curie isotherm depth from aeromagnetic data constraining shallow heat source depths in the central aeolian ridge southern tyrrhenian sea italy the salina lipari and vulcano volcanic ridge and the surrounding sea sectors aeolian archipelago southern tyrrhenian sea italy are characterized by vents responsible for a recent 40 ka 1889 1890 ad effusive and explosive subareal activity and repeated 56 to 7 ka in age submarine explosive eruptions from source areas located between lipari and vulcano a spectral depth estimation of the magnetic bottom using a fractal method on aeromagnetic data from vulcano lipari and salina volcanic ridge allows us to constrain the curie isotherm depth the elevated portion of the isotherm is between 2 and 3 km below salina and vulcano and about 1 km below lipari the curie depth results in the context of other geological and geophysical evidence suggest that the rise of the curie isotherm is mainly due to the occurrence of shallow heat sources such as magma ponds and associated hydrothermal systems the short wavelength magnetic anomaly field reflects magnetic contrasts from highly magnetized volcanic bodies low magnetization sediments and hydrothermally altered rocks borehole temperature data verify the curie temperature derived from the magnetic methods on the island of vulcano we conclude that the whole vulcano lipari and salina volcanic ridge is active and should be monitored

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

an extensive region of off ridge normal faulting earthquakes in the southern indian ocean an unusually large number of off ridge earthquakes have occurred within a broad region astride the southeast indian ridge the source mechanisms of nine of these earthquakes in lithosphere from 4 to 27 m y old have been determined with a formal inversion technique based on matching observed and synthetic p and sh waves all events occur in the oceanic mantle from 8 to 22 km below the seafloor the seismic moment and duration of faulting obtained from the inversion constrain the stress drop to lie between a few tens of bars and a few hundred bars for most of the events all of the off ridge earthquakes are characterized by normal faulting mechanisms with the t axes nearly parallel to the nearby ridge axis in sharp contrast to typical mid ocean ridge or intraplate earthquakes a variety of possible tectonic processes which might produce normal faulting in young oceanic lithosphere are considered as possible causes including bending stresses thermal stresses of various origins differential subsidence at a fracture zone off ridge volcanism latitudinal plate motions on a nonspherical earth basal shear stress from small scale convection and plate wide perturbations of the stress field due to the continental collision between india and asia no single mechanism is adequate to explain all the observed off ridge normal faulting earthquakes an apparent asymmetry in the observed seismicity with respect to the ridge suggests that the earthquakes on the antarctic and indian plates are caused by different processes the antarctic plate events can be satisfactorily attributed to the release of thermal and bending stresses associated with a residual depth anomaly between the kerguelen plateau and the southeast indian ridge a region which has been suggested as the site of a horizontal pipe for flow in the asthenosphere from the kerguelen hot spot to the vicinity of amsterdam and st paul islands on the southeast indian ridge the off ridge earthquakes in the indian plate are speculatively attributed to part of the plate wide response of the lithosphere to the ongoing continental collision

