Deep-Seated Seismicity in the Flores Sea, Indonesia

The Flores Sea region of eastern Indonesia lies within a tectonically complex portion of the Banda Arc, where the Indo-Australian plate subducts beneath the Eurasian plate. This subduction produces a steeply dipping Wadati-Benioff zone capable of generating earthquakes at depths exceeding 500 km. The area has a well-documented history of both intermediate and deep-focus events, reflecting ongoing plate convergence at rates of approximately 70 mm per year. On 27 February 2015 at 13:45 local time, a magnitude 7.0 earthquake occurred 150 km NNE of Maumere, Flores, at a focal depth of 552 km. The extreme depth placed the event well within the mantle portion of the subducting slab, resulting in minimal surface shaking across the region. No significant damage or casualties were reported on nearby islands. A subsequent strong earthquake of magnitude 7.3 struck the Flores Sea on 14 December 2021, approximately 48 km from the 2015 epicenter. Like its predecessor, the 2021 event originated at substantial depth and produced limited felt effects onshore. Both earthquakes illustrate the persistent deep seismicity that characterizes the Flores segment of the subduction zone. Historical records indicate that the Flores Sea has experienced recurrent deep events since instrumental monitoring began. These earthquakes typically occur along the steeply inclined seismic zone that extends from the surface trench to depths greater than 600 km. The geometry of the slab in this area allows for brittle failure at mantle pressures and temperatures, a phenomenon observed in only a few subduction zones worldwide. The combination of rapid convergence and slab geometry in the Flores region sustains the potential for future deep-focus earthquakes. Monitoring by regional seismic networks continues to track activity along this segment of the arc, providing data that improve understanding of mantle dynamics beneath eastern Indonesia.

References

USGS Earthquake Catalog (events 2015-02-27 and 2021-12-14) Global CMT Project focal mechanism solutions Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) regional reports