The spatio-temporal evolution characteristics of the MS6.0 Barkam earthquake sequence in Sichuan on June 10,2022
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摘要:
2022年6月10日马尔康MS6.0震群序列发生在松岗断裂与龙日坝断裂交会处,呈震群型活动特征。本文以四川地震台网目录为基础,对马尔康地震序列的参数特征开展研究,取该序列三次MS5.0地震之后各一个小时,作为 Ⅰ , Ⅱ , Ⅲ 共计三个阶段,进行对比分析。由于较大地震后短时间内目录遗漏的余震较多,为增大研究所需的地震样本量,首先采用模板匹配法进行微小地震识别,以补充完备目录,并利用识别的地震目录及台网目录分别计算马尔康MS6.0地震序列的b值、p值等参数。计算结果显示,相比于第 Ⅱ 和 Ⅲ 阶段,第 Ⅰ 阶段具有显著的低b值(0.59),随着时间的推移,序列b值逐渐上升,后两个阶段分别为0.84和0.86。第 Ⅰ 阶段低b值的结果反映了此阶段孕震区应力水平较高。另外,第 Ⅰ 阶段序列的p值为0.76,明显低于后两个阶段的1.81和1.64,反映出第 Ⅰ 阶段序列频次衰减速度较慢,应力释放不充分,而后两个阶段刚好相反,表明不同阶段序列的时间演化特征存在差异。综合分析认为,MS5.8地震是MS6.0地震的前震。序列西支与东支的参数计算结果呈现不一样的特征,可能与MS5.8前震序列发生在西支有关。
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关键词:
- 马尔康MS6.0震群 /
- 松岗断裂 /
- 序列参数 /
- 前震
Abstract:According to the China Earthquake Networks Center, at 00:03 on June 10, 2022, local time, an earthquake with MS5.8 (ML6.3) struck Barkam City (32.27°N, 101.82°E), Aba Prefecture, Sichuan Province, with a focal depth of 10 km. Subsequently, at 01:28 local time, another MS6.0 (ML6.5) earthquake occurred in the same location (32.25°N, 101.82°E), with a focal depth of 13 km. These two earthquakes were located in the southeast part of the Bayan Har block, approximately two kilometers apart. According to the definition of earthquake sequence type, the magnitude difference between the two earthquakes ΔM=0.2 constitutes a swarm type earthquake, which is hereinafter referred to as “MS6.0 Barkam earthquake swarm”. The MS6.0 Barkam earthquake swarm exhibited numerous minor earthquakes, up until 23:00 on June 30, 2022, a total of 4 821 earthquakes of magnitude 0 and above were recorded, including seven earthquakes with magnitude ranging from MS3.0 to MS3.9, three earthquakes with magnitude ranging from MS4.0 to MS4.9, two earthquakes with magnitude ranging from MS5.0 to MS5.9, and one MS6.0 earthquake. The largest aftershock recorded was an earthquake with MS5.2 (ML5.6) at 03:27 local time on June 10. Considering the significant event of an earthquake with MS5.8 preceding the occurrence of the main shock with MS6.0, it is important to investigate the characteristics of the MS5.8 earthquake sequence as a foreshock and the sequence features before and after the main shock. Existing seismological methods indicate that earthquake sequences can intuitively reflect differences in tectonic stress fields, seismic structures, and the seismogenic environment medium. Moreover, the rupturing properties of the main shock often influence the evolution of aftershock sequences. This study utilizes seismic data from the Sichuan regional seismic network, based on parameters such as the b-value and p-value of the sequence, in combination with regional structure and focal mechanism solution parameters of the strong aftershock, to investigate the spatio-temporal evolution characteristics of the Barkam MS6.0 earthquake swarm sequence and to explore the sequence evolution characteristics before major earthquakes in Barkam.
Regional geological settings and M≥5.0 historical earthquakes
The Barkam region is situated in the eastern part of the southeastern margin of the Bayan Har block within the Songpan-Garze orogenic belt. The Longriba fault in the studied area divides the eastern part of the Bayan Har block into two parts, including the secondary Longmenshan block on the east and the Aba secondary block on the west. The Longmenshan block primarily features the NE-trending Longmenshan fault zone, the nearly SN-trending Minjiang fault, and the Huya fault. On the other hand, the Aba secondary block, features a series of large-scale strike-slip faults trending NW that have exhibited activity in the Late Quaternary period. These faults, along with the Garze-Yushu fault, the Xianshuihe fault, and the east Kunlun fault at the southern and northern boundaries of the Bayan Har block, collectively constitute the tectonic framework of the Bayan Har block. The Barkam MS6.0 earthquake swarm occurred near the intersection of the NW-trending Songgang fault and the NE-trending Longriba fault within a relatively complex fault structure. The Songgang fault is approximately 100 kilometers in length, with a maximum width of about 300 meters, trending between 320° and 330°, dipping to the northeast with an angle of 50° to 70°. It extends from the northern slope of Mengbi Mountain on the south side of Barkam, along a NW direction, and disappears near Zoigê. This fault exhibits relatively complex activity, featuring characteristics of multiple periods of activity. Its southern segment is a Late Pleistocene active fault, and historical records indicate that it has experienced three MS≥5.0 earthquakes, with the largest MS6.0 earthquake occurred on October 8, 1941 in Heishui area. The northern segment of the fault does not exhibit obvious surface activity since the Late Pleistocene. Apart from the Maerkang MS6.0 earthquake swarm, no MS≥5.0 earthquake was shown in historical records. Minor earthquakes are relatively active in the central part of the fault, concentrated in deep-seated dense activity along the NW direction. This reflects the existence of a northwest-trending ruptured surface and suggests that the northern segment of the fault possesses potential seismic risk. The Longriba fault is considered to be resulted from the strong obstruction of the southeastern movement of the Bayan Har block from the South China block during the Late Cenozoic, and it constitutes the backthrust-overthrust tectonic system of the Longmenshan structural belt, bearing the role of crustal deformation on the eastern edge of the Qinghai-Xizang Plateau since the Late Cenozoic. This fault is a new fault primarily characterized by right-lateral strike-slip movement in the NE direction, consisting mainly of two parallel branch faults: the Longriba fault and the Maoergai fault. It exhibits a series of distinct fault landforms and demonstrates Late Quaternary activity. Historical records indicate that the Longriba fault has not experienced any MS≥5.0 earthquakes. The M≥5.0 earthquakes that occurred within a 50 km range from the epicenter were all earthquake swarms. There were a total of two swarms, which took place on September 26 and November 6, 1969, with MS5.1 and MS5.3 in Aba, and on September 5 and November 8, 1970, with MS5.5 and MS5.5 in Rangtang.
