cautious is acceptable, but excessive worry and panic should be avoided. The potential for earthquake disasters should not only be a concern for the people of Lombok, but all of us as Indonesian citizens living in a country prone to seismic activity should pay attention to this," Annisa emphasized.

She highlights that preparedness doesn't equate to panic or fear but rather serves as a proactive stance toward potential hazards. By acknowledging the potential for disasters, communities can shift their focus towards preventive measures to prevent large-scale casualties in the future. She also emphasizes to the public not to easily panic or be swayed by unverified rumors.

"We must be aware of the potential natural disasters around us, understand preventive efforts to reduce the risk of disasters, know how to evacuate ourselves, work together to lighten the burden on others, and educate ourselves as well as our families or closest communities about the potential disasters around us and the disaster mitigation efforts that can be taken," Annisa continued.

Annisa advocates emulating Japanese society, which has adapted to their country's earthquake and tsunami-prone conditions. She highlights the effectiveness of Japan's mitigation efforts. She suggests specific measures to implement in Indonesia, such as designated disaster evacuation zones and enforcing earthquake-resistant building standards.

"We can emulate Japan, which is very adaptive to its country's conditions. Fated to reside in an earthquake-prone region, it inevitably compels its citizens to adapt by strengthening disaster mitigation and constructing earthquake-resistant buildings," she explained.

Annisa emphasizes the importance of not being hasty in accepting information from unclear sources.

"Also, do not be hasty or rush to analyze and spread information, especially information that is not clear in its authenticity," she emphasized.

Given this situation, Annisa suggests that a simple step the government can take now is to incorporate disaster mitigation and evacuation materials into the Indonesian education curriculum. According to her, disaster education should begin at an early age.

"At the very least, education is key. Disaster mitigation should be taught starting from basic education. Additionally, there should be a widespread campaign of disaster mitigation education for adults and the elderly to increase awareness of potential disasters," she concluded.

Writer: Gede Arga Adrian

Editor: Al Habiib Josy Asheva

Translator: Farizal Luqman Majid

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