sediment subduction a probable key for seismicity and tectonics at active plate boundaries summary a model involving extensive occurrence of sediment subduction and viscous interaction of lithospheric plates at convergent zones is applied to derive simple relations between extremal values of seismic and global tectonic parameters the strength of mechanical coupling at the interface zone is defined as the maximum shear stress at the base of the over thrusting plate a test of these relations with the data set on great thrust earthquakes supports this model moreover it reveals independently of previous studies the main contrasting modes of subduction stress regimes continental island arc types and chilean mariana types an analysis of asperity categories in the framework of this model leads to the conclusion that the asperity type and probably the fault area of great earthquakes depends on the strength of coupling which in its turn is governed by global tectonics and the amount of subducted sediments the approach also yields a physically reasonable dependence of the seismic slip on the plate parameters which is in agreement with the data now available

exhumation and subduction erosion in orogenic wedges insights from numerical models 1 at oceanic margins syn convergent exhumation subduction erosion and inter plate coupling are intimately related but ample questions remain concerning their interaction and individual mechanisms to analyze these interactions for a thick skinned visco elastic wedge we focus on properly modeling stresses energies and topographies at the inter plate and wedge bounding interfaces using a coulomb frictional contact algorithm in this innovative plane strain free surface lagrangian finite element model fault dynamics is modulated by retreating subduction subduction is dynamically driven by slab pull due to a slab sinking in a semi analytic computationally favorable approximation of three dimensional induced mantle flow nodal trajectories show that continuous underthrusting of a slab induces a steady state corner flow through forced underplating and subsequent trenchward extrusion due to gravitational spreading this flow pattern confirms early proposed models of syn orogenic deep seated rock exhumation propelled by coexisting extension and continuous shortening at depth a distinct reduction in upward flowing material and accompanying decrease of exhumation velocities to millimeters per year as observed in nature is induced by a diversion of orogenic wedge material toward the mantle once a subduction channel is formed the key parameter affecting model evolution and spontaneous formation of a subduction channel is basal friction which modulates the amount of erosion however formation of a subduction channel entrance needs to be ensured through the deformability of the overriding plate which is influenced by applied pressure at the overriding plate tip and material properties the down dragging of the overriding plate is sufficient above a threshold inter plate shear stress of about 2 7 mpa

layered and laterally constrained 2d inversion of resistivity data in a sedimentary environment quasi layered models often can represent the actual geology more accurately than smooth minimum structure models we present a 2d inversion scheme with lateral constraints and sharp boundaries lci for continuous resistivity data all data and models are inverted as one system producing layered solutions with laterally smooth transitions the models are regularized through lateral constraints that tie interface depths or thicknesses and resistivities of adjacent layers a priori information used to resolve ambiguities and to add for example geological information can be added at any point of the profile and migrates through the lateral constraints to parameters at adjacent sites similarly information from areas with well resolved parameters migrates through the constraints to help resolve areas with poorly constrained parameters the estimated model is complemented by a full sensitivity analysis of the model parameters supporting quantitative evaluation of the inversion result a simple synthetic model proves the need for a quasilayered 2d inversion when compared with a traditional 2d minimum structure inversion a 2d minimum structure inversion produces models with spatially smooth resistivity transitions making identification of layer boundaries difficult a continuous vertical electrical sounding field example from sweden with a depression in the depth to bedrock supports the conclusions drawn from the synthetic example a till layer on top of the bedrock hidden in the traditional inversion result is identified using the 2d lci scheme furthermore the depth to the bedrock surface is easily identified for most of the profile with the 2d lci model which is not the case with the model from the traditional minimumstructure inversion

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

laterally constrained inversion of ground roll from seismic reflection records seismic reflection data contain surface waves that can be processed and interpreted to supply shear wave velocity models along seismic reflection lines the coverage of seismic reflection data allows the use of automated multifold processing to extract high quality dispersion curves and experimental uncertainties in a moving spatial window the dispersion curves are then inverted using a deterministic laterally constrained inversion to obtain a pseudo 2d model of the shear wave velocity a monte carlo global search inversion algorithm optimizes the parameterization when the strategy is used with synthetic and field data consistent final models with smooth lateral variations are successfully retrieved this method constitutes an improvement over the individual inversion of single dispersion curves

