Lombok Island forms part of Indonesia’s Lesser Sunda archipelago, positioned above the convergent boundary where the Indo-Australian Plate subducts beneath the Eurasian Plate along the Sunda Trench. This tectonic setting produces frequent moderate-to-large earthquakes and occasional earthquake swarms. The island’s geology comprises Quaternary volcanic deposits overlying older sedimentary and metamorphic basement rocks, with active faults such as the Lombok Strait Fault contributing to regional strain accumulation.
Seismic swarms in this environment typically reflect fluid migration or afterslip along subsidiary faults rather than a single mainshock-rupture sequence. Historical records since 1 January 2000 indicate only two such swarms have occurred in the monitored area, with the first registered in 2018.
Swarm PS20180819.2 began at 04:06 on 19 August 2018 and concluded at 01:30 on 20 August 2018, centered 25 km NNW of Labuan Lombok. Over 21 hours and 24 minutes the sequence comprised eleven events. Magnitudes ranged from 5.0 to 6.9, with focal depths predominantly near 10 km except for two deeper shocks at 16 km and 21 km.
The sequence opened with a 5.4 event at 04:06, followed 4 minutes later by the largest shock of 6.3 at 16 km depth. A 6.9 event at 21 km depth occurred later that afternoon at 14:56, triggering a rapid succession of five additional events between 15:04 and 16:38, all at 10 km depth and magnitudes 5.0–5.8. Three further shocks of 5.1–5.2 completed the swarm by early the following morning, the final event recorded at 6 km depth.
This temporal clustering, with no single dominant mainshock and a rapid decay in activity within 24 hours, is characteristic of swarm behavior in subduction-related volcanic arcs. Depths concentrated in the upper crust suggest activation of shallow crustal faults above the plate interface.
USGS Earthquake Catalog
Global CMT Catalog
Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) annual seismicity reports