significant unreported earthquakes in aseismic regions of the western pacific with data recorded by a deep ocean hydrophone array centrally located in the northwestern pacific basin near wake island significantly large earthquakes were found to have their epicenters within the interior of the pacific plate the signal noise ratio of the largest is about 40 1 at a distance of 2268 km although all of these earthquakes were unreported in national earthquake information service neis listings comparisons with the po so phases of other earthquakes reported by neis indicate that these earthquakes could have body wave magnitudes in excess of 5 0

subduction initiation at passive margins the scotian basin eastern canada as a potential example thermal evolution modeling based on the geometry of the scotian basin eastern canada and analysis of the shear resistance associated with lithospheric faulting are used to investigate the initiation of subduction at passive margins previous studies have demonstrated the difficulty of subduction initiation at mature passive margins however unique features along a passive margin may facilitate subduction initiation subduction may initiate along a part of the margin 1 by extensional or strike slip deformation rather than shortening 2 by deformation of the weaker continental lithosphere rather than oceanic lithosphere 3 by heating and weakening of oceanic lithosphere due to lateral heat transfer from adjacent continental lithosphere and 4 by heating and weakening of the lithosphere due to the passage of a mantle plume beneath the margin the values of shear resistance under these conditions are still higher than those calculated for ridge push indicating that subduction initiation at passive margins is difficult even with these weakening mechanisms however pulses of compression associated with plate reorganization may be enough to trigger subduction at such passive margins

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

investigation of microearthquake activity following an intraplate teleseismic swarm on the west flank of the southern east pacific rise between february 1991 and may 1992 33 intraplate earthquakes having body wave magnitudes between 4 3 and 6 0 were located on the west flank of the southern east pacific rise by the international seismological center seven months after the last teleseismic event we deployed four ocean bottom seismometers at the site of the teleseismic swarm one hundred and ninety two microearthquakes were located using p and s travel times of events recorded by three or more instruments during the 16 day deployment most of the microearthquakes were in a band about 30 km long and 6 km wide between and parallel to seamount chains in addition several events were distributed along a line perpendicular to the main seismicity band and parallel to the ridge axis the focal depths of the microearthquakes range from 1 to 15 km and most are between 5 and 12 km similar to the depth range of the teleseismic events hung and forsyth 1996 the composite p wave polarities indicate that the microearthquakes had a variety of focal mechanisms we developed a new grid search inversion technique that utilizes the p wave polarities and the empirically corrected ratios of p and s wave amplitudes to find the focal mechanisms of individual events within the acceptable travel time and amplitude misfits focal solutions are fairly stable normal faulting is found in the ridge parallel seismicity line the thrust and strike slip faulting in the main seismicity band is distinctly different from the exclusively normal faulting mechanisms of the teleseismic events there is no apparent depth dependence of fault types none of the existing models of the sources of stress ridge push thermoelastic stresses loading by local topographic features caldera collapse and north south extension of the pacific plate provides a satisfactory explanation for both the teleseismic swarm and microearthquakes we propose a new tectonic scenario in this scenario the lithosphere is prestressed by the cooling of the plate magma rising from the deeper mantle induces normal faulting ahead of the dike tips in the lower lithosphere which is already under extensional thermal stress producing the larger teleseismically detected events once the dikes propagate into the lithosphere the region surrounding the dikes behind the tips is compressed by the overpressure of magma depending on the geometry of the dikes the local orientations of the minimum principal stress and the local weaknesses in the lithosphere thrust or strike slip faulting the microearthquakes may occur

