Discrepancy between local and moment magnitudes attributed to radiated seismic energy

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.