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Scaling Relations of Earthquakes That Occurred in the Upper Part of the Philippine Sea Plate beneath the Kanto Region, Japan, Estimated by Means of Borehole Recordings

Acceleration source spectra of SH waves for 94 interplate and 74 intraplate earthquakes with a moment range of M 0 = 10 13 –10 18 (N m) are investigated using 634 strong-motion records obtained at 27 borehole and 7 surface hard-rock stations installed in the Kanto region, central Japan. Different scaling relations control the estimated acceleration source spectra for interplate and intraplate events. The estimated acceleration source spectra of interplate events are similar to the spectra expected from the barrier model, and those of intraplate events show a characteristic bump around the corner frequency. The amplitude of acceleration source spectrum of interplate events is, on the whole, larger than that of intraplate events for the same seismic moment. Seismic moment, M 0 , corner frequency, f c , and seismic energy, E , are evaluated from the estimated acceleration source spectrum for each earthquake. The seismic moment, M 0 , is nearly proportional to \(f_{c}^{-3}\) . The ratio of seismic energy to seismic moment is E/M 0 = 1.15–12.9 × 10 –5 for all events without a significant difference between interplate and intraplate earthquakes. This means that the estimated velocity source spectra for interplate and intraplate events with the same seismic moment are nearly identical. The dependence of apparent stress, σ A = E/M 0 , on seismic moment is weak and depends on tectonic setting. Apparent stress drop, 2σ A , is almost proportional to the Brune9s stress drop.