depth determination for shallow teleseismic earthquakes methods and results the depths of moderate sized shallow earthquakes can be routinely determined to an accuracy of several kilometers using teleseismic data alone making it possible to determine the vertical positions of earthquakes within the lithosphere this capability provides a valuable tool for plate tectonic studies since earthquake depths can be used to constrain lithospheric thermal and mechanical structure at plate boundaries and in intraplate regions two basic techniques are used for depth analyses surface wave spectra can be used to determine focal depths as mode excitation reflects the behavior of the eigenfunctions with depth the primary limitation on such studies stems from lateral heterogeneity of velocity structure the waveforms of body waves are also diagnostic of focal depth which controls the time separation between the direct arrival and near source surface reflections these studies can be limited by a partial trade off between depth and source time function duration the reliability of the body wave modeling approach now in general use is shown by excellent agreement between depths determined by different investigators using different algorithms the depth determination is robust to typical uncertainties in focal mechanism and near source structure the trade off with source time function duration generally does not produce major difficulties for small events ms 6 5 for larger events the trade off can be resolved using a approach in which for a given depth the time function yielding the best fit to a suite of seismograms is found by deconvolution deconvolution at a series of depths yields a misfit function whose minimum indicates the focal depth tests with synthetic data show that the deconvolution process yields reliable depth estimates even given uncertainties in focal mechanism and near source structure it successfully handles multiple sources moderate sized finite sources and horizontally propagating ruptures good results are obtained for vertically propagating ruptures until the fault area becomes so large that waveforms differ significantly from the point source approximation a number of important tectonic results have emerged from studies using these depth determination methods the maximum depth of oceanic intraplate seismicity increases with lithospheric age and is approximately bounded by the 750 c isotherm this depth roughly equals the flexural thickness of the lithosphere but is much less than the depth to the low velocity zone indicated by surface wave dispersion the maximum depth of continental intraplate seismicity is also temperature limited but by a lower temperature these observations are consistent with the rapid weakening of the appropriate rocks with depth predicted by standard temperature dependent rheologies the maximum depth of oceanic transform seismicity corresponds to an isotherm of about 400 c this temperature is lower than would be predicted from the intraplate results and may indicate that transforms are either hotter or weaker than expected the depths of ridge crest earthquakes large enough for teleseismic analysis are extremely shallow 0 8 km the depth distribution of seismicity in continental normal faulting regions provides constraints on the nature of the extension process at subduction zones precise depth determinations can resolve the upper and lower flexural regions in the bending downgoing plate precisely locate the plate interface and map the vertical extent and position of faulting in large earthquakes

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

tensional intraplate seismicity in the eastcentral pacific abstract we identify a region of enhanced intraplate seismicity west of the mathematicians ridge in the eastcentral pacific recent activity includes the december 2 1984 event ms 6 2 which was accompanied by a swarm of seismic activity including 45 recorded shocks in the 3 months preceding the earthquake and 10 aftershocks extending into 1986 modeling of the main shock p and sh waveforms and a grid search fit to the body wave amplitudes indicates normal faulting along a ne sw plane inversion of wwssn p waveforms suggests a depth of 14 km and a source time function duration of 7 s such figures preclude the anomalous low stress drops observed for documented volcanic earthquakes we obtain similar focal mechanisms for the october 5 foreshock mb 5 5 and the may 28 1986 aftershock mb 5 5 relocation shows that the large june 30 1945 earthquake with gutenberg magnitude 6 3 4 was an intraplate shock which occurred 400 km south of the 1984 epicentral region body waveform modeling and moment variance analysis of p sh love and rayleigh waves suggest a normal faulting mechanism with east west striking fault planes at a depth similar to the 1984 event small earthquakes also occurred near the 1945 epicenter in 1973 1978 and 1985 the seismicity occurs in tectonically complex lithosphere formed just before a re orientation of spreading on the mathematicians ridge associated with the elimination of offset along the clarion fracture zone seasat data indicate a significant gravity anomaly in the region of the 1945 event which may represent a fracture zone of substantial offset but no such features are found near the 1984 epicenter the earthquakes suggest that concentrations of normal faulting intraplate earthquakes found previously in the indian ocean occur elsewhere in the oceanic lithosphere

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

inverse modelling of the reversely magnetized shallow plumbing system hosting oil reservoirs of the auca mahuida volcano payeina retroarc neuquen basin argentina

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

geomorphic evidence for plio quaternary compression in the andean foothills of the southern neuquen basin argentina the southern central andes foothills are made of imbricate thick and thin skinned fold and thrust belts they were formed during cretaceous and miocene compressive periods by successive eastward propagation episodes of the orogenic front we analyze remnant terraces and alluvial deposits of the neuqun river and its tributaries to study the plio quaternary tectonic regime in the southern neuqun basin topographic profiles of remnant terraces show a crustal scale tens of kilometers anomaly above the chihuido south anticline this anomaly is accompanied by sedimentary aggradations downstream and upstream of the anticline another anomaly in the terrace profiles correlated with a knickpoint in the current neuqun river is observed farther east near aelo we interpret these anomalies to be the consequence of a plio quaternary uplift of the chihuidos and aelo anticlines folding and surface faulting of plio quaternary alluvial deposits show that this uplift is concomitant with the reactivation of the miocene orogenic front and of the predominantly thin skinned agrio fold and thrust belt from these data we infer that the southern neuqun basin is still shortening under a compressive stress regime and that the andean orogenic front is migrating farther to the east extensional features in the study area correspond mainly to thin skinned gravitational gliding resulting from crustal uplift thus we argue that the tectonic compressive regime is preserved in the southern neuqun basin throughout the plio quaternary though the slab may have steepened during this period

