地震前低频事件的实验研究

介绍了几种强震前超低频事件的观测证据,并通过实验,研究了产生和传播机理。初步结果表明,微破裂的集结和断层的破裂起始会出现超低频脉冲,并透射到空气中形成次声波。塑性体的冲击或超临界流体的膨胀比脆性破裂更容易激发（超）低频波,且不伴随明显高频辐射成分。这样可以较为合理地解释震前“平衡”背景下的（超）低频事件。对实验中破坏前的低频脉冲给出了断裂力学解释。断裂力学的实验和膨胀－－扩容理论证明了震前会产生断层的张裂隙,从而为超临界流体提供了运移空间。流体在运移过程中突然涨缩会激发（超）低频辐射波。讨论了改进（超）低频事件的观测条件问题。     

井水位前驱波现象与震源成核过程关系研究的现状

井水位前驱波现象的成因可能与震源过程有关，是大地震断层大破裂之前的慢破裂产生的，因此对前驱波现象的研究、震源过程与前驱波现象的对比分析、井－含水层系统对前驱波响应的理论分析和前驱波生成机理的研究是认识震源过程的重要途径之一。正确区分源兆、场兆和远兆，努力寻求3种前兆的信息特征，是震情综合分析所必须的工作。     

强地震前兆低频波的实验研究

声波透射实验表明，对于板状材料，声音的一部分能量可以透射到空气中。另一部分能量在板中产生简正波，使板产生较大的振幅，在空气中产生能被话筒接收到的声音信号，而信号的简正频率与板的尺寸有关。另外，破裂过程中的声发射实验证实：随着微破裂的集结和裂纹的扩展，所辐射波的频谱会逐渐向低频移动，而且当频率很低时，可以透射出岩石，被话筒接收到。根据尺度相似性原理，推广到野外，强地展前破裂集结可能会辐射出超低频波，这种超低频波不能被常规的地震仪记录到，但可以被超低频传感器或次声接收器记录到。在地震前，产生前兆的源非常复杂，大多数情况下可能并非脆性破裂，而是塑性蠕滑或者有流体的参与。而后两种情况产生的波的周期会更长，而且缺乏脆性破裂所固有的高频成分，这可能就是多数地震前缺乏前震的原因。     

井水位前驱波现象与震源成核之间关系的研究

经仔细研究了1999～2002年间全球48次Ms≥7.0以上、中国大陆12次Ms≥6.0以上地震前山西地区5口地下水位观测井强震前的前驱波事件,给出了井水位前驱波的识别方法.从理论与实验结果分析了前驱波与震源成核过程之间的关系;根据井一含水层系统的特征参数给出了不同类型观测井对前驱波响应能力的大小;根据记录地震前驱波信息所需要的条件,初步探讨了大震前驱波远程传播的机制.     

热红外震兆成因的模拟实验研究

对完整岩石破裂前和既存断层粘滑失稳前的红外辐射温度场进行了遥感观测研究,发现岩石破裂前出未来断层处出现条带状的红外热像,粘滑失稳前断层闭锁点出现升温现象,声发射测量表明,岩石破裂前已有大量微破裂产生,地壳中的既存断层和大震前的微小破裂均可能为震前地壳中的热能转移提供通道,从而导致地面热外震兆的出现。     

井水位长周期事件记录及其机理的讨论

选择了几口具有代表性的观测井, 并对这些井孔在强震前记录到长周期波现象的特征进行了分析。从理论上对地震前的长周期事件给出了较为合理的解释, 认为地震前一些敏感井水位长周期事件可能反映了断层蠕动、静地震与慢地震、断裂的预扩展和地震成核等。井— 含水层系统像一个长周期地震仪, 它为记录到有价值的前兆低频波提供了最佳的频率响应范围。     

2001年1月强震的前兆次声波测量及机理探讨

频谱分析结果表明 :(1) 2 0 0 1年 1月 (含两次 8级地震 )的测量结果与 1999年 1月～ 7月、 2 0 0 0年 4月～ 2 0 0 1年 4月观测结果具有相同的规律 ,强地震前约 10天内常能测到振幅很强、方向可测的地震前兆次声波。其三维动态频谱的典型特点是振幅由弱 (10Pa～ 2 0Pa)变强 (80Pa以上 ) ;最后的周期范围很宽 (2分～ 6 5分 ) ;波形明显不同于流星雨次声和大风次声。 (2 )强震前兆次声三维动态频谱的最大振幅不仅受震级大小影响 ,而且与震源距离及深度等其它因素都有关。 (3)强震前长周期波的产生机理 ,主要有断层预滑理论、断裂预扩展理论和流体撞击假说 ,而前兆次声波很可能是由地壳中的长周期波耦合到空气中形成。 (4 )岩石破裂实验证实主破裂前在岩石表面和空气中均收到较强的低频波。 (5 )若在北京和昆明各建足够大的三点阵测准波向 ,两直线相交 ,即可预测国内的震中位置 ,这就可能为临震预报提供一种较有效的新方法     

