GPS观测与数值模拟揭示的滇西北地区地壳形变特征及主要断层运动变化

滇西北地区地质构造复杂,强震孕育背景显著.本文利用滇西北地区不同期次(1999～2007期、2013～2018期)GPS观测数据解算地壳应变率场,分析地壳应变率场的动态演化特征.以不同期次GPS观测数据作为边界约束,利用二维有限元接触计算,模拟给出滇西北主要断裂带运动学特征与应力积累速率的变化;并结合汶川、芦山等历史强震和近场跨断层观测等资料,探讨滇西北地区地壳变形特征及其变形机理.结果表明:滇西北地区现今地壳变形特征表现为近东西向的拉张和近南北向的挤压,同时还叠加了南西—北东向的剪切变形,地壳应变率场在不同时段经历了动态调整,但最大主压应变方向在两个时段未发生改变,为NNW向.汶川地震前,滇西北地区主要断裂带的运动速率、运动性质与现有地质、形变结果较为一致;汶川、芦山等地震后,部分断裂带的运动性质与受力状态发生变化,本文分析认为与汶川等远场强震及断裂带近场中强震的发生有关,这些地震对滇西北地区局部应力场产生一定的影响,是部分断层运动性质发生变化的原因之一.     

最小二乘配置下的新疆地区地壳形变动态分析

为了研究新疆地区地壳形变动态变化特征,本文搜集整理了国内外发表的多期中国大陆GPS观测资料,获取1992～2016年间新疆地区的4期GPS实测速度场,运用最小二乘配置法对实测速度场进行插值,构建研究区地壳运动模型和应变率场。结果表明：实测速度场中,平均的GPS最小站间距由1992～2001年间的84.7 km减小到1992～2016年间的28.6 km,点位覆盖密度增大了约3倍;利用最小二乘配置法插值后的地壳运动模型与实测速度场具有很好的一致性,并且地壳运动模型与应变率场反映出1992～2016年间构造运动与变形较稳定,是对区域长期构造运动与变形的继承。研究区内的乌恰地区和伊塞克湖附近的面应变和最大剪应变变化最大,应变高值区逐渐自西向东扩张,反映出帕米尔高原对天山造山带的持续推挤还在增强;而塔里木盆地、准噶尔盆地和哈萨克斯坦块体的应变量基本不发生变化,说明3大块体内部基本不发生变形,反映了其刚性的特征;研究区最大剪应变率和地震发生率的相关性研究表明二者相关性显著,说明应变积累和地震活动有较好的对应关系。     

汶川、芦山地震前龙门山断裂带地壳形变特征对比分析

为研究2008年汶川地震和2013年芦山地震前地壳形变特征,本文利用1999—2015年四期GPS速度场和1990—2017年跨断层短水准资料,对跨断层GPS速度剖面、GPS应变率场、断层闭锁程度和滑动亏损、跨断层年均垂直变化速率等进行了分析讨论,总结了汶川和芦山地震前后龙门山断裂带三维地壳变形演化特征。结果表明,汶川地震前龙门山断裂带中、北段处于强闭锁状态、断层面应力应变积累水平很高,而龙门山断裂带西南段闭锁较弱、变形速率明显高于中北段、依然可以积累应力应变,汶川地震震源位于闭锁相对弱的部位,这可能是导致汶川地震自初始破裂点沿龙门山断裂带向北东方向单侧破裂,而震中西南方向断层并没有发生破裂的原因之一。汶川地震的发生引起龙门山断裂带西南段应力应变积累速率加快、断层闭锁程度增强、闭锁面积增大,这在一定程度上促进了芦山地震的发生,而芦山地震震源位于汶川地震前强闭锁和弱闭锁的高梯度过渡部位。因为芦山地震只释放了龙门山断裂带西南段有限的应变能,并没有显著缓解该段的地震危险性,所以汶川和芦山地震之间的地震空段以及芦山地震西南方向的地震空段,依然需要持续关注。此外,本文还收集和对比分析了多次6～9级地震前地壳变形特征,同样显示地震成核于闭锁高梯度带区域而非完全闭锁区域内部,并且随着震级升高闭锁断层面的长度也在增大,这一现象还需在高分辨率形变数据的帮助下进行深入研究和分析。     

GPS资料对青藏高原现今下地壳粘性特征的约束

假定GPS速度场在很大程度上受震间期地壳浅部发震层弹性应变积累的影响,文中以青藏地区现有GPS观测资料为约束,在较高精度地形数据和全球板块相对运动的NUVEL-1A模型基础上,借助数值模拟方法通过构建粘弹性动力学模型,分析了用下地壳介质流动和大陆深部断裂带内的延性应变集中等两种不同的大陆岩石圈深部变形机制对青藏地区现今...     