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

the elastic properties of the lithosphere beneath scotian basin abstract to assess the possibility that the north atlantic ocean may subduct at scotian basin east of canada we investigate the present compensation state of this deep basin a fourier domain analysis of the bathymetry depth to basement and observed gravity anomalies over the oceanic area east of nova scotia indicates that the basin is not isostatically compensated moreover the analysis emphasizes that in addition to the sediments density perturbations exist beneath the basin the load produced by the sediments and these density perturbations must have been supported by the lithosphere we simulate the flexure of the lithosphere under this load by that of a thin elastic plate overlying an inviscid interior it is shown that a plate with a uniform rigidity does not adequately represent the lithosphere beneath the basin as well as the oceanic lithosphere far from the basin rather the rigidity of the lithosphere directly beneath the basin is about one to two orders of magnitude smaller than elsewhere we relate this weakening to the thermal blanketing effects of the thick sediments and the fact that the lithosphere has a temperature dependent rheology we suggest that this weak zone would have a controlling effect on the reactivation of normal faults at the hinge zone of the basin that were formed during the break up of africa and north america and were locked in the early stages after the break up the weak zone would facilitate reactivation of the faults if tensional stresses were produced by possible reorientation of the spreading direction of the north atlantic ocean in the future the reactivation of the faults would create a free boundary condition at the hinge zone allowing further bending of the lithosphere beneath the basin and juxtaposition of this lithosphere to the mantle beneath the continent this may provide a favorable situation for initiation of slow subduction due to subsequent compressional forces

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

implications of oceanic intraplate seismicity for plate stresses driving forces and rheology abstract recent studies of earthquakes occurring within the oceanic lithosphere provide valuable information about stresses in the lithosphere plate tectonic driving forces and the rheology of the lithosphere asthenosphere and mantle focal mechanisms indicate that oceanic lithosphere older than 35 million years is almost entirely in deviatoric compression both compressional and extensional events occur in younger lithosphere in young lithosphere mechanisms generally indicate compressive stress in the spreading direction or extension oblique to the spreading direction extension in the spreading direction is not observed using the constraint of compression in the spreading direction in old lithosphere models of the stresses produced by a combination of ridge push and basal drag forces require the magnitude of the drag be less than a few bars for rapidly moving plates and a few tens of bars for slow moving plates assuming that basal drag results from mantle return flow the upper limits on drag can be converted to constraints on the viscosity structure of the asthenosphere and mantle using simple two dimensional models if return flow occurs in a mantle of viscosity 10 22 poise comparable to the results from glacial rebound studies the predicted basal drag is too high unless a thin asthenosphere with viscosity less than 10 19 to 10 20 poise depending on flow depth is present the intraplate stress data thus are consistent with the idea of oceanic plates largely decoupled from the underlying mantle the strength of the lithosphere is constrained by the maximum depth of oceanic intraplate seismicity which increases with lithospheric age and appears to be bounded by a 700 800 c isotherm this limiting depth is approximately equal to the flexural elastic thickness of the lithosphere and is consistent with experimental olivine rheologies which predict rapid weakening at high temperatures similar phenomena are important for estimating the fraction of seismic and aseismic slip on transform faults and in determining the extent of rupture for large trench normal faulting events

origin of the calderas and evolution of roccamonfina volcano roman region italy abstract the roccamonfina volcano consists of a volcanic fluvial succession dated 549 ka bp paleoauruncus overlain by a 546 ka bp stratacone main cone formed by leucite bearing lava and tephra the main cone was dissected by four nested calderas a ring fracture concentric with and outside of the central caldera is also present detailed mapping of the volcano inspection of landsat satellite images and radiometric dating establish that the gli stagli and northern calderas subsided between respectively 546 474 and 403 374 ka bp and that the summit part collapsed between 546 and 446 ka bp to form the central caldera a chronologically intermediate state dated in the range 376 323 ka bp was identified between the 549 374 ka bp leucite bearing high k series hks stage i and the 317 96 ka bp leucite free low k series lks stage ii this stage is made up of concurrent hks lks rocks stage ics the central gli stagli and northern calderas are older than blt and wtt as supported by the strong discrepancies between calculated volumes of these pyroclastic rocks and the volume of the caldera also caldera collapses due to sector sliding are improbable as tested by the absence of rock slide avalanche deposits the collapses occurred incrementally through repetitive draining of the magma chamber s resulting from a succession of moderate sized volcanic events gli stagli caldera was infilled by a lake early after collapse so that sediments alternated with leucite tephrite lava flows and tephra within the lake the sequence was overlain by a thick succession of young pyroclastic rocks downfaulting downsagging cut the flat lying gli stagli filling sequence resulting in the old valle caldera successive pyroclastic and debris flows derived by updoming of the central caldera deposits infilled the last structure and downfaulted part of the valle fill producing the young valle caldera