断层破裂传播速度与破坏性地震的烈度分布——浅源地震的断层不均匀性与短周期波动的多普勒效应

地震的等震线不仅反映了地震的大小,而且也反映了地震断层过程的类型和破裂速度.单侧破裂的断层过程呈现出蛋型的等震线,而双侧破裂的断层过程呈现出椭圆的等震线,等震线的长轴与短轴之比对于破裂传播速度十分敏感.本研究利用理论等震线与观测结果相比较的方法,确定了1964年日本新地震、1983年日本海中部地震、1975年中国海城地震和1976年中国唐山地震的破裂传播速度、断层类型以及断层走向.得到的断层破裂速度是剪切波的0.7——0.9倍.这些值比用长周期地震波所确定的要稍大.产生这种差异的原因是:对烈度起主要影响的短周期地震波强烈地依赖于小尺度的断层不均匀破裂过程,以及局部的破裂传播的开始和终止;而由长周期波所得到的破裂速度却反映了在整个断层上破裂传播的平均过程.根据等震线图所得出的断层类型及断层走向与其它独立方法的结果相一致.这意味着本方法可以应用于推断某些历史地震的断层类型、破裂传播速度及破裂传播方向.      

基于声发射实验结果讨论大震前地震活动平静现象的机制

基于岩石变形声发射实验结果讨论了大震前地震活动平静现象的机制。在双轴压缩、等位移速率加载条件下 ,挤压型雁列断层、含宏观凹凸体断层、Ⅲ型剪切断层等非连续断层在临近滑动失稳前 ,声发射活动均出现了明显的相对平静现象 ,表现为声发射事件的发生率和应变能释放水平显著降低。与此阶段相对应 ,断层带 (特别是非连续部位上新形成的一小段断层 )发生了蠕滑和匀阻化作用 ,并使得标本的差应力开始下降。根据实验结果提出 ,大地震前断层的“蠕滑 -匀阻化”过程是造成地震活动平静现象的可能机制     

临震预滑在青藏活动地块区的观测证据

实验室内大量岩石黏滑实验表明:对存在断层的岩石加压,岩石在黏滑失稳前会有预滑,预滑会出现在断层的某个区域.由于浅源地震也是断层失稳破裂的结果,近年来临震预滑作为一种地震前兆信息日益受到地震学界关注.但目前尚缺乏广泛的野外观测证据来证实黏滑实验中的预滑现象在构造地震前也普遍存在.本文对青藏活动地块区内姑咱地震台等12个台的应变仪和水位仪的记录作对比分析后发现:在汶川MW 7.9级地震前数月,各台陆续记录到了 一些"阶跃式"波形.在对观测结果可靠性讨论的基础上,用岩石黏滑实验结果和颗粒物理原理来解释这种"阶跃式"波形可能是预滑震相Xp.姑咱等12个地震台观测到预滑震相Xp表明:在青藏活动地块区临震预滑是普遍存在的.这种"准同步"的临震预滑,特别是2008年4月18日和5月6日2次幅度较大的预滑可能引发了汶川MW7.9级地震.因此,关注预滑,特别是临震预滑对预测未来强震有一定的前兆意义.     

Guided Waves from Sources Outside Faults: An Indication for Shallow Fault Zone Structure?

— Using 3-D numerical modeling of seismic wave propagation we investigate the possibility of generating fault zone (FZ) trapped wave energy from sources well outside a fault. The FZ is represented by a O(200 m) wide vertical low velocity layer in a half space. We find that FZ trapped waves can be excited from sources well outside the fault if (1) the low-velocity structure extemds only to shallow depth and the source is located at greater depth or (2) the structure of the low-velocity zone is such that only the shallow part of the FZ traps energy. FZ trapped waves are not excited from sources well outside a FZ continuous with depth. The results support, in conjunction with recent observational evidence, a model for natural faults with shallow trapping structures rather than ones that span the entire seismogenic zone. This may have implications for fault mechanics as well as for aspects of shaking hazard near faults.     

理解前兆异常变化机理和地震短临前兆

本文概要介绍了国内外近几年在地壳形变一地球自转日长变化十年尺度相关性、静寂地震、前驱波三方面的重要研究发现,并分析了这三项研究发现在地震前兆异常机理认识中的作用。初步分析表明：深部核幔耦合作用引起的地壳形变、地球物理场十年尺度波动,是数年至十年尺度地震场兆波动性变化的重要调制因素之一;源于断层脆韧转换带(过渡层)的静寂地震一慢地震缓慢破裂过程,是数天至数月尺度地震短期异常变化产生的重要原因;源于脆韧转换层上部或弹脆性层薄弱部分慢地震过程所辐射出来的低频前驱波(慢地震波),是目前已发现的可重复出现的短临前兆信号,具有数小时至7天左右的地震短临预报指标意义。     

Recent advances in imaging crustal fault zones: a review

Crustal faults usually have a fault core and surrounding regions of brittle damage, forming a low-velocity zone(LVZ) in the immediate vicinity of the main slip interface. The LVZ may amplify ground motion, influence rupture propagation, and hold important information of earthquake physics. A number of geophysical and geodetic methods have been developed to derive high-resolution structure of the LVZ. Here, I review a few recent approaches, including ambient noise cross-correlation on dense across-fault arrays and GPS recordings of fault-zone trapped waves. Despite the past efforts, many questions concerning the LVZ structure remain unclear, such as the depth extent of the LVZ. High-quality data from larger and denser arrays and new seismic imaging technique using larger portion of recorded waveforms, which are currently under active development, may be able to better resolve the LVZ structure. In addition, effects of the alongstrike segmentation and gradational velocity changes across the boundaries between the LVZ and the host rock on rupture propagation should be investigated by conducting comprehensive numerical experiments. Furthermore, high-quality active sources such as recently developed large-volume airgun arrays provide a powerful tool to continuously monitor temporal changes of fault-zone properties, and thus can advance our understanding of fault zone evolution.     

Including Vertical Fault Structures in Layered Groundwater Flow Models

Accurate representation of groundwater flow and solute transport requires a sound representation of the underlying geometry of aquifers. Faults can have a significant influence on the structure and connectivity of aquifers, which may allow permeable units to connect, and aquifers to seal when juxtaposed against lower permeability units. Robust representation of groundwater flow around faults remains challenging despite the significance of faults for flow and transport. We present a methodology for the inclusion of faults utilizing the unstructured grid features of MODFLOW-USG and MODFLOW 6. The method focuses on the representation of fault geometries using non-neighbor connections between juxtaposed layers. We present an illustration of the method for a synthetic fluvial aquifer. The combined impact of the heterogeneous aquifer and fault offset is clearly visible where channel features at different depths in the aquifer were connected at the fault. These results highlight the importance of representing fault features in groundwater flow models.     

Charge generation and propagation in igneous rocks

Various electrical phenomena have been reported prior to or concurrent with earthquakes such as resistivity changes, ground potentials, electromagnetic (EM), and luminous signals. Doubts have been raised as to whether some of these phenomena are real and indeed precursory. One of the reasons for uncertainty is that, despite decades of intense work, there is still no physically coherent model. Using low- to medium-velocity impacts to measure electrical signals with microsecond time resolution, it has now been observed that when dry gabbro and diorite cores are impacted at relatively low velocities, 100 m/s, highly mobile charge carriers are generated in a small volume near the impact point. They spread through the rocks, causing electric potentials exceeding +400 mV, EM, and light emission. As the charge cloud spreads, the rock becomes momentarily conductive. When a dry granite block is impacted at higher velocity, 1.5 km/s, the propagation of the P and S waves is registered through the transient piezoelectric response of quartz. After the sound waves have passed, the surface of the granite block becomes positively charged, suggesting the same charge carriers as observed during the low-velocity impact experiments, expanding from within the bulk. During the next 2–3 ms the surface potential oscillates, indicating pulses of electrons injected from ground and contact electrodes. The observations are consistent with positive holes, e.g. defect electrons in the O²⁻ sublattice, traveling via the O 2p-dominated valence band of the silicate minerals. Before activation, the positive holes lay dormant in the form of electrically inactive positive hole pairs (PHP), chemically equivalent to peroxy links, O₃X/^OO\XO₃, with X=Si⁴⁺, Al³⁺, etc. PHPs are introduced into the minerals by way of hydroxyl, O₃X–OH, which all nominally anhydrous minerals incorporate when crystallizing in H₂O-laden environments. The fact that positive holes can be activated by low-energy impacts, and their attendant sound waves, suggests that they can also be activated by microfracturing. Depending on where in the stressed rock volume the charge carriers are activated, they will form rapidly moving or fluctuating charge clouds that may account for earthquake-related electrical signals and EM emission. Wherever such charge clouds intersect the surface, high fields are expected, causing electric discharges and earthquake lights.     

