granite geomorphology and its geological controls serra da estrela portugal abstract serra da estrela is an elevated granite massif in central portugal characterized by extensive plateau surfaces incised by deep valleys affected by quaternary glaciation bounded by steep fault generated escarpments the presence of seven major textural variants of granite provides an opportunity to study the relationships between lithology and relief whereas dem analysis helped to show the relationships between lithology and topography objectively the higher ground is associated with fine to medium grained granites and is typified by planar surfaces of low gradient with occasional angular tors and rock pedestals block fields built by angular material are common in the parts that were not previously glaciated less elevated parts of the plateau are supported by medium to coarse grained granites and show more varied topography with an abundance of tors boulder piles and depressions lithological boundaries locally coincide with slope breaks but this is not the rule in the northern part of the massif a deep topographic basin has evolved in biotite granite whereas deeply incised valleys follow major fault lines geological controls show a hierarchy in that gross relief reflects the pattern of tectonic uplift and subsidence whereas lithology and then fracture patterns become more and more important if one focuses on smaller and smaller landforms

large earthquakes at the ibero maghrebian region basis for an eews large earthquakes mw 6 imax viii occur at the ibero maghrebian region extending from a point 12ow southwest of cape st vincent to tunisia with different characteristics depending on their location which cause considerable damage and casualties seismic activity at this region is associated with the boundary between the lithospheric plates of eurasia and africa which extends from the azores islands to tunisia the boundary at cape st vincent which has a clear oceanic nature in the westernmost part experiences a transition from an oceanic to a continental boundary with the interaction of the southern border of the iberian peninsula the northern border of africa and the alboran basin between them corresponding to a wide area of deformation further to the east the plate boundary recovers its oceanic nature following the northern coast of algeria and tunisia the region has been divided into four zones with different seismic characteristics from west to east large earthquake occurrence focal depth total seismic moment tensor and average seismic slip velocities for each zone along the region show the differences in seismic release of deformation this must be taken into account in developing an eews for the region

on the initiation of subduction estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation three environments proposed as preferential sites of incipient subduction are investigated passive continental margins transform faults fracture zones and extinct ridges none of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins the attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates this is the only tectonic force sufficient to trigger the nucleation of a new subduction zone the ubiquitous distribution of transform faults and fracture zones combined with the common proximity of these features to mature subduction complexes suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere

an example of intraplate neotectonic activity vilarica basin northeast portugal the vilarica tectonic basin nw iberian peninsula is presented as an example of intraplate neotectonic activity the regional geological setting is first introduced then followed by a brief view on the neotectonic activity in portugal the basin consists of an elongated depression approximately 20 km long and 2 to 3 km wide trending nne ssw it is located astride a late variscan strike slip fault the manteigas braganca fault suggesting that it might correspond to a structural depression related to reactivation movements on that fault mapping of neogene and quaternary cover deposits preserved in the basin and its vicinity was accomplished and numerous evidence of tectonic deformation tilting and faulting affecting the sediments were found other geomorphological criteria are discussed showing the structural origin of the vilarica basin and its neotectonic activity attending to its location on the manteigas braganca fault the vilarica depression is interpreted as a pull apart basin resulting from left lateral strike slip displacement on that fault in the quaternary average fault slip rates are estimated with values comprised between 0 2 mm yr and 0 5 mm yr these slip rates are typical of moderate to moderately high degrees of fault activity and are compatible with the regional faulting geomorphic expression and the relatively important historical and instrumental seismicity affecting the vilarica region finally a geodynamical model is proposed and the studied area is presented in the geodynamic framework

tsunami sedimentation associated with the lisbon earthquake of 1 november ad 1755 boca do rio algarve portugal a description is given of a distinctive marine deposit at boca do rio on the algarve coast of portugal it is proposed here that the sediment accumulation was deposited by the tsunami generated by the lisbon earthquake of 1 november ad 1755 the deposit exhibits sedimentary characteristics quite unlike other coastal sediment accumulations that are deposited by more moderate wave regimes these include laterally continuous sand layers chaotic pebble horizons large amounts of gravel sized shell debris and distinctive assemblages of marine microfossils a preliminary attempt is made here to use the empirical field and laboratory data to define more clearly the processes of coastal sedimentation that characterize tsunami runup the use of tsunami sediments as time synchronous marker horizons is also discussed

remote sensing of floodplain geomorphology as a surrogate for biodiversity in a tropical river system madre de dios peru the complex floodplains of large rivers offer a striking example of how geomorphology by dictating patterns in the frequency and duration of soil saturation and surface flooding influences ecosystem structure and function this study draws on multiple sources of data from remote sensing together with ground observations and water sampling to distinguish floodplain ecosystems in the madre de dios river a tributary of the amazon river in peru this remote tropical river meanders across sub andean alluvial deposits in a tectonically active region and creates floodplain surfaces of varying ages including terraces that are above the reach of present day river floods data from landsat etm optical multispectral jers 1 l band radar and the shuttle radar topography mission c band interferometric digital elevation models were integrated in an object oriented image analysis approach to distinguish five classes of floodplain vegetation vegetation classes generally correspond with successional age and reflect the activity of the riverine meander belt stage data for the river show erratic fluctuations and an annual range exceeding 8 m but the maximum depth of floodplain inundation varied from 1 m close to the river to approximately 0 1 m on more elevated terraces the major ion composition of standing waters on the floodplain during the dry season indicated the importance of emergence of local groundwater in maintaining saturated soils particularly further from the river where backswamp vegetation is distinct and includes palm swamps thus a hydrological continuum exists from deep but sporadic river inundation near the river to constant soil saturation by groundwater emergence in distal backswamps reflecting the geomorphological origin and age of the floodplain deposits this hydrogeomorphic continuum results in fundamental ecological differences the exceptionally rich biodiversity of the sub andean region may be ascribed in part to the enhanced biodiversity associated with fluvial geomorphological features and thus conservation planning must account for the diverse landforms created by fluvial dynamics

the eastern lower tagus valley fault zone in central portugal active faulting in a low deformation region within a major river environment abstract active faulting in the lower tagus valley central portugal poses a significant seismic hazard that is not well understood although the area has been affected by damaging earthquakes during historical times only recently has definitive evidence of quaternary surface faulting been found along the western side of the tagus river the location geometry and kinematics of active faults along the eastern side of the tagus valley have not been previously studied we present the first results of mapping and paleoseismic analysis of the eastern strand of the lower tagus valley fault zone ltvfz geomorphological paleoseismological and seismic reflection studies indicate that the eastern ltvfz is a left lateral strike slip fault the detailed mapping of geomorphic features and studies in two paleoseismic trenches show that surface fault rupture has occurred at least six times during the past 10 ka the river offsets indicate a minimum slip rate on the order of 0 14 0 24 mm yr for the fault zone fault trace mapping geomorphic analysis and paleoseismic studies suggest a maximum magnitude for the eastern ltvfz of m w 7 3 with a recurrence interval for surface ruptures 1 7 ka at least two events occurred after 1175 95 cal yr bp single event displacements are unlikely to be resolved in the paleoseismic trenches thus our observations most probably represent the minimum number of events identified in the trenches

stress and palaeostress in the betic rif cordilleras miocene to the present palaeostress orientation in the betic rif cordilleras can be determined for the miocene to the quaternary by analysis of brittle microstructures faults joints striated pebbles and stylolites in the central and northern south iberian domain 1 was subhorizontal and trended nw se from the early miocene to the present in the northern alboran domain from the burdigalian to the serravallian palaeostress ellipsoids were oblate with 3 subhorizontal and trending ene wsw in the southern part of the south iberian domain and in the northern part of the alboran domain betic cordillera from the tortonian to the quaternary palaeostress ellipsoids were variable in the granada region they were prolate with 1 subvertical but in the almeria region they were triaxial with 2 subvertical and 3 subhorizontal and trending ne sw to e w in the rif the alboran and african maghrebian domains were affected by palaeostresses with 1 subhorizontal varying from ne sw in tortonian times to n s in the plio quaternary a study of earthquake focal mechanisms by the right dihedra method indicates that the present day general stress field around the betic rif cordilleras is compressional with 1 subhorizontal and trending nw se in the betic rif cordilleras this general stress field is modified and the present day orientations of 1 2 and 3 are nearly the same as those of the palaeostresses in the pliocene and quaternary the distribution and evolution of stresses in the betic rif cordilleras from the miocene to the present can be correlated with the main neogene deformations extensional structures in the central and northern alboran domain and the southern part of the south iberian domain and compressional structures in the south iberian and african maghrebian domains and in the southern part of the alboran domain

seismotectonics of portugal and its adjacent atlantic area abstract a study of focal mechanisms of earthquakes and stress indicators are presented for portugal and its adjacent atlantic margins recently riberio et al 1996 published a work on this subject in this study we introduce new focal mechanisms of nine earthquakes we use these results together with 12 other focal mechanisms and the main tectonic features in the region to define more accurately the type of mechanism and the stress pattern of the region all the focal mechanisms used in this study differ partially but are compatible with the regional stress field the stress indicators resulting from focal mechanisms and other geological and geophysical data show that the portugal continental and its atlantic margins are under horizontal pressure in nnw sse direction with a greater proportion of strike slip and reverse oblique mechanisms for the whole area

