Tectonic Summary

The August 19, 2018, 14:56 UTC, M 6.9 earthquake near Belanting, Indonesia, occurred as the result of shallow thrust faulting on or near The Flores Back Arc Thrust. This event occurred 14 days after the August 5, 2018, M6.9 earthquake near Pulau Lombok, Indonesia which killed at least 476 people. Both events have a similar depth and mechanism. Preliminary focal mechanisms for the August 19 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 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 the Java Trench. This Flores region marks the transition between traditional oceanic plate subduction in the west (Australia plate beneath Sunda) to 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.

Prior to the August 5, 2018 earthquake two weeks prior to today's event, there had been six other events of M 6.5 or larger over the previous century. Four of these are likely to have occurred on the back arc thrust system; a M 6.5 in the Bali region to the west of Lombok in July 1976, and three events of M 6.5, M6.5, and M 6.6 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. Since the August 5, 2018 earthquake there have been 10 M 5 and greater earthquakes. Four of these have had magnitudes larger than M 6, including an M 6.3 earthquake ~10 hours prior to this August 19, 14:56 UTC earthquake. Both the August 19, 2018 event, and the August 5, 2018 earthquake, were M 6.9.

Event Details

Event Map

Tectonic Summary

Tectonic Summary

Historical Magnitude 4.5+ (100km)