鲜水河—安宁河断裂带深部构造特征、固体潮应力变化与地震触发相关性研究

本研究通过反演241,561条纵波和209,363条横波高质量的地震走时数据,获得了鲜水河、安宁河断裂带的P波、S波以及泊松比的三维结构模型,并结合相位分析与Schuster测试讨论了该断裂带上地震事件的触发与其深部构造以及固体潮所产生的应力变化三者之间的关系.结果表明,鲜水河—安宁河断裂带26.5°N、28°N、29.5°N以及31.5°N四个位置附近存在低速、高泊松比异常,可能暗示着深部流体(包括部分熔融物质)的上涌.这些大范围分布的流体提高了孕震层的孔隙流体压力,削弱了断层面之间摩擦力,在鲜水河—安宁河断裂带形成了一些具有高度地震活动性的区域.此外,相位分析与Schuster测试的结果表明,鲜水河—安宁河断裂带附近的地震触发与固体潮剪切应力的变化密切相关.综合分析的结果表明,固体潮与构造应力对地震触发可能存在某种“合”的关系.在分布着大量流体的区域,地震事件可能对固体潮更加敏感.     

地壳流体对地震活动的影响与控制作用

地壳中普遍存在着流体,不仅在地壳动力过程中发挥重要作用,而且也极有可能在地震孕育与发生过程中起着不为人知的重要作用。这样的认识,越来越得到地质学家与地球物理学者们的认可。本文概括介绍了地壳流体对地震活动的空间分布、震源深度与强震活动周期的影响与控制作用,并提出了地壳硬夹层孕震与流体促震的假设。最后还介绍了通过流体控制地震活动并实现减轻地震灾害的思想。     

2008年MS6.1攀枝花地震震源区多参数结构成像及发震机制研究

通过联合反演123,053个P、Pn、Pg震相和100,176个S、Sn、Sg震相数据,获得了2008年M_S6.1攀枝花地震震源及其周边区域的高分辨率三维纵、横波速度(V_P,V_S)和泊松比(σ)图像.结合研究区域地壳应力数据综合分析发现,攀枝花地震发生在高-低纵、横波速度转换带,并且在震源下方存在一显著的低V_S和高σ异常体延伸至下地壳.本研究认为,该构造特征主要是由于西侧坚硬的川滇菱形块体对来自深部流体或熔融物质具有一定的阻挡作用,绝大部分流体或熔融物质通过断裂带向东南侧的块体内部迁移,造成断裂带两侧块体的岩石物理属性差异较大所致.研究结果表明,攀枝花地震发生在剪切应力较强的元谋—绿汁江断裂带上,震源下方的流体或部分熔融物质被挤入至震源的断层或裂隙中,增加了震源区岩石的流体应力,降低了横波速度(V_S)、增加了岩石的泊松比(σ).我们推测,流体侵入在攀枝花地震形成上扮演了重要角色,来自于青藏高原下地壳的大量的流体或部分熔融物质被挤入震源区岩体的断层或裂缝中,这一过程增加了震源区的孔隙流体压力、减弱岩石的机械强度,同时岩石的静摩擦力增加,导致容易引起岩体脆性形变,从而诱发地震.     

Influence of fluids on deep crustal Jabalpur earthquake of 21, May 1997: Geophysical evidences

We present the geophysical evidences on the role of fluids for generation of the lower crustal Jabalpur earthquake (21 May 1997, mb 6.0, Mw 5.8), in the mid-continental fracture zone of the Indian Peninsular Shield. With a focal depth of 35 km, it indicates a high angled (< 62^∘ enclosed with maximum principal stress direction) reverse fault with small component of left-lateral strike slip in the lower crust. The Son-Narmada-Tapti (SONATA) magalineament, during the past two centuries, has experienced about 25 moderate to strong earthquakes; two of which namely the Son Valley (1927, M 6.5) and Jabalpur (21 May 1997) were disastrous. Historical earthquakes and recent earthquake swarms indicate a moderate to high seismicity in SONATA belt that is due to high strain accumulation, flexuring of the crust and neotectonic movements of the faults in the rift zones. By analyzing geophysical parameters such as Zero-Free air-based (ZFb) gravity anomalies (∼ −10 to –30 mGals), heat flow values (45–47 mWm⁻²), magneto-telluric values (1- Ohm m), strain rate (1.5 × 10⁻⁸) and failure stress conditions, we identify plausible causative factors for the occurrence of lower crustal earthquake in this region Fluids, due to dehydration of serpentinite in the lower crust, are suggested to be present in the earthquake source zone. The estimated pore-fluid factor for the Jabalpur earthquake (λ _v ) is 0.95. The diffusion of pore-pressure relaxation, represented as pressure perturbation generated by coseismic stress change was seen in the form of swarm activity two years prior to the Jabalpur earthquake. We suggest the existence of a deep pre-fractured zone with low shear stress (τ = 15–18 MPa) that indicates the presence of fluid filled fractured mafic material in the felsic crust, in critical state of unstable failure condition, and also fluid driven migration of swarm activity before the Jabalpur earthquake.     

龙门山断裂带多参数深部结构成像与地震成因研究

通过反演大量的纵、横波地震数据,获得了沿龙门山断裂带及周边区域的深部三维精细结构,结合前人二维大地电磁探测研究成果,提出龙门山断裂带地壳形变与深部速度结构和导电率不均匀性有关,探讨了2008年汶川和2013年芦山地震的诱发和产生与流体侵入及地壳形变的密切关系.本研究发现,2008年汶川地震发生在高速度、高泊松比和低电导率的区域,2013年芦山地震则位于高速度、低泊松比和低电导率的发震层.在上地壳中,四川前陆盆地的低速、低泊松比和低阻异常与松潘一甘孜地块的高速、高泊松比和高阻异常形成了鲜明的对比.在龙门山断裂带下方的两个低速和低阻块体,将龙门山断裂带分成南、中和北三段.我们的研究认为,这两个异常体与来自松潘甘孜地块的下地壳和(或)上地幔的局部熔融或流体侵入到龙门山断裂带的脆弱区有关.基于对汶川和芦山地震的余震分布特征及震源区的地震波速度、泊松比及电阻率参数分析,揭示了龙门山断裂带深部剧烈的地壳形变与流体应力积累对2008年汶川和2013年芦山地震的触发及其地震破裂过程具有重要的控制作用.     

The relationship between the deep-level structure in crust and brewing of strong earthquakes in Xingtai area

IntroductionWiththefurtherstudyonthefinestructureofcrustandapplicationoftheCTtechnologytotheresearchonthecontinentaldeepstructureinNorthChina,thenon-heterogeneityofcrustanduppermantleintheseareashasbeenclearlyacknowledged.Zeng,etal(1991)gavethefocalmodeloflargeearthquakeinNorthChina,inwhichthemostimportanttraitisthetransversenonuniformoftemperaturedistributiononthetopofthemantle.Liu,etal(1986)studiedtheseismictomographyofNorthChinaregion,andindicatedthattherearemanydifferentlowervelocityare…     

邢台地震前地壳形变异常的可能性物理机制

将地壳介质视为麦克斯威尔体 ,运用差分法和三维有限元方法 ,探讨了邢台分层地壳结构模型 （含高速体和低速体及深大断裂 ）中深部断裂加速蠕滑时 ,平均应力、水平最大剪应力和地表面垂直位移随时间演化的特征 ,计算结果表明 :（1 ）在地壳中上部 1 1km处 ,深部断裂的加速蠕滑急剧加速了水平最大剪应力的增加速率 ,可达数百倍 ,深部断裂的加速蠕滑是邢台强地震成核过程的开始 ,可实现地壳下部的能量向地壳中上部快速转移 ;（2 ）深部断裂的加速蠕滑引起的地面垂直位移变化与邢台地震震前的地表面垂直位移变化非常一致 .说明邢台地震震前地壳表面垂直方向的位移不仅与岩石的膨胀有关 ,而且可能与地壳内深部断裂的加速蠕滑密切相关 .     

