On 5 August 2018, a destructive and shallow earthquake measuring Mw 6.9 (ML 7.0 according to BMKG) struck the island of Lombok, Indonesia. It was the main shock following its foreshock, a nearby Mw 6.4 earthquake on 29 July. It was followed by a nearby 6.9 earthquake on 19 August 2018.
The epicentre was located inland, near Loloan Village in North Lombok Regency. The fault rupture spread to the north and reached the sea, creating tsunamis. Severe shaking was reported throughout the island, while strong shaking was reported on the neighboring islands of Bali and Sumbawa.
Widespread damage was reported in Lombok and Bali. Officials stated that at least 80% of the structures in North Lombok were damaged or destroyed. In the aftermath of the sequence of earthquakes in August, a total of 563 people were confirmed killed while more than 1,000 were confirmed injured. More than 417,000 people were displaced.
This earthquake is the largest and the strongest earthquake to have hit Lombok in recorded history. With more than 560 deaths, it is also the deadliest earthquake in the Lesser Sunda Islands since the 1992 Flores earthquake and tsunami. The earthquake later caused chains of earthquakes in West Nusa Tenggara with significant magnitude, which was deemed by officials as a rare event.
Tectonic setting
Indonesia lies on the Pacific Ring of Fire, an area of significant volcanic and tectonic activity. In particular, the archipelago is located between the Eurasian, Pacific and Australian tectonic plates. The Australian plate subducts beneath the Sunda plate at 50–75 mm (1.97–2.95 in) a year, forming the Sunda Trench. This activity caused the Mw 9.2 2004 Indian Ocean earthquake and tsunami, one of the strongest and most deadly earthquakes in recorded history.
Lombok in particular lies on the destructive plate boundary between the Australian plate and the Sunda plate. To the east of Bali, the plate boundary starts to involve a collision between the leading edge of the Australian continent and the eastern part of the Sunda Arc and the western end of the Banda Arc, also known as the Flores Back Arc Thrust Zone. The Sunda Arc has produced a large number of powerful and devastating earthquakes in the past, including the 1977 Sumba earthquake, 1994 Java earthquake and the 2006 Pangandaran earthquake and tsunami. In addition, the island also lies between two major geomagnetic anomalies of opposite signs. The Flores Back Arc Thrust system had in the past century produced at least 4 earthquakes stronger than Mw 6.5 in the region: a Mw 6.5 event in Bali to the west in 1976, alongside three events (Mw 6.5, 6.5, 6.6 in 2007 and 2009) in Sumbawa to the east.
The North Lombok area, where the earthquake occurred, has a track record of earthquakes in the past. One Mw 6.4 earthquake in 1979 killed 37 people, with a more recent Mw 5.7 event in 2013 causing extensive damage but no deaths. Simulations by scientists from the University of Mataram suggested that an earthquake stronger than Mw 6.0 could cause a small tsunami which would be 13–20 cm (5–8 in) in height, reaching Mataram within 18–20 minutes after the earthquake.
Geologically, the rocks close to the epicenter are primarily Tertiary to Quaternary volcanic sediments, with pre-Tertiary to Tertiary sedimentary and metamorphic rock. These soft rocks are thought to have caused an increase in the intensity of the earthquake.
Earthquake
The earthquake occurred on 19:46 local time, at a depth of 34.0 km (21.1 mi) (USGS) or 15.0 km (9.3 mi) (BMKG). Shaking was felt as far away as Sumbawa in the east and Trenggalek Regency in the west. Shaking was also felt in Pacitan Regency, East Java. The Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) stated that the epicentre of the earthquake was located inland, on the northeastern slopes of Mount Rinjani at 8°22′S 116°29′E / 8.37°S 116.48°E. It initially measured the earthquake's magnitude at Mw 6.8, before revising it to 7.0. The United States Geological Survey initially measured the earthquake at Mw 7.0, before revising it down to 6.9. It placed the epicenter at 8°17′13″S 116°27′04″E / 8.287°S 116.451°E, somewhat north of the BMKG estimate.
Caused by a shallow thrust fault on or near the Flores Back Arc Thrust, at which the Australian and Sunda plates underthrusts the Indonesian volcanic island arcs, the earthquake struck just a week after the magnitude 6.4 earthquake which had killed 20. According to BMKG, this earthquake was a foreshock to the Mw 6.9 earthquake. Despite the earthquake, no increase in activity was recorded for the nearby volcanoes of Rinjani and Agung. At certain areas in the northwestern parts of the island, the earthquake caused the ground to rise by as much as 25 centimetres (9.8 in).
The earthquake interrupted a ministerial security council held in Mataram, which was attended by Indonesian ministers Yasonna Laoly and Wiranto, Australian minister Peter Dutton, New Zealand minister Andrew Little, and Singaporean minister K. Shanmugam. Immediately following the earthquake, many residents evacuated to local mosques in fear of aftershocks.
Days following the earthquake, scientists from NASA and California Institute of Technology published a satellite image of the ground shift in Lombok. The image showed that the area in the earthquake's rupture had been lifted by 25 cm. Several portions of North Lombok were raised by 5–10 cm. The area which was located on the south of the rupture however had a decrease on its height, including Mount Rinjani which had lost 15 cm due to the earthquake.
Impact
As the earthquake occurred at a relatively shallow depth and a 7 magnitude, the earthquake caused severe shaking in the island of Lombok and strong shaking in the neighboring islands. Indonesian officials released maps of the perceived shaking. The largest city in Lombok and the provincial capital, Mataram, recorded a maximum intensity of VIII (Severe). The strongest shaking felt in the island of Bali was in Karangasem Regency with an intensity of VI (strong) while, on the island of Sumbawa, the strongest shaking was felt in Bima with a recorded maximum intensity of VI (strong).
