August 2018 Lombok earthquake

August 2018 Lombok earthquake
Lombok earthquake mosque ruin evacuation.jpg
A collapsed mosque in North Lombok
August 2018 Lombok earthquake is located in Indonesia
August 2018 Lombok earthquake
UTC time2018-08-05 11:46:37
ISC event612456510
Local date5 August 2018 (2018-08-05)
Local time19:46:35 (WITA)
Magnitude6.9 Mw
Depth31.0 km (19.3 mi)
Epicenter8°17′13″S 116°27′07″E / 8°17′13″S 116°27′07″E / -8.287; 116.452
FaultFlores Back Arc Thrust Fault
Areas affectedBali and West Nusa Tenggara, Indonesia
Max. intensityVIII (Severe)
Tsunami10.0 cm (3.9 in)[1]
Foreshocks6.4 Mw, 29 July 2018 (2018-07-29)
22:47:37 (UTC), 05:47:37 WITA
Aftershocks5.9 Mw, 9 August 2018 (2018-08-09)
05:25:31 (UTC), 597 total aftershocks
Casualties460 dead, 1,353 injured

On 5 August 2018, a destructive and shallow earthquake measuring Mw 6.9 (7.0 on Richter scale) struck the island of Lombok, Indonesia. It was the main shock following its foreshock, a nearby Mw 6.4 earthquake on 29 July.

The epicentre was located inland, near Loloan Village in North Lombok Regency. Its rupture spread to the north and reached the sea, creating tsunamis. Severe shaking was reported throughout the entire 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 structures in North Lombok were either damaged or destroyed. In the aftermath of the earthquake 460 people were confirmed killed while more than 1,000 were confirmed injured. More than 357,000 people were displaced.[2]

The earthquake is the largest and the strongest earthquake to have hit Lombok in recorded history. With more than 430 deaths, it is also the deadliest earthquake in Lesser Sunda Islands since the 1992 Flores earthquake and tsunami and the deadliest in Indonesia since the 2010 Mentawai earthquake.[3]

Tectonic setting

Tectonic setting of the region.

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.[4] 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.[5][6]

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.[7] In addition, the island also lies between two major geomagnetic anomalies of opposite signs.[8] The Flores Back Arc Thrust system had in the past century produced at least 4 earthquakes stronger than Mw 6.5: a Mw 6.5 event in Bali to the west on 1976, alongside three events (Mw 6.5, 6.5, 6.6 in 2007 and 2009) in Sumbawa to the east.[9]

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, with a more recent Mw 5.7 event in 2013 causing no deaths but extensive damage was reported. 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.[10]

Geologically, the rocks close to the epicentre are primarily tertiary to quartenary volcanic sediments, with pre-tertiary to tertiary sedimentary and metamorphic rock. The soft rocks are predicted to have caused an increase in the intensity of the earthquake.[11]