Anomalous Rayleigh-Wave Propagation Along Oceanic Trench

A clear later phase of amplitude larger than the direct surface wave packet was observed at stations in Hokkaido, Japan, for several events of the December 1991 off-Urup earthquake swarm in the Kuril Islands region. From its particle motion, this phase is likely to be a fundamental Rayleigh wave packet that arrived with an azimuth largely deviated from each great-circle direction. As its origin, Nakanishi (1992) proposed that the sea-trench topography in this area as deep as 10 km may produce a narrow zone of low velocity for Rayleigh waves of periods around 15 sec. Following this idea, we compute ray paths and estimate how Rayleigh waves would propagate if we assume that lateral velocity variations are caused only by seafloor topography. We confirm that thick sea water in the trench indeed produces the phase velocity of Rayleigh waves to be smaller than in a surrounding area by the degree over 100%. Such a low-velocity zone appears only in a period range from 12 to 20 sec. Although this strong low-velocity zone disturbs the direction of Rayleigh wave propagation from its great circle, the overall ray paths are not so affected as far as an epicentre is outside this low-velocity zone, that is, off the trench axis. In contrast, the majority of rays are severely distorted for an event within the low-velocity zone or, in other words, in the neighborhood of the trench axis. For such an event, a part of wave energy appears to be trapped in this zone and eventually propagates outwards due to the curvature or bend of trench geometry, resulting in very late arriving waves of large amplitude with an incident direction clearly different from great circles. This phenomenon is observed only at a very limited period range around 16 sec. These theoretical results are consistent with the above mentioned observation of Nakanishi (1992).     

Modified ray-mode (phase) theory: Understanding counter-wind propagation effects from atmospheric explosions

We have incorporated horizontal winds into ray-mode theory including the full spectrum of acoustic–gravity waves for a perfectly stratified, range-independent, steady-state model atmosphere for frequencies from 10^?4 to ～10 Hz. This approach has also been applied to a specific atmospheric propagation problem that has long defied a solution, namely counter-wind propagation arrivals at a location ～300 km up-wind of the source. Our modified ray-mode theory predicts reliable up-wind solutions, but only if small-scale sound speed fluctuations were added to the mean seasonal sound speed profiles. Since full-waveguide theory and modified ray-mode mode theory incorporate diffraction and scattering propagation effects, we have performed additional analyses to determine the mechanism through which these fluctuations produce the up-wind signals. We have concluded that the dominant mechanism is through diffraction due to the presence of semi-permanent turbulence and internal gravity waves located near the stratopause.     

Finite-Difference Modeling and Dispersion Analysis of High-Frequency Love Waves for Near-Surface Applications

Love-wave propagation has been a topic of interest to crustal, earthquake, and engineering seismologists for many years because it is independent of Poisson’s ratio and more sensitive to shear (S)-wave velocity changes and layer thickness changes than are Rayleigh waves. It is well known that Love-wave generation requires the existence of a low S-wave velocity layer in a multilayered earth model. In order to study numerically the propagation of Love waves in a layered earth model and dispersion characteristics for near-surface applications, we simulate high-frequency (>5 Hz) Love waves by the staggered-grid finite-difference (FD) method. The air–earth boundary (the shear stress above the free surface) is treated using the stress-imaging technique. We use a two-layer model to demonstrate the accuracy of the staggered-grid modeling scheme. We also simulate four-layer models including a low-velocity layer (LVL) or a high-velocity layer (HVL) to analyze dispersive energy characteristics for near-surface applications. Results demonstrate that: (1) the staggered-grid FD code and stress-imaging technique are suitable for treating the free-surface boundary conditions for Love-wave modeling, (2) Love-wave inversion should be treated with extra care when a LVL exists because of a lack of LVL information in dispersions aggravating uncertainties in the inversion procedure, and (3) energy of high modes in a low-frequency range is very weak, so that it is difficult to estimate the cutoff frequency accurately, and “mode-crossing” occurs between the second higher and third higher modes when a HVL exists.