Basic situation of the sequence
Using the multi-stage positioning method to locate the Barkam MS6.0 earthquake sequence. The results indicate that the sequence is situated to the northeastern side of the Songgang fault, with an overall parallel distribution along the NW-SE direction, forming two branches running in an east-west direction, parallel to the Songgang fault. The western branch spans approximately 12 km in length and 3 km in width, while the eastern branch extends about 15 km in length and 2 km in width, with a separation of roughly 2 km between the two. The sequence experienced a total of three MS≥5.0 earthquakes, with the MS5.8 earthquake occurred closer to the eastern side of the western branch, the MS6.0 earthquake located within the eastern branch, and the largest aftershock of MS5.2 situated to the east of the eastern branch. Prior to the MS6.0 earthquake, seismicity was primarily distributed along the western branch, while after the MS6.0 earthquake, the activity shifted to the eastern branch, indicating the spatial migration over time. Additionally, the distribution of seismicity is more scattered in the western branch and more concentrated in the eastern branch, suggesting potential differences in the seismicity patterns between the two branches and indicating that the sequence does not occur on a single fault structure, but rather on different branch faults.
Using the CAP (cut and paste) method to calculate the focal mechanism solutions of the MS5.8, MS6.0, and some MS≥3.5 earthquakes of the sequence. The results revealed that the MS5.8 earthquake had best double-couple solutions with strike 324°, dip 76°, and slip 0° for nodal plane I, while strike 234°, dip 90°, and slip 166° for nodal plane Ⅱ . The MS6.0 earthquake had best double-couple solutions with strike 329°, dip 90°, and slip −3° for nodal plane I, and strike 58°, dip 87°, and slip −180° for nodal plane Ⅱ . Both main shocks and the focal mechanisms of the larger aftershocks exhibited consistent strike-slip motion, which is consistent with the regional predominance of reverse and strike-slip faulting. The strike of nodal plane I in the NW direction is consistent with the strike of the Songgang fault near the epicenter. Furthermore, the focal mechanism solutions for some MS≥3.5 events of the Barkam MS6.0 earthquake sequence indicated a concentrated depth distribution ranging from 5 to 8 km (Table 1). Combining the results of precise positioning and the study of the sequence’s seismogenic structure by Long et al (2023), it is suggested that the seismogenic structure of the Barkam MS6.0 earthquake swarm is complex, which is resulted from the simultaneous activity of several faults of different scales. These faults are located close to the Songgang fault, not exposed at the surface, and may be connected to the Songgang fault at depth, indicating that the seismogenic structure may be a concealed branch fault of the Songgang fault. It is noteworthy that there are traces of a NE-oriented distribution within the sequence, implying that the seismicity in this area may exhibit conjugate rupture characteristics.
Early sequence parameter evolution characteristics
The early temporal evolution of the sequence encapsulates the processes of nucleation and stress changes. The analysis of the temporal evolution characteristics is helpful to understand the mechanism and development of the sequence. In commonly used statistical seismological models, the regression parameter b value in the Gutenberg-Richter (G-R) relationship and the p value in the modified Omori’s law carry specific physical properties and are widely considered as statistical quantities characterizing the sequence. The G-R relationship is expressed as lgN=a-bM, where b value represents the maximum likelihood solution $ {b}={\mathrm{lg}\mathrm{e}}/ ( {\overline{M}-{M}_{\mathrm{C}}} ) $, a value represents the overall level of seismic activity, N represents seismic frequency, MC represents the minimum completeness magnitude, $ \overline{M} $ represents the average magnitude, and lge=0.4343. The rock fracture experiments indicate that the b value decreases with the increase of stress level. A smaller b value reflects higher regional stress levels, while a larger b value indicates lower stress levels. So the b value, as a means of assessing regional stress accumulation levels, has been widely used in seismic hazard assessment and post-earthquake trend analysis. Laura and Stefan (2019) studied the b values of the earthquake sequences of the AmatriceNorcia MW6.2, MW6.6 in central Italy on August 24 and October 30, 2016; the Kumamoto MW6.5, MW7.3 sequence in Japan; and the Tohoku MW7.3, MW9.0 sequence in Japan. They concluded that the b value of the foreshock sequence would significantly decrease. Jiang et al (2021) found that the b value of the Yongping MS6.4 earthquake sequence in Yunnan showed a decrease followed by fluctuation before and after the main shock, reflecting the intense stress adjustment state in the sequence’s continuous process. Wang et al (2023) discovered the phenomenon of b value decreasing after the foreshock and increasing after the main shock, clearly reflecting the development process of the foreshock-main shock-aftershock sequence. In the actual calculation process, calculation error of the b value is obtained through the construction of a bootstrap process. Additionally, assuming N=1, M corresponds to the theoretical maximum earthquake magnitude Mmax, expressed as Mmax=a/b. The expression of the modified Omori’s law is n(t)$ ={K}/{{ ( t+c ) }^{p}} $, where n(t) represents the number of aftershocks at time interval t after the main shock, K represents the aftershock occurrence rate, and p is referred to as the aftershock frequency decay coefficient, representing the rate of sequence decay. A larger p value indicates faster decay, while a smaller p value suggests slower decay, typically varying between 0.9 and 1.5. Its variation characteristics may be related to the uneven structure, temperature changes, and stress accumulation levels in the crust. Discretizing frequency statistics over time periods can result in information loss, thus we obtained the p value and other coefficients in cumulative frequency form. The final integrated form of the modified Omori’s law is: when p≠1, $ N ( t ) = K[{c^{1- p}}- { ( t + c ) ^{1- p}}] /{{ ( p - 1 ) }}$, and when p=1, N(t)=Kln(c+t). In the actual inversion process, both equations are simultaneously calculated, and the fitting error is computed to select the smaller error value as the final p value calculation result. Due to the small sample size, conventional least squares methods may lead to local optimal solutions, hence genetic algorithms were used to solve the parameters. This study attempts to calculate the b value, p value, and other sequence parameters to identify evidence of the MS5.8 serving as a foreshock to the MS6.0 in the Barkam earthquake sequence.
