Slow slip and aseismic deformation episodes associated with the subducting Pacific plate offshore Japan, revealed by changes in seismicity

Aseismic phenomena, including slow slip, can alter the surrounding seismicity. We here investigate how seismicity can be used in order to reveal episodes of aseismic deformation. An objective method is proposed that accounts for both earthquake interactions and transient loading. Applying it to the 1990-2011 (pre-Tohoku) seismicity of the Japan subduction zone, we find several significant instances of aseismic transients. Small-scale and short-duration transients are favored updip of the subducting plate. Large-scale transients are mostly observed offshore Ibaraki prefecture, in a partly decoupled zone that extends downdip. The four most intense of such transients have occurred periodically every 5.9years and are likely due to slow-slip episodes. Other aseismic phenomena, including possible fluid intrusion in the outer rise, are also detected. Finally, the seismicity in January and February 2011, close to the epicenter of the megathrust Tohoku earthquake, is found to be due to aseismic loading, confirming previous studies, although this transient is only one among others and is not the most intense nor the most significant for the 21year long period studied here.