Seismic Hazards of the Bengkulu Region, Sumatra

Bengkulu lies on the southwestern coast of Sumatra, Indonesia, directly above the Sunda megathrust where the Indo-Australian plate subducts beneath the Eurasian plate at rates of 5–6 cm per year. This convergent margin produces frequent large-magnitude earthquakes and has shaped both the regional geology and historical disaster record.

The subsurface structure consists of an accretionary prism, forearc basin, and volcanic arc. The megathrust interface dips gently at 10–15° near the trench and steepens to 30–40° beneath the coast, allowing rupture over broad areas. Bengkulu sits roughly 100–130 km landward of the trench, placing it within the zone of strong shaking for interface events.

On 12 September 2007 at 11:10 local time, an Mw 8.4 earthquake struck 122 km southwest of Bengkulu at a focal depth of 34 km. The rupture initiated near the deeper part of the seismogenic zone and propagated up-dip and along-strike, generating a modest tsunami that reached heights of 1–2 m along the coast. Ground accelerations exceeded 0.3 g in Bengkulu city, damaging unreinforced masonry and triggering landslides in the Barisan Mountains.

Seven years earlier, on 4 June 2000, an Mw 7.9 event occurred 103 km south of Bengkulu. That quake ruptured a shallower portion of the same megathrust segment and was followed by an energetic aftershock sequence lasting several weeks. Both events released strain accumulated since the great 1833 and 1861 earthquakes that had previously ruptured adjacent portions of the Sunda interface.

Paleoseismic and coral-microatoll records indicate recurrence intervals of 200–250 years for Mw ≥ 8.5 earthquakes along this stretch of the margin. GPS measurements show current locking depths of 20–40 km, consistent with the 2007 hypocenter and implying continued elastic strain buildup. The presence of the Mentawai Islands offshore further focuses slip deficit, increasing the likelihood of future multi-segment ruptures.

Local geology amplifies hazard. Thick Quaternary alluvial fans and deltaic sediments beneath Bengkulu city exhibit low shear-wave velocities, producing site amplification factors of 1.5–2.0. Steep volcanic slopes in the hinterland are prone to co-seismic landslides, while coastal subsidence during megathrust events exacerbates inundation risk.

Modern monitoring by Indonesia’s BMKG and international networks has improved early-warning capability since 2007, yet rapid urbanization along the coast continues to raise exposure. Mitigation measures now emphasize tsunami vertical-evacuation structures, updated building codes, and community education programs informed by the 2000 and 2007 lessons.

References

USGS Earthquake Catalog (event parameters 2000 and 2007)
Global Centroid Moment Tensor Project
Sieh et al., 2008, Nature Geoscience (coral records)
Bradley et al., 2019, Geophysical Research Letters (GPS locking)