Tsunami
Following the earthquake, a tsunami warning was issued by BMKG for the North coast of Lombok. With a maximum expected height of just 50 cm (20 in), the tsunami struck at three locations, the maximum of which was at 13.5 cm (5.3 in) in Carik, North Lombok, and the warning was removed later. However, the warning had caused panic as the residents attempted to evacuate to higher ground. In Gili Trawangan, residents and tourists were evacuated to the hills in expectation of a tsunami. In Gili Air and Gili Meno there are no hills, so locals waited in expectation of a tsunami in the small islands' open fields. A tsunami somewhat larger than what BKMG expected hit Kayangan, with a height of 2 m (6 ft).
Aftershocks
By 18 August, the BMKG had reported as many as 664 aftershocks, most of which were below Mw 5.9 magnitude.
There were two major aftershocks. The first one occurred on 9 August at 13:25 local time. It struck the island of Lombok at a depth of 12 km and measured at Mw 5.9 (USGS) or 6.2 (BMKG), and was felt as far away as East Java. Shaking was felt in the neighboring islands of Bali and Sumbawa and lasted for at least 10 seconds. Officials stated that the intensity of the shaking in Lombok was V-VI MMI (medium-strong). Panicked residents ran out to the streets. Dozens of buildings which had been badly damaged by the mainshock reportedly collapsed. Widespread damage was also reported in West Nusa Tenggara's provincial capital Mataram. There were also reports of landslides which buried at least 4 people in Gangga-Kayangan area, Rescue efforts were abruptly stopped due to the earthquake. Authorities confirmed that 24 people were injured and 6 were killed, including two volunteers from the Indonesian Red Cross, by the 9 August aftershock.
The second one occurred on 18 August at 23:10 local time. The earthquake was measured at Mw 6.3 (USGS) or ML 6.5 (BMKG). It struck at a depth of 7.9 km. 4 minutes earlier, a 5.4 magnitude earthquake had just struck Lombok. There were reports of landslides in Mount Rinjani. There were 2 deaths.
Casualties
As of 25 August 563 people had been confirmed killed by the 5 August and subsequent events. Most victims were killed by falling debris due to collapsed buildings. Deaths occurred in all regencies and cities on the island – with most of the deaths being in North Lombok Regency. Of the 563 deaths in Lombok, 471 were in North Lombok, 45 in West Lombok (including 7 in the Gili Islands), 12 in Mataram, 26 in East Lombok and 2 in Central Lombok Regency. Officials confirmed that 2 people had been killed in Bali, both were in Denpasar while 58 people in Bali were injured. All killed by the earthquake were Indonesian citizens.
Hundreds of people were injured, overloading the capacity of medical sites and forcing them to be treated outside. Many injuries were also reported in Karangasem Regency, the easternmost part of neighboring Bali.
On 6 August at 07:45 local time, the regent of North Lombok Regency, Najmul Akhyar, confirmed on live television that the death toll had risen to more than 100, as reports from his districts revealed that at least 5 people had been killed in each district. At 10:00 a.m local time, the Head of Data, Information and Public Relations Center of BNPB, Sutopo Purwo Nugroho, said in a press conference that at least 91 people had been killed, all Indonesians, while 209 people were seriously wounded in the earthquake.
Later that afternoon, Sutopo revised the death toll to 98 people as another 7 bodies were recovered from the West Lombok Regency while the number of injured was raised to 236. He added that the current death toll is not final and more information will come from the villages that are cut off by the earthquake.
There were many reports of collapsed mosques in Lombok. As the earthquake struck during isha prayer, many people were trapped after the mosques they were in collapsed onto them. In Lading-Lading village, North Lombok Regency, more than 50 people were trapped inside the Jabal Nur Mosque, while some local residents stated that there were hundreds. Dozens of people were also reportedly trapped inside the Jamiul Jamaah Mosque in Karangtangsor Village. On 7 August, officials announced that they had rescued one worshipper from the rubble, who was found alive by the rescuers. Three bodies were also recovered during the search and rescue operation.
On 8 August, the death toll was raised by the Indonesian National Disaster Management Board to 131 as more bodies were retrieved by rescue workers. Data assessment conducted by authorities revealed that more than 1,470 people had been seriously wounded and more than 165,000 people had been displaced by the earthquake. He added that there were still some areas that couldn't be reached by rescuers.
On 18 August 469 people were confirmed dead. Authorities, however, warned that the death toll may rise. On 25 August, the Indonesian Ministry of Social Affairs confirmed that 565 people had been killed by all of the August tremors.
Damage
Widespread damage was reported in Lombok Island. Officials stated that structures in North Lombok Regency, in which the epicentre of the earthquake was located, reportedly suffered the greatest damage. Telecommunications went down and blackouts were reported throughout Lombok. Thousands of houses were damaged, many of which had already been struck by the foreshock. A lack of construction regulations and technical knowledge of builders regarding earthquake resistance, coupled with low economic ability, exacerbated the damage. At least three bridges reportedly had collapsed in Lombok.
Immediately after the earthquake, most of Mataram lost electrical power. Although some parts of Lombok still received electricity, the load was only 50 MW compared to the normal 220 MW. Streets were jammed across Lombok as traffic lights were unusable and roads were blocked by debris. The Indonesian National Electricity Company (PLN) stated that electricity had been restored in major parts of Lombok hours after the quake, even though around 25% of the area in Lombok hadn't been restored.
In Gili Trawangan, severe damage was reported to the island's cafes and resorts. As multiple convenience stores were abandoned by their owners, cases of looting were widely reported across the island.