【作者】 胡进军；

【导师】 谢礼立；

【作者基本信息】 中国地震局工程力学研究所 ， 防灾减灾工程及防护工程， 2009， 博士

【摘要】 近断层地震动是造成近断层区工程结构破坏的重要因素,近断层地震动的特征受到诸多因素的影响,其中断层破裂传播的方向性是影响近断层地震动及其分布特征的关键因素之一。方向性效应的影响因素众多,比如断层破裂的速度、破裂的方向、断层的倾角、震源的深度、破裂的模式、破裂起始位置以及断层面上的局部震源参数等,但是这些参数是如何影响方向性效应的呢?特别是一般认为断层的破裂速度接近于剪切波传播速度时,会产生明显的方向性效应,但是当破裂速度超过剪切波传播速度时是否会产生方向性效应?如果存在,又会对近断层地震动的特征产生如何影响?为了回答这些问题,本研究首先分析了方向性效应的相关概念,接着采用数值方法模拟了各种震源相关参数的变化对方向性效应的影响,并且对模拟结果和实际近断层强地震动的方向性特征进行了分析,最后专门研究了地震破裂过程中的超剪切破裂现象及其对近断层地震动方向性效应的影响。研究时分别从地震动的加速度、速度和位移三方面,系统地分析了其三分量的峰值、频谱和持时特征,研究的主要内容和结论如下:1.论述了方向性效应的相关概念和影响因素,分析了方向性效应与破裂传播效应以及多普勒效应之间的区别与联系。研究表明:①经典的多普勒效应或所谓的多普勒频移,一般是指由单一频率的波源产生的一列波在介质中传播时,由于波源与观测点的相对位置的改变引起的单位时间内观测点接受到的波的个数的改变,而且波在传播过程中不会出现叠加和干涉现象。②断层的破裂传播效应不能等同于简单的移动源效应或多普勒效应,因为在断层的破裂过程中,断层面上的点有一定的滑动时间继续振动产生地震波,因而波源不止一个,即多波源产生的相同或者相近频率的波在观测点处的叠加和相长干涉。③从实际强震观测数据的角度来分析,由于地震断层破裂过程的复杂性,很难产生与多普勒效应和移动源效应中类似的频移现象。2.基于选取的集集地震强震记录,系统地分析了方向性效应对近断层地震动的峰值、反应谱和持时的影响。研究表明方向性效应对近断层地震动的峰值、反应谱和持时的强度以及分布有重要的调制作用,具体表现为:①方向性效应使得破裂前方的峰值增大,使破裂后方的峰值减小。②方向性效应对反应谱的2.0s以后的中长周期段影响更加明显,并且随着周期的增大,方向性效应越来越显著。③方向性效应使得破裂前方的能量持时减小,破裂后方的能量持时增大。④方向性效应对断层垂直分量影响最大,断层平行和竖向分量次之。⑤方向性对加速度、速度和位移地震动的影响依次增大,即方向性效应对长周期地震动影响显著。3.数值模拟了各种断层模型下的近断层地震动,分析了震源深度、破裂起始点位置、破裂速度(常破裂速度和变破裂速度)、断层的倾角以及双侧破裂等震源相关参数对方向性效应的影响。以下分别说明:(1)方向性效应对地震动峰值、反应谱和持时影响的基本特征:①方向性效应使得断层破裂前方和破裂后方的地震动峰值有极不对称的分布,在破裂前方的很大区域内由于地震辐射的能量在很短时间内几乎同时到达,使得地震动峰值较大,反应谱谱值较高,能量持时较低;而在破裂后方,由于辐射的地震波依次到达,使得能量分布比较均匀,因此地震动的峰值较小、反应谱值较低,能量持时较小。②方向性效应对地震动的不同分量的影响不同,垂直于断层(FN)的分量受到方向性效应影响和控制最为显著,而平行于断层(FP)和竖向(UP)分量受到的影响相对较小。③方向性效应使得沿断层走向的破裂前方有明显的“显著影响区域”,不同分量的显著影响区域不同,对于FN分量,其显著影响区域可从断层末端到达3倍的断层长度距离范围;而对于FP和UP分量,其显著影响区域则明显小于FN分量,约从断层的中段到断层的末端或不超过1.5倍的断层长度的范围。(2)震源深度对方向性效应的影响:①随着震源深度的增加,地震动的峰值降低,但是通过对不同震源深度情况下峰值、频谱和持时参数的分析表明,虽然随着震源深度的增加地震动的峰值降低了,但是破裂的前方和后方的地震动的差异依然存在并且比较明显。②随着震源深度的增加,方向性效应的显著影响区域逐渐向破裂前方移动。③随着震源深度的增加方向性显著影响角逐渐增大,也就是说,受到方向性效应影响的范围扩大了,但是其影响的地震动强度并没有增大。(3)断层的破裂起始位置对方向性效应的影响:①破裂起始点越靠近地表,地震动的峰值越大,而且破裂起始点的位置对地震动的加速度影响最大,对速度和位移的影响次之。②断层的破裂起始点位置对地震动的FN分量的峰值影响较大,即随着初始破裂位置的下移,地震动FN分量的峰值降低,但是初始破裂点的位置对FP和UP分量的影响很小。③随着破裂初始位置的下移,各观测点的各周期对应的反应谱谱值降低,但是其沿破裂方向的变化趋势没有改变,也就是说断层的初始破裂位置对反应谱沿破裂方向的方向性特征影响不大。④初始破裂位置的变化对持时的影响不大,不同破裂起始位置的持时均在破裂前方的方向性显著影响区域较小且很接近,而后随着破裂起始位置的下移持时略有增大。(4)不同破裂速度对方向性效应的影响:①随着破裂速度逐渐接近于剪切波速,在破裂前方的一定区域内,地震动的峰值增大,反应谱值增大,持时略有下降。此区域与不同的分量有关,对于FN分量,此显著影响的区域可达到3倍的断层长度的距离,而对于FP和UP分量,其显著影响区域一般要小于1.5倍的断层长度。②破裂速度对地震动的加速度、速度和位移的影响比较相似,而且不同破裂速度下的显著影响区域并没有太大差别。(5)断层的倾角对方向性效应的影响:由于受到震源剪切错位的辐射模式的影响,使得与断层面上滑动方向垂直的分量上的地震动明显大于其它两个方向,因而当断层倾角越大时,与断层面上滑动方向垂直的地震动越接近于地表的FN分量,因此FN分量的方向性效应越明显;当倾角越小时,与断层面上滑动方向垂直的地震动越接近于地表的UP分量,因此UP分量的方向性效应越明显。(6)变破裂速度(非均一的破裂速度)对方向性效应的影响:①变化的破裂速度也能产生方向性效应,但是与常破裂速度相比,变破裂速度引起的方向性效应的峰值要明显小于常破裂速度。②变破裂速度使得在断层末端到破裂后方的观测点的各周期的反应谱值均大于均一的破裂速度情况下的值;而在断层末端到破裂前方的广大范围内观测点的各周期的反应谱值均小于均一破裂速度情况下的值。③常破裂速度和变破裂速度都会产生方向性效应,但是变破裂速度情况下破裂前、后方的持时差异比常破裂速度时的小;而且,一般说来,除了FN分量的破裂后方的观测点外,同一观测点在常破裂速度下的持时要小于变破裂速度情况下。