3D upper crustal density structure of the Deccan Syneclise,Central India

A constrained 3D density model of the upper crust along a part of the Deccan Syneclise is carried out based on the complete Bouguer anomaly data. Spectral analysis of the complete Bouguer gravity anomaly map of the study region suggests two major sources: short wavelength anomalies (<100 km) caused primarily due to the density inhomogeneities at shallow crustal level and long wavelength anomalies (>100 km) produced due to the sources deeper than the upper crust. A residual map of the short wavelength anomalies is prepared from the complete Bouguer anomaly using Butterworth high‐pass filter (100 km cut‐off wavelength). Utilizing the constraints from deep resistivity sounding, magnetotellurics and deep seismic sounding studies, 2.5D density models have been generated along 39 profiles of this region. The mismatch between the calculated response of the a priori 2.5D model with the residual (short wavelength) gravity anomalies is minimized by introducing high‐density intrusive bodies (≥2.81 g/cm³) in the basement. With these 2.5D density models, the initial geometry of our 3D density model, which includes alluvium, Deccan trap, Mesozoic sediment and high‐density intrusive bodies in the basement up to a depth of 7 km (upper crust), is generated. In the final 3D model, Deccan trap extends from 200 m to nearly 1700 m below the 90–150 m thick Quaternary sediment. Further down, the sub‐trappean Mesozoic sediment is present at a depth range of 600–3000 m followed by the basement. The derived 3D density model also indicates six intrusive bodies of density 2.83 g/cm³ in the basement at an average depth of about 4–7 km that best fits the residual gravity anomaly of the study area.     

Recent crustal movements and earthquakes in the area of the Tornquist Zone

Summary The present state of knowledge on recent crustal movements, stresses and earthquakes around the Tornquist Zone has been collected. It is found that the Tornquist Zone is neither geodetically nor seismically active, but it separates regions of different earthquake activity.Members of the working group on recent crustal movements around the Tornquist Zone: H. Lykke Andersen, M.C. Oncescu, V. Mocanu, E. and P. Bankwitz, G. Grünthal, and S. Gregersen.     

Three-dimensional modelling of crustal temperature and MOHO heat flow density in Central Europe and adjacent areas

For the Central European area three-dimensional model techniques are used to estimate the temperature-depth distribution within the Earth's crust, the temperature at the crust-mantle boundary, the MOHO heat flow density, and the lateral variations of the heat production within the granitic upper crust. Regularization methods are used for the inversion of the surface heat flow density. The calculations were done for a 1 × 1 degree grid. Evidently, for single geothermal provinces the 200°, 400°, 600°, and 800° isotherms are running parallel to the crust-mantle boundary which implies that this discontinuity might be interpreted in terms of an isotherm.     

地形变强震前兆指标探讨

在多年形变前兆研究基础上，对形变应变前兆的复杂性进行了讨论，并对典型的形变应变短临前兆实例进行了总结，找出其共性。以描述其共性特征为目标，给出了异常指标K的计算模型，似可用该指标结合资料背景和实践经验进行综合分析，对某些6级以上强震作出中短或短临阶段的预报。     

The crustal fluid evolution and the causes of earthquakes (III)

This paper is the third one of a series of three papers on the fluid evolution of the crust-upper mantle and the causes of earthquakes. Based on the last two papers, a model of the crustal resistivity structure and the deep-seated fluid evolution is presented, and also a seismogeny theory is set up, which is called the potential-kinetic energy transformation model. In this model, the crustal deep-seated fluid evolution is considered to take the most important effect on the seismogenic process. Taking the Tangshan M7.8 earthquake of 1976 as an example, the earthquakes occurred in a pull-apart rifting basin are analyzed, and finally the crust outgassing in the seismogenic processes is discussed, referring to the here-presented theory of seismogeny. This paper is supported by the Eight Five-Year Target of the State Seismological Bureau, China.     

Anomaly of the ionospheric electron density close to earthquakes: Case studies of Pu'er and Wenchuan earthquakes

The electron density recorded onboard the DEMETER satellite was analyzed to search for possible anomalies before earthquakes both in space and time. To distinguish pre-earthquake anomalies from the other anomalies related to geomagnetic activity, data were filtered using the K_p index. The analysis is based on the comparison of data recorded closely to earthquakes in space and time and past data for the same area. In analyzing data around the time and location of the Pu’er and Wenchuan earthquakes, obvious anomalies in electron density were found close to the epicenters, and some remarkable disturbances were detected before the earthquakes occurred. The results were finally compared with those of previous works that used the same data but employed different analysis methods. Good agreement was found which suggests that these anomalies have a close relation to the earthquake preparation.     

Effects of steep topography on the flow and stratification near Palmyra Atoll

Two interdisciplinary cruises aimed at relating the ecology of marine fish populations to oceanographic conditions were fielded during the late summer and late winter seasons near Palmyra Atoll (5.9°N, 162.1°W) in the Line Islands. Ocean current and hydrographic measurements revealed interaction of the flow with the steep topography. During the first cruise (August/September 1990) satellite-tracked surface drifters and acoustic Doppler current profiler (ADCP) measurements showed a strong eastward setting North Equatorial Counter Current (NECC) with maximum speeds exceeding 1 m s^–1 at 80 m depth approximately. This current turned southeastward on closer approach to Palmyra. The drifter paths exhibited excursions with zonal wavelength of approximately 250 km, meridional amplitude of 25 km and period of approximately 5 days. During the second cruise (February/March 1992), the ADCP-derived speeds of the NECC were weaker (maxima approximately 33 cm s^–1) while the relative geostrophic flow component was of magnitude similar to 1990 and the signal of zonal geostrophic currents reached much deeper to approximately 650 m depth (150 m in 1990). Doming isopycnals beneath the surface mixed layer as well as thick (10–25 m) internal mixed layers were found near Palmyra during both cruises, with slightly different positions relative to the island. The discontinuous vertical temperature profiles may have been a result of strong boundary mixing due to breaking internal waves on Palmyras steep slopes. In the immediate vicinity of the island variations in flow speed, stratification and mixing in both the alongshore and cross-isobath directions were observed. Overall, the current speeds were reduced during February/March 1992, the peak time of the 1991–1993 warm event in the tropical Pacific. While parameters of turbulent two-dimensional wake theory are suggestive of formation and shedding of eddies in the lee of the island, no direct observations of circular motions were made in either expedition.Responsible Editor: Hans Burchard     

强震前地壳形变时空演化分析

1 研究背景 自从Reid通过对1906年美国加州旧金山大地震发生前后大地测量观测资料分析提出弹性回跳理论以来,地壳形变测量在地震预报及科学研究中发挥着愈来愈重要的作用. 弹性回跳理论从震源区出发,较好解释了地震蕴育过程中断层附近的趋势变形.但是,震源区只是蕴震系统的一部分,无法同周围的、深部的构造环境和动力环境分割开...     

Central Chile: An example of quasi-static crustal behavior

Abstract Indications of transient crustal displacement associated with the 3 March 1985, M, = 7.8, Central Chile earthquake are evidenced by various observational devices. Almost half a meter of coastal uplift at localities close to the epicentral region was detected by repeated leveling lines. A tide gauge at Valparaiso revealed minor coseismic coastal subsidence (∼ 10 cm) continuously developing after the earthquake. Two limnigraphs, 27 km apart, that were situated at the extremes of Rapel Lake to the south of the leveling line, have recorded continuously the equipotential lake level for more than 10 years, providing a permanent very-long-base tiltmeter. The water level difference at the two limnigraphs as a function of time resembles a ramp function, beginning approximately at the time of the earthquake occurrence and gradually developing over a period of 10 months with a maximum amplitude of 120 mm or 4.4 p radians in tilt. The shape of the time-dependent tilt is mimicked by the sea level signal recorded at Valparaiso, ∼ 100 km away from Rapel Lake, showing a maximum coastal subsidence of 0.6 m. Comparisons of sea level changes produced by the 1971, M, = 7.5, earthquake indicates that they represent rupture in different portions along the seismogenetic region as well as a different rupture mode. Gravity surveys carried out in three different pre- and post-seismic epochs, along the segment of the leveling line which shows major coseismic uplift, indicate that the whole region has subsided, post-seismically, 10 em in 5 years. These observations are interpreted in terms of a variable slip dislocation model. Inversions show that it is the more than 2 m of fault displacement in 10 months of post-seismic movement along the contact between the Nazca and South America plates, which is interpreted to be responsible for the time-dependent elevation changes.     

