四川盆地深部地壳结构——深地震反射剖面探测

四川盆地位于扬子地块的西北部,被褶皱构造带所围绕,受周缘构造带的侧向挤压作用,盆地卷入了多期次和多边界的构造变形,为开展盆山耦合作用及多边界、多期次构造叠加与复合关系的研究提供了不可多得的理想野外实验室.为揭示四川盆地地壳结构,本文通过对3条不同时间采集的深地震反射剖面数据进行拼接联线处理,获得跨越四川盆地的330 km深地震反射偏移成果剖面,揭示了四川盆地地壳上地幔细结构：沉积层从西北向东南逐渐变薄,在龙门山前沉积层厚度超过15 km,在华蓥山下沉积层减薄到~8 km,且褶皱变形形成华蓥山薄皮褶皱冲断带;莫霍面出现在13~15 s（双程走时）,埋深约40~45 km;并发现从下地壳延伸至地幔的东南向的倾斜反射,从13 s向下延伸至18 s,结合四川盆地及其周边地区其他地球物理和地球化学花岗岩同位素年龄等资料,我们认为这些倾斜反射层是扬子克拉通地台西北缘发生的新元古代俯冲的遗迹.

     

Upper crustal P-wave velocity structure beneath two volcanic areas in northern Iran

Detailed information about the crustal structure is essential for better understanding the occurrence and mechanisms of earthquakes and volcanoes. Here we present a study of the upper crustal P-wave velocity structure of two seismically and volcanically active areas in northern Iran using the two-dimensional Pg travel time tomography method. The imaging results suggest low velocities in the upper crust beneath the Damavand and Sahand-Sabalan volcanic areas in the central and western parts of northern Iran, respectively. The upper crustal low velocities in these two areas roughly coincide with previously imaged low Pn velocity anomalies, suggesting that the Late Cenozoic volcanic activity was probably caused by the upwelling of hot materials from the mantle. The image feature of the Pg velocity structure beneath the Sahand-Sabalan volcanic area further indicates that the hot materials stored in the upper crust beneath Sahand may be larger in size than those stored beneath Sabalan. Comparison of the Pg velocity images with the earthquake distribution in north Iran suggests that earthquakes mainly occur at moderately low velocity or low to high velocity boundary areas instead of significantly low or high velocity regions. The anisotropy results show that the Pg wave fast direction is consistent with the GPS direction at high Pg velocity areas and the fast direction is inconsistent with the GPS direction but consistent with the strike direction of faults at low velocity areas. Our new upper crustal structural images provide the basic observation for better understanding of the regional seismicity and volcanism, and link the surface geological phenomena to deep crustal and mantle processes associated with the active tectonics in northern Iran.     

2-D crustal structure of Changbaishan-Tianchi volcanic region determined by seismic traveltime inversion

IntroductionTrial-and-error forward modeling of wide-angle seismic reflection/refraction traveltimes for 2-D velocity structure is extremely time-consuming, even for experienced data interpreters. For wide-angle seismic reflection/refraction experiments that consist of numerous shots along a single line, it is quite difficult through repeated trial-and-error forward modeling to construct a 2-D model that fits the data within acceptable limits (Cerveny, et al, 1977; ZHANG, et al, 200 . In ad…     

安阳及邻区三维地壳速度结构研究

应用区域地震台网1985～2008年的地震到时资料,对安阳及邻区地壳三维速度结构和震源参数进行了联合反演,获得了该区地壳的三维速度结构图像.成像结果表明:研究区浅部地形速度分布特征为:凹陷盆地速度较低,山脉位于速度的过渡区,隆起区多位于高速区,速度横向分布差异较大;上、中地壳速度横向分布存在非均匀性,与区域构造分布特征...     

