Abstract:

Underground fluid plays a vital role in the process of earthquake preparation and occurrence. Water level observation wells with good pressure and sealing are usually called crustal strain-sensitive indicators. In groundwater micro-dynamic observation, information such as earthquake precursor anomalies can be identified by using tidal response characteristics of well water level. In this paper, the water level data of Yanshou Station, Tonghe Station, Zhaodong Station, Gannan Station and Suihua Beilin Station, which can be used for tidal analysis in 18 water level observation wells in the Heilongjiang area since 2016, are selected, and the tidal factors of diurnal waves in the diurnal wave group of water level tidal response of each well are obtained by using Vinidkov harmonic analysis method. Based on the tidal analysis of the whole-point observation data in Heilongjiang and its surrounding areas without earthquakes above M_S4.0, with no obvious interference of water level and high accuracy and continuity of water level, the maximum tidal factor of the diurnal wave is extracted as the background value of diurnal wave of well water level tidal response at each station. Combined with the significant earthquakes around each station, the abnormal variation characteristics of diurnal wave height before and after Ningjiang M_S5.0, M_S5.7 and M_S5.1 earthquakes in Songyuan, Jilin Province are extracted.
When the earthquake preparation reaches the final stage, the triggering effect of the external environment will become the key factor. This research shows that the horizontal tidal force of the day and month is closely related to the occurrence of large earthquakes, and the tidal force of the earth's tide before moderate and strong earthquakes has obvious modulation and triggering effect on seismicity. In the study of the relationship between earthquakes and tidal triggering, it is the most intuitive and effective method to analyze the degree of modulation triggering of earthquakes in a certain region and the modulation anomaly characteristics of small earthquakes near the epicenter before earthquakes. Using M_L3.0 earthquakes in Heilongjiang Province and its adjacent areas, this paper selects a time window length: 1 year, time step length: 3 months, space window length: 150km, space step length: 0.5°×0.5°, and a lower limit of the number of earthquakes as 5, calculates the spatial anomaly area of modulation ratio one year before the earthquake, extracts the stress modulation anomaly near the focal area before the moderate and strong earthquake in Songyuan, Jilin Province, and further discusses the relationship between diurnal wave anomaly and small earthquake modulation in the process of earthquake preparation and occurrence.

The results show that: 1)The background change of diurnal wave tide factor of well water level tide response is relatively stable, the anomaly is easier to identify and has a high signal-to-noise ratio. 2)Before the Ningjiang earthquake in Songyuan, Jilin Province, the diurnal wave anomaly of the tidal response of the well water level was synchronous and morphologically consistent, mainly represented by the matching anomaly of three or more stations. 3)The Ningjiang earthquake in Songyuan, Jilin, occurred within 2.6 months after the end of the matching anomaly of the Sunday wave height, with the shortest of only 7 days, and has obvious short-term and imminent characteristics. The duration and amplitude of the anomaly are related to the magnitude of the earthquake. 4)Before the Ningjiang earthquake in Songyuan, Jilin Province, there was a low-value anomaly in the modulation ratio of small earthquakes with M_L≥3.0 in the focus area, which was mainly characterized by short-impending features. It may have the same physical meaning as the lower or lower b-value before the earthquake.
The diurnal wave anomalies in the tidal response of the well water level reflects the change in the stress state within the structure. The modulation ratio of small earthquakes can better reveal that the tectonic stress in the focal area reaches or approaches a critical state. The combined analysis of the two helps identify and capture short-term and imminent anomalies of earthquake precursors. Studying the tidal response of well water level and the modulation of small earthquakes may be an effective way to explore earthquake precursor information related to tidal force triggering during earthquake preparation and occurrence.

Key words: well water level, moderately-strong earthquake, tidal response, stress modulation

摘要：

地下流体在地震孕育和发生过程中发挥着至关重要的作用。文中从黑龙江地区18个水位观测井中, 筛选出2016年以来符合一定条件的延寿台、 通河台、 肇东台、 甘南台和绥化北林台的井水位数据, 应用维尼迪柯夫调和分析方法进行潮汐分析, 得到井水位潮汐响应全日波群中的周日波潮汐因子, 分析了吉林松原宁江 M_S5.0、 M_S5.7、 M_S5.1 地震前后周日波的高值异常变化特征, 并结合震源区M_L≥3.0小地震调制比异常, 进一步探讨了周日波异常与小地震调制作用的关系。结果显示: 1)周日波潮汐因子背景变化相对稳定, 异常更容易被识别, 具有较高的信噪比。2)吉林松原宁江地震前, 井水位潮汐响应的周日波异常具有准同步性和形态一致性, 主要表现为3个及以上台站的配套性异常。3)吉林松原宁江地震发生在周日波高值配套性异常结束后的2.6个月内, 最短仅为7d, 具有明显的短临特征, 且异常持续时间、 异常幅度与震级大小相关。4)吉林松原宁江地震前震源区M_L≥3.0小地震调制比存在低值异常, 主要表现为短临特征, 井水位潮汐响应的周日波异常反映内部应力状态的改变, 小地震调制比则能更好地揭示震源区构造应力达到或接近临界状态, 对二者进行联合分析有助于识别和捕捉地震前兆短临异常。

关键词: 井水位, 中强地震, 潮汐响应, 应力调制