Study on Nonlinear Statistical Characteristics of Surface/Downhole Response Spectrum Ratio and Influencing Factors
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摘要: 浅地表软弱覆盖层对地震动的影响具有强非线性,但因观测记录样本量过小,难以通过参考谱比法等直接方法予以可靠分析。本文基于日本KiK-net台网136个竖向钻井台阵获取的141881组加速度记录的统计分析,研究了地表/井下反应谱比值随地震动强度变化的非线性变化规律与主要影响因素。利用变窗口尺度的滑动窗口平均法加强数据线性度,并确定了与地震动强度相关的台站最小记录样本量,统计结果表明地表/井下反应谱比值平台值随地震动强度变化的非线性衰减指数为−0.24~−0.08。利用套索(Lasso)算法统计回归发现,本研究12个场地特性表征参数中,30 m平均剪切波速(
$ {V}_{\mathrm{s}30} $ )、场地卓越频率与地表/井下反应谱比值非线性衰减指数具有较好的正相关性;较弱强度地震动作用下地表/井下傅里叶谱比值峰值、地表/井下反应谱比值峰值与地表/井下反应谱比值非线性衰减指数具有较好的负相关性。Abstract: The influence of shallow soft overburden on ground motion is strongly nonlinear, but it is difficult to make reliable analysis by direct methods such as reference spectral ratio method because of the small sample size of observation records. In this study, based on the statistical analysis of 141881 sets of acceleration records obtained from 136 vertical borehole array in Japan’s KiK-net network, the nonlinear variation of surface/downhole response spectrum ratio and its main influencing factors of field motion intensity were studied. The sliding window average method with variable window scale was used to enhance the linearity of the data, and the minimum sample size related to the ground motion intensity was determined. The statistical results show that the nonlinear attenuation index of ground motion intensity variation is between −0.24 and −0.08 for the platform value of the surface/underground response spectrum ratio. According to Lasso regression statistics, the average shear wave velocity of 30 m ($ {V}_{S30} $ ) and site predominant frequency have a good positive correlation with the nonlinear attenuation index of surface/downhole response spectrum ratio among the 12 site characteristic parameters studied. However, the peak values of the surface/downhole fourier spectrum ratio and the surface/downhole response spectrum ratio are negatively correlated with the nonlinear attenuation index of the surface/downhole response spectrum ratio under the weak ground motion.-