形变场多重分形谱异常与强震关系研究

以平均形变速率梯度概率为变量, 研究了唐山地区、甘肃河西地区和北京地区多期垂直形变场图像的动态多重分形特征及其与强震的关系。结果表明, 震前一至数年形变场图像的多重分形谱Dq 值域变宽, 即- q 区上翘, + q区下降现象, 在强震孕育发生的过程中, 形变场时空表现出从准均匀到不均匀的演化趋势。结合地震活动和岩石力学实验结果认为, 形变场不均匀化可能是中长期地震复杂性前兆的一个重要标志。     

Empirical correlation of PGA,spectral accelerations and spectrum intensities from active shallow crustal earthquakes

Empirical correlation equations between peak ground acceleration, spectral acceleration, spectrum intensity, and acceleration spectrum intensity are developed. The correlation equations are developed for shallow crustal earthquakes using the Next Generation Attenuation (NGA) ground motion database, and four of the NGA ground motion prediction equations (GMPEs). A particularly novel aspect of the present study is the explicit consideration of epistemic uncertainty in the correlation equations due to both the adopted ground motion database and GMPEs. The resulting correlation equations enable the joint consideration of these four ground motion intensity measures in ground motion selection using frameworks such as the generalized conditional intensity measure approach. Copyright © 2011 John Wiley & Sons, Ltd.     

Seismological evidence for the convergence of crustal stress orientation before large earthquakes

Earthquakes are caused by the failure of faults,driven by tectonic stress build-up in the Earth’s crust.To study the earthquake preparation process and assess regional earthquake potentials,it is vitally important to understand the crustal stress evolution process and identify its change in pattern associated with the seismogenic process.In this study we investigate the focal mechanism orientations of earthquakes in southern California from 1982 to 1999,basing on a focal mechanism catalog from Hauksson.We f...     

Observation and analysis of low-frequency crustal earthquakes in Lake Van and its vicinity,eastern Turkey

Lake Van is located in very intensely deformed tectonic settings in the eastern Turkey. The surrounding area of Lake Van has major tectonic structures; NE and NW trending active conjugate strike-slip faults, pull-apart basins along these faults, E-W trending active thrust faults, folds, compressional ramp basins, N-S trending folds, NE-SW aligned continental collision-related volcanoes and N-S trending tensional cracks functioned as magma conduits. Since the area is tectonically very active, the earthquake activity within the basin and the surrounding area of Lake Van is very high. In this study, we have analysed the records of VANB broadband station of Kandilli Observatory and Earthquake Research Institute's network for the seismic events (2000–2004) that occurred in the basin and in the vicinity of Lake Van. Based on the spectral analysis of earthquake data, we have mainly observed three types of seismic events; hybrid event, long period event and tremor. We have suggested that the observed hybrid, long period and tremor type of seismic events that occurred in the study area are related to the upward moving materials and stress loading in the mid and lower crust based on the geophysical data.     

Wave disturbances induced by crustal earthquakes: Case study of two strong earthquakes in the Caucasian-Anatolian sector of the Alpine Mediterranean mobile belt

The display conditions of strong earthquakes in the Caucasian-Anatolian sector of the Mediterranean mobile belt are analyzed with allowance for the instrumental observations by titlmeters and magnetic variometers at the North Caucasian geophysical observatory of Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences (NCGO IPE RAS) (Elbrus volcanic region) and at the magnetic stations operated by the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (IZMIRAN). It is demonstrated that the geophysical information obtained during the preparatory stages of the earthquakes on March 8, 2010 and January 19, 2011 in Turkey and the North Caucasus, respectively, reflect the fine structure of the anomalous wave processes of the preparation and evolution of the crustal earthquakes. The results provide a general idea of the geomagnetic activity and some characteristic features of the induced anomalous disturbances in the Caucasian-Anatolian sector of the Alpine Mediterranean mobile belt, which precede and accompany strong seismic events.     

中强震对地壳应力场的影响——以盈江地区5次中强震为例

为了研究中强震对震源区应力场的影响,本研究以盈江地区2008—2014年间发生的5次中强震为例,深入研究这5次中强震对盈江地区地壳应力场的时空影响.为了探究中强震对应力场的时空影响,我们对盈江地区地壳应力场进行了以下研究：根据5次中强震发生时间,分时段反演应力场以考察应力场在时间上是否存在变化;根据震源机制解的空间分布,将研究区分为南部和北部反演应力场,以确定应力场是否存在空间差异;随机抽取震源机制解反演应力场以便确定应力场是否存在时空差异.研究结果表明：(1) 5次中强震对盈江地区应力场产生的扰动较小,没有引起盈江地区应力场在时间上的显著变化;(2) 在研究时段内,盈江地区地壳应力场在时空上可以视为均匀应力场;(3) 盈江地区的地壳应力场为走滑型,最大主应力轴呈NNE-SSW向,最小主应力轴呈SEE-NWW向,最大和最小主应力轴倾角近水平,中间主应力轴倾角近直立.

