facies distribution and textural variation in faro drift contourites velocity fluctuation and drift growth abstract detailed study of seismic profiles piston cores and bottom photographs from the faro drift on the southern margin of portugal has led to a better understanding of drift development and its relationship to bottom current circulation data on the contourite facies characteristics and the surface microphysiography have been published elsewhere here we concentrate on sediment distribution and geometry longitudinal trends in facies types mean grain size sedimentary structures and composition can be interpreted in terms of relative intensity of currents over different parts of the drift these are generally more intense in the marginal channels and at the upstream or eastern end of the drift three different scales of vertical variation of facies can be identified at the large scale 300 500 m of sediment has accumulated over 4 6 ma in a regular vertical succession due to the northward progradation of the drift at the medium scale the upper 20 30 m of sediment shows alternating phases of active lateral progradation and uniform vertical accumulation that may correlate with episodes of more and less current activity related to high and low sea level stands respectively over the past 0 3 ma at the small scale the topmost 2 3 m of sediment deposited in approximately 0 03 ma shows three zones of coarser grained sediments separated by finer grained contourites this sequence can also be interpreted in terms of long term fluctuation in bottom current activity although the signal is clearly complex this kind of analysis of sedimentary drifts can lead to more accurate reconstruction of paleocirculation patterns

mud volcanism in the gulf of cadiz results from the ttr 10 cruise a new deep water mud volcano field between 2000 and 3500 m water depth was discovered in the deep south portuguese margin as well as several new mud volcanoes in the south spanish and western moroccan margins of the gulf of cadiz during the ttr 10 training through research unesco ioc cruise in july august 2000 this work followed the discovery of a large mud volcano field in the gulf of cadiz first investigated during the ttr 9 cruise gardner 2001 geophys res lett 28 339 342 kenyon et al 2000 ioc technical series no 56 the discoveries were made based on a seamap side scan sonar mosaic and multibeam bathymetry seabeam collected in the area by the naval research laboratory nrl washington dc usa in 1992 kindly released for this purpose single channel seismics long range side scan sonar okean tv controlled grab hull mounted 3 5 khz profiler and coring were used to investigate several seafloor features observed on the side scan sonar imagery in the south iberia spanish and moroccan margins of the gulf of cadiz which were confirmed to be mud volcanoes the typical structures related to fluid venting in the gulf of cadiz are essentially represented by conical mud volcanoes with diameters ranging from several tens of meters to 4 km and heights that can reach 200 m some of these structures appear to be aligned along major conjugate ne sw and nw se trending faults that can be identified on the side scan sonar imagery the new field discovered in the south portuguese margin is the deepest in the gulf of cadiz area and includes three new mud volcanoes bonjardim olenin and carlos ribeiro which seem to be quite active with near surface gas hydrate occurrence and a high saturation in h2s and hydrocarbon gases mainly methane in the mud breccia and overlying pelagic sediments gas hydrates were recovered from the bonjardim mud volcano an intensely gassified mud breccia with one fragment of semi consolidated claystone with a thin bituminous veneer at the surface was recovered from the carlos ribeiro mud volcano the fauna recovered consists mainly of pogonophoran worms belonging to several species and undetermined species of foraminifera three new mud volcanoes were also discovered in the nw moroccan margin rabat student and jesus baraza these show a richer fauna that includes several species of molluscs polychaetes pogonophoran worms crustaceans echinoderms and some fragments of dead coral madrepora and lophelia carbonate crusts were recovered from the student mud volcano the ginsburg mud volcano discovered during the previous ttr 9 cruise was revisited and gas hydrates recovered once again a new mud volcano was also discovered in the spanish margin tasyo where evidence was found of coral build ups on the hard substratum of the mud volcanic edifice

the quest for the africa eurasia plate boundary west of the strait of gibraltar the missing link in the plate boundary between eurasia and africa in the central atlantic is presented and discussed a set of almost linear and sub parallel dextral strike slip faults the swim1 faults that form a narrow band of deformation over a length of 600 km coincident with a small circle centred on the pole of rotation of africa with respect to eurasia was mapped using a new swath bathymetry compilation available in the area offshore sw portugal these faults connect the gloria fault to the rif tell fault zone two segments of the plate boundary between africa and eurasia the swim faults cut across the gulf of cadiz in the atlantic ocean where the 1755 great lisbon earthquake m 8 5 8 7 and tsunami were generated providing a new insight on its source location

gas charged sediments and large pockmark like features on the gulf of cadiz slope sw spain abstract gassy sediments and large seafloor pockmarks are identified from seismic profiles in the gulf of cadiz slope gas charged sediments are represented by zones of acoustic turbidity and other acoustic anomalies on seismic profiles most of the gas is believed to be biogenic in origin resulting from the decay of organic matter contained within rapidly developed regressive shelf margin deltas a group of large asymmetric seafloor pockmarks also occurs downslope of the gassy area both on the seafloor surface and deeply buried in the sediment column sediment sorting by the mediterranean undercurrent seems to act as a controlling factor for the development of each type of gas originated feature the area where the strong mediterranean undercurrent outflow impinges the seafloor is covered by sandy sediments of high porosity which allow the interstitial gas to escape through the sediment pores whereas the areas affected by weaker currents are covered by clays and the gas tends to accumulate in the sediment beneath impermeable horizons

deep water sediment wave fields bottom current sand channels and gravity flow channel lobe systems gulf of cadiz ne atlantic a study of the seafloor of the gulf of cadiz west of the strait of gibraltar using an integrated geophysical and sedimentological data set gives new insights into sediment deposition from downslope thermohaline bottom currents in this area the mediterranean outflow mo begins to mix with north atlantic waters and separates into alongslope geostrophic and downslope ageostrophic components changes in bedform morphology across the study area indicate a decrease in the peak velocity of the mo from 1 m s 1to 0 5 m s 1 the associated sediment waves form a continuum from sand waves to muddy sand waves to mud waves a series of downslope oriented channels formed by the mo are found where the mo starts to descend the continental slope at a water depth of a700 m these channels are up to 40 km long have gradients of 0 5 a fairly constant width of a2 km and a depth of a75 m sand waves move down the channels that have mud wave covered levees similar to those seen in turbidite channel levee systems although the channel size and levee thickness do not decrease downslope as in typical turbidite channel systems the channels terminate abruptly where the mo lifts off the seafloor gravity flow channels with lobes on the basin floor exist downslope from several of the bottom current channels each gravity flow system has a narrow slightly sinuous channel up to 20 m deep feeding a depositional lobe up to 7 km long cores from the lobes recovered up to 8 5 m of massive well sorted fine sand with occasional mud clasts this work provides an insight into the complex facies patterns associated with strong bottom currents and highlights key differences between bottom current and gravity flow channel levee systems the distribution of sand within these systems is of particular interest with applications in understanding the architecture of hydrocarbon reservoirs formed in continental slope settings

a two dimensional p sv hybrid method and its application to modeling localized structures near the core mantle boundary a p sv hybrid method is developed for calculating synthetic seismograms involving two dimensional localized heterogeneous structures the finite difference technique is applied in the heterogeneous region and generalized ray theory solutions from a seismic source are used in the finite difference initiation process the seismic motions after interacting with the heterogeneous structures are propagated back to the earth s surface analytically with the aid of the kirchhoff method anomalous long period sks spdks observations sampling a region near the core mantle boundary beneath the southwest pacific are modeled with the hybrid method localized structures just above the core mantle boundary with lateral dimensions of 250 to 400 km can explain even the most anomalous data observed to date if s velocity drops up to 30 are allowed for a p velocity drop of 10 structural shapes and seismic properties of those anomalies are constrained from the data since synthetic waveforms are sensitive to the location and lateral dimension of seismic anomalies near the core mantle boundary some important issues such as the density change and roughness of the structures and the sharpness of the transition from the structures to the surrounding mantle however remain unresolved due to the nature of the data

mapping geoelectric fields during magnetic storms synthetic analysis of empirical united states impedances empirical impedance tensors obtained from earthscope magnetotelluric data at sites distributed across the midwestern united states are used to examine the feasibility of mapping magnetic storm induction of geoelectric fields with these tensors in order to isolate the effects of earth conductivity structure we perform a synthetic analysis calculating geoelectric field variations induced by a geomagnetic field that is geographically uniform but varying sinusoidally with a chosen set of oscillation frequencies that are characteristic of magnetic storm variations for north south oriented geomagnetic oscillations at a period of t0 100 s induced geoelectric field vectors show substantial geographically distributed differences in amplitude approximately a factor of 100 direction up to 130 and phase over a quarter wavelength these differences are the result of three dimensional earth conductivity structure and they highlight a shortcoming of one dimensional conductivity models and other synthetic models not derived from direct geophysical measurement that are used in the evaluation of storm time geoelectric hazards for the electric power grid industry a hypothetical extremely intense magnetic storm having 500 nt amplitude at t0 100 s would induce geoelectric fields with an average amplitude across the midwestern united states of about 2 71 v km but with a representative site to site range of 0 15 v km to 16 77 v km significant improvement in the evaluation of such hazards will require detailed knowledge of the earth s interior three dimensional conductivity structure

strong ground motion from the michoacan mexico earthquake the network of strong motion accelerographs in mexico includes instruments that were installed under an international cooperative research program in sites selected for the high potenial of a large earthquake the 19 september 1985 earthquake magnitude 8 1 occurred in a seismic gap where an earthquake was expected as a result there is an excellent descripton of the ground motions that caused the disaster

tectonics and mud volcano development in the gulf of cadiz many structures associated with fluid escape including mud volcanoes mud carbonate mounds pockmarks and slides have been identified and characterized in the gulf of cadiz most of the mud volcanoes following ne sw and nw se main trends are found at 350 2000 m depth in the eastern domain of the gulf of cadiz which corresponds to the betic rifean margin scattered mud volcanoes have also been recognized on the lower slope at 2300 3900 m depth the major tectonic structures are thrust faults extensional faults strikeslip faults and diapirs all these tectonic structures have provided escape pathways for overpressured material and fluids or have favoured upward fluid movement along the sedimentary column and eventually the build up of mud volcanoes in this work we present images of the mud volcano plumbing systems and the relationship between regional tectonics and mud volcano development seismic profiles acquired during the tasyo 2000 and mvseis ttr 15 cruises are used to image and interpret the link between the mud volcano edifices and the subsurface tectonic structures

basin structure estimation by waveform modeling forward and inverse methods we introduce a technique for using broadband seismograms recorded from earthquakes at local and regional distances to refine basin structure for the region outside the basin we assume a one dimensional 1d crustal model and analytical techniques grt to propagate the energy from sources to the basin edge where the motions are then interfaced with a 2d finite difference algorithm wen and helmberger 1996 we parameterize the basin section by isovelocity layers with linear dipping segments between control points the control point depths are allowed to vary to improve the modeling of waveform data of stations inside the basin the comparison between data and synthetics is qualified by a fitness function defined by two factors the timing shift required for best alignment and the correlation coefficient the procedure was applied to a strong motion waveform profile across the extended los angeles basin produced by the 1992 landers california earthquake to refine the velocity structure using sensitivity testing and forward modeling only the correlation coefficient and amplitude were used because absolute timing was unknown the procedure was extended to a direct waveform inversion by employing a conjugate gradient approach which uses numerical derivatives numerical tests using the new inversion process with synthetic data demonstrate that it is possible to recover a detailed basin structure if a sufficient amount of high quality data exists