intraplate seismicity of the pacific basin 1913 1988 we establish here a comprehensive database of intraplate seismicity in the pacific basin relocation and analysis of 894 earthquakes yield 403 reliable intraplate earthquakes during 1913 1988 these numbers do not include earthquake swarms which account for another 838 events most of the remainder 304 events are actually plate boundary earthquakes that have been erroneously located in intraplate regions a significant number occur in recent years when location capabilities should have guarded against this situation relocations involve a careful linear inversion of p and s arrivals accompanied by a monte carlo statistical analysis we have also attentively removed the high number of clerical errors and nuclear tests that exist in epicenter bulletins

oceanic intraplate seismicity the occurrence of seismicity at the surface of the globe largely along preferential lines now recognized as mid oceanic ridges and subduction zones was key evidence for the development of the theory of plate tectonics in the 1960s indeed many plate boundaries especially some of the southern less accessible ridges were drawn at least initially on the basis of this evidence alone the mere concept of intraplate seismicity may then appear as somewhat of a paradox if not as a failure of the whole theory however and as shown later in this paper the relatively low level of this seismicity warrants its consideration as a perturbation in the general framework of nearly rigid plates additionally intraplate earthquakes are of great value since in the oceanic environment they are our only clue to the state of stress of the lithosphere thereby providing us with critical insight into the forces responsible for its motion understanding para meters that control these earthquakes is made easier with respect to a continental situation by the generally younger age of the ocean floor and the much simplified tectonic history of any given oceanic province on the other hand seismic detection capabilities are greatly reduced at sea because of both logistics and generally higher seismic attenuation thus our knowledge of many aspects of oceanic intraplate seismicity is still rudimentary it is in this general framework that the present paper reviews the following points

strain rate dependency of oceanic intraplate earthquake b values at extremely low strain rates we discovered a clear positive dependence of oceanic intraplate earthquake oceq b values on the age of the oceanic lithosphere oceq b values in the youngest 20 ma oceanic lithosphere exceed 1 5 which is significantly higher than the average worldwide earthquake b value around 1 0 on the other hand the b value of intraplate earthquakes in the ninety east sumatra orogen where oceanic lithosphere has an anomalously higher strain rate compared with normal oceanic lithosphere is 0 93 which is significantly lower than the oceq b value about 1 9 with the same age 50 110 ma thus the variation in b values relates to the strain rate of the oceanic lithosphere and is not caused by a difference in thermal structure we revealed a negative strain rate dependency of the b value at extremely low strain rates 1 5 in oceanic lithosphere 20 ma old imply that future improvement in seismic observation will capture many smaller magnitude oceqs which will provide valuable information on the evolution of the oceanic lithosphere and the driving mechanism of plate tectonics

intraplate seismicity of the southern part of the pacific plate we present a compilation of all the known seismicity of the part of the pacific plate located south of the equator this includes historical earthquakes dating back to 1937 in addition to the gilbert islands swarm of 1981 1983 we identify 60 events at 32 epicenters in the magnitude range 4 3 6 9 our results indicate that the level of seismicity of the southern pacific plate is comparable to that of other plates including continent bearing ones most of the seismic activity is concentrated in the youngest parts of the plate but a few large events are present in cretaceous lithosphere with the exception of events from the gilbert islands swarm earthquakes located in lithosphere 35 ma and older are usually compatible with the release of horizontal compressional stress due to the gravitational process known as ridge push in younger areas the situation is much less clear cut the final geometry of the focal mechanisms is controlled by local tectonics in particular due to the miocene reorientation of the spreading at the east pacific rise the bulk of the pacific plate is particularly vulnerable to ridge push and this explains the numerous strike slip events and the absence of magnitude 6 events in most of the plate correlations of epicenters with zones of weakness or bathymetric features are frequent but not universal finally a number of areas with high seismic release are identified as potential targets for future exploration