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

potential field modeling of collapse prone submarine volcanoes in the southern tyrrhenian sea italy 1 hydrothermal alteration may weaken volcanic rocks causing the gravitational instability of portions of active volcanoes with potentially hazardous collapses here we show high resolution multibeam magnetic and gravity surveys of the marsili seamount the largest active volcano of europe located in the southern tyrrhenian back arc basin these surveys reveal zones with exceptionally low densities and with vanishing magnetizations due probably to the comminution of basalts during hyaloclastic submarine eruptions and to their post eruptive hydrothermal alteration the location of these regions correlates with morphological data showing the occurrence of past collapses similar evidence has been obtained from pre existing data at vavilov seamount another older volcanic system in the tyrrhenian back arc basin here a large volume of at least 50 km3 may have collapsed in a single event from its 40 km long western flank given the similarities between these volcanoes a large collapse event may also be expected at marsili

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

a procedure for modelling shallow dislocation sources summary expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented the results of this procedure are used to generate synthetic p sv and sh waveforms for various assumptions of stress drop

contemporary stress orientations from borehole breakout analysis in the southernmost flat slab boundary andean retroarc 32 44 and 33 40 s 1 horizontal stress directions have been determined in the southernmost flat slab boundary andean retroarc between 32 44 and 33 40 s within cuyo basin argentina these directions were obtained from the borehole breakout analysis of 42 wells using four arm caliper data the mean maximum horizontal stress shmax direction for the whole region is 104 1 with a 95 confidence interval of 8 1 the present day stress field has an approximately preferred e w trend maximum horizontal stress direction consistent with the plate boundary forces 80 and the topographic forces near e w the calculated shmax directions are near the expected values but some local deviations were observed the shmax rotates from an e w orientation in the south to a nw se orientation to the north of this sector of the andean retroarc a regional variation in the stress field can be observed when these results for the cuyo basin are analyzed together with those presented in a previous study in the neuquen basin to the south the maximum horizontal stress varies from nw ne along this combined section of the andean retroarc with the e w shmax directions in the northern neuquen basin consistent with those observed in the southern sector of cuyo basin these variations in the stress field orientation appear related with the topography geometry from the analysis between the mean shmax obtained and the acting forces it can be concluded that the topographic control on the horizontal stress field seems to be dominant in the cuyo basin and in the north of neuquen basin to the south of neuquen basin the horizontal stress field should be mainly controlled by the plate boundary forces

historical seismicity near chagos a complex deformation zone in the equatorial indian ocean abstract over the last twenty years chagos bank has a seismicity rate disproportionate to its supposed intraplate location earthquake relocation also shows a high seismicity rate in pre wwssn time 1912 1963 with seven events located off of the central indian ridge including large events in 1912 m 6 8 and 1944 m 7 2 this study uses the moment variance technique a systematic search for the mechanism which best fits p pp sh love and rayleigh amplitudes to determine the focal mechanisms of two pre wwssn earthquakes a test with a recent event of known mechanism demonstrates that accurate focal parameter determination is possible even when only a few good records are available moment variance analysis shows a thrust faulting mechanism for the 1944 event northeast of chagos bank near the chagos laccadive ridge and a strike slip focal mechanism for a smaller 1957 event west of chagos bank the 1944 event one of the largest oceanic intraplate earthquakes known moment 1 4 1027 dyne cm indicates that the chagos seismicity reflects not an isolated occurrence of normal faulting as previously thought but rather regional tectonic deformation extending northeast of chagos bank and including thrust normal and strike slip events this seismicity and previously studied seismicity near the ninetyeast ridge and central indian basin suggest a broad zone of deformation stretching across the equatorial indian ocean this zone contains all known magnitude seven oceanic intraplate earthquakes not associated with subduction zones or continental margins suggesting that elsewhere such extensive deformation occurs only along plate boundaries this study proposes that a slow diffuse plate boundary extends east from the central indian ridge to the ninetyeast ridge and north to the sumatra trench a recent plate motion study confirms this boundary and suggests that it separates the australian plate from a single indo arabian plate

