汶川地震热异常与构造应力关联的数值模拟

引用本文

朱传华, 单新建, 张国宏, 焦中虎, 张迎峰, 李彦川, 乔鑫. 汶川地震热异常与构造应力关联的数值模拟[J]. 地震地质, 2019,41(6): 1497-1510.
ZHU Chuan-hua, SHAN Xin-jian, ZHANG Guo-hong, JIAO Zhong-hu, ZHANG Ying-feng, LI Yan-chuan, QIAO Xin. TWO DIMENSIONAL MODEL ON RELATION BETWEEN THERMAL ANOMALY BEFORE WENCHUAN EARTHQUAKE AND TECTONIC STRESS[J]. SEISMOLOGY AND GEOLOGY, 2019,41(6): 1497-1510. 复制到剪切板

DOI:10.3969/j.issn.0253-4967.2019.06.012
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汶川地震热异常与构造应力关联的数值模拟

朱传华, 单新建^*, 张国宏, 焦中虎, 张迎峰, 李彦川, 乔鑫

中国地震局地质研究所, 地震动力学国家重点实验室, 北京 100029

*通讯作者: 单新建, 男, 1966年生, 研究员, 主要从事地壳形变观测与动力学研究, E-mail: xjshan@163.com。

〔作者简介〕朱传华, 男, 1989年生, 2017年于中国石油大学(华东)获地质资源与地质工程专业博士学位, 主要从事热流固耦合的数值模拟研究, E-mail: zhu_chuanhua@163.com。

收稿日期: 2019-04-23

基金项目: 国家自然科学基金(41631073, 41701409)、重大自然灾害监测预警与防范重点研发项目(2018YFC1503602)和中国地震实验场专项(2018CSES0205)共同资助

摘要

地震孕育的过程中, 在一定时空范围内会出现热异常现象, 该异常可能与构造活动存在密切联系, 能否利用与构造活动相关的物理过程解释该热异常现象, 目前尚未取得共识。为进一步揭示构造应力与地震热异常的关系, 文中基于二维多孔介质热流固(THM)耦合模型, 模拟了汶川地震前由于断层应力释放引发的热异常演化特征, 发现流体对流和岩体应力致热效应导致的热异常现象在空间分布、时间演化、异常量级上皆与观测数据有较好的一致性。结果显示: 汶川地震前, 受断层应力释放的影响, 热异常现象主要出现在断裂带及其相邻的上盘区域, 沿断层走向呈带状分布, 以增温异常为主, 断裂带区域可发生短时降温; 断裂带区域热异常受流体对流和应力致热作用联合控制, 增温强度先增大后减小, 通常可产生高于1K量级的空气增温; 非断裂带区域的增温强度主要取决于流体对流作用的强弱, 在地震前逐渐增强, 对渗透率的响应显著, 通常当渗透率≥10^-13m²时, 才能出现1K量级的空气增温异常。文中分析认为, 地震前断层应力释放引起的流体热对流和应力致热作用可以解释地震前的热异常现象。

关键词: 地震前; 汶川地震; 孔弹性模拟; 应力致热

中图分类号:P315.72 文献标志码:A 文章编号:0253-4967(2019)06-1497-14

TWO DIMENSIONAL MODEL ON RELATION BETWEEN THERMAL ANOMALY BEFORE WENCHUAN EARTHQUAKE AND TECTONIC STRESS

ZHU Chuan-hua, SHAN Xin-jian, ZHANG Guo-hong, JIAO Zhong-hu, ZHANG Ying-feng, LI Yan-chuan, QIAO Xin

State Key Laboratory of Earthquake Dynamics,Institute of Geology,China Earthquake Administration,Beijing 100029,China

Abstract

It has been reported that there is thermal anomaly within a certain time and space preceding an earthquake, and previous research has indicated potential associations between the thermal anomaly and earthquake faults, but it is still controversial whether physical processes associated with seismic faults can produce observable heat.
Based on rock experiments, some scholars believe that the convective and stress-induced heat associated with fault stress changes may be the cause of those anomalies. Then, did the thermal anomaly before the Wenchuan earthquake induced by the fault stress change?It remains to be tested by numerical simulations on the distribution and intensity of thermal anomalies. For example, is the area of thermal anomaly caused by the fault stress changes before the earthquake the same as the observation?Is the intensity the same?To clarify the above questions, a two-dimensional thermo-hydro-mechanical(THM)finite element model was conducted in this study to simulate the spatial and temporal variations of thermal anomalies caused by the underground fluid convection and rock stress change due to the tectonic stress release on fault before earthquake. Results showed that the simulated thermal anomalies could be consistent with the observed in magnitude and spatio-temporal distribution. Before the Wenchuan earthquake, deformation-related thermal anomalies occurred mainly in the fault zone and its adjacent hanging wall, which are usually abnormal temperature rise, and occasionally abnormal cooling, occurring in the fault zone after the peak temperature rise. In the fault zone, the thermal anomaly is usually greater than the order of 1K of the equivalent air temperature and is controlled by the combined effect of fluid convection and stress change. The temperature increases first and then decreases before the earthquake. In the hanging wall, it’s weaker than that of the fault zone, mainly depending on the convection of the fluid. The temperature gradually increases before the earthquake and is dramatically affected by the permeability. Usually, only when the permeability is larger than 10^-13m², can the air temperature rise higher than 1K occur. The results of this study support the view that fluid convection and stress change caused by fault slip before the earthquake can produce observable air temperature anomalies.

Keyword: pre-earthquake; Wenchuan earthquake; poroelastic model; stress induced warming

文章图片

0 引言

研究与构造应力相关的热异常分布规律, 可加深我们对地震热异常的认识。地震前一定时空范围内会出现热异常现象, 其与发震断层之间存在一定的空间关联, 如多发于震中附近、沿发震构造线性迁移等(张治洮, 1994; 强祖基等, 1998; 徐秀登等, 2001; 单新建等, 2005; Tramutoli et al., 2005; Ouzounov et al., 2007; Lisi et al., 2010; Pergola et al., 2010; Zoran, 2012)。那么, 地震热异常是否由断层应力异常引起?马瑾等(2012)通过断层黏滑实验研究发现断层在进入亚失稳阶段时, 断层及两盘总体上会由应力积累状态转变为应力释放状态。并且, 基于物理实验结果, 前人认为与断层应力变化相关的应力致热效应和流体热对流可能是地震热异常发生的原因(Qiang et al., 1991; 耿乃光等, 1992; Wu et al., 1998; Tronin et al., 2002; 刘力强等, 2004; 刘善军等, 2004; 陈顺云等, 2009)。

物理实验研究虽然从理论上证明了构造应力变化引发热异常的可能性, 然而要证实热异常是否由构造应力所引发, 仍有待于通过数值模拟手段对断层应力释放引发的热异常分布和量级进行检验。例如, 地震前断层应力释放引起的热异常所发生的区域与观测结果是否相同?应力致热与流体热对流形成的热异常量级与观测值是否相符?基于宏观多孔介质THM耦合理论, 数值模拟断层应力释放所导致的应力致热和热对流异常变化, 是回答上述问题的有效手段。丁留伟(2013)利用该理论模拟了地震前由于断裂带渗透率的增强导致流体沿断裂带快速向上运移、进而使其升温的过程, 但该研究未涉及地震前断层应力的释放过程。

