Significant afterslip contribution to postseismic deformation in Sado Island following the 2024 Noto Peninsula earthquake: insights from two dense GNSS observation networks

Abstract This study investigated postseismic deformation resulting from the 2024 Noto Peninsula earthquake on January 1. Particular focus was placed on the displacement observed in Sado Island, which is located several tens of kilometers east of the eastern end of the fault, where the effects of the complex rupture processes of the mainshock are relatively limited. Using dense Global Navigation Satellite System (GNSS) data, including the Geospatial Information Authority of Japan and SoftBank Corp. sites, postseismic horizontal displacement over 5 months following the mainshock was estimated to be directed northwestward. To explore the driving mechanism of the postseismic deformation, we modeled Maxwell viscoelastic relaxation. However, this mechanism could not account for the observed horizontal displacement direction, suggesting that factors other than viscoelastic relaxation, such as the contribution of afterslip, dominated in Sado Island. A model incorporating afterslip on the downdip extension of the coseismic fault off the western side of Sado Island was developed in addition to viscoelastic relaxation. The assumed afterslip fault largely explained the northwestward displacement. However, the displacement predicted solely from the downdip extension was insufficient to fully explain the observed deformation in the central part of Sado Island. By including afterslip on the northern extension of the fault, the model more accurately reproduced the GNSS data. The afterslip was found to occur complementarily to the coseismic slip. In contrast, with slight differences in mechanical conditions, if the assumed afterslip had occurred simultaneously with the coseismic slip, it could have generated a significant tsunami. This finding underscores the potential for future tsunami hazards along the active fault zones in the Sea of Japan. Graphical Abstract