morphotectonics of the iberian peninsula a morphotectonic interpretation of the relief of the iberian peninsula ip is presented based on an analysis of its gross forms several geological geophysical and geomorphological methods were used in order to build up a morphostructural sketch three main categories were established using the approach of rantsman 1979 territorial units 1 megablock 9 macroblocks 34 mesoblocks 1 374 blocks and 2 523 microblocks morphostructural alignments 2 first 8 second 20 third and 1 fourth rank and 43 knots between morphostructural alignments second fourth rank the main seismic activity is concentrated on the first and second rank lineaments and some important epicenters are located near the lineament intersections the origin of the earthquakes in the vicinity of such knots can be explained by the forcing pushing of macroblocks westwards the existence of earthquakes along the lineaments may be explained by tension from the present study it appears that earthquake occurrence in the ip is due principally to stress concentrations around morphotectonic zones a seismotectonic interpretation of the ip is also presented on this map three zones are distinguished a b c each of them with a different active level and dimensions

the ocean continent boundary off the western continental margin of iberia ii crustal structure in the tagus abyssal plain summary an 80 km long reversed seismic refraction line line 5 was shot over the tagus abyssal plain off portugal the main p wave reflected and refracted phases were modelled both for traveltime and amplitude the resulting p wave velocity depth model has the following features a an extremely thin crust of about 2 km b the absence of oceanic layer 3 and c very low upper mantle velocities between 7 6 and 7 9 km s this very unusual seismic velocity crustal structure is quite unlike thinned continental crust but is remarkably similar to the seismic crustal structures found at atlantic fracture zones and in particular to the structures found in profiles shot along the transform valley and near ridge transform intersections a magnetic anomaly chart seems to allow the possibility of several fracture zones one of which could intersect the centre of line 5 as an alternative to the fracture zone hypothesis we show that if the oceancontinent transition in the tagus abyssal plain is located at about 11 30 w in a symmetric position with respect to the ocean continent transition in the conjugate south newfoundland basin then magnetic anomalies can be modelled simply by assuming sea floor spreading west of 11 45 w at 10 mm yr beginning at m11 time 133 myr bp and blocks of rifted continental crust to the east the location of the proposed ocean continent transition in the tagus abyssal plain is marked by a well defined n s linear magnetic anomaly which is adjacent to the oldest sea floor spreading block east of the proposed ocean continent transition there is an increase in the depth to basement similar to that found east of the ocean continent transition in the iberia abyssal plain and elsewhere this model also allows us to explain why purdy s 1975 seismic refraction line a ar in the tagus abyssal plain cannot be interpreted as a conventional reversed pair because most of line a was shot over the ocean continent transition zone and most of line ar over thinned continental crust remarkably similar velocity depth structures to that under line 5 are found close to the ocean continent transition zone off the whole of western iberia in areas which show no clear evidence of fracture zones therefore it appears more likely that the seismic structure of line 5 is due to its proximity to the ocean continent transition than to a local association with a fracture zone and further that its structure is typical of this transition off the western margin of iberia we also suspect that the low upper mantle velocities associated with the ocean continent transition indicate the widespread occurrence of serpentinized peridotite

ground motion simulation in the lower tagus valley basin throughout history the lower tagus valley ltv region has been shaken by several earthquakes including some with moderate to large magnitudes and with sources located inside the basin for example the 1344 m6 0 and 1909 m6 0 earthquakes previous simulations bezzeghoudet al natural hazard 69 1229 1245 2011 have revealed strong amplification of the amplitude waves in the region because of the effect of the low velocity sediments that fill the basin the model used in those simulations was updated in this work by including new high resolution geophysical and geotechnical data available for the area seismic reflection aeromagnetic gravimetric deep wells standard penetration tests and geological data to contribute to improved assessment of seismic hazard in the ltv we simulated propagation of seismic waves produced by moderate earthquakes in a 3d heterogeneous medium by using elastic finite difference wave propagation code the method successfully used by grandinet al geophys j int 171 1144 1161 2007 involves evaluation of the seismic potential of known faults in the area studied and three dimensional seismic ground motion modelling by use of finite difference methods on the basis of this methodology we calculated the ground motion for the april 23 1909 benavente portugal earthquake mw 6 0 in dense grid points then computed the synthetic isoseismic map of the area by use of appropriate relationships between seismic intensity mmi and peak ground velocity pgv the synthetic results in contrast with available macroseismic and instrumental data enable validation of the source models proposed for the area identification of the sources of historical earthquakes and could also indicate which areas are more exposed to seismic ground motion

some results of the short time ocean bottom seismological observations in the central part of the azores gibraltar fault summary an investigation of the seismicity of the area in which the high magnitude earthquakes of 1931 may 20 1941 november 25 and 1975 may 26 were generated was carried out in 1988 august by the institute of oceanology of the academy of sciences of ussr in collaboration with the national institute for meteorology and geophysics of portugal five seismographs were installed by the r v akademic kurchatov on the sea bottom in that area for 13 days at depths ranging from 3400 m to 5100 m a total of 99 earthquakes was registered the magnitudes ml of the events ranging from 0 5 to 3 0 most of the epicentres located in this investigation are probably situated along the azores gibraltar fault or along the fault associated with it trending nw se which is considered to be responsible for the 1975 earthquake an alignment of epicentres trending approximately ne sw probably connected with a fault is also proposed

simulations of strong ground motion in sw iberia for the 1969 february 28 ms 8 0 and the 1755 november 1 m 8 5 earthquakes ii strong ground motion simulations summary this is the second paper of a series of two concerning strong ground motion in sw iberia due to earthquakes originating from the adjacent atlantic area the aim of this paper is to use the velocity model that was proposed and validated in the companion paper for seismic intensity modelling of the 1969 m s 8 0 and 1755 m 8 5 8 7 earthquakes first we propose a regression to convert simulated values of peak ground velocity pgv into modified mercalli intensity mmi in sw iberia and using this regression we build synthetic isoseismal maps for a large m s 8 0 earthquake that occurred in 1969 based on information on the seismic source provided by various authors we show that the velocity model effectively reproduces macroseismic observations in the whole region we also confirm that seismic intensity distribution is very sensitive to a small number of source parameters rupture directivity fault strike and fault dimensions then we extrapolate the method to the case of the great m 8 5 8 7 1755 earthquake for a series of hypotheses recently proposed by three authors about the location of the epicentral region the model involving a subduction

connecting onshore structures in the algarve with the southern portuguese continental margin the carcavai fault zone abstract the algarve is located a few hundred kilometres north of the crossing of the e w eurasia africa plate boundary and is characterised by a moderate seismicity with some important historical and instrumental earthquakes causing loss of lives and significant material damages the area is affected not only by plate boundary generated earthquakes but also by local events capable of generating moderate to large earthquakes the assessment of onshore local sources and its connections with the plate border is therefore of vital importance for an evaluation of the regional seismic hazard this paper discusses the application of geophysical data to study a large fault zone which is the offshore prolonging of the carcavai fault zone cf an onshore outcropping structure more than 20 km long which is seen to deform sediments of plio quaternary age offshore and onshore aeromagnetic data offshore gravimetric and seismic reflection data shows the existence of a long over 200 km wsw ene trending fault zone affecting the palaeozoic basement with a normal geometry which is probably segmented by nnw sse to n s faults seismic data shows that this fault zone has been reactivated as a left lateral strike slip fault and inverted in the cenozoic with the upthrust of the northwestern block in agreement with the onshore cf characteristics recent work carried out onshore and offshore near the coastline that shows deformation of plio quaternary sediments suggests that this is an active fault some of the faults segments have instrumental seismicity associated though faults very rarely rupture along its entire length several fault segments have a length of about 30 km and may produce an earthquake of magnitude about7 the proximity of the onshore segment to the city of faro and of the offshore segments to the main population centres of the algarve makes it a serious threat to the algarve

subduction related structures in the north iberian margin the oblique convergence of eurasia and iberia since the early cretaceous caused the formation of the pyrenean intracontinental collisional orogen in the east and progressed to continent ocean collision with subduction of the bay of biscay oceanic crust beneath the north iberian margin in the west two deep multichannel seismic profiles iam 12 and escin 4 integrated with gravity modeling and other geological and geophysical data provide the crustal scale architecture of this margin and its tectonic evolution during the convergence the north iberian margin is modeled with a south or south southeast dipping oceanic crust beneath the outer part of the continental shelf mesozoic basins on the shelf were inverted during the tertiary and compressional activity continued until recent times in the escin 4 section while a shallower probably neogene age basin is subjected to active recent erosion in the iam 12 section in the oceanic areas a marginal trough deepens and widens toward the east as a result of the regional east dip of the oceanic basement the accretionary prism increases in size from west to east 18 56 km and its internal structure and morphology varies along strike the prism is buried by postcorivergence sediments in both sections and in the iam 12 section appears to have been active at least during lutetian to burdigalian times the crustal scale structure of the north iberian margin is that of an arrested subduction zone in which a remnant oceanic basin was being consumed near two continental plates that collided obliquely