首都圈钻孔应变资料信息合成

一、引言由于全球的及区域的构造运动,或其它因素,地壳中一些部位可能受到较强的地应力作用,逐渐积累应变能,最终导致地壳岩层的变形、失稳、破坏而发生地震.搞好地震预报,一方面需研究不同地区地壳的应力、应变状态与介质性质;另一方面还需依据岩石破     

昆仑山与汶川强烈地震对青藏块体东北缘地壳运动及应变积累的影响

充分应用GPS、区域水准、跨断层等地形变观测资料, 借助非震负位错反演, 结合地震活动, 综合研究2001年11月14日昆仑山Ms8.1级地震对青藏块体东北缘地壳运动与应变积累的影响。通过分析笔者认为, 青藏块体内部Ms8.1级巨大地震能量释放产生的扰动应力场对NE向主压背景应力场的调制作用, 导致块体边界构造区域地壳差异运动和应力应变状态发生变化。其中, 对阿尔金断裂东段与祁连山断裂带西段的应变积累状态反映减缓为主的影响, 而对祁连山断裂带中东段则反映促进应变积累为主的影响。这种影响是一定时期、一定程度的。近期青藏块体东北缘区域应力场处于昆仑山、玉门、民乐等地震后的恢复状态。而由2008年5月12日汶川Ms8.0地震后有限的GPS、跨断层短水准观测资料, 可推测汶川地震对甘川交界东段-甘川陕交界区应变积累状况一定程度促进影响。     

青海玉树M_S7.1地震发震过程的数值模拟

根据玉树地区的地应力场、速度场和断层展布,对青海玉树2010年4月14日MS7.1级地震发震机理进行了数值模拟。将围岩看成弹性体,断层看成具有应变软化的弹塑性体,断层和围岩组成统一的地质介质系统。在地应力、孔隙压力及边界位移的作用下,应力逐渐积累,当达到断层摩擦破坏强度时,断层产生应变软化,断层突然滑动,能量突然释放,应力突然下降,形成地震。模拟结果表明:玉树7.1级地震是在印度板块向北推挤,青藏高原向东南侧向挤压,在玉树地区形成主压应力为北东80°方向的水平应力场,使甘孜-玉树断裂带产生左旋走滑错动形成的。计算结果给出了应力降、能量释放量、断层走滑错动量、地震复发周期、应力积累速度等重要参数,模拟结果与野外调查资料具有较好的一致性。     

GPS技术应用于中国地壳运动研究的方法及初步结果

文中主要就中国利用GPS等空间测地资料研究地壳运动、构造变形 ,以及用于地震预测探索方面 ,从方法技术和近年来取得的一些初步结果进行了概要性论述。介绍了利用GPS技术资料研究地壳水平运动速度场、水平应变场、建立地壳运动模型等方法研究的进展。由GPS观测给出的地壳水平运动初步结果表明 :中国大陆现时水平运动在全球参考系中为整体向东 ,并兼有顺时针扭转运动。西部地区构造形变强烈 ,整个青藏块体及其边界带 ,以及新疆西部是应变值最高的区域 ,水平应变场主压应变优势分布方向为近NE向 ,空间差异显著 ,反映了印度板块碰撞推挤和青藏块体强烈构造运动的影响。中国大陆东部水平运动的差异性不显著。强震分布于地壳运动的大小、方向显著变化的区域 ,大地震通常发生在水平剪应变高值区或其边缘 ,尤其是与区域主干断裂的构造活动背景相一致的剪应变率高值区。     

晋中盆地构造变形与地裂缝活动GPS监测分析

基于“数字地震网络工程”2007~2010年GPS监测数据,获取了晋中盆地现今地壳形变特征,并进一步利用椭球面块体应变模型研究了晋中盆地现今地壳应变场特征,据此分析了其与地质灾害发生的内在关系,揭示了盆地内地质灾害多发的成因.研究结果表明:晋中盆地现今地壳整体朝南东向运动,在盆地中部清徐—太谷区域表现出明显的拉张运动;晋中盆地现今地壳应变场整体以NW-SE向拉张为主,其中清徐—太谷、介休是应变高值区也是地震、地裂缝灾害多发区,最大主张应变量达1.8×10-6/a,且区域拉张应变方向与地裂缝呈近似垂直关系,进一步证实了晋中盆地地裂缝的强构造属性.     