2008年5月12日, 青藏高原东缘的龙门山断裂带发生了8.0级汶川地震, 震中位于断裂带中段(图1a, b)。地震发生后, 众多学者通过回溯性研究发现, 地震前发生了多种热异常现象, 最早始于震前1a之前, 多始于震前1个月内, 沿断裂带降温与区域性升温现象并存(图1c— f)。同时, 王凯英等(2012, 2018)基于紫坪铺水库区域台网记录的震前近4a的震中区486个小震的震源机制资料, 运用碎裂分析法得到震源区断层面上的应力演化过程, 发现2007年6月之前震中区以应力积累为主, 之后转变为应力释放, 空间上向2个应力异常高值区集中, 周围区域协同弱化, 最后2008年汶川地震则发生于应力集中和卸载区间的梯度带上, 与物理实验中断层亚失稳阶段的应力演化特征一致(马瑾等, 2012), 表明汶川地震前北川断裂已开始应力卸载过程。上述研究为断层应力释放与热异常关系的研究提供了基础资料。

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图 1 汶川地震构造背景及震前热异常概况
a 研究区位置图, 位置如图中红色方框所示, 黑色粗实线为板块边界, 背景图为SRTM 90m DEM数据^①(① http://srtm.csi.cgiar.org。); b研究区构造背景图, 黑色实线表示活动断层(徐锡伟等, 2016), 红色实线表示2008汶川地震地表破裂带(Xu et al., 2009), 蓝色箭头及圆圈为GPS速度及误差(Gan et al., 2007), 红色虚线框为图2b中GPS数据的范围, 红色虚线为模型剖面位置, 红色五角星为2008年汶川地震震中, 震中和震源机制解来自GCMT^②(② https://www.globalcmt.org。); c 2007年OLR增强区域(Jing et al., 2013), 断层及震中同图b; d 震前潜热通量增强区(严研等, 2008); e 震前及同震降温区(陈顺云等, 2013); f 震前热异常现象的时间演化特征, 横轴为距汶川地震的时间, 冷色调表示降温, 暖色调表示升温, 颜色浓度的变化表示异常强度的变化趋势, 颜色无变化表明无相关强度变化信息Fig. 1 Structural setting and pre-seismic thermal anomalies of the Wenchuan earthquake.

为明确汶川地震前的热异常现象是否与发震断层的应力释放有关, 本文利用多孔介质THM耦合理论模拟了地震前断层应力释放作用下的应力致热与流体热对流的演化过程, 分析了二者引起的热异常分布及强度, 初步揭示了地震前热异常与构造过程的协同演化特征。研究表明, 热异常模拟结果与观测结果在空间分布和量级上有良好的一致性, 支持汶川地震前的热异常现象与断层应力释放存在关联的观点。该研究可深化我们对与构造相关的地震热异常的认识。

1 THM耦合物理方程

连续介质THM耦合理论主要基于孔弹性和热弹性耦合理论发展而来, 其分别通过胡克定律描述弹性形变、达西定律描述多孔介质内部流体渗流和傅里叶定律描述热传导(Biot, 1962; Bear et al., 1990; Jing et al., 2003; Nield et al., 2013)。

1.1 多孔介质孔弹性方程

孔弹性理论用来描述多孔弹性介质变形过程中与内部流体流动之间相互作用的过程。本文的本构方程来自Biot(1962)提出的孔弹性理论:

$σ = σ_{ex} + Cε - α_{B} p_{f} I$ (1)

其中, σ 为柯西应力张量, ε 为弹性应变张量, α _B为Biot-Willis系数, p_f为流体孔隙压力。 C为排水、定孔隙压力条件下的弹性张量, I为特征张量。 σ _ex用于表示预应力及黏弹性应力的贡献等。本文采用Kelvin-Voigt黏弹性模型, 黏性应力贡献为

$σ_{vis} = 2 η \overset{\cdot}{ε} = 2 Gτ \overset{\cdot}{ε}$ (2)

其中, η 为黏滞系数, G为剪切模量, τ 为松弛时间。

根据Biot理论, 流体压力与多孔介质体积应变和流体增量呈线性关系, 即:

$p_{f} = \frac{1}{S} (V - α_{B} ε_{vol})$ (3)

其中, ς 为流体增量, ε _vol为多孔介质体积应变, 储水系数S为

$S = \frac{ε_{p}}{K_{f}} + (α_{B} - ε_{p}) \frac{1 - α_{B}}{K_{d}}$ (4)

ε _p为孔隙度, K_f流体体积模量, K_d为多孔介质排水体积模量。

孔弹性理论模型中用达西定律描述多孔介质中流体的流动, 根据质量守恒定律, 有

$\frac{\partial}{\partial t} (ρ_{f} ε_{f}) + \nabla \cdot (ρ_{f} u) = Q_{m}$ (5)

达西渗流模型为

$u = - \frac{κ}{μ} (\nabla p_{f} + ρ_{f} g \nabla D)$ (6)

储水模型为

$\frac{\partial}{\partial t} (ρ_{f} ε_{f}) = ρ_{f} S \frac{\partial p_{f}}{\partial t}$ (7)

一定体积的多孔介质内流体体积增量, 即孔隙体积增量为

$Q_{m} = - ρ_{f} α_{B} \frac{\partial}{\partial t} ε_{vol}$ (8)

最终, 流体连续性方程可写成:

$ρ_{f} S \frac{\partial p_{f}}{\partial t} + \nabla \cdot (ρ_{f} u) = - ρ_{f} α_{B} \frac{\partial}{\partial t} ε_{vol}$ (9)

式中, u为达西速度张量, κ 为多孔介质的渗透率, μ 为流体的黏度, ∇p_f为流体压力梯度, ρ _f为流体密度, g为重力加速度, ∇D为高程梯度。

1.2 热弹性方程

地壳内部传热机制主要有热对流、热传导和应力致热3种机制, 本文采用局部热均衡理论, 即假设局部区域岩体与流体的温度相同, 而忽略二者热交换的过程, 因为本文主要考虑地表总热量变化, 不具体关注岩石和流体的温度差异(Bear et al., 1990; Nield et al., 2013)。

考虑热传导和热对流的热守恒方程为

${(ρ C_{p})}_{eff} \frac{\partial T}{\partial t} + ρ C_{p} u \cdot \nabla T + \nabla \cdot q = Q + Q_{ted}$ (10)

$q = - k_{eff} \nabla T$ (11)

${(ρ C_{p})}_{eff}$ 为恒压有效体积热容, 定义为

${(ρ C_{p})}_{eff} = θ_{p} ρ_{p} C_{p, p} + (1 - θ_{p}) ρ C_{p}$ (12)

ρ 为流体密度, C_p为流体恒压热容, C_{p, p}为固体恒压热容, q为传导热通量, u为达西速度, Q为热源(汇), k_eff为有效热传导系数, 由流体k、固体k_p导热系数及其孔隙度(1-θ _p)共同决定:

$k_{eff} = θ_{p} k_{p} + (1 - θ_{p}) k$ (13)

应力致热, 即应力变化生成的热量为

$Q_{ted} = - αT : \frac{d S}{d t}$ (14)

式中, α 为热膨胀系数, S为第二Piola-Kirchhoff应力张量。

设空气等效增温值为Δ T, 其可通过累积CT或DT通量Δ Q表示:

$Δ T = \frac{Δ Q}{C ρ_{air}}$ (15)

式中, C为空气恒容热容, ρ _air为空气密度。

根据耦合方程(式(10)), 地表热量(Q)主要受岩石传导热通量、应力致热(DT)和流体热对流(CT)三者共同影响。在边界条件恒定的前提下, 三者会呈现恒定或线性变化的特征, 当断层发生应力释放, CT和DT则将首先表现出非线性, 即热异常。由于传导热通量取决于温度梯度, 仅可在CT和DT产生异常后, 才会进一步出现异常, 因此, 后文将重点分析CT和DT的特征。