In this sequence, three MS≥5.0 events occurred within three hours of the sequence’s onset, indicating rapid sequence development. When we attempted to analyze the evolution characteristics between these three events, the short time intervals resulted in the signals of smaller earthquakes submerged in strong seismic waves, leading to incomplete aftershock records and insufficient statistical sample size. In order to increase the required seismic sample size for the study, a template matching method was used to detect missed earthquakes in the data from June 10th. Using 764 seismic events recorded by the network as templates, events with a correlation coefficient R≥0.85 were identified as individual earthquake events, and DBSCAN method was employed for earthquake correlation, ultimately obtaining 1 713 precise earthquake records, which is 2.2 times as much as the number of templates used. Since the identified earthquakes were similar to the template earthquakes in terms of location and nearly identical in terms of source mechanism solutions, the magnitudes of the missed earthquakes can be obtained by comparing their maximum S-wave amplitudes with the corresponding templates. The supplemented missed earthquakes mainly occurred before 10:00 a.m. , indicating significant interference of strong seismic waves in the manual identification of earthquakes, and also demonstrating the effectiveness of template matching in picking up missed earthquakes. The GFT method was used to compare and analyze the network catalog and the supplemented earthquake catalog before 10:00 a.m. , calculating the minimum completeness magnitudes, showing ML1.8 and ML1.3, respectively, proving the effectiveness of the template matching method in supplementing small magnitude earthquakes.
On June 10, three seismic events of MS≥5.0 divided the seismic sequence into different stages. To ensure data comparability and control variables, each stage was defined as Ⅰ , Ⅱ , and Ⅲ , with each stage beginning one hour after each of the three MS≥5.0 events. For each stage, the Gutenberg-Richter-Frohlich (GFT) method was used to calculate the minimum completeness magnitude (MC) of the seismic sequence. Events above the MC were selected, and their b values, Mmax values, and p values were calculated. The analysis aimed to identify potential differences in stress states and seismic activity characteristics across the different stages and to understand the early evolution patterns of the sequence. We have listed the amount of data used and the calculation results for each stage in Table 2. From the table, it can be seen that the sample sizes for each stage (above the completeness magnitude) were all over 50, with 69, 57, and 66 events, respectively, ensuring robust calculation results. The MC values for the three stages were ML2.1, ML2.3, and ML1.7, indicating a significant impact of strong earthquakes on short-term monitoring capabilities, with the degree of impact varying with the magnitude. Compared to stages Ⅱ and Ⅲ , stageⅠexhibited a significantly lower b value (0.59). Over time, the b value gradually increased, with the subsequent two stages being 0.84 and 0.86, respectively. The low b value of stage Ⅰ suggests a high stress level in the seismic zone, indicating that the MS5.8 earthquake could be considered as a foreshock of the MS6.0 earthquake. The Mmax values also demonstrated differences across the stages, with an Mmax value of ML5.0 in stage Ⅰ , the highest among the three stages. The subsequent stages showed a gradual decrease of Mmax from ML4.2 to ML3.7, reflecting the potential foreshock nature of the seismic sequence in stage Ⅰ .
The a value of stage Ⅰ (3.1) was not significantly different from that of stage Ⅲ (3.2), but was lower than that of stage Ⅱ (3.6), suggesting that the a value alone cannot distinguish between foreshocks and aftershocks. When calculating the p value for events above the MC for each stage, the p value for stage Ⅰ was 0.76, significantly lower than those of the subsequent two stages, which were 1.81 and 1.64, respectively. This suggested that the seismic sequence in stage Ⅰ exhibited slower decay and insufficient stress release, while the subsequent two stages were opposite, possibly indicating differences in the temporal evolution between the foreshock sequence and the aftershock sequence.
Conclusions
The spatial distribution characteristics of the Barkam MS6.0 earthquake swarm in the region indicated a significant spatial migration feature, suggesting that the sequence was not a simple single rupture event, but a complex seismic sequence. Given the presence of two branches in the sequence, the evolution characteristics over time were discussed separately for the east and west branches. Calculations for the west branch yielded MC of ML1.5, b value of 0.71, and p value of 0.81, while the corresponding parameters for the east branch were ML1.6, 0.93, and 0.94, respectively. The differences in the parameters between the west and east branches reflected that the two branches were formed not by a single fault structure. The lower b and p values in the west branch indicated insufficient stress release, a high stress level, and slow decay, possibly related to the occurrence of the foreshock sequence.
