Magnitude 7.1 Deep Earthquake Recorded Northeast of Gili Air, Indonesia

On 28 August 2023 at 19:55 local time, a magnitude 7.1 earthquake struck 180 km north-northeast of Gili Air, Indonesia, at a focal depth of 500 km. The event originated within the subducted slab of the Indo-Australian Plate beneath the Eurasian Plate, a tectonic setting that characterizes much of the Indonesian archipelago. The region forms part of the Sunda subduction zone, where the Indo-Australian Plate converges with the Eurasian Plate at rates of approximately 6–7 cm per year. This convergence produces both shallow crustal earthquakes and deeper events within the Wadati-Benioff zone, extending to depths exceeding 500 km. Deep-focus earthquakes such as this one typically result from phase transitions and dehydration reactions in the descending oceanic lithosphere rather than brittle failure near the surface. Seismic activity in the broader Lombok and northern Bali region has been documented throughout the instrumental record. While many destructive events occur at shallow depths along the megathrust, intermediate- and deep-focus earthquakes are also common due to the steep geometry of the subducting slab. The 28 August 2023 event aligns with this pattern, occurring well below the seismogenic zone that produces most damaging ground shaking. No foreshocks or aftershocks were associated with this particular earthquake in the provided record of strong events since 2000. Its great depth limited felt intensities at the surface, consistent with the attenuation of seismic waves traveling through the mantle.

Tectonic and Historical Context

Indonesia experiences some of the world’s highest rates of seismicity because of its position at the junction of four major plates. The ongoing subduction has shaped the volcanic arcs of Java, Bali, and Lombok while generating both megathrust and intraslab earthquakes. Historical catalogs document great earthquakes along the Sunda Trench, yet deep events at 400–600 km depth occur regularly without producing widespread surface damage.

References

USGS Earthquake Catalog Global CMT Project Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG)