Late Quaternary Faulted Landforms and Determination of Slip Rate of Jinqanghe Segment of Maya Snow Mountain Fault
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摘要: 祁连山东段发育了多条大型活动断裂,如近东西向展布的天桥沟-黄羊川断裂及北西西向展布的金强河断裂、毛毛山断裂、老虎山断裂等,在马雅雪山北麓、宝泉山隆起北缘还发育了一条整体呈北西-北西西向展布的马雅雪山断裂。其中,前人已对天桥沟-黄羊川断裂、金强河断裂、毛毛山断裂、老虎山断裂的晚第四纪活动开展了大量的研究,相比而言,马雅雪山断裂的研究程度还较低,其最新构造活动特征及其与区域主干活动断裂之间的关系等尚不清楚。马雅雪山断裂构成了天祝盆地与南部山体、丘陵的分界线,迹线清晰,断层三角面、断层槽谷多见,局部冲洪积阶地可见线性展布的断层陡坎,显示出断裂在晚第四纪有一定的活动。本研究对马雅雪山断裂西部的金强河段开展了实地调查,重点对马营沟及小黑刺沟2处的阶地断层陡坎开展了高精度地形地貌测量及阶地地貌面定年,对滑动速率进行了厘定。研究结果表明,马雅雪山断裂金强河段晚第四纪活动显著,断裂最近强震活动发生在8.21~3.43 ka BP,晚更新世晚期以来的垂直滑动速率为0.45~0.63 mm/a。Abstract: Several large active faults have developed in the eastern section of the Qilian Mountains, including the near east-west trending Tianqiaogou-Huangyangchuan fault, the northwest-west trending Jinqianghe fault, Maomaoshan fault, Laohushan fault, and the northwest to northwest-west trending Maya Snow Mountain fault. While extensive research has been conducted on the late Quaternary activity of the Tianqiaogou-Huangyangchuan, Jinqianghe, Maomaoshan, and Laohushan faults, the Maya Snow Mountain fault remains comparatively under-researched, particularly in terms of quantitative studies. The recent activity characteristics and relationship with the regional active faults remain unclear. The Maya Snow Mountain fault, which extends approximately 150 km and passes through Tianzhu city and several villages, serves as the boundary between the Tianzhu basin and the southern mountains and hills. Fault triangles and troughs are common along this fault, and linear fault scarps are visible on some alluvial and proluvial terraces, suggesting activity during the late Quaternary. Given its proximity to populated areas, further investigation into its activity is crucial. In this study, we focus on the Jinqianghe segment of the Maya Snow Mountain fault, located in the fault's western section. We conducted high-precision topographic and geomorphic measurements, dated terraces, and determined fault slip rates at the Maying River and Xiaoheici River fault scarps. Our findings indicate that the Jinqianghe segment of the Maya Snow Mountain Fault was significantly active during the late Quaternary and remains active into the early Holocene. A strong earthquake occurred at 8.21 ~ 3.43 ka BP, with vertical slip rate since the late Pleistocene estimated at 0.45 to 0.63 mm/a.
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Key words:
- Maya Snow Mountain fault /
- Late Quaternary /
- Faulted landform /
- Slip rate
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引言
由构造运动引起的地应力和地倾斜变化,在大地震发生前可能发生异常变化。由于台站建立在地壳表层,位于岩石圈与大气圈、水圈、生物圈的交界面上,其观测不可避免地受到诸多因素影响(中国地震局监测预报司,2008)。许多台站能观测到地应力(变)和倾斜的年度变化,且幅度因台站条件不同而各异,造成这种随季节变化的因素包括气温、气压、降水、地下水位的变化等,温度引起热弹性变形也被用来解释地应力和地形变的年变化(曹建玲等,2005)。Berge(1975)曾对正弦变化的沿水平地表传播的热波动引起的地应变和地倾斜给出了理论解。曹建玲等(2005)利用有限元方法考虑在平行山脊的地形条件下,气温年变化造成的应力和倾斜的幅度及其随深度的分布特征。就目前的认识水平,外界干扰与地震前兆的变化形态有时难以区分(王梅等,2004)。于桂春等(2011)仅定性地分析了气温与阿合奇地震台地倾斜变化关系。邢喜民等(2016)研究了温度、水位、气压对乌什体应变的影响,利用分段回归研究气象因素对精河、库尔勒水平摆的影响(邢喜民等,2015),然而并没有研究定量剔除气象因素之后的形变观测与周边强震之间的关系。
由于地壳构造的复杂性和震源区的不可直观性,地震的孕育和发生、其成因和机制等问题,至今尚无完满的解答(杨龙,2015)。众多大地震的震例研究表明,前兆异常会或多或少地出现在地震孕育过程的不同阶段(吴中海等,2013)。根据中国地震台网测定,2017年8月9日07时27分,新疆博尔塔拉蒙古自治州精河县发生6.6级地震(44.27°N,82.89°E)。本文首先尝试利用相关、回归分析研究气温对精河伸缩仪、水管仪的影响特征,并将其所造成的干扰定量剔除;其次,通过对回归残差进行分析,研究精河6.6级地震前精河伸缩仪、水管仪的异常特征,以期推动地震数值预测方面的研究。
1. 台站、仪器概况
精河地震台(44.62°N,82.97°E)位于新疆维吾尔自治区博尔塔拉蒙古自治州精河县境内,海拔387.3m,处于博罗科努、准格尔南缘、科古琴三大断裂带交汇区,该断裂以北为准噶尔中新生代凹陷和围绕准噶尔古老地块形成的准噶尔-北天山古生代褶皱系;断裂以南为塔里木地台北缘的多旋回地槽在晚古生代末期形成的天山褶皱系。历史上该区曾多次发生6.0级以上的中强地震。精河地震台与精河6.6级地震分布如图 1所示。
精河台观测山洞东西向为主洞,进深约90m,南北洞长约12m,宽约2.2m,洞内年温差0.5℃,日温差小于0.1℃,精河FSQ水管仪、SSY-Ⅱ型伸缩仪于1990年投入使用,水管、伸缩北南向长9.04m,东西向长29.98m(朱令人,2002)。2011年台站进行了数字化改造,仪器观测精度高,稳定性好,抗干扰能力强。精河水管、伸缩台站观测环境、观测条件均达到观测规范要求。
2. 气温对水管仪、伸缩仪影响的定量剔除
为研究精河水管仪EW向、伸缩仪NS向分量与气温之间的关系,选取2013年以来水管仪、伸缩仪和气温的观测数据为研究对象。由日均值曲线图(图 2)可以看出,精河水管仪EW向具有趋势E倾、伸缩仪NS向具有趋势压缩的变化,而精河的气温却没有与之一致的趋势变化。根据干扰异常识别与排除的4个“相关性”原则(车用太等,2011),认为精河水管仪EW向、伸缩仪NS向的趋势性变化与气温无关。选用最佳逼近多项式对精河水管仪EW向、伸缩仪NS向进行拟合,去掉2分量观测资料趋势,结果见图 3。
通过研究精河水管仪、伸缩仪各分量与气温的相关系数和滞后天数之间的关系,得到了相关系数与滞后天数的计算结果(表 1)及关系曲线(图 4)。同时,为判定观测数据与气温的相关特征,绘制了其与气温的散点图,如图 5所示。
表 1 精河水管仪、伸缩仪各分量和气温的相关系数与滞后天数Table 1. Correlation coefficient of the measurements of Jinghe station测项 水管仪NS 水管仪EW 水管仪EW去倾 伸缩仪NS 伸缩仪NS去倾 伸缩仪EW 相关系数 -0.6064926 0.286082 0.7645674 -0.7441875 -0.8625605 0.8993703 滞后天数 33 92 92 82 82 73 由表 1及图 5可知,为定量剔除气温对精河水管仪、伸缩仪各分量的影响,以气温作为自变量,选用适合的回归模型进行回归分析。为了方便记录,记定点形变观测数据为y,气温为x(℃),残差为ε,并通过可决系数评价各回归方程的拟合效果,其结果见表 2。