基于GPS速度场的四川地区1999～2018年应变场特征演化分析

基于1999～2018年GPS水平运动速度场数据,解算并分析了四川“Y”形构造区各周期网格速度场、地壳应变率场,并讨论了近20年尺度的地壳应变场演化过程。研究表明：1）2008年汶川地震前1999～2007期GPS速度场相对稳定,整体“Y”型构造区地壳运动变化不大,但汶川地震后龙门山断裂带发生较大变化,由4.0 mm/a增至10.0 mm/a。2）1999～2007年,整个四川“Y”型构造区应变场演化特征微弱,而汶川地震之后的两个周期,最大剪应变自龙门山山前断裂向西到汶川一带,形成了由高到低、平行于龙门山断裂带走向的高密度梯度带。龙门山断裂带以ES或EES向的主压应变为主,其量值变化范围为 5.0×10^-8 /a～12.0×10^-8 /a;鲜水河断裂由震前主拉应变,改为震后近EW向的主压应变特征。面膨胀结果则显示龙门山断裂带由震前低密度梯度带瞬间变为平行于龙门山断裂带走向的高密度变化区。3）2008年汶川地震和2013年芦山地震是最为重要的时刻分割点。近20年的应变率场变化,更似一个“时间—地壳构造运动”的大轮回,目前四川“Y”型构造区整体处于2008年汶川地震前较为稳定的活动周期。龙门山断裂带仍值得我们做出更为深入的研究。     

Gravitational potential as a source of earthquake energy

Some degree of tectonic stress within the earth originates from gravity acting upon density structures. The work performed by this “gravitational tectonics stress” must have formerly existed as gravitational potential energy contained in the stress-causing density structure.According to the elastic rebound theory (Reid, 1910), the energy of earthquakes comes from an elastic strain field built up by fairly continuous elastic deformation in the period between events. For earthquakes resulting from gravitational tectonic stress, the elastic rebound theory requires the transfer of energy from the gravitational potential of the density structures into an elastic strain field prior to the event.An alternate theory involves partial gravitational collapse of the stress-causing density structures. The earthquake energy comes directly from a net decrease in gravitational potential energy. The gravitational potential energy released at the time of the earthquake is split between the energy released by the earthquake, including work done in the fault zone and an increase in stored elastic strain energy. The stress associated with this elastic strain field should oppose further fault slip.     

量子地震模型:对汶川地震的解释

从大量的地震参数分析可知,在由同一主震引发的所有的余震发生的位置都不同。在发生地震之前,地震的震中位置都是一些相互独立、互不连续且发生了弹性应变的单元个体,笔者称这些独立不连续的弹性应变单元称为应变量子。综合前人研究成果,笔者建立了一个地震模型。运用该模型对2008年5月12日发生在中国汶川地震作了解释:由于在龙门山断裂带周围形成了许多的应变量子,这些应变量子的形成阻碍了龙门山断裂的运动,产生滑移亏损,从而造成在地震前测量到的龙门山断裂带速度场变化很不显著。2008年5月12日14时28分时,在龙门山断裂周围已形成应变量子中的其中一应变量子最先达到它的最大储能,释放它所储存的应变能,引发了汶川Ms8.0地震。此次地震产生的地震波引发了龙门山断裂周围地壳应力的重分布,使得其他应变量子提前达到最大储能,释放出能量,于是触发成千上万次余震。此外,我们或许可以通过观测断裂滑移速率的变化情况来预测断裂周围应变量子的形成,从而来预测该断裂是否存在潜在地震的可能。     

