关键词: 背景噪声, 漾濞M_S6.4, 波形互相关, 波速变化

Abstract:

At 21:48:34 on May 21, 2021, a M_S6.4 earthquake occurred in Yangbi County(25.67°N, 99.87°E), Dali Prefecture, Yunnan Province, with a focal depth of 8km. The epicenter is located in Cangshanxi Town, Yangbi County. The earthquake occurred on a secondary fault zone on the west side of the Weixi-Qiaohou-Weishan Fault. The focal mechanism is right-lateral strike-slip with a small amount of normal faulting, and the earthquake sequence is the foreshock-mainshock-aftershock type. The Yangbi M_S6.4 earthquake is the largest earthquake in Yunnan Province after the Jinggu M_S6.6 earthquake on October 7, 2014, and the largest earthquake in the northwestern Yunnan after the Yongsheng M_S6.0 earthquake on October 27, 2001.
The preparation and occurrence of earthquakes are related to changes of regional stress field. The accurate measurement of changes in seismic wave velocity to monitor the changes in crustal stress over time is an effective way to make physical earthquake predictions. Studies have shown that before a strong earthquake, the physical properties of the medium in the crust will change, and this change can be reflected by changes in seismic wave velocity. Therefore, according to the temporal and spatial change characteristics of the seismic velocity in the crust, the physical properties of the crustal medium can be known. Ambient noise has a higher time sampling rate than natural seismic sources, is much more economical than artificial seismic sources, and has the advantages of repeatability, economy and environmental protection, so it is very suitable for tracking changes in the physical properties of the internal structure of the earth's crust.
In the previous work, we explored the application of seismic data and ambient noise cross-correlation technology to earthquake tracking, analysis and prediction. Based on the continuous waveform data of the Yunnan seismic network, the Rayleigh wave relative travel time offset time series of the station pairs are generated regularly and applied to the daily earthquake situation analysis and judgment. The occurrence of the Yangbi M_S6.4 earthquake is a good test for applying the results of wave velocity measurement from ambient noise to the earthquake analysis and prediction practice. Therefore, based on the continuous broadband vertical component waveform data recorded by the station pairs near the epicenter of the Yangbi M_S6.4 earthquake, and using the ambient noise wave velocity measurement method, the relative travel time offsets of the Rayleigh wave between the stations are obtained. A retrospective study of the regional wave velocity changes before the Yangbi M_S6.4 earthquake was performed to gain deeper insights into the mechanism of earthquake preparation. This study also provides a good reference sample for earthquake cases to measure the regional underground medium wave velocity changes and capture the characteristics of wave velocity anomalies before strong earthquakes in Yunnan area by using ambient noise technology.
Based on the continuous broadband vertical component waveform data recorded by 7 stations near the epicenter of the Yangbi M_S6.4 earthquake(4 stations of Yunnan network and 3 of Xiaguan network)from January 1, 2019 to May 21, 2021, the empirical Green's functions between 17 station pairs were extracted from the background noise cross-correlations. In the frequency range of 0.1~0.5Hz, the relative travel time offsets of the direct Rayleigh waves in the very day's empirical Green's function and the reference empirical Green's function of the station pair are measured directly using the cross-correlation time delay calculation method. On this basis, based on the Rayleigh wave relative travel time offset time series of 17 station pairs, with ±1.5 times the standard deviation as the anomaly threshold, the abnormal changes in the regional wave velocity around the epicenter of the Yangbi earthquake five months before the Yangbi earthquake were analyzed. In addition, based on the YUL--TUS, HDQ--BAS, EYA--CHT and YUL--CHT station pairs, the methods of “single station pair” and “multi-station pair combined average” are used to analyze the time-varying characteristics of the relative travel time offset near the epicenter. Conclusions are drawn as follows:
(1)From the perspective of the overall change of the abnormal station pairs, in the five months before the Yangbi M_S6.4 earthquake, the distribution of abnormal station pairs in relative travel time offsets of the Rayleigh wave experienced a change process from scattered distribution in the outer periphery to concentration to the epicenter, and during the whole change process, the abnormal station pairs mainly show negative anomalies(the relative travel time deviation exceeds -1.5 times the standard deviation line).
(2)The analysis of the relative travel time offset of Rayleigh waves based on the combined stations of YUL-TUS, HDQ-BAS, EYA-CHT and YUL-CHT near the epicenter shows that about 5 months(158 days)before the Yangbi M_S6.4 earthquake, the wave velocity of the underground medium near the epicenter showed an obvious accelerating trend, the amount of relative travel time offset change was -0.35%.
(3)The abnormal station pairs before the earthquake were concentrated near the epicenter, and dominated by negative anomalies. The wave velocity of the underground medium near the epicenter before the earthquake showed a significant acceleration trend, which was an obvious phenomenon observed before the Yangbi earthquake. Whether the above-mentioned change characteristics have to exist before strong earthquake requires further examination with more earthquake cases.
(4)Based on seismic waveform data, this paper uses ambient noise cross-correlation and cross-correlation time delay calculation methods to obtain the relative travel time offset of Rayleigh waves of station pairs and analyze the changes in wave velocity near the epicenter. The physical meaning is clear. The wave velocity measurement by ambient noise does not depend on a specific seismic source, and the data results have a high time sampling rate and can be updated every day. The method of measuring wave velocity from ambient noise is expected to become a new method and new approach for earthquake prediction.

Key words: ambient- noise, Yangbi M_S6.4 earthquake, waveform cross-correlation, wave velocity change