** Progress in Earth and Planetary Science is the official journal of the Japan Geoscience Union, published in collaboration with its society members.
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Research
Solid earth sciences
202406202406
Quantifying the expansion rates of aftershock zones for magnitude-7 class earthquakes around the Japanese archipelago
Yuta Mitsui, Yuya Utagawać Ayaka MiyamotoYuta Mitsui, Yuya Utagawa, Ayaka Miyamoto
Aftershocks, Expansion rates, Seismic b value, Interplate earthquakes, Intraplate earthquakes, Stress condition
(Left) Magnitude 7 class earthquakes targeted in this study. (Center) Example of the estimation of the expansion rate of the aftershock zone. (Right) Relationship between the expansion rate of the aftershock zone and the b value.
Earthquakes (mainshocks) trigger sequences of aftershocks, the frequency of which diminishes following a power-law decay, while the spatial domain of these aftershocks extends logarithmically over time. The delineation of the aftershock zone can be modulated by variables beyond the magnitude of the mainshock, encompassing the location of the fault (whether the fault is at a plate boundary), the depth at which the event occurs, and the prevailing local stress conditions. Here, we evaluate the expansion rate of aftershock zones by analyzing earthquakes of magnitude-7 class in the vicinity of the Japanese archipelago. Prior studies have offered approximate assessments of expansion rates; however, our approach involves the utilization of a straightforward algorithm for the automated estimation of this metric, facilitating the compilation of a catalog. Across the dataset, no pronounced correlations were discerned between the expansion rate and other examined parameters. Yet, an inverse relationship is identified between the expansion rate and the b value of aftershocks for mainshocks occurring at plate boundaries. This observation suggests that the expansion rate of aftershock zones predominantly mirrors the stress field following the mainshock. Such a pattern is not detected in mainshocks occurring within the plate's interior. While the expansion rate of aftershock zones is likely influenced by various factors, aftershock zones may expand more rapidly with higher differential stress in areas surrounding hypocenters of major interplate earthquakes of magnitude 8 or 9.
