【作者】 罗艳；

【导师】 陈颙； 倪四道；

【作者基本信息】 中国科学技术大学 ， 固体地球物理， 2010， 博士

【摘要】 地震预测是地震研究工作中的一个重要内容。开展地震预测研究工作,需要探测地壳中应力状态、断层形态和流变特征等性质,而震源参数(深度、机制解、破裂方向、破裂速度等)可提供这些信息。震源参数中的机制解反映区域应力状态,震源深度反映介质流变性质(脆性-韧性转换),而地震破裂方向可用来确定发震断层。与数量少且分布集中的强震相比,中小地震(M2-M5)数量众多,地理分布广泛,为探测广大地区的地壳应力状态提供了可能。此外,强震的余震多为中小地震,大量中小余震可为主震发震机理研究提供重要信息。对于强震震源参数,可以利用长周期波形结合简单一维模型,已有很多成熟方法。然而中小地震P波信号主要能量集中在短周期,只有在高频段内才有足够的信噪比,然而在高频段内P波振幅受地壳不均匀性、尖锐间断面等三维效应影响,较难直接运用简单一维模型计算的理论波形来确定震源参数。本文一方面利用已有准确震源参数的地震标定短周期的P波幅度,得到台站振幅放大因子(Amplitude Amplification Factor, AAF),从而使短周期P波振幅可以有效地应用于中小地震震源参数求解；另一方面,由于地壳中的一些界面是影响体波振幅的重要因素,本文还对莫霍面、沉积层-结晶基底界面等界面进行了研究。本文首先使用接收函数方法研究首都圈地区的莫霍面深度,然后利用接收函数中首到波峰和直达P波到时差确定沉积层厚度,此外还利用井下摆记录中的直达S波与其在地表反射波到时差和波形,研究了该地区近地表S波速度结构和Q值结构。研究结果表明首都圈地区莫霍面深度西厚东薄,区域构造方向呈NE—NN方向,平均厚度约为34km,泊松比分布则有明显的分块特征,泊松比高值区对应于岩石比较破碎的多条活动断裂带交汇区。发现浅部100米的平均S波速度低于300m／s,当深度增加到500m时S波速增加到800m／s,平均速度梯度为0.8(m／s)／m。为了考察速度结构对高频地震波振幅的影响,本文还研究了速度结构模型与SmS振幅的关系,探讨了超临界距离上SmS震相对强地面振动衰减关系的控制作用。通过对2008年云南盈江地震的实际波形记录和理论地震图的分析表明,SmS振幅大小与当地地壳及莫霍面结构有关。对于地壳结构较简单地区,能在莫霍面形成很强的反射波,这意味着当破坏性地震发生时,在超临界震中距附近,地壳结构相对简单有利于SmS发育的地区,强地面运动主要受控于SmS波。由于SmS的大振幅强能量,地面运动会加强2-3倍,这表明在超临界距离的地面震动衰减关系和超临界距离之前的衰减关系有明显差别。地震深度是震源参数中一个重要参数,P-SV系列的深度震相(sPL,sPg,sPmP,sPn)已广泛应用于震源深度的确定中,能大大改善震源深度的确定精度。但目前利用切向分量上SH系列的深度震相(sSmS)来确定震源深度的研究很少。而在地震波的切向分量上,由于没有P-SV波的耦合,波形相对简单,SmS和sSmS震相更易辨认。为检验用sSmS与SmS震相确定地震深度的可行性,本文通过一系列一维地壳速度模型计算理论地震图,发现SmS和sSmS只有在高频(＞0.2 Hz)、速度结构简单,且震源深度不能过浅的情形下才可使用。因此P-SV系列应该是优先使用的深度震相,而SH系列只能作为辅助深度震相。由于SH系列的深度震相只适用于速度结构简单的地区,而龙门山地区地质构造复杂,因此本文主要利用SP系列的近震深度震相sPL, sPg, sPmP, sPn与其主震相到时差,联合使用CAP长周期波形反演、面波与S波振幅谱比方法确定了汶川地震300多个中小余震的深度。研究结果表明,汶川余震深度优势分布在5-15km范围,最深地震为2008年05月25日Ms6.2余震,该次余震也是汶川地震最强的余震,中心震源深度为19±2km。在汶川地震破裂区中段,既没有深度小于5km的浅震,也没有深于15km的地震。在破裂区东北端,有很多浅震(1～3km深),其中2008年7月24日发生在青川与宁强交界的一次Ms5.7级地震及其余震序列深度均浅于3km。由于实地考察和InSAR研究没有发现汶川主破裂带在这个地区有地表破裂,所以本文认为这个地震序列是由于深部破裂引起浅部应力增加而导致完整岩体的新鲜破裂,而非沿着汶川主震破裂带发生。对于浅源地震由于震源浅,深度震相与直达P波十分接近,因此震相识别困难,难以用深度震相来确定震源深度。但是一般的浅源地震具有短周期面波发育的特点,本文使用短周期面波和远震波形拟合的方法研究了2010年1月31日遂宁—潼南M5.0地震,确定该地震深度约为1～3km,位于厚达6km的沉积盖层之内。一般而言只有结晶基底才有足够的强度积累大量的应变能然后突然释放形成破坏性地震,而沉积盖层则由于富含孔隙、流体因此较软弱而不能积累足够的能量形成大的地震。但此次破裂性地震却发生在中生代沉积盖层里,虽然文献中有古生代沉积岩中发生4级地震的报道,但是中生代沉积盖层中发生高达5级地震的案例尚属首例。此次地震给破坏性地震的孕育机理研究提供了罕见的案例。除震源深度之处,震源机制、破裂方向、破裂尺度和破裂速度也是描述震源的重要参数,本文使用经台站振幅放大因子AAF校正后的短周期体波进行波形反演,研究了2009年美国加州Inglewood M4.7地震序列,确定了14个M2.0以上小震的震源机制解。然后,用已知机制解的小震波形作为经验格林函数(EGF),反演了两个较大地震(Mw4.6,Mw3.8)的破裂方向,破裂尺度以及破裂速度。研究结果表明主震的震源深度约8km,断层面解为145／77／139(strike/dip/rake),与Newport-Inglewood断层方向一致,且余震主震机制解相近。这两个较大地震的破裂方向分别为154°和143°,与Newport-Inglewood断层走向一致。此外,主震表现出高破裂速度特点,与Inglewood断层为成熟断层的特征一致。用P波反演的破裂尺度和破裂速度计算SH波的理论地震图,对比实际波形,二者得到了很好的符合,从而验证了P波反演结果的可靠性。本文为中小地震震源参数研究提供了系统的框架：在较好了解了地壳间断面结构的基础上,对主要震相进行幅度和到时改正,从而更好地确定中小地震震源参数。更多还原

【Abstract】 Earthquake prediction, one of the most important fields in seismology, depends on the information on stress state, fault feature and rheology in the crust. Source parameters including depth, focal mechanisms, rapture directivity and rupture velocity provide abundent such information. Focal mechanism reflects regional stress state and focal depth shows rheology feature of medium, while rupture directivity can be used to determine the fault on which the earthquake occurs. Large numbers of small and moderate earthquakes (M2-M5) distribute widely, which make it possible to explore the crust physical state. Moreover, most aftershocks of strong earthquake are small and moderate events, which provide important information on focal mechanism of mainshock.For macroseism, the mature long period waveforms inversion methods are applied to determine source parameters. For small and moderate events, energy of P waves concentrate at short period, so