source process and tectonic implications of the great 1975 north atlantic earthquake summary the atlantic segment of the africa europe plate boundary has usually been interpreted as a transform boundary on the basis of the bathymetric expression of the gloria fault and dextral strike slip first motion mechanisms aligned along the azores gibraltar line of seismicity the 1975 may 26 earthquake ms 7 9 was assumed to fit into this framework because it occurred in the general area of this line and has a similar first motion focal mechanism strike 288 dip 72 slip angle 184 however several anomalies cast doubt on this picture the event is abnormally large for an oceanic transform event a sizeable tsunami was excited the aftershock area is unusually small for such a large event and most significantly the epicentre is 200 km south of the presumed plate boundary the rayleigh wave radiation pattern indicates a change in focal mechanism to one with a significant dip slip component the short duration of the source time history 20 s as deconvolved from long period p waves the lack of directivity in the rayleigh waves and the small one day aftershock area suggest a fault length less than 80 km one nodal plane of the earthquake is approximately aligned with the trace of an ancient fracture zone we have compared the pasadena 1 90 record of the 1975 earthquake to that of the 1941 north atlantic strike slip earthquake 200km to the nnw and confirmed the large size of the 1941 event m 8 2 the non colinear relationship of the 1975 and 1941 events suggests that there is no well defined plate boundary between the azores and gibraltar this interpretation is supported by the intraplate nature of both the 1975 event and the large 1969 thrust event 650 km to the east this study also implies that the largest oceanic strike slip earthquakes occur in old lithosphere in a transitional tectonic regime

earthquakes in the davie ridge madagascar region and the southern nubian somalian plate boundary the largest earthquake sequence mb up to 6 4 and m0 up to 5 1018 nm to have struck eastern africa within the past 60 years occurred in may and june of 1985 off the coast of the tanzania mozambique border the epicenters are located on the northern extension of the davie ridge a prominent north south trending bathymetrie feature accompanied by distinct gravity anomalies a joint hypocenter location of the 14 largest events in this sequence appears as a cluster of epicenters with no apparent lineation al teleseismic distances p and sh waves generated by the two largest events of this sequence have nearly identical waveforms on the long period records of the worldwide standard seismograph network a formal inversion of the data shows a pure normal faulting mechanism on nodal planes striking slightly west of north at least two point sources are necessary to model each event one at a depth of 40 10 km and the other at about 20 5 km on a regional scale normal faulting seems to characterize the scattered seismicity throughout the region between madagascar and the eastern arm of the east african rift zone even though oceanic lithosphere dominates part of the region and no topographic expression of a rift system can be identified since normal faulting events of up to 25 5 km deep also characterize the scattered seismicity west of the topographic expression of the western arm of the rift zone at about the same latitude we propose that the southern termination of the nubian somalian plate boundary consists of a diffuse zone of east west extension up to 2000 km wide

folding of oceanic lithosphere folding of the lithosphere just south of the bay of bengal appears as 1 undulations in acoustic basement topography and 2 as linear geoid undulations in the seasat altimeter data from the seasat data we find that the east west trending folds have wavelengths ranging from 130 to 250 km and clustering about 190 km the horizontal gravity disturbances due to the folds range in amplitude from 15 to 50 mgal elastic models of oceanic lithosphere have in the past been used to demonstrate the implausibility of lithosphere buckling or folding in response to compression these elastic models typically predict that compressive stresses of about 5 gpa are required to buckle oceanic lithosphere with an age comparable to that of the northeastern indian ocean 40 70 ma these stresses exceed the strength of lithospheric rock we use an elastic plastic model to show that oceanic lithosphere of this age should have a net compressive strength equal to about 12 of the elastic buckling stress we further demonstrate that loads approaching the net compressive strength cause the lithosphere to fold with a wavelength about 200 km i e the wavelength observed from seasat our results reinforce earlier speculation that this folding may be related to the himalayan orogeny

accurate source depths and focal mechanisms of shallow earthquakes in western south america and in the new hebrides island arc synthetic seismograms are matched to long period p waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms accurate depths are obtained for a large sample of moderate sized 6 ms 7 events which produced relatively simple p waveforms and do not require intensive analysis the estimated error in the determination of depth is about 5 km or less for most of the events considered the procedure is applied to two suites of events one in south america 83 events and one in the new hebrides 61 events in these two areas of contrasting styles of subduction the accurate depths provide new information on the bending of descending plates near trenches the seismicity and tectonics of the upper plate and the geometry and structure of the zone of contact between the upper and descending plates depths of suboceanic earthquakes which occurred near the trench and which have either tensional and compressional horizontal stress axes agree well with chapple and forsyth s 1979 preferred model of a bending elastic perfectly plastic plate however an unusually deep event occurred beneath the fiji plateau in an area of young ocean floor supposed to have been formed by sea floor spreading processes only since late miocene time the calculated depth of the event 48 km places it below the lithosphere asthenosphere boundary derived from thermal models vertical cross sections through the shallow parts of the subduction zones show that most of the earthquakes with thrust faulting focal mechanisms can be interpreted to be interplate events located in a thin 10 km thick curved zone of contact dipping arcward or landward from the trench axis however a few events with focal mechanisms similar to the interplate events do not fit on the inferred thin zone of contact these events would be accommodated by an interplate zone 15 km thick in both subduction zones interplate events occur at depths between about 15 and 50 km however the downdip width of the south american interplate zone is 2 to 3 times larger than that in the new hebrides most of the intraplate events near the interplate boundary are located in the descending plate in the upper plate seismicity is more pronounced in the back arc regions of the two subduction zones thus our results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc depths of shallow earthquakes in the eastern andes and sub andean regions of south america range between about 9 and 36 km indicating that both the upper and middle crust are seismically active about 75 of the events occurred in the upper 25 km furthermore the focal mechanisms indicate that from ecuador to argentina the crust of the eastern side of the andean cordilleran is in a state of east west compression oriented nearly parallel to the direction of convergence between the nazca and south american plates the depths the orientations of the earthquake nodal planes and the close relationships to quaternary tectonics imply basement involved crustal shortening in much of the eastern andes and sub andean zones of peru and argentina