lateral propagation effects observed at parkway new zealand a case history to compare 1d versus 2d site effects abstract the importance of 2d or 3d site effects has been shown by theoretical studies however there is a lack of observational evidence of site effects due to lateral heterogeneities we have evaluated 1d and 2d site effects in the data obtained by a dense seismograph array that operated for more than 2 months in the alluvial basin of parkway new zealand 1d site effects were evaluated using three independent techniques spectral ratios relative to a reference station spectral ratios of horizontal relative to vertical ground motion and a generalized inversion scheme site effects due to lateral heterogeneity were investigated in detail for two events using frequency wavenumber analysis our results show that it is not possible to separate 1d from 2d site effects using frequency domain techniques this explains the scatter obtained from standard 1d data analysis the 1d resonance peaks will be more or less contaminated with laterally propagating waves for different events

high resolution stratigraphy of the mediterranean outflow contourite system in the gulf of cadiz during the late pleistocene the impact of heinrich events abstract a detailed high resolution stratigraphic analysis of the mediterranean outflow contourite system at the continental slope of the gulf of cadiz has been carried out through the correlation between a dense network of seismic reflection profiles sparker airgun 3 75 khz and parametric echosounder topas calypso giant piston and standard gravity cores from such correlation we determine a stacking pattern constituted by four main seismic units a d that are internally structured into ten subunits each subunit shows a single sequence formed by transparent seismic facies at the base to smooth parallel reflectors of moderate to high amplitude facies at the top being well correlated in the cores with a coarsening upward sequence the latest pleistocene holocene deposits form glacial interglacial depositional sequences related to cycles with a frequency range below the milankovitch band that corresponds to millennial timescale climatic changes such as dansgaard oeschger 1 5 ka and bond cycles 10 15 ka oxygen isotope records of planktonic foraminifera and the occurrence of ice rafted debris ird in the most recent contourite subunits show clear evidence of the influence of the north atlantic climatic conditions especially the climatic heinrich events h in the slope sedimentation of the gulf of cadiz and then in the circulation of the mediterranean outflow water mow the coarser contourite deposits are mostly associated with the last glacial maximum younger dryas and heinrich events on the central area of the middle slope during globally cooler conditions the mow was denser so that it was more active in deeper areas than today on the other hand during warm periods the mow became less dense favoring an increased intensity of the mow on the distal area of the upper slope therefore spatial and vertical fluctuations of the mow contourite system are strongly affected by global climate and oceanographic changes being clearly influenced by iceberg discharges and probably also by the resumption of thermohaline circulation in the north atlantic ocean during ice melting periods

mud volcano gas hydrates in the gulf of cadiz this paper presents new geochemical data on gas hydrate bearing mud volcanoes discovered for the first time in the gulf of cadiz during cruises ttr9 and ttr10 of the r v professor logachev in 1999 2000 the estimated gas hydrate content is 3 16 of sediment volume and 5 31 of pore space volume estimated values of the water isotopic composition for the ginsburg mud volcano are very heavy for 18o up to 53 and light for d up to 210 gas released from the hydrates contains 81 of c1 and 19 of c2 the inferred source of the gas in the hydrates is enriched in c2 c6 5 indicating that the gas has a thermogenic origin gas hydrate of cubic structure ii should be formed from a gas of such composition it is interpreted that the composition of the mud volcano fluid corresponds to deep oil basins below the gulf of cadiz

the mediterranean outflow splitting a comparison between theoretical models and canigo data abstract the mediterranean outflow in the atlantic is vertically subdivided into two main cores that are identified by temperature and salinity maxima hydrological data collected west of the strait of gibraltar and off the south and south west coasts of portugal in september 1997 have been used in two different models to shed some light on the mechanisms that cause a splitting of the mediterranean outflow these models are i a steady one dimensional streamtube model and ii a local time dependent model that includes cross stream variations the streamtube model applied at the northern respectively the southern part of the outflow appears capable of reproducing the observed density difference between the two cores the vertical separation however was underestimated the results from the local model suggest that a bump in the bottom topography may be a determining factor for the splitting tendency of the flow

the electric field in northern england and southern scotland implications for geomagnetically induced currents summary magnetotelluric mt data in the form of mt tensors are used to estimate directly the size and spatial distribution of the electric field in northern england and southern scotland with the aim of predicting the flow of geomagnetically induced currents gic in power networks in the region mt and geomagnetic deep sounding data from a number of different field campaigns at a period of 750 s are employed the mt data are cast in the form of telluric vectors which allow a joint hypothetical event analysis hea of both geomagnetic deep sounding and mt data this analysis reveals qualitatively the pervasive effects of electric field distortion in the region two approaches are taken to understand how the spatial structure of the regional electromagnetic field is affected by local distortions and what the origin of these distortions might be the dimensionality and form of electric field distortion of the mt tensors is investigated using the weaver et al and bahr classification schemes and by examining the misfit of a galvanic distortion model as a function of rotation angle at sites where the galvanic distortion model is found to be appropriate the regional mt tensors are recovered using tensor decomposition techniques it is found that recovering the regional mt response reconciles the geometry of induced currents implied by the mt data with that of the magnetic variation anomalies lilley s central impedances are used to calculate rotationally invariant effective telluric responses in the southern uplands the magnitude of the effective telluric response is approximately 0 25 0 5 mv km 1 nt 1 but as the southern uplands fault is approached it rises steadily to 3 mv km 1 nt 1 in the midland valley the effective telluric response is approximately 0 5 mv km 1 nt 1 which rises steadily to 2 5 mv km 1 nt 1 as the southern uplands and highland boundary faults are approached to the southeast and northwest respectively therefore the increase in the magnitude of the effective telluric response correlates with the approach of a major tectonic boundary such as the southern uplands fault these results show that the induced electric field strength varies considerably throughout the central scotland region in addition the hea indicates that due to lateral changes in conductivity structure the direction of the electric field deviates significantly from the regional direction implied by the polarization azimuth of the primary geomagnetic induction therefore any attempts to model the flow of gic in the region need to account for the spatial variation of both the magnitude and azimuth of the electric field

geophysical images of the deep crust the iapetus suture the iapetus suture arguably the most fundamental lineament of british and irish structure has been previously identified on the birps winch deep seismic reflection profiles offshore as a nw dipping feature new depth migrated interpretations of these and other reflection data show good correlation of structure for 100 km along strike in the northern irish sea towards magnetotelluric stations onshore in the north of england and southern uplands of scotland three different methods of inverting data from these sites all reveal a thin nw dipping slab of high conductivity a contour map of the suture trace dipping at 15 25 nw can be constructed down to and through the moho at 28 km depth this map suggests crustal stretching below the solway basin the mylonitic suture zone is characterized by good seismic reflection layering high conductivity presumably fluid produced and a steep increase of p wave velocity over the few kilometres of its width crustal refraction experiments lispb and cssp provide useful velocities but locally misleading structure so that the combination of the seismic reflection and geoelectric methods may be the most powerful means of determining deep crustal structure with a good degree of both vertical and lateral resolution

the contourite depositional system of the gulf of cadiz a sedimentary model related to the bottom current activity of the mediterranean outflow water and its interaction with the continental margin the present paper is mostly a synthesis of work conducted on the continental margin of the gulf of cadiz using a broad database collected by several cruises and projects supported by the spanish research council and the nrl and naval oceanographic office usa including bathymetry sidescan sonar imagery seismic profiles sediment cores submarine photographs and physical oceanographic data these data have enabled us to establish a detailed understanding of the morphologic development of the margin its pliocene and quaternary stratigraphy and a full characterization of the contourite depositional system cds generated by the mediterranean outflow water mow the northern margin of the gulf of cadiz shows the following distinct features a an active compressive framework where the cadiz allocthonous unit provides an unstable substratum for late miocene pliocene and quaternary sedimentation b a relative lack of submarine canyons except in the western area of the algarve margin c a very broad continental slope that lacks a marked continental rise d a middle slope dominated by along slope processes driven by the mow which has generated a complex cds during the pliocene and quaternary and e an irregular lower slope and abyssal plain region dominated by down slope processes that is partly detached from an upper slope source region the cds is composed of both depositional and erosive features the main depositional features are characterised by sedimentary wave fields sedimentary lobes mixed drifts plastered drifts elongated mounded and separated drifts and sheeted drifts the main erosive features are contourite channels furrows marginal valleys and moats these various depositional and erosive features have a specific location along the margin and their detailed distribution is essential to understand the present and past interaction of the mow with the middle slope based on this distribution five morphosedimentary sectors have been identified within the cds which from east to west are 1 proximal scour and sand ribbons 2 overflow sedimentary lobe 3 channels and ridges 4 active contourite drifts and 5 submarine canyons the development of the cds has been controlled in general by the pliocene and quaternary environmental and paleoceanographic changes and by the morphology of the margin but in detail the development of each of these sectors is related to systematic deceleration of the mow as it flows westwards to the interaction with the margin bathymetry and to the effects of coriolis force our comprehensive sedimentary model for the cds defines the gulf of cadiz margin as a mixed contourite turbidite system with a detached combined drift fan morphology this is different from many other contourite influenced margins where the contourite processes are dominant on the middle slope and separated from the down slope processes which are characteristic on the lower slope and abyssal plains

vast fields of hydrocarbon derived carbonate chimneys related to the accretionary wedge olistostrome of the gulf of cadiz we report the first discovery and sampling of vast fields of hydrocarbon derived carbonate chimneys along the gulf of cadiz continental slope at depths between 500 and 1200 m a large variety of carbonate chimneys more than 200 samples were recovered from four different areas named the diasom tasyo gbf guadalquivir basin and esf east moroccan fields observations from an underwater camera revealed a spectacular high density of pipe like chimneys some of them longer than 2 m lying over the sea floor and some protruding from muddy sediment local fissures and alignment of isolated chimneys were also observed suggesting that their distribution is controlled by fault planes chimneys collected show a wide range of morphological types spiral cylindrical conical mushroom like and mounded with numerous nodule protuberances and ramified fluid channelways the chimneys are mainly composed of authigenic carbonates ankerite fe bearing dolomite and calcite with abundance of iron oxides forming agglomerates of pseudo pyrite framboids n 18 o isotopic values vary from 0 7 to 5 5x whereas n 13 c values indicate that chimney carbonates are moderately depleted in 13 c ranging from 346x to 320x pdb interpreted as formed from a mixture of deep thermogenic hydrocarbons and shallow biogenic methane these vast fields of carbonate chimneys imply new considerations on the importance of hydrocarbon fluid venting in the gulf of cadiz and moreover on the active role of the olistostrome accretionary wedge of the gibraltar arc b 2003 elsevier science b v all rights reserved