regional magnetic and gravity anomaly correlations of the southern tyrrhenian sea abstract the complex magnetic and gravity anomaly fields of the southern tyrrhenian sea provide a record of the complicated properties and evolution of the underlying crust geologic interpretation of these anomalies is hindered by the effects of anomaly superposition and source ambiguity inherent to potential field analysis a common approach to minimizing interpretational ambiguities is to consider analyses of anomaly correlations spectral correlation filters are used to separate positively and negatively correlated anomaly features based on the correlation coefficient given by the cosine of the phase difference between common wavenumber components this procedure is applied to reduced to pole magnetic and first vertical derivative gravity anomalies for mapping correlative crustal magnetization and density contrasts adding and subtracting the standardized outputs of the filters yield summed slfi and differenced dlfi local favorability indices that respectively highlight positive and negative feature correlations in the anomaly data sets correlative maxima mainly reflect volcanic structures and secondarily intrusive bodies and pre tortonian carbonates of the maghrebian chain and the basement rocks of the sardinia eastern margin correlative minima mostly mark sediment filled peri tyrrhenian structural basins related to the pliocene extensional tectonics and intra slope marine depressions related to post pliocene and still active compressional tectonics off northern sicily prominent inverse anomaly correlations mainly reflect crustal features around the southern margin of the tyrrhenian sea that include higher density lower magnetization pelagic to terrigenous and flysch type nappes of the sicilian maghrebian chain as well as lower density higher magnetization sediments filling depressions of the chain and syn rift sediments of southeastern sardinia

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

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

investigating the strength and trend of seismic anisotropy in the western part of makran subduction zone and southeast of iran abstract the makran subduction zone located in southeastern iran and southern pakistan extends approximately 900 km along the eurasian arabian plate boundary where the arabian oceanic plate subducts beneath the eurasian plate using broadband seismic data recorded in the western part of makran in the southeast of iran seismic anisotropy was studied to better understand geodynamic processes of this subduction zone we analyzed shear wave splitting of core refracted shear wave phases sks skks and pks recorded by 17 broadband stations located in the western makran and southeast of iran from 542 earthquakes mw 6 0 that occurred between 2004 and 2017 at epicentral distances between 90 145 few stations close to the trench line exhibit a trench oblique trend in the west to a trench prependicular trend to the east the magnitude of splitting delay times suggests a mantle source of anisotropy our findings show that the majority of the fast axes of seismic anisotropy at farthest stations in the backarc region are oriented in the nw se direction with delay times created by crust and mantle varying between 0 8 and 1 3 s the orientations of these fast directions are almost parallel to surface structural features for stations near the minab strike slip fault system the transition from the makran subduction zone to the zagros collision zone the fast directions are sub parallel to the strike of this fault system indicating that simple shear is likely the main source of anisotropy

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

magmatic events and sour crude oils in the malargue area of the neuquen basin argentina abstract heavy medium oils 14 5 27 9 api in a studied field of malargue area in the neuquen basin argentina have associated gas that produces between 0 and 4000 ppm h 2 s and 0 6 103 kg h 2 s day being able to discriminate between biological and inorganic h 2 s sources is essential to the oil field s economy h 2 s associated with anaerobic bacteria sulfate reduction bsr might be mitigated using biocompetitive technologies or bactericides whereas abiotic h 2 s from geological sources can be controlled only by sequestration in surface facilities and oil field management if the h 2 s distribution in different oil pay zones field compartments and origin are well known the isotopic signatures of the h 2 s 34 s range between 2 3 and 7 8 which suggests magmatic sources laccoliths in the malargue area associated with tertiary magmatic events were mapped out many decades ago petrographic descriptions of igneous samples are consistent with andesitic magmas that bore sulfur bearing fluids which are considered the principal source of h 2 s associated hydrothermal fluids as recorded by secondary minerals induced rapid tsr thermochemical sulfate reduction reactions as documented by mineral phases seen in sem most of the h 2 s neuquen basin has been linked to bsr bacterial sulfate reduction however we document a geological origin for the h 2 s which is tied to magmatic events organic geochemistry and fluid inclusion data allow for an early local generation of hydrocarbons linked to burial followed by a second pulse of light hydrocarbons gas and h 2 s associated with the magmatic event water geochemistry and the lithology of oil pay zones are used to predict the distribution of h 2 s in each well wells with less than 20 ppm of h 2 s are linked to meteoric waters and siliciclastic pay zones

