The role of fluids in lower-crustal earthquakes near continental rifts

The occurrence of earthquakes in the lower crust near continental rifts has long been puzzling, as the lower crust is generally thought to be too hot for brittle failure to occur. Such anomalous events have usually been explained in terms of the lower crust being cooler than normal. But if the lower crust is indeed cold enough to produce earthquakes, then the uppermost mantle beneath it should also be cold enough, and yet uppermost mantle earthquakes are not observed. Numerous lower-crustal earthquakes occur near the southwestern termination of the Taupo Volcanic Zone (TVZ), an active continental rift in New Zealand. Here we present three-dimensional tomographic imaging of seismic velocities and seismic attenuation in this region using data from a dense seismograph deployment. We find that crustal earthquakes accurately relocated with our three-dimensional seismic velocity model form a continuous band along the rift, deepening from mostly less than 10 km in the central TVZ to depths of 30-40 km in the lower crust, 30 km southwest of the termination of the volcanic zone. These earthquakes often occur in swarms, suggesting fluid movement in critically loaded fault zones. Seismic velocities within the band are also consistent with the presence of fluids, and the deepening seismicity parallels the boundary between high seismic attenuation (interpreted as partial melt) within the central TVZ and low seismic attenuation in the crust to the southwest. This linking of upper and lower-crustal seismicity and crustal structure allows us to propose a common explanation for all the seismicity, involving the weakening of faults on the periphery of an otherwise dry, mafic crust by hot fluids, including those exsolved from underlying melt. Such fluids may generally be an important driver of lower-crustal seismicity near continental rifts.     

Three-dimensional crustal velocity structure of P-wave in East China from wide-angle reflection and refraction surveys

The 3-D crustal structure of P-wave velocity in East China is studied based on the data obtained by wide-angle seismic reflection and refraction surveys.The results suggest that a deep Moho disconti-nuity exists in the western zone of the study region,being 35―48 thick.High-velocity structure zones exist in the upper crust shallower than 20 km beneath the Sulu and Dabie regions.The cause of high-velocity zones is attributable to high-pressure metamorphic(HPM) and ultra-high-pressure metamorphic(UHPM) terran...     

Uppermost mantle structure of the North China Craton: Constraints from interstation Pn travel time difference tomography

The uppermost mantle is the key area for exchange of heat flux and material convection between the crust and lithospheric mantle. Spatial variations of lithospheric thinning and dynamic processes in the North China Craton could inevitably induce the velocity heterogeneity in the uppermost mantle. In this study, we used Pn arrivals from permanent seismic stations in North China and surrounding regions to construct a tomographic image of the North China Craton. The tomographic method with Pn travel time difference data were used to study the velocity variations in the uppermost mantle. Pn velocities in the uppermost mantle varied significantly in the Eastern, Central and Western blocks of the North China Craton. This suggests that the lithosphere beneath different blocks of the North China Craton have experienced distinct tectonic evolutions and dynamic processes since the Paleozoic. The current uppermost mantle has been imprinted by these tectonic and dynamic processes. Fast Pn velocities are prominent beneath the Bohai Bay Basin in the Eastern Block of the North China Craton, suggesting residuals of the Archean lithospheric mantle. Beneath the Tanlu Fault Zone and Bohai Sea, slow Pn velocities are present in the uppermost mantle, which can be attributed to significant lithospheric thinning and asthenospheric upwelling. The newly formed lithospheric mantle beneath Yanshan Mountain may be the dominant reason for the existence of slow Pn velocities in this region. Conversely, the ancient lower crust and lithospheric mantle already have been delaminated. In the Central Block, significant slow Pn velocities are present in Taihangshan Mountain, which also extends northward to the Yinchuan-Hetao Rift on the northern margin of the Ordos Block and Yinshan Orogen. This characteristic probably is a result of hot asthenospheric upwelling along the active tectonic boundary on the margin of the Western Block. The protracted thermal erosion and underplating of hot asthenospheric upwelling may induce lithospheric thinning and significant slow velocities in the uppermost mantle. Fast velocities beneath the Western Block suggest that the thick, cold and refractory Archean lithospheric keel of craton still is retained without apparent destruction.     