effectiveness of geophysical surveys for water wells relocation in overexploited aquifers the example of maggiore and traversola valleys northwestern italy aquifer overexploitation is a common problem in drinking water management this will become more and more important given the general reduction of water resources to overcome overexploitation one of the most adopted solutions is the relocation of extraction wells to establish the positions of new wells a precise knowledge of the hydrogeological setting is required specific surveys are therefore necessary to obtain information over wide investigation zones geophysical methods particularly electromagnetic and electrical can be useful with this aim in the present paper a case history on the combined use of electric resistivity tomography and time domain electromagnetic soundings is reported surveys have been performed within the maggiore and traversola valleys to investigate the uppermost part of the quaternary deposits hosting the near surface aquifer the electromagnetic data have been inverted with a spatially constrained approach by assuming a quasi 1d model of the subsurface geophysical surveys allowed for depicting the depth and lateral continuity of the supposed aquifer level in the surveyed area up to a depth of about 200 m and proposing potential positions for well relocation

subduction zones an introduction to comparative subductology abstract the subduction model for trench arc back arc systems was deduced more or less as a logical consequence of sea floor spreading many features in these systems such as thrust type inter plate earthquakes and deep earthquakes along wadati benioff zones are readily explained by the subduction model but other features such as extensional spreading and high heat flow in back arc regions and arc volcanism are difficult to explain an approach to solve these problems may be to recognize the existence of the two basically different modes of subduction one chilean type causing a compressional stress regime in the arc and back arc regions the other mariana type causing a tensional stress regime implications of the two types of the mode of subduction for some tectonic problems such as vertical movement and sediment accretion at trenches are discussed with special reference to the results of recent dsdp active margin drilling in discussing the possible causes of the two modes it is suggested that the large scale thermal and stress regimes in the back arc areas may not be the direct products of simple subduction but of some additional factors involving much greater energy such as mantle flow associated with the motion of major plates finally the possible great importance of various types of collision accretion and erosion at subduction zones is emphasized

relationship between subduction and seismicity in the mexican part of the middle america trench two catalogs of earthquakes in the mexican part of the middle america trench are analyzed to elucidate principal relations between structure of the subducting cocos plate and seismicity a catalog of historical events that have occurred during the last two centuries with large magnitudes ms 6 0 is used to obtain cumulative seismic moment mo and seismic moment release rate m 0 distributions along the mexican subduction zone the catalog of instrumentally observed earthquakes from 1963 to 1990 international seismological center and u s geological survey with 4 5 mb 6 0 is applied to investigate background seismicity for the region the strength of coupling between the cocos and north american plates would be expected to grow gradually from the southeast to the northwest according to the variation of convergence rate v and age a of the cocos plate this correlates in general with a steady reduction in background seismicity and a slight average increase of m 0 and seismic energy release rate w at the sites where the main fracture zones of the cocos plate tehuantepec o gorman orozco and rivera undergo the subduction the general correlation breaks down the background seismicity increases at fracture zones while m0 and m 0 decrease significantly this feature is interpreted as a drop of the coupling at the areas where transform faults are being subducted seismic slip rates along the trench obtained from m 0 are lower then the values of plate convergence rates but the average seismic slip is in agreement with the estimates from the v model interaction between lithospheric plates at convergent zones through the viscous layer of subducted sediments variability of m 0 and seismic slip rate in relation with tectonics should be taken into account when the seismic gap model is being used for the prediction of strong earthquakes an examination of space time plots for the historical catalogs supposes a probable tendency of northwest migration of strong events with a rate of 10 km yr

the earthquake deformation cycle recurrence and the time predictable model geodetic and geologic observations from a handful of well studied great plate boundary earthquakes provide a basis for exploring those features of the deformation cycle having the strongest influence on recurrence estimation in ideal circumstances the time predictable model requires knowledge of only the seismic slip and time of the last earthquake and the rate of relative plate motion or fault slip practically indirect measurements must usually suffice local coseismic strain changes rather than seismic slip and local deformation rate rather than slip rate permanent nonrecoverable deformation often roughly half the coseismic strain offset near great thrust earthquakes complicates the inference of interplate seismic slip owing to short and long term postseismic transients commonly 20 40 of the coseismic strain drop deformation rates are variable and not simply related to the plate motion rate at strike slip plate boundaries these complications can sometimes be avoided and seismic slip and slip rate can be obtained more or less directly when these favorable circumstances are absent and when knowledge of the deformation cycle is incomplete recurrence accuracy can be improved by empirically correcting for unknown elements of the cycle systematic features in the spatial distribution of the transients and permanent deformation near subduction zones help identify the regions where these corrections are largest and where the shortcomings of the empirical approach are most severe