earthquakes in the aseismic regions of the western pacific operators of a deep ocean hydrophone array near wake island in the northwest pacific basin have located several previously unreported earthquakes within 20 of wake island in the aseismic interior of the pacific plate and have suggested high attenuation in the source region to explain why these events have not been reported by seismological agencies the largest such earthquake occurred on 1983 june 29 in the kiribati region readings were submitted to the international seismological centre but the earthquake was excluded from the final analysis because of doubt about its location and interfering arrivals from other earthquakes closer examination now provides a well determined seismological location and a magnitude of 4 7 mb neither amplitudes nor frequency content of australian records of the kiribati earthquake support the existence of a region of abnormally high attenuation beneath the source we suggest that the higher magnitudes reported from the hydrophone measurements result from the efficient coupling of energy from shallow intraplate earthquakes in the pacific basin into the po so waveguide

fingernails and diet stable isotope signatures of a marine hunting community from modern uummannaq north greenland c n ad s stable isotope compositions of fossil human bones are frequently used in palaeodiet studies but results from strictly marine communities are scarce modern hunting and fishing inuit greenlanders in the uummannaq district in north greenland are known to live from a diet high in local marine food seal whale halibut combined with basic foods bread cereals dairy products imported from denmark and thus provide an excellent test population here were report diet anamnesis data and stable isotope compositions of fingernails from 82 uummannaq district inuit individuals and a control group of 32 danes living in denmark the uummannaq fingernails are significantly enriched in 13c 15n 34s compared to the danish group t test 99 conf level calculated from isotope modelling the uummannaq inuit get on average 50 of their diet range 28 to 68 from marine food placed high in the trophic system reflecting high proportions of carnivorous marine mammals seal in the diet and the most 15n enriched individuals have trophic positions close to polar bears comparisons to other human hair and fingernail studies reveal the uummannaq inuit to be the most heavy isotope enriched modern community analysed so far the consistency of our data lead us to conclude that the fingernail isotope signatures are reliable measures of modern human diet composition and that fingernail isotope analyses can provide an easy quantitative supplement to other diet studies sulphur isotope analyses in special may provide unambiguous results in terrestrial to marine mixing systems

anisotropic horizontal thermal contraction of young oceanic lithosphere inferred from stress release due to oceanic intraplate earthquakes how freely the oceanic lithosphere contracts horizontally due to thermal contraction is important information because it reflects the boundary condition of the oceanic lithosphere which includes information regarding the magnitude of driving resisting forces of plate tectonics we investigated the horizontal thermal contraction of young oceanic lithosphere using an analysis of the intraplate stress release due to oceanic intraplate earthquakes oceqs and numerical simulations the stress release due to oceqs in young oceanic lithosphere 5 15 ma shows significant differences between the spreading directional component and the ridge parallel component the extensional stress release of the ridge parallel component is six times as large as that of the spreading directional component while the compressional stress release of the ridge parallel component is one seventh that of the spreading directional component we conducted a numerical simulation of the thermal stress evolution of the oceanic lithosphere to investigate how the difference in the horizontal contraction rates between the spreading direction and the ridge parallel direction can explain the observed anisotropic stress release the result indicates that young oceanic lithosphere 5 15 ma barely contracts in the ridge parallel direction only 0 30 of the spreading directional contraction rate while it contracts freely in the spreading direction due to the weakness of the oceanic ridge strength and the low viscosity asthenosphere from the results we constrained the magnitude of the basal traction working on the bottom of the oceanic lithosphere to be smaller than 0 44 mpa

state of stress and age offsets at oceanic fracture zones and implications for the initiation of subduction abstract the recycling of oceanic lithosphere back into the earth s interior through subduction is a central component of plate tectonics the process by which new subduction zones initiate however remains poorly understood several different mechanisms for subduction initiation have been proposed including passive margin collapse aided by sediment loading and or rheological weakening due to the presence of volatiles and forced convergence across a zone of preexisting lithospheric weakness in this paper we focus on the latter type of model which identifies three conditions necessary for subduction initiation a zone of weakness such as a fracture zone an age and therefore density offset along the fracture zone and significant normal compressive stress which leads to shortening we identify regions on the present day earth which meet these conditions and which may correspond to regions of relatively likely subduction initiation in the near future using a digital seafloor age model we have created a database of oceanic fractures and quantified the associated age offsets we have evaluated the state of stress on these lithospheric weak zones using two different global stress models we find that the conditions needed to initiate subduction via the forced convergence model are relatively rare on the present day earth and that there is little indication of incipient subduction at regions identified as relatively likely for subduction initiation using the same technique we have evaluated the state of stress and seafloor age offset at regions of inferred present day incipient subduction and find that most of these regions are not associated with both high far field compressive stresses and large age and thus density offsets subduction has likely initiated via forced convergence across preexisting zones of lithospheric weakness in the past but our results indicate that the conditions needed for this type of subduction initiation are rare on the present day earth

