Characteristics of Far-Field Surface Ruptures Caused by Two Recent Strike-Slip Earthquakes: Insights into Fault Displacement Prediction

Abstract Probabilistic fault displacement hazard analysis (PFDHA) is essential for assessing the safety of critical facilities, both near the fault and in far-field areas (more than 3 km away). However, the conventional equations for strike-slip faults have limitations in reliably assessing far-field fault displacement hazards because of incomplete far-field surface rupture data, nonunique surface rupture classifications, and a failure to consider the triggering of pre-existing active faults. Recently, surface ruptures caused by two strike-slip earthquakes, the 2016 Kumamoto and the 2019 Ridgecrest earthquakes, have been intensively investigated based on remote sensing techniques and field surveys. In this study, we used these surface rupture data to revise conventional equations for strike-slip faults to capture characteristics of the attenuation of fault displacement in the far field as a function of distance from the subsurface earthquake source fault, without classifying surface ruptures as principal or distributed. In the revised equations, we introduced a new term related to the distance from the mapped, pre-existing active faults, and we considered reactivation of those faults and undetected faults around them. Our revised equations using individual earthquake datasets showed that both the conditional probability of surface rupture and the conditional probability of exceedance on the surface ruptures increase markedly near mapped, pre-existing active faults. The proximity to mapped, pre-existing active faults is more important than the conventional approach in far-field fault displacement prediction. The fault displacement characteristics seemed to differ at each tectonic setting. Revised equations developed in other regions where robust and extensive surface rupture data are available could be utilized for PFDHA at far-field sites with similar tectonic settings. The insights gained from this study might contribute to the improvement of PFDHA.