Seismic Activity Around and Under Krakatau Volcano,Sunda Arc: Constraints to the Source Region of Island Arc Volcanics

There is general agreement that calc-alkaline volcanic rocks at convergent plate margins are genetically related to the process of subduction (Ringwood, 1974; Maaloe and Petersen, 1981; Hawkesworth et al., 1997). However, opinions on the mode and site of generation of primary magma for island arc volcanism differ substantially. The site of generation of calc-alkaline magma is thought to be either in the mantle wedge (Plank and Langmuir, 1988; McCulloch and Gamble, 1991) or in the subducting slab (White and Dupré, 1986; Defant and Drummond, 1990; Edwards et al., 1993; Ryan and Langmuir, 1993). We present seismological evidence in favour of the latter concept. A distinctive seismicity pattern around and under the Krakatau volcano was identified during systematic studies of the SE Asian convergent plate margins by means of global seismological data. A column-like cluster of events, probably associated with the dynamics of the volcano, is clearly separated from the events in the Wadati-Benioff zone. The accuracy of hypocentral determinations of the events of the cluster does not differ from the accuracy of the events belonging to the subducting slab. The depths of the cluster events vary from very shallow to about 100 km without any apparent discontinuity. On the other hand, there is a pronounced aseismic gap in the Wadati-Benioff zone directly beneath the volcano at depths between 100-150 km. The Krakatau cluster connects this aseismic gap to the volcano at the surface. The pervasive occurrence of earthquakes in the continental wedge between the subducting slab and the Earth surface bears witness to the brittle character of the continental lithosphere and casts doubt on the existence of large-scale melting of mantle material. The aseismic gap (Hanu and Vank, 1985), interpreted by us as a partially melted domain occurring in subducted slabs in practically all active subduction zones that reach depths greater than 100 km, is here used as evidence for the location of the primary source region of island arc volcanics in the subducting plate.     

对日本俯冲带与IBM俯冲带俯冲特征的地球物理研究:来自重力与震源分布数据的启示

日本俯冲带与IBM俯冲带位于太平洋板块、菲律宾海板块和欧亚板块三者的交汇地带,是典型的"俯冲工厂"地区,具有重要的研究意义.本文利用震源分布资料与卫星重力数据对日本俯冲带与IBM俯冲带进行了研究.通过空间重力异常反映了俯冲带地区的区域构造形态,在此基础上基于艾利模式计算了均衡异常以反映地壳均衡特征.利用震源分布资料,分别从垂直俯冲带走向与沿俯冲带走向划定了横截剖面(cross-sections)进行了地震提取,讨论了俯冲带地区的Wadati-Benioff带形态特征,并借助于俯冲带地震等深线图直观描述了俯冲带的俯冲形态.在日本俯冲带与伊豆—小笠原俯冲带各选取了一条典型剖面进行了重力2.5D反演,研究了俯冲带地区的壳幔结构特征.研究结果表明,九州—帕劳海脊与IBM岛弧在均衡异常上存在差异,前者已逐渐趋向于地壳均衡.IBM的Wadati-Benioff带存在明显的南北差异,反映出伊豆—小笠原俯冲板片停留在了660km转换带中,而马里亚纳俯冲板片很可能垂直穿过了这一转换带,造成这种南北差异的原因与板块相对运动、岩石圈黏性和年龄差异以及俯冲板片的重力效应等因素有关.在IBM的中部和南部存在板片撕裂现象.日本俯冲带的俯冲洋壳密度随俯冲深度变化较小,洋幔存在一定程度的蛇纹岩化,地幔楔蛇纹岩化作用不典型,海沟处有一范围较小的含水畸变带;伊豆—小笠原俯冲带俯冲洋壳密度随深度增大而明显增大,洋幔蛇纹岩化程度较日本俯冲带低,地幔楔蛇纹岩化作用强烈,板块交汇处存在明显的蛇纹岩底辟.日本俯冲带与IBM俯冲带一线自北向南板片俯冲变陡,两侧板块耦合度降低,与俯冲带两侧的板块运动速率差异有关.     

