1. Based on the information from Meteorology, Climatology, and Geophysics Agency (BMKG), there was a series of big tectonic earthquakes happened at the north part of Lombok, which were on 29th July 2018 with magnitude of 6.4 Scale Richter (6.4 SR), on 5th August with magnitude of 7 SR, and on 19th August with magnitude of 6.9 SR. The depth of those three earthquakes was 10 – 32 km under the sea level, that can be categorized as a shallow tectonic earth quake. A tectonic earth quake with a big magnitude, and comes from a shallow depth has a destructive impact yet only limited to certain area (does not destroy the whole Lombok Island). Those three earthquakes are very damaging at the north up to east part of Lombok Island, nevertheless the destructions are getting less at the south part of Lombok.
According to data of earthquakes distribution from BMKG, the source of earthquake on 5th August 2018 was relatively located at the north-west from the source of earth quake on 19th August 2018. The main and subsequent source of earthquake distribution tends to be at the constant line, which moves from west to east, at the north part of Lombok. This is a sign of an earth quake caused by Sesar Backarc Thrust (Sesar Naik Busur Belakang Flores).
In the observation period on 19th August 2018 – 23rd August 2018, after the main earthquake, at 15.00 WITA, 268 subsequent earthquakes have been recorded and 15 of them were felt in Lombok.
2. Analysis and Conclusion
• Flores Backarc Thrust is an active sesar, and also the source for a series of tectonic earth quake at the north part of Lombok.
• Up to this date, there is no expert or technology in the world that can predict when and where should earth quake happen and how big the magnitude is.
Based on the data given at Picture 1, the source of the main and subsequent earth quake on 5th and 19th August 2018 relatively moves to east side, therefore hopefully the source of earth quake will not moves to west. If this is true, it will be less likely for earth quake happen in the north part of Bali due to Flores Backarc Thrust.
• On Picture 2, the graphic of daily subsequent earthquakes shows decreasing in term of amount and energy (magnitude). Based on the experience of several/similar earth quake happened in Indonesia, it can be concluded that Flores Backarc Thrust is shifting in to a new balance, it also means that the subsequent earthquake will soon stop, and Flores Backarc Thurst will be stable/locked, and will be active in the future and produce earth quake once it has enough energy, yet we cannot predict when or where it will happen.
• Lombok earthquake has a large energy/magnitude of 6.0 to 7.0 SR, with the depth of the source of shallow earthquakes, ranging from 10 to 30 km below sea level, has a large destructive power, in a limited wide range, only on the north does not have an overall damaging impact on Lombok Island.
• Infrastructure damage occurs in areas with a characteristic soil layer containing a lot of sand, which has the properties of loose granules, not compact, thus increasing (high amplification) earthquake shocks. Infrastructure built on the ground contains a lot of sand, if it does not use a shock-resistant earthquake structure, it will experience mild to severe damage. The area in question is in the middle to North Island of Lombok, including the Narmada, Gunungsari, Senggigi and other coastal areas. While the city of Mataram is safe against earthquake shocks, because the base rock is hard, in the form of hard, strong and thick lava breccia, which is to cushion earthquake shocks.
3. Mount Agung Volcanic Activity
Volcanic eruptions with a height of volcanic ash columns of more than 10 km from sea level can pose a danger to land and air. The threat of volcanic eruptions on the ground in the form of hot clouds, rain of ash, throwing volcanic material with small size to volcanic bombs. The threat of eruption in the air, in the form of fine dust particles that can reach a far range from the source of eruption (depending on the direction and speed of the wind) and very dangerous for flight. Because fine dust enters the jet engine of an aircraft, it can damage and stop the jet engine suddenly.
Volcanic activity of Mount Agung in Karangasem Regency, Bali Province entered the highest level, namely Level IV (Awas/Warning) on September 22, 2017. At that time, officials handling volcanic eruption disaster mitigation, stated that the eruption of Mount G. could achieve Volcanic Eruption Index V, similar with an eruption in 1963, which lasted for one year.