geophysical evidence for post miocene rotation of the island of viti levu fiji and its relationship to the tectonic development of the north fiji basin geophysical studies of the island of viti levu fiji show largeamplitude magnetic anomalies of up to 1500 nt to be located above the tavua volcanic center field sampling of the volcanic rocks from the intrusive and extrusive centers of the volcano show the rock samples to have initial intensities of natural remanent magnetization of up to 380 a m radiometric dating of fifteen oriented samples show an age range of 2 9 to 7 0 m y b p for the suite of rocks collected from the mba basaltic group paleomagnetic analysis of the dated rocksshows declination vectors ranging from 325 9 for an age of 2 9 0 2 m y b p to 290 4 for an age of 7 0 0 4 m y b p as a group the paleomagnetic declination values of the samples systematically decrease with age this decrease is attributed to progressive anticlockwise rotation of viti levu during the process of formation of the north fiji basin a linear least squares analysis of the paleomagnetic data indicates that the island has rotated in an anticlockwise direction through 90 during the past 7 m y at a rate of 13 7 per million years declination values as low as 245 measured for some sites not dated suggest that anticlockwise rotation of viti levu and development of the north fiji basin could have commenced as early as 10 m y b p following the breakup of the ancient solomons new hebrides fiji lau island arc large rotations of microplates may have occurred along other island arcs and marginal basin systems elsewhere in the western pacific and such rotational histories may reflect in detail the evolutionary development of these arcs and basins

geoid anomalies over gorringe ridge north atlantic ocean abstract gorringe ridge is a strong uplifted block of oceanic crust and upper mantle lying at the eastern end of the azores gibraltar plate boundary the geoid over this structure derived from seasat altimeter data exhibits a 9 m height anomaly with a north south lateral extension smaller than 200 km an attempt is made to interpret this geoid together with the gravity anomalies and with the seismicity which has been compiled as a function of depth it is first shown that the flexure of the oceanic lithosphere due to the ridge loading does not provide a good fit of the geoid anomalies and probably should be discarded as it assumes a continuous unfractured elastic plate models involving local heterogeneities are then tested the comparison of the observed geoid anomalies with the anomalies due to the uncompensated relief indicates that the topographic high has no shallow compensation uncompensated models previously proposed to explain the gravity anomalies are tested using the geoid one model purdy and bonnin in bonnin 11 which involves an uplift of upper mantle material at depth generates too strong geoid anomalies and must be discarded another model which represents a nascent subduction zone le pichon et al 25 fits both the gravity and geoid anomalies but leads to difficulties in explaining the deep seismicity north of gorringe ridge a model in isostatic equilibrium is also able to fit both gravity and geoid anomalies this model involves a deep root of density 3 0 g cm 3 as has been previously proposed for many oceanic ridges and plateaus this model is compatible with the deep seismicity but the origin of this low density material at great depth is up to now an unresolved question more likely dynamical models taking into account the forces induced by the convection related to the slow plate convergence in this area will have to be considered

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 i velocity model summary this is the first 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 build and calibrate a velocity model that will be used in the companion paper for seismic intensity modelling of the 1969 m s 8 0 and 1755 m 8 5 8 7 earthquakes taking into account the geological evolution of the region since the palaeozoic we build a 3 d velocity model down to the moho discontinuity substantially different from a simple 1 d layered model the velocity model presented in this paper is built a priori using information originating from a variety of geological and geophysical studies its resolution is sufficient to simulate realistically seismic wave propagation in the low frequency f 0 5 hz range which is the most significant for the study of the destructive effects of large earthquakes at a regional scale to validate the model we compute synthetic seismograms for three recent earthquakes of moderate magnitudes 4 6 m w 5 3 located offshore in the most seismically active area in the region synthetics are generated using a wave propagation code based on the finite difference method which was chosen for its simplicity and accuracy in the frequency range considered in this study 0 1 0 5 hz we compare simulated waveforms with threecomponent seismograms for 9 different stations traveltimes of the direct p waves and the amplitude of ground motion are accurately reproduced at all stations the frequency content of the seismograms fits the observations especially for the lowest frequencies investigated 0 1 0 3 hz for each earthquake the estimated seismic moment is in good agreement with values obtained by other authors using different methods we conclude that the velocity model provides encouraging results for the computation of low frequency seismograms in the region and can be used for the study of larger earthquakes for which the radiated wavefield has a predominant low frequency spectrum

intraplate earthquakes and the state of stress in oceanic lithosphere abstract the sources of stress responsible for oceanic intraplate earthquakes and their usefulness for inferring the tectonic stress field are investigated through a synthesis of the source mechanisms of 58 of the largest such events determined by inversion of long period p and sh waveforms further constraints are provided by the global distribution of oceanic intraplate seismicity and a calculation of the seismic moment release per unit volume of seismogenic lithosphere as a function of lithosphere age although frequently associated with relict tectonic features large oceanic intraplate earthquakes are generally reliable indicators of the state of stress in the source region most oceanic intraplate seismicity occurs in young lithosphere

