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

Abstract

AbstractWe calculated radiated seismic energy (ES), seismic moment (M0), and moment magnitude (MW) and then determined the ES–ML, ES–M0, M0–ML, and MW–ML relations for the 2021 Shoufeng earthquake sequence (2.5 < ML < 6.3), where ML is the local magnitude. Notably, a crossover magnitude was detected as ML = 4.0 for the earthquake sequence. For ML < 4.0, we obtained logM0 ∝ ML, MW ∝ 0.67ML, and a low ES/M0, indicating a low average stress drop; for ML > 4.0, logM0 ∝ 0.67ML, MW ∝ ML, and a high ES/M0 were present, and then there was a high average stress drop. These derived relations implied that source duration (T) is independent of M0 for ML < 4.0. Moreover, the M0 ∝ T3 relation seemed able to interpret those relations for ML > 4.0. Nevertheless, the ES–ML relation remains logES ∝ 2.0ML for 2.5 < ML < 6.3. From this study, the derived relations could not predict the source parameters for ML > 6.3 events. This might indicate that ML saturates beyond ML 6.3. Through such analyses, we not only established the relations among source parameters but also elucidated the basic physics of the earthquake sequence.