山东地区地壳及上地幔结构研究

利用布设在山东省境内的宽频带流动地震观测台阵和国家地震局固定地震观测台站记录的地震数据, 应用接收函数和SKS波分裂方法, 研究山东地区的地壳与上地幔结构, 得到该区域的地壳厚度、地壳平均P波与S波的波速比以及SKS波分裂延迟的分布情况。结果表明, 山东地区地壳厚度范围为28~39 km; 胶南隆起的北段和南段以及鲁西隆起北侧济阳凹陷的地壳厚度小于32 km, 鲁西隆起下方的地壳比较厚。研究区 P波与S波的波速比主要分布在1.67~1.94之间, 鲁西隆起西南部和胶南隆起北段该比值小于1.75, 可能是由中上地壳增厚以及下地壳减薄和拆沉造成。鲁西隆起南北P波与S波的波速比差异反映地壳活动的差异。地幔物质的各向异性显示, 山东地区西部的地壳减薄和拆沉可能仍在进行。     

Crustal P-wave velocity structure in Lower Yangtze region： Reinterpretation of Fuliji-Fengxian deep seismic sounding profile

A deep seismic sounding profile in this paper, from Fuliji in Anhui Province to Fengxian of Shanghai City, is located at eastern China (Fig. 1). The field work was jointly accomplished by the Chinese Geological and Mineral Bureau, the China Seismological …     

Velocity structure of the crust and uppermost mantle in the boundary area of the Tianshan Mountains and the Tarim Basin

A portable 3-component broadband digital seismic array was deployed across the Tianshan orogenic belt (TOB) to investigate the lithospheric structure. Based on receiver function analysis of the teleseismic P-wave data, a 2-D S-wave velocity profile of the boundary area of the TOB and the Tarim Basin was obtained at the depths of 0--80 km.Our results reveal a vertical and lateral inhomogeneity in the crust and uppermost mantle. Four velocity interfaces divide the crystalline crust into the upper, middle and lower crust. A low velocity zone is widely observed in the upper-middle crust. The depth of Moho varies between 42 and 52 km. At the north end of the profile the Moho dips northward with a vertical offset of 4--6 km, which implies a subduction front of the Tarim Basin into the TOB. The Moho generally appears as a velocity transitional zone except beneath two stations in the northern Tarim Basin, where the Moho is characterized by a typical velocity discontinuity. The fine velocity structure and the deep contact deformation of the crust and upper most mantle delineate the north-south lithospheric shortening and thickening in the boundary area of the TOB and the Tarim Basin, which would be helpful to constructing the geodynamical model of the intracontinental mountain-basin-coupling system.     

S-wave velocity of the crust around Tianshan Mountains inverted from seismic ambient noise tomography

We process ambient noise data from seismic stations deployed in central Asia to determine the crustal shear wave velocity structure beneath the Tianshan Mountians and surrounding area. About 748 inter-station Rayleigh wave empirical Green’s functions have been recovered to estimate the phase velocity dispersions over periods from 6 to 50 s using the image transformation technique. Results show that for short periods (6–20 s), the distribution of Rayleigh wave phase velocities is generally consistent with surface geology, with high velocities corresponding to mountain ranges and low velocities to sedimentary basins. Along two profiles, which trend from NE-SW and NW-SE, the shear wave velocity shows a pair of high velocity anomalies dipping in opposite directions beneath the Tianshan Mountains. At shallow depths, those high velocity anomalies roughly correlate with areas where the mountain front and the surrounding basin are connected. The profiles also show a narrow zone beneath the Tianshan Mountains, which may represent a route for the upwelling from upper mantle. Those observations suggest that the underthrusting of the Tarim Basin and Kazakh Shield combine with the weakness of the crust, which is heated by the upwelling from upper mantle, may play an important role on the reactivation of the Tianshan Mountains associated with the India-Eurasia collision.     

Imaging the Indian subcontinent beneath the Himalaya

The rocks of the Indian subcontinent are last seen south of the Ganges before they plunge beneath the Himalaya and the Tibetan plateau. They are next glimpsed in seismic reflection profiles deep beneath southern Tibet, yet the surface seen there has been modified by processes within the Himalaya that have consumed parts of the upper Indian crust and converted them into Himalayan rocks. The geometry of the partly dismantled Indian plate as it passes through the Himalayan process zone has hitherto eluded imaging. Here we report seismic images both of the decollement at the base of the Himalaya and of the Moho (the boundary between crust and mantle) at the base of the Indian crust. A significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths. North of the Himalaya, the lower Indian crust is characterized by a high-velocity region consistent with the formation of eclogite, a high-density material whose presence affects the dynamics of the Tibetan plateau.     