Deep seismic reflection profiling revealing the complex crustal structure of the Tongling ore district

Deep seismic reflection profiling has been the dominant method for probing the deep structure of continental crust since the initiation of the COCORP in 1974[1,2]. Over the past few decades, this tool has been applied to diverse geologic features from the Appalachian area of the United States, to the Rhine-graben area in Western Europe and to the Tibetan Pla-teau[36]. In recent years, the Australian Geodynamic Cooperative Research Center ( AGCRC ) has applied this technique to the stu…     

Intensity attenuation for active crustal regions

We develop globally applicable macroseismic intensity prediction equations (IPEs) for earthquakes of moment magnitude M _W 5.0?C7.9 and intensities of degree II and greater for distances less than 300?km for active crustal regions. The IPEs are developed for two distance metrics: closest distance to rupture (R _rup) and hypocentral distance (R _hyp). The key objective for developing the model based on hypocentral distance??in addition to more rigorous and standard measure R _rup??is to provide an IPE which can be used in near real-time earthquake response systems for earthquakes anywhere in the world, where information regarding the rupture dimensions of a fault may not be known in the immediate aftermath of the event. We observe that our models, particularly the model for the R _rup distance metric, generally have low median residuals with magnitude and distance. In particular, we address whether the direct use of IPEs leads to a reduction in overall uncertainties when compared with methods which use a combination of ground-motion prediction equations and ground motion to intensity conversion equations. Finally, using topographic gradient as a proxy and median model predictions, we derive intensity-based site amplification factors. These factors lead to a small reduction of residuals at shallow gradients at strong shaking levels. However, the overall effect on total median residuals is relatively small. This is in part due to the observation that the median site condition for intensity observations used to develop these IPEs is approximately near the National Earthquake Hazard Reduction Program CD site-class boundary.     

Three-dimensional crustal P-wave velocity structure in the Yangbi and Eryuan earthquake regions,Yunnan, China

A magnitude 5.5 earthquakes occurred in Eryuan County, Dali Bai Autonomous Prefecture, Yunnan Province, China, on March 3. And a magnitude 5.0 earthquake occurred in the same place on April 17, 2013, i.e., 45 days later. Then, on May 21, 2021, multiple earthquakes, one with magnitude 6.4 and several at 5.0 or above, occurred in Yangbi County, Dali Bai Autonomous Prefecture, Yunnan Province, China. All of these occurred in the Weixi-Qiaohou-Weishan fault zone. In this study, 1,874 seismic events in Yangbi and Eryuan counties were identified by automatic micro-seismic identification technology and the first arrivals were picked up manually. Following this, a total of 11,968 direct P-wave absolute arrivals and 73,987 high-quality P-wave relative arrivals were collected for joint inversion via the double difference tomography method. This was done to obtain the regional three-dimensional fine crustal P-wave velocity structure. The results show that the travel time residuals before and after inversion decreased from the initial –0.1–0.1 s to –0.06–0.06 s. The upper crust in the study area, which exhibited a low-velocity anomaly, corresponded to the basin region; this indicated that the low-velocity anomaly in the shallow part of the study area was affected by the basin. Results also showed some correlation between the distribution of the earthquakes and velocity structure, as there was a low-velocity body Lv1 with a wide distribution at depths ranging from 15–20 km in the Yangbi and Eryuan earthquake regions. In addition, earthquakes occurred predominantly in the high-low velocity abnormal transition zone. The low-velocity body in the middle and lower crust may be prone to concentrating upper crustal stress, thus leading to the occurrence of earthquakes.     