2 汶川地震二维有限元模型

汶川地震震中位于龙门山断裂带中段, 断层产状稳定, 且以逆冲运动为主(Shen et al., 2009; Xu et al., 2009; Wang et al., 2011), 因此, 根据二维平面应变理论可用垂直于断层的二维模型近似模拟龙门山断裂带中段的三维模型, 模型位置如图1b所示。模型的构建、离散及求解均在有限元商业软件COMSOL Multiphysics中进行。

2.1 多孔介质几何模型

几何模型主要涉及2条发震断层的产状、断裂带厚度及地壳结构(图2a)。断层产状采用Wang等(2011)以GPS和InSAR同震形变观测数据为约束反演获得的最优发震断层产状, 具体为浅部倾角35° 的彭灌断裂(PF)、浅部倾角55° 的北川断裂(BF), 逐渐过渡到倾角为7° 的滑脱面上(图2a)。基于汶川地震后的钻探及野外露头观测数据, 将断层破碎带的厚度设置为200m(王焕等, 2010; Togo et al., 2011; Li et al., 2013)。地壳结构则按照地层速度差异构建成2个分层的块体(Tao et al., 2015; Liu et al., 2016)(表1), 模型深度为60km, 宽160km(图2a)。假设岩石孔隙中充满水, 水的黏度、热膨胀性、可压缩性等参数均为前人(Ross et al., 1977)测试获取的不同温压条件下的实验结果。

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图 2 汶川地震热流固耦合模拟有限元模型
a 几何模型及边界条件, 模型整体施加重力加速度g, 西部边界速度v=2mm/a, 流体压力p=1atm+静水压力, 温度T=293.15K+21.5K/km× D; b 模型地表速度与GPS观测速度对比, GPS点位见图1b, 横轴为沿断层倾向的距离; c 断层累计滑移量(陈棋福等, 2018)与断层强度衰减曲线, Δ E为降低的杨氏模量; d 网格尺寸Fig. 2 HTM finite element model of the Wenchuan earthquake.

表1 模型参数表 Table1 Parameters of the model

形变场	深度D/km		泊松比ν		ρ /kg· m^-3	杨氏模量E/Pa	黏滞系数η /Pa· s		参考文献
松潘-甘孜地块	0~22		0.25		2 500~2 683	(3.75~9.70)× 10^10①	1× 10²³		Tao et al., 2015; Liu et al., 2016
	22~42		0.26		2 754~2 773	(1.10~1.2)× 10¹¹	4× 10²⁰
	42~60		0.27		2 959~3 270	1.30× 10¹¹	5× 10¹⁹
四川盆地	0~19		0.25		2 500~2 683	(3.75~9.70)× 10¹⁰	1× 10²³		Tao et al., 2015; Liu et al., 2016
	19~32		0.26		2 754~2 773	(1.10~1.20)× 10¹¹	1× 10²²
	32~45		0.27		2 835~2 959	(1.30~1.70)× 10¹¹	8× 10²¹
	45~60		0.26		2 959~3 270	1.70× 10¹¹	6× 10²⁰
流场						温度场
断层孔隙度θ		0.10		Li et al., 2014; 陈建业等, 2011		岩石恒压热容 ${C_{p,}}_{p}$ /J· (kg· K)^-1		850
地壳孔隙度θ		0.02				流体恒压热容C_p/J· (kg· K)^-1		2 400
渗透率κ _l/㎡		1× 10^-15 × e^(-0.043×^P⁾		Tsutsumi et al., 2004; Chen et al., 2013		导热系数κ /W· (m· K)^-1		2.90		An et al., 2007
动力学黏度μ /Pa· s		8.8× 10^-4				岩石热膨胀系数α /K^-1		2.20× 10^-6		陈顺云等, 2009
Biot-Willis系数α _B		0.47		Hudson, 1993		流体热膨胀系数α /K^-1		(2.72~17.1)× 10^-5		273.15~373.15K
流体密度ρ /kg· ${m^{-}}^{3}$		1 000				参考温度T_ref/K		290.78		冯学民等, 2004
空气密度ρ /kg· ${m^{-}}^{3}$		1.22				空气恒容热容C[J/(kg· K)]		718

注 ①(3.75~9.70)× 10¹⁰ 表示0~22km深度范围内杨氏模量由3.75× 10¹⁰ 线性增加至9.70× 10¹⁰; 未标注参考文献的参数均来自COMSOL Multiphysics内置材料。

表1 模型参数表 Table1 Parameters of the model

2.2 主要参数的选取依据

应力致热的强度主要通过热膨胀系数描述(式(14)), 根据陈顺云等(2009)关于岩石应力加载和卸载过程中温度响应的实验数据, 设热膨胀系数为2.2× 10^-6/K。

热对流受流体渗流速度及其内能影响。对渗流速度影响最大的参数为岩石的渗透率, 基于汶川地震断裂带岩石测试结果, 本文采用随压力(P)变化的渗透率k=10^-15× e^(-0.043^P⁾(Tsutsumi et al., 2004; Chen et al., 2013)。内能的高低取决于选取的参考温度, 本文假设与地表气温相等时内能为0, 根据冯学民等(2004)对中国150个地温观测点的研究, 将气温和参考温度均设置为比地温低2.37K。

2.3 边界条件

对于形变场, 地表为自由边界, 西部边界速度根据GPS数据设置为2mm/a(Gan et al., 2007)(图2b), 底部和东部边界保持切向自由, 法向固定(图2a)。流场四周为开放边界, 地表施加1个大气压力, 其余边界处为静水压力加1个大气压力(图2a)。温度场四周同样为开放边界, 边界处的温度与下文模型的初始值一致(图2a)。模型采用三角形网格, 最大网格长2km, 地表网格最大尺寸为0.2km, 地表断层处网格尺寸约40m(图2d)。

模拟中假设1个地震周期为3 000a(张培震等, 2008), 2 998a断层浅部(深度< 15km)开始应力释放, 震前断层应力释放采用弱化断层强度的方法处理(Hu et al., 2012)。断层的应力释放速度基于震前重复地震事件获取的断层累计滑移量拟合得到(Li et al., 2011; 陈棋福等, 2018)(图2c)。考虑到断层深部(深度> 15km)强度较弱(Scholz, 1998), 应力积累较小, 其应力释放早于2 998a, 这与Li等(2018)的GPS观测结果相符。

2.4 模型初始值

稳态的初始场是提取热异常等非线性特征的关键。进行瞬态计算之前, 首先通过稳态计算获取应力场和流场的初始值。以龙门山中段的应力测量数据为约束, 施加随深度(D)变化的水平应力S_H=34MPa/km× D+2.7MPa(Wang et al., 2015; 秦向辉等, 2018)和边界条件, 将孔弹性和黏弹性均衡后的结果作为应力场和流场的初始场。温度场的初始条件根据现今的温度梯度设置为21.5℃/km(Li et al., 2015)。

3 计算结果及讨论

为直观展示断层应力释放引起的热异常现象— — 流体热对流(CT)和应力致热(DT)的非线性变化特征, 分别引入CT与DT的一阶时间导数dCT/dt和dDT/dt。当断层未发生应力释放时, dCT/dt和dDT/dt=0, 此时无热异常发生; 当断层应力释放后, dCT/dt和dDT/dt> 0则表示出现增温异常, < 0则表示出现降温异常。本节论述了计算结果所揭示的断层应力释放导致的热异常的时空演化特征, 并与观测结果在空间分布与量级上进行了对比。

3.1 热异常的时空演化特征

在热异常的空间分布上, 地表CT主要表现为增温异常, 沿断裂带走向在其上盘形成宽约20km的增温条带(图3a)。 DT同样以增温异常为主, 但强度较低(图3b)。在汶川地震前的热异常观测研究中, Jing等(2013)发现震前龙门山断裂带及其西侧上盘区域震前1a OLR增强, 严研等(2008)发现震前沿断裂带潜热通量增强的现象(图1c, d), 分布区域与本次模拟结果相似, 且该结果同时也符合前人对热异常与发震构造空间关系的认识— — 地震前出现热异常的区域位于震中区附近或沿发震构造线性分布。