historical seismicity and seismotectonic context of the great 1979 yapen and 1996 biak irian jaya earthquakes we present relocations of over 220 historical and recent earthquakes in the northwestern part of irian jaya in the context of the large earthquakes of 1979 and 1996 our results document continuous activity on a 420 km segment of the sorong fault with a possible extension over an additional 330 km to the west we also show that some level of activity did take place on the new guinea trench prior to the 1996 biak earthquake and relocate a large m pas 7 4 event on 02 april 1947 to the trench at 138 e we speculate that the large earthquake of 26 may 1914 may also have taken place on the new guinea trench we study the pattern of activity following the 1979 yapen earthquake which triggered stress release in the pandaidori islands also the location of stress transfer following the 1996 biak earthquake

thermoelastic stress how important as a cause of earthquakes in young oceanic lithosphere we present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere less than 35 m y in age in support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m y where cooling rates are relatively high most near ridge earthquakes have focal depths between the moho and the depth of the 800 c isotherm thrust and strike slip events dominate the shallowest seismicity while the focal mechanisms of the deeper events are generally characterized by normal faulting to assess the importance of thermal stress in the generation of near ridge earthquakes we model the lithosphere as an elastic half space in which the response to cooling is computed using the method of thermoelastic displacement potentials a key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere a further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half space cooling models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust and normal faulting earthquakes near mid ocean ridges the addition of a ridge push stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m y but probably contributes little to the stress field at younger ages though these simple models are limited by several simplifying assumptions particularly in the immediate vicinity of the ridge axis they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere

geoid lows over deep sea trenches in many instances negative gravity anomalies over trenches are slightly displaced toward the inner slope of the subduction complex indicating an accumulation of low density material this effect known for many years in gravity data is also observed in geoid data we have analyzed regularly spaced geoid height profiles from the seasat satellite across the aleutian kuril mariana and tonga kermadec trenches regional variations of the offset between topography and geoid minima have been recorded along the aleutian trench the offset increases from a few kilometers in the eastern part to a few tens of kilometers up to 40 km in the central part between 170 w and 180 w and then decreases again to less than 10 km in the western aleutians along the kuril trench the offset is more or less constant 20 km except for a larger offset up to 30 km at the southern end of the arc along the mariana and tonga trenches the geoid low is in near coincidence with the trench axis while in the southern end of the kermadec trench a small offset is detected the displacement of the geoid low away from the trench axis is related to the width of accreted material beneath the inner trench slope no offset is observed in regions of very narrow accretionary prism whereas a 30 km offset may result from an accretionary prism of 60 km in width supported without isostatic compensation by the subducting elastic lithosphere geoid lows centered over the axis of trenches with narrow accretionary prism e g mariana and tonga are almost completely explained by the seawater trench topography alone unlike displaced geoid lows which require an additional negative density contrast on the other hand in areas of very extensive accretionary prism as in the eastern aleutians the situation is complicated by sediment loading and additional flexure of the descending plate which insures partial compensation of the prism