Seismic properties in the eastern part of the South Aegean volcanic arc

The South Aegean active volcanic arc lies along the 150-km seismic isodepth of a Benioffzone and consists in andesitic, dacitic and rhyolitic volcanoes of an orogenic calc-alkaline type. In the eastern part of the arc there are two main volcanic sites in the Nisyros and Kos islands. High shallow and intermediate depth seismic activity occurs in this volcanic area. Seismological data concerning the 1911–1980 period have been used to investigate seismic properties in that area. Two, distinct regions with different seismotectonic features have been defined. The internal region (Nisyros active volcano and its proximity) is characterized by a shallow, thin seismogenetic layer with abnormally highb-value, locally concentrated stresses, low seismicity and highly heterogeneous structure. These features are probably due to a magmatic body intruded in shallow depths within the crust. The possibility of magma formation in the upper surface of the descending slab or within the upper mantle wedge overlying this slab in the Nisyros-Kos area is herein discussed. On the contrary, the remaining (external) region, including the non-active volcanic island of Kos, is characterized by normal for tectonic shocksb-value, thick seismogenetic layer and high seismicity. It seems that in this region there is not a magmatic intrusion at least in shallow depths within the crust.     

Deep structure,volcanism and Wadati-Benioff zone of the northwestern pacific convergent margin

The morphology of the Wadati-Benioff zone in the region of Kamchatka, the Kurile Islands and Hokkaido, based on the distribution of 6319 earthquake foci, has verified the existence of an intermediate-depth aseismic gap and its relation to active andesitic volcanism. It appeared that deep-focus earthquakes in this region belong to a paleosubduction zone activated by an intermediate-depth collision with the active subduction zone in the area of Hokkaido. A system of deep seismically active fracture zones was delineated in the continental plate and confirmed by the results of deep seismic sounding. Two of these fractures, dipping toward the subduction zone, may be considered as the principal feeding channels for active and Holocene volcanoes of the continental volcanic bels of Kamchatka.     

Recent geodynamic evolution of convergent plate margins and the reconstruction of fossil plate boundaries

Summary The discovery of paleoplates buried in the upper mantle leads to an interpretation of the subduction as a discontinuous process running in cycles and shifting the place of its operation in or against the direction of ocean floor spreading. This mechanism explains the distribution of calc-alkaline volcanism of different age in fossil convergent plate boundaries. The establishment of regular spatial correlation of the aseismic gap in the Wadati-Benioff zones with the distribution of calc-alkaline volcanism enables to reconstruct fossil plate boundaries and to define allochtonous terranes in apparently homogeneous continental plates. The hampering effect of the ocean floor morphology and of the fragments of continental plates approaching the trench, which substantially influences the rates of subduction and the geodynamic history of active continental margins in different domains along the trench, allows us to understand the complicated geological development of continental wedges in fossil convergent plate margins. The establishment of the segmented nature of active subduction zones and the dramatic morphology of the lower limit of the active subducted slab along the trench help us to interpret extensive lateral gaps in volcanic chains overlying active as well as fossil subduction zones.     

Nishinoshima volcano in the Ogasawara Arc: New continent from the ocean?

Nishinoshima, a submarine volcano in the Ogasawara Arc, approximately 1 000 km south of Tokyo, Japan, suddenly erupted in November 2013, after 40 years of dormancy. Olivine‐bearing phenocryst‐poor andesites found in older submarine lavas from the flanks of the volcano have been used to develop a model for the genesis of andesitic lavas from Nishinoshima. In this model, primary andesite magmas originate directly from the mantle as a result of shallow and hydrous melting of plagioclase peridotites. Thus, it only operates beneath Nishinoshima and submarine volcanoes in the Ogasawara Arc and other oceanic arcs, where the crust is thin. The primary magma compositions have changed from basalt, produced at considerable depth, to andesite, produced beneath the existing thinner crust at this location in the arc. This reflects the thermal and mechanical evolution of the mantle wedge and the overlying lithosphere. It is suggested that continental crust‐like andesitic magma builds up beneath submarine volcanoes on thin arc lithosphere today, and has built up beneath such volcanoes in the past. Andesites produced by this shallow and hydrous melting of the mantle could accumulate through collisions of plates to generate continental crust.     