Figure Option
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图 3 汶川地震前断层应力释放形成的热异常
a 断层应力释放后的dCT/dt分布, 冷色调表示降温异常, 暖色调表示增温异常, 白色方框所示为放大区域; b 断层应力释放后的dDT/dt分布, 图示同图a, 须注意色标量级的差异; c 断层上盘单位面积dCT/dt与单位体积dDT/dt的强度对比, 注意双纵轴量级的差异, 取值点分别为图a和b中的P₁; d 断裂带内部dCT/dt与dDT/dt的强度对比, 取值点分别为图a和b中的P₂; e 不同应力释放速度下断裂上盘P₁点处的dCT/dt, 纵轴为断层强度的弱化程度, 横轴为弱化所需时间, 白色等值线为7d内单位面积Δ CT引起的单位体积空气的等效增温值; f 不同应力释放速度下断裂带内P₂点处的dDT/dt, 白色等值线为7d内单位体积Δ DT引起的单位体积空气的等效增温值Fig. 3 Pre-earthquake thermal anomalies induced by fault slip.

断裂带内部的CT与DT有不同的演化特征。 DT以增温异常为主, 且增温强度随时间先增强后减弱(图3b, c)。而CT在断裂带内增温与降温异常并存(图3a), 且降温强度随时间呈增强趋势(图3c)。在CT和DT的联合作用下, 断裂带内增温异常先强后弱, 最终可转变为降温异常(图3d)。汶川地震前的热异常演化也存在类似特征。 Singh等(2010)发现震前几日震中区亮温出现升温异常, 震前1d降温; 陈顺云等(2013)发现自震前数日起龙门山断裂带及相邻断裂发生由南向北依次降温的现象(图1f)。另外, 在1976年龙陵地震(黄广思, 1993)、 2005年九江5.7级地震(陈梅花等, 2007)、 2003年墨西哥Colima 7.3级地震和2004年Sumatra 9.0级地震(Dunajecka et al., 2005; Ouzounov et al., 2007)等震例中, 皆有学者报道增温异常先升后降的现象。

3.2 热异常的量级

地震前热异常量级的不确定性大, 气温增温范围可由1K以下到20K不等, 但多数报道为数K(Sugisaki et al., 1980; 马宗晋等, 1982; 王林瑛等, 1984; 张治洮, 1994)。那么, 断层应力释放形成热异常能否使空气增温达到相同的量级呢?本节计算了不同应力释放强度和渗透率条件下的热异常强度, 并将其转换为等效的空气增温值, 与观测结果进行了对比。

前文研究表明, 断层上盘的热异常主要源自CT的异常, 图3e绘制了不同应力释放强度下的dCT/dt分布。可见, 随着弱化速度的增加, 增温异常逐渐增强, 当断层弱化速度< 30d时, dCT/dt基本处于相同的量级。我们计算了地表7d内单位面积累积Δ CT引起的单位体积空气等效增温值(参数见表1), 结果显示CT异常可以形成1K量级的升温(图3e)。注意, 图3e的结果所采用的渗透率量级为10^-13m², 是北川断层附近原岩渗透率的上限值(陈建业等, 2011)。计算结果表明, dCT/dt与渗透率线性相关, 当渗透率的量级为10^-16m²时, 则等效的空气增温值仅为1mK量级。

断层应力释放强度在模型中主要通过断层弱化程度和弱化速度来表征, 弱化程度越强、速度越快, 则断层应力释放越强。断层弱化程度用断层的最终强度表示(E-Δ E, E为初始杨氏模量); 弱化速度通过断层强度改变Δ E所需的时间表示。经过测试, 要保证地表速度模拟值与GPS观测值相符, 断层最大弱化程度约0.01E。根据热异常发生的时间, 将断层的弱化速度设置为震前数日至数月不等(表2)。

表2 断层应力释放强度参数表 Table2 Parameters of stress release on fault

断裂带内部的增温异常则主要源自DT的作用。图3f绘制了不同应力释放强度下的dDT/dt分布, 可见, 断裂带的增温强度明显高于非断层区域, 当断层的弱化速度< 30d时, 仅需 “ 微弱的” 断层活动即可造成高于1K量级的空气升温(图3f)。

3.3 不足之处

地震热异常的发生过程涉及地下、地表及大气层不同分层的众多物理化学过程, 本次模拟研究仅局限于地下岩体应力致热和流体热对流, 且仅从能量守恒的角度计算系统中新增热量可导致的空气增温异常, 未考虑热从流体、岩体到空气交换过程中的时间效应、能量损失和传递比率等, 实际中该过程可能对热异常现象存在一定影响, 这有待于进一步深入研究。

此外, 在模型构建方面: 1)研究中依据平面应变理论, 用二维剖面模型近似三维模型的方法模拟龙门山断裂带中段的断裂几何和应力分布, 但无法代表断裂产状与应力状态差异较大的北段, 因此进一步开展三维模拟仍十分必要; 2)模型中采用弱化孕震深度(0~15km)断层强度的方法模拟物理实验中 “ 断层及周围区域协同弱化” 的特征, 但该方法无法模拟断层 “ 局部” 的应力集中现象。然而, 本文主要关注热异常的区域性分布特征及变化趋势, 因此, 我们认为该方法不会影响得出的结论; 3)断裂带内部应力致热效应在地表至深约500m处与整条断层的温度变化趋势相反(图3b), 初步研究显示该现象和模型中松潘-甘孜地块与四川盆地的地壳结构不同有关, 但地表温度异常变化趋势与介质结构的关系仍有待深入研究。

4 结论

本文开展了汶川地震前THM耦合的数值模拟研究, 模拟与断层应力释放相关的热异常现象的时空演化特征, 发现其与观测结果在时空分布和量级上有一定的相似性, 本研究结果支持流体热对流和应力致热是汶川地震热异常产生的重要机制的观点。

汶川地震前断层应力释放导致断裂带及其上盘区域流体热对流和应力致热作用强度改变, 进而发生热异常。断裂带区域的热异常受二者联合控制, 表现为增温异常先强后弱, 最后出现短期降温现象。断裂带上盘区域的热异常主要受流体热对流作用影响, 表现为沿发震断层的带状升温现象。

我们将模型地表的CT和DT产生的新增热量转换为同体积空气的增温值, 发现断裂带区域增温强度较强, 空气升温异常通常都高于1K量级; 而非断裂带区域增温强度主要受渗透率影响, 通常当渗透率≥ 10^-13m²时, 非断裂带区域才能导致1K量级的升温。

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2011

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陈建业, 杨晓松, 党嘉祥, 等. 2011. 汶川地震断层带结构及渗透率[J]. 地球物理学报, 54(7): 1805—1816.

CHEN

Jian-ye

, YANG

Xiao-song

, DANG

Jia-xiang

, et al. 2011. Internal structure and permeability of Wenchuan earthquake fault[J]. Chinese Journal of Geophysics, 54(7): 1805—1816(in Chinese).

Cross-fault permeability measurement was conducted on the Wenchuan earthquake fault. The results showed that this earthquake fault consists of fault core with low permeability (2. 4 X 10(-19) - 3. 8 X 10(-16) m(2)), highly permeable damaged zone (3. 7 X 10(-16) - 3. 0X 10(-15) m(2)) and the fractured protolith (6. 0x 10(-18) - 4. 3 X 10(-13) m(2)) (40 MPa effective pressure), among which fresh gouges on the main slip surface are the most impermeable. Impeded by the impermeable protolith and fault gouge, cross-fault fluid flow was inhibited and fluid activity was confined within the damaged zone. This recognition is consistent with microstructure and grain size distribution of fault rocks. Condition of co-seismic thermal pressurization was calculated and the results showed that at depth greater than 2 km, the main slip zone of Wenchuan earthquake has the characteristics of low permeability for thermal pressurization to occur.