A Characteristic Analysis of the Dynamic Evolution of Preseismic- Coseismic-Postseismic Interferometric Deformation Fields Associated with the M 7.9 Earthquake of Mani, Tibet in 1997

By using the D-InSAR technique,we have acquired the temporal-spatial evolution images of preseismic-cosesimci-postseismic interferometric deformation fields associated with the M 7.9 earthquake of Mani,Tibet on 8 November 1997.The analysis of these images reveals the relationships between the temporal-spatial evolution features of the interferometric deformation fields and locking, rupturing,and elastic restoring of the source rupture plane,which represent the processes of strain accumulation,strain release,and postseismic restoration.The result shows that 10 months prior to the Mani event,a left-lateral shear trend appeared in the seismic area,which was in accordance with the earthquake fault in nature.The quantity of local deformation on the north wall was slightly larger than that on the south wall,and the deformation distribution area of the north wall was relatively large.With the event impending,the deformation of the south wall varied increasingly,and the deformation center shifted eastward.Two and half monthd before the event,the west side of the fault was still locked while the east side began to slide,implying that the whole fault would rupture at any moment.These features can be regarded as short-term precursors to this earthquake.Within the period from 16 April 1996 to two and half months before the earthquake,the most remarkable deformation zones appeared in the north and south walls,which were parallel to and about 40 km apart from the fault,with accumulated local displacements of 344 mm and 251 mm on the north and south walls,respectively.The south wall was the active one with larger displacements.Five months after the earthquake,the distribution feature of interferometric fringes was just opposite to that prior to the event,expressing evident right-lateral shear.The recovered displacements are～179 mm on the north wall and～79 mm on the south wall,close to the east side of the fault.However,in the area of the south wall far from the fault there still existed a trend of sinistral motion.The deformation of the north wall was small but recovered fast in a larger area,while the active south wall began to recover from the east section of the fault toward the WSW.     

Heterogeneous constrictional deformation in a ductile shear zone resulting from the transposition of a lineation-parallel fold

We use new (micro-)structural, petrofabric, strain and vorticity data to analyze the deformation path in a mesoscopic quartz mylonite zone. The mylonite zone resulted from the complete transposition of a stretching lineation-parallel isoclinal fold. Symmetric cleft-girdle quartz c-axis fabrics were recorded in the middle domain, which occupies the inner limbs of the precursor isoclinal fold, while asymmetric cleft- and crossed-girdle fabrics were observed in the upper and lower domains that represent the outer limbs. Constrictional strain, with increasing k values towards the middle domain, is inferred from petrofabric and 3D strain data. Oblique grain shape fabrics yield vorticity estimates of 0.72–0.90 in the zone. However, in the middle domain, pure shear dominated deformation is suggested by orthorhombic crystallographic fabrics. Strain rate is constant throughout the zone; a strain decrease towards the zone center implies that deformation ceased earlier in the middle domain. The data indicates that fold transposition and subsequent mylonitization started as pure-shear-dominated constrictional deformation and progressively changed to simple-shear-dominated, plane strain. During this flow path the asymmetric quartz c-axis fabrics likely developed by depopulation of cleft-girdle maxima rather than from the synthetic rotation of fabric maxima itself.     

Relationships between InSAR Seismic Deformation and Fault Motion Sense, Fault Strike, and Ascending/Descending Modes

Based on the working principle of satellite radars, the earthquake deformation field measured by interferometric synthetic aperture (InSAR) is the projection of ground displacement associated with the seismogenic fault in the line of sight (LOS) of the satellite. However, LOS projections are complex, and are not only related to the ascending/descending modes and incidence angles of SAR data, but also related to the strike and motion senses of the fault. Even for the same earthquake, the LOS deformation derived from different ascending/descending data can be almost identical in one case, but quite different in another case, which makes the interpretation of InSAR seismic deformation and its comparison with field observations difficult. In this study, we undertook a quantitative analysis of the relationships between LOS observation sensitivity of InSAR and fault strike, fault motion sense, and ascending/descending modes, as well as 3D deformation fields. We studied the features and differences of the LOS deformation fields in different types of earthquakes using ascending/descending modes, with a particularly detailed analysis of the relations for a strike-slip type of earthquake. We also summarized the characteristics of LOS deformation fields of faults with different strikes and optimal observational data modes. Taking the strike-slip Yushu earthquake and the normal Gaize event as examples, we used SAR data of the ascending/descending modes to verify the results of quantitative calculations. These analyses will not only provide a more reasonable interpretation of InSAR seismic deformation fields and but also help understand the differences of seismic deformation fields revealed by data with different observational modes, therefore promoting the application of InSAR technology in seismology.     