Earthquake distribution and volcanism in Kamchatka,Kurile Islands,and Hokkaido Part 1: Kamchatka and northern Kuriles

Summary The morphology of the Wadati-Benioff zone in the region of Kamchatka and Northern Kuriles, based on the distribution of 1102 earthquake foci, verified the existence of an intermediate depth aseismic gap and its relation to active andesitic volcanism. A system of deep seismically active fracture zones, genetically connected with the process of subduction, was delineated in the continental plate and confirmed by the results of deep seismic sounding. Two of these fractures, dipping toward the subduction zone, may be considered as the principal feeding channels for active and Holocene volcanoes of the continental volcanic belts of Kamchatka.     

Influence of fluids and magma on earthquakes: seismological evidence

In this paper, we present seismological evidence for the influence of fluids and magma on the generation of large earthquakes in the crust and the subducting oceanic slabs under the Japan Islands. The relationship between seismic tomography and large crustal earthquakes (M=5.7-8.0) in Japan during a period of 116 years from 1885 to 2000 is investigated and it is found that most of the large crustal earthquakes occurred in or around the areas of low seismic velocity. The low-velocity zones represent weak sections of the seismogenic crust. The crustal weakening is closely related to the subduction process in this region. Along the volcanic front and in back-arc areas, the crustal weakening is caused by active volcanoes and arc magma resulting from the convective circulation process in the mantle wedge and dehydration reactions in the subducting slab. In the forearc region of southwest Japan, fluids are suggested in the 1995 Kobe earthquake source zone, which have contributed to the rupture nucleation. The fluids originate from the dehydration of the subducting Philippine Sea slab. The recent 2001 Geiyo earthquake (M=6.8) occurred at 50 km depth within the subducting Philippine Sea slab, and it was also related to the slab dehydration process. A detailed 3D velocity structure is determined for the northeast Japan forearc region using data from 598 earthquakes that occurred under the Pacific Ocean with hypocenters well located with SP depth phases. The results show that strong lateral heterogeneities exist along the slab boundary, which represent asperities and results of slab dehydration and affect the degree and extent of the interplate seismic coupling. These results indicate that large earthquakes do not strike anywhere, but only anomalous areas which can be detected with geophysical methods. The generation of a large earthquake is not a pure mechanical process, but is closely related to physical and chemical properties of materials in the crust and upper mantle, such as magma, fluids, etc.     

Morphology of the Wadati-Benioff zone,andesitic volcanism,and active fracture zones in Central America

Summary The morphology of the Wadati-Benioff zone in the region of Central America, based on the distribution of 1377 earthquake foci, verified the existence of an intermediate aseismic gap and its relation to active andesitic volcanism, and the non-uniformity of subduction due to the hampering effect of the main structural features of the subducting Cocos plate. Four deep seismically active fracture zones, genetically connected with the process of subduction, and three fracture zones manifesting the possible boundary between the Americas and Caribbean plates were identified in the continental wedge.     

Dissecting large earthquakes in Japan: Role of arc magma and fluids

We synthesized information from recent high-resolution tomographic studies of large crustal earthquakes which occurred in the Japanese Islands during 1995–2008. Prominent anomalies of low-velocity and high Poisson's ratio are revealed in the crust and uppermost mantle beneath the mainshock hypocenters, which may reflect arc magma and fluids that are produced by a combination of subducting slab dehydration and corner flow in the mantle wedge. Distribution of 164 crustal earthquakes ( M 5.7–8.0) that occurred in Japan during 1885–2008 also shows a correlation with the distribution of low-velocity zones in the crust and uppermost mantle. A qualitative model is proposed to explain the geophysical observations recorded so far in Japan. We consider that the nucleation of a large earthquake is not entirely a mechanical process, but is closely related to the subduction dynamics and physical and chemical properties of materials in the crust and upper mantle; in particular, the arc magma and fluids.     