modern tectonic stress field in the mediterranean region evidence for variation in stress directions at different scales summary a compilation of more than one thousand stress indicators which include in situ stress measurements focal mechanisms microtectonic and other geological data allowed us to reconstruct the modern stress field in the mediterranean region and the surrounding area average stress directions at different scales have been reconstructed by means of a linear interpolation method this method takes into account the distribution scale and quality of stress data the results of the interpolation at plate scale allow us to recognize slightly deformed regions such as the northwestern european platform where average maximum horizontal stress direction is oriented roughly nnw sse subparallel to absolute and relative plate velocity directions other regions such as the caucasus alps and pyrenees where recent tectonic deformation and seismicity are present display important variations of stress directions the reconstruction of the average stress directions at different scales within the french alps pointed out that the average stress field pattern may vary from one scale to another nevertheless variations of stress directions at a given scale are consistent with the kinematics of faults of the same scale

seismotectonics of the ibero maghrebian region abstract a study of the seismotectonics of the ibero maghrebian region is presented as deduced from main geological features seismicity and focal mechanism data the hypocentral distribution of shallow intermediate depth and deep earthquakes is analyzed the direction of the stress distribution is inferred from the focal mechanism of 24 selected events a seismotectonic framework summarizing these results together with the main geological features shows that a nw se general direction of horizontal compressive stresses is present in the region with a localized zone of horizontal tensions in the betics and the alboran sea the presence of intermediate depth and deep earthquakes is tentatively explained as caused by two consecutive processes of subduction

far field tsunami simulations of the 1755 lisbon earthquake implications for tsunami hazard to the u s east coast and the caribbean article i nfo the great lisbon earthquake of november 1st 1755 with an estimated moment magnitude of 8 5 9 0 was the most destructive earthquake in european history the associated tsunami run up was reported to have reached 5 15 m along the portuguese and moroccan coasts and the run up was significant at the azores and madeira island run up reports from a trans oceanic tsunami were documented in the caribbean brazil and newfoundland canada no reports were documented along the u s east coast many attempts have been made to characterize the 1755 lisbon earthquake source using geophysical surveys and modeling the near field earthquake intensity and tsunami effects studying far field effects as presented in this paper is advantageous in establishing constraints on source location and strike orientation because trans oceanic tsunamis are less influenced by near source bathymetry and are unaffected by triggered submarine landslides at the source source location fault orientation and bathymetry are the main elements governing transatlantic tsunami propagation to sites along the u s east coast much more than distance from the source and continental shelf width results of our far and near field tsunami simulations based on relative amplitude comparison limit the earthquake source area to a region located south of the gorringe bank in the center of the horseshoe plain this is in contrast with previously suggested sources such as marques de pombal fault and gulf of cadiz fault which are farther east of the horseshoe plain the earthquake was likely to be a thrust event on a fault striking 345 and dipping to the ene as opposed to the suggested earthquake source of the gorringe bank fault which trends ne sw gorringe bank the madeira tore rise mtr and the azores appear to have acted as topographic scatterers for tsunami energy shielding most of the u s east coast from the 1755 lisbon tsunami additional simulations to assess tsunami hazard to the u s east coast from possible future earthquakes along the azores iberia plate boundary indicate that sources west of the mtr and in the gulf of cadiz may affect the southeastern coast of the u s the azores iberia plate boundary west of the mtr is characterized by strike slip faults not thrusts but the gulf of cadiz may have thrust faults southern florida seems to be at risk from sources located east of mtr and south of the gorringe bank but it is mostly shielded by the bahamas higher resolution near shore bathymetry along the u s east coast and the caribbean as well as a detailed study of potential tsunami sources in the central west part of the horseshoe plain are necessary to verify our simulation results

deformation in a hyperslow oceanic rift insights from the tectonics of the sao miguel island terceira rift azores the evolution of hyperslow oceanic rifts like the terceira rift tr in the azores is still poorly understood here we examine the distribution of strain and magmatism in the portion of the tr making up the nubia eurasia plate boundary we use sao miguel island because it stretches most of the tr width which allows to investigate the tr s architecture and shedding light on tr s age and mode of deformation from topography and structural analysis and new measurements of 380 faults and dikes we show that 1 sao miguel has two main structural directions n150 and n110 mostly concentrated in the eastern part of the island as an onshore continuation of the faults observed offshore in the ne n110 faults and sw n140 tr walls 2 a new n50 n80 fault system is identified in sao miguel 3 fault and dike geometries indicate that eastern sao miguel comprises the tr s northern boundary and the lack of major faults in central and western sao miguel indicates that rifting is mostly concentrated at master faults bounding the tr based on tr s geometry structural observations and plate kinematics we estimate that the tr initiated between 1 4 and 2 7 ma ago and that there is no appreciable seafloor spreading associated with rifting based on plate kinematics on the new structural data and on sao miguel s structural and volcanic trends we propose that the eastern two thirds of sao miguel lie along a main tr related transform fault striking n70 n80 which connects two widely separated n130 n150 tr trending segments

seismicity source mechanisms and tectonics of the azores gibraltar plate boundary abstract a study and partial revision of the seismicity and source mechanisms of earthquakes are presented for the oceanic plate boundary of eurasia and africa from the azores islands to the strait of gibraltar focal mechanisms of 36 earthquakes wave form modelling of 4 and seismic moments and dimensions of 28 have been calculated these results indicate that the boundary is divided into three sections with different characteristics the azores the central section and the east section the azores section has high seismicity of moderate magnitude and a low slip rate 0 76 cm yr the predominant mechanism is normal or transform faulting with horizontal tensions on average in a n25 e direction in the central section large earthquakes occur with strike slip right lateral motion along faults in an e w direction there is a high slip rate 3 39 cm yr the east section is of more complex nature large earthquakes also occur but the predominant mechanism is reverse faulting with a horizontal pressure axis in the n30 w direction resulting in underthrusting of africa from the slip vectors the pole of rotation for the african plate is located at 28 2 n 21 1 w nearer to the boundary than previous locations close to the continent there is a continuation in the ocean crust of some structural features of the iberian peninsula

abnormal historic sea surface fluctuations sw england abstract this paper presents a series of documentary accounts of abnormal sea surface changes that have affected the coastline of sw england during historic times an attempt is made to consider possible origins for individual events although it is recognised that certain episodes of coastal flooding appear to have no simple explanation however certain coastal floods appear to have been generated by tsunamis triggered by far field offshore earthquakes

the 1998 faial earthquake azores evidence for a transform fault associated with the nubia eurasia plate boundary abstract with very few exceptions m 4 tectonic earthquakes in the azores show normal fault solution and occur away from the islands exceptionally the 1998 shock was pure strike slip and occurred within the northern edge of the pico faial ridge fault plane solutions show two possible planes of rupture striking ene wsw dextral and nnw sse sinistral the former has not been recognised in the azores but is parallel to the transform direction related to the relative motion between the eurasia and nubia plates therefore the main question we address in the present study is do transform faults related to the eurasia nubia plate boundary exist in the azores knowing that the main source of strain is related to plate kinematics we conclude that the sinistral strike slip nnw sse fault plane solution is not consistent with either the fault dip ca 65 which is typical of a normal fault or the ca ene wsw direction of maximum extension both are consistent with a normal fault as observed in most major earthquakes on faults striking around nnw sse in the azores in contrast the dextral strike slip ene wsw fault plane solution is consistent with the transform direction related to the anticlockwise rotation of nubia relative to eurasia altogether tectonic data measured ground motion observed destruction and modelling are consistent with a dextral strike slip source fault striking ene wsw furthermore the bulk clockwise rotation measured by gps is typical of bookshelf block rotations observed at the termination of such master strike slip faults therefore we suggest that the 1998 earthquake can be related to the wsw termination of a transform ene wsw fault plane solution associated with the nubia eurasia diffuse plate boundary

seismotectonics of portugal and its adjacent area in the atlantic abstract new elements on the seismicity of portugal and new focal mechanism solutions of earthquakes with epicentres situated off the coast of the portuguese mainland and in the azores region are presented historical seismicity data show that in the territory of the portuguese mainland there are active faults that are responsible for earthquakes that have caused important damage and many casualties however most of the intraplate earthquakes with epicentres situated in the portuguese mainland or near the shore are normally of small magnitude and this renders difficult their interpretation in the light of focal mechanisms a solution for one earthquake with magnitude 5 and epicentre at the nazare submarine canyon is presented southwestwards of cape st vincent there is an important seismic zone responsible for high magnitude earthquakes such as that of 1 november 1755 this zone is situated in the region where the extension of the messejana fault into the ocean joins with the azores gibraltar fault the seismicity of the area situated between the western coast of the portuguese mainland and the azores increases approximately along the 15 w meridian from the latitude of the azores gibraltar fault up to 44 n focal mechanisms of earthquakes with epicentres situated along this line show very similar solutions the interpretation of the focal mechanism solutions of the earthquakes with epicentres situated in the studied area shows that the stress field trends approximately nw se it is assumed that this stress field results from the interaction of the eurasian and african plates however this direction is not maintained in the azores region

geophysical characterization of the ota vila franca de xira lisbon sesimbra fault zone portugal summary this paper focuses on the reprocessing of seismic reflection profiles aeromagnetic and seismicity data to locate and characterize the ota vila franca de xira lisbon sesimbra fault zone the studied structure is sited in the lower tagus valley an area with over 2 million inhabitants that has experienced historical earthquakes causing many casualties serious damage and economical losses e g 1531 january 26 and 1909 april 23 earthquakes whose tectonic sources are mostly unknown the fault zone trends nne ssw to n s is located near the eastern border of the mesozoic lusitanian basin and partially delimits the lower tagus cenozoic basin at the west mostly hidden under the cenozoic sedimentary fill according to the data presented here the normal structures that compose the fault zone were reactivated in cenozoic times with positive inversion and the development of splays towards the east the fault zone shows three distinct segments with different behaviour in conformity with their various orientations relative to the nw se maximum compressive stress the northern segment splays into a series of nne ssw oriented east verging imbricate thrusts which merge to the west into a major reverse fault that resulted from the tectonic inversion of the former normal fault bordering the