南北地震带地壳结构多参数成像及强震触发机制研究

通过反演由大量的纵、横波地震数据组成的综合数据集,获得了南北地震带地壳的多参数三维精细结构,探讨和分析了南北地震带的高地震活动性和强震频发的原因.成像结果表明,尽管1976年松潘一平武地震(M7.2)与2008年汶川地震(M8.0)以及2013年芦山地震(M7.0)均发生在高速、低泊松比异常区域,并且在其震源的下方均有一低速、高泊松比异常区域.我们认为,上述三个地震的触发与流体侵入导致的地壳形变之间有密切的联系.1955年炉霍地震(M7.4)和1973年康定地震(M7.1)均发生在鲜水河断裂带上,其震源中心区域表现为低速、高泊松比异常,可以解释为下地壳中的流体沿断层面上涌.在震源区的周边区域兼有高速、高泊松比异常,低速、高泊松比异常以及高速、低泊松比异常,可能分别与含流体的岩石、沿断裂带发育的变质岩以及坚硬的克拉通块体对应.流体的侵入不仅能够改变断层面上的应力情况,还能降低岩石骨架的岩石力学强度,进而触发地震.1970年云南通海大地震(M7.1)发生在哀牢山一红河断裂带附近的曲江断裂上,其震源处于高速度、低泊松比异常与低速度、高泊松比异常之间的边界区域,被认为是流体挤压后的应变能积累,最终导致脆性破裂,以至于发生地震.根据本次研究获得的多参数结构图像,结合前人的研究成果,我们认为南北地震带地壳强烈形变与流体侵入是造成该区域地震活动性较高及强震频发的两个主要因素.     

Connection between earthquakes and deep fluids revealed by magnetotelluric imaging in Songyuan,China

Songyuan is the most earthquake prone area in northeast China.Since 2006,earthquakes have occurred in the area in the form of swarms,with a maximum magnitude of M_L5.8.There is much controversy about the cause of the Songyuan earthquakes.We attempted to determine the cause using a three-dimensional electrical conductivity structure inverted from a regional network of magnetotelluric data in the Songyuan area.The L-BFGS inversion method was applied,with a fullimpedance tensor data set used as the inversion input.Combined with an evaluation of the earthquake locations,the resistivity model revealed a northeast-oriented hidden fault running through the Songyuan earthquake area(SEA),which was speculated to be the preexisting Fuyu-Zhaodong Fault(FZF).Our resistivity model also found an apparent lithospheric low-resistivity anomaly beneath the earthquake area,which breached the high-resistivity lithospheric mantle and stalled at the base of the crust.A petrophysical analysis showed that this lower crustal low-resistivity anomaly was most likely attributed to hydrated partial melting,which could release water into the lower crust during later magma emplacements.While weakening the strength of the FZF,these ascending fluids also increased the pore pressure in the fault,further reducing the shear strength of the fault.Shear stress action(a fault strike component of the east-west regional compress),together with possible near-surface disturbances,may drive the fault to slip and trigger the earthquakes in Songyuan.It is possible that the continuous replenishment of fluids from the deeper mantle forces the Songyuan earthquakes into the form of swarms.We infer that the Songyuan earthquakes could be attributed to a combination of preexisting faults,regional stress,and deep fluids associated with plate subduction,and near surface disturbances might induce the earthquakes in advance.The Songyuan earthquakes are inherently induced earthquakes,fed by deep fluids.     

区域应力作用引起的北天山地区地下流体异常特征分析

大量观测事实表明 ,赋存于地壳深部的流体 (水、气、油 )作为弹性介质 ,对深层地壳应力的分布、变化、转移过程及地震孕育、发生等有着较为灵敏的反映。地震的形成与区域构造应力活动强度密切相关 ,这种大范围的区域应力作用会形成多个应力集中点 ,在众多应力集中点上可产生地下流体前兆异常变化。在对新疆地下流体数十年的监测中 ,发现强震前出现大量与地震有关的由区域构造应力活动引起的前兆异常 ,异常范围广且种类繁多 ,它们在成因、特征上与震源应力活动的前兆异常明显不同 ;这种地下水位地震前兆异常场 ,不能完全孤立地与单个地震事件相对应 ,应充分考虑到它与一组或系列地震有关 ,而这些地震往往与块体挤压、断裂活动等区域性的应力活动有关。因此 ,正确区分并掌握其不同的特性 ,对地震预报具有一定的意义     

Fluid regime in the source and preparation area of an earthquake

Peculiarities of the fluid regime in the source and in the area of preparation of an earthquake which accommodates the evolution of the precursors are considered. The qualitative characteristics and the conditions of migration of the fluids, as well as their influence on the disjunctive deformations of the crust are discussed. During the development of the earthquake source, the fluid regime in this area depends on the redistribution of the fluids and their inflow from the outside. The fluid inflow and the increase in the fluid pressure are mainly due to the filtering from the subvertical permeable faults and the metamorphic dehydration of the rocks in the walls of the main fault. The fluids may also be supplied to the source from the near-surface horizons, and most of the induced earthquakes are associated with this process. These earthquakes can be considered as large-scale natural experiments which can shed light on the contribution of fluids in this phenomenon. A particular role in the mechanism of natural seismicity is played by the inflow of high-pressure fluids through the faults from the deep zones of the section.     