(7)双侧破裂方向性效应分析:①双侧破裂对FN和FP、UP分量的影响不同,对于FN分量,随着震中距的增大,在断层长度范围内的地震动峰值逐渐增大,并且在断层的末端峰值逐渐下降,但是靠近断层的观测点的峰值最大。对于FP分量,观测点越靠近震中峰值越大,并且随着震中距的增加峰值逐渐减小。②双侧破裂时反应谱以破裂开始点为中心,在两个破裂方向分别出现方向性效应,使得在破裂末端区域的地震动的反应谱谱值加大,但是相比而言,方向性效应对FN分量反应谱的影响更为明显,并且在破裂的方向上,FN分量要比FP和UP分量衰减的慢得多。③方向性效应对于不同分量的持时影响程度不同,并且持时场的空间变化很复杂。4.引入了超剪切破裂的概念,从实际地震观测、实验室实验以及理论和数值模拟方面给出了地震超剪切破裂的证明,并简述了超剪切破裂产生的原因。通过数值方法模拟了不同超剪切破裂速度下的近断层地震动,分析结果表明:超剪切破裂也能产生方向性效应,但是超剪切破裂引起的方向性效应与常规破裂速度(即破裂速度小于剪切破裂)时有不同的特点:①在峰值方面,超剪切破裂速度的变化对地震动三分量的影响不同。对于FN分量,随着超剪切破裂速度的增大,地震动的峰值逐渐减小,而且显著影响区域也逐渐减小,并且随着超剪切破裂速度的增大,方向性显著影响角逐渐增大;对于FP和UP分量,随着超剪切破裂速度的增大,地震动的峰值并没有减小,反而有增大的趋势,但显著影响区域逐渐减少,方向性显著影响角逐渐增大。另外,随着超剪切破裂速度的增大,FN分量峰值大于FP分量峰值的观测点逐渐减少。②在反应谱方面,对于FN分量,随着超剪切破裂速度的增大,反应谱的谱值逐渐减小;但是对于FP和UP分量,随着超剪切破裂速度的增大,其反应谱却出现增大的趋势。③在持时方面,对于FN分量,随着超剪切破裂速度的增大,破裂前方的持时显著影响区域在逐渐减少,并且破裂前方的持时值逐渐增大,破裂后方的持时值逐渐减少,但是破裂后方的持时仍然大于破裂前方,并且根据断层距不一样,相差的程度也不相同;对于FP和UP分量,随着超剪切破裂速度的增大,破裂后方的持时变化不大,但是破裂前方的变化显著,明显不同于FN分量的就是,随着超剪切破裂速度的增大,破裂前方的持时逐渐增大,甚至超过了破裂后方的持时。5.简述了汶川地震的震源参数、断层模型和烈度分布的特点,分析了汶川地震的近断层地震动方向性特征,并从房屋震害和人员伤亡的分布特征间接佐证了方向性效应。研究表明:①在峰值方面,破裂前方场点的峰值要比破裂后方的大,而且随着断层距的增大,破裂前方和后方的差距逐渐减小。②在反应谱方面,破裂前方场点的平均反应谱显著高于破裂后方的平均反应谱,并且当周期大于2s时,破裂前、后的谱比最高可以达到4倍左右。③在持时方面,破裂后方场点的持时明显大于破裂前方的,差别可达2~4倍。最后,通过工程震害和人员伤亡特征佐证了此次地震的破裂方向性效应。更多还原

【Abstract】 The nature of rupture directivity caused by a predominately unilateral propagating fault with similar rupture speed to the local shear-wave velocity is a key factor in characterizing the near-fault ground motions. This study focuses on the fault related parameters that contribute to the directivity effect, including the fault rupture speed, rupture direction, fault dip, focal depth, rupture model and the local parameters on the fault plane. Among all the factors mentioned above, the super-shear rupture speed is the one that this study paid special attention to. So is there still exist rupture directivity under super-shear rupture speed of the fault? And in what extent will the super-shear rupture affect the strong ground motions? To answer these questions, three aspects of studies were conducted, including the theoretical analysis, numerical modeling and analysis of the real near-fault recordings and simulated ground motions. Through analysis of peak ground motion, response spectra and significant duration of the fault-normal, fault-parallel and vertical components of ground motions, some conclusions were drawn.1. The directivity effect, Doppler effect and rupture propagation effect are three but closely related concepts in characterizing the phenomenon of a rupturing fault. Firstly, the classical Doppler effect describes the frequency shift between the radiated and received waves of a moving source with single frequency. This is caused by the relative movement of the receiver and source, and it is only a changing of received numbers of wave in a given time but no wave superposition