Nests of intermediate depth (70–160 km) earthquakes adjacent to active volcanoes during 1963–1982

Uniform maps of circumpacific convergence zones constructed by the same projection and scale reveal three main associations between intermediate depth seismicity and contemporaneous volcanic activity. First, in areas of high seismicity there are usually few active volcanoes. Second, in areas of low to moderate seismicity aseismic domains can often be recognized beneath the active volcanoes. Third, adjacent to the aseismic domains there are nests of seismicity located a few 10's of km away from the active volcanoes usually in the direction of the trench. More than 40 such nests are identified in the circumpacific and Indonesian arcs. These three associations may be related to melting near the upper surface of the underthrust slab. A fluid phase, either silicate melt or a hydrous fluid released by dehydration of the slab, would lower the strength of the slab and create aseismic zones beneath the active volcanoes. Nests of earthquakes can develop by stress concentration at the margins of the weak zones. Areas not volcanically active have little or no fluid phase and a higher level of seismicity.     

New advances of seismic tomography and its applications to subduction zones and earthquake fault zones: A review

Abstract There have been significant advances in the theory and applications of seismic tomography in the last decade. These include the refinements in the model parameterization, 3-D ray tracing, inversion algorithm, resolution and error analyses, joint use of local, regional and teleseismic data, and the addition of converted and reflected waves in the tomographic inversion. Applications of the new generation tomographic methods to subduction zones have resulted in unprecedentedly clear images of the subducting oceanic lithosphere and magma chambers in the mantle wedge beneath active arc volcanoes, indicating that geodynamic systems associated with the arc magmatism and back-arc spreading are related to deep processes, such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab. High-resolution tomographic imagings of earthquake fault zones in Japan and California show that rupture nucleation and earthquake generating processes are closely related to the heterogeneities of crustal materials and inelastic processes in the fault zones, such as the migration of fluids. Evidence also shows that arc magmatism and slab dehydration may also contribute to the generation of large crustal earthquakes in subduction regions.     

Earthquake distribution and volcanism in Kamchatka,Kurile Islands,and Hokkaido

Summary The morphology of the Wadati-Benioff zone in the region of Central Kuriles, based on the distribution of 1202 earthquake foci, verified the existence of an intermediate depth aseismic gap and its relation to active andesitic volcanism. Several deep seismically active fracture zones of different tectonic function were delineated in the continental plate and confirmed by the results of deep seismic sounding. A seismically active fracture zone, found in front of the Kamchatka-Kurile trench, indicates the segmented nature of fracturing of the subducting Pacific plate.     

东北日本地震波速度、VP/VS和各向异性结构:对俯冲带水迁移过程的探讨

本文利用区域地震初至波到时数据,通过地震层析成像研究获得了东北日本俯冲带上地幔（深至约150 km）的P波速度（V_P）、S波速度（V_S）、V_P/V_S和P波各向异性结构.结果表明,低速及高V_P/V_S比异常体主要分布在火山下方的下地壳和地幔楔中,其与低频地震的分布吻合,该区域与俯冲板块脱水所释放的流体及其导致的部分熔融密切相关;俯冲的太平洋板块内可能由于脱水脆化导致的双层地震带区域则没有表现出整体的高V_P/V_S值,其可能与俯冲板块内部含水矿物含量有关;俯冲板块内双重地震带区域及上覆地幔楔薄层主要表现为与海沟平行的方位各向异性和正的径向各向异性,其可能是由于含水矿物的脱水使橄榄石晶格结构发生了从A型到B型的变化所引起的.我们研究表明,结合地震波速度和各向异性结构能够加深对俯冲带内水运移过程的认识.     