short polarity intervals within the matuyama transitional field records from hydraulic piston cored sediments from the north atlantic abstract detailed sampling of two short magnetozones within the matuyama chronozone recorded at dsdp site 609 49 86 n 335 77 e confirms that one the cobb mountain subchronozone 1 12 ma is a very short full normal polarity interval and that the other the older interval is a record of a geomagnetic excursion which occurred at approximately 1 55 ma the cobb mountain subchron lasted approximately 25 000 years one third the duration of the jaramillo subchron the normal polarity interval is bounded by two transition zones which document an antisymmetry in the sequence of directions in the reverse to normal and normal to reverse polarity transitions we interpret the antisymmetry as reflecting a dependence upon the sense of the reversal without significant changes in the relative contributions of non dipole terms the polarity interval recorded at 1 55 ma lasted only 8 800 years with what may be regarded as full polarity directions observed across only 3 cm of stratigraphic section this feature is interpreted as an excursion of the geomagnetic field and appears to be correlative with the gilsa subchron similarities between the transition bounding these two magnetozones suggest that these features occur as the result of the same process or triggering mechanisms in the earth s outer core

volcanism accompanying back arc basin development in the southwest pacific abstract the age and composition of magmatism in fiji and to a lesser extent vanuatu and tonga are evaluated in the light of the fragmentation which has sundered the island arc system they once collectively constituted any of six magmatic phenomena during the past 10 m y may be consequences of adjacent plate reconfigurations trondhjemitic plutonism about 10 m y change from tholeiitic to more calc alkaline volcanism between 10 and 6 m y eruption near the rifted arc margin of voluminous crustally derived high si low k rhyolites 7 m y ago shoshonite dominated basaltic volcanism 5 5 to 3 0 m y change to ocean island like basaltic volcanism at 3 0 0 2 m y and geochemically peculiar as well as tectonically anomalous acid andesites 0 7 3 0 m y ago in kandavu southwestern fiji crustal derivation of the low k rhyolites may reflect maximal heating during initial rifting shoshonites may accompany extreme regional extension above a still arc like source eruption of ocean island like basalts begins once subducted lithosphere no longer contributes to erupted melts or separates fiji from ocean island sources coeval eruption of hybrid andesites may accompany oblique subduction of south fiji basin crust beneath kandavu along the northeast hunter fracture zone

crustal structure of the madeira tore rise eastern north atlantic results of a dobs wide angle and normal incidence seismic experiment in the josephine seamount region summary during november december 1988 an extensive geophysical data set was collected over the josephine seamount which is located at the northeasterly end of the madeira tore rise in the eastern north atlantic the josephine seamount lies at the intersection of the madeira tore rise and azores gibraltar fracture zone the latter representing the africa eurasia plate boundary in this part of the eastern north atlantic from this data set a 275 km long explosive refraction line has been modelled together with wide angle airgun profiles seismic reflection and gravity data the velocity depth model shows that the crust either side of the seamount is typically oceanic in character however beneath the seamount there exists a region of anomalously high velocity and crustal thickening to a depth of about 17 18 km gravity modelling also suggests that the josephine seamount is compensated by a crustal root and that the josephine seamount madeira tore rise system is in local isostatic equilibrium calculations of the flexural rigidity and effective elastic thickness of the lithosphere in this region suggest that the madeira tore rise formed contemporaneously with the lithosphere on which it lies this age of crustal loading is consistent with the proposal that the madeira tore rise is an aseismic ridge which formed at or near the mid atlantic ridge

deep earthquakes in the southwest pacific a tectonic interpretation this paper examines the spatial distribution of deep earthquakes in the tonga fiji new hebrides region of the southwest pacific our interpretation emphasizes the complex cenozoic tectonics of the pacific indo australian plate boundary as a primary control on the distribution and deformation of subducted lithosphere most deep earthquakes in the interarc region are associated with the contorted pacific plate lithosphere subducted at the tonga trench however anomalous groups of deep earthquakes located west of the tonga zone are unrelated to the present plate configuration tectonic reconstructions of the region to 8 m y b p provide circumstantial evidence that 1 the anomalous events west of the tonga zone occur in two pieces of detached lithosphere subducted at the vitiaz and proto new hebrides trenches during the late miocene 2 the flattening of the inclined seismic zone in northernmost tonga is related to the rapid opening of the lau basin since 4 m y b p and 3 the sharp westward curvature of the tonga seismic zone in this area coincides with a preexisting bend in the late miocene vitiaz arc the sharpness of its present curvature appears to be a secondary effect of shear flow in the lower mantle and compression between detached vitiaz and attached tonga lithosphere thus much of the contortion of the subducted lithosphere beneath tonga fiji may be produced by local tectonic interactions rather than collision of the slab with an impenetrable boundary in the midmantle

earthquake source mechanisms from body waveform inversion and intraplate tectonics in the northern indian ocean double couple point source parameters for 11 of the largest intraplate earthquakes in the northern indian ocean during the last 20 y were determined from a formal inversion of long period p and sh waveforms nine of the events have centroid depths at least 17 km below the seafloor well into the upper mantle two have centroid depths as great as 39 km using the source mechanisms of these earthquakes we distinguish two major intraplate tectonic provinces in the northern indian ocean to the west of the ninetyeast ridge in the southern bay of bengal intraplate earthquakes have thrust faulting mechanisms with p axes oriented n s the centroid depths of these earthquakes range from 27 to 39 km below the seafloor lithospheric shortening in this region is thus accomplished by thrust faulting in the strong core of the oceanic upper mantle while other geophysical evidence suggests that shallow sedimentary and crustal layers apparently deform predominantly by folding in the immediate vicinity of the ninetyeast ridge earthquakes display strike slip mechanisms with left lateral motion on planes parallel to the ridge this type of faulting occurs from at least 10 s to the northern end of the ninetyeast ridge near 10 n where the ridge meets the sunda arc seismic activity diminishes to the east of the ninetyeast ridge but is also characterized by strike slip faulting despite these variations in deformational style the inferred orientation of greatest compressive stress in the northern indian ocean displays a consistent long wavelength pattern over a large portion of the indian plate varying smoothly from nearly n s in the bay of bengal to nw se in the northeastern indian ocean this plate wide stress pattern and the high level of intraplate seismicity in the northern indian ocean are likely the results of substantial resistance along the himalayan continental collision zone to the continued northward motion of the western portion of the indian plate oceanic intraplate earthquakes in other regions where the level of deviatoric stress associated with the long wavelength part of the stress field is likely to be smaller need not be comparably reliable indicators of the plate wide stress field