and interference. Secondly, for rupture propagation effect, the slipping of a point on the fault plane still continues after the surpass of the rupture front, so it may lead to a constructive interference of wave radiated from the multiple wave source, thus it is different in nature from the finite moving source and Doppler effect. Thirdly, the rupture process of a real earthquake fault is so complex that it is difficult to generate a frequency shift similar to the Doppler effect.2. Analysis on strong ground motions of the Chi-Chi earthquake indicates that the rupture directivity has an orientation modulation effect on the motion parameters. To be specific, firstly, directivity increases the amplitude of motion and decreases the attenuation rate in the forward direction, while it decreases the amplitude of motion and increases the attenuation rate in the backward direction. Secondly, the response spectra with periods larger than 2 sec are much more sensitive to the rupture directivity compared with that of the short periods, and the bigger the period, the larger the directivity effect appeared. Thirdly, the significant duration is getting longer with increasing epicenter distance in the forward direction. Lastly, the fault-normal component is the most sensitive to be affected by the directivity effect.3. Analysis on the related source parameters indicates that the focal depth, hypocenter location, rupture velocity, fault dip and rupture model play important roles in affecting the strong ground motion’s directivity.(1) Modeling results of a typical unilateral rupture fault indicate that there is an asymmetry distribution of amplitudes, spectral ordinates and durations in the ground surface. The peak ground motion and response spectral ordinates in the forward direction get much higher than that of the backward direction with similar rupture distance, while the duration parameter is in the opposite.(2) Variation of focal depth also has an obvious effect on the directivity. With increasing of focal depth, the amplitude of ground motion is getting more and more smaller, while the significantly affected areas in the forward direction is getting more and more far from the epicenter, and the directivity affected areas are getting more and more bigger.(3) The hypocenter location has a similar effect with the focal depth in the study, and in general, the more closely the hypocenter located to the ground surface, the bigger the amplitude and spectral ordinate is. And also the hypocenter parameter has a different effect on fault-normal and fault-parallel components.(4) The uniform rupture velocity with different value has an effect on the amplitude, spectral ordinate and duration. It is concluded that larger rupture velocity increases the amplitude and spectral ordinate but decrease the duration. And for different rupture velocity in the study, the significantly affected areas show little difference.