The subducted slab of Yangtze continental block beneath the Tethyan orogen in western Yunnan

The western Yunnan area is a natural laboratory with fully developed and best preserved Tethyan orogen in the world. Seismic tomography reveals a slab-like high velocity anomaly down to 250 km beneath the western Yunnan Tethyan orogen, to its west there is a low-velocity column about 300 km wide. In the region from Lancangjiang to Mojiang an obvious low velocity in the lower crust and uppermost mantle overlies on the slab. Synthesizing the available geological and geochemical results, the present paper demonstrates that this slab-like high velocity anomaly is a part of the subducted plate of Yangtze continental segment after the closure of Paleotethys. The collision of India and Eurasia continent starting from 50–60 MaBP might trigger thermal disturbance in the upper mantle and cause the uprising of asthenosphere, in that case the subducted Yangtze plate could be broken off, causing Cenozoic magmatic activities and underplating in the Lancangjiang-Mojiang region.     

The subducted slab of Yangtze Continental block beneath the Tethyan orogen in western Yunnan

The western Yunnan area is a natural laboratory with fully developed and best preserved Tethyan orogen in the world. Seismic tomography reveals a slab-like high velocity anomaly down to 250 km beneath the western Yunnan Tethyan orogen, to its west there is a low-velocity column about 300 km wide. in the region from Lancangjiang to Mojiang an obvious low velocity in the lower crust and uppermost mantle overlies on the slab. Synthesizing the available geological and geochemical results, the present paper demonstrates that this slab-like high velocity anomaly is a part of the subducted plate of Yangtze Continental segment after the closure of Paleotethys. The collision of India and Eurasia continent starting from 50-60 MaBP might trigger thermal disturbance in the upper mantle and cause the uprising of asthenosphere, in that case the subducted Yangtze plate could be broken off, causing Cenozoic magmatic activities and underplating in the Lancangjiang-Mojiang region.     

鸟取MJMA 7.3级地震震源区地震层析成像研究

采用2000年日本鸟取MJMA7.3级地震的907个余震及其地方震的24 756个P波和22 547个S波到时,确定鸟取地震震源区的P波、S波和泊松比的三维结构.在震源区地震波速变化幅度达4%,泊松比变化幅度达9%.在11 km深度发现高波速和低泊松比异常,表明这一部位为刚性的粗糙断层面,形成断裂带的锁定结构,它的破裂产生鸟取地震的主震.低波速和高泊松比异常在震源区之下的地壳中出现,这与菲律宾板块俯冲带的脱水过程和岩浆活动有关,导致Daisen弧后火山的形成.流体运动和岩浆活动,对鸟取地震的成核和破裂过程有重大的影响.     

下地幔可以发生地震吗?

大部分的地震发生在比较浅的位置,但是部分地震的深度可以达到600 km以上.目前已有的破裂机制预测地震只存在于上地幔中,但是一些地震的初步定位深度却可以超过670 km(1998年2月9号Okhotsk海地震,地震目录中被定为678 km).研究表明大多数浅震是由于断层失稳,沿着已有的断层面突然滑动引发的,而深震的发震机制目前尚无定论,尤其是下地幔中是否发生地震对于研究深震机制可以提供重要的约束.因为地球中的横向不均匀性,地震的绝对深度有较大的误差,所以我们根据观测地震图中明显的三重值特征,对上述那次深地震进行波形模拟,把相对深度和绝对深度结合起来,最终确定地震实际发生在670公里间断面以上,因此我们认为目前目录中那些最深的地震仍然发生在上地幔中.     

Seismic P-wave tomography in eastern Tibet: Formation of the rifts

For better studying the relationship between the rifts and deep structure, a detailed P-wave velocity structure under eastern Tibet has been modeled using 4767 arrival times from 169 teleseismic events recorded by 51 portable stations. In horizontal slices through the model, a prominent low-velocity anomaly was detected under the rifts from the surface to a depth of ~250 km; this extends to a depth of ~400 km in the vertical slice. This low-velocity anomaly is interpreted as an upper mantle upwelling. The o...     

强震孕育过程中地震类型的动态变化及实验研究

 讨论了1965年以来云南地区强震孕育过程中地震类型的动态变化,发现:云南地区发生6.7级以上强震前数月至数年,云南省内均有2次以上5级双震或群震型地震发生;部分强震前数月至数年,至少有3次以上前主震型地震密集发生.强震发生在这些前主震、双震、群震型地震150 km以外地区.典型地区地震类型的追踪研究也表明:宁蒗地区发生5级双震、前主震型地震以及腾冲地区发生6级群震型地震,云南地区均发生6.5级和7级以上强震,但地点远在宁蒗、腾冲200 km以外地区.实验中,随着载荷的增大,岩样的破裂类型经历了主余破、孤立破→群破→前主破→破裂类型多元化的过程.震例和实验研究结果的对照显示:地震的类型不仅与发震地区地质构造有关,也与地壳介质所处的应力状态有关.     