seismotectonic constraints on the convergence rate between the rivera and north american plates there are two significantly different types of models for the convergence rate between the rivera and north american plates the first type the high rate model bandy 1992 predicts convergence rates of approximately 5 0 cm yr near the southern end of the rivera north america subduction zone and between 2 0 and 3 0 cm yr at its northern end in contrast the second type the low rate model e g demets and stein 1990 predicts convergence rates of between 2 0 and 3 3 cm yr near the southern end of the rivera north america subduction zone and between 0 6 and 1 7 cm yr at its northern end seismotectonic relationships which relate seismic characteristics of subduction zones maximum magnitudes maximum seismic depths etc to plate tectonic parameters convergence rates age of the oceanic lithosphere etc provide a means of distinguishing between the two different models three such relationships suggest that the rivera north american and cocos north american convergence rates should be roughly equal across the rivera cocos plate boundary favoring the high rate model employing the high rate model one can evaluate the magnitude and distribution of the strike slip component of forearc motion vss produced by oblique convergence between the rivera and north american plates the analysis indicates both a progressive increase and clockwise reorientation of vss northwestward along the plate contact zone of the rivera north america subduction zone such a distribution in vss should produce a northwestward movement of and nw se oriented extension within the interior of the jalisco block consistent with previous proposals of jalisco block motions also such a distribution in vss should produce a slight clockwise rotation of the jalisco block in the vicinity of bahia de banderas consistent with paleomagnetic data

a geophysical and geochemical approach for seawater intrusion assessment in the acquedolci coastal aquifer northern sicily vertical electrical sounding ves surveys and chemical analyses of groundwater have been executed in the coastal plain of acquedolci northern sicily with the aim to circumscribe seawater intrusion phenomena this urbanized area is representative of a more general problem interesting most of mediterranean littoral areas where intensive overdraft favors a heavy seawater intrusion through the coastline aquifer resistivity seems decisively to be conditioned by the chlorine and magnesium content in the main aquifer of the region schlumberger ves together with piezometric and chemical physical information of groundwater allowed us to perform hydrogeological and geophysical elaborations to propose the occurrence of a relatively narrow belt marked by fresh salt water mixing in the considered plain pollution risk studies have been already realized by authors with the proposal to identify by parametric and synthetic zoning of significant hydrogeological elements the most vulnerable sectors in detail an intrinsic vulnerability mapping has been already performed applying the well known sintacs system this paper intends to give in this sector of sicily an example of integration of different methodologies showing the role of geophysics to describe the degradation of aquifers on the whole as well as to assess their pollution risk better

estimation of aquifer hydraulic parameters from surficial geophysical methods a case study of keritis basin in chania crete greece summary knowledge of aquifer parameters is essential for the management of groundwater resources conventionally these parameters are estimated through pumping tests carried out on water wells few boreholes may be available and carrying out pumping tests at a number of sites may be costly and time consuming the application of geophysical methods in combination with pumping tests provides a cost effective and efficient alternative to estimate aquifer parameters a geophysical method is used to obtain aquifer characteristics that are estimated through the pumping tests a correlation is established between these parameters at other sites where pumping has not been carried out in this way the entire investigation area could be covered to characterize an aquifer system this study has been carried out in the keritis basin in chania crete greece where the aquifer characteristics are required for the management of groundwater in the region

the archean bif hosted cuiaba gold deposit quadrilatero ferrifero minas gerais brazil abstract the cuiaba gold deposit is located in the northern part of the quadrilatero ferrifero minas gerais state brazil the region constitutes an archean granite greenstone terrane composed of a basement complex ca 3 2 ga the rio das velhas supergroup greenstone sequence and related granitoids 3 0 2 7 ga which are overlain by the proterozoic supracrustal sequences of the minas the stratigraphy of the cuiaba area is part of the nova lima group which forms the lower part of the rio das velhas supergroup the lithological succession of the mine area comprises from bottom to top lower mafic metavolcanics intercalated with carbonaceous metasedimentary rocks the gold bearing cuiaba banded iron formation bif upper mafic metavolcanics and volcanoclastics and metasedimentary rocks the metamorphism reached the greenschist facies tectonic structures of the deposit area are genetically related to deformation phases d 1 d 2 d 3 which took place under crustal compression representing one progressive deformational event e n the bulk of the economic grade gold mineralization is related to six main ore shoots contained within the cuiaba bif horizon which range in thickness between 1 and 6 m the bif hosted gold orebodies 4 ppm au represent sulfide rich segments of the cuiaba bif which grade laterally into non economic mineralized or barren iron formation transitions from sulfide rich to sulfide poor bif are indicated by decreasing gold grades from over 60 ppm to values below the fire assay detection limit in sulfide poor portions the deposit is gold only and shows a characteristic association of au with ag as sb and low base metal contents the gold is fine grained up to 60 m and is generally associated with sulfide layers occurring as inclusions in fractures or along grain boundaries of pyrite the predominant sulfide mineral 90 vol gold is characterized by an average fineness of 0 840 and a large range of fineness 0 759 to 0 941 the country rocks to the mineralized bif show strong sericite carbonate and chlorite alteration typical of greenschist facies metamorphic conditions textures observed on microscopic to mine scales indicate that the mineralized cuiaba bif is the result of sulfidation involving pervasive replacement of fe carbonates siderite ankerite by fe sulfides gold mineralization at cuiaba shows various features reported for archean gold lode deposits including the 1 association of gold mineralization with fe rich host rocks 2 strong structural control of the gold orebodies showing remarkable down plunge continuity 3 km relative to strike length and width up to 20 m 3 epigenetic nature of the mineralization with sulfidation as the major wall rock alteration and directly associated with gold deposition 4 geochemical signature with mineralization showing consistent metal associations au ag as sb and low base metal which is compatible with metamorphic fluids