从断裂力学观点研究地震的破裂过程和地震预报

把断裂力学中的应变能释放率公式和裂纹错开位移公式运用到地震破裂中来,再用震级-能量公式logε_e=α₁M+α₂,对于走向滑动、倾向滑动和圆盘形剪切破裂找到了震源参数和地壳应力状态之间的内在联系,汇总在表2。对于走滑断层情形,关系式如下: （1） （2） （3） （4）应力降 （5）平均应力 （6）用M_o和L²W求区域应力公式式中,M为震级,L、W为断层长度和宽度,η为地震效率,τ₀为区域剪切应力,τ_y为剪切屈服强度,μ为刚性系数,v为泊松比,为平均位错,M₀为地震矩。 利用（6）式或（1）式,在实验室测出地壳状态下屈服强度τ_y,后,可用地震观测资料算出区域剪切应力τ₀。 上述关系式和目前流行的震源参数之间的关系式有很大的差别。原因是以往只考虑了破裂的初态和终态,没有考虑破裂过程。而断裂力学恰恰是考虑了破裂过程。     

Fluid involvement in normal faulting

Evidence of fluid interaction with normal faults comes from their varied role as flow barriers or conduits in hydrocarbon basins and as hosting structures for hydrothermal mineralisation, and from fault-rock assemblages in exhumed footwalls of steep active normal faults and metamorphic core complexes. These last suggest involvement of predominantly aqueous fluids over a broad depth range, with implications for fault shear resistance and the mechanics of normal fault reactivation. A general downwards progression in fault rock assemblages (high-level breccia-gouge (often clay-rich) → cataclasites → phyllonites → mylonite → mylonitic gneiss with the onset of greenschist phyllonites occurring near the base of the seismogenic crust) is inferred for normal fault zones developed in quartzo-feldspathic continental crust. Fluid inclusion studies in hydrothermal veining from some footwall assemblages suggest a transition from hydrostatic to suprahydrostatic fluid pressures over the depth range 3–5 km, with some evidence for near-lithostatic to hydrostatic pressure cycling towards the base of the seismogenic zone in the phyllonitic assemblages. Development of fault-fracture meshes through mixed-mode brittle failure in rock-masses with strong competence layering is promoted by low effective stress in the absence of thoroughgoing cohesionless faults that are favourably oriented for reactivation. Meshes may develop around normal faults in the near-surface under hydrostatic fluid pressures to depths determined by rock tensile strength, and at greater depths in overpressured portions of normal fault zones and at stress heterogeneities, especially dilational jogs. Overpressures localised within developing normal fault zones also determine the extent to which they may reutilise existing discontinuities (for example, low-angle thrust faults). Brittle failure mode plots demonstrate that reactivation of existing low-angle faults under vertical σ₁ trajectories is only likely if fluid overpressures are localised within the fault zone and the surrounding rock retains significant tensile strength. Migrating pore fluids interact both statically and dynamically with normal faults. Static effects include consideration of the relative permeability of the faults with respect to the country rock, and juxtaposition effects which determine whether a fault is transmissive to flow or acts as an impermeable barrier. Strong directional permeability is expected in the subhorizontal σ₂ direction parallel to intersections between minor faults, extension fractures, and stylolites. Three dynamic mechanisms tied to the seismic stress cycle may contribute to fluid redistribution: (i) cycling of mean stress coupled to shear stress, sometimes leading to postfailure expulsion of fluid from vertical fractures; (ii) suction pump action at dilational fault jogs; and, (iii) fault-valve action when a normal fault transects a seal capping either uniformly overpressured crust or overpressures localised to the immediate vicinity of the fault zone at depth. The combination of σ₂ directional permeability with fluid redistribution from mean stress cycling may lead to hydraulic communication along strike, contributing to the protracted earthquake sequences that characterise normal fault systems.     