Key words:
- Strong motion /
- Site effect /
- Response spectrum /
- Nonlinear /
- Vertical borehole arrays
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引言
2014年8月3日16点30分,云南省昭通市鲁甸县发生强烈地震(简称鲁甸“8·03”地震),震中位于27.1°N,103.3°E,震级MS 6.5,震源深度12km(民政部国家减灾中心,2014)。根据民政部门的统计,地震造成617人死亡,112人失踪,3143人受伤,大量房屋损毁。本次地震震级高,震源浅,震区地质环境复杂,地形陡峻,诱发了大量崩塌、滑坡等次生山地灾害(李西等,2014;陈兴长等,2015)。本文旨在野外调查和实地勘测的基础上,分析地表破裂的方式和性质,为地震次生山地灾害预测与减灾提供依据。
1. 研究区域
地震发生在青藏高原东南缘著名的鲜水河-安宁河-则木河-小江断裂带东侧的昭通-莲峰断裂系上(李西等,2014;王腾文等,2015)。区内断裂发育(图 1),构造对工程区域稳定存在影响的断裂主要有则木河断裂(线走向主要为NE向,F13)、大凉山断裂(F4)、小江断裂(F17)、昭通-鲁甸断裂(F11)、五莲峰断裂(F10)、马边-盐津断裂(F8)、会泽-彝良断裂(F19)和包谷垴-小河断裂(F28)等。
王家坡不稳定斜坡位于红石岩堰塞湖的北东方向,红石岩村的北面,牛栏江的右岸,河流流向在此由近南北向转为近东西向,不稳定斜坡处于河流转弯的弧形下部。研究区多属构造侵蚀、溶蚀为主的高中山区和中山区,这里山高谷深,地形陡峻,切割剧烈,基岩多裸露,岩体主要为强风化白云岩、白云质灰岩和风化残坡积土。地势总体上呈西高东低之势。地区的相对高差在1000m以上,属于强侵蚀高中山峡谷地形地貌区。山脉延伸方向与地层及构造线走向大体一致,河谷深切多呈“V”型或“U”型河谷,谷底比降较大,天然比降约6.6‰。
2. 发震构造
根据云南地震实测资料(图 2),2014年8月3日地震震中东西向最大水平加速度达985gal(刘爱文等,2014),垂直加速度约504gal,震中及枢纽工程区地震影响烈度为Ⅸ度。本次地震Ⅵ度及以上烈度区总面积为10350km2,极震区烈度达Ⅸ度(帅向华等,2014),包括龙头山镇大部分地区及火德红镇和包谷垴乡部分区域。等震线长轴呈NNW至NW向展布,反映出引起地震灾害的地震动沿NNW至NW向衰减较NEE向慢,衰减较慢的方向一般与发震断层走向或破裂扩展方向一致,故根据等震线长轴走向判断发震断裂为NW向的包谷垴-小河断裂。该断裂位于川滇块体东侧的凉山次级块体南缘的昭通-莲峰断裂带内,断层走向NNW,近直立,由数条断续展布的断层组成,南东起于包谷垴以北的月亮山一带,北西经龙头山、乐红、小河、满天星,止于东坪一带,长度约47km(王腾文等,2015)。据现场调查,断裂地表线性构造不明显,破碎带宽约5—20m,主要由断层角砾岩、片状岩及构造透镜体组成,含少量糜棱岩及断层泥,胶结中等。断裂形成于加里东期,断错了寒武系至二叠系等各地层单元,具有长期的活动历史。
3. 地表破裂特征
3.1 地表破裂分布
鲁甸地震造成的王家坡强变形区地表破裂带大致位于27°06′30″N,103°27′20″E,是一条整体走向N45°W—N50°W的左旋走滑破裂带,NW走向的具有明显左行走滑特征的包谷垴-小河断裂为2014鲁甸地震的发震构造(李西等,2014),现场调查发现,单条地表破裂又以2组方向为主,一组为N45°W—N55°W,另一组为N25°E—N65°E(图 3),其中部分地表破裂走向与主断裂近平行,主要分布在强变形区两侧,部分地表破裂走向与主断裂斜交,少量与主断裂垂直。
