The Lombok region of Indonesia lies along the Sunda Arc, a tectonically active margin formed by the northward subduction of the Indo-Australian Plate beneath the Eurasian Plate. Convergence rates average 6–7 cm per year, generating frequent shallow to intermediate-depth earthquakes along the megathrust and associated back-arc structures such as the Flores Thrust. The island’s geology also reflects Quaternary volcanism, notably at Mount Rinjani, whose magma system interacts with regional faulting.
Between 22:47 UTC on 28 July 2018 and 02:26 UTC on 30 July 2018, a swarm of 28 earthquakes was recorded in this setting. The sequence began with a magnitude 6.4 event at 14 km depth. Within the first hour, five additional shocks between magnitude 4.7 and 5.4 occurred, all at 10 km depth. Subsequent activity remained shallow, with the majority of events nucleating between 5 km and 13 km. Magnitudes declined rapidly after the initial cluster, although several magnitude 4.0–4.8 events continued through the night of 28–29 July.
Over the full 27-hour 38-minute duration, the swarm released energy through a dense series of small-to-moderate events rather than a single dominant mainshock. Depths stayed consistently crustal, consistent with faulting above the subduction interface. The largest aftershock reached magnitude 5.3 at 01:50 UTC on 29 July, again at 10 km. By the final recorded event—a magnitude 3.1 shock at 5 km—the sequence had produced 28 locatable earthquakes, illustrating the typical swarm pattern of rapid onset, high event rate, and gradual decay without a clear aftershock sequence governed by Omori’s law.
Such swarms in the Lombok area are commonly linked to fluid migration along pre-existing faults or to transient stress changes induced by nearby volcanic processes at Rinjani. The 2018 swarm preceded the larger 5 August magnitude 6.9 mainshock by roughly one week, suggesting it may have contributed to progressive loading of the Flores Thrust system.
SeismoSight internal classification for Swarm S20180729.2
USGS Earthquake Catalog (regional tectonic framework)
Global CMT Project (subduction zone parameters)