Early Afterslip Deformation Captured by Sub-Daily Kinematic GPS following the 2010 Mw 7.8 Mentawai Earthquake

On 25 October 2010, an Mw 7.8 earthquake occurred west of the Mentawai Islands, releasing energy from a shallow offshore fault and generating complex ground deformation patterns. Although surface deformation has been monitored for over a decade, the precise geometry of the slipping zone remains unresolved. This study investigates the event using sub-daily kinematic GPS observations from the Sumatra GPS Array (SuGAr) to analyze coseismic and early postseismic deformation following the rupture. The resulting displacement field reveals limited horizontal motion of approximately 20–25 cm and vertical subsidence of about 4–6 cm near the islands, suggesting that most slip was concentrated offshore toward the trench. In the subsequent days, the GPS time series displays a gradual decay in motion, which can be modeled with a logarithmic function and characteristic timescales of roughly five days. This transient response is interpreted as an early afterslip evolving downdip of the main rupture area. These findings underscore the heterogeneous nature of the plate interface, where seismic and aseismic processes occur concurrently over short timescales. Future work will estimate early afterslip using a time-dependent inversion model to further characterize the rupturing zone and clarify the transition between seismic and aseismic deformation.