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

are subduction zones invading the atlantic evidence from the southwest iberia margin subduction initiation at passive margins plays a central role in the plate tectonics theory however the process by which a passive margin becomes active is not well understood in this paper we use the southwest iberia margin sim in the atlantic ocean to study the process of passive margin reactivation currently there are two tectonic mechanisms operating in the sim migration of the gibraltar arc and africa eurasia convergence based on a new tectonic map we propose that a new subduction zone is forming at the sim as a result of both propagation of compressive stresses from the gibraltar arc and stresses related to the large scale africa eurasia convergence the gibraltar arc and the sim appear to be connected and have the potential to develop into a new eastern atlantic subduction system our work suggests that the formation of new subduction zones in atlantic type oceans may not require the spontaneous foundering of its passive margins instead subduction can be seen as an invasive process that propagates from ocean to ocean

contourite erosive features caused by the mediterranean outflow water in the gulf of cadiz quaternary tectonic and oceanographic implications abstract contourite depositional systems have been the focus of much recent research but still relatively little is known about contourite erosive features and their associated processes based on multibeam bathymetry side scan imagery and different resolution seismic records a detailed description and classification of the major erosive submarine valley features of the contourite depositional system of the gulf of cadiz middle slope is presented for the first time four types of erosive features have been differentiated including contourite moats contourite channels marginal valleys and large isolated furrows and interpreted in terms of their tectonic and oceanographic implications during the quaternary the study of the distribution and characteristics of erosive features is essential to better understand the present and past interaction of the mediterranean outflow water mow with the middle slope seafloor and allows us to propose a new and more precise scheme for the mow circulation patterns this scheme includes a main along slope circulation responsible for the excavation of the contourite moat and channels and a secondary down slope circulation responsible for the erosion of marginal valleys and isolated furrows three evolutionary stages have been observed in the development of the erosive system during the quaternary that can be related to changes in the distribution and splitting of the mow as a consequence of the segmentation of the ne sw diapiric ridges by neotectonic effects 1 early pleistocene to mid pleistocene linear diapiric ridges 2 mid pleistocene to late pleistocene diapiric reactivation and 3 late pleistocene to holocene main recent phase of diapiric ridges segmentation and rotation this study provides important regional clues for establishing the evolution of the erosive features in relation with neotectonic effects and represents a good example of the potential of erosive features as evidences for the reconstruction of the paleoceanography and recent tectonic changes

magnetotelluric analysis using mohr circles the mohr circle most commonly met in the analysis of mechanical stress is used to depict magnetotelluric impedance information taking the real and quadrature parts of magnetotelluric tensors separately the magnetotelluric concepts of two dimensionality three dimensionality skew and anisotropy are then all given quantitative expression on a diagram as are various magnetotelluric invariants in particular a new invariant the central impedance becomes evident in a discussion of effective impedances some insight is gained into impedance rotations and an anisotropy angle is defined analogous to skew angle mohr circles are also tested to depict the effects of the shear and twist operations on a regionally two dimensional structure generally the application of shear or twist results in an impedance tensor with a mohr circle of typical three dimensional form

multidisciplinary study of mud volcanoes and diapirs and their relationship to seepages and bottom currents in the gulf of cadiz continental slope northeastern sector abstract the seabed morphology type of sediments and dominant benthic species on eleven mud volcanoes and diapirs located on the northern sector of the gulf of cadiz continental slope have been studied the morphological characteristics were grouped as i fluid escape related features ii bottom current features iii mass movement features iv tectonic features and v biogenic related features the dominant benthic species associated with fluid escape hard substrates or soft bottoms have also been mapped a bottom current velocity analysis allowed the morphological features to be correlated with the benthic habitats and the different sedimentary and oceanographic characteristics the major factors controlling these features and the benthic habitats are mud flows and fluid escape related processes as well as the interaction of deep water masses with the seafloor topography mud volcano eruptions give rise to mud flows and or aqueous fluid seepage these processes sustain chemosynthesis based communities closely associated with fluid seepage large depressions in the nearby area are influenced by collapse related phenomena where active fluid escape and the erosive effect of bottom currents have been identified when the extrusion activity of the mud volcano is low and the seepage is diffuse authigenic carbonates form within the edifice sediments the bottom current sweeps the seafloor from the se to the nw when the velocity is moderate sedimentary contourite processes take place on both sides of the edifices at high velocities the authigenic carbonates may be exhumed and colonised by species associated with hard substrates small carbonate mounds are found at the summits of some volcanoes and diapirs living corals have been found on the tops of the shallowest mud volcanoes revealing different oceanographic conditions and strong bottom currents that favour the availability of nutrients and organic particles the edifices affected by very high current velocities are located in the channels where erosive processes dominate

seabed morphology and hydrocarbon seepage in the gulf of cadiz mud volcano area acoustic imagery multibeam and ultra high resolution seismic data abstract extensive mud volcanism mud diapirism and carbonate chimneys related to hydrocarbon rich fluid venting are observed throughout the spanish portuguese margin of the gulf of cadiz all the mud volcanoes and diapirs addressed in this paper lie in the region of olistostrome accretionary complex units which were emplaced in the late miocene in response to nw directed convergence between the african and eurasian plates the study area was investigated by multibeam echo sounder high and ultra high resolution seismic profiling dredging and coring the structures observed on multibeam bathymetry at water depths between 500 and 1300 m are dominated by elongate mud ridges mud cones mud volcanoes and crater like collapse structures ranging in relief from 50 to 300 m and size from 0 8 to 2 km in diameter the main morphotectonic features named the guadalquivir diapiric ridge gdr and the cadiz diapiric ridge cdr are longitudinally shaped diapirs which trend ne sw and consist of lower middle miocene plastic marly clays the gdr field and the tasyo field which consist of mud volcanoes and extensive fluid venting related to diapiric ridge development are described in this paper the gdr field is characterised by numerous single sub circular mud volcanoes and mud cones the single mud volcanoes are cone shaped features with relatively gentle slopes of 3 6 consisting of several generations of mud breccia deposition with indications of gas saturation degassing structures and the presence of h2s the mud cones have asymmetric profiles with steep slopes of up to 25 and contain large surficial deposits of hydrocarbon derived carbonate chimneys and slabs the tasyo field is characterised by an extensive concentration of small sub circular depressions oval and multi cone mud volcanoes and large sediment slides mud volcanoes in this area are characterised by moderate slopes 8 12 have bathymetric relief ranging from 100 to 190 m and consist of sulphide rich mud breccia calcite chimneys carbonate crusts and chemosynthetic fauna pogonophora tube worms we propose that all these hydrocarbon seepage structures are related to lateral compressional stress generated at the front of the olistostromic accretionary wedge this stress results in the uplifting and squeezing plastic marly clay deposits additionally the compressional stress at the toe of the olistostrome forms overpressurised compartments which provide avenues for hydrocarbon enriched fluids to migrate

early cenozoic two phase extension and late cenozoic thermal subsidence and inversion of the bohai basin northern china abstract the bohai basin is one of a family of early cenozoic extensional basins that lie along the eastern margin of asia from russia to vietnam initial extension was probably triggered by subduction roll back of the oceanic pacific plate from the asian continent there were two phases in the bohai basin s rift history the earlier paleocene early eocene phase resulted in the deposition of the kongdian formation and the fourth lowest member of the shahejie formation in a series of elongate half grabens these half grabens have master faults with a nne ssw orientation secondary normal faults are typically clockwise oblique to the master faults indicating a component of dextral transtension deposition was focused in the west and south of the present basin these rocks are mainly alluvial fan and fluvial red beds the architecture of the basin underwent an important change at ca 43 45 ma middle eocene beginning with the deposition of the third member of the shahejie formation in part these sediments were deposited in the same half grabens as the kongdian formation but the bozhong depression in the central part of the basin originated at this time and became the major depocentre the bozhong depression superficially resembles a pull apart basin it formed when continued transtension of the earlier tertiary fault systems to the east and west created an extensional overlap between them during this phase the basin as a whole had a geometry with elements typical of both pull apart and transtensional basins regional extension in many east asian basins ended at the end of the oligocene probably because of the onset of transpression within eastern asia caused by the collision of australia with the philippine sea plate dextral transpression caused minor inversion of some of the earlier normal faults in bohai but as a whole the basin began to subside in a post rift phase of thermal subsidence that has lasted until the present day

significance of mud volcanism 1 mud volcanism and diapirism have puzzled geoscientists for 2 centuries they have been described onshore and offshore in many places on earth and although they occur in various tectonic settings the majority of the features known to date are located in compressional tectonic scenarios this paper summarizes the main thrusts in mud volcano research as well as the various regions in which mud volcanism has been described mud volcanoes show variable geometry up to tens of kilometers in diameter and several hundred meters in height and a great diversity regarding the origin of the fluid and solid phases gas predominantly methane water and mud may be mobilized at subbottom depth of only a few meters but in places can originate from several kilometers depth with minor crustal or mantle input the possible contribution of mud extrusion to global budgets both from quiescent fluid emission and from the extrusive processes themselves is important in regions where mud volcanoes are abundant such as the collision zones between africa and eurasia fluid flux through mud extrusion exceeds the compaction driven pore fluid expulsion of the accretionary wedge also quiescent degassing of mud volcanoes may contribute significantly to volatile budgets and hence to greenhouse climate

mediterranean undercurrent sandy contourites gulf of cadiz spain abstract the pliocene quaternary pattern of contourite deposits on the eastern gulf of cadiz continental slope results from an interaction between linear diapiric ridges that are perpendicular to slope contours and the mediterranean undercurrent that has flowed northwestward parallel to the slope contours and down valleys between the ridges since the late miocene opening of the strait of gibraltar coincident with the northwestward decrease in undercurrent speeds from the strait there is the following northwestward gradation of sediment facies associations 1 upper slope facies 2 sand dune facies on the upstream mid slope terrace 3 large mud wave facies on the lower slope 4 sediment drift facies banked against the diapiric ridges and 5 valley facies between the ridges the southeastern sediment drift facies closest to gibraltar contains medium fine sand beds interbedded with mud the adjacent valley floor facies is composed of gravelly shelly coarse to medium sand lags and large sand dunes on the valley margins compared to this the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand further northwestward sediment drift grades to biogenous silt near the faro drift at the portuguese border because of the complex pattern of contour parallel and valley perpendicular flow paths of the mediterranean undercurrent the larger scale bedforms and coarser grained sediment of valley facies trend perpendicular to the smaller scale bedforms and finer grained contourite deposits of adjacent sediment drift facies the bottom current deposits of valleys and the contourites of the cadiz slope intervalley areas are distinct from turbidite systems the valley sequences are not aggradational like turbidite channel levee complexes but typically exhibit bedrock walls against ridges extensive scour and fill into adjacent contourites transverse bedform fields and bioclastic lag deposits both valley and contourite deposits exhibit reverse graded bedding and sharp upper bed contacts in coarse grained layers low deposition rates and a regional pattern of bedform zones textural variation and compositional gradation the surface sandy contourite layer of 0 2 1 2 m thickness that covers the gulf of cadiz slope has formed during the present holocene high sea level because high sea level results in maximum water depth over the gibraltar sill and full development of the mediterranean undercurrent the late pleistocene age of the mud underlying the surface sand sheet correlates with the age of the last sea level lowstand and apparent weak mediterranean undercurrent development thus the cyclic deposition of sand or mud layers and contourite or drape sequences appear to be related to late pliocene and quaternary sea level changes and mediterranean water circulation patterns since its pliocene origin the contourite sequence has had low deposition rates of

three dimensional simulation of a magnitude 7 75 earthquake on the san andreas fault simulation of 2 minutes of long period ground motion in the los angeles area with the use of a three dimensional finite difference method on a parallel supercomputer provides an estimate of the seismic hazard from a magnitude 7 75 earthquake along the 170 kilometer section of the san andreas fault between tejon pass and san bernardino maximum ground velocities are predicted to occur near the fault 2 5 meters per second and in the los angeles basin 1 4 meters per second where large amplitude surface waves prolong shaking for more than 60 seconds simulated spectral amplitudes for some regions within the los angeles basin are up to 10 times larger than those at sites outside the basin at similar distances from the san andreas fault

