Fourteen‐Year Acceleration Along the Japan Trench
Résumé
Abstract An acceleration of the background seismicity and a shortening of the slow slip events recurrence intervals in the Boso peninsula (Japan) suggest a slow unlocking of the Philippine Sea‐North America (PHS‐NAM) subduction interface from 1990 to 2011. Motivated by these previous observations, we used GPS (Global Positioning System) time series to study the 1997–2011 evolution of interface locking offshore Honshu with a specific focus on the Kanto region. We newly processed the GPS data in double difference and analyzed them with a trajectory model that accounts for seismic and aseismic variations, and that includes an inter‐seismic acceleration term. We inverted the surface acceleration obtained, on both the Pacific‐North America (PAC‐NAM) and the PHS‐NAM interfaces. The inverted slip rate changes over time compare well with previous studies: we infer slip deceleration between – N and acceleration between – N (PAC‐NAM), with a maximum amplitude of 1.75 mm/ corresponding to an equivalent geodetic locking change of 0.3 over the 14 years. More notably, our analysis reveals a novel and robust slip acceleration South of N that we interpret as an unlocking of the PAC‐NAM interface. It is located noticeably far from the 2011 Tohoku earthquake rupture and is therefore unlikely connected to it. The slip acceleration inverted from the surface displacements is comparable to the changes observed in the seismicity rate. Our results further demonstrate that inter‐seismic slip rate can significantly evolve over years to decades, and suggest a simple relationship between the seismicity and the slip on the subduction interface.
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