(5) Dip angle of the fault model has a special effect on the ground motion. In a word, big dip angle leads to a distinct directivity effect in the fault-normal component, while small dip angle leads to a distinct directivity effect in the vertical component.(6) Nonuniform rupture speed along the fault length also generates directivity on the ground motions. Compared to the constant rupture speed, the variable rupture speed leads to relatively smaller amplitude and spectral ordinate, but a higher duration in the same rupture distance.(7) Bilateral rupture of the fault can also generate directivity effect in the two end of the rupture propagating. Analysis to the simulated ground motion indicates that the directivity effect generated by a bilateral rupture is more complex than that of a unilateral rupture. And it also has different effect on the fault-normal and fault-parallel components.4. The concept of super-shear rupture and its validation were introduced, through analysis of ground motion under various super-shear rupture speeds, results indicate that super-rupture also leads to directivity effect but there appear some difference compared with the sub-shear ruptures. Firstly, the amplitude of fault-normal component decreases with increasing super-shear rupture speed, while the fault-parallel component increases with increasing super-shear rupture speed. And the significantly affected areas in the forward direction decrease with increasing super-shear rupture speed. Secondly, as for the response spectra ordinate, it bears some similar features with the amplitude aspect. Thirdly, in general, the duration in the forward direction for fault-normal component decreases with increasing super-rupture speed, while the fault-parallel component is in opposite.5. A preliminary analysis of directivity effect on strong ground motions in Wenchuan earthquake is conducted. Firstly, in general, the peak ground acceleration in the forward direction is bigger than that of the backward direction, and the difference between the forward and backward depends on the closest rupture distance. Secondly, the average response spectra in the forward direction is higher than that of the backward direction, especially for periods higher than 2 sec, and the maxim ratio of forward and backward average spectra can reaches 4. Thirdly, for strong ground motion duration, the difference between the forward and backward direction some times reaches 2~4 times on average. Lastly, the distribution of building collapse ratio and human mortality ratio in direction along and perpendicular to the fault strike are adopted to additionally prove the directivity effect.更多还原