the simulation of magnetic variation anomalies using single station data abstract hypothetical event maps of the vertical field derived from single station magnetic variation transfer functions are biased by the anomalous horizontal fields an iterative scheme proposed earlier is used to estimate and correct for the anomalous fields the scheme here is applied to single station transfer functions from 90 sites in southern scotland and northern england it converges to a stable solution in four iterations and the results compare favourably with those predicted in the southern part of the area using measured inter station transfer functions the resultant hypothetical event maps of the anomalous horizontal fields show that the northumberland trough and southern uplands conductivity anomalies are both strikingly linear features which correlate more closely than previously supposed with other geological and geophysical observations

three dimensional vp and vp vs velocity models of the los angeles basin and central transverse ranges california we use p and s arrival times from 5225 earthquakes and 53 explosions recorded by the southern california seismographic network to invert for the three dimensional p velocity v p and the p and s velocity ratio v p v s in the central transverse ranges and the los angeles basin to model long wavelength features of the velocity structure we invert for the models by interpolating models determined successively from a sparse medium to a dense grid with 40 20 and 10 km spacing of horizontal grid nodes layers of grid nodes are placed at depths of 1 4 6 8 12 16 and 20 km the data variance decreased about 80 in the gradational inversion ample data from the 1994 northridge and other earthquake sequences the rich background seismicity and the dense station distribution along with controlled sources made the model well resolved except along the edges to the southwest in the offshore region and at depths greater than 20 km the v p model images the shape of the los angeles and east ventura basins down to depths of 8 and 12 km three low velocity areas at 1 km depth in the los angeles basin that coincide with recent sediment depositional areas are also imaged the north edge of the peninsular ranges the santa monica and the san gabriel mountains form discontinuous high velocity ridges extending to depths of 20 km the high v p v s ratios in the near surface are consistent with high pore fluid pressures in the basin sediments at depth beneath the east ventura basin the high v p v s and high v p suggest the presence of ophiolitic assemblages or mid miocene volcanics in contrast a body with normal v p and low v p v s that is identified in the upper crust beneath santa monica bay may be a fragment of quartz rich continental crust the v p model of both the ventura and the los angeles basins shows features that suggest deformation of the hanging wall or basin closure at seismogenic depths of 16 km the hypocenters of moderate sized and large earthquakes are located within or adjacent to high v p velocity bodies in most cases these high velocity bodies form the upper block consistent with shortening of the lower crust as described in thick skinned tectonic interpretations

deep crustal geoelectric structure beneath the northumberland basin in terms of lateral variations in conductivity structure the southern southern uplands and northumberland basin are characterized by a region of attenuated vertical magnetic fields with small spatial gradients reflecting the presence of a substantial conducting zone five magnetotelluric data sets from the region have been analysed to provide accurate and unbiased estimates of the impedance tensor the response data are used to investigate the deep geoelectric crustal structure of the region three appropriate sets of response data have been subjected to two construction algorithms for 1 d inversion the geoelectric profiles recovered identify a deep crustal conducting zone underlying the northumberland basin the zone modelled as a layered structure dips steeply from mid crustal depths underneath the northumberland basin to lower crustal depths to the nw the structure thus correlates in location and geometry with a deep crustal reflecting wedge detected offshore by a deep seismic reflection profile

two dimensional interpretation of three dimensional magnetotelluric data an example of limitations and resolution summary interpretation of magnetotelluric mt data for three dimensional 3 d regional conductivity structures remains uncommon and two dimensional 2 d models are often considered an adequate approach in this paper we examine 2 d interpretation of 3 d data by considering the synthetic responses of a 3 d structure chosen specifically to highlight the advantages and limitations of 2 d interpretation 2 d models were obtained from inversion of the synthetic 3 d data set with different conditions noise and distortion applied to the data we demonstrate the importance of understanding galvanic distortion of the data and show how 2 d inversion is improved when the regional data are corrected prior to modelling when the 3 d conductive structure is located below the profile the models obtained suggest that the effects of finite strike are not significant if the structure has a strike extent greater than about one half of a skin depth in this case the use of only tm mode data determined better the horizontal extent of the 3 d anomaly when the profiles are located away from the 3 d conductive structure the use of only tm mode data can imagine phantom conductive structures below the profile in this case the use of both polarizations produced a better determination of the subsurface structures it is important to note that the main structures are identified in all the cases considered here although in some cases the large data misfit would cause scepticism about features of the models

carbonate mounds in a mud volcano province off north west morocco key to processes and controls abstract this paper presents a new cluster of carbonate mounds discovered in 2002 in the gulf of cadiz off morocco r v belgica 2002 in water depths of 500 to 600 m amidst a field of giant mud volcanoes multibeam bathymetry side scan sonar imagery and 2d seismics are analyzed to present four mound provinces 1 the pen duick mound province on the pen duick escarpment 2 the renard mound province on the renard ridge 3 the vernadsky mound province on the vernadsky ridge and the al idrisi mound province on the gas blanked sediments above the buried al idrisi ridge video imagery and surface samples are described to ground truth the different mound areas the paradox is that nearly no live corals are presently being observed at the surface of the mounds while the mound cores display throughout a high number of reef forming cold water coral fragments scleractinians in association with numerous associated fauna formerly inhabiting the econiches provided by the coral framework environmental and oceanographic conditions during the recent past glacials stadials were probably more favourable for cold water coral growth pore water analyses in on mound cores at the south eastern edge of pen duick escarpment give evidence of focused higher methane fluxes and sulphate reduction rates on mounds than in the surrounding sediments cores from several mounds display horizons of strong corrosion and dissolution of the coral fragments a three phase model for carbonate mound evolution in these settings is proposed 1 in a first stage external controls positive oceanographic and environmental conditions the presence of an active planktonic food chain based on a high primary production and a suitable substrate are responsible for the initiation of cold water coral growth 2 once the cold water corals established an initial framework sedimentation becomes an important factor controlling mound growth the cold water corals baffle the sediments 3 throughout mound growth the mound may episodically be affected by diagenetic processes responsible for aragonite dissolution and probably carbonate precipitation

separation of local and regional information in distorted gds response functions by hypothetical event analysis summary electromagnetic investigations are usually intended to examine regional structures where induction takes place at a given period range however the regional information is often distorted by galvanic eiects at local conductivity boundaries bahr 1985 and groom bailey 1989 developed a physical distortion model for decomposing the mt impedance tensor based upon local galvanic distortion of a regional 2 d electromagnetic eld we have extended their method to predict the magnetic variation elds created at an array of sites the magnetic response functions at periods around 1000 s may be distorted by large scale inhomogeneities in the upper or middle crust in this period range the data measured by a magnetometer array contain common information that can be extracted if the data set is treated as a unit for example by using hypothetical event analysis with this technique it is always possible to recover the regional strike direction from distorted data even if a strong spatially varying regional vertical eld component is present in the data set the determination of the regional impedance phases on the other hand is far more sensitive to deviations from the physical distortion model the approach has been used to investigate the iapetus data set for the array which covers an area of 200 km 300 km in northern england southern scotland the technique revealed a common regional strike azimuth of ca n125 0 e in the period range 500 2000 s this direction diiers from the strike indicated by the induction arrows which seem in uenced mainly by local current concentrations along the east weststriking northumberland trough and a ne sw striking mid crustal conductor both impedance phases are positive and diier by ca 1 0 0 which supports the assumptions of distortion elds in the data set and that the regional structure is 2 d

a geophysical approach to the granite batholith under the eastern southern uplands scotland from our interpretation of the bouguer gravity and aeromagnetic anomalies in south east scotland we conclude that a massive granite batholith underlies the greater part of the eastern southern uplands the granite model which we computed earlier from gravity anomalies in the tweeddale area fits the observed magnetic anomalies closely if a normal magnetization of 0 095 a m 1 is assigned similar to values found for exposed local granites further gravity modelling shows that apart from the tweeddale boss the granite shallows to less than 1 km near lammer law in east lothian and extends north of the lammermuir fault a model for the east lothian volcanics was computed from their aeromagnetic anomalies then their gravitational effect was combined with that estimated for the devonian and carboniferous sediments and the result stripped off the observed gravity field the residual gravity anomalies were used to generate a two dimensional model for the granite north of the lammermuir fault the expected tectonic consequences of a massive granite batholith in the eastern southern uplands are compared with the known development of faults and sedimentary basins around its margins

the seismic response of the los angeles basin california using strong motion data recorded in the los angeles region from the 1992 mw 7 3 landers earthquake we have tested the accuracy of existing three dimensional 3d velocity models on the simulation of long period 2 sec ground motions in the los angeles basin and surrounding san fernando and san gabriel valleys first the overall pattern and degree of long period excitation of the basins were identified in the observations within the los angeles basin the recorded amplitudes are about three to four times larger than at sites outside the basins amplitudes within the san fernando and san gabriel valleys are nearly a factor of 3 greater than surrounding bedrock sites then using a 3d finite difference numerical modeling approach we analyzed how variations in 3d earth structure affect simulated waveforms amplitudes and the fit to the observed patterns of amplification significant differences exist in the 3d velocity models of southern california that we tested magistrale et al 1996 graves 1996a hauksson and haase 1997 major differences in the models include the velocity of the assumed background models the depth of the los angeles basin and the depth location and geometry of smaller basins the largest disparities in the response of the models are seen for the san fernando valley and the deepest portion of the los angeles basin these arise in large part from variations in the structure of the basins particularly the effective depth extent which is mainly due to alternative assumptions about the nature of the basin sediment fill the general ground motion characteristics are matched by the 3d model simulations validating the use of 3d modeling with geologically based velocity structure models however significant shortcomings exist in the overall patterns of amplification and the duration of the long period response the successes and limitations of the models for reproducing the recorded ground motions as discussed provide the basis and direction for necessary improvements to earth structure models whether geologically or tomographically derived the differences in the response of the earth models tested also translate to variable success in the ability to successfully model the data and add uncertainty to estimates of the basin response given input scenario earthquake source models

a high resolution magnetotelluric survey of the iapetus suture zone in southwest scotland summary magnetic variation mv surveys show a 40 km wide zone of high electrical conductivity in the middle crust extending across the southern uplands of scotland from the north sea to galloway a new magnetotelluric mt survey across the conductor in galloway was designed to improve the resolution of structure in the depth range 1 15 km and at the same time assess the improvement in resolution that could be achieved in a practical situation by optimizing data acquisition processing analysis and modelling a 40 km long profile was measured doubled for part of its length at a separation of 1 km with a typical site spacing of 2 km or less it was supplemented by shorter parallel and orthogonal lines giving a total of 40 sites galvanic distortion models involving regional 2 d structures gave poor fits to the impedance tensors both for the array as a whole and for individual sites confirming the picture from induction arrow maps that many sites were influenced by 3 d structures on scales of a few kilometres and upwards other indicators of the electrical strike regional and local mv measurements the spatial structure of the groom bailey regional impedance phase and of the rotated off diagonal phase gave values more in agreement with the geological strike of n52 e which was adopted as the most appropriate coordinate system into which to resolve the data the mt impedances were accordingly rotated into directions n38 w tm and n52 e te and inverted using the 2 d code of rodi mackie the investigation explored the sensitivity of the outcome to a wide range of starting model resistivities and roughness parameters and to different data subsets the te mode data were insensitive to structural detail and the best fit achieved was unacceptable suggesting the influence of structure outside the plane of the section the tm mode data could be fitted satisfactorily by varying the model roughness and gave the best control on the geometry of the conductive bodies forward modelling showed that the model generated from the tm mode also fitted the mv data adequately the inversion resolved the conductive zone into distinct blocks with edges matching faults mapped in the surface geology the leadhills fardingmullach and orlock bridge faults and the moffat valley lineament in the resistivity image the faults are vertical structures disrupting other features to a depth of 15 km which agrees with the view that although they originated as thrusts they were reactivated by strike slip motion during oblique closure of the iapetus ocean within some of the blocks are highly conductive regions no more than 4 km wide the most likely explanation is graphite or other metallic mineralization localized in shear zones

