Very strong and shallow M7.0 earthquake hits Lombok region, Indonesia

Very strong and shallow M7.0 earthquake hits Lombok region, Indonesia

A very strong and shallow earthquake registered by the USGS as M6.9 hit Indonesia's Lombok region at 11:46 UTC on August 5, 2018 (18:46 local time). The agency is reporting a depth of 10.5 km (6.5 miles). EMSC is reporting M7.0 at a depth of 20 km (12.4 miles). BMKG is reporting M7.0 at a depth of 15 km (9.3 miles). This earthquake can have a medium humanitarian impact based on the magnitude and the affected population and their vulnerability. It hit just a few days after shallow M6.4 on July 28 in which 17 people died.
The epicenter was located 2.4 km (1.5 miles) ESE of Loloan, 34.5 km (21.4 miles) NW of Labuan Lombok (population 10 000), 51.2 km (31.8 miles) NE of Mataram (population 318 647), 52.6 km (32.7 miles) NNE of Praya (population 35 183) and 66 km (41 miles) NE of Lembar (population 10 000).
This is just 14 km (8 miles) south of Rinjani stratovolcano. 
There are 3 780 000 people living within 100 km (62 miles).
Some 180 000 people are estimated to have felt very strong shaking, 2 774 000 strong, 971 000 moderate and 14 940 light.
Strong aftershocks are shaking the region.
The USGS issued a yellow alert for shaking-related fatalities and economic losses. Some casualties and damage are possible and the impact should be relatively localized. Past yellow alerts have required a local or regional level response.
Estimated economic losses are less than 1% of GDP of Indonesia.
Overall, the population in this region resides in structures that are vulnerable to earthquake shaking, though resistant structures exist. The predominant vulnerable building types are unreinforced brick with concrete floor and concrete/cinder block masonry construction.
Recent earthquakes in this area have caused secondary hazards such as tsunamis that might have contributed to losses.

Estimated population exposure to earthquake shaking

Selected cities exposed

Regional seismicity

Ground failure

Geological summary

The quake occurred as the result of the shallow thrust fault on or near The Flores Back Arc Thrust. Preliminary focal mechanisms for the earthquake indicates slip occurred on either a shallow, south-dipping thrust fault or on a steep, north-dipping reverse fault. At the latitude of this earthquake, the Sunda and Australia plates converge in a north-south direction at a rate of about 70 mm/yr. The location of this earthquake is consistent with it occurring as southward-oriented thrust faulting on the Flores Back Arc Thrust.
At the location of this event, the Sunda plate underthrusts the Indonesian Arc to the south, along with the Flores Back Arc Thrust local to this earthquake, and along the Wetar Back Arc Thrust farther to the east. To the south, the Australia plate also underthrusts the arc to the north, along with the Java Trench. This Flores region marks the transition between traditional oceanic plate subduction in the west (Australia plate beneath Sunda) to a continental collision in the east. As a result, two opposite-verging thrust systems are evident in cross-sections, and the northward subducting slab of the Australia plate exists at depth beneath the Indonesian Islands and the Flores Sea. The focal mechanism solution and depth of this earthquake indicate it is associated with the Flores Back Arc Thrust system, rather than the deeper Australia slab.
In the region surrounding the August 5, 2018 earthquake, there have been six other events of M6.5 or larger over the previous century. Four of these are likely to have occurred on the back arc thrust system; a M6.5 in the Bali region to the west of Lombok in July 1976, and three events of M6.5, M6.5, and M6.6 in the Sumbawa region to the east of Lombok in November 2007 and November 2009. The Sumbawa earthquakes were associated with several deaths, hundreds of injuries, and hundreds of houses being destroyed.

Updates

This quake hit just a few days after strong and shallow M6.4 on July 28 which killed at least 17 people, injured more than 150 and destroyed thousands of homes.
Aftershocks were numerous and as large as M5.7.
The earthquakes caused remobilization of ash deposits on Rinjani as well as landslides. There were 1 226 visitors to the Mount Rinjani National Park Area at the time, and, according to news articles, about 690 climbers were on the volcano and had to be rescued. One person in the national park died from rockfalls.
The Alert Level remained at 2 (on a scale of 1-4) and the public was warned not to approach the crater within a 1.5-km radius (0.9 miles).
Featured image credit: USGS

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