[Chen Jian-Ye; Yang Xiao-Song; Dang Jia-Xiang; He Chang-Rong; Zhou Yong-Sheng; Ma Sheng-Li] China Earthquake Adm, State Key Lab Earthquake Dynam, Inst Geol, Beijing 100029, Peoples R China.

对汶川地震断层带进行了跨断层的渗透率测量. 结果显示汶川地震断层由低渗的核部(2.4×10 -19 ~ 3.8×10 -16 m 2 )、高渗的破碎带(3.7×10 -16 ~3.0×10 -15 m 2 )以及含裂隙原岩(6.0×10 -18 ~4.3×10 -13 m 2 )组成(有效压力 40 MPa),其中新鲜断层泥具有最低的渗透率. 断层泥和两侧原岩由于渗透率低,阻碍流体跨断层带流通, 断层带内的流体活动局限在高渗的破碎带中, 这一结果与断层岩显微结构和粒度分布特征能很好的吻合. 对汶川地震同震热压作用发生的条件进行了探讨, 结果显示, 约2 km以下汶川地震主滑动带具有发生热压作用所要求的低渗透率特征.

... 注意, 图3e的结果所采用的渗透率量级为10^-13m², 是北川断层附近原岩渗透率的上限值(陈建业等, 2011) ...

2007

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陈梅花, 邓志辉, 王煜, 等. 2007. 2005年九江5. 7级地震前后地表热动态初步研究[J]. 地震地质, 29(3): 617—626.

CHEN

Mei-hua

, DENG

Zhi-hui

, WANG

, et al. 2007. Primary study on the variation of surface thermal flux before and after the M_S5. 7 earthquake of 2005 in Jiujiang, Jiangxi[J]. Seismology and Geology, 29(3): 617—626(in Chinese).

The temporal and spatial variations of surface latent heat flux(SLHF)and diagnostic air temperature at 2m before and after the M S 5.7 earthquake that occurred between Ruichang and Jiujiang in Jiangxi province on the 26th November 2005 are summarized in this paper.It is found that before the earthquake significant SLHF anomalies and air temperature anomalies occurred in the epicentral area and its vicinity.The air temperature anomalies are found from the 2nd to 13th November,2005 and concentrated at the epicentral area and its southern area.Then two days later,that is,from the 4th to 15th November 2005,significant SLHF anomalies occurred in the epicentral area and its northern area where a lot of lakes distribute along the active faults.During the anomalous period the SLHF and air temperature at 2m exceeded the sum of 26 years average and 1.5 times of their standard deviation for the same day.Both anomalies had maintained for 12 days with a peculiar distribution associated with the tectonic active zone.It is considered that both of air temperature anomalies and SLHF anomalies are behaviors of thermal flux from underground prior to earthquake.SLHF anomalies occurred over regions with abundant water,whereas air temperature anomalies occurred over land.

1. Institute of Geology, China Earthquake Administration, Beijing 100029, China; 2. Institute of Earthquake Science, China Earthquake Administration, Beijing 100036, China; 3. Nancheng Meteorological Administration, Jiangxi Province, Fuzhou 344700, China

2005年11月26日九江-瑞昌5.7级地震前出现了显著的地表气温异常和潜热通量异常。二者在时间上相关,空间上相邻;既有联系,又有区别。地表气温异常出现于2005年11月2日至13日,主要分布在震中及其南侧,呈NE向展布,与江西北部主要的构造带走向一致。地表气温出现异常2d后,在震中地区及其北侧,沿构造带分布的大片水域及周边地区出现了显著的潜热通量异常,持续时间从11月4日至15日。2种异常的量值都超过了26a的平均值与1.5倍均方差之和,持续时间长,形态孤立,与地质构造相关,有别于气象因素导致的异常。震前气温异常和潜热通量异常都是地震前地下的热量增加导致的不同表现形式,在水体分布较广的区域大多表现为潜热通量异常,而在水体较少的区域则表现为气温异常。

... 7级地震(陈梅花等, 2007)、 2003年墨西哥Colima 7 ...

2018

0.0

... 陈棋福等, 2018)(图2c) ...

2009

0.0

... 陈顺云等, 2009) ...

... 2 主要参数的选取依据应力致热的强度主要通过热膨胀系数描述(式(14)), 根据陈顺云等(2009)关于岩石应力加载和卸载过程中温度响应的实验数据, 设热膨胀系数为2 ...

2013

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陈顺云, 马瑾, 刘培洵, 等. 2013. 利用Terra和Aqua卫星地表温度探索汶川地震同震热响应[J]. 地球物理学报, 56(11): 3788—3799.

CHEN

Shun-yun

, MA

Jin

, LIU

Pei-xun

, et al. 2013. Exploring co-seismic thermal response of Wenchuan earthquake by using land surface temperatures of Terra and Aqua satellites[J]. Chinese Journal of Geophysics, 56(11): 3788—3799(in Chinese).

There are many problems which should be solved when thermal infrared information is applied to exploring the current tectonic activities. One of these problems is to obtain the reliable, testable information related to tectonic activity through the satellite thermal infrared image. From the viewpoint of signal processing, thermal information changes largely before and after the strong earthquake. Specially, the co-seismic thermal response is the breakthrough point to investigate current tectonic activity. In this paper, the MODIS/Terra and MODIS/Aqua land surface temperature products are applied to detecting the co-seismic response of Wenchuan earthquake. The effects of non-tectonic factors on land surface temperature are carefully considered. Firstly, the stable background of land surface temperature is extracted from land surface temperature. Then, influences of atmosphere circulation and mankind activity are gotten rid of, with consideration of spatial frequency difference between tectonic and non-tectonic activities. At last, the co-seismic temperature-dropping response of Wenchuan earthquake is found in the left temperature field which stems from the in-situ factors. Furthermore, the spatial distribution of co-seismic response is in accordance with the mechanical mode related to the fault activity. According to the relationship between temperature response and stress-strain, this regional lowering temperature is equivalent to the co-seismic unloading or tension.

Chen Shun-Yun 1 ;Ma Jin 1 ;Liu Pei-Xun 1 ;Liu Li-Qiang 1 ;Hu Xiao-Yan 1 ;

如何从卫星热红外遥感影像中提取出可靠的、能够验证的、与构造活动有关的热信息,是利用热红外信息研究构造现今活动亟待完善的问题之一.从信息提取的角度看,地震前后所对应的物理变化最为强烈,同震热响应可能是利用热信息探索现今构造活动潜在的突破点.本文以MODIS/Terra和MODIS/Aqua两颗卫星的地表温度为基础,探索汶川地震同震热响应.首先,去除了由地形等静态因素和太阳年周期组成的稳定背景;其次,去除大气环流的影响,获得由本地因素产生的原地温度场.最后,在原地温度场中,发现了汶川地震的同震温度响应.经过检验,上述温度响应的空间展布形态与发震断层的活动方式产生的力学效应吻合.从应力变化与温度响应的关系看,本文相当于借助热信息获得了汶川地震同震拉张或卸载区的空间展布情况.

... 陈顺云等(2013)发现自震前数日起龙门山断裂带及相邻断裂发生由南向北依次降温的现象(图1f) ...

2013

0.0

... 丁留伟(2013)利用该理论模拟了地震前由于断裂带渗透率的增强导致流体沿断裂带快速向上运移、进而使其升温的过程, 但该研究未涉及地震前断层应力的释放过程 ...