destructive strong ground motion in mexico city source path and site effects during great 1985 michoacan earthquake simultaneous consideration of source path and site effects on ground motion during the michoacan earthquake of 1985 allows us to draw coherent conclusions regarding the roles played for the disaster in mexico city by the rupture process the mode of propagation of the waves between the epicentral zone and mexico city and the local amplification in contrast to the horizontal component which showed dramatic amplification for the 2 to 3 sec motion at lake sediment sites we observe almost identical vertical displacement seismograms containing ripples with 2 to 3 sec period throughout the mexico city valley whether the recording site is on the lake sediments or on hard rock we therefore conclude that the 2 to 3 sec motion responsible for the destruction of mexico city was present in the incident field after performing a phase analysis we interpret the signal as the superposition of long period rayleigh waves and short period lg with a dominant period of about 3 sec the analysis of the teleseismic records indicates that the radiation of this event is enhanced for waves around the 3 sec period except in the case of stations for which an anomalous path effect is suspected the records present ripples appearing a few seconds after the beginning of the signal the characteristics of near fault records show that the rupture process consists of the growth of a smooth crack the numerical simulation indicates that the 3 sec period ripples can be explained by a series of changes of the rupture front velocity we examine two alternative source models associated with different crustal models to explain the characteristics of the vertical displacements recorded in mexico city our preferred model attributes the cause of the enhanced 3 sec motion to the irregularity in the rupture propagation in addition to the effect of the local conditions in mexico city this interpretation leads to a very coherent scenario of what happened from the start of the failure on the fault up to the destruction in mexico city this example illustrates the need to consider simultaneously source path and site effects in order to understand strong ground motions

distribution and circulation of water masses in the gulf of cadiz from in situ observations from data collected during golfo 2001 survey a study was conducted on the circulation and distribution of water masses in the gulf of cadiz in the spring of 2001 the general surface circulation in the gulf of cadiz is anticyclonic with

factors contributing to the catastrophe in mexico city during the earthquake of september 19 1985 the extensive damage to high rise buildings in mexico city during the september 19 1985 earthquake is primarily due to the intensity of the ground shaking exceeding what was previously considered credible for the city by mexican engineers there were two major factors contributing to the catastrophe resonance in the sediments of an ancient lake that once existed in the valley of mexico and the long duration of shaking compared with other coastal earthquakes in the last 50 years both of these factors would be operative again if the guerrero seismic gap ruptured in a single earthquake

modeling the jiyang depression northern china using a wave field extrapolation finite difference method and waveform inversion we study the 2d seismic velocity structure of the south edge of the jiyang depression in the bohai bay basin northern china using recordings from 28 broadband seismic stations and a method combining an shear horizontal sh forward synthetic calculation and waveform inversion the forward synthetic cal culations are performed by a finite difference fd method with its computational domain localized in the basin area and input motions at the base of the model ex trapolated from the displacement recorded at a nearby hard rock station in waveform inversion the shapes of the strata are varied to search for a best fitting model based on the fit of relative timing amplitude and phase of the synthetics to the seismic data numerical experiments using synthetic data and test models demonstrate the validity resolution and sensitivity of our technique with this technique we obtain two cross sections of the south edge of the jiyang depression the bohai bay basin northern china the waveform inversions using the seismic data from four teleseis mic events show consistent results for both cross sections those inversion results are also in agreement with a geological cross section available in the region our models estimate that the south edge of the jiyang depression is juxtaposed against the luxi uplift and its structures our inversion results also reveal a blind fault in the region and features suggesting major extensions in paleogene

influence of the atlantic inflow and mediterranean outflow currents on late quaternary sedimentary facies of the gulf of cadiz continental margin abstract the late quaternary pattern of sedimentary facies on the spanish gulf of cadiz continental shelf results from an interaction between a number of controlling factors that are dominated by the atlantic inflow currents flowing southeastward across the cadiz shelf toward the strait of gibraltar an inner shelf shoreface sand facies formed by shoaling waves is modified by the inflow currents to form a belt of sand dunes at 10 20 m that extends deeper and obliquely down paleo valleys as a result of southward down valley flow a mid shelf holocene mud facies progrades offshore from river mouth sources but atlantic inflow currents cause extensive progradation along shelf toward the southeast increased inflow current speeds near the strait of gibraltar and the strong mediterranean outflow currents there result in lack of mud deposition and development of a reworked transgressive sand dune facies across the entire southernmost shelf at the outer shelf edge and underlying the mid shelf mud and inner shelf sand facies is a late pleistocene to holocene transgressive sand sheet formed by the eustatic shoreline advance the late quaternary pattern of contourite deposits on the spanish gulf of cadiz continental slope results from an interaction between linear diapiric ridges that are oblique to slope contours and the mediterranean outflow current flowing northwestward parallel to the slope contours and down valleys between the ridges coincident with the northwestward decrease in outflow current speeds from the strait there is the following northwestward gradation of contourite sediment facies 1 upper slope sand to silt bed facies 2 sand dune facies on the upstream mid slope terrace 3 large mud wave facies on the lower slope 4 sediment drift facies banked against the diapiric ridges and 5 valley facies between the ridges the southeastern sediment drift facies closest to gibraltar contains medium fine sand beds interbedded with mud the adjacent valley floor facies is composed of gravelly shelly coarse to medium sand lags and large sand dunes on the valley margins by comparison the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand because of the complex pattern of contour parallel and valley perpendicular flow paths of the mediterranean outflow current the larger scale bedforms and coarser grained sediment of valley facies trend perpendicular to the smaller scale bedforms and finer grained contourite deposits of adjacent sediment drift facies radiocarbon ages verify that the inner shelf shoreface sand facies sedimentation rate 7 1 cm kyr mid shelf mud facies maximum rate 234 cm kyr and surface sandy contourite layer of 0 2 1 2 m thickness on the cadiz slope 1 12 cm kyr have deposited during holocene time when high sea level results in maximum water depth over the gibraltar sill and full development of the atlantic inflow and mediterranean outflow currents the transgressive sand sheet of the shelf and the mud layer underlying the surface contourite sand sheet of the slope correlate respectively with the late pleistocene sea level lowstand and apparent weak mediterranean outflow current

euro seistest determination of the geological structure of the volvi basin and validation of the basin response the volvi basin northern greece is a 5 km wide graben of neogene age it has been chosen by european union directorate general xii for science research and development in climatology and natural hazards as a test site euro seistest for studying site effects in one of europe s most seismic active areas the site is located in the epicentral area of the 1978 ms 6 5 earthquake that struck the city of thessaloniki the geometry velocity and stratigraphy of the basin were not well known consequently a number of geophysical and geotechnical studies were performed to determine the 2d geometry and the dynamic characteristics vp vs qs of the valley investigations included 78 refraction tests 20 surface wave inversions and 8 borehole seismic tests as well as geotechnical tests this article presents the methodology used for the geophysical investigation and the results in its center the graben depth reaches 190 m and at least four normal faults have been located along the section the overall graben structure and the fault locations have been confirmed by the records of a shot generated at one edge of the valley and by the measurements of a local earthquake wave propagation appears to be strongly influenced by the thickness of the sediments which in turn is controlled by fault displacements simple 2d modeling of the basin response was performed and the results are in good agreement with weak motion data confirming the validity of the seismic structure inferred from seismic prospecting

evidence for sediment eruption on deep sea floor gulf of mexico a large crater has been discovered on the sea floor gulf of mexico in a water depth of 2176 meters deep tow high resolution imagery shows that the crater is cut into a low hill surrounded by near surface concentric faults approximately 2 million cubic meters of ejected sediment forms a peripheral debris field the low hill and faults may be related to mud diapirism or intrusion of gas hydrates into near surface sediments a recent eruption evacuated sediments from the crater apparently because of release of overpressured petrogenic gas

detailed evaluation of site response estimation methods across and along the sedimentary valley of volvi euro seistest using records from the euro seistest area near thessaloniki greece we studied empirically the amplification effects of a sedimentary valley one of the main advantages of this site is the very detailed knowledge of the near subsurface structure due to exhaustive geophysical and geotechnical measurements and data analyses jongmans et al 1998 the uniqueness of the seismological data set is the very dense network 31 stations were installed perpendicular and parallel to the valley axis with minimum interstation distances of 250 m the mean ampli fications along both axes estimated from 13 local events were imaged using the traditional spectral ratio technique variations of the mean amplifications due to the choice of different data subsets were tested by the use of up to 89 local events supplemented by four teleseismic events offering good low frequency resolution results obtained using the traditional spectral ratio technique were compared with results from a generalized inversion technique the h v ratio technique a coda wave technique and nakamura s technique advantages and disadvantages of each method are outlined especially from a practical point of view

revising the 1759 kattegat earthquake questionnaires using synthetic wavefield analysis abstract in aseismic regions the lack of large earthquakes in modern times makes the study of historical earthquakes essential as such events often serve as the yardstick for estimating seismic hazards since the largest earthquakes with few exceptions took place hundreds of years ago we may claim that their recurrence times are very large or alternatively their magnitudes are biased upwards scandinavia is no exception in this regard and one interesting event in particular is the kattegat earthquake of 22 december 1759 it is unique in the sense that a macroseismic questionnaire was circulated in zealand within days of its occurrence the puzzling feature here is that intensity ranges from vii to i across zealand but then given i iii for hamburg and other north german cities we have chosen two avenues in reassessing the magnitude of the kattegat earthquake namely 1 2d finite difference fd waveform modelling to evaluate possible amplitude amplifications due to sub surface basin structures and 2 re assessing the intensity reports of the 1759 kattegat earthquake in light of the new european macroseismic scale ems 98 our 2d fd results show that the presence of the danish and the north german basins led to shear wave amplitude amplification by factors of 10 and 5 respectively this is equivalent to a significant increase or overestimation of the area of perception tied to i iii the re assessment of the macroseismic observations in light of the above results and the ems 98 downgrade the kattegat earthquake ms magnitude from 5 6 to 5 1

methods used in the u s s r to reduce near surface inhomogeneity effects on deep magnetotelluric sounding abstract the effects of near surface inhomogeneities on deep magnetotelluric sounding are considered methods are suggested allowing for and ruling out the near surface effects examples for practical application of these methods are given

contourite processes associated with the mediterranean outflow water after its exit from the strait of gibraltar global and conceptual implications we characterize the eastern gulf of cadiz proximal to the strait of gibraltar using a multidisciplinary approach that combines oceanographic morphosedimentary and stratigraphic studies two terraces upper and lower were identified along the middle slope they are composed of several associated morphologic elements including two large erosive channels which allow us to determine a new and more detailed understanding of the mediterranean outflow water mow pathway and its deceleration upon exiting the strait of gibraltar there is evidence for along slope circulation and additional secondary circulation oblique to the main flow the present upper core of the mow flows along the upper terrace and the lower core flows along the lower terrace however the lower terrace shows larger and better defined erosive features on the seafloor than does the upper terrace we attribute this to a denser deeper and faster mow circulation that prevailed during past cold climates development of the present features started ca 3 8 3 9 ma but the present morphology was not established until the late pliocene early quaternary 3 2 to older than 2 0 ma when the mow was enhanced coeval with global cooling a sea level fall and an increase in thermohaline circulation we propose a direct link between the mow and the atlantic meridional overturning circulation and therefore between the mow and both the northern hemisphere and global climate our results have enabled a better understanding of a major overflow related to an oceanic gateway and are of broad interest to geologists climatologists oceanographers and petroleum geologists