the 2006 slow slip event and nonvolcanic tremor in the mexican subduction zone the last decade featured an explosive sequence of discoveries of slow slip events sse and nonvolcanic tremor nvt in different subduction zones and continental faults many observations show that sse is usually associated with an increased nvt activity but it is not clear yet if those events are the result of the same process or are independent expressions of a common underlying seismotectonic source a large sse in central mexico occurred in 2006 during the meso american subduction experiment mase which provided continuous observations of the nvt for the years 2005 2007 gps and abundant seismic data show that although the nvt energy increased notably during the 2006 sse the two phenomena were separated spatially and not completely synchronized in time significant nvt episodes occur during the period between sses suggesting again that large slow slip events and nvt observed in the mexican subduction zone are of different origins the results presented here contribute to uncovering the nature of these two separate phenomena that have been indistinguishable in some other regions

multivariate statistical analysis to investigate the subduction zone parameters favoring the occurrence of giant megathrust earthquakes abstract the observed maximum magnitude of subduction megathrust earthquakes is highly variable worldwide one key question is which conditions if any favor the occurrence of giant earthquakes mw 8 5 here we carry out a multivariate statistical study in order to investigate the factors affecting the maximum magnitude of subduction megathrust earthquakes we find that the trench parallel extent of subduction zones and the thickness of trench sediments provide the largest discriminating capability between subduction zones that have experienced giant earthquakes and those having significantly lower maximum magnitude monte carlo simulations show that the observed spatial distribution of giant earthquakes cannot be explained by pure chance to a statistically significant level we suggest that the combination of a long subduction zone with thick trench sediments likely promotes a great lateral rupture propagation characteristic of almost all giant earthquakes

seismic strain rate along the middle america trench reveals significant differences between cocos north america and cocos caribbean convergence summary centroid moment tensor solutions for thrust faulting earthquakes in the last 27 yr are used to obtain and compare seismic strain rate associated to the subduction process along the cocos north america and along the cocos caribbean convergent margins in both cases the eigenvectors of the strain rate tensor align along radial and tangential directions with respect to their respective euler pole seismic strain rate along the cocos north america plate boundary is about 2 8 10 7 yr 1 a value close to those previously found along the japan and aegean convergent margins strain rate along the cocos caribbean margin is one order of magnitude smaller namely 3 7 10 8 yr 1 the relative plate velocity is also calculated using seismic moment tensors we find a relative plate convergence of 1 13 myr 1 for the cocos north america plate margin and 0 13 myr 1 for cocos caribbean this means that 80 per cent of the plate motion along the cocos north america plate margin is expressed as thrust faulting earthquakes as compared to the velocity given by global plate motion models in contrast only about 10 per cent of the cocos caribbean relative motion is translated into thrusting events similar results are obtained when a 102 yr earthquake database is used we suggest that differences in strain rate and percentage of plate motion are due to differences in the behaviour of the overriding plate and or the central america forearc sliver being detached from the caribbean plate and being incorporated into the cocos plate