In summary, the seismic cluster of the MS6.0 event in Barkam on June 10, 2022 occurred near the intersection of the Songgang fault and the Longriba fault, indicating a complex fault structure. The seismic cluster exhibited rich small earthquakes, slow overall decay, and segmental spatial characteristics. Several conclusions were drawn from the analysis:
1) The Barkam MS6.0 earthquake swarm occurred near the intersection of the NW-trending Songgang fault and the NE-trending Longriba fault, indicating a complex fault structure. The precise positioning results showed that the sequence was located along the NE direction of the Songgang fault, with an overall NW-SE orientation and parallel east and west branches. The east and west branches exhibited spatial migration features and different spatial distribution patterns. The source mechanisms of the larger earthquakes in the sequence were consistent, all being strike-slip type, which aligns with the regional background and the nature of the Songgang fault. It is speculated that this sequence was caused by several different-sized faults, which are close to the Songgang fault, not exposed at the surface, and may be connected to the Songgang fault at depth, representing concealed branch faults of the Songgang fault.
2) Using template matching, 1 713 precise earthquake events were identified, which was 2.2 times as much as the number of templates used. This reflected the significant interference of strong seismic waves on manual earthquake identification and demonstrates the effectiveness of template matching in detecting missed earthquakes.
3) By dividing the sequence into three stages following each MS5.0 event, the b and p values were compared and analyzed. The results showed a significantly lower b values in stage Ⅰ , reflecting a higher stress level in the seismic zone during this stage. The M5.8 earthquake can be considered as a foreshock before the MS6.0 earthquake. Additionally, the p value for stage Ⅰ was significantly lower than the subsequent two stages, indicating differences in the temporal evolution between the foreshock sequence and the aftershock sequence.
4) When analyzing the east and west segments separately, we found that the west segment exhibited lower b and p values, indicating insufficient stress release, high stress level, and slow decay, possibly related to the occurrence of the MS5.8 foreshock sequence in this segment.
5) In summary, the comprehensive analysis suggests that the MS5.8 earthquake was a foreshock of MS6.0 event in Barkam. Temporally, the MS5.8 earthquake sequence before the main shock exhibited characteristics of low b and p values. Spatially, the west segment of the MS5.8 earthquake sequence also showed lower b and p values, indicating slow decay of the foreshock sequence of the Barkam MS6.0 earthquake swarm and the presence of insufficient stress release and high stress levels in the foreshock area.