2004

0.0

... 内能的高低取决于选取的参考温度, 本文假设与地表气温相等时内能为0, 根据冯学民等(2004)对中国150个地温观测点的研究, 将气温和参考温度均设置为比地温低2 ...

1992

0.0

... 耿乃光等, 1992 ...

1993

0.0

... 另外, 在1976年龙陵地震(黄广思, 1993)、 2005年九江5 ...

2004

0.0

刘力强, 陈国强, 刘培洵, 等. 2004. 岩石变形实验热红外观测系统[J]. 地震地质, 26(3): 492—501.

LIU

Li-qiang

, CHEN

Guo-qiang

, LIU

Pei-xun

, et al. 2004. Infrared measurement system for rock deformation experiment[J]. Seismology and Geology, 26(3): 492—501(inChinese).

The infrared signals produced during rock deformation are very faint and tiny,so that the measurements of them require excellent capabilities of detection system and strict working environment. The reliability of the results of infrared experiment,therefore,depends greatly on the reasonable technical specifications of infrared photodetector,high sensitivity recorder for temperature field and effectively controlled thermal background. All of the key parameters are discussed in detail in this paper based on laboratory infrared observation. The features for an infrared camera include spectral band,digitizing resolution,view field etc. For capturing the optimal infrared images,it is suggested that the spectral range should be 7～13μm,the observation range should be from 0.5m to infinity,and high-density focal plane array should be employed as far as possible. In order to confirm the temperature from infrared image,a surface temperature recorder with multi thermometers should be employed as an eyewitness working alongside the camera. A realistic experiment system has been developed based on the afore mentioned discussion. Aside from the traditional functions of infrared camera,the system is fitted with high-speed data transmission component. The infrared focal plane array has 320×240 pixels,and 25 frames of temperature field data are saved into the hard disk per second. Some steel beam bending and steel cylinder compressing experiments were made for testing the system. The tests have proved that the system is reliable and the data quality is improved greatly.

State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquae Administration, Beijing 100029, China

岩石变形导致的温度变化极其微弱,对观测系统与环境有严格要求。正确选择红外观测系统的参数,配备高灵敏度的温度场观测设备,以及有效地控制环境热背景决定了变形热红外实验结果的可靠性。在对噪声与干扰源进行详细分析的基础上,提出了进行热红外实验所需要的各主要设备的关键技术指标,并建立了一套相应的实验系统。典型材料样品实验表明,在考虑到变形热红外实验的各种特殊要求,认真选择观测设备,并进行了严格的误差校正之后,可以获得可靠的变形热过程信息。

... 刘力强等, 2004 ...

2004

0.0

刘善军, 吴立新, 王川婴, 等. 2004. 遥感-岩石力学(Ⅷ): 论岩石破裂的热红外前兆[J]. 岩石力学与工程学报, 23(10): 1621—1627.

LIU

Shan-jun

, WU

Li-xin

, WANG

Chuan-ying

, et al. 2004. Remote sensing-rock mechanics(Ⅷ): TIR omens of rock fracturing[J]. Chinese Journal of Rock Mechanics and Engineering, 23(10): 1621—1627(in Chinese).

There are many omens existing before rock fracturing，which is an important research issue of rock mechanics and rock engineering. Based on the fundamental experiments on remote sensing-rock mechanics，the thermal infrared radiation (TIR) omens of rock fracturing are systematically analyzed and studied. The research contents of this paper include the types and features of TIR omens，the occurring time of TIR omens，the spatio-temporal features of TIR omens，and its mechanism. It is concluded that (1) there are two kinds of TIR omens for rock fracturing，i.e.，TIR image anomaly and curve of average TIR temperature and time anomaly，which reflect the spatial and temporal features of TIR omens respectively，(2) the TIR image anomaly is classified as high-temperature strip and low-temperature strip，and the curve of average TIR temperature and time anomaly is classified as temperature drop，rapid rise of temperature and rise-from-drop of temperature，(3) the omens occur before peak load at the moment when load is 77%～87% of the peak load，and they are tightly related to the formation of micro-cracks，(4) the possibility of omens of shearing fracturing with high confining pressure or high compressive stress is higher than that with low confining pressure or low compressive stress，and the possibility of omens for pure-shear will be less and its occurring time is close to the fracturing moment，and (5) the mechanism for the TIR omens of rock fracturing is the comprehensive effect of rock thermo-elasticity and rock friction.

岩石破裂存在多种前兆，它是岩石力学和岩石工程中的一项重要研究内容。在总结多年遥感-岩石力学基础实验研究的基础上，对岩石破裂的热红外前兆进行了系统总结与分析，包括：前兆表现形式及其类型、前兆的时间及空间特征、前兆的机理等。研究表明：(1) 岩石破裂的热红外前兆有平均红外幅射温度-时间曲线异常和热像异常2种形式，分别反映前兆的时空特征；(2) 热像异常分为高温条带和低温条带两种类型，平均红外幅射温度-时间曲线异常分为降温、升温加速和降温转升3种类型；(3) 前兆出现在岩石载荷峰前s = (0.77～0.87)sp 的应力区间内，与微破裂带的形成密切相关；(4) 高围压和压性强的剪性破裂出现前兆的几率要大于低围压和压性弱的剪性破裂，而随着破裂向纯剪过渡，前兆出现的几率降低，且前兆发生的时间越接近岩石破裂时刻；(5) 岩石破裂热红外前兆机理是岩石热弹和摩擦热效应的综合。

... 刘善军等, 2004 ...

2012

0.0

... 那么, 地震热异常是否由断层应力异常引起?马瑾等(2012)通过断层黏滑实验研究发现断层在进入亚失稳阶段时, 断层及两盘总体上会由应力积累状态转变为应力释放状态 ...

... 同时, 王凯英等(2012, 2018)基于紫坪铺水库区域台网记录的震前近4a的震中区486个小震的震源机制资料, 运用碎裂分析法得到震源区断层面上的应力演化过程, 发现2007年6月之前震中区以应力积累为主, 之后转变为应力释放, 空间上向2个应力异常高值区集中, 周围区域协同弱化, 最后2008年汶川地震则发生于应力集中和卸载区间的梯度带上, 与物理实验中断层亚失稳阶段的应力演化特征一致(马瑾等, 2012), 表明汶川地震前北川断裂已开始应力卸载过程 ...

1982

0.0

... 马宗晋等, 1982 ...

1998

0.0

... 强祖基等, 1998 ...

2018

0.0

... 秦向辉等, 2018)和边界条件, 将孔弹性和黏弹性均衡后的结果作为应力场和流场的初始场 ...

2005

0.0

... 单新建等, 2005 ...

2010

0.0

王焕, 李海兵, 裴军令, 等. 2010. 汶川地震断裂带结构、岩性特征及其与地震活动的关系[J]. 第四纪研究, 30(4): 768—778.

WANG

Huan

, LI

Hai-bing

, PEI

Jun-ling

, et al. 2010. Structural and lithologic characteristics of the Wenchaun earthquake fault zone and its relationship with the seismic activity[J]. Quaternary Sciences, 30(4): 768—778(in Chinese).