the volcanoseismic swarms of 1981 1983 in the tahiti mehetia area french polynesia during the years 1981 1983 three intense seismic swarms took place in the tahiti mehetia area of french polynesia at the presumed location of the society island hot spot the 1981 swarm featured 4000 earthquakes with a maximum magnitude ml 4 3 in the immediate vicinity of the island of mehetia the 1982 swarm occurred along the flank of the major teahitia seamount and involved more than 9000 events a second swarm occurred at teahitia in 1983 and involved 3000 events although no precise constraint can be placed on the depth of individual events from their travel times to polynesian stations features in the evolution of the mehetia swarm are generally consistent with the probable ascent of a magma body toward the surface in the case of teahitia the recording of abundant tremors of both high and low frequency particularly intense during the 1983 swarm is directly similar to cases of documented volcanic eruptions the swarms are interpreted as episodes of active volcanism part of the process of building the next island in the chain appendix tables are available with the entire article on micro fiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 005 2 50 payment must accompany order

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

payenia volcanic province in the southern andes an appraisal of an exceptional quaternary tectonic setting abstract the southern volcanic zone of the andes has a quaternary basaltic province along the retroarc which has a unique tectonic setting the payenia volcanic province covers an area larger than 40 000 km2 between 33 30 and 38 south latitudes with an estimated volcanic volume of about 8387 km3 erupted through more than 800 volcanic centers in the last 2 ma the mainly basaltic province developed above the san rafael block is subdivided in three segments characterized by the cerro nevado llancanelo payun matru tromen and auca mahuida volcanic fields together with hundreds of minor monogenetic basaltic centers the analysis of the different segments shows the formation of a common basalt plateau with intraplate signature from south to north between 2 0 and 1 7 ma which reached the 35 s to the north above this plateau monogenetic centers as nihuil vn 1 433 ma and cerro chato at 1 352 ma are developed followed by the large polygenetic center of cerro nevado 3980 m a s l at 1 320 ma this plateau was broken by a series of normal faults that produced volcanic cone alignments such as the nnw trending mancha jarilla lineament in the central part at about 1 0 ma extension shifted to the eastern margin of the san rafael block which concentrates tens of monogenetic centers between 0 9 and 0 7 ma extension then migrated towards the foothills in the west where many monogenetic cones were erupted through nw trending normal faults between 0 5 and 0 435 ma the collapse of the large diamante caldera at 0 445 ma coincides with that period subsequent volcanism was concentrated in 1 the payun matru volcanic field with the eruption of cerro payen between 0 272 and 0 261 ma the payun matru shield volcano with polygenetic eruptions at least since the last 0 233 ma and with the caldera formation bracketed between 0 168 0 004 ma and 0 082 0 001 ma followed by several eruptions until 7000 yrs and even historical ones and in 2 the tromen volcano where younger than 0 2 ma eruptions took place and historical eruptions were reported the understanding of these eruptions in time and space combined with geophysical data indicates the geometry of an important crustal attenuation beneath payenia associated with a hot sublithosphere the late miocene uplifted san rafael block collapsed in the early pleistocene as a consequence of the steepening of the subducted slab and the injection of hot asthenosphere produced the quaternary payenia volcanic province melts of the lower crust along the principal cordillera at these latitudes are responsible for the quaternary calderas ignimbritic flows and rhyolitic volcanism that express the crustal delamination of the andes the payun matru volcanic field concentrates this asthenospheric flow in the present