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引言
固体地球内部含有大量气体,这些气体是最活跃且最易运移的物质组分,尤其是地幔中的气体或挥发份会发生脱气作用而被运移至壳内或地表(陶明信等,2005),并且在一定的构造条件及地表覆盖条件下富集,表现为某些气体成分的浓度偏高,从而反映出固体地球内部的构造信息及深部地球化学信息。基于气体的构造地球化学观测已广泛应用于地震地质等相关研究之中,包括断裂构造活动、隐伏型活动断裂探测、地震与火山活动观测等(Fu et al,2005,2017a;Weinlich et al,2006;Lombardi,Voltattorni,2010;Li et al,2013;Neri et al,2016)。地下气体组分(H2,Rn和CO2等)能够客观地、灵敏地反映地壳的应力应变状态(李营等,2009)。而地震活动前后,通常能够从断裂带上捕获到气体组分浓度异常的信息。Ito等(1999)对日本中部约罗(Yoro)活动断裂带的分析结果表明,1998年名古屋(Nagoya)M5.4地震及之前4次微震的震前、震时均观测到了流体H2异常;Barman等(2016)在印度西孟加拉地邦巴克斯瓦尔(Bakreswar)地热活动区的观测研究表明,222Rn的异常信号与相关地区的地震活动高度相关;Fu等(2017b)分析了2016年我国台湾美农ML6.6地震前两周内,位于不同位置的土壤气观测站点同步观测到的土壤Rn异常,其结果反映出孕震阶段土壤气体浓度的变化;Weinlich等(2016)在捷克波西米亚地震活动区,观测到2008年新教堂(NovýKostel)震群事件震前CO2浓度降低,偏离正常的年变曲线,并在地震发生时异常结束。因此,H2,Rn和CO2等气体的地球化学特征与地震活动之间存在某种确定的联系,可以将其作为前兆监测的有效手段之一。同时,气体地球化学在隐伏断层探测方面也有广泛的应用,Fu等(2005)在台湾南部通过多剖面多气体组分的观测确定了He和CO2气体异常与潮州隐伏活动断裂之间的联系;Yuce等(2017)通过Rn和CO2浓度的观测证实了阿米克盆地死海(Dead Sea)断裂与卡拉苏(Karasu)断裂之间推测的隐伏断裂存在。大量活动断层的地球化学探测研究也表明,断裂带土壤气体浓度曲线特征不仅能显示断层的位置,而且能反映断裂的规模、产状、活动性等地壳内部构造信息(刘菁华等,2006;周晓成等,2011;张慧等,2013;李源等,2018)。然而,气体地球化学在活断层探测方面的应用更多地受制于地壳地球物理条件、地质构造特征以及覆盖层介质的物性状况等,在缺乏翔实的地球物理探测资料的前提条件下,气体地球化学探测结果与地质构造间的确切关系往往较模糊。在 “新乡市活动断层探测与地震危险性评价” 项目的支持下,通过地球物理与跨断层联合钻孔详勘等方法获取了汤东断裂带的精确位置、构造特征以及活动年代等信息。本文拟通过多期跨断层气体地球化学流动监测,分析汤东断裂带土壤气体的分布特征,进一步探讨气体地球化学观测的构造基础,并获取气体地球化学观测的关键指标。
目前,汤阴地堑地下流体监测的主要手段为地下水位、水化学等指标,气体地球化学监测基础相对薄弱。因此,在汤东断裂带开展气体地球化学研究可以为汤东活动断裂带的进一步精准地球化学监测提供理论基础,亦可为区域经济社会发展规划与震害防御工作提供科学依据。
1. 研究区概况和研究方法
1.1 研究区概况
汤阴地堑位于太行山隆起区与华北平原沉降区的交会区域(图1)。汤阴地堑是太行山隆起与内黄隆起之间的一个北东向凹陷,其东西边界分别受汤东断裂带和汤西断裂控制,南北分别被新乡—商丘断裂和安阳南断裂所控制。新构造运动时期,汤阴地堑受NNE-NE向断裂控制,汤东断裂带和汤西断裂形成了北东向地堑,其基底为三叠系地层,主要发育于古近纪,最大厚度达2 500 m;新近纪以后,汤阴地堑在整体下沉的基础上继承性下降,最大沉积厚度约为800 m,其中第四系沉积厚度较薄(中国地震局地球物理勘探中心,2016)。本文主要研究目标为汤东断裂带主断裂,是汤阴地堑的东部构造边界,走向NNE,倾向NW,以正断层活动为主,全长约为100 km,断层倾角在近地表约为50—60°,向下逐渐变缓,在4 000—4 500 m深度近于水平,是典型的铲型断层,其最新活动时代为晚更新世早期(中国地震局地球物理勘探中心,2016)。深地震反射剖面结果表明,汤东断裂带下方存在一条高角度断裂带,该断裂带切割了岩石圈底界,属于岩石圈尺度的深大断裂(中国地震局地球物理勘探中心,2016),具有大地震发生的构造背景。该断裂带是本文研究区的主要发震构造,规模较大、活动性较强,对河南省新乡市城区有着较高的地震危险性。公元344年M6卫辉东、1737年M5½封丘、1814年M5½汤阴和2008年ML4.8封丘地震均发生在该断裂带周边。
1.2 研究方法
在充分参考汤东断裂带地球物理与跨断层详勘资料(中国地震局地球物理勘探中心,2016)的基础上,选取跨断层联合钻孔详勘两条剖面XZ1和XZ2 (图1),即南端的张河村与北端的邢李庄所在位置进行跨断层土壤气流动观测。每条测线以断裂带为中心,以15—30 m为间距布设测点,依次进行气体观测。野外监测工作分别于2017年10月、2018年1月和6月进行,测量时用钢钎在测点并列打两个深度约为0.8 m的钻孔,将气体取样器置于孔内,封住孔口,用硅胶管连接便携式Rn、痕量H2监测仪和便携式多组分监测仪,分别进行土壤H2,Rn和CO2浓度的现场测定。Rn浓度用Alpha GuARD P2000测氡仪测定,仪器灵敏度50 cpm/(kBq·m−3),仪器校准误差小于3%;H2浓度用杭州超距ATG-300H便捷式测氢仪测定,仪器检出限为5×10−9 L,仪器校准误差小于5%;CO2浓度用德尔格多气体检测仪测定,仪器灵敏度为0.1%,量程为5%。由于土壤层的气体释放不稳定,监测过程中观测数据不断变化,因此在每测点气体浓度测定期间(15 min内), H2和Rn分别测量5个和10个观测数据,H2取最大值作为该测点气体观测值,Rn取最后8个观测数据的平均值作为观测值。最后,运用EXCEL和SPSS13.0软件对所获观测数据进行统计分析处理。
2. 结果与分析
2.1 断裂带气体分布及季节动态
张河测线位于汤东断裂带南端,全长800 m,分别于2017年10月和2018年1月、6月进行了分期测量,数据统计结果列于表1。可以看出,H2浓度介于1.07×10−6—110.40 ×10−6,且各测量期次间H2浓度表现为6月显著高于10月和1月(p<0.05,p表示显著水平),而1月又略高于10月。总体上,H2浓度在测线上分布的离散程度较高,空间差异较大,其中,6月H2浓度的四分位间距和标准差最大,峰值与背景值的比值(峰背比)较大,测点间浓度差异程度显著,对断裂破碎带的指示作用最为明显。而Rn浓度介于4.64—46.70 kBq/m3之间,各测量期次之间未呈显著差异,6月Rn浓度略高于其它两期。总体上,Rn浓度在测线上分布的差异较明显,其中,6月Rn浓度的四分位间距、标准差以及峰背比最大,数据离散程度较高,对断裂破碎带的指示效果较明显。虽然仅测量了1月和6月两期的CO2浓度,且测点间距较大,但CO2浓度总体上依然能够较好地指示断裂破碎带的位置,且与两期次所测H2和Rn浓度的吻合程度较高。CO2浓度值介于0.15%—5% (超过仪器量程),且6月浓度显著高于1月(p<0.05)。
