The 2023 M7.6 Deep Earthquake Near Tonga: Seismic Context and Regional Geology

On 10 May 2023 at 16:02 UTC, a magnitude 7.6 earthquake struck 82 km west-northwest of Hihifo, Tonga, at a focal depth of 210 km. This event stands as the sole strong earthquake (M7.0 or greater) recorded in the immediate vicinity since 1 January 2000. Its substantial depth placed the hypocenter well within the subducting slab, limiting felt intensities at the surface and reducing the likelihood of significant damage or tsunami generation. Tonga occupies a tectonically dynamic position along the Tonga subduction zone, where the Pacific Plate descends beneath the Tonga microplate at rates exceeding 15 cm per year. This rapid convergence produces one of Earth’s most active seismic belts, with earthquakes occurring across a wide range of depths from shallow crustal events to those exceeding 600 km. The 210 km depth of the May 2023 event classifies it as an intermediate-focus earthquake, typical of the steeply dipping Wadati-Benioff zone that characterizes the region. The Tonga-Kermadec arc system has evolved over millions of years through continuous subduction, resulting in a narrow forearc, active volcanism, and back-arc spreading in the Lau Basin. Deep seismicity here often exhibits complex rupture patterns influenced by phase transitions in the slab, such as the olivine-spinel transformation near 400 km. Historical records document recurrent large events, yet the 2023 quake remains the only M7.6 recorded within 100 km of Hihifo since systematic modern monitoring began. Because of its depth, the earthquake produced minimal ground shaking onshore. Regional seismic networks recorded aftershocks clustered near the mainshock, consistent with stress release inside the cold slab core. No significant tsunami was observed, aligning with the expectation that deep events rarely displace the water column sufficiently to generate hazardous waves. Ongoing monitoring by international agencies continues to refine models of slab geometry and stress accumulation beneath Tonga. The 2023 event supplies additional data on the mechanical behavior of the subducting Pacific Plate at intermediate depths, contributing to improved understanding of seismic hazards along this rapidly converging margin.

References

USGS Earthquake Catalog (event details and magnitude) Global CMT Project (focal mechanism and depth verification) Tonga Geological Services (regional seismic context)