Signal Noise Ratio is high enough only at this band. But the amplitudes of observed P waves are quite different from those of synthetics at this band, thus it can not be used to invert source parameters directly. With determined source parameters, short period (0.5-2Hz) P waves of the these events are compared with synthetic ones to handle amplitude variability with Amplitude Amplification Factor (AAF) correction, which is then used in modeling source mechanism of the small and moderate earthquakes. On the other hand, interfaces in the crust may affect body wave amplitude, and hence interfaces such as Moho discontinuity and basin basement were also investigated in this study.Receiver functions were calculated with teleseismic P waveforms recorded at broadband and short period stations of the Capital Digital Seismic Network. H-Kappa method was adopted to obtain the thickness and Poisson ratio of the crust. The results show that the crust thickeness gradually thickens from SE to NW with an average thickness of 34 km. The crustal Poisson ratios are sensitive to geological structure and high ratios are considered to be related to active faults. S wave velocity in the top 100m of the crust is lower than 300 m/s, and it reaches 800 m/s as depth increases to 500 m with an average gradient of 0.8 m/s which are obtained from differential travel time between direct S wave and reflected wave at the free surface with borehole station records. To demonstrate the effect of velocity structure on amplitude of high frequency waves, the relationship between velocity models and SmS amplitude was studied, and the effect of SmS amplitude over the critical distance on the strong ground motion was investigated. The foreshock and aftershocks of the 21 August 2008 Yingjiang earthquake (Mw5.9) were used in this study. For the data recorded at Wangding station which is 100 km far away from epicenter, large SmS phases were observed with amplitudes of 2 to 5 times greater than the direct S wave. Comparisions with the synthetic seismograms indicate that such large-amplitude SmS phases were produced by simple crustal structure of Wangding region which allows a large Moho reflection. This suggests that when damaging earthquakes occur in regions of simple crustal structures, Moho reflections will produce amplified strong motions at distance around 100km depending on the local structure.Focal depth is one of the most important source parameters, which can be well determined by depth phases of P-SV series such as sPL, sPg, sPmP, sPn, greatly improving the resolution of depth determination. But so far, the depth phases of SH series have been rarely used to determine focal depth. In fact, SmS and sSmS phases could be easily indentified due to there are no P-SV coupling with relatively simple waveforms on tangential componet. To test the feasibility of determing focal depth with SmS and sSmS phases, we calculated synthetic seimograms on a set of 1D crustal velocity models, finding out that SmS and sSmS are obvious phases only in high frequency(>0.2Hz), simple crustal velocity model, and deep focal depth(>10km). Therefore, we drawed the results that the depth phases of P-SV series are preferential, while those of SH series are supplementary.Since depth phases of SH series is feasible only in regions with simple velocity structures and the geological structures near Longmen Mountain faults are very complicated that the crustal structures of eastern and western sides near LongMengShan faults are very different, we determined focal depths of over 300 small to moderate afthershocks of the Wenchuan