depositional setting and structural evolution of the archean perseverance volcanogenic massive sulfide deposit matagami mining district quebec canada the perseverance volcanogenic massive sulfide deposit 5 1 mt at 15 8 zn 1 24 cu 29 4 g t ag 0 4 g t au matagami district abitibi greenstone belt consists of four pipelike orebodies discordant to local bedding in a shallow dipping portion of the south flank volcanic succession most of the ore is hosted by rhyolitic lavas of the watson unit 2725 9 0 8 ma and is overlain by a thinly laminated tuffaceous unit known as the key tuffite as well as by rhyodacitic lavas of the dumagami unit 2725 4 0 7 ma the presence of sulfide zones predominantly hosted in the footwall rhyolite relicts of key tuffite within the ore and intricate sulfide replacement of the laminated tuff are all features consistent with the formation of most of the deposit via subseafloor replacement the permeability of steep synvolcanic structures likely controlled the migration of metal bearing fluids and simultaneously allowed the downward infiltration of seawater causing mixing cooling and sulfide deposition in the subseafloor environment stratigraphic relationships suggest that massive sulfide formation was active at perseverance while tuffaceous sedimentation was ongoing on the seafloor progressive growth of the massive sulfides along subvertical structures was accompanied by the development of pipelike sericite chlorite talc alteration halos both the hanging wall and the footwall units are characterized by significant mgo mass gains k 2 o na 2 o mass losses and high alteration index values e g chlorite carbonate pyrite index alteration index however these geochemical changes are less intense and widespread in the hanging wall than in the footwall the record of a decreasing alteration above the orebodies was likely caused by the emplacement of the rhyodacite while massive sulfide deposition and hydrothermal activity were still ongoing primary relationships between mineralization and the host lithofacies were overprinted by intense deformation during the main event of compression induced by regional north south shortening deformation is controlled by contrasting rheologies within the sulfide assemblage and between the orebodies and their host rocks structural modifications include the transposition of sulfide ores subparallel to the main foliation the formation of piercement veins at the interface with the host rocks and the generation of a series of secondary textures within the orebodies including a vertical mineralogical banding the distribution of cu in the deposit is interpreted to be affected by the mechanical remobilization of chalcopyrite during deformation whereas the zonation of zn is likely primary hydrothermal alteration halos adjacent to the orebodies have accommodated significant strain as indicated by the presence of tight folds in the key tuffite unit and a marked schistosity strain localization in the vicinity of the deposit highlights the relative ductility of the ore assemblage with respect to the host volcanic succession and resulted in a spatial association between hydrothermal alteration and deformation despite the lack of a genetic relationship

factors affecting seismic moment release rates in subduction zones the amount of shallow seismic activity in subduction zones varies greatly from region to region we quantify this seismicity by calculating seismic moment release rates and seismic slip rates for 24 subduction zones to calculate the moment release we sum the seismic moment for all interplate thrust type events with surface wave magnitude ms 7 0 occurring from 1904 to 1980 we present a time history plot of the seismic moment release for each subduction zone these exhibit the differences in the seismic release patterns for subduction zones where the time window of our study is not representative the total moment release is corrected using information on repeat times the moment release rates are compared with various subduction parameters in order to determine which factors influence the degree of coupling these parameters include the age of the subducting lithosphere absolute velocities of the upper and subducting plates convergence velocity and length maximum depth and dip of the wadati benioff zone the moment release rate decreases as the age of the subducting lithosphere increases when the zones belonging to a single subducting plate are considered this age versus moment release relation is consistent for the zones in which the pacific cocos philippine sea and indian plates are subducting the moment release rates for the subduction zones in which the pacific plate is subducting are much higher than for zones of other plates with similar age the age versus moment release relation holds among the zones which belong to one subducting plate however zones with similar ages but belonging to different plates do not have the same moment release rates this suggests that within a single plate the age is the dominating factor affecting the strength of seismic coupling but that each plate as a whole has a characteristic moment release budget zones with retreating upper plates tend to have lower moment release rates the moment release rate does not increase with convergence velocity no simple relationship was found between these two parameters the moment release rate depends most clearly on the age of the subducting lithosphere and the absolute velocity of the upper plate these are the two independent subduction zone parameters among the variables considered the other variables depend on these two parameters

the thermal structure of the subduction thrust within accretionary and erosive margins abstract we investigate differences in the thermal structure of the subduction thrust between accretionary and erosive margins using a finite element model global averages of plate margin geometries sediment thickness plate age and convergence rate are used to construct generic models of accretionary and erosive margins of these parameters our analysis shows that the largest uncertainty in these models is the geotherm for the incoming oceanic plate despite these uncertainties the subduction thrust of similarly aged accretionary margins is slightly warmer than erosive margins primarily due to the effect of sediment insulating the subduction thrust at accretionary margins if the updip limit of seismicity is thermally controlled warmer accretionary margins suggest shallower seismogenic updip limits counter to the observation that erosional margins are more likely to generate tsunamigenic earthquakes this discrepancy can be reconciled if frictional heat generation at erosive margins is larger than at accretionary margins

