Mechanisms Behind Aseismic Slip Pulsing on the Northeast Japan Subduction Zone: Insights from Time-Dependent Modelling of GPS and Repeating Earthquakes

Aseismic slip is a key component of the moment budget of subduction zones, in some cases it accounts for more than half the total slip. It has been suggested that the propagation of aseismic slip as slow slip sequences precedes large megathrust events, such as the Tohoku-oki earthquake, M_w 9.0, 11^th March 2011. Complex patterns of aseismic slip in space and time have been observed using either GPS data or repeating earthquakes, yet the physical mechanisms driving this behaviour are still poorly understood and have implications for seismic hazard assessment. Therefore, in this study both these datasets are combined in a time-dependent inversion, to investigate the spatiotemporal distribution of aseismic slip in northeast Japan, during the interseismic period of 21^st March 1996 to 29^th October 2003.

Daily displacements recorded by ~800 GEONET stations are associated with slip on the subducting plate interface using a time-dependent inverse modelling scheme. The bounds on slip are defined by the location and slip of repeating earthquakes. Initial results show significant slip heterogeneity along-strike and down-dip. The majority of the upper part of the plate, at depths of 5 – 30 km, is locked except for two patches at 37° and 40° latitude. At 30 – 70 km depth there are three regions of aseismic slip, which exhibit periodic acceleration and deceleration throughout the interseismic period; in the south near Tokyo, offshore Sendai, and the northernmost part of Honshu at 41.5° latitude. Within these regions, patches with an average velocity of 7 to 8 cm/yr show large fluctuations in velocity, which in some cases precede the smaller variations observed for patches with an average velocity of 2 to 4 cm/yr. These observations are discussed in terms of the changes in effective normal stress, and the heterogeneous frictional properties of the northeast Japan subduction zone.