observations and numerical modelling of the mediterranean outflow splitting and eddy generation during the canigo project the generation of dipolar eddies in the mediterranean water mw layers off south portugal was recorded by subsurface floats from february to november 1998 the role of the bottom topography on the spreading of the mw in the gulf of cadiz and on the generation of these mw eddies was studied in the framework of the medtop project some of the hydrological and velocity observations collected during medtop in 2001 and 2002 also revealed the association of anticyclones meddies and cyclones in the region just south of the portimao canyon the evolution of these structures is studied with subsurface floats deployed in the vortices the splitting of the mw plume in the gulf of cadiz and the generation mechanisms of the mw vortices in the region from portimao canyon to the estremadura promontory are investigated using a bottom following numerical primitive equations model in a regional ocean configuration incorporating the realistic topography of the iberian continental slope the model reproduces the mw division into different cores in the gulf of cadiz as well as the generation of baroclinic dipoles downstream of portimao canyon the role of the bottom topography in the eddy generation process is discussed

the cenozoic tectonic evolution of the great north china two types of rifting and crustal necking in the great north china and their tectonic implications abstract the great north china underwent widespread continental rifting during cenozoic time that caused extension and necking of the crust in this area and formed four major rift systems in this paper we provide geological and geophysical evidence that rifting and extension in the western part of the great north china differs from that in its eastern part in age and nature the space time relationships show that the origins of the western rift group of the great north china are probably related to the tibet plateau tectonics whereas those of the eastern rift group are related to the subduction events in the western margin of the pacific plate furthermore the quaternary tectonic regime in the eastern part of the great north china differs from the neogene tectonic pattern and shows relations with western rifts we speculate that during the quaternary the processes that caused rifting in the west have spread further east and now affect the eastern part of the great north china

wave equation traveltime inversion this paper presents a new traveltime inversion method based on the wave equation in this new method designated as wave equation traveltime inversion wt seismograms are computed by any full wave forward modeling method we use a finite difference method the velocity model is perturbed until the traveltimes from the synthetic seismograms are best fitted to the observed traveltimes in a least squares sense a gradient optimization method is used and the formula for the frechet derivative perturbation of traveltimes with respect to velocity is derived directly from the wave equation no traveltime picking or ray tracing is necessary and there are no high frequency assumptions about the data body wave diffraction reflection and head wave traveltimes can be incorporated into the inversion in the high frequency limit wt inversion reduces to ray based traveltime tomography it can also be shown that wt inversion is approximately equivalent to full wave inversion when the starting velocity model is

a geology based 3d velocity model of the los angeles basin sediments abstract seismic hazard studies of the los angeles basin area require a realistic seismic velocity model we use geologic information about depth to crystalline basement depths to sedimentary horizons uplift of sediments and surface geology in a velocity depth age function to construct a three dimensional velocity model in earthquake location tests the model predicts travel times satisfactorily and in earthquake ground motion simulations the model correctly determines the timing and amplitude of late arriving waves

lateral variation in the sedimentary structure of west bohai bay basin inferred from p multiple receiver functions the widely distributed cenozoic sediments in the bohai bay basin give rise to noticeable modification of broadband teleseismic p waveforms at one station in the west bohai bay basin the observed amplitude of tangential p receiver func tions is significantly above the noise level and sedimentary reverberations e g the p type wave pppp remain one of the most prominent features in both the radial and tangential components to investigate the lateral heterogeneity structure under this site a 3d raytracing technique is used to compute the teleseismic p wave response and a fast simulated annealing algorithm is applied to the simultaneous inversion of radial receiver functions for different backazimuths an upper crustal structure con sisting of shallow dipping sedimentary layers with low seismic velocities and large poisson s ratios is proposed to interpret the observed seismic data the west dipping interfaces we obtained are consistent with the north northeast south southwest sur face geology in north china and tertiary extensional fault structures may be re sponsible for the formation of dipping sedimentary layers

hydrocarbon derived ferromanganese nodules in carbonate mud mounds from the gulf of cadiz mud breccia sediments and clasts as nucleation sites abstract more than 500 fe mn nodules were sampled during the anastasya 01 cruise tasyo project along the continental margin of the gulf of cadiz eastern central atlantic at the confluence of the mediterranean sea with the atlantic ocean where extensive nodule fields were discovered based on wide previous studies that included swath bathymetry multi channel and very high resolution seismic reflection gravimetry magnetism and underwater photography surveys nodules were collected at water depths ranging from 850 to 1000 m associated with hydrocarbon derived ankerite and dolomite chimneys and crusts thirty six selected samples among the various morphological types were used for the laboratory analysis of physical properties morphology colour surface texture sphericity weight and size mineralogy xrd optical and electronic microscopy and geochemistry xrf aas icp ms epma gc ms s c and o isotopes nodules show a wide range of sizes densities weights and morphologies nodules are formed of multiple millimetre thick layers of fe and mn oxyhydroxides surrounding the nucleus composed of early middle miocene plastic marl the textures developed by the fe and mn oxyhydroxides layers are massive laminated detrital and mottled to dendritic goethite lepidocrocite mn oxides 7 a manganates and 10 a manganates quartz and phyllosilicates are the main components accessory minerals are calcite dolomite siderite rhodochrosite kutnohorite pyrite chalcopyrite potassium feldspar zircon rutile ilmenite and chlorite fe mn carbonates from the siderite rhodochrosite continuous series are forming part of the marl nuclei framboidal filamentous and globular textures are observed in fe mn oxides and pyrite suggesting biogenic origin nodules show a high mean abundance of fe 38 6 moderate mn 6 0 and low contents of trace metals and rees compared to the average content of deep seabed polymetallic nodules from other oceanic areas the mn fe ratio ranges from 0 07 to 0 25 the studied nodules hold in their oxide layers hydrocarbons n alkanes derived from marine bacterial activity aromatic hydrocarbons in the form of phenanthrene are also present which is characteristic of mature petroleum the structure mineralogy and chemical composition in the studied nodules are similar to those of diagenetic hydrogenetic continental margin nodules rather than deep sea nodules we suggest that the formation of this type of nodule occurred through a combined diagenetic hydrogenetic growth process involving fluid venting from deep seated hydrocarbon reservoirs bio mineralization processes and bottom current erosion

interaction of tectonic and depositional processes that control the evolution of the iberian gulf of cadiz margin abstract this study provides an integrated view of the growth patterns and factors that controlled the evolution of the gulf of cadiz continental margin based on studies of the tectonic sedimentologic and oceanographic history of the area seven sedimentary regimes are identified but there are more extensive descriptions of the late cenozoic regimes because of the larger data base the regimes of the mesozoic passive margin include carbonate platforms which become mixed calcareous terrigenous deposits during the late cretaceous early tertiary the oligocene and early miocene terrigenous regimes developed in contrast over the active and transcurrent margins near the african iberian plate boundary the top of the gulf of cadiz olistostrome emplaced in the late miocene is used as a key horizon to define the post orogenic depositional regimes the late miocene progradational margin regime is characterized by a large terrigenous sediment supply to the margin and coincides with the closing of the miocene atlantic mediterranean gateways the terrigenous drift depositional regime of the early pliocene resulted from the occurrence of high eustatic sea level and the characteristics of the mediterranean outflow currents that developed after the opening of the strait of gibraltar the late pliocene and quaternary regimes are dominated by sequences of deposits related to cycles of high and low sea levels deposition of shelf margin deltas and slope wedges correlate with regressive and low sea level regimes caused by eustasy and subsidence during the highstand regimes of the holocene inner shelf prograding deltas and deep water sediment drifts were developed under the influence of the atlantic inflow and mediterranean outflow currents respectively a modern human cultural regime began 2000 years ago with the roman occupation of iberia human cultural effects on sedimentary regimes may have equalled natural factors such as climate change interplay of tectonic and oceanographic controls dominated the evolution of the cadiz margin during the cenozoic depositional sequences formed where the tectonic setting provided the accommodation space and the shape of the deposits has been greatly influenced by the strong unidirectional atlantic inflow currents on the shelf and mediterranean outflow currents on the slope the entire cycle of the inflow and outflow deposition along the margin has been controlled first by the tectonic evolution of the betic and rif gateways which become closed during the late miocene and after the messinian by the opening of the strait of gibraltar strong current development during eustatic sea level highstands of the pliocene and quaternary has controlled deposition because of maximum sill depths at gibraltar for water circulation lowstand sea levels slowed circulation and resulted in mud drapes over the slope and regressive stratigraphic sequences over the shelf more recently the human industrial revolution has caused heavy metal contamination of sediment and water over the cadiz margin human activity also has affected sedimentation rates because of deforestation that caused increased depositional rates near undammed rivers and decreased rates where rivers have been dammed future research efforts will need to focus on 1 the effect of increased mediterranean outflow caused by river damming plus global warming and the increased outflow as a potential trigger for new ice ages 2 assessments of geologic hazards for planning man made shoreline structures developing offshore petroleum resources and maintaining undersea communications cables and 3 confirmation of the general geologic history of the cadiz margin

the betic orogen and the iberian african boundary in the gulf of cadiz geological evolution central north atlantic abstract the study of the gulf of cadiz on the basis of multichannel seismic profiles and wells illustrates the stratigraphy and tectonics the evolution of the southern iberian margins was more complex than in most north atlantic margins since it entailed several phases of rifting convergence and strike slip motions three main tectonic provinces surround the internal zones of the gibraltar arc orogenic belt these include in the iberian margin of the gulf of cadiz the flysch units of the campo de gibraltar complex the betic external zones and the neogene basins of the guadalquivir valley fault bounded blocks of flysch and subbetic units crops out over large areas of the southeastern iberian shelf the basement of the northwestern area in contrast is represented by the paleozoic rocks of the hercynian massif of iberia half graben structures determined the main structural trends of the margin during the mesozoic which were affected by inversion structures during the neogene compressional stages the mesozoic and lower cenozoic units are best observed in wells and seismic profiles from the northern area these units are either obscure below a thick olistostrome deposit or are absent in most of the rest of the gulf of cadiz seven lithoseismic units from triassic to upper oligocene and another seven neogene and quaternary units are identified based on the relationship to the depositional sequence and the emplacement of the olistostrome the first tectonic phase was characterized by a passive margin which was controlled by the development of half graben extensional structures and carbonate platforms this evolution comprises the mesozoic and early cenozoic ocean spreading in the north atlantic induced extensional tectonics which deformed the cretaceous syn tectonic post aptian deposits increased amounts of terrigenous materials were supplied to the margin from aptian to albian times controlling depositional patterns while terrigenous siliciclastic facies replaced the jurassic carbonate platforms from middle eocene to early miocene the margin was influenced by the relative motions of iberia and africa and the development of the alpine orogeny the iberian african boundary in the gulf of cadiz experienced transpression and the mesozoic basins probably underwent subduction the emplacement of an olistostrome took place in the gulf of cadiz towards the central atlantic basin plains during the tortonian the end of the olistostrome emplacement during the late miocene coincides with accelerated tectonic subsidence while thick progradational and aggradational depositional sequences were developed the occurrence of closely juxtaposed regions of compression and extension during the miocene may reflect the westwards progression of the gibraltar arc mountain front over a subducting thinned tethys crust when the motion between iberian and africa was n s to nnw sse oriented the migration of the arcuate mountain front into the eastern gulf of cadiz is attributed to a mechanism of collision induced delamination of the mantle lithosphere in contrast to the betic rif belts however there is no geophysical evidence to postulate that continental collision took place in the gulf of cadiz during the cenozoic