a tensile flexural model for the initiation of subduction we argue that subduction may be initiated at passive continental margins without shortening the lithosphere overcoming the lithosphere s high compressive strength requires special circumstances and these make it difficult to explain the nearly complete recycling of old sea floor instead we present a model that predicts tensile decoupling of the continental and oceanic lithosphere passive rifting and foundering of the seafloor beneath material welling up in the rift this occurs because the lithosphere in a new ocean basin establishes mechanical continuity with the continent at a depth comparable to mid ocean ridges later subsidence at the margin is therefore inhibited by flexure which implies shear stresses that promote fault slip and tensile stresses necessary to balance the component of the plate s weight directed down the margin slope we show that this tension can more than offset ridge push in our model an important additional tension arises from basal shear tractions resisting the plate s motion away from the mid ocean ridge although these tractions cannot be evaluated with confidence slip on a high angle fault decouples the oceanic and continental lithosphere when shear stresses arising from flexure and the applied tension exceed the lithosphere s shear strength under these loads a passive rift then forms allowing a mantle column to rise to the height of mid ocean ridges over 3 km above the old seafloor and flow onto the surface in a gravity current this load flexes the plate downwards which enhances the flow and lets the old oceanic lithosphere founder this model is consistent with the presence of oceanic material in continental forearcs and the youth of ophiolites when they are obducted as both might be explained by rifting a margin and underthrusting the juvenile crust formed there boninites in ophiolite complexes and tectonites at their bases show depleted and hydrated geochemistries consistent with the melting of rifted mantle lithosphere that receives volatiles from foundering oceanic crust

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

intraplate deformation in the samoa gilbert ralik area a prelude to a change of plate boundaries in the southwest pacific okal e a woods d f and lay t 1986 intraplate deformation in the samoa gilbert ralik area a prelude to a change of plate boundaries in the southwest pacific in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 69 71 intense intraplate seismic activity has recently been identified in the west central pacific along a line extending from samoa through the gilbert islands and the ralik fracture zone to the caroline islands over the past 50 years this seismic line has contributed an estimated 15 30 of the entire intraplate seismicity for the pacific plate hawaii excepted focal mechanisms available for ten of the largest events during the 1981 1983 gilbert islands swarm with horizontal compressional stress oriented at right angles to the direction of motion of the plate cannot be reconciled with simple models of intraplate stresses we speculate that they express large scale deformation of that part of the pacific plate related to incipient detachment of a microplate and relocation of the pacific australian subduction zone some 1000 km to the north of its present position

ontong java plateau and late neogene changes in pacific plate motion aided by a new geometrical technique called hot spotting we recently proposed a new model for the absolute motion of the pacific plate this model called wk97 assumed the hawaiian hotspot is under loihi and that there was a change in plate motion at 3 6 ma the model located the louisville hotspot close to the hollister ridge a shallow volcanic ridge associated with a geoid high seismicity and recent volcanism most of the tectonic implications of wk97 however were not addressed furthermore although several recent studies suggest that the geochemistry of hollister lavas may indicate a mixture between louisville melt material and pacific antarctic ridge mid ocean ridge basalt these studies strongly disagree with wk97 and consider it seriously flawed here we discuss numerous tectonic predictions of wk97 and present a large amount of observational evidence in support of it we find that a late neogene collision between the ontong java plateau and the northern margin of the australia plate appears to have altered the motion of the pacific plate as inferred from hotspot volcanism by forcing it to rotate counter clockwise this rotation about a low latitude pole appears to have induced right lateral shear stress along the entire pacific plate divergent boundary which resulted in the formation of extensional transform faults microplates and propagating ridges this change in motion also appears to have triggered concomitant circum pacific tectonism including trench migration and back arc rifting the wk97 suggests that long lived plume fed volcanism on the pacific plate appears very limited perhaps only to five to six hotspots hawaii louisville cobb caroline and perhaps marquesas and bowie the geometry of these chains are consistent with our model despite some evidence for volcanic age progressions most other pacific neogene hotspots are most likely crack spots i e sites of extensional volcanism at preexisting zones of weakness reactivated by plate stresses indeed several volcanic ridges such as hollister ridge near the louisville hotspot appear to have formed in response to stresses following the change in absolute plate motion wk97 also obviates the need for simultaneous different directions of mantle flow in the south central pacific transpression at the san andreas and alpine strike slip faults and aleutian arc explosivity since 6 ma also appear to be consequences of the changes in pacific plate motion furthermore both the wk97 model and observed relative plate motions suggest that adjoining plates have changed their absolute motions as well and that large changes in absolute plate motion do not necessarily imply large changes in relative plate motion our findings support slab pull as the dominant plate tectonic driving force highlight the rheological difference between strong oceanic lithosphere and weak continental crust and suggest that wk97 presently provides a unifying albeit preliminary framework for relating both intraplate and circum pacific tectonism and volcanism in the late neogene