该区域发育有3条主要的同震地表破裂,可划分为长约444m的北东段、长约294m的中段和长约381m的南西段等3个相对独立的地震地表破裂段,分别为强变形区的左侧地表破裂边界、后缘地表破裂和右侧地表破裂边界,最大左旋位移分别为0.33m、0.62m和0.55m。除后缘地表破裂延伸方向为N60°W外,左侧地表破裂边界、右侧地表破裂边界的延伸方向均为N40°W,后缘地表破裂将左、右2条地表破裂边界相连。地表破裂的延伸长度普遍较长,总体在6m以上,最长的一条地表破裂长度可达320m,在延伸过程中以塌陷坑、拉裂槽出露。根据现场勘查及推断,该条地震地表破裂带南东起于王家坡滑坡后缘,北西止于斜坡缓坡、基岩出露的交界处。张开程度在垂直方向上呈现上宽下窄的形态,向深部逐渐尖灭。在平面形态上,北西向地表破裂张开程度较小,南东向张开程度较大,具有从深部沿SSE方向向浅地表逐步扩展的破裂过程。破裂缝内充填物主要有2类,一类为第四系残积土泥质填充,另一种充填物为全-强风化碎石土填充(图 4(a)及(b))。
3.2 典型地表破裂性质
根据现场调查,王家坡地表破裂方式主要有剪切破裂、张剪切破裂、压剪切破裂、张性破裂和地震鼓包等(孙鑫喆等, 2010, 2012;张桂芳等,2011)。其中,最典型的剪切破裂发育在强变形区北西侧,主要为LF6的北西段(破裂位置见图 3),以及一种仅见左旋走滑分量的地表破裂单元,例如P型剪切破裂(图 5(a)),其主要特征是破裂走向与发震断层走向基本一致,破裂面近于直立,两侧以同震左旋走滑位移为主,差异升降运动不明显,并伴随产生一些次级地表破裂(图 5(b))。王家坡地表破裂带最为发育的基本破裂单元是张剪切破裂,其走向N50°W±20°,同时兼有左旋走滑分量和垂直于破裂走向的张开分量或正断倾滑分量,局部地段伴随有南西盘数十厘米的正断层状下降现象。主要集中在强变形区中、后侧(图 5(c))。王家坡张剪切破裂常呈雁行或左行右阶斜列状组合,呈整体走向N65°W±5°的地震地表破裂带,拉开宽度20—30cm,与地表破裂带之间存在着小于等于30°的夹角。最典型的张剪切破裂为LF5,走向N70°W,整体呈现北东盘上升、南西盘下降的趋势,可见相关的小拉分盆地或陷落坑(图 5(d))。压剪切破裂主要分布在王家坡地表破裂带南西侧,以LF6南东段为主,走向N42°E—N48°E,与地表破裂带整体走向基本一致,形成了南西盘抬升、北东盘下降的陡坎(图 5(e))。与之对应的后缘地表破裂LF5,其南西盘相对北东盘出现很明显的下错(图 5(f)),这在一定程度上说明了强变形区的变形方向及变形程度,经现场实测,整体的滑动方向为S10°W—S20°W。王家坡张性破裂走向一般为N65°E—N80°E,呈张开状或出现南侧盘块体下降现象(图 5(g)),与剪切破裂之间的差异在于张性破裂呈雁行斜列。张性破裂主要分布在2处,一处为地表破裂带南西侧,为王家坡坡度陡缓交接的部位;另外一处为王家坡滑坡后缘,在接近后壁的地方形成张性地表破裂(图 5(h))。王家坡地表破裂带上的鼓包主要出现在LF19处。在地震作用下,LF19同震左旋走滑位移逐渐增大,不断挤压临近的基岩,造成裂缝前缘隆起,局部形成鼓包,从形态特征上看,多为圆弧状鼓包(图 5(i))。
4. 地表破裂组合方式
通过对王家坡不稳定斜坡的高分辨率遥感影像的分析,以及野外考察表明,王家坡不稳定斜坡的地表破裂是由剪切破裂、张剪切破裂、压剪切破裂、张性破裂和鼓包等基本破裂单元组合而成,在不同的区域有着不同的组合方式(董彦芳等,2012;周春景等,2014;李海兵等,2015;罗文行等,2015)。王家坡地表破裂主要的排列组合方式有3种。第一种,由强变形区右侧主要的3条走向为N45°W—N50°W的左旋走滑破裂组成,呈左行左阶羽列,也是该区域最为常见的排列组合方式。第二种为左行右阶羽列,多为出现于局部的地表破裂。第三种为雁行斜列式,主要由强变形区SW侧前缘张性破裂构成,在雁列区局部出现拉分构造。地表破裂类型和基本组合特征等显示出王家坡潜在不稳定斜坡上的地表破裂带具有左旋走滑的性质。
5. 成因初析
通过对以上几个典型地表破裂的调查研究发现,地表破裂在一般情况下被认为是断层在地表的表现(Yeats等,1997;Xu等,2006;Zhang等,2010),根据地表破裂的特征可以发现,其中左、右地表破裂边界与发震断层的出露位置一致,是断层错动引起的;而部分地表破裂与断层的位置不重合,其成因分为2种,一种是发震断层导致的一些次级地表破裂,另一种是地震引发的滑坡后缘破裂。
6. 结论
(1)根据野外调查和分析,2014年8月3日16点30分发生的云南鲁甸地震,在王家坡潜在不稳定斜坡上所造成的地表破裂带是一条总体走向为N45°W—N50°W的地表破裂带。