短期线状集中分布地磁日变化感应电流异常机理初步解释——以2012年唐山4.8级地震为例

本文分析了2012年唐山4.8级地震前,震中附近出现的短期原地重现线状集中分布地磁日变化感应电流异常的时空变化特征,及其与地震、中-下地壳和上地幔高导层的关系,进一步证实了短期原地重现线状集中分布感应电流的走向与中-下地壳和上地幔高导层顶面界埋深走向一致,认为其机理可能是深部热流体的上涌导致壳幔高阻体出现带有上拱性质的拆离滑动,深部上涌的热流体和高导层内热流体侵入高导层内电阻相对较高的地区,高导层出现短时间高导电流通道,当地磁日变化感应电流扫描经过高导电流通道时,感应电流会呈线状集中分布于此,并基于趋肤效应分布于其顶面附近。由于重现异常是发生在震源下方中-下地壳和上地幔高导层的地震异常,且该异常不同于震源附近及其震源至地表的地震异常,因此对推进地震孕育与发生机理研究可能有一定作用。此外研究还发现,地震虽然主要位于重叠段的端部,但更有可能位于中-下地壳重叠段的端部,这一发现对日常震情跟踪中应用该异常确定未来地震位置有一定帮助。     

Nitrogen in fluids effecting retrogression of granulite facies gneisses: a debatable mantle connection

The abundance patterns of nitrogen, and chlorine in retrogressed granulite facies gneisses from southern East Greenland exhibit strong enrichment in the vicinity of small-scale shear zones. Sulfur in the shear zones occurs at the same concentration levels as in the adjacent country rock, but is depleted in the transition zone between shear zone and country rock. Within the shear zone sulfur occurs as sulfate, whereas in the country rock granulites it occurs as sulfide. Recrystallization of rock in the shear zone to scapolite-bearing, hornblende-absent assemblages, along with changes in the major element chemistry, demonstrates that these zones define migration pathways of chemically reactive fluids. Consideration of the computed fluid compositions, and of the mass ratios of chlorine/sulfur and nitrogen/sulfur demonstrate that the fluid equilibrated with continental crust prior to its passage through gneisses in the study area. Previous suggestions have been made that the mantle may act as a source region for nitrogen-rich fluids. However, equilibration of these S-, N- and Cl-rich fluids with crustal material precludes the use of element abundances to identify a mantle signature; the bulk of these fluid constituents must be considered crustal derived.     

Role of crustal fluids in triggering the West Bohemia/Vogtland earthquake swarms: Just what we know (a review)

We summarise the results of seismological studies related to triggering mechanisms, driving forces and source processes of the West Bohemia/Vogtland earthquake swarms with the aim to disclose the role of crustal fluids in the preparation, triggering and governing of the swarms. We present basic characteristics distinguishing earthquake swarms from tectonic mainshock-aftershock sequences and introduce existing earthquakes swarm models. From the statistical characteristics and time-space distribution of the foci we infer that self-organization is a peculiarity of West Bohemia/Vogtland swarms. We discuss possible causes of the foci migration in these swarms from the viewpoint of co-seismic and/or post-seismic stress changes and diffusion of the pressurized fluids, and we summarize hitherto published models of triggering the 2000-swarm. Attention is paid to the source mechanisms, particularly to their non-shear components. We consider possible causes of different source mechanisms of the 1997-and 2000-swarms and infer that pure shear processes controlled solely by the regional tectonic stress prevail in them, and that additional tensile forces may appear only at unfavourably oriented faults. On data from the fluid injection experiment at the HDR site Soultz (Alsace), we also show that earthquakes triggered by fluids can represent purely shear processes. Thus we conclude that increased pore pressure of crustal fluids in the region plays a key role in bringing the faults from the subcritical to critical state. The swarm activities are mainly driven by stress changes due to co-seismic and post-seismic slips, which considerably depend on the frictional conditions at the fault; crustal fluids keep the fault in a critical state. An open question still remains the cause of the repeatedly observed almost simultaneous occurrence of seismic activity in different focal zones in a wider area of West Bohemia/Vogtland. The analysis of the space-time relations of seismicity in the area between 1991 and 2007 revealed that during a significant part of this time span the seismicity was switching among distant focal zones. This indicates a common triggering force which might be the effect of an increase of crustal-fluid pore-pressure affecting a wider epicentral region.     