Co-seismic displacements associated to the Molise (Southern Italy) earthquake sequence of October–November 2002 inferred from GPS measurements

The 2002 earthquake sequence of October 31 and November 1 (main shocks Mw = 5.7) struck an area of the Molise region in Southern Italy. In this paper we analyzed the co-seismic deformation related to the Molise seismic sequence, inferred from GPS data collected before and after the earthquake, that ruptured a rather deep portion of crust releasing a moderate amount of seismic energy with no surface rupture. The GPS data have been reduced using two different processing strategies and softwares (Bernese and GIPSY) to have an increased control over the result accuracy, since the expected surface displacements induced by the Molise earthquake are in the order of the GPS reliability. The surface deformations obtained from the two approaches are statistically equivalent and show a displacement field consistent with the expected deformation mechanism and with no rupture at the surface. In order to relate this observation with the seismic source, an elastic modeling of fault dislocation rupture has been performed using seismological parameters as constraints to the model input and comparing calculated surface displacements with the observed ones. The sum of the seismic moments (8.9 × 10¹⁷ Nm) of the two main events have been used as a constraint for the size and amount of slip on the model fault while its geometry has been constrained using the focal mechanisms and aftershocks locations. Since the two main shocks exhibit the same fault parameters (strike of the plane, dip and co-seismic slip), we modelled a single square fault, size of 15 km × 15 km, assumed to accommodate the whole rupture of both events of the seismic sequence. A vertical E–W trending fault (strike = 266°) has been modeled, with a horizontal slip of 120 mm. Sensitivity tests have been performed to infer the slip distribution at depth. The comparison between GPS observations and displacement vectors predicted by the dislocation model is consistent with a source fault placed between 5 and 20 km of depth with a constant pure right-lateral strike-slip in agreement with fault slip distribution analyses using seismological information. The GPS strain field obtained doesn't require a geodetic moment release larger than the one inferred from the seismological information ruling out significant post-seismic deformation or geodetic deformation released at frequencies not detectable by seismic instruments. The Molise sequence has a critical seismotectonic significance because it occurred in an area where no historical seismicity or seismogenic faults are reported. The focal location of the sequence and the strike-slip kinematics of main shocks allow to distinguish it from the shallower and extensional seismicity of the southern Apennines being more likely related to the decoupling of the southern Adriatic block from the northern one.     

Co-seismic gravity changes in the Koyna-Warna region: Implications of mass redistribution

Koyna-Warna Region (KWR) is one of the known sites for reservoir triggered seismicity. The continued triggered seismicity over the five decades is restricted to a region of about 600–700 sq. km, which provides a unique opportunity to monitor geophysical anomalies likely to be associated with seismicity of the region. Present study confers temporal gravity changes recorded by gPhone and GRACE satellite and interprets observed changes in conjunction with seismological, geodetic (cGPS) observations and groundwater level measurements. GRACE data suggest that seasonal vertical deformation due to hydrological loading is ～ 2 cm, which corroborates with continuous GPS observations. Seasonal hydrological loading of the region, which is in a phase of reservoir loading, might be influencing the critically stressed KWR leading to the seasonal seismicity of the region. The gPhone gravity data distinctly show co-seismic gravity signals for eight earthquakes of M_w > 2 and gravity anomalies show positive correlation on a logarithmic scale with earthquake released energy. To investigate the cause of gravity changes, an estimate is made for 14^th April 2012 earthquake for M_w 4.8 using fault dislocation model. The recorded gravity changes of 189 μGal by gPhone located at a distance of 28 km from the hypocentre is much more than the estimate of ～0.1 μGal calculated for M_w 4.8 Koyna earthquake. Therefore, it is inferred that co-seismic gravity signals for eight earthquakes are primarily caused due to redistribution of mass at shallow depth.     

