Radiated seismic energy from the 2021 M-L 5.8 and M-L 6.2 Shoufeng (Hualien), Taiwan, earthquakes and their aftershocks

Abstract

We calculated radiated seismic energy (E-S), seismic moment (M-0), and moment magnitude (M-W) and then determined the E-S-M-L, E-S-M-0, M-0-M-L, and M-W-M-L relations for the 2021 Shoufeng earthquake sequence (2.5 < M-L < 6.3), where M-L is the local magnitude. Notably, a crossover magnitude was detected as M-L = 4.0 for the earthquake sequence. For M-L < 4.0, we obtained logM(0) proportional to M-L, M-W proportional to 0.67M(L), and a low E-S/M-0, indicating a low average stress drop; for M-L > 4.0, logM(0) proportional to 0.67M(L), M-W proportional to M-L, and a high E-S/M-0 were present, and then there was a high average stress drop. These derived relations implied that source duration (T) is independent of M-0 for M-L < 4.0. Moreover, the M-0 proportional to T-3 relation seemed able to interpret those relations for M-L > 4.0. Nevertheless, the E-S-M-L relation remains logE(S) proportional to 2.0M(L) for 2.5 < M-L < 6.3. From this study, the derived relations could not predict the source parameters for M-L > 6.3 events. This might indicate that M-L saturates beyond M-L 6.3. Through such analyses, we not only established the relations among source parameters but also elucidated the basic physics of the earthquake sequence.