the gilbert islands republic of kiribati earthquake swarm of 1981 1983 abstract a major swarm of intraplate earthquakes at the southeastern end of the gilbert islands chain 3 5 s 177 5 e commenced in december 1981 and lasted through march 1983 no seismicity had been reported in the historical record in this region prior to 1981 but during the swarm 217 events with mb 4 0 were located by the neis with 86 events having mb 5 0 the source region is quite remote and the uniform detection level for the neis is for mb 4 8 a b value of 1 35 is found for the swarm using the maximum likelihood method four events in the sequence were large enough mb 5 6 5 9 to determine focal mechanisms teleseismically using body and surface wave analysis these events are found to have a range of mechanisms from predominantly thrust with a significant oblique component to purely strike slip the compression axes are consistent for all four events with horizontal orientation trending nne ssw this orientation is perpendicular to the direction of plate motion the events are located at depths between 15 and 20 km placing them deep in the oceanic crust or in the upper mantle no obvious bathymetric feature can be related to the fault plane orientations though there is an offset in the island chain near the epicenters while some characteristics of the swarm suggest a magmatic origin the nature of the focal mechanisms the location of the swarm and the large accumulated moment release of the sequence favor a tectonic origin

seismic detection of underwater volcanism the example of french polynesia we present a review of the principal methods used for the seismic detection and identification of active underwater volcanism based on our experience in french polynesia in particular we descrobe the 5 year activity in the tahiti mehetia area during which more than 32000 earthquakes were detected by the polynesian network we discuss the use of the following three types of seismic waves conventional mostly body waves seismic tremor andt waves propagated in the low velocity acoustic channel of the ocean for each of these waves we discuss the principal characteristics of the signals their spectral content the type of information they provide on the activity of the volcano and the various limitations faced by their use in detection or monitoring of underwater volcanic edifices we present a review of the principal swarms monitored by the polynesian network and discuss their characterization as either volcanic or tectonic

plate tectonic models for indian ocean intraplate deformation abstract the equatorial region of the conventionally defined indo australian plate has long been recognized as containing a type example of intense intraplate deformation we trace the development of tectonic models for the area to illustrate techniques for the analysis of such deformation the identification of anomalous seismicity near the ninetyeast and chagos laccadive ridges demonstrated the existence of the deformation focal mechanisms from recent and historic earthquakes showed strike slip motion occurring along the ninetyeast ridge seismic moment data allowed the rate to be estimated similar studies showed north south tension in the chagos bank region and north south compression in the region between the ninetyeast and chagos ridges global plate motion studies indicated non closure of the indian ocean triple junction suggesting the conventional plate geometry was inadequate for a rigid plate description of the area gravity and marine geophysical data indicated intense north south compressional deformation south of the bay of bengal these observations are reconciled by a plate motion model in which australia and india lie on distinct plates divided by a boundary that intersects the central indian ridge near the equator in this model arabia usually considered a separate plate has negligible motion relative to india the resulting euler vector for australia relative to indo arabia lies just east of the central indian ridge and predicts approximately 0 5 1 5 cm yr compression in the central indian basin and 1 5 2 cm yr strike slip motion along the northern ninetyeast ridge consistent with the seismological and geophysical data in contrast to conventional oceanic plate boundaries the boundary deformation is distributed over a wide zone this diffuse nature may reflect either the boundary s recent inception or slow rate of motion analysis of seismicity and deformation in the boundary zone should offer insights into the mechanics of its development and its implications for the evolution of plate boundaries

stress magnitude estimates from earthquakes in oceanic plate interiors we propose a method to estimate stress magnitudes in oceanic plate interiors from focal depths and focal mechanisms using a depth dependent rheology we show it is possible to estimate the differential stress 1 3 averaged over some reference lithospheric thickness the resolving power of the method is investigated by evaluating the effect of uncertainties in parameters that are involved in the analysis we apply the method to the central indian ocean where intraplate seismicity is high from well studied earthquakes we estimate differential stresses of the order of hundreds of rnegapascals this result is consistent with the high level of stress that was found from numerical model calculations by cloetingh and wortel 1985 1986 from the few intraplate events in the pacific plate we also estimate differential stresses in this area

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