Deep seismic sounding investigation into the deep structure of the magma system in Changbaishan-Tianchi volcanic region

IntroductionThe Changbaishan-Tianchi volcano, located in Jilin Province, northeast of China, is one of the well-known active volcanoes in China. In the year 1 215(15, the volcano underwent an eruption that is believed to be one of the greatest explosive eruptions on earth in the past 2000 years with regard to its scale and violence. The eruption amounted to over 100 km3, and the eruption cylinder reached a height of more than 30 km, the volcanic ashes floated and fell south of Hokkaido, Jap…     

Three-dimensional structure of inclined sheet swarms: Implications for crustal thickening and subsidence in the volcanic rift zones of Iceland

Inward-dipping (cone) sheet swarms and an associated central volcano are well-exposed in the deeply-eroded Tertiary crust of Vatnsdalur, Skagi Peninsula region, northern Iceland. Spatially registered orientations of 389 mafic sheets, mapped in three distinct sheet swarms define both the overall shape and magmatic source of each swarm. The Vatnsdalur sheet swarms consist of planar inward-dipping sheets that collectively define a conical shape rather than a bowl- or trumpet-shape as have been found in swarms in other locations. In the best exposed swarm, three-dimensional projection of mafic sheets into the subsurface defines two distinct foci, which are interpreted as the magmatic sources of two temporally distinct sub-swarms. These results help to establish the influence of inclined sheet intrusion on crustal accretion at central volcanoes. The geometry of the swarm constrains the thickness of material that was added to the crust during sheet intrusion. When combined with estimates of surface relief, we calculate that 2.2 to 4.1 km of subsidence were required beneath the central volcano in order to accommodate the intrusion of the sheet swarm. Similar processes of crustal thickening and subsidence likely occur in a wide variety of both continental rift and mid-ocean ridge systems where magmatic activity is focused at central volcanoes.     

Modeling crustal structure of the south-eastern Fennoscandia

We present new seismic velocity models of the crust and uppermost mantle along two refraction and wide-angle reflection profiles in the southern Fennoscandia: the Pribalt and 1-EB profiles. Some new results obtained along the Coast and the Baltic Sea profiles are also presented. The intercept time method and ray tracing are used for the modeling. The study shows that the lateral variations are small in the velocity structure of?? the crust up to the depth of 20?C25 km. The most significant lateral variations are observed in the Moho discontinuity topography and in the seismic velocities in the lower crust. In Paleoproterosoic Svekofennian domain, besides the well-known Moho depression in southern Finland, another Moho depression is revealed in the region from the Gotland Island to the Gulf of Riga. We suggest that this depression can correspond to the unknown crustal unit (we call it the Gotland-Riga belt). The Moho depth increases from the average of 40?C45 km to 55 km in this belt. The Moho depression is filled by the matter with velocities of 6.8?C7.1 km/s. Deep faults inclined to the north and strong variations of the mantle velocities are typical for the uppermost mantle of the Gotland-Riga belt.     

Average crustal structure of Sweden

Summary Records obtained at the permanent stations of the Swedish seismograph network from explosions carried out in Scandinavian waters in June 1969 are evaluated. The study includes determination of velocities for all crustal phases observed, furthermore of layer thicknesses, Poisson ratios and amplitude ratios. The purpose of the study is partly to provide a first approximation to the crustal structure in Sweden, partly to provide regional data for location of earthquakes and explosions in the area in the future. Average velocities (km/sec) are forPn 7.88±0.05,Pg1 6.25±0.08,Pg2 5.70,Sn 4.58±0.04,S ^* 3.70±0.04,Sg1 (Lg1) 3.58±0.03,Sg2 (Sg) 3.40±0.03,Rg 3.02±0.07. The average thickness is 12 km for the granitic layer, and 23 km for the basaltic layer, thus making the average crustal thickness equal to 35 km. Relative amplitudes plotted versus distance complete the dynamical side of the study and they are useful for identification of waves. A regional travel-time table is presented for the distance range 0°–10° with entries for each 0.1° and including all crustal phases read.     

Deep crustal reflection survey in central Israel

The results of the first deep crustal reflection survey in Israel are described. The line crosses central Israel in a SE-NW direction from near the Dead Sea rift to the Mediterranean sea. The data reveals a correlation between the reflectivity and type of crust as previously determined on the basis of magnetic and crustal refraction data. To the southeast, in an area which is thought to be underlain by continental crust, the upper crust is mostly transparent while the lower crust shows distinct reflections. Near the Mediterranean sea, where the crystalline crust was previously shown to be thinner with higher magnetization and of possible oceanic origin, the upper crust is reflective while in general the lower crust is significantly less reflective. The lateral boundary between these two patterns of crustal reflectivity coincides with the magnetic boundary which was previously used to differentiate between the two types of crusts. Detailed analysis of crustal reflections does not seem to be justified with the present data except in special cases. One such case is the determination that the uppermost basement in the coastal plain is highly reflective, possibly due to metamorphic layering. Comparison of the deep reflection data with MT and seismic refraction data indicates possible correlation in the southeastern part of the line of high conductivity with lower crust reflections and of refraction Moho with the termination of lower crustal reflections. This correlation does not extend into the northwestern part of the section in which the crust is, most probably, no longer continental.     

Pre-deccan trap topography in central India and crustal warping in relation to Narmada rift structure and volcanic activity

A study of the geology of the Dhar Forest, the Pachmarhi plateau and the area around Bari in Central India has led to the conclusion that the pre-Deccan Trap topography which was completely covered by the lava flows and is being exposed with spectacular clarity by the process of exhumation, had much the same relief as the present land surface. The geomorphological studies of the Vindhyan and Pachmarhi plateau suggest a characteristic rise and increasing separation of different planation surfaces towards the edges of the Narmada rift valley and indicate upwarping movements in several distinct stages. In considering the possible causes of the upwarping movements special significance is attached to epeirogenic movements probably representing the various stages of Himalayan orogeny. The succession of events in Peninsular India suggests that these upwarping movements were caused by rising magma which led to the fissure eruptions of the Deccan Trap lavas; which presumably took place during a period of tension in the upwarped area. The problem of the origin of the Narmada rift structure is discussed and evidence is adduced to show that the final sinking of the crest of the upwarped area has caused the out-pouring of the Deccan Trap lavas. The individual lava flow with their typical field and microscopic characteristics maintaining their interflow differences have been traced over long distances. These studies have led to the correlation of the flows between the measured sections. Further, as regards the cause of the higher elevation of the base of the basal flow in Katangi (1950), the possibility of a post-Deccan Trap upwarping movement is briefly considered.     

Mean crustal velocity: a critical parameter for interpreting crustal structure and crustal growth

Mean crustal velocity is a critical parameter for genesis of continental crystalline crust because it is a function of mean crustal composition and therefore may be used to resolve continental crustal growth in space and time. Although the best values of mean crustal velocity are determined from wide-angle reflection measurements, most studied here necessarily come from vertical averages in crustal refraction determinations. The mode of 158 values of mean crustal velocity is 6.3 km/s, a velocity which corresponds to a mean crustal composition of granodiorite to felsic quartz diorite; Archean crust may be slightly more mafic. Mean crustal velocities range from 5.8 to 7.0 km/s. The lowest values invariably are found in thermally disturbed rift zones and the highest values correspond to velocities in gabbro. Velocities in island arcs may be as low as 6.0 km/s but are typically 6.5–6.9 km/s which corresponds to andesitic composition; estimates of island arc composition are andesitic. If values of mean crustal velocity are not biased, this observation suggests that continental crust did not grow simply by addition of island arc material. Possibilities are that crust formed from fusion of island arcs and was later changed to more felsic composition by addition of material from the mantle or that the late Archean episode of major crustal growth did not involve processes similar to younger island arcs. Some crustal blocks might be changed in composition and thickness by such processes as underplating, interthrusting, necking and sub-crustal erosion. Specially designed experiments are suggested to determine this parameter so critical for understanding genesis of continental crust.     

