[S-CG45] Science of slow-to-fast earthquakes

[SCG45-48] Hierarchical Repeating Earthquakes with Strong Downward Directivity

*Dawei Gao^1,2, Satoshi Ide² (1.Central South University, 2.The University of Tokyo)

Keywords:Hierarchical Repeating Earthquakes, Hypocenter-repeaters , Centroid-repeaters, Hierarchical Structure, Strong Downward Directivity

Any two earthquakes can be similar at the beginning or the end of their rupture process. This similarities yield hypocenter- and centroid-repeaters as two endmembers of complex but similar rupture processes, although the latter has been usually considered as repeating earthquakes. Despite the critical role of both types of repeaters in advancing our understanding of earthquake physics, their rupture characteristics and hosting fault structures remain poorly understood. Here, we present the first systematic investigation of the detailed rupture features of such repeaters that occur around and within various hierarchical structures, based on 19 years (2004-2022) of high-quality waveform data from 150 subduction-type earthquake sequences (4,127 events) along the Tohoku-Hokkaido megathrust. Through state-of-the-art hypocenter-centroid relative locating and rigorous repeater identification, we analyze 43 well-constrained sequences (2,298 events) beneath onshore-nearshore regions, primarily located downdip of the seismogenic zone, revealing four key findings: (1) Clustered rupture initiation and termination: 79% and 74% of sequences show statistically significant hypocenter and centroid clustering at the 95% confidence level, respectively, highlighting the deterministic influence of local fault heterogeneity on rupture initiation and termination; (2) Similarity in hierarchical rupture growth: 58% of sequences exhibit co-located large (M>=4.5) and small (M=<4.0) earthquake pairs within ~100 m proximity, forming 247 hypocenter-repeater pairs, implying widespread hierarchical fault structures along the plate interface and probabilistically limited predictability of earthquake size; (3) Prevalent centroid-repeaters but limited rupture repeatability: A remarkably high percentage (over 60%) of M >= 3.2 earthquakes belong to 148 families of centroid-repeaters, yet fewer than 30% of these families fully replicate their rupture process, revealing hidden complexity in “identical” repeaters and their previously unrecognized limited predictability; (4) Downward directivity: 44% of events with resolvable hypocenter-centroid offsets exhibit pronounced downward rupture propagation, potentially amplifying seismic shaking and elevating hazard levels in coastal cities. Taken together, our study unifies and generalizes the concepts of hierarchical repeaters, encompassing both hypocenter- and centroid-repeaters, reveals the significance of small-scale hierarchical structures in controlling earthquake repeatability, and introduces a hierarchical rupture framework for earthquake early warning and probabilistic forecasting.