The 1977 Sumba earthquake series: Evidence for Slab pull force acting at a subduction zone

Abstract

The great 1977 Sumba earthquake occurred at the eastern Sunda trench, just west of the collision of Australian continental lithosphere with the island arc. The length of the aftershock zone of this normal‐faulting earthquake is about 200 km. Aftershocks are concentrated 65–115 km east of the main shock epicenter, with very few aftershocks in a 50‐km‐long segment that spans the main shock epicenter. Relocated hypocenters and focal mechanism data are consistent with normal faulting throughout the upper 28 km of the oceanic lithosphere. There is no evidence for thrust faulting of the deeper aftershocks. These data imply that the neutral bending surface must be at least 35–40 km deep. A second aftershock zone, about 180 km northwest of the main shock, became active immediately following the main shock, but events were concentrated during days 50–52. This zone is a 70‐km‐long lineation that trends toward the main shock epicenter and reflects right‐lateral, strike‐slip faulting within the subducted oceanic plate. Seismicity exists to a depth of about 650 km in the very old plate beneath the Sunda‐Banda arc, and that plate's negative buoyancy causes very large slab pull forces. Great interface thrust earthquakes are absent at the Sumba region, and slab pull forces are inferred to have partially decoupled the subducted plate from the overriding plate. This decoupling permits slab pull stresses to be guided updip to the region of the Sumba main shock. Such shallow‐acting slab pull provides a bending moment at the trench and explains the deformation and timing observed for the entire Sumba earthquake series. In this model, slab pull forces stretch the subducted plate until the increasing stresses at the shallow subduction zone lead to a subduction zone earthquake. Postseismically, the released oceanic plate undergoes a pulse of downdip strain, returning the plate to a less extended state. The moment of this downdip plate motion could exceed the seismic moment of the main shock.