Several series of eruptions of ash and smoke occurred, at the end of November 2017, an effusive eruption was suspected with the release of lava, with a volume of about 20 million cubic meters, exposed in the crater of Agung. Indeed, after the effusive eruption, the energy and volume of the magma remaining in the body of the Mount Agung is very small. Based on visual and instrumental monitoring data, Mount Agung has gone through a volcanic crisis, almost reaching the end of its field and is unlikely to have a large eruption in the near future.
According to the Real Data of Seismic Amplitude Measurement (RSAM) of Mount Agung, which reflects the energy of volcanic activity, did not show any signs of increased volcanic activity.
A graph of RSAM G. Agung is presented before, during and after the occurrence of 3 major energy tectonic earthquakes in Lombok on 29 July, 5 August and 19 August 2018. RSAM shows an increase in energy (suddenly) occurring at and after a tectonic earthquake. Increased RSAM was not due to volcanic activity of Mount Agung but by the 3 main tectonic earthquakes and its subsequent earthquakes that occurred in Lombok.
4. Analysis and Conclusion
• Tectonic earthquakes and their subsequent occurrences on the northern part of Lombok Island did not affect volcanic activity of Mount Agung in Bali.
• Effusive eruption at the end of November 2017, which was suspected by the exposure of 20 million cubic meters of lava in the crater of Agung G. indicates the end of the volcanic crisis of Mount Agung. So, it is less likely, even to be ruled out, that a great eruption will occur in the near future. Minor eruptions are unlikely to occur, with a maximum eruption of 3 km from the peak. This minor eruption was triggered by the presence of 20 million cubic meters of lava in the process of cooling.
• Based on scientific data and facts as well as personal experience of mitigating volcanic eruption disaster, the status of Agung G. must be dropped from Watch/Standby (Level III) to Advisory/Level II. Until now, there was no significant volcanic activity, there were no scientific data and facts that supported the status of Agung G. Still Level III/Watch.
Preparations for the IMF-WBG Annual Meetings activities in Nusa Dua, Bali, before until the end of October 2018, should be carried out without worries and excessive anxiety from the threat of eruption of Mount Agung and tectonic earthquakes which currently occur on the northern part of Lombok Island, because:
a. Based on data on the distribution of the main tectonic earthquake sources and subsequent earthquakes, it tends to shift towards the East, it is unlikely that the source of the earthquake will migrate towards the West/ North part of Bali. Based on the data on the number and energy of the aftershock tectonic earthquake shows a decrease, meaning that Flores Backarc Thrust is headed to stable.
b. Based on data on volcanic activity presented on the RSAM chart, it does not show an increase in volcanic activity since the effusive eruption at the end of November 2017. This means that Mount Agung has passed a volcanic crisis. A series of tectonic earthquakes in the northern part of Lombok Island do not affect volcanic activity of Mount Agung. Then Mount Agung is unlikely, and can even be ruled out a major eruption will occur until the end of October 2018.
Based on scientific data and facts, we advise the agency which authorizes to handle volcanic eruption disaster mitigation reduces the status of Mount Agung from Level III to Level II. This decline in status is very necessary to provide a sense of security and comfort from the threat of eruption of Mount Agung for tourists (domestic and foreign) and preparation to the holding of the IMF-WBG Annual Meetings. A sense of security and comfort to all parties is very important, considering the Balinese economy is very dependent on the tourism sector and national and international meetings/ meetings / congregations.
This report is limited, not to be contested with reports from agencies authorized to handle and mitigate earthquake disasters and volcanic eruptions.
This report has nothing to do with the ESDM Earthquake and Volcanic Disaster Mitigation report, because since November 1, 2017, I am no longer the Geological Disaster Expert at the Ministry of Energy and Mineral Resources.
I am fully responsible for this report.
Jakarta, August, 2018.