Transient crustal movement in the northern Izu–Bonin arc starting in 2004: A large slow slip event or a slow back-arc rifting event?

• GNSS stations on Izu Islands, Japan show eastward movement which transiently accelerated in 2004.
• We compare three different mechanisms possibly responsible for this transient accelerated movement.
• Postseismic deformation, temporary activation of the back-arc rifting, and large slow slip event are considered as the possible cause of the acceleration.
• Result indicates that the first possibility is less likely, and the second and the third is more likely.
• Current evidence support the third possibility to be more likely responsible to the acceleration.

The Izu–Bonin arc lies along the convergent boundary where the Pacific Plate subducts beneath the Philippine Sea Plate. Horizontal velocities of continuous Global Navigation Satellite System stations on the Izu Islands move eastward by up to ~ 1 cm/year relative to the stable part of the Philippine Sea Plate suggesting active back-arc rifting behind the northern part of the arc. Here, we report that such eastward movements transiently accelerated in the middle of 2004 resulting in ~ 3 cm extra movements in 3 years. We compare three different mechanisms possibly responsible for this transient movement, i.e. (1) postseismic movement of the 2004 September earthquake sequence off the Kii Peninsula far to the west, (2) a temporary activation of the back-arc rifting to the west dynamically triggered by seismic waves from a nearby earthquake, and (3) a large slow slip event in the Izu–Bonin Trench to the east. By comparing crustal movements in different regions, the first possibility can be shown unlikely. It is difficult to rule out the second possibility, but current evidence support the third possibility, i.e. a large slow slip event with moment magnitude of ~ 7.5 may have occurred there.