2000年云南姚安地震余震区的速度和衰减结构层析成像分析

文中根据2000年云南姚安6.5级地震后,流动数字化地震监测台网记录到的地震序列走时和波形资料,采用震源位置和速度结构联合反演的方法,反演了震源区的V_P、V_S和Q_S的三维结构。 层析成像结果显示,姚安6.5级地震发生在低V_P、高Q_S扰动异常的边缘,大部分余震均发生在高、低V_P、Q_S过渡区域内。 V_P的低值异常表明姚安震区存在一条NWW向断裂带,且异常深度达到10 km左右。 该文通过研究地震序列的分布特征并结合震源区速度结构和衰减结构的特点,进一步探讨了姚安地震的发震构造。     

CT在肝癌LP－TAE术后的应用价值（附30例）

作者对３０例原发性肝癌ＬＰ－ＴＡＥ治疗前后有ＣＴ检查的病例进行分析，讨论了碘油选择性沉积在肿瘤内的机理．ＬＰ－ＴＡＥ术后肿瘤区域四种成份的ＣＴ表现．碘油在肿瘤内的三种分布类型及其意义。重点讨论了肝癌ＬＰ－ＴＡＥ术后ＣＴ检查，可以从以下几方面对疗效观察提供帮助，即肿瘤大小变化、碘油聚积量及分布情况、肿瘤区密度变化、子结节的发现和治疗效果、术后并发症的发现等．     

京津唐地区地壳结构与强震的发生──I．P波速度结构

用１９８４－１９９１年发生在京津唐地区（３７°－４１°Ｎ，１１２．５°－１２０°Ｅ）的２５０５个地震的２２２３４条Ｐ波到时进行的层析成像，揭示了地震活动性与速度图像间的联系：１．公元１０００年以来该区强震震中在上地壳的投影大多分布在高速块体内或高速块体与低速块体相交地带，偏高速体的一侧．２．唐山地震和三河－平谷地震所处的构造背景相似，均未依傍大断层，处在介质速度变化剧烈部位．３．宁河与唐山、滦县分别位于规模大小、位置深浅不同的两个不同的高速块体处，可以解释唐山地震序列震源深度分布东北浅西南深的总体特征．４．唐山地震可能是由一条北西向的地下隐伏断层扩展的结果．     

Perturbation methods for two special cases of the time‐lapse seismic inverse problem

Scattering theory, a form of perturbation theory, is a framework from within which time‐lapse seismic reflection methods can be derived and understood. It leads to expressions relating baseline and monitoring data and Earth properties, focusing on differences between these quantities as it does so. The baseline medium is, in the language of scattering theory, the reference medium and the monitoring medium is the perturbed medium. The general scattering relationship between monitoring data, baseline data, and time‐lapse Earth property changes is likely too complex to be tractable. However, there are special cases that can be analysed for physical insight. Two of these cases coincide with recognizable areas of applied reflection seismology: amplitude versus offset modelling/inversion, and imaging. The main result of this paper is a demonstration that time‐lapse difference amplitude versus offset modelling, and time‐lapse difference data imaging, emerge from a single theoretical framework. The time‐lapse amplitude versus offset case is considered first. We constrain the general time‐lapse scattering problem to correspond with a single immobile interface that separates a static overburden from a target medium whose properties undergo time‐lapse changes. The scattering solutions contain difference‐amplitude versus offset expressions that (although presently acoustic) resemble the expressions of Landro ( 2001 ). In addition, however, they contain non‐linear corrective terms whose importance becomes significant as the contrasts across the interface grow. The difference‐amplitude versus offset case is exemplified with two parameter acoustic (bulk modulus and density) and anacoustic (P‐wave velocity and quality factor Q) examples. The time‐lapse difference data imaging case is considered next. Instead of constraining the structure of the Earth volume as in the amplitude versus offset case, we instead make a small‐contrast assumption, namely that the time‐lapse variations are small enough that we may disregard contributions from beyond first order. An initial analysis, in which the case of a single mobile boundary is examined in 1D, justifies the use of a particular imaging algorithm applied directly to difference data shot records. This algorithm, a least‐squares, shot‐profile imaging method, is additionally capable of supporting a range of regularization techniques. Synthetic examples verify the applicability of linearized imaging methods of the difference image formation under ideal conditions.     

The Al Hoceima Mw 6.4 earthquake of 24 February 2004 and its aftershocks sequence

The Al Hoceima Mw 6.4 earthquake of 24 February 2004 that occurred in the eastern Rif region of Morocco already hit by a large event in May 1994 (Mw 5.9) has been followed by numerous aftershocks in the months following the event. The aftershock sequence has been monitored by a temporary network of 17 autonomous seismic stations during 15 days (28 March–10 April) in addition to 5 permanent stations of the Moroccan seismic network (CNRST, SPG, Rabat). This network allowed locating accurately about 650 aftershocks that are aligned in two directions, about N10-20E and N110-120E, in rough agreement with the two nodal planes of the focal mechanism (Harvard). The aftershock alignments are long enough, about 20 km or more, to correspond both to the main rupture plane. To further constrain the source of the earthquake main shock and aftershocks (mb > 3.5) have been relocated thanks to regional seismic data from Morocco and Spain. While the main shock is located at the intersection of the aftershock clouds, most of the aftershocks are aligned along the N10-20E direction. This direction together with normal sinistral slip implied by the focal mechanism is similar with the direction and mechanisms of active faults in the region, particularly the N10E Trougout oblique normal fault. Indeed, the Al Hoceima region is dominated by an approximate ENE-SSW direction of extension, with oblique normal faults. Three major 10–30 km-long faults, oriented NNE-SSW to NW-SE are particularly clear in the morphology, the Ajdir and Trougout faults, west and east of the Al Hoceima basin, respectively, and the NS Rouadi fault 20 km to the west. These faults show clear evidence of recent vertical displacements during the late Quaternary such as uplifted alluvial terraces along Oued Rihs, offset fan surfaces by the Rouadi fault and also uplifted and tilted abandoned marine terraces on both sides of the Al Hoceima bay.However, the N20E direction is in contrast with seismic sources identified from geodetic inversions, which favour but not exclusively the N110-120E rupture directions, suggesting that the 1994 and 2004 events occurred on conjugate faults. In any event, the recent seismicity is thus concentrated on sinistral N10-20E or N110-120E dextral strike-slip faults, which surface expressions remain hidden below the 3–5 km-thick Rif nappes, as shown by the tomographic images build from the aftershock sequence and the concentration of the seismicity below 3 km. These observations may suggest that strain decoupling between the thrusted cover and the underlying bedrock and highlights the difficulty to determine the source properties of moderate events with blind faults even in the case of good quality recorded data.     

A note on three-dimensional scattering and diffraction by a hemispherical canyon–I: Vertically incident plane P-wave

The three-dimensional scattering by a hemi-spherical canyon in an elastic half-space subjected to seismic plane and spherical waves has long been a challenging boundary-value problem. It has been studied by earthquake engineers and strong-motion seismologists to understand the amplification effects caused by surface topography. The scattered and diffracted waves will, in all cases, consist of both longitudinal (P-) and shear (S-) shear waves. Together, at the half-space surface, these waves are not orthogonal over the infinite plane boundary of the half-space. Thus, to simultaneously satisfy both zero normal and shear stresses on the plane boundary numerical approximation of the geometry and/or wave functions were required, or in some cases, relaxed (disregarded). This paper re-examines this boundary-value problem from the applied mathematics point of view, and aims to redefine the proper form of the orthogonal spherical-wave functions for both the longitudinal and shear waves, so that they can together simultaneously satisfy the zero-stress boundary conditions at the half-space surface. With the zero-stress boundary conditions satisfied at the half-space surface, the most difficult part of the problem will be solved, and the remaining boundary conditions at the finite canyon surface will be easy to satisfy.     

Inhomogeneities near the core-mantle boundary evidenced from scattered waves: A review

Statistical properties of small-scale inhomogeneities (wavelengths between 20 and 70 km) near the core-mantle boundary are inferred from scattered core waves. Observations of scattered core waves at large seismic arrays and worldwide networks indicate that the inhomogeneities have a global nature with similar characteristics. However, there may exist a few regions having markedly stronger or weaker strengths. Scattering by volumetric inhomogeneities of about 1% inP-wave velocity in the lower mantle or by about 300 m of topographic relief of the core-mantle boundary can explain the observations. At present it is not possible to rule out either of these two alternatives, or a combination of both.     

利用首波重建基底速度图象

地震层析技术(Seismic Tomography)正在作为一种新技术引入到地震学的研究中,但由于地学中存在着非完全投影和模型参数间的耦合,使得该技术的应用受到了限制。本文讨论了用首波走时资料作为投影数据,重建基底或莫霍界面的速度图象的方法,同时亦可利用该方法对震源深度做进一步的修正。计算中采用了代数重建法,并就改善ART和SIRT这两种算法的稳定性及收敛速度,提出了较合理的权因子。通过数值模拟还对分辨和方差的影响进行了分析。 利用海南岛人工爆破地震Ⅰ、Ⅱ测线非纵剖面的P_8波走时资料,初步得到了该区基底面的速度图象。结果表明,基底的低速区分布与地表的两条断裂带吻合得较好,说明北东向的春江-中和断裂与干冲-木裳断裂穿过了基底。断裂带内的分段特征也在图象中有所表现     

Scattering and dissipation of seismic waves in the presence of nonlinearity

The processes of seismic wave scattering and dissipation are examined in a medium that, in addition to being inhomogeneous and anelastic, is nonlinear and seismically active (seismic emission). In such a medium, there is a complex interrelation between the effects of nonlinearity, scattering and dissipation. Thus, nonlinear interactions between the various components (primary, scattered and induced) of a developing wavefield cause nonlinear (or wave-on-wave) scattering and, by transferring part of the wave energy to the high-frequency region, contribute to its scattering and dissipation. On the other hand, whereas dissipation opposes the accumulation of nonlinear effects by reducing the wave amplitudes, scattering assists it by increasing the propagation distance (and hence the interaction time).Estimates based on results of field experiments involving vibrators indicate that, as a rule, scattering on inhomogeneities is much stronger than nonlinear scattering, and that nonlinear effects may often dominate dissipative ones.The nonlinearity of the transmitting medium explains observedQ-value anomalies, and its seismic activity explains the existence ofP coda and the temporal changes in codaQ.