This paper focuses on the research of the Yingxiu Beichuan fault zone, which is the central fault of Longmuanshan fault belt. Detailed study was taken on the fault zone in Bajiaomiao village，Hongkou town at the coseismic surface rupture zone of Wenchuan earthquake Combining with analyses of the drilling cores from the No.1 Well of the Wenchuan Earthquake Fault Scientific Drilling(WFSD-1)Project，the composition features and structure of the Longmenshan fault belt was discussedOur research indicates that Yingxiu Beichuan fault zone is composed by many small sub faults(damage zone).The thickness of the gouge in the fault zone ranges from several millimeters to 25 centimeters，which is consistent with the fault characters recorded in the drilling cores of the WFSD-1.The width of Yingxiu Beichuan fault zone is about 120m viewed from the outcrops in Bajiaomiao villageMore than 80 small sub faults that contain gouge distributed in this areaOnly several millimeters to approximately 2 centimeters gouge can be formed from the experiences in Taiwan Chelungpu fault Drilling Project(TCDP)and Wenchuan Earthquake Fault Scientific Drilling(WFSD) Project，so we can infer that each layer of the gouge in Yingxiu Beichuan fault zone might be produced by 1 to 13 large earthquakes at leastThe total thickness of the gouge in this area is about 150cm，which indicates at least 183 earthquake events and suggests that strong earthquake repeatedly occurred along the Yingxiu Beichuan fault zoneEach earthquake does not completely slip along the principal slip zone(PSZ)of the older earthquake，but rather along the edge of the gougeAccording to the gouge statistics of the whole fault zone，seismic events have the obvious tendency towards the foot wall，and the thickness of gouge is proportional to the activity of the fault，indicating that the width of fault zone has the directly relation with the number of times and evolution history of earthquake eventRepeated earthquakes maybe the main cause of formation of Longmenshan Moutain.

①China University of Geosciences, Beijing 100083; ②Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037; ③Key Laboratory of Continental Dynamics of Ministry of Land and Resources, Beijing 100037; ④Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081; ⑤The Sixth Geological Survey of Jiangsu Bureau of Geology and Mineral Resources, Donghai 222300

文章以龙门山中央断裂带映秀-北川断裂带为研究对象，重点对汶川地震破裂带南段虹口乡八角庙地区地表断裂带进行了详细研究，并结合汶川地震科学钻探1号孔(WFSD-1)岩芯部分研究成果，探讨龙门山中央断裂带的物质组成及其结构特征。研究表明，映秀-北川断裂带由很多次级小规模断裂（破碎带）以及夹持其中的块体所构成，其中断层泥的厚度由几毫米到25cm不等，与WFSD-1岩芯记录的地震断裂现象基本一致。从虹口乡八角庙露头来看，映秀-北川断裂带整体宽约120m，分布有近80条含有断层泥的次级断裂带。以台湾车笼埔断裂钻探项目（TCDP）和汶川地震科学钻探（WFSD）研究来看，一次大地震只能形成几毫米至约2cm厚的断层泥，推断映秀-北川断裂带中每层断层泥至少发生过1次到13次地震，该区总厚度约150cm的断层泥中发生地震次数至少为183次，说明沿着映秀-北川断裂带重复发生过多次强地震活动。每次地震活动并不完全沿袭老的地震断裂主滑移带滑动，而是沿着断层泥边部区域滑动。从整个断裂带中断层泥分布特征来看，地震断裂活动具有向断裂下盘迁移的趋势，并且断层泥的厚度与断裂活动性成正比关系，表明断裂带宽度与地震活动次数及其演化历史有着成因上的直接联系，多次地震活动叠加可能是龙门山形成的主要成因。

... 基于汶川地震后的钻探及野外露头观测数据, 将断层破碎带的厚度设置为200m(王焕等, 2010 ...

2018

0.0

2012

0.0

王凯英, Rebetsky

, 刁桂苓, 等. 2012. 2008年汶川地震前震源区的异常应力状态[J]. 地震地质, 34(1): 11—16. doi: 103969/j.issn.0253-4967.2012.01.002.

WANG

Kai-ying

, Rebetsky

, DIAO

Gui-ling

, et al. 2012. Stress state of Wenchuan area before the 2008 strong earthquake[J]. Seismology and Geology, 34(1): 11—16(in Chinese).

Using the cataclastic analysis method, this paper tries to make an analysis on the focal mechanism data of 486 small earthquakes that occurred at the epicenter of the Wenchuan earthquake and its surrounding areas in more than three years before the 2008 Wenchuan earthquake. The result shows that obvious stress change occurred at the seismic source and its surrounding areas around June 2007 before the Wenchuan earthquake, manifested in two high numerical value areas of abnormal stress state. Meanwhile, the formation process of the above areas was accompanied by the drop of stress level of the Longmenshan central fault. The ultimate strong earthquake occurred on the stress gradient belt between the high stress area and the low stress area. The evolution process of stress level before the Wenchuan earthquake indicates that earthquake nucleation phenomenon turned up before the strong earthquake. One result can be inferred that there was an abnormal process of accelerated movement of the whole Bayankala block before the Wenchuan strong earthquake.

1. State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China; 2. Schmidt Joint Institute of Physics of the Earth, Russian Academy of Sciences, Moscow 123810, Russia; 3. Earthquake Administration of Hebei Province, Shijiazhuang 050012, China

我们收集了紫坪铺水库区域台网记录到的2004年8月16日至2008年5月12日汶川地震前发生在震中区及附近区域的486个小震震源机制资料,运用碎裂分析法对这些震源机制数据进行分析,获得了汶川地震前3年多时间内震源区应力水平随时间的演化过程。结果显示,强震发生前震源及附近区域约在2007年6月发生了应力突变,出现了2个应力异常高值区,其形成过程同时伴随着龙门山中央断裂的应力水平降低,最终强震发生在高应力区和低应力区之间的应力梯度带上。汶川地震前应力状态的演化过程,类似于实验研究中出现的非均匀断层失稳前的地震成核现象,符合地震成核的宏观表现。巴颜喀拉块体的南边界-鲜水河断裂的短基线测量也显示出,在同一时间段鲜水河断裂南段由左旋滑动转为异常的右旋错动,反映了汶川强震前整个巴颜喀拉块体有向东加速运动的异常过程。

1984

0.0

... 王林瑛等, 1984 ...

2016

0.0

2001

0.0

... 徐秀登等, 2001 ...

2008

0.0

严研, 邓志辉, 陈梅花, 等. 2008. 基于双温度差分双源模型的遥感方法反演四川汶川地区的潜热通量[J]. 地震地质, 30(4): 1085—1094.

YAN

Yan

, DENG

Zhi-hui

, CHEN

Mei-hua

, et al. 2008. Retrieving the latent heat flux of Wenchuan area of Sichuan Province using a dual-temperature difference method[J]. Seismology and Geology, 30(4): 1085—1094(in Chinese).

Research shows that anomalous increase of latent heat flux was discovered before several earthquakes.Basing on the Modis data,high-spectral data and ground meteorological data of the working area,we retrieved the sensible heat flux and calculated the latent heat flux from surface energy balance equation using a simplified two-layer model(dualtemperature-difference method).In this paper,we also discussed the characteristics of this model,and retrieved the latent heat flux of the area where the Longmenshan Fault goes through.Finally,we found that there were significantly high values of latent heat flux in the epicenter area of the M S 8.0 Wenchuan earthquake and the surrounding area of Longmenshan Fault 48 hours before the earthquake,the latent heat flux on May 12 reached the maximum of 300～400W/m 2 ,which might be the anomalous increase of latent heat flux before the M S 8.0 Wenchuan earthquake.

Institute of Geology, China Earthquake Administration, Beijing 100029, China

研究表明,在很多地震前都出现了潜热通量异常。文中基于Modis遥感数据,结合研究区(四川省汶川地区)的高光谱数据及地面观测的气象数据,利用一种简化的双层模型(双温度差分双源模型)反演显热通量,并通过地表能量平衡方程计算研究区的潜热通量。还探讨了该模型的特点,并对龙门山断裂带通过地区进行了潜热通量的遥感反演,发现在2008年5月12日汶川 M S 8.0地震前48h内,研究区内的震中地区、龙门山断裂周边地区潜热通量值出现了明显的高值,5月12日中午12时7分沿龙门山断裂带潜热通量值达到300～400W/m 2 ,可能是汶川8.0级地震前的异常反应。

... 在汶川地震前的热异常观测研究中, Jing等(2013)发现震前龙门山断裂带及其西侧上盘区域震前1a OLR增强, 严研等(2008)发现震前沿断裂带潜热通量增强的现象(图1c, d), 分布区域与本次模拟结果相似, 且该结果同时也符合前人对热异常与发震构造空间关系的认识#cod#x02014 ...

