The 2021 Timor-Leste Earthquake: Tectonic Setting and Regional Implications

On 29 December 2021 at 18:25 UTC, a magnitude 7.3 earthquake struck approximately 125 km north-northeast of Lospalos, Timor-Leste. The event originated at a depth of 165.4 km, placing it well within the mantle portion of the subducting slab. This intermediate-depth earthquake produced strong shaking across eastern Timor-Leste and parts of neighboring Indonesian islands but resulted in limited reported damage, consistent with its depth and distance from population centers. Timor-Leste occupies a geologically complex zone at the collision boundary between the Australian continental margin and the Banda Arc. The island sits above the northward-subducting Australian plate, where convergence has produced the Timor Trough to the south and an active fold-and-thrust belt onshore. The 165 km focal depth indicates rupture within the steeply dipping slab beneath the arc rather than on the shallow megathrust, a common occurrence in this region where the subducting lithosphere remains seismically active to depths exceeding 200 km. Since 1 January 2000, the M7.3 event of 29 December 2021 stands as the sole magnitude 7.3 or greater earthquake recorded within 200 km of Lospalos. Earlier instrumental records show that large events in the broader Banda Arc typically cluster either along the Timor Trough or within the deeper portions of the subducting slab. The 2021 earthquake therefore highlights the persistent seismic hazard posed by intermediate-depth seismicity even in the absence of recent great shallow thrust events. The high seismic activity reflects ongoing oblique convergence at roughly 70 mm per year. Crustal shortening is accommodated partly by thrusting within the accretionary complex and partly by strike-slip faulting along the island’s interior. Deep earthquakes such as the 2021 event provide important constraints on slab geometry and thermal structure, helping refine models of subduction dynamics in the region. Long-term geological evidence, including uplifted coral terraces and deformed sedimentary sequences, documents repeated episodes of rapid uplift throughout the Quaternary. These observations align with geodetic measurements that continue to record active convergence across the Timor-Leste region. Although the 2021 earthquake did not generate a tsunami, the potential for shallow thrust events along the Timor Trough remains a key consideration for regional hazard assessment.

References

USGS Earthquake Catalog (event page for 2021-12-29 M7.3 Timor-Leste) Global CMT catalog (moment tensor solution for 2021-12-29 event) Hamilton, W. (1979). Tectonics of the Indonesian Region. USGS Professional Paper 1078.