re evaluation of seismic intensities and relocation of 1969 saint vincent cape seismic sequence a comparison with the 1755 lisbon earthquake seismic intensity for the february 28 1969 mw 7 8 earthquake have been re evaluated using original documents in local archives such as contemporary newspapers council minutes monographic studies among other sources for spain portugal and morocco and answers to macroseismic questionnaires for morocco this information is used to plot a new intensity map for the whole region affected by the earthquake portugal spain and morocco the intensity values vary from viii to ix in the e w coast of algarve southern portugal to ii iii furthermore we have relocated the hypocentres for main shock and 24 aftershocks using a new 3d crustal velocity model for the gulf of cadiz region and a non linear probabilistic location methodology most of them previously lacking a depth estimate the new locations show an e w distribution of epicenters with focus located in the uppermost mantle most of them with depths between 30 and 50 km no earthquakes have been located at depths shallower than 30 km a comparison between peak ground accelerations pgas estimated from the observed intensities for the 1969 and the lisbon 1755 earthquakes and synthetic pga values generated assuming two different scenarios using the 1969 and 2009 earthquakes for the 1755 lisbon event shows that the observed damage produced by the 1755 earthquake may be better explained assuming a reverse dip slip mechanism oriented in ne sw direction similar to that of the 2009 earthquake rather than assuming focal mechanism similar to that of the 1969 earthquake

intraplate seismicity in the atlantic ocean basin a teleseismic catalog a comprehensive compilation and reassessment of intraplate seismicity in the atlantic ocean basin is presented for the years 1918 through 1990 of the 846 earthquakes whose original bulletin epicenters were located in atlantic ocean crust and were at least 2 from active plate boundaries 289 were found to be reliably intraplate a significant number 237 of the original epicenters were actually mislocated interplate earthquakes that located to active plate boundaries upon relocation or whose 95 confidence locations determined through monte carlo simulations intersected seismically active boundaries many of these erroneous locations particularly in regions surrounding the mid atlantic ridge frequently appear in global seismicity maps the resulting reliable catalog of intraplate seismicity is not randomly distributed but shows significant clustering in space and time much of the seismicity is associated with the locations of hot spots such as the bermuda canary and cape verde islands seismicity is also clustered around other tectonic features such as the diffuse plate boundary between north america and south america and the fossil spreading ridge between greenland and north america while we did not examine intraplate earthquakes within 2 of the mid atlantic ridge and some of our unreliable near ridge earthquakes may be truly intraplate we do not find an indication within our reliable data set of an increase in intraplate seismicity with increased proximity to the mid atlantic ridge beyond our 2 limit

seismological evidence for a delamination process in the lithosphere under the alboran sea summary the continental collision between the african and eurasian plates in the iberian peninsula is solved with a nw se compression with an extensional basin the alboran sea to explain this paradox several models have been proposed including the subduction process of a lithospheric plate in this study a relocation of the seismicity of the area using a jhd algorithm for the period 1950 1995 has been performed which shows a specific hypocentre pattern that is not compatible with plate subduction in addition the direction of the stresses acting in the area has been determined through a comprehensive study of all existing focal mechanisms together with calculations for 19 new earthquakes with sufficient data to determine the focal solution after considering all these data the absence of intermediate shocks in the central part of the alboran basin and the predominantly thrusting mechanisms at the borders of the basin with pressure axes perpendicular to the front we consider a delamination process as a possible mechanism acting in the area

impacts of nonuniform flow on estimates of vertical streambed flux 1 the use of inverse one dimensional 1 d analytical methods for estimating vertical stream aquifer exchange flux is now commonplace however the application of such simple models can lead to significant errors in estimates of vertical exchange flux where the model assumptions are violated in real systems an idea that is gaining acceptance in the literature is that the presence of nonvertical flow is such a violation however it is shown here that nonvertical flow by itself will not necessarily lead to errors in vertical flux estimation but rather that significant errors can stem from nonuniform convergent divergent flow fields and or hydrodynamic dispersion even within uniform flow fields nonuniform flow may also be expected in some cases to create discrepancies between flux estimates made on the basis of vertical head gradient measurements and those made using 1 d analytical heat tracer methods significant differences are observed in the estimates of heat derived fluxes obtained by the amplitude ratio and phase shift time series methods when convergent and divergent flows are apparent such differences may potentially be used to infer that convergent or divergent flow is occurring and that a 1 d analysis is inappropriate