salt water intrusion mapping by geoelectrical imaging surveys the geoelectrical imaging method is a tool for mapping the intrusion boundary between fresh water and saline water due to its inherent capability to delineate the lateral changes in pore water salinity a new field survey technique that can be used for environmental and geotechnical investigations has been developed this study evaluates the multiscale survey technique as a tool employed in electrical imaging to detect the salt water intrusion boundary in yan state of kedah northwest malaysia the technique was incorporated into these surveys and it has proved to be a robust method for accurately mapping the fresh water saline water boundary the resulting resistivity sections from these surveys were consistent with both the available geological and borehole information from the area and the previous resistivity surveys conducted by the geological survey of malaysia at those sites

relation between subduction megathrust earthquakes trench sediment thickness and upper plate strain 1 giant earthquake moment magnitude mw 8 5 forecasts for subduction zones have been empirically related to both tectonic stresses and geometrical irregularities along the subduction interface both of these controls have been suggested as able to tune the ability of rupture to propagate laterally and in turn exert an important control on giant earthquake generation here we test these hypotheses and their combined influence by compiling a dataset of trench fill thickness a proxy for smoothing of subducting plate relief by sediment input into the subduction channel and upper plate strain a proxy for the tectonic stresses applied to the subduction interface for 44 segments of the global subduction network we statistically compare relationships between upper plate strain trench sediment thickness and maximal earthquake magnitude we find that the combination of both large trench fill 1 km and neutral upper plate strain explains spatial patterns of giant earthquake occurrence to a statistically significant degree in fact the concert of these two factors is more highly correlated with giant earthquake occurrence than either factor on its own less frequent giant earthquakes of lower magnitude are also possible at subduction zones with thinner trench fill and compressive upper plate strain extensional upper plate strain and trench fill 0 5 km appear to be unfavorable conditions as giant earthquakes have not been observed in these geodynamical environments during the last 111 years

strata bound fe co cu au bi y ree deposits of the idaho cobalt belt multistage hydrothermal mineralization in a magmatic related iron oxide copper gold system mineralogical and geochemical studies of strata bound fe co cu au bi y rare earth element ree deposits of the idaho cobalt belt in east central idaho provide evidence of multistage epigenetic mineralization by magmatic hydrothermal processes in an iron oxide copper gold iocg system deposits of the idaho cobalt belt comprise three types 1 strata bound sulfide lenses in the blackbird district which are cobaltite and less commonly chalcopyrite rich with locally abundant gold native bismuth bismuthinite xenotime allanite monazite and the be rich silicate gadolinite y with sparse uraninite stannite and bi tellurides in a gangue of quartz chlorite biotite muscovite garnet tourmaline chloritoid and or siderite with locally abundant fluorapatite or magnetite 2 discordant tourmalinized breccias in the blackbird district that in places have concentrations of cobaltite chalcopyrite gold and xenotime and 3 strata bound magnetite rich lenses in the iron creek area which contain cobaltiferous pyrite and locally sparse chalcopyrite or xenotime most sulfide rich deposits in the blackbird district are enclosed by strata bound lenses composed mainly of cl rich fe biotite some deposits have quartz rich envelopes whole rock analyses of 48 co and or cu rich samples show high concentrations of au up to 26 8 ppm bi up to 9 16 wt y up to 0 83 wt rees up to 2 56 wt ni up to 6 780 ppm and be up to 1 180 ppm with locally elevated u up to 124 ppm and sn up to 133 ppm zn and pb contents are uniformly low 821 and 61 ppm respectively varimax factor analysis of bulk compositions of these samples reveals geochemically distinct element groupings that reflect statistical associations of monazite allanite and xenotime biotite and gold detrital minerals chalcopyrite and sparse stannite quartz and cobaltite with sparse selenides and tellurides significantly cu is statistically separate from co and as consistent with the general lack of abundant chalcopyrite in cobaltite rich samples paragenetic relations determined by scanning electron microscopy indicate that the earliest y ree be mineralization preceded deposition of co cu au and bi allanite xenotime and gadolinite y commonly occur as intergrowths with and inclusions in cobaltite the opposite texture is rare monazite in contrast forms a poikiloblastic matrix to cobaltite and thin rims on allanite and xenotime reflecting a later metamorphic paragenesis allanite and xenotime also show evidence of late dissolution and reprecipitation forming discordant rims on older anhedral allanite and xenotime and separate euhedral crystals of each mineral textural data suggest extensive deformation of the deposits by folding and shearing and by pervasive recrystallization all during cretaceous metamorphism sensitive high resolution ion microprobe u pb geochronology by aleinikoff et al 2012 supports these paragenetic interpretations documenting contemporaneous mesoproterozoic growth of early xenotime and crystallization of megacrystic a type granite on the northern border of the district these ages are used together with mineralogical and geochemical data from the present study to support an epigenetic iocg model for fe co cu au bi y ree deposits of the idaho cobalt belt a sulfide facies variant of iocg deposits is proposed for the blackbird district in which reducing hydrothermal conditions favored deposition of sulfide minerals over iron oxides this new model includes mesoproterozoic vein mineralization and related fe cl metasomatism that formed the biotite rich lenses a predominantly felsic magmatic source for metals in the deposits given their local abundance of y rees and be and a major sedimentary component in the hydrothermal fluids based on independent sulfur isotope and boron isotope data for sulfides and ore related tourmaline respectively