表 1 汤东活动断裂带土壤气H2,Rn和CO2浓度分布特征Table 1. Statistics on characteristics of soil H2,Rn and CO2 concentrations on Tangdong active fault zone测线 指标 时间 测点数 最大值 最小值 平均值 中值 下四
分位上四
分位四分位
间距标准差 峰背比 背景值 张
河
村H2/10−6 10月 34 23.70 1.07 6.29 5.00 2.38 10.67 8.29 5.25 4.39 8.93 1月 32 44.42 2.78 8.47 6.02 4.36 8.14 3.78 8.32 6.75 6月 30 110.40 1.58 21.47 13.65 5.61 26.10 20.49 25.85 8.02 Rn/(kBq·m−3) 10月 34 38.14 8.56 18.20 17.56 14.08 21.47 7.39 6.66 2.25 17.38 1月 32 37.35 9.76 17.54 17.01 13.45 19.81 6.36 5.72 2.21 6月 30 46.70 4.64 19.68 17.52 13.63 25.05 11.42 8.89 2.58 CO2 1月 17 0.54% 0.15% 0.29% 0.22% 0.19% 0.40% 0.21% 0.12 % 1.99 — 6月 16 5.00% 0.73% 2.00% 1.47% 0.92% 3.13% 2.21% 1.34% 2.78 邢
李
庄H2/10−6 10月 33 82.19 11.41 37.62 34.83 22.76 49.09 26.33 17.77 2.35 41.20 1月 30 185.3 10.6 58.70 44.48 19.71 77.17 57.46 47.57 3.98 6月 30 87.79 0.27 34.81 30.94 16.26 44.26 28.01 24.73 2.82 Rn/(kBq·m−3) 10月 33 62.60 10.11 28.39 24.91 19.69 36.72 17.04 12.67 2.38 29.00 1月 30 62.21 14.0 35.29 33.93 24.08 45.28 21.20 12.48 1.81 6月 30 59.96 7.52 24.58 22.01 12.69 33.35 20.66 14.02 2.71 CO2 1月 16 0.78% 0.16% 0.38% 0.36% 0.19% 0.50% 0.31% 0.20% 2.22 — 6月 14 2.00% 0.52% 1.09% 0.98% 0.63% 1.53% 0.91% 0.51% 2.04 邢李庄测线位于汤东断裂带北端,全长约为700 m,土壤气测量工作与张河测线同期进行。此测线上,H2浓度介于0.27×10−6—185.30 ×10−6之间,各测量期次间差异明显,1月H2浓度显著高于6月和10月(p<0.05),10月又略高于6月,其中1月四分位间距、标准差和峰背比最大,H2浓度的差异程度较高,对断裂位置的指示作用明显。Rn浓度介于7.52—62.60 kBq/m3,各期次数值差异明显,1月Rn浓度显著高于6月和10月,10月又略高于6月,与各期次的H2浓度相似。CO2浓度介于0.16%—2.00%之间,6月显著高于1月(p<0.05),总体上,CO2浓度分布也能指示断裂带的存在,尤其与Rn浓度分布的吻合程度高。
2.2 断裂带气体活动背景值
取两测线各期次对应测点的H2浓度和Rn浓度的平均值作为各测点综合观测值,对数据进行K-S非参数检验,除张河测线的H2 (p=0.03)外,其它气体的浓度综合观测值均符合正态分布,因此取综合观测值的平均值作为各组分的背景值。为避免极值对背景值所产生的影响,剔除极值后取平均值作为组分活动背景值(Walia et al,2009;Zhou et al,2017),本文将大于平均值加2倍均方差的值视为极值,以剔除极值后的背景值加2倍均方差作为气体异常点判定下限(Baubron et al,2002;Fu et al,2005),异常幅度以峰背比来表征。
图2给出了张河村测线H2,Rn和CO2浓度的分布情况。可见该测线的H2和Rn浓度各期次观测值的分布特征相似,高值点的重现性较好,观测结果的可信度高(图2a,c)。从3期对应测点的平均值来看,平均值曲线与各期次曲线间有较好的对应关系,H2和Rn浓度高值点的同步性较好(图2b,d);H2和Rn气体背景值分别为(8.93±3.92)×10−6,(17.38±4.28) kBq/m3。在90—185 m距离处,H2和Rn的浓度同步出现高值异常,峰背比分别为5.1和2.3。此外,在480 m距离处H2浓度也出现高值异常(图2b),Rn在此处虽未达到异常限,但也同步出现峰值(图2d)。与此同时,CO2浓度在H2和Rn浓度异常部位也同步出现峰值(图2e)。
将测点各组分浓度值与相应组分最大值之比作为各组分浓度的相对值,相对值分析结果(图2f)显示,H2,Rn和CO2浓度的曲线形状相似,高值点吻合程度高。地球物理与跨断层联合钻孔详勘结果表明,主断裂在距离测线320 m处(中国地震局地球物理勘探中心,2016)。本文的监测结果显示,主断裂以西135 m和230 m距离处,以及该主断裂以东130 m距离处,H2,Rn和CO2浓度显示同步相对高值(图2f),与地球物理与跨断层联合钻孔详勘结果基本一致,表明观测气体组分对断裂带有较好的指示作用。
图3给出了邢李庄测线H2,Rn和CO2浓度分布情况。该测线各期次土壤H2和Rn浓度的分布特征相似,高值点重现性较好(图3a,c)。从3个期次对应测点的平均值(图3b,d)可以看出,平均值曲线与分期测量曲线间有较好的对应关系,曲线形态能够反映断裂带的位置;该测线H2和Rn浓度的背景值分别为(41.20±16.64)×10−6和(29.00±8.28) kBq/m3;H2浓度在测线270 m和330—345 m距离处出现高值异常,而Rn在270—285 m处出现同步异常,此外Rn在30—60 m处也显示高值异常。而CO2浓度在H2和Rn同步异常的部位也出现峰值(图3e)。