the separation of madagascar and africa identification of a sequence of east west trending magnetic anomalies of mesozoic age in the western somali basin helps define the position of madagascar in the gondwana reconstruction the anomalies are symmetric about ancient ridge segments and are flanked to the north and south by the jurassic magnetic quiet zone the motion of madagascar relative to africa was from the north and began in the middle jurassic about the same time as the initial breakup of gondwanaland sea floor spreading ceased when madagascar assumed its present position in the early cretaceous

seismotectonics of the fiji plateau and lau basin abstract focal mechanism solutions of shallow earthquakes in the fiji plateau and lau basin provide significant contributions to the understanding of the recent tectonic movements beneath these marginal seas left lateral shear zones are identified on the linear shallow seismic zone extending from north of the fiji islands to the northern end of the lau basin in the central area of the fiji plateau and in the northern part of the lau basin nearly nw se trending opening tectonics is suggested nearly e w striking compressional tectonics is inferred near the western edge of the hazel holme fracture zone

euler deconvolution in a radial coordinate system this paper introduces the conversion of euler s equation from a cartesian coordinate system to a radial coordinate system and then demonstrates that for sources of the type 1 r n where r is the distance to the source and n is the structural index it can be solved at each point in space without the need for inversion for a known structural index it is shown that although the distance to the source that is obtained from euler s equation depends on the structural index used the direction to the source does not for some models such as the gravity and magnetic response of a contact calculation of the analytic signal amplitude of the data is necessary prior to the application of the method effective noise attenuation strategies such as the use of moving windows of data points are also discussed the method is applied to gravity and magnetic data from south africa and yields plausible results

seaward extension of the east african rift the continental margin of northern mozambique is edged by a north south trending alignment of submarine peaks forming the davie ridge fig la from earlier studies1 5 the ridge has been interpreted as a remnant transform fault that controlled the late jurassic early cretaceous drift of madagascar and the opening of the somali basin6 8 during md40 macamo cruise june 1984 7 500 km of water gun seismic reflexion lines have been implemented throughout the region where two flat bottomed troughs the north south trending kerimbas and lacerda deep grabens interrupt the continental slope alongside the ridge these grabens cut into tertiary sediments correspond westward to a miocene tilting or a recent normal fault and eastward to an old discontinuity several times reactivated and located above an abrupt uplift of the moho the two grabens are separated from each other by a rigid region of thickened crust affected by intense seismicity and supporting volcanoes we report here that the eastern branch of the east african rift joins the submerged grabens across the tanzanian continental shelf fig 1b

magnetic lineations in marginal basins of the western pacific this paper separates the small oceanic basins around the western perimeter of the pacific ocean into marginal basins that have formed through back arc extension and those that apparently have not and reviews our knowledge of magnetic lineation patterns observed in possible and probable back arc basins magnetic lineations in these basins resemble lineations commonly associated with the world s mid oceanic spreading systems indicating that similar processes of crustal accretion occur in both tectonic environments in some back arc basins of the southwestern pacific magnetic lineation and other evidence suggest that back arc basins can evolve through the interaction and growth of multi plate systems because of the small time and space characteristics of back arc basins compared with the world s major oceanic spreading systems tectonic conditions favourable for the generation of back arc basins are either relaxed rapidly or easily interrupted models proposed to account for back arc basin development include a local models where back arc extension is mechanically driven by the downgoing slab and b global plate kinematics models where conditions favourable for back arc extension are governed by the motion of the overriding plate relative to the trench axis

reconstruction of madagascar and africa evidence from the davie fracture zone and western somali basin new seismic reflection gravity and magnetic data from offshore east africa allow the davie fracture zone to be traced from 11 s to its intersection with the kenyan coast at 2 s constraining the relative motion of madagascar and africa seasat derived free air gravity anomalies and slope rise positive magnetic anomalies observed in shipboard data help to locate the continent ocean boundaries cob off the shore of east africa and madagascar seismic reflection data further document a diapirprovince off madagascar presumably conjugate to that observed off kenya and somalia the dhow and very large crude carrier vlcc basement ridges are complex features and do not appear to be simple fracture zones owing their existence entirely to the separation of madagascar and africa from these data we determine a predrift fit of madagascar and africa involving a 14 2 rotation of madagascar to africa about a pole at 10 n 150 e the geometry of the reconstruction adheres to seismic and potential field data indicating the oceanic nature and extent of the comoros basin and of the somali basin between kenya and the seychelles and it does not conflict with onshore or offshore stratigraphy timing of the opening of the western somali basin is constrained by mesozoic marine magnetic anomalies and extrapolation to the interpreted cob and occurred between approximately 165 and 130 ma