deep seismic sounding surveys and earthquake prediction abstract in the tokai region facing the pacific ocean in the central part of japan a major earthquake is expected to occur in the near future during the last 80 years omaezaki southernmost cape has subsided by 40 cm the structure in the region is known through a number of deep seismic sounding profiles shot in the region this information and the force derived from sinking of the plate is used to estimate the accumulated strain energy

life of an oceanic plate cooling time and assimilation time an oceanic plate undergoes two different thermal stages in its life from its creation at the spreading center to its assimilation under the subduction zone first the cooling stage and secondly the heating stage introducing the terms cooling time and assimilation time as measures of intervals of the former and latter stages we discussed the quantitative correlation between the two the cooling time ac is defined as the time required for the oceanic plate to move from a spreading center to a trench and is equivalent to the age of the oceanic plate when it just reached the trench the assimilation time tm is defined as the time required for the subducting plate to lose the ability to generate earthquakes since it began to subduct we estimated the values tm and ac at 50 sites in major subduction zones of the world the result indicates a positive linear relation between ac and tm simply expressed by tm ac9 2 ma while mckenzie s subduction model and the thickening plate model theoretically expect the relationship to be expressed as tm ac the discrepancy between our result and the theoretical model suggests the delay of effective heating of the subducting plate under the subduction zone and or the rapid increase of the plate thickness around the spreading center

quasi 3d modeling of airborne tem data by spatially constrained inversion we present a new methodology spatially constrained inversion sci that produces quasi 3d conductivity modeling of electromagnetic em data using a 1d forward solution spatial constraints are set between the model parameters of nearest neighboring soundings data sets models and spatial constraints are inverted as one system the constraints are built using delaunay triangulation which ensures automatic adaptation to data density variations model parameter information migrates horizontally through spatial constraints increasing the resolution of layers that would be poorly resolved locally sci produces laterally smooth results with sharp layer boundaries that respect the 3d geological variations of sedimentary settings sci also suppresses the elongated artifacts commonly seen in interpretation results of profile oriented data sets in this study sci is applied to airborne time domain em data but it can also be implemented with other ground based or airborne data types

seismic coupling and uncoupling at subduction zones seismic coupling has been used as a qualitative measure of the interaction between the two plates at subduction zones kanamori 1971 introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern pacific subduction zones a quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables age of the subducting lithosphere and convergence rate the largest earthquakes occur in zones with young lithosphere and fast convergence rates while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane asperities are strong regions that resist the motion between the two plates the zones with the largest earthquakes have very large asperities while the zones with smaller earthquakes have small scattered asperities this observation can be translated into a simple model of seismic coupling where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface while the variation in asperity size is used to establish a connection between earthquake size and tectonic stress it also implies that plate age and rate affect the asperity distribution plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the preferred trajectory i e the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity indirect influences are many including oceanic plate topography and the amount of subducted sediments all subduction zones are apparently uncoupled below a depth of about 40 km and we propose that the basalt to eclogite phase change in the down going oceanic crust may be largely responsible this phase change should start at a depth of 30 35 km and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change