the largest recorded earthquake swarm intraplate faulting near the southwest indian ridge the most prolific source of moderate and large earthquakes along the mid ocean ridge system is located near the atlantis ii transform along the southwest indian ridge a remarkable swarm of 48 earthquakes was recorded from 1925 to 1933 which included 29 events with m 6 0 and 4 events with m 7 0 examination of the global earthquake catalog suggests this is the largest known earthquake swarm relocated events cluster around 33 8 s 58 0 e approximately 100 km east of the southern intersection of the atlantis ii transform and the spreading ridge and epicentral uncertainties derived from both standard least squares and resampling techniques preclude locations along the plate boundary multiple event relocation shows that seven 1977 1983 events mb 5 1 5 6 are distributed across a 40 km diameter region at the location of the 1925 1933 swarm events mechanisms determined for three of the recent events using waveform inversion indicate normal faulting with variable strike slip components at depths of 12 17 km neither the nodal planes nor the tensional axes show consistency between the three mechanisms grid search fits to body waveforms from the larger 1925 1933 swarm earthquakes suggest normal faulting mechanisms similar to the recent events with depths of 11 20 km and seismic moments ranging up to 1 8 1027 dyn cm the earthquake depths and tensional mechanisms are consistent with release of thermoelastic stress which may be enhanced at this location by thermal bending moments associated with the transform fault the total moment release of the 1925 1933 swarm 7 1027 dyn cm is greater than the cumulative moment of all known oceanic intraplate seismicity since 1963 3 1027 dyn cm this suggests that intraplate stress release is dominated by infrequent but highly energetic episodes of seismicity and that estimates of intraplate moment release taken from observations over a limited time period underestimate intraplate seismicity levels

mapping the miocene farallon ridge jump on the pacific plate a seismic line of weakness abstract bathymetric and magnetic data are used to obtain estimates on the pacific and nazca plates of the boundaries separating lithosphere generated at the old farallon ridge from the more recent one created at the present day east pacific rise an excellent correlation is found with the sites of known teleseismically recorded intraplate seismicity suggesting that these boundaries which are lines of age discontinuity in the plate must be zones of weakness of the lithospheric plate in particular the so called region c identified by okal et al as a major site of seismic release sits on a small piece of farallon plate in the immediate vicinity of the northern extension of the fossil roggeveen rise cut across by the east pacific rise during the ridge jump

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

contemporary stress orientations in the andean retroarc between 34 s and 39 s from borehole breakout analysis 1 in this paper we present the results of the analysis of borehole breakouts from 115 wells drilled within neuquen basin in the andean retroarc between 34 and 39 s argentina the first order present day stress orientation in the andean retroarc is expected to be mainly controlled by the plate boundary forces azimuth 80 and the topographic forces e w the obtained maximum horizontal stress shmax has a preferred trend with a resultant direction of azimuth 88 7 and a 95 confidence interval of 13 3 consistent with the expected trend the horizontal stress trajectory map achieved for this region shows that the shmax along the study area is not completely uniform to the north of colorado river the shmax shows an ese tendency interpreted as significant influenced by the topographic forces to the south of colorado river shmax has an ene trend similar to the expected based on plate boundary forces to the southeast of the region a ne direction was found probably showing a basement structural control in the stress field geometry the stress orientations obtained for the whole region show that plate boundary forces drag basal and topographic forces are strongly controlling the stress direction distribution