subduction seismicity and tectonics in the lesser antilles arc we have studied the mechanisms of 17 earthquakes along the lesser antilles subduction zone to examine a site where very old lithosphere subducts at a slow convergence rate no large thrust earthquakes occurred during the 1950 1978 study period the three large magnitude seven events are all normal faults one is a normal faulting event seaward of the trench its aftershock sequence includes strike slip events on differently oriented faults probably due to lateral block motion in response to the main shock a second indicates extension within the slab at depth these observations suggest that subduction in this region is primarily decoupled and aseismic unless the time interval studied is unrepresentative the third normal fault earthquake occurred within the upper plate with fault planes perpendicular to the arc and trench this unusual geometry may represent a flexural response to the subduction of the barracuda ridge a major bathymetric high with uncompensated excess mass at depth which seems analogous to flanking ridges found along some mid atlantic ridge fracture zones thus the barracuda ridge is not buoyant and does not affect benioff zone dip strike slip faulting occurs at depth in the subduction zone along a concentration normal to the arc and maymore indicate a fossil fracture zone there is no direct evidence in the shallow seismicity for the hypothetical north america south america caribbean triple junction through some of the oceanic intraplate seismicity is consistent with such a boundary less

active tectonics of the alpine himalayan belt between western turkey and pakistan summary over 80 new fault plane solutions combined with satellite imagery as well as both modern and historical observations of earthquake faulting are used to investigate the active tectonics of the middle east between western turkey and pakistan the deformation of the western part of this region is dominated by the movement of continental material laterally away from the lake van region in eastern turkey this movement helps to avoid crustal thickening in the van region and allows some of the shortening between arabia and eurasia to be taken up by the thrusting of continental material over oceanic type basement in the southern caspian mediterranean makran and black sea thus central turkey bounded by the north and east anatolian strike slip faults is moving west from the van region and overrides the eastern mediterranean at two intermediate depth seismic zones one extending between antalya bay and southern cyprus and the other further west in the hellenic trench the motion of northern iran eastwards from the van region is achieved mainly by a conjugate system of strike slip faults and leads to the low angle thrusting of iran over the southern caspian sea the seismicity of the caucasus shows predominantly shortening perpendicular to the regional strike but there is also some minor elongation along the strike of the belt as the causcasus overrides the caspian and black seas the deformation of the eastern part of this region is dominated by the shortening of iran against the stable borders of turkmenistan and afghanistan the north east direction of compression seen in zagros is also seen in north east iran and the kopet dag where the shortening is taken up by a combination of strike slip and thrust faulting large structural as well as palaeomagnetic rotations are likely to have occurred in ne iran as a result of this style of deformation north south strike slip faults in southern iran allow some movement of material away from the collision zone in ne iran towards the makran subduction zone where genuinely intermediate depth seismicity is seen within this broad deforming belt large areas such as central turkey nw iran azerbaijan central iran and the southern caspian appear to be almost aseismic and therefore to behave as relatively rigid blocks surrounded by active belts 200 300 km wide the motion of these blocks can usefully be described by poles of rotation the poles presented in this paper predict motions consistent with those observed and also predict the opening of the gulf of iskenderun ne of cyprus the change within the zagros mountains from strike slip faulting in the nw to intense thrusting in the se and the relatively feeble seismicity in se iran baluchistan this description also explains why the north south structures along the iran afghanistan border do not cut the east west ranges of the makran within the active belts surrounding the relatively aseismic blocks a continuum approach is needed for a description of the deformation even though motions at the surface may be concentrated on faults the evolution of fault systems within the active zones is controlled by geometric constraints such as the requirement that simultaneously active faults do not in general intersect many of the active processes discussed in this paper particularly large scale rotations and lateral movement along the regional strike are likely to have caused substantial complexities in older mountain belts and should be accounted for in any reconstructions of them

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

deep earthquakes beneath the fiji basin sw pacific earth s most intense deep seismicity in stagnant slabs abstract previous work has suggested that many of the deep earthquakes beneath the fiji basin occur in slab material that has been detached and foundered to the bottom of the transition zone or has been laid down by trench migration in a similar recumbent position since nowhere else in the earth do so many earthquakes occur in slabs stagnated in the transition zone these earthquakes merit closer study accordingly we have assembled from historical and modern data a comprehensive catalogue of the relocated hypocenters and focal mechanisms of well located deep events in the geographic area between the bottoms of the main vanuatu and tonga wadati benioff zones two regions of deep seismogenesis are recognized there i 163 deep shocks have occurred north of 15 s in the vityaz group from 1949 to 1996 these seismological observations and the absence of other features characteristic of active subduction suggest that the vityaz group represents deep failure in a detached slab that has foundered to a horizontal orientation near the bottom of the transition zone ii another group of nearly 50 outboard deep shocks occur between about 450 and 660 km depth west of the complexly buckled and offset western edge of the tonga wadati benioff zone their geometry is in the form of two or possibly three small circle arcs that roughly parallel the inferred motion of tonga trench migration earthquakes in the southernmost of these arcs occur in a recumbent high seismic wavespeed slab anomaly that connects both to the main inclined tonga anomaly to the east and a lower mantle anomaly to the west van der hilst r 1995 complex morphology of subducted lithosphere in the mantle beneath the tonga trench nature vol 374 pp 154 157 both groups show complexity in their focal mechanisms the major question raised by these observations is the cause of this apparent temporary arrest in the descent of the tonga slab into the lower mantle we approach these questions by considering the effects of buoyant metastable peridotite in cold slab material that was detached and rapidly foundered or was buckled segmented and laid out in the transition zone