the 1980 1997 and 1998 azores earthquakes and some seismo tectonic implications abstract we have studied the focal mechanisms of the 1980 1997 and 1998 earthquakes in the azores region from body wave inversion of digital gdsn global digital seismograph network and broadband data for the 1980 and 1998 shocks we have obtained strike slip faulting with the rupture process made up of two sub events in both shocks with total scalar seismic moments of 1 9 1019 nm mw 6 8 and 1 4 1018 nm mw 6 0 respectively for the 1997 shock we have obtained a normal faulting mechanism with the rupture process made up of three sub events with a total scalar seismic moment of 7 7 1017 nm mw 5 9 a common characteristic of these three earthquakes was the shallow focal depth less than 10 km in agreement with the oceanic type crust from the directivity function of rayleigh lr waves we have identified the nw se plane as the rupture plane for the 1980 and 1998 earthquakes with the rupture propagating to the se slow rupture velocity about of 1 5 km s has been estimated from directivity function for the 1980 and 1998 earthquakes from spectral analysis and body wave inversion fault dimensions stress drop and average slip have been estimated focal mechanisms of the three earthquakes we have studied together with focal mechanisms obtained by other authors have been used in order to obtain a seismotectonic model for the azores region we have found different types of behaviour present along the region it can be divided into two zones zone i from 30 w to 27 w zone ii from 27 w to 23 w with a change in the seismicity and stress direction from zone i in zone i the total seismic moment tensor obtained corresponded to left lateral strike slip faulting with horizontal pressure and tension axes in the e w and n s directions respectively in zone ii the total seismic moment tensor corresponded to normal faulting with a horizontal tension axis trending ne sw normal to the terceira ridge the stress pattern for the whole region corresponds to horizontal extension with an average seismic slip rate of 4 4 mm yr

geophysical methods applied to fault characterization and earthquake potential assessment in the lower tagus valley portugal abstract the study region is located in the lower tagus valley central portugal and includes a large portion of the densely populated area of lisbon it is characterized by a moderate seismicity with a diffuse pattern with historical earthquakes causing many casualties serious damage and economic losses occurrence of earthquakes in the area indicates the presence of seismogenic structures at depth that are deficiently known due to a thick cenozoic sedimentary cover the hidden character of many of the faults in the lower tagus valley requires the use of indirect methodologies for their study this paper focuses on the application of high resolution seismic reflection method for the detection of near surface faulting on two major tectonic structures that are hidden under the recent alluvial cover of the tagus valley and that have been recognized on deep oil industry seismic reflection profiles and or inferred from the surface geology these are a wnw ese trending fault zone located within the lower tagus cenozoic basin across the tagus river estuary porto alto fault and a nne ssw trending reverse fault zone that borders the cenozoic basin at the w vila franca de xira lisbon fault vertical electrical soundings were also acquired over the seismic profiles and the refraction interpretation of the reflection data was carried out according to the interpretation of the collected data a complex fault pattern disrupts the near surface first 400 m at porto alto affecting the upper neogene and at least for one fault the quaternary with a normal offset component the consistency with the previous oil industry profiles interpretation supports the location and geometry of this fault zone concerning the second structure two major faults were detected north of vila franca de xira supporting the extension of the vila franca de xira lisbon fault zone northwards one of these faults presents a reverse geometry apparently displacing holocene alluvium vertical offsets of the holocene sediments detected in the studied geophysical data of porto alto and vila franca de xira lisbon faults imply minimum slip rates of 0 15 0 30 mm year three times larger than previously inferred for active faults in the lower tagus valley and maximum estimates of average return periods of 2000 5000 years for m 6 5 7 co seismic ruptures

present day state of stress in northern morocco from focal mechanism analysis abstract the use of focal mechanisms of micro earthquakes and moderate teleseisms in the al hoceima area northern morocco enables the determination of the state of stress in the southern part of the gibraltar arc two different states of stress were determined 1 a ene wsw extension in the imzouren area probably with similar magnitudes of 2 and 1 which may be controlled by pre existing structures and 2 a strike slip regime in beni boucetta tamassint and beni abdallah areas 1 nnw sse 3 ene wsw in the two first areas and 1 ne sw 3 nw se in the latter contrary to recently proposed ideas the mean state of stress determined with the help of moderate teleseisms corresponds to a strike slip regime with a nnw sse oriented main compressional axis and a ene wsw horizontal extension similar to the stress field determined from neotectonic studies and somewhat different from that determined from the el asnam aftershock sequence of 1980