present day kinematics of the plate boundary zone between africa and europe from the azores to the aegean anticlockwise rotation of africa relative to europe around an euler pole in the eastern atlantic causes plate convergence in the central mediterranean towards n20 w at rate considered a priori to be 5 10 mm yr 1 however previous summaries of the present day deformation sense in the central mediterranean are inconsistent with observations in southern italy recent investigations of italy and adjacent parts of north africa enable local deformation sense observed to be reconciled with sense predicted from relative plate motions suggested angular velocity of the african plate relative to europe is 0 07 myr 1 around a pole near 21 n 21 w this predicts 2 6 mm yr 1 maximum extension rate towards n64 e near the azores consistent with the minimum local accretion of new oceanic crust in the past few millions of years and 4 3 mm yr 1 plate convergence towards n20 w in the central mediterranean at 13 e further east the northern margin of the african plate can be regarded as the gargano seismic zone in the central adriatic sea the northern adriatic sea is a microplate rotating anticlockwise relative to europe at 0 3 myr 1 around a pole in northwestern italy causing northeastward shortening in yugoslavia and up to 4 mm yr 1 extension in central and northern italy the african plate is deforming internally in tunisia and northern libya extending obliquely in direction e10 s at rate up to 5 5 mm yr 1 the resultant velocity relative to europe in the ionian and southern adriatic seas 5 mm yr 1 towards n50 e explains the observed relative velocity across southern italy kinematic consistency requires present day shortening rate east of calabria associated with subduction on the tyrrhenian sea benioff zone and tyrrhenian sea extension rate to be both zero or equal to within 1 mm yr 1

the use of geophysical prospecting for imaging active faults in the roer graben belgium as part of a paleoseismological investigation along the bree fault scarp western border of the roer graben various geophysical methods electrical profiling electromagnetic em profiling refraction seismic tests electrical tomography ground penetrating radar gpr and high resolution reflection seismic profiles were used to locate and image an active fault zone in a depth range between a few decimeters to a few tens of meters these geophysical investigations in parallel with geomorphological and geological analyses helped in the decision to locate trench excavations exposing the fault surfaces the results could then be checked with the observations in four trenches excavated across the scarp geophysical methods pointed out anomalies at all sites of the fault position the contrast of physical properties electrical resistivity and permittivity seismic velocity observed between the two fault blocks is a result of a differences in the lithology of the juxtaposed soil layers and of a change

physical characteristics of subduction interface type seismogenic zones revisited 1 based on global earthquake catalogs the hypocenters nodal planes and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007 assuming that the seismogenic zone coincides with the distribution of 5 5 m 7 earthquakes the subduction interface seismogenic zones were mapped for 80 of the trench systems and characterized with geometrical and mechanical parameters using this database correlations were isolated between significant parameters to identify cause effect relationships empirical laws obtained in previous studies were revisited in light of this more complete accurate and uniform description of the subduction interface seismogenic zone the seismogenic zone was usually found to end in a fore arc mantle rather than at a moho depth the subduction velocity was the first order controlling parameter for variations in the physical characteristics of plate interfaces determining both the geometry and mechanical behavior as such the fast subduction zones and cold slabs were associated with large and steep plate interfaces which in turn had large seismic rates the subduction velocity could not account for the potential earthquake magnitude diversity that was observed along the trenches events with mw 8 5 preferentially occurred in the vicinity of slab edges where the upper plate was continental and the back arc strain was neutral this observation was interpreted in terms of compressive normal stresses along the plate interface large lateral ruptures should be promoted in neutral subduction zones due to moderate compressive stresses along the plate interface that allow the rupture to propagate laterally

subducted sediment thickness and mw 9 earthquakes i measure the thickness of subducted sediment ss beneath the decollement in the fore arc wedge and show that the average value of ss over a subduction zone segment ss is greater than 1 3 km in segments where mw 9 earthquakes have occurred and less than 1 2 km in segments without such large earthquakes in a previous study i showed that the stress drop of large earthquakes mw 7 averaged over a subduction zone segment is larger in segments where mw 9 earthquakes have occurred than in segments without such an event it has also been shown that is linearly related to 1 the pore fluid pressure ratio in the interplate megathrust in this study i revise the previous estimates of and and show that there is a positive correlation between ss and 1 i present a model that relates ss to 1 based on the porous flow of h2o in the subducted sediments which gives a theoretical basis for the correlation between ss and the combination of these parameters thus provides a better indicator for identifying segments where mw 9 earthquakes may occur based on this i propose that the tectonic environments where such huge events are likely to occur are 1 near collision zones 2 near subduction of spreading centers and 3 erosive margins with compressional fore arcs near the japanese islands se hokkaido is prone to such an event but the nankai trough is not