transition from oblique subduction to collision in the northern luzon arc taiwan region constraints from bathymetry and seismic observations we systematically study the detailed bathymetry seismicity and source parameters of large and moderate sized earthquakes that occurred in the northern luzon arc taiwan region between 1964 and 1996 the purpose is to characterize the transition from oblique subduction to regional collision in terms of the distribution of morphological features the characteristics of seismogenic structures and the corresponding state of strain to the south of 21 5 n the existence of a subduction zone is clearly shown by both bathymetry and seismicity the subducted eurasia slab reaches a maximum depth of 200 km and the depth decreases sharply as it approaches taiwan focal mechanisms indicate that the subducted slab is in downdip extension for depths 150 km but switches to downdip compression at greater depths to the north of 23 n collision is clearly the predominant process with broad deformation on both sides of the suture in between the transition is accommodated by a distributed thrust deformation zone in the frontal portion of the accretionary prism closure of the forearc basin and the right lateral ne sw striking taitung canyon fault zone tcfz these structures resolve the differential movements between the subduction and collision zones and will repeatedly change their positions as the collision propagates southwestward the present day plate boundary follows the axis of the manila trench in the south broadens significantly in the zone of transition and finally becomes a zone of several tens of kilometers in width across eastern taiwan the deformation front on the other hand is located between the western foothills and coastal plain in taiwan then connects to the manila trench to the south seismic strain tensors indicate that the region to the north of tcfz has a compression rate 15 times larger than that to the south the northern luzon arc shows characteristics of oblique subduction but the effects of collision are not as significant within the subducted eurasian lithosphere as comparing to the subducted philippine sea plate beneath the southernmost ryukyu arc

palaeomagnetic excursions aborted reversals and transitional fields recordings of palaeomagnetic excursions have revealed apparent field behaviour ranging from rather erratic directional movements and loopings1 4 to single highly defined events5 7 excursions of the latter variety show what seems to be a rapid change in direction such that the path of the associated virtual geomagnetic poles vgps is well constrained in longitude such behaviour is not unlike that observed at the onset of some recorded polarity transitions7 9 indeed it has been suggested that excursions may occur during unsuccessful or aborted reversals2 5 7 10 we show here that available records of palaeomagnetic excursions together with our present understanding of field behaviour associated with geomagnetic reversals strongly support this hypothesis

gps constraints on africa nubia and arabia plate motions summary we use continuously recording gps cgps and survey mode gps sgps observations to determine euler vectors for relative motion of the african nubian arabian and eurasian plates we present a well constrained eurasia nubia euler vector derived from 23 igs sites in europe and four cgps and three sgps sites on the nubian plate 0 95 4 8 n 21 8 4 3 e 0 06 0 005 myr 1 we see no significant 1m m yr 1 internal deformation of the nubian plate the gps nubian eurasian euler vector differs significantly from nuvel 1a 21 0 4 2 n 20 6 0 6 e 0 12 0 015 myr 1 implying more westward motion of africa relative to eurasia and slower convergence in the eastern mediterranean the arabia eurasia and arabia nubia gps euler vectors are less well determined based on only one cgps and three sgps sites on the arabian plate the preliminary arabia eurasia and arabia nubia euler vectors are 27 4 1 0 n 18 4 2 5 e 0 40 0 04 myr 1 and 30 5 1 0 n 25 7 2 3 e 0 37 0 04 myr 1 respectively the gps arabia nubia euler vector differs significantly from nuvel 1a 24 1 1 7 n 24 0 3 5 e 0 40 0 05 myr 1 but is statistically consistent at the 95 per cent confidence level with the revised euler vector reported by chu gordon based on a re evaluation of magnetic anomalies in the red sea 31 5 1 2 n 23 0 2 7 e 0 40 0 05 myr 1 the motion implied in the gulf of aqaba and on the dead sea fault dsf by the new gps nubia arabia euler vector i e ignoring possible sinai block motion and possible internal plate deformation grades from pure left lateral strike slip in the gulf and on the southern dsf with increasing compression on the central and northern dsf with relative motion increasing from 5 6 to 7 5 mm yr 1 1m m yr 1 from south to north along the northern dsf i e north of the lebanon restraining bend motion is partitioned between 6 1m m yr 1 left lateral motion parallel to the fault trace and 4 1m m yr 1 faultnormal compression relative motions on other plate boundaries including the anatolian and aegean microplates derived from the gps euler vectors agree qualitatively with the sense of motion indicated by focal mechanisms for large crustal earthquakes m 6 where data are available on fault slip rates on plate bounding faults north anatolian fault east anatolian fault dead sea fault red sea rift they are generally lower than but not significantly different from the full plate motion estimates suggesting that the majority of relative plate motion is accommodated on these structures