Sangay volcano, Ecuador: structural development, present activity and petrology

Sangay (5230 m), the southernmost active volcano of the Andean Northern Volcanic Zone (NVZ), sits 130 km above a >32-Ma-old slab, close to a major tear that separates two distinct subducting oceanic crusts. Southwards, Quaternary volcanism is absent along a 1600-km-long segment of the Andes. Three successive edifices of decreasing volume have formed the Sangay volcanic complex during the last 500 ka. Two former cones (Sangay I and II) have been largely destroyed by sector collapses that resulted in large debris avalanches that flowed out upon the Amazon plain. Sangay III, being constructed within the last avalanche amphitheater, has been active at least since 14 ka BP. Only the largest eruptions with unusually high Plinian columns are likely to represent a major hazard for the inhabited areas located 30 to 100 km west of the volcano. However, given the volcano's relief and unbuttressed eastern side, a future collapse must be considered, that would seriously affect an area of present-day colonization in the Amazon plain, 30 km east of the summit. Andesites greatly predominate at Sangay, there being few dacites and basalts. In order to explain the unusual characteristics of the Sangay suite—highest content of incompatible elements (except Y and HREE) of any NVZ suite, low Y and HREE values in the andesites and dacites, and high Nb/La of the only basalt found—a preliminary five-step model is proposed: (1) an enriched mantle (in comparison with an MORB source), or maybe a variably enriched mantle, at the site of the Sangay, prior to Quaternary volcanism; (2) metasomatism of this mantle by important volumes of slab-derived fluids enriched in soluble incompatible elements, due to the subduction of major oceanic fracture zones; (3) partial melting of this metasomatized mantle and generation of primitive basaltic melts with Nb/La values typical of the NVZ, which are parental to the entire Sangay suite but apparently never reach the surface and subordinate production of high Nb/La basaltic melts, maybe by lower degrees of melting at the periphery of the main site of magma formation, that only infrequently reach the surface; (4) AFC processes at the base of a 50-km-thick crust, where parental melts pond and fractionate while assimilating remelts of similar basaltic material previously underplated, producing andesites with low Y and HREE contents, due to garnet stability at this depth; (5) low-pressure fractionation and mixing processes higher in the crust. Both an enriched mantle under Sangay prior to volcanism and an important slab-derived input of fluids enriched in soluble incompatible elements, two parameters certainly related to the unique setting of the volcano at the southern termination of the NVZ, apparently account for the exceptionally high contents of incompatible elements of the Sangay suite. In addition, the low Cr/Ni values of the entire suite—another unique characteristic of the NVZ—also requires unusual fractionation processes involving Cr-spinel and/or clinopyroxene, either in the upper mantle or at the base of the crust.     

Geochemistry and origin of Archean volcanic rocks from the Upper Keewatin assemblage (ca 2.7 Ga), Lake of the Woods Greenstone Belt, Western Wabigoon Subprovince, Superior Province, Canada

Abstract   Volcanic rocks from the Upper Keewatin assemblage ( ca 2720 Ma) were geochemically classified into five groups; komatiites, tholeiitic rocks having near-flat primitive mantle-normalized abundance patterns, Nb-enriched basalts and andesites (NEBA) plus normal calc-alkaline (NCA) rocks, adakites and shoshonites. The adakites having [La/Yb]_N >30 and <30 were probably derived from felsic magmas formed by partial melting of a subducted slab at relatively greater and smaller depths, respectively. Ascending adakite magmas, by interaction with the overlying mantle wedge, decreased in Al₂O₃ / Y ratio and selectively lost high-field strength elements, thereby forming mantle sources for both NEBA + NCA and shoshonite magmas. Under the influence of a mantle plume, the source of komatiites, the NEBA + NCA magmas were generated from that part of the mantle wedge metasomatized by adakite magmas having [La / Yb]_N <30, and tholeiitic magmas from unmetasomatized part of the same mantle wedge. Magmas of both adakites having [La / Yb]_N >30 and shoshonites were generated in a normal Archean Arc system setting.     

Discontinuous Nature of the Lower Part of the South American Wadati-Benioff Zone in The Arica Elbow Region

The Arica Elbow region represents that part of Andean South America where the azimuth of the strike of the Peru-Chile trench changes from 150° to 190°. The area under study is roughly bounded by latitudes 17 °S and 23 °S, The shape of the Wadati-Benioff zone was studied in terms of the distribution of ISC hypocentres dated between 1964 and 1993. A system of 22 vertical cross-sections, perpendicular to the trench axis, and a map of epicentres was used to derive the detailed shape of the Wadati-Benioff zone of the presently descending slab. The distribution of earthquake foci indicates a fingerlike shape of the lower part of the Wadati-Benioff zone beneath the aseismic gap. The slab length shows small changes around 350 km in the northern sections, pronounced length oscillations between 350 and 750 km in the neighbouring central sections and a constant value of 650 km in the southern sections. The dip and thickness of the Wadati-Benioff zone are practically constant in all sections. Fault plane solutions, separated spatially into three zones, were used to estimate the state of stress in the slab.     