松辽盆地地壳精细结构研究

基于东北地区已有的宽频带流动台阵远震数据,利用波场延拓和分解的H-β网格搜索法,对松辽盆地的沉积层及地壳结构进行了深入分析。结果显示:松辽盆地的沉积层厚度为0.2—2.5 km,整体呈现中央坳陷区厚、边缘薄且西南地区最薄的分布特征;研究区地壳较薄,厚度介于24—34 km之间,其横向变化特征与沉积层厚度分布具有一定的对应性。依据沉积层和地壳的厚度计算了地壳伸展系数,其平均值接近于以往接收函数研究估测的岩石圈伸展因子。因此,本文推测松辽盆地在伸展构造过程中,其地壳和岩石圈的减薄以纯剪切模式为主。此外,松辽盆地具有较高的地壳平均波速比v_P/v_S,暗示盆地下方岩石圈地幔的减薄过程中可能存在岩浆的底侵作用。      

云南腾冲火山区地壳及岩石圈厚度研究

使用云南腾冲火山监测台网9个宽频带地震台站的远震数据,采用P波和S波接收函数的方法研究了腾冲火山区的地壳厚度、泊松比值以及岩石圈和软流圈分界面(LAB)深度.研究结果表明: 1) 云南腾冲火山区的地壳厚度约在33.5~38.0 km之间; 2) 火山区的泊松比主要集中范围为0.26~0.32,其中6个台站均大于0.29,推测与地壳镁铁质成分的增加有关并且可能存有2个岩浆囊; 3) 火山区的岩石圈厚度在78.2~88.0 km,较周边地区明显隆起且横向差异较大.腾冲火山区岩石圈的明显穹隆,由软流圈上涌(地幔热物质上升)引起岩石圈的拉张与减薄所致.     

首都圈地壳网格化三维结构

通过对首都圈地区不同时期的18条深地震测深(DSS)测线资料的重新统一处理,形成采样网格密度为0.25°×0.25°×（2～5）km的速度网格化数字地壳;通过对三维数据的可视化,得到了首都圈地壳不同走向、不同圈层的截面图像;从不同的视角、不同的方面探讨了控制首都圈地壳的北部燕山隆起、西南部太行隆起和东南部裂陷盆地等三大地质单元的构造特征以及接触带张家口-渤海断陷带的构造性质,进一步研究首都圈地壳内部结构构造与灾害性地震的孕发机制的关系.     

长白山—镜泊湖火山区地壳结构接收函数研究

利用71个远震的波形资料,用接收函数方法提取了布设在长白山—镜泊湖火山区的34个宽频带流动数字地震台站的接收函数,通过对接收函数反演,获得了台站下方的S波速度结构.研究结果表明,沈阳—敦化一线莫霍面深度32～33km,向西地壳厚度加厚,到长春附近地壳厚度约为36km.在天池火山口莫霍面深度为达38km,而镜泊湖火山口森林的莫霍面深度约为39km.总体看研究区的地壳厚度是南浅北深.长白山天池火山口附近地下10km左右有一明显的低速层存在;镜泊湖火山口森林附近30km也可能有低速体存在;研究发现莫霍面上S波速度梯度在火山口附近和远离火山口有明显区别.在火山口附近其莫霍面的S波速度梯度比非火山口地区的S波速度梯度明显小,说明火山口下与一般的地壳莫霍面结构有差别.研究发现沈阳—敦化一线两侧的莫霍面深度有较大变化,其位置与地表的敦化—密山断裂基本一致,说明敦化—密山断裂是研究区的一条非常重要的地质构造带.     

中国大陆东部强震区和火山区人工地震探测研究综述

介绍了东北、华北、华东和华南的强震区和火山区的地震测深研究成果,揭示了强震区和火山区的地壳深部构造背景.分析表明,地壳深部断裂、中下地壳低速层、速度结构的差异、波速比异常、泊松比和岩性的不同、上地幔顶部隆起、莫霍界面较大的起伏、复杂的壳幔过渡带、滑脱构造、深部岩浆活动等构造特征与东部地区强震形成和发生有较为密切的关系.