subduction of aseismic ridges beneath the caribbean plate implications for the tectonics and seismic potential of the northeastern caribbean normal seafloor entering the puerto rico and northern lesser antillean trenches in the northeastern caribbean is interrupted by a series of aseismic ridges on the north and south american plates these topographic features lie close to the expected trend of fracture zones created about 80 110 m y ago when this seafloor was formed at the mid atlantic ridge the northernmost of the ridges that interact with the lesser antillean subduction zone the barracuda ridge intersects the arc in a region of high seismic activity some of this seismicity including a large shock in 1974 occurs within the overthrust plate and may be related to the deformation of the caribbean plate as it overrides the ridge a large bathymetric high the main ridge is oriented obliquely to the puerto rico trench and intersects the subduction zone north of the virgin islands in another cluster of seismic activity along the inner wall of the trench data from a seismic network in the northeastern caribbean indicate that this intersection is also characterized by both interpolate and intraplate seismic activity magnetic anomalies bathymetric trends and the pattern of deformed sediments on the inner wall of the trench strongly suggest that the main and barracuda ridges are parts of a formerly continuous aseismic ridge a segment of which has recently been overridden by the caribbean plate reconstruction of mid miocene to recent plate motions also suggest that at least two aseismic ridges and possibly fragments of the bahama platform have interacted with the subduction zone in the northeastern caribbean the introduction of these narrow segments of anomalous seafloor into the subduction zone has segmented the arc into elements about 200 km long these ridges may act as tectonic barriers or asperities during the rupture processes involved in large earthquakes they also leave a geologic imprint on segments of the arc with which they have interacted a 50 km landward jump of the locus of island arc volcanism occurred in late miocene time along the northern half of the lesser antilles we postulate that the subduction of a segment of seafloor of anomolously thick crust being more buoyant than adjacent seafloor resulted in a marked shoaling in the dip of the descending slab and therefore a shift of the locus of volcanism in the region near western puerto rico and eastern hispanolia plio pleistocene interaction with a similar feature in this case a part of the bahama platform about 3 4 m y ago led to a jump in the locus of subduction as evidenced by a gap in the downgoing seismic zone that segment of the bahama platform interferred with the subduction process and was subsequently sutured onto the caribbean plate when the boundary jumped about 60 km to the northeast the maximum size of historic shallow earthquakes along the lesser antillean arc varies from about 7 0 7 5 in the center of the arc where the dip of the shallow part of the plate boundary is steep to 8 0 8 5 along the northern part of the arc where the dip is shallow the interaction of anomalous seafloor as along the northern lesser antilles can lead to the development of a wider than normal zone of interplate contact and hence to earthquakes that are larger than those associated with more typical seafloor entering subduction zones major seismic gaps and regions of high seismic potential currently exist along the northern lesser antilles and to the north of puerto rico both gaps are bounded by anomalous features on the downgoing plate the intersection of these features with the plate boundary created large asperities that may be good places to search for precursors to future large earthquakes a great shock in 1787 may have ruptured an existing seismic gap north of puerto rico between 65 and 67 w thus that gap can be expected to eventually rupture again in a great shock and not to accommodate plate motion by totally aseismic processes

magnetic anomalies and tectonic fabric of marginal basins north of new zealand detailed airborne magnetic studies conducted over the region of the s w pacific marginal basins extending from the solomon islands to new zealand suggest that three major phases of basin formation and island arc development have occurred in this region development of the tasman sea took place during the late cretaceous paleocene development of the basins to the east of the tasman sea occurred predominantly during the oligocene as well as during the upper miocene to recent the south fiji basin consisting of the kupe and minerva abyssal plains is marked by the presence of possibly two rrr triple junction spreading centers that were active between the times of anomalies 13 to 7 36 25 5 m y the kupe abyssal plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center the western limb appears to have been subducted beneath the present site of the three kings rise this seafloor spreading phase calculated half spreading rate of 35 mm yr was coincident with the overthrusting phase of the new caledonia ultramafic rocks during that period active volcanism along the then continuous solomons new hebrides fiji lau island arc was taking place magnetic anomalies from 1 to 4 0 8 m y b p are seen to extend along a clearly defined lineation pattern over the north fiji basin analysis of the magnetic anomalies suggests that during the upper miocene at about the time of magnetic anomaly 4 8 m y the solomons lau island arc commenced breaking up through the development of an active spreading center calculated half spreading rate of 35 mm yr located between the new hebrides and fiji portions of the island arc portions of the northeastern sector of the minerva abyssal plain were subducted beneath the north fiji basin during this period between 0 8 m y b p at the time of about anomaly 3 to 2 4 5 3 5 m y the development of the north fiji basin extended into the lau basin and havre trough thus separating the tonga and lau ridges as a result a distinct series of active spreading centers developed southward along the axis of the havre trough lau basin the havre trough is marked by present seismic and submarine volcanic activity residual magnetic anomalies 1 2 0 1 8 m y b p form a symmetrical pattern around the axis of the trough extending south from the lau basin to the active taupo white island volcanic zone of new zealand phases of development of the marginal basins described in this paper may be correlated with phases of changes in the direction and rate of pacific plate motion

the tangshan earthquake sequence and its implications for the evolution of the north china basin the 1976 tangshan earthquake sequence that occurred on the northern margin of the north china sedimentary basin is one of the strongest and most destructive intracontinental earthquake sequences ever recorded by worldwide long period seismograph networks we have studied the source process of the six largest events of this earthquake sequence by formally inverting long period p and sh wave seismograms from the world wide standardized seismographic network our analysis shows that the main shock was caused by motion on at least three fault segments each with a different orientation the initial and dominant faulting occurred on two segments of a north northeast trending right lateral strike slip fault system the last stage of faulting had a significant component of thrusting and occurred on an east northeast trending subsidiary fault at the southern end of the main fault the largest aftershock occurred at the northeastern end of the main fault and had a nearly pure normal faulting mechanism with east west striking nodal planes this event occurred in a pull apart region where the north northeast trending main strike slip fault system steps to the right this region subsided by more than 1 m during the earthquake sequence to the north of this step the main fault is delineated by numerous aftershocks whose mechanisms are by and large consistent with right lateral slip on this segment the large event in 1945 m 6 3 probably occurred on this part of the fault a region of substantial subsidence up to 1 5 m associated with the earthquake sequence was also observed in the south where the main fault system takes another step to the right two of the large aftershocks occurred nearby and had strike slip mechanisms the overall faulting process of the tangshan sequence is well characterized by right lateral slip on a set of right stepping faults the mechanisms and the points of nucleation of the strongest shocks and the extent of the ruptures appear to be closely tied to preexisting fault geometries a compilation of the mechanisms of large earthquake sequences in the north china basin strongly suggests that the present deformation of the entire basin is dominated by motions on right lateral strike slip systems trending north northeast the basin seems to have formed and evolved as a large transtensional structure at an en echelon step of a large scale strike slip system the deformation within the basin is both strike slip and extensional and appears to occur on smaller scales in an approximately selfsimilar fashion as evidenced by the deformation during the tangshan sequence this interpretation provides an explanation for the variable rate of subsidence in both space and time and the scatter in measured heat flow values as well as the overall shape of the north china basin it also provides an explanation for occasional thrust faulting earthquakes that occur apparently as a result of geometrical complexities of the strike slip faults transpression these observations are otherwise difficult to fit into a simple model of regional crustal extension and thermal subsidence