earthquake using depth phases and CAP inversion at long period which employed amplitude ratios between surface waves and S wave. The results show the epicenter mainly locates at 5-15km deep and the largest depth is around 21km, while the shallowest one is only 1km. A Ms 5.7 aftershock occurred at Qingchuan, northeast end of the LongMengShan fault on Jul 24, 2008, featuring thrust mechanism with a 3km source centroid depth. The shallow focal depth is confirmed using the sPL phase recorded by a station at an epicentral distance of 15km. The observed large amplitude of Rg at a distance of 15km implies a depth of 3km or less. Dozens of aftershocks’ sPL waveforms are also analyzed and less than 3km depths were obtained from it. On the other hand, both geological observations and InSAR studies report there is no surface breaks in this region. It is strongly suggested that this aftershock sequence initiates a fresh rupture in intact rocks which were triggered by stress increment raisen from the deep co-seismic rupture of the Wenchuan mainshock.Shallow earthquakes with potential of great earthquake disaster are very dangerous. The depth phases are very close to Pg in arrival time because of shallow depth, which makes it difficult to identify them. However, generally the shallow earthquakes are characterized by developmental surface waves. Waveform modeling of short period surface waves and teleseismic body waves is performed to study 2010 Suining-Tongnan M5.0 earthquake. The source depth of 1-3km indicates that this event occur in the over six kilometer’s sediment layer. In general, only basement rocks are strong enough to accumulate strain energy for destructive earthquake. While sediment layers are porous and full of fluids, thus they are too soft to accumulate enough strain energy. This event provides an unusual but typical case for seismogenic mechanism.Besides source depth, focal mechanism, rupture directivity, rupture length and rupture speed are also important parameters for describing source. Short period body waves calibrated with station amplitude amplification factor (AAF) inversion approach is applied to study the 2009 Inglewood M4.7 earthquake sequence in California. Using this approach, we have determined 14 focal mechanism solutions of small events with ML down to 2.0. Then records of small events with well determined source mechanism was taken as Empirical Green’s Function (EGF), and the source parameters including rupture directivity, rupture length and rupture speed of two largest events (M>3.5) obtained with such EGF. The results show a source depth of 8km and fault solution of 145/77/139(strike/dip/rake) for mainshock. One of the fault planes has similar strike to that of Newport-Inglewood fault. The rupture propagation directions of the two largest events are N154 and N143 respectively, coinciding with strike of Newport-Inglewood fault. The high rupture velocity may be related to the fact that the Newport-Inglewood fault is a mature fault. To confirm the results from P wave inversion, we first used the rupture velocity and rupture scale to obtain duration time of SH wave and then calculate SH synthetics. Comparison between synthetics and observed SH waveforms demonstrated validity of results inferred from P waves.We try to provide a frame of systematically studying source parameters of small to moderate earthquakes. The amplitudes and arrival times of main phases could be corrected on the base of the well understood fine-scale velocity model, which is required by studying source parameters of small and moderate earthquakes with good resolution.更多还原