Temporal changes in surface afterslip along the 2016 Kumamoto earthquake rupture revealed by repeated field surveys

Surface afterslip can persist over several years, with displacements reaching tens of centimeters. Such deformations can damage structures, making it crucial to understand their temporal changes for effective post-earthquake reconstruction. Following the 2016 Kumamoto earthquake sequence, surface afterslip was observed at seven sites along a 5.7 km section of the fault. This segment is the junction between two adjacent active faults, where surface ruptures occurred during both the foreshock and the mainshock. Based on repeated field measurements, the surface afterslip persisted for 6–7 years following the mainshock. The cumulative displacement amounted to 10–40 cm. Compared with previous studies, its persistence is relatively long, whereas the displacement is comparable to reported values. The postseismic and coseismic displacements were nearly identical at the southern site of the fault, suggesting that distinguishing between coseismic and postseismic displacements in paleoseismic trenching or geomorphic offset studies can be challenging. Surface afterslip, which can persist for several years, may cause further damage to surface and subsurface infrastructure that has been repaired or restored following earthquakes. When planning post-earthquake restoration, it is necessary to determine whether surface afterslip is occurring and to estimate when it will cease.