We investigated the factors contributing to variations in local magnitude (ML) among earthquakes of comparable moment magnitude (Mw). Specifically, we examined how radiated energy accounts for the difference between Mw and Mjma (∆M = Mw − Mjma), where Mjma is a form of ML routinely estimated by the Japan Meteorological Agency (JMA). To this end, we estimated the radiated energy (ER), seismic moment (M0), and moment-scaled radiated energy (eR = ER/M0) for small earthquakes (2.0 ≤ Mjma ≤ 3.0) in a highly active earthquake cluster near the Fukushima–Ibaraki border, northeastern Japan. We first determined source spectra by removing the site and path effects from the observed S-wave spectra and then estimated ER, M0, and eR for 9,538 earthquakes. The estimated eR was distributed around 10−5, comparable to previous estimates for crustal earthquakes, but varied by one order of magnitude, suggesting diversity of earthquake radiation processes. Mw was systematically larger than Mjma in the analyzed magnitude range (2.0 ≤ Mjma ≤ 3.0), consistent with previous studies, and likely reflecting the site and path effects. Notably, Mjma showed a stronger agreement with the energy magnitude (ME), derived from ER, than with Mw, suggesting that Mjma is more strongly controlled by ER than by M0. Consistent with this interpretation, ∆M systematically increased with decreasing eR among earthquakes with similar Mw. These trends persisted even after removing the depth and Mw dependence of eR. These results suggest that the diversity of Mjma at a given Mw reflects variability in dynamic rupture processes. Graphical abstract

Discrepancy between local and moment magnitudes attributed to radiated seismic energy

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2026年7月1日

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Masaki Orimo,
Keisuke Yoshida,
Toru Matsuzawa,
Akira Hasegawa,
Mare Yamamoto

摘要

We investigated the factors contributing to variations in local magnitude (M_L) among earthquakes of comparable moment magnitude (M_w). Specifically, we examined how radiated energy accounts for the difference between M_w and M_jma (∆M = M_w − M_jma), where M_jma is a form of M_L routinely estimated by the Japan Meteorological Agency (JMA). To this end, we estimated the radiated energy (E_R), seismic moment (M₀), and moment-scaled radiated energy (e_R = E_R/M₀) for small earthquakes (2.0 ≤ M_jma ≤ 3.0) in a highly active earthquake cluster near the Fukushima–Ibaraki border, northeastern Japan. We first determined source spectra by removing the site and path effects from the observed S-wave spectra and then estimated E_R, M₀, and e_R for 9,538 earthquakes. The estimated e_R was distributed around 10⁻⁵, comparable to previous estimates for crustal earthquakes, but varied by one order of magnitude, suggesting diversity of earthquake radiation processes. M_w was systematically larger than M_jma in the analyzed magnitude range (2.0 ≤ M_jma ≤ 3.0), consistent with previous studies, and likely reflecting the site and path effects. Notably, M_jma showed a stronger agreement with the energy magnitude (M_E), derived from E_R, than with M_w, suggesting that M_jma is more strongly controlled by E_R than by M₀. Consistent with this interpretation, ∆M systematically increased with decreasing e_R among earthquakes with similar M_w. These trends persisted even after removing the depth and M_w dependence of e_R. These results suggest that the diversity of M_jma at a given M_w reflects variability in dynamic rupture processes.

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