inversion of electromagnetic induction data for the iapetus suture zone in the uk abstract the iapetus suture zone isz corresponding to the join between the european crust of northern england and the north american derived crust of scotland has been examined by numerous seismic and induction experiments over a 20 year period the university of edinburgh electromagnetic em induction group is in the process of building a database comprising magnetotelluric and magnetic variation stations from this region in this paper we present 2d inversions of selected induction sites along roughly north south traverses across the isz in northern england and southern scotland the first inversion involves both the transverse electric te and transverse magnetic tm modes for 10 audio magnetotelluric approximately 100 0 1 hz sites care has been taken to remove the effects of static shifts this inversion resolves a conductive feature beginning under the southern northumberland trough at depths of 10 km dipping to the south below the alston block in the second inversion 12 longer period magnetic variation sites were added to the original data set improving the resolution of this feature which correlates well with a dipping bright reflector detected by the north east coast nec seismic reflection experiment in addition the southern uplands anomaly is imaged no north dipping mid to deep crustal conductor is observed which would correlate to north dipping palaeozoic subduction under laurentia

looking for clues to paleoceanographic imprints a diagnosis of the gulf of cadiz contourite depositional systems a new morphosedimentary map of the gulf of cadiz is presented showing the contourite depositional system on the gulf s middle slope this map is constructed from a broad da tabase provided by the spanish research council and the u s naval research laboratory our map shows that this contourite depositional system comprises five morphosedimentary sectors 1 proximal scour and sand ribbons 2 overflow sedimentary lobe 3 channels and ridges 4 contourite deposition and 5 submarine canyons the gulf of cadiz con tourite depositional system stems directly from the interaction between mediterranean out flow water and the seafloor its morphosedimentary sectors are clearly related to the sys tematic deceleration of the mediterranean outflow waters westward branches bathymetric stress on the margin and the coriolis force the slope s depositional system can be consid ered as a mixed contourite and turbidite system i e a detached combined drift and fan

emplacement of a passive margin evaporitic allochthon in the betic cordillera of spain the triassic section of the external domain of the betic cordillera is rootless and thus allochthonous the frontal accretionary wedge of the guadalquivir allochthon consists dominantly of triassic evaporites and red beds forming a melange with upper cretaceous paleogene deep water sedimentary rocks throughout the unit jurassic rocks are absent it is here proposed that the widespread triassic evaporites of the guadalquivir allochthon were originally emplaced as gravitational allochthonous masses in a passive margin setting much like the widespread allochthonous salt of the texas louisiana gulf of mexico thus on the betic passive margin allochthonous evaporites were first emplaced during the late cretaceous paleogene later during neogene time these evaporites were overthrusted as an accretionary wedge to form the guadalquivir allochthon a schematic reconstructed position of the guadalquivir allochthon places the original passive margin allochthon in its continental slope setting

mud volcanoes revealed and sampled on the western moroccan continental margin we surveyed and sampled two active and 3 inactive mud volcanoes on the atlantic continental margin of morocco the active mud volcanoes named yuma and ginsburg are each about 4 km in diameter rise between 150 250 meters above the seafloor and are the first active methane related mud volcanoes to be identified in this region the inactive volcanoes range in diameter from 1 3 km with relief between 50 and 80 meters gravity cores from the crest of the active mud volcanoes yielded methane hydrates and mud breccia deposits chemosynthetic communities of pogonophora worms and the bivalve solemya were found on the surface of the active mud volcanoes sediment cores from the inactive volcanoes contained hemipelagic sediments rich in foraminifera and ahermatypic coral debris overlying mud breccia

structure and evolution of the olistostrome complex of the gibraltar arc in the gulf of cadiz eastern central atlantic evidence from two long seismic cross sections reflection profiles characterize the structure and the upper mesozoic to cenozoic deposits of the gulf of ca diz region two long ene wsw multichannel seismic lines ca 400 500 km long are analyzed to study the evolution of the area from the continental shelf to the horseshoe and seine abyssal plains the huge allochthonous deposits emplaced in this region the socalled olistostrome of the gulf of cadiz are described in terms of three different domains on the basis of the seismic architecture the main tectonic features and the nature of the basement oceanic or continental the eastern domain extends along the continental shelf and upper and middle slope and corresponds to the offshore extension of the betic rifean external front it is characterized by salt and shale nappes later affected by extensional collapses the central domain develops along the lower slope between the betic rifean front and the abyssal plains and is characterized by a change in dip of the allochthonous basal surface and the basement the allochthonous masses were emplaced by a combined gravitational and tectonic mechanism the northern boundary of this domain is marked by the occurrence of an outstanding wnw ese trending thrust fault with a strike slip component termed here as the gorringe horseshoe fault the westernmost domain corresponds to the abyssal plains where the distal emplacement of the allochthonous body takes place it is characterized by thrust faults affecting both the sedimentary cover and the oceanic basement the allochthonous masses show a less chaotic character and the thickness decreases notably these domains represent different evolutionary steps in the mechanisms of emplacement of the allochthonous units the eastern domain of the allochthonous units was emplaced as part of the pre messinian orogenic wedge related to the collision that gave rise to the betic rifean belt whereas the allochthonous wedge of the central and western domains were emplaced later as a consequence of the ne sw late miocene compression that continues in present times

implications for the lithospheric geometry of the iapetus suture beneath ireland based on electrical resistivity models from deep probing magnetotellurics

large scale slope failure involving triassic and middle miocene salt and shale in the gulf of cadiz atlantic iberian margin sheets of salt and ductile shale advancing beyond the thrust front of the gibraltar arc iberian moroccan atlantic continen tal margin triggered downslope movements of huge allochth onous masses these allochthons represent the cadiz nappe which detached from the gibraltar arc along low angle normal faults and migrated downslope from the iberian and moroccan continental margins towards the atlantic ocean extensional tectonics initiated upslope salt withdrawal and downslope diapirism during large scale westward mass wasting from the shelf and upper slope low angle salt and shale detachments bound by lateral ramps link extensional structures in the shelf to folding thrusting and sheets of salt and shale in the gulf of cadiz from backstripping analyses carried out on the depo centres of the growth fault related basins on the shelf we infer two episodes of rapid subsidence related to extensional collapses these were from late tortonian to late messinian 200 400 m myr 1 and from early pliocene to late pliocene 100 150 m myr 1 the extensional events that induced salt movements also affected basement deformation and were probably associated with the westward advance of frontal thrusts of the gibraltar arc as a result of the convergence between africa and eurasia the complexities of salt and or shale tectonics in the gulf of cadiz result from a combination of the deformations seen at convergent and passive continental margins

should the electric line be straight in magnetotelluric surveys the usual description of electric field measurements in terms of potential differences is not entirely adequate at high frequencies in general the telluric electric field is non conservative and voltage measurements depend on the contour described by the cable a simple error analysis helps to recognize those situations where systematic errors may be significant for present day standards

site effects in parkway basin comparison between observations and 3 d modelling summary this paper is a contribution towards filling the large gap that currently exists between theoretical and observational studies of site effects we present results of numerical modelling of ground motion in parkway valley a small alluvial valley in new zealand we use a 3 d finite difference code and compute ground motion for the vertical incidence of plane waves p and s the results are compared with a thorough analysis of site effects in this basin presented previously based on the records obtained using a dense temporary seismic array the results of the simulations confirm the observation that ground motion in parkway basin is dominated by locally generated surface waves diffracted surface waves are generated mainly towards the south this direction does not depend on the polarization of the incident waves we obtain a very good agreement between observed and predicted transfer functions for the two horizontal components of stations located towards the centre of the valley our model clearly performs less well towards the edges of the basin where the details of the real structure are missing in our model our results suggest that the 3 d shape of the basin may be more important than a detailed knowledge of the subsoil mechanical properties in predicting ground response a correct estimate of the absolute amplification level however requires a correct estimate of the impedance contrast but this also applies to any modelling of site response

a study on the response of a soft basin for incident s p and rayleigh waves with special reference to the long duration observed in mexico city abstract the responses of two types of soft basins for incident sh sv p and rayleigh waves in a 2 d elastic half space are investigated with special reference to the long duration of strong motions observed in mexico city during the michoacan mexico earthquake of 1985 first the difficulty for simple 1 d models to reproduce the later part of the accelerogram observed in mexico city is shown then the effect of a 2 d deep basin structure of a relatively large scale 10 km wide and 1 km deep and a moderate impedance ratio 2 5 is studied the results show that surface waves generated at the edges of the basin are clearly propagating inside the basin back and forth however the increase in duration by these surface waves is 20 sec at most introducing another much softer layer inside the basin we found that the calculated durations and envelop shapes become very similar to the observed ones because of the slowness of surface wave propagation these theoretical studies suggest that the exceptionally long duration observed in mexico city might be caused by a strong constructive interaction of soft surface layers with a deep basin structure beneath the city

propagational corrections for basin structure landers earthquake transfer functions appropriate for correcting for propagational distortion caused by basin structures 2d given a hard rock 1d response are developed the transfer functions are generated theoretically using a hybrid method where the finite difference technique is employed only in the basin this allows for efficient computation and the ability to separate path contributions due to body waves and surface waves the procedure is used to explain some strong motions observed in the los angeles basin relative to the hard rock site pasadena during the 1992 landers earthquake

causes of high crustal conductivity beneath the iapetus suture zone in great britain summary two magnetic variation anomalies in southern scotland and northern england have been linked to the position of the iapetus suture previous magnetotelluric measurements along a 140 km profile that crosses both anomalies have been supplemented by new high quality broad band observations groom bailey decomposition of the impedance and hypothetical event analysis of the magnetic variation data agree in defining different strike directions for the two halves of the mt profile n50 e for the northern part over the southern uplands and n90 e for the southern part over the northumberland trough te and tm mode apparent resistivity and phase data at representative frequencies have been inverted for the two sub profiles separately using the smooth inverse of smith booker 1990 the uppermost layer has a low resistivity is variable in thickness and correlates well with the known position and thickness of carboniferous sedimentary rocks the second layer has a very high resistivity thousands of m and reaches the surface where the lower palaeozoic metamorphic rocks of the southern uplands crop out a relatively rapid transition to low resistivities occurs at depths of between 8 and 16 km the conducting layer appears to be quasi continuous but where the profile crosses the anomalies identified by magnetic variation measurements the conductance increases and the upper surface is shallower the spatial coverage of the magnetic variation data has enabled us to extrapolate conductive features away from the line of section and project the electrical image onto the nec vertical incidence seismic reflection profile in the north sea there is excellent agreement between a number of features in the acoustical and electrical images the shallowing of the low resistivity layer to form a narrow wedge like feature corresponds to the offshore position of the stublick fault while to the north of the fault the top of the layer coincides with a south dipping reflector thought to be a thrust however the zones of high conductance and high lower crustal reflectivity do not in general correlate the good conductor beneath he northumberland trough spans two zones which were differentiated on ihe nec profile in terms of their reflectivity the shape of the conductor s upper surface in the vicinity of the stublick fault agrees well with the model of chadwick holliday 1991 who proposed that the iapetus suture was a whole crustal shear with a gently dipping central ramp the coincidence between the present day low resistivity layer and a surface of weakness that was active 300 400 myr ago is much more readily explained in terms of mineralogy the presence of graphite than the presence of fluids there is also strong support for interpreting the northern conductor in the same way its upper surface is relatively flat and occupies the position predicted as the horizontal detachment surface over which wedges of the southern uplands rocks were thrust its northeastward extension is sampled by a group of carboniferous vents from which xenolith suites of crust and upper mantle origin have been obtained the middle to upper crust is believed to be represented by quartzo feldspathic gneisses that contain abundant graphite