(2)该地震地表破裂带由剪切破裂、张剪切破裂、压剪切破裂、张性破裂和鼓包等典型破裂样式所组成,由左旋走滑破裂(左行左阶羽列)、左行右阶羽列和雁行斜列式等组合方式。地表破裂类型和基本组合特征显示出王家坡潜在不稳定斜坡上的地表破裂带具有左旋走滑的性质。
(3)地表破裂的产生方式有2种,一种是同震地表破裂,另一种是震后形成的滑坡后缘破裂。
致谢: 感谢裴向军教授在野外关于地表破裂分析所提出的宝贵意见。 -
表 1 不同地震动强度分组下的最小可信样本量
Table 1. Minimum reliable sample size for different ground motion intensity groups
组别 1 2 3 4 5 6 7 8 9 10 11 12 分组标准/cm·s−2 0.178~
0.5620.316~
1.0000.562~
1.7801.000~
3.1601.780~
5.6203.160~
10.0005.620~
17.80010.000~
31.60017.800~
56.20031.600~
100.00056.200~
177.800>100.000 n0平均值/个 30 27 25 21 17 14 10 7 5 3 2 1 表 2 场地条件表征参数
Table 2. The site characterization parameters selected in this paper
参数 含义 定义 参考文献 $ {V}_{\rm{se}} $ 我国规范定义的等效剪切波速 ${V}_{\rm{se}}={d}_{0}/{\displaystyle\sum }_{i=1}^{n}\left({d}_{i}/{V}_{\mathrm{s}i}\right)$ 中华人民共和国住房和城乡建设部等,2010 $ {V}_{\mathrm{S}30} $ 美国规范定义的平均剪切波速 ${V}_{\mathrm{S}30}=30/{\displaystyle\sum }_{i=1}^{n}\left({d}_{i}/{V}_{\mathrm{s}i}\right)$ BSSC,2001(美国NEHRP规范) $ {B}_{30} $ 剪切波速剖面梯度 $ {\mathrm{log}}_{10}{V}_{\mathrm{s}}\left(z\right)={B}_{Z\mathrm{m}\mathrm{a}\mathrm{x}}{\mathrm{log}}_{10}\left(z\right)+{A}_{Z\mathrm{m}\mathrm{a}\mathrm{x}}\pm {\sigma }_{Z\mathrm{m}\mathrm{a}\mathrm{x}} $ Régnier等,2013 $ {Z}_{0.5} $ 0.5 km/s覆盖层厚度 剪切波速达0.5 km/s时的土层深度 中华人民共和国住房和城乡建设部等,2010 $ {Z}_{0.8} $ 0.8 km/s覆盖层厚度 剪切波速达0.8 km/s时的土层深度 Zhu等,2020 $ {Z}_{1.0} $ 1.0 km/s覆盖层厚度 剪切波速达1.0 km/s时的土层深度 Zhu等,2020 $ {V}_{\mathrm{s}\mathrm{a}\mathrm{v}} $ 整个土层沉积平均剪切波速 地表到$ {V}_{\mathrm{s}}>800 $ m/s的“地震基岩”
土层厚度内的走时平均剪切波速Pitilakis等,2013 $ {f}_{\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{d}} $ 场地卓越频率 弱震平均傅里叶谱放大系数峰值对应的频率 Régnier等,2013 $ {f}_{\mathrm{s}} $ 场地基本频率 弱震HVSR曲线峰值对应的频率 陈国兴等,2020 $ {T}_{\mathrm{s}} $ 场地基本周期 $ {f}_{\mathrm{s}} $的倒数 陈国兴等,2020 $ {A}_{\mathrm{s}} $ 弱震平均傅里叶谱放大系数峰值 — Régnier等,2013 $ {A}_{\mathrm{s}\mathrm{r}} $ 弱震平均反应谱放大系数峰值 — -
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