A STUDY ON THE REGIONAL WAVE VELOCITY CHANGES BEFORE THE MS6.4 YANGBI EARTHQUAKE USING AMBIENT NOISE

doi:10.3969/j.issn.0253-4967.2021.05.009

Abstract

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

摘要：

文中基于2021年5月21日漾濞M_S6.4地震震中附近7个台站记录的2019年1月1日—2021年5月21日的宽频带垂直分量连续波形数据, 通过背景噪声互相关提取17个台站对间的经验格林函数, 在0.1~0.5Hz频率范围内, 采用互相关时延计算方法获取台站对的当天经验格林函数与参考经验格林函数中直达Rayleigh波的相对走时偏移, 对漾濞地震前的区域波速变化进行研究。结果表明: 1)漾濞M_S6.4地震前5个月以来, 震中附近区域的Rayleigh波相对走时偏移异常台站对经历了从外围较远处零散分布向震中附近集中分布的变化过程, 且整个变化过程中, 异常台站对主要以负异常(相对走时偏移超过-1.5倍标准差线)为主; 2)基于YUL-TUS、 HDQ-BAS、 EYA-CHT和YUL-CHT的多台站对组合平均的Rayleigh波相对走时偏移结果显示, 漾濞M_S6.4地震发生前约5个月, 震中附近地下介质的波速存在明显加快的趋势, 相对走时偏移变化量为-0.35%。

关键词: 背景噪声, 漾濞M_S6.4, 波形互相关, 波速变化

CLC Number:

P315.3⁺1

YANG Jian-wen, JIN Ming-pei, YE Beng, GAO Qiong, CHEN Jia, ZHANG Hua-ying, DENG Jia-mei. A STUDY ON THE REGIONAL WAVE VELOCITY CHANGES BEFORE THE M_S6.4 YANGBI EARTHQUAKE USING AMBIENT NOISE[J]. SEISMOLOGY AND EGOLOGY, 2021, 43(5): 1171-1187.

杨建文, 金明培, 叶泵, 高琼, 陈佳, 张华英, 邓嘉美. 利用背景噪声研究漾濞M_S6.4地震前区域波速变化[J]. 地震地质, 2021, 43(5): 1171-1187.

Figures/Tables 11

Fig. 1 Distribution of seismic stations, earthquake epicenters and station pairs.

Fig. 2 Cross-correlation calculation results of YUL-HDQ station pair on the day's empirical Green's function and reference empirical Green's function.

Fig. 3 Station pairs distance-time diagram of reference empirical Green's function.

Fig. 4 The variation curve of the relative travel time offset of Rayleigh wave at the YUL-HDQ station pair.

图 5 Anomalous changes in regional wave velocity before the Yangbi MS6.4 earthquake.

Fig. 6 Distribution of the station pairs near the epicenter of Yangbi earthquake.

Fig. 7 The variation curve of the relative travel time offset of Rayleigh wave at the YUL-TUS stations.

Fig. 8 The variation curve of the relative travel time offset of Rayleigh wave at the HDQ-BAS stations.

Fig. 9 The variation curve of the relative travel time offset of Rayleigh wave at the EYA-CHT stations.

Fig. 10 The variation curve of the relative travel time offset of Rayleigh wave at the YUL-CHT stations.

Fig. 11 The variation curve of the relative travel time offset of the Rayleigh wave at the combined station pairs.

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