North Lombok Fault Found

The earthquake occurred on Sunday, August 5, 2018, at 18:45:35 WIB. Based on information from BMKG the epicenter of the earthquake was at coordinates 8.37° South Latitude and 116.48° East Longitude, with a magnitude of 7.0 MW (moment magnitude) at a depth of 15 km. Previously, on July 29, 2018 there was an earthquake with magnitude 6.4 MW and a depth of 10 km. Aftershocks with a magnitude of 6.2 MW occur on August 9, 2018.

The third series of earthquakes caused heavy damage to buildings and infrastructure in the Regency. North Lombok, East Lombok, West Lombok and the City of Mataram and caused 392 victims (BNPB sources). The Geology Agency sent the Earthquake Emergency Response Team and the Land Movement to the location to map geological damage.

In addition, the Geological Agency provided assistance in the form of drilling ground water to meet the availability of clean water in several locations in the affected areas.

Sesar Naik North Lombok

The results of the analysis of the Geological Agency Emergency Response Team were severely damaged with VIII MMI due to the earthquake M6.4 concentrated in Malempo Hamlet, Obel Obel Village; Ketapang Hamlet, Madayin Village in Sambelia District and Sajang Village, Sembalun District, East Lombok District. In all three locations there were cracks of land trending east-west. This crack causes heavy damage to the building it passes.

Earthquake M7.0

Heavy damage with VIII MMI concentrated in Tampes Village, Selengan Village; Braringan Village, Kayangan Village, Kayangan Sub district, and Sambik Bengkol Village, Gangga Sub district, North Lombok. The disaster area is composed of Quaternary deposits in the form of a cluster of young volcanoes (tuff, volcanic breccia, lava) which have undergone weathering and coastal alluvial deposits. The characteristics of Quaternary deposits tend to increase earthquake shocks.

Field observations and detailed mapping show surface deformation or surface rupture and soil cracks which cause damage to roads and buildings. Surface faults were found in Sambik Bengkol Village, Gangga District; Beraringan Village, Kayangan Village, Kayangan District; and Selengan Village, Kayangan District. Surface cracks and faults found in all three areas are generally west – east.

According to the analysis of the Agency’s Geological Emergency Response Team the surface fault, which has a western direction – east indicates a thrust fault with a vertical off set in the village of Sambol, Kayangan and Selengan sambi, varying from 2 cm to a maximum of 50 cm. The vertical off set distribution is a new fault identified after the earthquake on August 5, 2018. The Geological Agency’s Emergency Response Team calls this surface fault a Naik Fault of North Lombok west-east which forms a fault zone with a distribution north-south. This Naik fault in North Lombok is estimated to be associated with the Naik Fault of Flores, which until now has been identified in the Flores Sea north of Lombok, Sumbawa, Flores to Wetar. This surface fault causes severe damage to the area it passes.

Liquefaction or land leaching is the reduction of bonding between water-saturated soil due to earthquake shock so that the saturated layer of water is like a liquid mass. This melting soil mass radiates to the surface through soil cracks. Liquefaction causes damage to all types of buildings on it. The liquefaction location is spread in Gangga, Kayangan and Bayan Districts. All buildings that were traversed by cracks, soil cracks and liquefaction all collapsed / were severely damaged.

Damage to buildings at the disaster site is caused by several factors :

• The building was designed not to be earthquake resistant
• Close distance to the source of the earthquake (active fault) so that it will experience a strong earthquake shock
• Built on young volcanic deposits
• Water saturated alluvial deposits
• Soil cracking process
• Surface magnification and liquefaction

Until now, there has been no increase in volcanic activity in Indonesia including Rinjani Volcano and  Agung Volcano, which are located close to the earthquake center. Before the earthquake, the activity status of Rinjani Volcano was at level II (Alert) and Agung Volcano at level III (Standby), and after the earthquake the activity status was still constant.

Mitigation Efforts

Earthquakes cannot be estimated when, where, and how big the magnitude is. Earthquake mitigation efforts carried out by the Center for Volcanology and Geological Disaster Mitigation – the Geological Agency are by conducting research to identify and characterize the source of earthquakes, then calculate the magnitude of earthquake shocks that can hit a region probabilistically over a period of time. The magnitude of the shock is used as a reference in designing buildings in a place. The potential for earthquake shock hazards in an area has been sought for mitigation through Geological Agency products in the form of Earthquake Disaster Prone Areas (KRB) Maps. KRB Map of the NTB Provincial Earthquake was published in 2012. Estimated earthquake shocks in Kab. North Lombok and East Lombok in VII-VIII MMI are symbolized in yellow on the map.

The earthquake of July 29, 2018 (M6.4), the August 5, 2018 (M7.0) earthquake and the August 9 earthquake (M6.2) had the same mechanism and were estimated from the same source. Based on several studies, the source of the earthquake causing the Lombok earthquake has the potential to cause earthquakes with the strength of the M7.4 magnitude maximum. If you look on to that information, then the occurrence of an earthquake at the same source, with greater strength M7.0, has a small probability.

Technical Recommendations

1. Citizen to remain vigilant about the occurrence of aftershocks.
2. Citizen encouraged to stay calm and follow instructions from BPBD officials and the local government, and not easily provoked by irresponsible issues regarding the earthquake and tsunami.
3. Vital buildings, strategic and inviting the concentration of many people (offices, shop houses, markets, schools, etc.) are built to follow the principles of earthquake resistant buildings.
4. In order to avoid building on the lower, middle and upper steep slopes that have undergone weathering because it will potentially occur landslides triggered by earthquakes.
5. Buildings located in the land shift zone and soil cracks in large and long dimensions to be shifted about 20 meters from the main cracks, namely in Sambik Bengkol Village, Gangga District; Beraringan Village, Kayangan Village, Kayangan District; and Selengan Village, Kayangan District.
6. Buildings located in the liquefaction zone can be rebuilt by applying earthquake resistant building rules.
7. Dissemination, simulation, and earthquake and tsunami disaster management training in North Lombok and East Lombok districts should be carried out regularly.
8. In order for the North Lombok and East Lombok Regency Governments to immediately revise the RTRW (Regional Spatial Planning) based on maps of geological disaster prone areas issued by the Geological Agency, the Ministry of Energy and Mineral Resources includes earthquakes, tsunamis, volcanoes and soil movements.
9. So that the North Lombok and East Lombok Regency Governments include geological disaster material (volcanic eruptions, earthquakes, tsunamis, and ground movements) into the education curriculum.

Source: Center for Volcanology and Geological Disaster Mitigation, Geological Agency, Ministry of Energy and Mineral Resources