Plate subduction,and generation of earthquakes and magmas in Japan as inferred from seismic observations: An overview

A dense nationwide seismic network recently constructed in Japan has been yielding large volumes of high-quality data that have made it possible to investigate the seismic structure in the Japanese subduction zone with unprecedented resolution. In this article, recent studies on the subduction of the Philippine Sea and Pacific plates beneath the Japanese Islands and the mechanism of earthquake and magma generation associated with plate subduction are reviewed. Seismic tomographic studies have shown that the Philippine Sea plate subducting beneath southwest Japan is continuous throughout the entire region, from Kanto to Kyushu, without disruption or splitting even beneath the Izu Peninsula as suggested in the past. The contact of the Philippine Sea plate with the Pacific plate subducting below has been found to cause anomalously deep interplate and intraslab earthquake activity in Kanto. Detailed waveform inversion studies have revealed that the asperity model is applicable to interplate earthquakes. Analyses of dense seismic and GPS network data have confirmed the existence of episodic slow slip accompanied in many instances by low-frequency tremors/earthquakes on the plate interface, which are inferred to play an important role in stress loading at asperities. High-resolution studies of the spatial variation of intraslab seismicity and the seismic velocity structure of the slab crust strongly support the dehydration embrittlement hypothesis for the generation of intraslab earthquakes. Seismic tomography studies have shown that water released by dehydration of the slab and secondary convection in the mantle wedge, mechanically induced by slab subduction, are responsible for magma generation in the Japanese islands. Water of slab origin is also inferred to be responsible for large anelastic local deformation of the arc crust leading to inland crustal earthquakes that return the arc crust to a state of spatially uniform deformation.     

Co and postseismic characteristics of Indian sub-continent in response to the 2004 Sumatra earthquake

The M_w 9.3 Sumatra earthquake of December 26, 2004 caused extensive coseismic displacements globally, measurements of which were made essentially using modern geodetic techniques. This earthquake induced considerable perturbation in stress distribution as far as ∼8000 km away from the epicenteral region, which is tending to relax to its normal rates as seen from postseismic transient deformation. The monitoring of crustal displacements from strategically located sites using GPS provides coseismic as well as postseismic deformation that facilitates the understanding of the fault geometry, elastic thickness, postseismic relaxation mechanisms, rheology and earthquake recurrence time interval.We investigated coseismic and postseismic GPS derived displacements in Indian region together with the GPS data collected from Andaman and Sumatra region. It is found that while EW displacements are significantly large in peninsular India, those in the region to the north of Central India Tectonic Zone (CITZ) are relatively small. We could delineate the postseismic transients from position time series and interpreted them in terms of viscoelastic relaxation. It is inferred that the postseismic deformation is characterized by a power-law viscoelastic flow in the mantle. In Indian peninsula region, the timescale parameter of the exponential decay (τ = 250 days) would require an extremely low viscosity for the upper mantle. Relying on the prevailing coseismic and postseismic displacement fields, the present study also reflects upon the contemporary litho-tectonics of the Indian sub-continent.     

深圳横岗-罗湖断裂中南段活动特征及现今构造变形监测

横岗-罗湖断裂是深圳断裂带的主干断裂,控制着深圳地区的地震构造环境。采用多种手段、方法对该断裂第四纪以来的活动性及现今构造变形进行研究十分重要。作者通过野外地质地貌调查、组合探槽开挖、断层带物质年龄测试(TL、ESR)等手段,分析了横岗-罗湖中南段第四纪活动期次、活动时代。初步认为,横岗-罗湖断裂在第四纪至少经历了4期构造活动,其最新活动时代在第四纪中更新世晚期,约0.14~0.31 Ma。为了定量监测该断裂现今构造变形量及变形速率,对断裂进行了动态形变监测、GPS形变监测、地应力及应变监测。监测数据表明,断裂现今变形速率在0.05~0.5 mm/a之间,属于中等偏下活动水平。断层活动方式以右旋正断与左旋逆冲交替进行,逆冲活动幅度较大,左旋活动略具优势,表明断裂现今变形具有螺旋式渐进的特征。同时,在监测时间段内,2010年11月19日深圳发生了MS2.8地震。通过分析应变仪和砂土应力仪记录的数据,发现在深圳MS2.8地震前后存在明显异常,这可能为地震前兆监测提供有效方法。