2008

0.0

张培震, 徐锡伟, 闻学泽, 等. 2008. 2008年汶川8. 0级地震发震断裂的滑动速率、复发周期和构造成因[J]. 地球物理学报, 51(4): 1066—1073.

ZHANG

Pei-zhen

, XU

Xi-wei

, WEN

Xue-ze

, et al. 2008. Slip rates and recurrence intervals of the Longmen Shan active fault zone, and tectonic implications for the mechanism of the May 12 Wenchuan earthquake, 2008, Sichuan, China[J]. Chinese Journal of Geophysics, 51(4): 1066—1073(in Chinese).

The great Wenchuan earthquake of May 12, 2008 occurs on the Longmen Shan fault zone which forms a prominent section of the seismically very active seismic belt called the North-South Trending Seismic Belt by Chinese seismologists. There have been 3 earthquakes with magnitudes from 6 to 6 1 / 2 along the Longmen Shan fault zone during more then 2000 year documented history of the Sichuan province. Previous active faulting studies indicate slow (less than 3 mm/yr ) slip rate across the Longmen Shan fault zone. Why such a strong earthquake occurred in the Longmen Shan fault zone? What are the characteristics of the May 12 earthquake? What is the tectonic mechanism of the earthquake? On the basis of geological studies of the earthquake surface rupture zone and pre-earthquake GPS measurements in the region, we try to understand the questions mentioned above. The May 12 earthquake, 2008 is caused by displacement along the Yingxiu-Beichuan fault along which a more than 200 km long surface ruptures was formed. Another strand of the Longmen Shan fault zone, Guanxian-Jiangyou fault also ruptured as indicated by more then 60 km long surface ruptures. GPS measurements before the earthquake suggest that slip rate across the entire Longmen Shan fault zone does not excess about 2 mm/yr, and does not excess 1 mm/yr across individual fault strand. These data agree with seismogeological studies and historical seismicity of the Sichuan province. Using maximum coseismic displacement obtained from seismogeological studies and inversions from seismic waves, the recurrence intervals of great earthquakes, such as the 2008 Wenchuan earthquake, can be estimated to be 2000 ~6000 years. The Longmen Shan fault zone has a high dipping angle (more than 50°~60°) near surface and low-angle at depth (15~20 km). This kind of listric shape favors significant strain or energy accumulation to form great earthquake. The May 12 Wenchuan earthquake, 2008 is characterized by slow strain accumulation, long recurrence interval, and significant damage power. It is a new type of earthquake event that deserves further studies.

1 State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China;2 Sichuan Seismological Bureau, Chengdu 610041, China

2008年5.12汶川大地震发生在中国大陆南北地震带中段.由于龙门山断裂带历史上只发生过3次6～6 1 / 2 级强震，而且其晚第四纪构造活动速率很低，以至于对其潜在地震危险性认识不足.为什么在龙门山地区突发大地震，该地震具有哪些特征？其成因机制是什么？本文在地震地质科学考察的基础上，利用震前的GPS观测结果，试图对上述问题进行一些初步的思考和探讨.结果表明，5.12汶川大地震是龙门山断裂带的映秀—北川断裂突发错动的结果，地表上形成200多公里长的地表破裂带；灌县—江油断裂在地震中也发生了破裂，形成的地表破裂带长达60多公里.震前的GPS观测表明，横跨整个龙门山断裂带的滑动速率不超过～2 mm/yr，单条断裂的活动速率不超过～1 mm/yr，与地震地质研究结果和历史地震记录相一致.利用地震地质考察和地震波反演得到的最大同震位移可以获得相当于5.12汶川大地震的强震复发周期为2000～6000年.龙门山断裂带发育在破裂强度很大的变质杂岩体中，断裂带本身在剖面上呈“犁形”或“铲形”结构，有利于能量积累，形成破坏性巨大的地震.所以，5.12汶川大地震是一次低滑动速率、长复发周期和高破坏强度的巨大地震，是一种值得高度重视和深入研究的新的地震类型.

... 模拟中假设1个地震周期为3 000a(张培震等, 2008), 2 998a断层浅部(深度#cod#x0003C ...

1994

0.0

... 地震前一定时空范围内会出现热异常现象, 其与发震断层之间存在一定的空间关联, 如多发于震中附近、沿发震构造线性迁移等(张治洮, 1994 ...

... 张治洮, 1994) ...

2007

0.0

1990

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... Bear et al ...

... 2 热弹性方程地壳内部传热机制主要有热对流、热传导和应力致热3种机制, 本文采用局部热均衡理论, 即假设局部区域岩体与流体的温度相同, 而忽略二者热交换的过程, 因为本文主要考虑地表总热量变化, 不具体关注岩石和流体的温度差异(Bear et al ...

1962

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... 1 THM耦合物理方程连续介质THM耦合理论主要基于孔弹性和热弹性耦合理论发展而来, 其分别通过胡克定律描述弹性形变、达西定律描述多孔介质内部流体渗流和傅里叶定律描述热传导(Biot, 1962 ...

... 本文的本构方程来自Biot(1962)提出的孔弹性理论: ...

2013

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... Chen et al ...

2005

0.0

... 0级地震(Dunajecka et al ...

2007

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... 3 边界条件对于形变场, 地表为自由边界, 西部边界速度根据GPS数据设置为2mm/a(Gan et al ...

2012

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... 15km)开始应力释放, 震前断层应力释放采用弱化断层强度的方法处理(Hu et al ...

0.0

2013

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2003

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... Jing et al ...

2013

0.0

... Li et al ...

2014

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2015

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... 5℃/km(Li et al ...

2011

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... 断层的应力释放速度基于震前重复地震事件获取的断层累计滑移量拟合得到(Li et al ...

2018

0.0

... 15km)强度较弱(Scholz, 1998), 应力积累较小, 其应力释放早于2 998a, 这与Li等(2018)的GPS观测结果相符 ...

2010

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... Lisi et al ...

2016

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... Liu et al ...

2013

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... Nield et al ...

2007

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... Ouzounov et al ...

2010

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1991

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... 并且, 基于物理实验结果, 前人认为与断层应力变化相关的应力致热效应和流体热对流可能是地震热异常发生的原因(Qiang et al ...

1977

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... 假设岩石孔隙中充满水, 水的黏度、热膨胀性、可压缩性等参数均为前人(Ross et al ...

1998

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... 15km)强度较弱(Scholz, 1998), 应力积累较小, 其应力释放早于2 998a, 这与Li等(2018)的GPS观测结果相符 ...

2009

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... 2 汶川地震二维有限元模型汶川地震震中位于龙门山断裂带中段, 断层产状稳定, 且以逆冲运动为主(Shen et al ...

2010

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... Singh等(2010)发现震前几日震中区亮温出现升温异常, 震前1d降温 ...

1980

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2015

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... 地壳结构则按照地层速度差异构建成2个分层的块体(Tao et al ...

2011

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... Togo et al ...

2005

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... Tramutoli et al ...

2002

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2004

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2015

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... 7MPa(Wang et al ...

2011

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... Wang et al ...

... 断层产状采用Wang等(2011)以GPS和InSAR同震形变观测数据为约束反演获得的最优发震断层产状, 具体为浅部倾角35#cod#x000b0 ...

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2009

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2012

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... Zoran, 2012) ...