H2和Rn浓度相对值的分析结果显示,二者的曲线形态特征相似,尤其在270 m处高值异常部位吻合得非常好(图3f)。地球物理与跨断层联合钻孔详勘结果显示,主断裂位于测线210 m处(中国地震局地球物理勘探中心,2016)。本文的气体监测结果显示,H2和Rn浓度在主断裂以西60 m处出现同步高值异常,与地球物理与跨断层联合钻孔详勘结果基本一致,表明气体地球化学异常与断裂破碎带之间存在较好的对应关系,气体组分高值异常为断裂存在的地表显示。
3. 讨论与结论
3.1 气体地球化学特征的构造基础
断裂带气体异常是地壳应力应变特征的反映,在构造应力作用下,岩层产生构造变形,地球内部气体沿着断裂及伴生裂隙等地壳薄弱地带逸散到大气中,从而造成地表土壤层中气体组分浓度异常(周晓成等,2012)。断裂破碎带的存在增强了岩层及土壤的渗透性,为地下不同来源气体的迁移提供了良好的通道。众多断层气观测实践表明,在断裂带附近可观测到土壤H2,Rn和CO2等气体组分异常,且气体异常程度与岩层的破碎程度密切相关(Fu et al,2005;刘舒波等,2012;Kumar et al,2017;Yuce et al,2017)。汤东断裂带的观测结果显示,沿张河村测线与邢李庄测线的H2,Rn和CO2气体高值异常均出现在主断裂附近,主要异常点出现在断裂带上盘位置距主断裂60—130 m距离处,张河村测线的气体观测效果更为明显,各观测期次间各观测组分的同步性较好。地球物理与跨断层联合钻孔详勘结果表明,张河村测线上主断裂的上断点埋深为77 m,邢李庄测线上断点埋深为114 m,主断裂的最新活动时期为晚更新世早期(中国地震局地球物理勘探中心,2016)。本文气体观测结果与地球物理探测结果之间的对应关系较好,土壤气体高值异常是断裂破碎带存在的表征,H2,Rn和CO2能敏感地指示断裂带的位置。断裂带的高渗透性为气体运移提供了优越条件,同时也是气体地球化学观测的构造基础。然而气体高值异常点出现在主断裂以西的上盘位置,而非主断裂带所在位置,这与汤东断裂的构造变形性质和活动习性密切相关。该断裂为右旋走滑正断型,走向为NEN,倾向NW,断裂错断过程中上盘更容易形成大量的次级断裂或裂隙,这些裂隙构成了内部气体运移的通道,导致气体组分在上盘构造的有利部位强烈释放。而主断裂本身可能因为挤压、走滑被断层泥、糜棱岩等物质胶结,通透性降低、导气作用弱化从而导致气体不易逸散。汤东断裂带的地球物理勘探结果(中国地震局地球物理勘探中心,2016)也表明,在主断裂以西存在多条次级断裂,次级断裂和裂隙的存在对地壳深浅部气体的释放有积极作用。
3.2 断裂带气体来源及迁移
土壤气体成分来源多样,由来自地幔及地壳深部、地壳浅部、沉积层以及大气的气体成分在近地表土壤层混合交织而成,且成因复杂。不同来源的H2在近地表混合,构成气体活动的背景值,断裂带的存在极大地增强了土层的渗透性,增强了深部与浅部来源间的联系,使断裂带的特征突显于背景之上。H2的生成机制主要包括:① 新鲜矿物表面的水岩作用(Sugisaki et al,1983),② 超基性铁镁质岩体在一定水热条件下的氧化还原反应(Seyfried et al,2007;Yoshizaki et al,2009),③ 含碳酸盐岩岩层中来自深部的富含CO2,CH4,H2,H2O的化学活动性流体与碳酸盐岩围岩发生化学反应(Sun et al,2017),④ H2O在放射性条件下的辐射分解作用(Dubessy et al,1988),⑤ 土壤中的生物作用等(Eisbrenner,Evans,1983;Sugimoto,Wada,1995;Peters,Conrad,1996)。对于汤东断裂而言,H2来源可能与上述①,③,⑤过程有关,汤东断裂带主断裂是晚更新世以来的活动断裂,断裂错动过程中,新生的破裂岩体表面的新鲜矿物与水之间发生水岩作用,流体中质子的活性被改变,在新形成的矿物表面生成硅醇基,进而生成大量的氢离子和氢气(Sugisaki et al,1983;Saruwatari et al,2004)。同时,汤阴地堑所在的中下地壳存在低速体,在其东侧下部存在一条高角度的深大断裂(刘保金等,2012;中国地震局地球物理勘探中心,2016)。在此种构造条件下,汤东断裂切穿沉积盖层,深入结晶基底,通过深大断裂与低速体甚至上地幔连通,富含CO2,CH4,H2,H2O的化学活动性流体沿该断裂带向上运移,与寒武系、奥陶系灰岩相互作用并通过机制③生成部分H2,继而沿断裂带逸出。此外,生物作用生成的H2也可能是H2的重要来源。位于汤阴地堑北部的安阳南断裂的痕量氢定点连续监测(观测井深8 m)数据的周年动态分析结果表明,H2浓度与地温、气压呈极显著正相关关系(p<0.001),12月至1月的H2浓度达到峰值。而张河村测线的数据监测结果显示,夏季(6月底)H2浓度显著高于1月和10月,这可能与两条测线表层的土壤质地有关,张河村测线为砂质土壤,质地疏松多孔,而邢李庄测线为黏质土壤,孔隙度相对较低。夏季,在强烈的生物作用下,表层土壤生物活动生成的H2在裂隙和孔隙发育的部位强烈逸出。
汤东断裂带的Rn观测结果也显示,Rn与H2、CO2在断裂带附近同步出现高值异常。Rn主要来自土壤或岩层中铀、钍、镭等放射性物质衰变。Ciotoli等(2007)指出,氡是断裂和地震活动最有效的示踪剂之一,对地震的构造过程有重要的指示作用,对地震活动前的构造应力积累也有很好的反映(Fu et al,2008;Walia et al,2008)。土壤Rn浓度异常与活动断裂的位置密切相关(Kumar et al,2017)。由于Rn的地球化学性质,222Rn不能通过扩散迁移机制进行长距离传输,但可通过载气输送到岩石或地表的孔隙空间中(Yuce et al,2010,2017)。而CO2作为Rn的优良载气,通过CO2的作用使Rn长距离迁移,因此能通过Rn与CO2的关系对Rn的迁移机制进行深入解析(Sciarra et al,2018)。在这种被动的迁移机制的作用下,二者在Rn与CO2的散点图中可能表现为Rn浓度随CO2浓度的增加而增加;此外,Rn浓度的增加也可能是由于气体的浅部循环使得Rn在土壤孔隙空间中的积累所致。从图4a可以看出:Ⅰ部分Rn的浓度不随CO2浓度而增加,高浓度的Rn可能主要由于土壤浅部循环积累而形成;Ⅱ部分则表现为Rn浓度随CO2浓度快速增加,Rn浓度的增加则可能反映地壳深部信息;而Ⅲ部分则表现为CO2浓度快速增加,而Rn变化缓慢,高浓度CO2可能是由于CO2的水平迁移作用造成的。而图4b是夏季(6月底)的观测结果,可以看出Rn可以分为两部分,Ⅰ部分与图4a相似,Ⅱ部分则包含图4aⅡ和Ⅲ部分信息。对比图4a与图4b可以看出,冬季观测结果能将Ⅲ部分划分开来,但夏季则会同Ⅱ混合在一起,主要是由于夏季生物化学作用强烈,CO2浓度快速增加,掩盖了图4a中的Ⅱ部分信息。由此可知,汤东断裂带气体可能包括深部来源信息。同时,研究区也存在深部物质来源的构造基础,汤阴地堑所在的中下地壳存在低速体,且汤东断裂下部存在一条高角度深大断裂(刘保金等2012;中国地震局地球物理勘探中心,2016)。Zhang等(2016)等对北京西部盆岭山区的泉水溶解气体3He和4He与δ13C研究表明,包含幔源信息的同位素值与地震层析成像P波速度负异常之间有很好的一致性。至于汤东断裂带深部气体来源及贡献尚需进一步进行气体成分分析与同位素组成研究。