a finite difference approach for simulating ground responses in sedimentary basins quantitative modelling of the nile valley egypt summary on 1992 october 12 egypt was struck by an earthquake with m b 5 9 or equivalently m s 5 2 it occurred southwest of greater cairo cairo and giza and was widely felt from alexandria in the north to aswan in the south this earthquake also caused widespread destruction and damage mostly concentrated in greater cairo along the nile valley and in the nile delta area the extent of damage caused by this earthquake motivated this study we computed a 2 d finite difference fd synthetic wavefield for a prototype model of the nile valley with a maximum sedimentary basin thickness of 5 km and assumed uniform crustal thickness of 35 km a variant of the latter model tested is a thin crust moho bump beneath the basin we used a 2 5 hz ricker wavelet source that excites both p and or s waves initiated as point sources focal depths vary from 5 to 20 km to simulate different scenarios we investigated how various model features such as source location focal depth and epicentral distance deep sedimentary basin near surface attenuation and crustal thinning beneath the sedimentary basin may influence the wavefield across the nile valley the outstanding result of the wavefield simulations is that the nile valley sedimentary basin and the associated strong impedance contrast produce significant focusing and amplification effects it was found that the peak amplitudes might vary by one order of magnitude in the 3 8 hz frequency band across the nile valley the high vulnerability of the buildings in greater cairo in combination with focusing amplification in the nile valley explain qualitatively the extensive damage in the city itself even for moderate size earthquakes in fact the predicted area of high amplification coincides closely with that of greatest housing damage the effects of near surface attenuation in the uppermost 500 m within the basin and or crustal thinning beneath the basin is less significant these results are interesting on two accounts 1 the extent of variation in the synthetic wavefield will always be less than that observed but unfortunately there are no egyptian records to corroborate these results and 2 wavefield variations are significantly larger than those deduced from ranges of q variations and seismotectonic modelling as used in most seismic risk analysis these results imply that the wavefield simulations can be very informative and provide realistic results in particular when accurate structural model information is available and even more so if the synthetics can be corroborated with dense seismic network recordings

modeling the seismic response of basin edge structure by a sequential waveform method a sequential waveform method is developed to simulate the seismic response of basin edge structure excited by a plane incident p wave the full procedure involves a a previous parameterization of the investigated model using the seismic wave velocities and depths of the sedimentary stratifications b an input motion determined from the records at stations installed on hard rock c forward computation of the p sv elastic wave field by means of a two dimensional finite difference fd method d the optimization of the model vector using simulated annealing technique and comparing the simulated seismic response of the tested structure with the observed wave field e the correction of the initial model by trial and error by testing the differences between synthetics and observed data and f the final solution obtained by iteration using the conjugate gradient algorithm the search of an optimal basin edge model has been parallel processed by varying the shapes and velocities of strata on the basis of the fitting of relative timing amplitude and phase between the output and the observed data the input motion and sensitivity have been checked and the validity of the method has been demonstrated by numeric analysis using the teleseismic records generated by 7 earthquakes recorded at 26 broadband seismic stations we have studied the seismic velocity structure of the southern edge of the jiyang depression located in the bohai bay basin northern china two cross sections show an agreement between the velocity results and the geological sections available in the region in addition we obtain evidence of three hidden faults under the sections and features that suggest major extensions at the paleogene

the el arraiche mud volcano field at the moroccan atlantic slope gulf of cadiz the el arraiche field is a new mud volcano field discovered near the moroccan shelf edge in the gulf of cadiz that consists of 8 mud volcanoes in water depths from 200 to 700 m the largest mud volcano in the field al idrissi mud volcano is 255 m high and 5 4 km wide the cluster was discovered during a survey with the rv belgica and studied further during leg 2 of the ttr 12 survey onboard the r v prof logachev the 2002 surveys yielded detailed multibeam bathymetry over a 700 km 2 study area dense grids of high resolution seismic data deep tow sub bottom profiles sidescan sonar mosaics over the major structures selected video imagery lines video guided grab samples dredge samples gravity cores and box cores were collected for groundtruthing purposes eight mud volcanoes in water depths from 200 to 700 m cluster around two sub parallel anticlines and associated active extensional faults rock clasts and regional seismic data locate the el arraiche field over a late miocene pliocene extensional basin the onset of mud volcanic activity is estimated at about 2 4 ma and probably roots in the cretaceous miocene accretionary wedge stacked outflows are visible up to a depth of about 500 m below the sea floor the occurrence of long lived mud volcanoes bear witness to continued overpressure generation at depth either by in situ oil and gas generation or by focussed flow and accumulation in the area geochemical analyses of pore water from cores demonstrate the presence of thermogenic hydrocarbon processes the activity of the mud volcanoes is indicated by the thickness of hemi pelagic sediments covering extruded mud breccia the

magnetic variation anomalies in northern england and southern scotland single station transfer functions linking the time variations of the vertical and horizontal components of the magnetic field at stations in northern england and southern scotland have been compiled into a uniform data set from hypothetical event analysis we show here that there are two distinct anomalies in the borders region one runs south west to north east immediately to the south east of the southern uplands fault the second follows the northumberland basin and seems to exist because the conductive sedimentary rocks filling the basin create a link between the irish and north seas if the lapetus suture is marked by a conductivity anomaly as has been suggested1 2 these results place it beneath the southern uplands unless it is masked by the surface conductor in the northumberland basin

effects of sedimentary layers on surface wave propagation abstract earthquake ground motions in the period range from 3 to 10 sec observed in the kanto plain japan during the western nagano earthquake of 1984 were predominantly trapped within sedimentary layers the dispersion observed in the transverse component on sedimentary rock is similar to the dispersion calculated for the fundamental love wave in a 1 d velocity model derived from the results of the seismic surveys forward modeling with a 2 d finite difference method shows good agreement with observed seismograms when realistic 2 d underground structures derived from seismic surveys are used

seismic waveform modeling in the los angeles basin the proximity of several recent earthquakes to the los angeles sedimentary basin provides an opportunity to isolate the effects of the basin on wave propagation the 4 october 1987 aftershock m l 5 3 of the whittier narrows sequence and the 28 june 1991 sierra madre mainshock m l 5 8 are on a similar azimuth to stations overlying the deepest part of the basin a distinctive feature of records from basin stations recording the 4 october after shock is the large amplitude of multiple s ss etc the multiples have up to twice the amplitude of the direct s phase on the tangential component at such a short range less than 25 km a horizontal seismic velocity gradient is needed to turn rays rapidly enough for large amplitude multiples to form a forward modeling approach is employed using finite difference numerical techniques that produce double couple point source solutions a model based on a recent geologic cross section constructed for the east edge of the los angeles basin generates more phases than are seen in the seismic records simpler models based on dipping layers with low shear velocities in the top few layers fit the data better the seismic velocity depth and dip of the layers are varied to fit the timing between the direct p the direct s and the first s multiple the timing and amplitude of the direct and first multiple s pulses are well modeled though the phase of the first multiple does not match the data including a steeply dipping west edge in the basin model has little effect on the synthetic waveforms except at distances near that basin edge the amplitude of ss is greatest in the deepest part of the basin where it is two times larger than direct s the coda duration increases from 8 sec to more than 20 sec from the ne to the sw the sierra madre mainshock occurred about 25 km to the ne of the whittier narrows sequence the model for whittier narrows was extended to this distance with a shallow basin between whittier and the sierra madre hypocenter to simulate the san gabriel sedimentary basin phases generated by the edge of the deep basin continue to dominate the synthetic waveforms but this model generates a lengthy coda this study shows that specific phases with frequencies up to 1 hz that have traveled through deep sedimentary basins can be explained by two dimensional seismic velocity models

late glacial to holocene history of the mediterranean outflow evidence from benthic foraminiferal assemblages and stable isotopes at the portuguese margin records of benthic foraminiferal assemblage variations and benthic 13c along 12 sediment cores from the western iberian margin between 36 and 42 n at water depths from 820 to 3580 m are used to monitor fluctuations of the mediterranean outflow water mow during the past 30 ka the chronostratigraphy of the cores is based on planktonic 18o records 14c ams dating and the recognition of heinrich events h1 through h4 increased abundances of suspension feeding benthic foraminifers denoted as epibenthos group closely match areas where the recent mow core layers impinge on the continental slope at 800 and 1300 m water depth and near bottom current velocities are enhanced elevated epibenthos group abundances increased benthic 13c and sedimentological evidence for winnowing and erosion are found in glacial sections up to the earliest termination i in cores at water depths between 1600 and 2200 m off southern portugal the combined evidence reveals enhanced current activity at these depths due to a deep glacial mow the mow advection at the portuguese margin during the last glacial was about 700 m deeper than today conceivably forced by increased mow density due to higher salinity and colder temperatures of mediterranean waters the deep mow current gradually decreased in strength and shoaled to 1300 m water depth during the termination and early holocene a shallow mow core layer became active with the onset of termination i at depths between 600 and 1000 m both the shallow and deep mow current culminated during the younger dryas period the present flow pattern with two mow core layers centred at 800 and 1300 m water depth was established between 7 5 and 5 5 ka

bedform velocity matrix the estimation of bottom current velocity from bedform observations a wide variety of bedforms both depositional and erosional in origin has been recognized on the deep seafloor and attributed to the influence of bottom currents these range in scale from those visible in bottom photographs centimeter to decimeter to those recorded with seafloor bathymetric imaging meter to kilometer in many cases it has been possible to provide some quantification of substrate grain size and flow velocity responsible for each bedform type we have synthesized both our own and published data in order to present a bedform velocity matrix which facilitates the estimation of bottom current velocity based on bedform type despite imperfections we believe this to be a valuable model for assessing strength and variability of bottom currents that can have a significant influence on the siting of submarine cables pipelines and other seafloor installations

local and regional induction in the british isles electric currents induced in the shallow seas and deep ocean around the british isles have a profound effect upon the electromagnetic fields observed at stations on the land the configuration of anomalous currents changes with frequency and causes corresponding changes in the geomagnetic transfer functions magnetic variations have been recorded at a dense network of sites in southern scotland and northern england single station transfer functions have been used to generate hypothetical event maps of the anomalous vertical field and hence to infer the configuration of the anomalous internal currents at periods exceeding 2000 s the vertical field is dominated by the effects of electric currents to the west presumably in the atlantic ocean in the period range 400 2000 s anomalous currents are concentrated in a thin sheet comprising the shallow seas the t hick sequences of post caledonian sedimentary rock which underlie them and the extensions of the sedimentary basins into the land the response of the individual basin is determined not only by its local conductivity structure but also by the extent of its connection to the shallow seas i e its regional importance within the conductive sheet at periods less than 200 s on the other hand the anomalous fields at inland sites are principally determined by the local geological structure these results confirm conclusions reached from theoretical studies of electromagnetic induction in a heterogeneous surface layer park orange madden that electromagnetic response data measured in a 3 d environment such as the northumberland basin must be interpreted using 3 d models if one or 2 d models are used the data must be corrected on the basis of regional measurements