Upper-Scale Structural Evaluation of the Forearc Weak Suture Zone and the Pacific Plate Interface — An Analysis Based on Precursor Data (No.1778) —

Yoshikazu Hiraoka

This study reinterprets precursor data (No.1778) published by the Yatsugatake South Base Observatory using the framework of seismology and tectonics, in order to evaluate the upper-scale structural potential of the forearc weak suture zone and the Pacific Plate interface beneath the Kanto Basin. The precursor data are not used for earthquake prediction; instead, they are treated as aggregate indicators of crustal stress release and incorporated as input parameters for structural modeling. Thirteen magnitude-equivalent values reported in the official bulletin were converted into rupture length and rupture area using empirical scaling laws (Matsuda, 1975; Wells & Coppersmith, 1994). The total rupture area reaches approximately 5.6×10⁴ km², corresponding to a moment magnitude of Mw 8.7. Although the forearc weak suture zone alone cannot account for such a rupture area, a composite rupture model involving both the suture zone and the low-angle Pacific Plate interface provides a structurally plausible scenario. This study does not aim to predict any specific earthquake. Rather, it evaluates the structural upper limits implied by precursor-derived aggregate quantities and examines the tectonic conditions under which large-scale rupture could theoretically occur.