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

constraints on the motion of madagascar with respect to africa abstract three constraints on the motion of madagascar relative to africa are investigated in order to arrive at a model for madagascar s drift these are 1 fracture zone morphology and direction in the somali basin and mozambique channel 2 the relative positions of the carboniferous to jurassic palaeomagnetic poles for madagascar and africa and 3 the crustal structure along a geophysical traverse between lamu kenya and mahe seychelles the evidence for the bunce and molnar 1977 pole for the rotation of madagascar relative to africa defined by the davie dhow vlcc and ars fracture zones is critically examined and accepted it is used to rotate madagascar northwards back to a position that brings the carboniferous to jurassic palaeomagnetic poles for madagascar and africa into good agreement the rotation of approximately 16 places madagascar in the north of the somali basin with its northern tip at the southwestern end of the chain ridge it moves madagascar almost exactly the length of the extended davie fracture zone accepting that this position may have been the pre drift position of madagascar some interesting correlations with the lamu mahe crustal section emerge continental crust appears to extend 300 km offshore from lamu and thus abuts precisely the continental crust of rotated madagascar between 300 km and 1200 km from lamu the crust is probably oceanic of jurassic cretaceous age this would have been left in the wake of madagascar as it drifted southwards four fracture zones are seen in this crust a major one appears in the crustal section about 700 km from lamu the dhow fracture zone at 850 km may be associated with a large step up to the east in basement which has acted as an efficient dam to sediments from africa and the vlcc and ars are smaller fracture zones the easternmost part of oceanic crust between 1100 and 1200 km may have been formed in the wake of the farquhar islands drifting southwards with madagascar more likely however it could have formed as the seychelles bank area drifted away from madagascar having previously moved southwards with madagascar eastwards from 1200 km thicker possibly continental crust adjoins the seychelles bank this model assumes that the comores basin is oceanic in origin which may not be the case if it is continental a rotation of madagascar of only 9 or 10 can be accommodated moving the island almost exactly the length of the topographic expression of the davie fracture zone but providing a poor fit of the palaeomagnetic poles

earthquakes associated with diffuse zones of deformation in the oceanic lithosphere some examples abstract we review the results of source parameters of earthquakes associated with the eastern end of the azores gibraltar plate boundary the davie ridge near madagascar and the horizon bank in the north fiji basin to characterize the nature of present day tectonics in diffuse zones of deformation in the oceanic lithosphere in comparison with typical plate boundaries diffuse zones of deformation are in general characterized by complex morphological expressions and scattered seismicity of up to several hundred kilometers width although the average rate of displacement across these regions is not resolvable by current models of global plate reconstruction the deformation is not truly intraplate in nature because strain is concentrated and often distinct tectonic boundaries can be identified earthquakes with seismic moments as large as 8 10 20 nm have occurred in bands of scattered seismicity that are associated with diffuse zones of deformation in the oceanic lithosphere in two cases the zones of diffuse deformation continue into the continental lithosphere east of gibraltar and north of the davie ridge however the largest earthquakes occur in the oceanic part of these two zones in contrast to previous reports the depth of earthquakes is quite shallow in regions where a mixture of focal mechanisms occur the consistent parameter is usually the orientation of the p or the t axes instead of that of the nodal planes or slip vectors thus as in the continental lithosphere zones of diffuse deformation in the oceanic lithosphere seem to be better described by a regional compression or extension field rather than by displacement on discrete faults

development of an earthquake early warning system based on earthworm application to southwest iberia abstract the main aim of this study is to demonstrate the feasibility of an earthquake early warning system eews to warn of potentially destructive earthquakes in the cape san vicente sv and gulf of cadiz gc areas in the southwest iberian peninsula based on real time broadband stations existing in the region historic earthquakes in this region include the 1755 lisbon m w 8 5 and 1969 sv m w 7 8 events this study illustrates the design configuration and first results of an eews prototype based on the u s geological survey earthworm tools and developed at the institut cartografic i geologic de catalunya icgc system main functionalities are real time data acquisition processing p wave picking event detection earthquake hypocentral location and magnitude estimation from previously calculated peak displacement p d and predominant period c of the p wave signal data archiving and earthquake early warning dissemination the prototype was put into operation after a setup period during which several simulations were carried out to establish the optimal settings after the first year of operation one felt earthquake occurred in the area of study location and magnitude results are fairly good compared with the instituto geografico nacional ign catalog lead times obtained are on the order of tens of seconds for the majority of targets which are long enough to mitigate damage for a large area of the southern coasts of portugal and spain due to the gc and sv earthquakes preliminary results for this prototype demonstrate the possibility of a regional reliable and effective eews in southwest iberia

a diffuse plate boundary model for indian ocean tectonics it is suggested that motion along the virtually aseismic owen fracture zone is negligible so that arabia and india are contained within a single indo arabian plate divided from the australian plate by a diffuse boundary the boundary is a zone of concentrated seismicity and deformation commonly characterized as intraplate the rotation vector of australia relative to indo arabia is consistent with the seismologically observed 2 cm yr of left lateral strike slip along the ninetyeast ridge north south compression in the central indian ocean and the north south extension near chagos