图 4 2018年1月(a)和6月(b)汤东断裂Rn浓度与CO2浓度的相关性Ⅰ表示气体浅部循环,Ⅱ表示可能包含部分深部来源气体,Ⅲ表示气体水平迁移Figure 4. The relationships between Rn and CO2 concentration in Tangdong active fault zoneⅠ indicates that Rn mainly comes from shallow gas circulation,Ⅱ indicates that the fault gases could contain partial deep-source information,Ⅲ indicates that CO2 mainly comes from gas horizontal migration综上,汤东断裂带两测线不同测量期次H2,Rn和CO2浓度存在较好的同步性与重现性,断裂带附近显示高值异常,与活断层探测结果之间有较好的对应关系,气体高值异常是断裂存在的表征,且H2,Rn和CO2能敏感地指示断裂带位置。气体异常主要受断裂构造控制,汤东断裂带下方的深大断裂和汤阴地堑中下地壳的低速体对深部气体释放有重要作用,气体组分可能蕴含部分地壳深部信息。鉴于H2,Rn和CO2作为构造地球化学观测指标的敏感性与有效性,应对目标断裂的构造活动性进行长期连续观测,同时进一步开展气体组分来源与贡献研究,以强化气体地球化学指标对断裂构造活动性的指示意义,增强区域防震减灾工作的有效性。
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图 1 马尔康及邻区区域构造和MS≥5.0历史地震震中分布图
断裂数据引自徐锡伟等(2016);板块边界数据引自张培震等(2003)
Figure 1. Map of Barkam and neighboring regional geological settings and epicenters of historical MS≥5.0 earthquakes
Fracture data are after Xu et al (2016),and plate boundary data are after Zhang et al (2003)
Table 1 Focal mechanism solutions of the 10 June 2022 Barkam MS5.8,MS6.0 events and some MS≥3.5 aftershocks
Origin time
a-mo-d h:minMW M Best fit
depth/kmNodal plane Ⅰ Nodal plane Ⅱ strike/° dip/° slip/° strike/° dip/° slip/° 2 022-06-10 00:03 5.3 5.8 7 324 76 0 234 90 166 2 022-06-10 01:28 5.79 6.0 6 329 90 −3 59 87 −180 2 022-06-10 03:27 5.2 251 84 −175 160 85 −6 2 022-06-10 04:37 4.46 4.4 6 339 76 0 249 90 166 2 022-06-10 04:54 3.98 3.9 8 155 84 9 64 81 174 2 022-06-14 18:11 4.38 4.4 5 149 87 22 58 68 177 表 1 2022年6月10日马尔康MS5.8,MS6.0震群序列主震及部分MS≥3.5余震震源机制解
Table 1 Focal mechanism solutions of the 10 June 2022 Barkam MS5.8,MS6.0 events and some MS≥3.5 aftershocks
发震时间
年-月-日 时:分MW M 节面Ⅰ 节面Ⅱ 最佳拟合
深度/km走向/° 倾角/° 滑动角/° 走向/° 倾角/° 滑动角/° 2 022-06-10 00:03 5.3 5.8 324 76 0 234 90 166 7 2 022-06-10 01:28 5.79 6.0 329 90 −3 59 87 −180 6 2 022-06-10 03:27 5.2 251 84 −175 160 85 −6 2 022-06-10 04:37 4.46 4.4 339 76 0 249 90 166 6 2 022-06-10 04:54 3.98 3.9 155 84 9 64 81 174 8 2 022-06-14 18:11 4.38 4.4 149 87 22 58 68 177 5 注:由于6月10日3时27分发生的MS5.2地震存在波形叠加现象,因此选用HASH方法求取该地震震源机制解,该方法无法求取震源深度及矩震级。 Table 2 List of seismic parameters and results used in different stages
Stage Time period MC Number of events
with M≥MCa b Mmax p Ⅰ 2 022-06-10 00:03−01:03 2.1 69 3.1 0.61±0.10 5.0 0.76 Ⅱ 2 022-06-10 01:27−02:27 2.3 57 3.6 0.84±0.34 4.2 1.81 Ⅲ 2 022-06-10 03:27−04:27 1.7 66 3.2 0.86±0.10 3.7 1.64 表 2 不同阶段计算所用的地震参数及结果列表
Table 2 List of seismic parameters and results used in different stages
时间段 MC M≥MC地震次数 a b Mmax p 第 Ⅰ 阶段 2 022-06-10 00:03—01:03 2.1 69 3.1 0.61±0.10 5.0 0.76 第 Ⅱ 阶段 2 022-06-10 01:27—02:27 2.3 57 3.6 0.84±0.34 4.2 1.81 第 Ⅲ 阶段 2 022-06-10 03:27—04:27 1.7 66 3.2 0.86±0.10 3.7 1.64 注:MC为最小完整性震级,a为地震活动率,b为地震频率随震级增加的衰减率,p为余震频度衰减系。 -
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