     

形变数字化资料对邢台地区地震前兆反应的研究

使用张家口、怀来、赤城、宽城、易县和涉县6个台站共计43个测项的数字化资料,针对2002~2005年邢台地区发生的3个M>4级的中强地震,进行了地震前兆和同震反应研究。发现我省数字化形变资料对3次邢台4级以上地震的形变前兆反应主要是以应变为主,包括体应变和线应变,并且异常形态以固体潮畸变和抖动为主。同震反应比较明显,主要以同震阶跃为主。     

四川威远及邻区中小地震活动特征及地壳精细结构研究

利用2019年以来在四川荣县-威远布设的密集地震台站,以及部分固定台站记录到的近震资料,采用双差层析成像方法获得了高分辨率的浅层地壳三维速度结构和震源位置.重定位结果显示,研究区内中小地震多数呈南北向条带状分布,与已知地表断层的分布无明显关联.地震震源深度主要集中在2~5 km深度之间.研究区中小地震活动与速度结构变化具有相关性,在5 km以浅,地震多分布在S波高速异常区;在7~10 km深度范围,地震多发生在P波和S波的高、低速异常转换带.综合重定位和速度剖面结果,推测研究区内沉积层厚度约4~6 km,而一些中强地震多发生在结晶基底顶部.在研究区深部,黄桷坡断层以北地震区比荣县地震区具有更高的P波速度,在相似的应力状态下,力学性质更强的黄桷坡断层以北地区更难以破裂,可能是该地区更晚发生地震的缘由.

     

山东地区地壳速度与中强地震的关系

利用山东区域地震数字台网提供的数据,通过三维层析成像方法结合山东地质构造实际情况,对山东区域进行联合走时反演地壳速度结构,获得该区分辨率较高的三维速度结构图像。结果表明：不同深度上的速度分布反映出山东地区深部构造的某些重要信息,为进一步研究该区的活动构造及地震参数计算提供科学依据。在地壳速度结构与中强地震活动的关系上,再次显示出中强震多发生在速度结构高速单元与低速单元的过渡带附近,说明中强震的孕育与震源区周围介质的不均匀性有关。     

Gravity field separation, density inversion and crustal tectonics in Kang-Dian region

IntroductionTethys-Himalya tectonic belt extends from west to east across Europe, Asia and Africa, and stops at the longitude of 104(E in China. The famous North-south active tectonic belt located in China is a part of China-Mongolia central-axis strong earthquake belt and the huge crustal thickness undulation belt. The tectonic belt is a strong extrusion, tension and shear region. It is synthesized by topography gradient belt, faults, Cenozoic basin and strong earthquake belt on north-so…     

青藏高原地壳的三维密度结构和物质运动

应用区域重力场小波多尺度分析和反演于青藏区后,得到6个地壳等效层密度扰动图件,刻划了地壳三维密度结构,为研究地壳构造和物质运动提供了重要佐证.研究表明在青藏高原地壳内密度变化有以下三个规律.1)从上地壳到下地壳,平面分布上低密度区的分布范围逐渐扩大;在下地壳只有刚性克拉通地体才有显示高密度.2)从上地壳到下地壳,平面分布上密度扰动区的尺度逐渐扩大;到下地壳高或低的密度区不仅数量大为减少,而且边界更加清晰.3)从上地壳到下地壳,青藏高原南部的低密度带不断向北移动,反映印度陆块向欧亚大陆的向北俯冲.青藏高原下地壳密度高的克拉通地体有羌塘、柴达木和巴颜喀拉三个;而昆仑山、阿尔金山、祁连山和冈底斯地块都属于低密度的中新生代构造活动单元.拉萨地块也是低密度地块,在中下地壳它与冈底斯地块相连,应归属于中新生代构造活动单元.松潘甘孜地块在下地壳为低密度,但在上中地壳逐步变为高密度,并与巴颜喀拉克拉通地体连接.这种情况可能反映巴颜喀拉地体的上地壳随印澳板块俯冲向东南方向挤出.青藏高原低密度的物质也由下地壳向上挤出,在中上地壳体积迅速减小.由于下地壳低密度的物质向上挤出,中地壳密度高的克拉通地体会相应发生裂解,使克拉通地块的数目增加.高原北缘的下地壳低密度物质侧向挤出的枝杈有三支;其中一支从西昆仑到天山,另一支从龙门山西秦岭到银川盆地.第三支从高原南缘理塘到大理.它们可能反映下地壳管道流,宽度约180~300 km.7级以上地震震中都位于下地壳低密度物质侧向挤出枝杈周围,可能与下地壳管道流位置吻合.