中地壳的水和水岩相互作用实验及其地球物理涵义

本文重点报道了高温高压下流体与流体-岩石相互作用实验结果,提供了中地壳条件下流体性质和水岩反应速率数据.这些数据有助于理解中地壳的一些地球物理现象.作者进行了25℃~435℃和22~39 MPa条件下水-岩相互作用反应动力学实验.同时,研究水在近临界区至超临界区的性质.一般地说,中地壳大致位于10(15)至25 km的深度范围.各地的地壳厚度不同,但是中地壳高导-低速层的深度范围十分相似.中地壳的顶界温度处于300℃,底界大致为450℃范围,压力高达200 MPa以上.流体-岩石相互作用实验表明:硅酸盐矿物和岩石的硅最大溶解速率出现在300℃~400℃.此时,硅酸盐矿物格架解体.通常,地壳里普遍存在水、流体.地壳构造活动导致断裂空隙、减压、流体流动.这时,有可能导致中地壳处于300℃~450℃流体的压力减低,由超临界区进入临界态、亚临界态.这会引发强烈流动的水与岩石相互作用.溶解反应导致岩层的硅淋失,硅的强烈淋失又会导致硅酸盐矿物格架解体,岩石崩塌.同时,进一步促进流体的流动.实验表明300℃~400℃下的强烈水岩相互作用促进了岩石破坏,并有可能影响岩层的地球物理性质,如高导层出现.另外,实验和理论研究表明处于300℃~400℃流体具有高电导率性质.这些水岩相互作用会使中地壳出现高导-低速层.     

深部流体在地震孕育和发生过程中的作用

提出了几个难以用断层弹性回跳模型解释的地震实例，指出必须重视地壳上地幔结构的探测和深部流体的研究。然后用简单型说明，没有流体在存在，则不可能发生岩体间快速的错动。重点探讨发生在地壳内10-25km深处的强震孕育和发生机理，分析了深部流体的迁移和岩体的错动。最后结合实例，给出了在发生深部流体迁移的条件下，一次强震释放的能量和岩体错动的最大加速度。     

Magma Intrusions and Earthquake Swarm Occurrence in the Western Part of the Bohemian Massif

We are proposing a hypothesis that earthquake swarms in the West Bohemia/Vogtland seismoactive region are generated by magmatic activity currently transported to the upper crustal layers. We assume that the injection of magma and/or related fluids and gases causes hydraulic fracturing which is manifested as an earthquake swarm at the surface. Our statements are supported by three spheres of evidence coming from the western part of the Bohemian Massif: characteristic manifestations of recent geodynamic activity, the information from the neighbouring KTB deep drilling project and from the 9HR seismic reflection profile, and the detailed analysis of local seismological data. (1) Recent manifestations of geodynamic activity include Quaternary volcanism, rich CO ₂ emissions, anomalies of mantle-derived ³ He, mineral springs, moffets, etc. (2) The fluid injection experiment in the neighbouring KTB deep borehole at a depth of 9 km induced hundreds of micro-earthquakes. This indicates that the Earth's crust is near frictional failure in the western part of the Bohemian Massif and an addition of a small amount of energy to the tectonic stress is enough to induce an earthquake. Some pronounced reflections in the closely passing 9HR seismic reflection profile are interpreted as being caused by recent magmatic sills in the crust. (3) The local broadband seismological network WEBNET provides high quality data that enable precise localization of seismic events. The events of the January 1997 earthquake swarm are confined to an extremely narrow volume at depths of about 9 km. Their seismograms display pronounced reflections of P- and S-waves in the upper crust. The analysis of the process of faulting has disclosed a considerable variability of the source mechanism during the swarm. We conclude that the mechanism of intraplate earthquake swarms generated by magma intrusions is similar to that of induced seismicity. As the recent tectonic processes and manifestations of geodynamic activity are similar in European areas with repeated earthquake swarm occurrence (Bohemian Massif, French Massif Central, Rhine Graben), we assume that magma intrusions and related fluid and gas release at depths of about 10 km are the universal cause of intraplate earthquake swarm generation     

强地震前兆异常特征与深部流体作用探讨

根据华北几次强震前后地下流体和重力异常特征,结合大地测深资料探讨了地震发生前后深部介质的变化和地壳浅层应力场变化特征。地震发生前震中区地下水位异常主要表现为下降型,气体异常呈上升型,重力和形变也出现异常;在外围地区地震前地下水位异常以上升型为主。 海城地震和大同–阳高地震前地下水位上升、下降异常有方向性分布的趋势,呈 X 形分布,这意味着地壳浅层应力场是压扭型的,并存在垂向应力的作用。在孕震过程中,地球深部流体向上运移,使得震源区及其附近的岩石膨胀、密度降低,震中区地壳浅层应力场呈拉张型,造成地下水储容积增大,地下水位出现了下降型异常。 在特定的构造单元内,地下流体、重力异常的空间分布特征能较好地反映地球深部物质运动和地壳浅层应力场特征。