The rapid determination of radiated seismic energy of MS6.4 Yangbi earthquake

Radiated seismic energy is a fundamental physical quantity that can be used to represent the dynamic characteristic of the earthquake source, and it is a reliable indicator of the high-frequency component radiated by the seismic source which goes into the seismic waves. Estimating radiated seismic energy has an important role in seismic hazard assessment, quantification of earthquake research, and engineering seismology. Therefore, it is of great significance for the rapid determination of the seismic radiated energy after an earthquake as a reference. After the Yangbi earthquake on May 21, 2021 in Yunnan Province, we measured the radiated seismic energy for the Yangbi earthquake based on the broadband recordings provided by the Global Seismograph Network. The results show that the radiated seismic energy is estimated to be 1.6×1013J; the corresponding Me value is 5.9, which is smaller than MW; The difference between the single station value and the average value is less than 0.2 units in the determination of energy magnitude for more than 70% of the stations; Based on the results of the seismic moment obtained by Global Centroid Moment Tensor(GCMT), it can be concluded that the energy-moment ratio is 9.9×10-6, which is considerably weaker than the global average value of (4×10-5) for the strike-slip earthquakes. Several previous studies showed that due to the difference in the source rupture process, even earthquakes with small differences in seismic moment and focal mechanism that occurred in the same seismotectonic area may have obviously different amounts of radiated seismic energy. Therefore, we compared this event with Jinggu earthquake that occurred on October 7, 2014. Although the moment magnitudes and focal mechanisms of the two earthquakes are similar, and the epicenter distance is only 290km apart, the area of the same isoseismal for the Jinggu earthquake is significantly larger than the Yangbi earthquake. Earthquake intensity indicates the level of ground shaking, which is associated with the high-frequency components of the seismic waves radiated from the seismic source. Thus, we compared the energy release characteristic of the two earthquakes. The results show that the energy-moment ratio of the Jinggu earthquake(1.58×10-5)is 1.6 times higher than that of this earthquake(9.9×10-6); Comparing with the time-domain analysis alone, the time-frequency analysis performed with the S-transform allows us to quantify important details about the source process information provided by the seismic recordings. According to the results of the time-frequency analysis via the S-transform of these two earthquake waves recorded by the same station, the recording of the Jinggu earthquake had much larger spectral amplitudes at frequencies between 0.5 and 1.5Hz during 30~50s after the P-arrival, while this phenomenon was not observed in the Yangbi earthquake. It can be seen that the high-frequency energy of the Jinggu earthquake is higher than that of the Yangbi earthquake. In summary, the Yangbi earthquake belongs to a low energy release efficiency strike-slip event, the difference in seismic energy radiated per unit seismic moment led to the different area of the same isoseismal. The relatively high-frequency seismic wave is firmly related to the near-fault seismic hazard. Therefore, we suggest that apart from the difference in focal depth, geological condition and building structure, the difference in energy release efficiency is one of the reasons for the different areas of the same isoseismal with the earthquakes that have the same moment magnitude, the low energy release of the Yangbi earthquake limits the capacity to generate enormous numbers of casualties, damage to the environment and critical infrastructure. © 2021, Editorial Office of Seismology and Geology. All right reserved.