表 2 精河水管仪、伸缩仪与气温回归分析结果Table 2. Results of regression analysis of water tube tiltmeter, extensometer components and temperature in Jinghe station测项 回归方程 可决系数 拟合效果 原值 修正值 水管仪NS向 $ y = - 0.18801{x^2} - 3.45022x + 792.66731 + \varepsilon $ 0.434 0.4333 一般 水管仪EW向 $ y = 7.7023x - 63.2397 + \varepsilon $ 0.5846 0.5843 一般 伸缩仪NS向 $ y = - 335.723x + 2780.482 + \varepsilon $ 0.744 0.7439 较好 伸缩仪EW向 $ y = 14.65{x^2} + 652.7x - 12590 + \varepsilon $ 0.8513 0.8511 较好 通过回归分析得到的回归残差序列见图 6。由上述分析可知,气温是影响精河水管仪、伸缩仪年变的主要影响因素,气温与水管仪、伸缩仪各分量具有准线性关系,且水管仪NS、EW向,伸缩仪NS、EW向分别滞后气温天数约为33、92、82、73天。
3. 精河6.6级地震前的异常特征分析
由图 6可以看出,在剔除气温对精河水管仪、伸缩仪的影响之后,2013年起水管仪NS分量持续N倾,并于2016年5月22日出现转向S倾的异常,S倾幅度达0.53",精河6.6级地震发生后异常恢复;2014年沙湾5.0级地震前,水管仪EW分量出现E倾变化,并于2015年9月由E倾转为W倾,之后发生了新源5.0级地震;在精河6.6级地震前,2017年3月24日水管仪再次出现E倾异常变化,至精河地震发生时E倾幅度达0.28",地震发生后持续E倾,2017年9月29日E倾结束,异常恢复。此外,自2013年起精河伸缩仪NS分量变化较平稳,而自2016年9月10日起出现拉张的异常变化,2017年5月20日拉张速率减慢,精河6.6级地震发生后出现压缩变化,异常恢复;2017年以来伸缩仪EW向虽有拉张迹象,但异常并不明显。由精河6.6级地震构造应力场图(图 7)可以看出,精河地震台附近区域的构造应力场水平最大主应力优势方向为近NS向,区域压应力轴为近NS向,这说明精河6.6级地震前伸缩仪NS分量的趋势压缩异常是可靠的。
4. 结论与讨论
形变台站(测点)作为多输入、单输出(观测值)的系统,在观测值序列中不仅包含着来自地球内部的地形变信息,还包含着各种影响因素的信息。观测值序列既然是多种信息的综合,必然也是可分离的(中国地震局监测预报司,2008)。
本文利用相关、回归分析研究了气温对精河水管仪、伸缩仪的影响特征,并尝试定量剔除气温对其影响,结果表明精河水管仪、伸缩仪观测数据的趋势性变化与气温无关;气温与精河水管仪、伸缩仪各分量具有准线性关系;气温是影响精河水管仪、伸缩仪各分量年变形态的主要影响因素;精河水管仪NS、EW向,伸缩仪NS、EW向滞后气温天数分别为33、92、82、73天;另外,对回归残差序列进行了分析,在精河6.6级地震前出现水管仪NS分量S倾0.53",EW分量E倾0.28"以及2016年9月10日起伸缩仪的压缩变化等异常变化。通过对比图 2、图 6可知,剔除气象因素对形变观测数据的影响后,震前异常特征与幅度将更接近由地震引起的异常变化。今后,可通过对大量震例进行总结,尝试建立异常幅度、异常持续时间等异常特征与地震关系,以期推动地震数值预测的发展。
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表 1 样品光释光测试参数及年龄
Table 1. OSL dating results of the samples
样品编号 埋深/m U/(μg·g−1) Th/(μg·g−1) K/% 含水量/% 环境剂量率/(Gy·ka−1) 等效剂量/Gy 年代/(ka BP) OSL-MY-3 0.87 2.67±0.06 14.4±0.19 2.14±0.02 2.05 4.87±0.36 35.64±1.05 7.33±0.58 OSL-MY-4 0.49 2.75±0.05 14.8±0.38 2±0.01 6.31 4.59±0.32 37.02±1.68 8.06±0.67 OSL-MY-5 0.53 2.71±0.06 14±0.36 1.9±0.01 3.59 4.54±0.33 156.74±7.78 34.56±3.02 OSL-MY-6 0.55 2.56±0.02 14.7±0.28 2.23±0.02 5.05 4.8±0.34 44.43±2.28 9.25±0.81 OSL-MY-7 0.6 3.21±0.03 17±0.19 2.32±0.02 5.13 5.33±0.38 18.29±0.57 3.43±0.27 OSL-MY-8 0.45 2.49±0.03 14.1±0.27 2.02±0.02 2.57 4.66±0.34 2.34±0.09 0.5±0.04 -
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