安宁河裂谷的“活化”

安宁河裂谷位于"攀西裂谷"的中轴部位,近期研究发现,裂谷区有显著的地壳引张作用,存在地幔隆起与热对流。通过野外实地调查,结合地球物理、断裂、沉积、地震、地应力和地形变等资料,认为安宁河裂谷区现在正处于明显的构造活化状态。这种活化受上地壳塑性流动产生的水平挤压和地幔隆起产生的应力张量的共同作用。据此提出一个新的裂谷活化地壳力源模型,认为在研究区特定的地球动力环境下,以安宁河裂谷为中心,再现岩浆活动和地壳裂陷并非不可能。     

Origin of the Changbai intraplate volcanism in Northeast China： Evidence from seismic tomography

Seismic images of the mantle beneath the active Changbai intraplate volcano in Northeast China determined by teleseismic travel time tomography are presented. The data are measured at a new seismic network consisting of 19 portable stations and 3 permanent stations. The results show a columnar low-velocity (-3%) anomaly extending to 400 km depth under the Changbai volcano. High velocity anomalies are visible in the mantle transition zone, and deep earthquakes occur at depths of 500--600 km under the region,suggesting that the subducting Pacific slab is stagnant in the transition zone, as imaged clearly also by global tomography.These results suggest that the Changbai intraplate volcano is not a hotspot like Hawaii but a kind of back-arc volcano related to the upwelling of hot asthenospheric materials associated with the deep subduction and stagnancy of the Pacific slab under northeast Asia.     

Deep structure at northern margin of Tarim Basin

In this paper, a 2D velocity structure of the crust and the upper mantle of the northern margin of the Tarim Basin （TB） has been obtained by ray tracing and theoretical seismogram calculation under the condition of 2D lateral inhomogeneous medium using the data of seismic wide angle reflection/refraction profile from Baicheng to Da Qaidam crossing the Kuqa Depression （KD） and Tabei Uplift （TU）. And along the Baicheng to Da Qaidam profile, 4 of the 10 shot points are located in the northern margin of the TB. The results show that the character of the crust is uniform on the whole between the KD and TU, but the depth of the layers, thickness of the crust and the velocity obviously vary along the profile. Thereinto, the variation of the crust thickness mainly occurs in the middle and lower crust. The Moho has an uplifting trend near the Baicheng shot point in KD and Luntai shot point in TU, and the thickness of the crust reduces to 42 km and 47 km in these two areas, respectively. The transition zone between the KD and TU has a thickest crust, up to 52 km. In this transition zone, there are high velocity anoma- lies in the upper crust, and low velocity anomalies in the lower crust, these velocity anomalies zone is near vertical, and the sediment above them is thicker than the other areas. According to the velocity distributions, the profile can be divided into three sections： KD, TU and transition zone between them. Each section has a special velocity structural feature, the form of the crystalline basement and the relationship between the deep structure and the shallow one. The differences of velocity and tectonic between eastern and western profile in the northern margin of the Tarim Basin （NMTB） may suggest different speed and intensity of the subduction from the Tarim basin to the Tianshan orogenic belt （TOB）.     

Three-dimensional deformation caused by the Bam, Iran, earthquake and the origin of shallow slip deficit

Our understanding of the earthquake process requires detailed insights into how the tectonic stresses are accumulated and released on seismogenic faults. We derive the full vector displacement field due to the Bam, Iran, earthquake of moment magnitude 6.5 using radar data from the Envisat satellite of the European Space Agency. Analysis of surface deformation indicates that most of the seismic moment release along the 20-km-long strike-slip rupture occurred at a shallow depth of 4-5 km, yet the rupture did not break the surface. The Bam event may therefore represent an end-member case of the 'shallow slip deficit' model, which postulates that coseismic slip in the uppermost crust is systematically less than that at seismogenic depths (4-10 km). The InSAR-derived surface displacement data from the Bam and other large shallow earthquakes suggest that the uppermost section of the seismogenic crust around young and developing faults may undergo a distributed failure in the interseismic period, thereby accumulating little elastic strain.     

Locating seismic scatterers at the base of the mantle beneath eastern Tibet with PKIKP precursors

Clear PKIKP precursors were observed from the Lanzhou CTBTO seismic array.We measured their incident angles,arriving azimuths and differential travel times with respect to the PKIKP arrivals using array analysis techniques.These measurements allowed us to locate the scatterers that generated the observed precursors.We found that the scatterers are located in the lowermost mantle beneath eastern Tibet,which is featured by a high-velocity anomaly based on previous tomographic studies.The high velocity anomaly...