Melihat Kembali Gempa Lombok 2018

GEMPA Lombok 2018 is a rare and interesting event to understand its behavior. The up-and-coming seismicity scheme gave panic and confusion, especially residents living in Lombok, Bali and Sumbawa. There are at least six earthquakes that have a magnitude of more than 5.5. The magnitude 6.4 earthquake which took place on July 29, 2018 was the beginning of a series of 2018 Lombok Earthquakes. With the event process, this earthquake was triggered by the presence of a fault fault in the north of Lombok. Based on the BNPB's initial report on July 30, 2018, this earthquake caused 16 lives and more than 10,000 buildings were damaged. Whereas BMKG noted, there were at least 585 incidents of aftershocks up to 07.00 hours, August 5, 2018.

At 6:46 p.m., the earthquake magnitude 6.9 at the hypocenter depth of 34 km hit Lombok on the north side again. With capability, the second earthquake is bigger than the first earthquake.

Of course this incident has resulted in a large number of fatalities and damaged buildings in Lombok, Bali, and some on the west side of Sumbawa. Passed four days after the second earthquake, exactly on August 9, 2018 at 12:25 WIB, an earthquake with a capacity of 5.9 returned. The 3rd place of the earthquake is more west and different from the first earthquake and the second which are both in the northern part of Lombok. Around 10 days after the exact 3rd earthquake on August 19, 2018, we were again surprised by two earthquakes with a greater capacity than magnitude 6.0 taking place in Lombok where the earthquake was more east.

The two earthquakes had a magnitude of 6.3 taking place at 11:10 WIB with a hypocenter depth of 7.9 km and a magnitude of 7.0 taking place at 21.56 WIB with a hypocenter depth of 25 km. On August 25, 2018, an earthquake magnitude of 5.5 takes place in eastern Lombok or more precisely on the west side of Sumbawa. This earthquake can be called the 6th earthquake of a series of Lombok earthquakes whose magnitude is more than 5.5. In addition to earthquakes with relatively smaller strengths, the BMKG recorded aftershocks that took place in Lombok whether it was felt or not, more than 2000 incidents.

Figure 1. Distribution of earthquakes that took place in Lombok, six large earthquakes followed by blue combined black circles that provide info on the process of the rise in fault concentration. While the red circle is the distribution of aftershocks that take place from July 29 to September 10, 2018. Data on the concentration process and earthquake hypocenter are obtained from the USGS catalog. Based on data from the National Body for Disaster Management (BNPB), the overall damage caused by a series of Lombok 2018 earthquakes was 71,962 damaged housing units, 671 damaged educational facilities, 52 health facilities, 128 worship facilities and infrastructure facilities.

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Data on the victims were 460 people died, 7,733 injured, 417,529 people moved. Predicted momentary losses done by BNPB due to the Lombok Earthquake to trillions. This figure does not include losses caused by a decrease in visits of local and foreign travelers.

Tectonics and seismic history in Lombok The location of Bali and Nusa Tenggara has a difficult and active tectonic order. The presence of a subduction zone in the south called the collision zone between the Indo-Australian Ocean Crust Plate and the Eurasian Continent Plate. One of the implications of the existence of collision activities in this zone is the occurrence of earthquakes. While in the northern part of Bali and Nusa Tenggara, the tectonic situation is dominated by the presence of activities in the back arc of Flores which consists of two fragments. Based on the Indonesian Earthquake Hazard Map 2017, the two fragments are the Bali Fragment and Lombok Fragment Sumbawa. Not only the two fragments, the eastern and western parts of Lombok were squeezed by several fragments of tectonic arrangement. In the west

Lombok has two fragments, namely Lombok North and Lombok Central. While in the eastern part of Lombok there are three fragments, namely Sumbawa North, Sumbawa Central and Sumbawa South. McCaffrey & Nabelek explained the tectonic dynamics and the history of seismicity in Bali and the Nusa Tenggara in a paper entitled Earthquake, Gravity, and The Origin of the Bali Basin: An Example of a Nascent Continental Fold-and-Thrust Belt. The paper was published in the Journal of Geophysical Research in 1987. Based on this paper, there were at least seven earthquake incidents since 1963 in Bali and Lombok with relatively large magnitudes. The 7 earthquakes were the earthquake on May 18, 1963, May 22, 1963, 2 earthquakes on July 14, 1976, May 30, 1979, October 20, 1979, and December 17, 1979 (Figure 2).

Figure 2. Distribution of earthquakes in 1963, 1976 and 1979 that took place in Bali and Lombok. In figure 2 above, it can be seen if with the place, the earthquakes took place in the western part of Lombok and north of Bali. Whereas for the north side of Lombok it tends to not be an earthquake with the relevant magnitude. With the scheme of the earthquake incident process, the earthquake that took place in 2018 was almost the same as the earthquake that took place in 1963, 1976, and 1979. The process was a reverse fault. This provides a view if the earthquakes were spurred by upward fault activities in northern Bali and Lombok. The 2018 Lombok earthquake was different from the 1963, 1976 and 1979 earthquakes. It was also possible that the earthquake incidents in 1963, 1976, and 1979 would be repeated again. Surely we did not expect the earthquake incident in 1963, 1976 and 1979 to repeat itself again.