东北地区660km间断面附近波速结构研究

我国东北地区位于西北太平洋和达-贝尼奥夫俯冲带前缘,其深部速度结构对理解板块俯冲行为以及地幔物质的交换有重要意义.利用区域三重震相模拟方法,对中国地震观测台网记录到的两个深源地震P波和SH波波形数据,进行了相对到时和波形的拟合,获得了我国东北地区660 km间断面附近波速结构.结果表明,研究区域下方的间断面没有发生明显...     

Magmatic response to early aseismic ridge subduction: the Ecuadorian margin case (South America)

A geochemical and isotopic study of lavas from Pichincha, Antisana and Sumaco volcanoes in the Northern Volcanic Zone (NVZ) in Ecuador shows their magma genesis to be strongly influenced by slab melts. Pichincha lavas (in fore arc position) display all the characteristics of adakites (or slab melts) and were found in association with magnesian andesites. In the main arc, adakite-like lavas from Antisana volcano could be produced by the destabilization of pargasite in a garnet-rich mantle. In the back arc, high-niobium basalts found at Sumaco volcano could be produced in a phlogopite-rich mantle. The strikingly homogeneous isotopic signatures of all the lavas suggest that continental crust assimilation is limited and confirm that magmas from the three volcanic centers are closely related. The following magma genesis model is proposed in the NVZ in Ecuador: in fore arc position beneath Pichincha volcano, oceanic crust is able to melt and produces adakites. En route to the surface, part of these magmas metasomatize the mantle wedge inducing the crystallization of pargasite, phlogopite and garnet. In counterpart, they are enriched in magnesium and are placed at the surface as magnesian andesites. Dragged down by convection, the modified mantle undergoes a first partial melting event by the destabilization of pargasite and produces the adakite-like lavas from Antisana volcano. Lastly, dragged down deeper beneath the Sumaco volcano, the mantle melts a second time by the destabilization of phlogopite and produces high-niobium basalts. The obvious variation in spatial distribution (and geochemical characteristics) of the volcanism in the NVZ between Colombia and Ecuador clearly indicates that the subduction of the Carnegie Ridge beneath the Ecuadorian margin strongly influences the subduction-related volcanism. It is proposed that the flattening of the subducted slab induced by the recent subduction (<5 Ma?) of the Carnegie Ridge has permitted the progressive warming of the oceanic crust and its partial melting since ca. 1.5 Ma. Since then, the production of adakites in fore arc position has deeply transformed the magma genesis in the overall arc changing from ‘typical’ calc-alkaline magmatism induced by hydrous fluid metasomatism, to the space- and time-associated lithology adakite/high-Mg andesite/adakite-like andesite/high-Nb basalts characteristic of slab melt metasomatism.     

深部俯冲板片三维构造重建及其几何学、运动学研究——以汤加—克马德克地区俯冲板片为例

西南太平洋板块与澳大利亚板块之间的汤加—克马德克俯冲带,是研究地球动力学最重要的区域之一.本文研究根据MIT-P08地震数据,结合板块构造边界、地震活动分布、海岸地形数据等,基于GOCAD软件平台建立三维地震层析成像,对西南太平洋板块的汤加—克马德克俯冲板片进行三维解释.地震层析成像显示汤加—斐济地区地幔至少存在三个"高速"异常体.早期汤加—克马德克俯冲板片穿过地幔转换带,并进入下地幔,最大深度达1600 km.三维构造模型揭示了汤加—克马德克板片在深度600~800 km处存在断折形变,该俯冲板片去褶皱恢复后,测量其俯冲的最大位移达2600 km.汤加—克马德克板片开始快速俯冲的时间至少在30 Ma之前,平均移动速率约为68~104 mm /a.俯冲板片三维构造重建和恢复,可以有效揭示俯冲板片几何学、运动学,为研究深源地震成因、地球深部变化过程和动力学机制提供约束.