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

thermal stress and the spacings of transform faults bathymetric charts are used with satellite altimeter profiles to locate major ridge transform intersections along five spreading ridges the ridges are the mid atlantic ridge the east pacific rise the chile rise the pacific antarctic rise and the southeast indian ridge analysis of these data show spacings between transform faults w increase linearly with spreading rate w 6 28 m y this linear correlation is explained by a thermoelastic model of a cooling strip of lithosphere spreading at a rate the traction free boundaries of the thin elastic strip simulate cracks in the lithosphere at transform faults a two dimensional thermoelastic solution for the in plane stress shows the largest stress component is tensional and parallel to the ridge stresses are zero at the ridge and increase as age to a maximum value at an age of w 4 all stress components are small for ages greater than w when the transform spacing is large compared with the spreading rate w 100 m y thermal stresses exceed the strength of the lithosphere for ages between 0 and 30 ma the observed maximum ratio of transform spacing to spreading rate w 10 m y results in low thermal stresses that only exceed the strength of the lithosphere for ages less than 1 ma thus transform faults relieve most of the thermal stress model predictions also agree with earthquake studies showing that normal faults in young lithosphere have tensional axes aligned with the ridge moreover oceanic intraplate earthquakes rarely occur in lithosphere older than 30 ma as predicted by the model these and other geophysical observations confirm turcotte s hypothesis that transform faults are thermal contraction cracks

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

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

imaging and modelling the subsurface structure of volcanic calderas with high resolution aeromagnetic data at vulcano aeolian islands italy in this paper we present a magnetic model of the subsurface structure of vulcano island based on high resolution aeromagnetic data three profiles across the most intense magnetic anomalies over the piano and fossa calderas were selected for the magnetic modelling which was constrained by structural and volcanological data previous geophysical models paleomagnetic data and borehole stratigraphy obtained from two deep wells the interpretation of the magnetic sources represents a significant contribution to the understanding of the piano and fossa calderas underlying structure providing us with evidence of the lateral discontinuity between them at depth we propose that the positive magnetic anomalies in the piano caldera area are caused by a the remnants of an early submarine volcano b an outcropping dyke swarm related to the feeding system of the primordial vulcano phase beneath mt saraceno and c the presence of a non outcropping dyke system intruded along a ne sw oriented intra caldera fault beneath the eastern part of the piano caldera offshore to the west the magnetic anomaly map suggests the presence of a submarine volcanic structure not revealed by bathymetric data which could represent the eruptive centre the presence of which has been indirectly deduced from the outcrop of eastern dipping lavas on the western seashore magnetic modelling of the fossa caldera points to the presence of a highly magnetized cone like body inside the fossa cone centred beneath the oldest crater rims we interpret this body as a pile of tephritic lavas emplaced in an early phase of activity of the fossa cone suggesting that the volume of mafic lavas that erupted at the beginning of the construction of the fossa edifice was more significant than has previously been deduced furthermore the presence of a magnetized body inside the fossa cone implies that high temperatures are contained in very limited spaces do not affect its bulk inner structure and are restricted to fumarolic conduits and vents in addition structures beneath the western and northern part of the fossa caldera are revealed to have null or low magnetization which can be ascribed to the presence of pyroclasts and hyaloclastites in this area as well as to a large volume of hydrothermally altered materials this suggests that the hydrothermal system with a very limited extension at present affected a larger area in the past especially beneath the western part of the caldera

regional stress field of the indian plate we have calculated the regional stress field in the indian plate implementing dependence of slab pull and ridge push on the age of the oceanic lithosphere in the finite element procedure the high level of the calculated stress field order of a few kbar and the dominance of compression in the plate are consequences of the unique dynamic situation of the present day indian plate the calculated stress field explains the concentration of intraplate deformation in the region of the ninetyeast ridge and yields insight into the variations in stress directions in the australian continent

the deep structure of the eolian arc filicudi panarea vulcano sector in light of gravity magnetic and volcanological data abstract the magnetic and gravimetric methods of investigation have been applied to the central sector of the eolian arc combined with geovolcanological data for the purpose of defining the structural framework in which the volcanism developed the processing of the magnetic and gravimetric data involved 3d modeling of the volcanic bodies and the application of a depth separation filter in order to subtract from the measured field the effect caused by the subaerial and submarine volcanic edifices and by all surface inhomogeneities magnetic and gravimetric 2 5d modeling on selected profiles has also been done in a joint interpretation of the magnetic and gravimetric data constrained by volcanological knowledge the interpretation of the final maps and profiles has demonstrated the existence of extensional tectonics resulting in the development of horst and graben volcanism occurs in the graben areas in particular the salina lipari vulcano magmatic axis has developed in a very deep articulated graben of lithospheric importance and completely filled by volcanic deposits and intrusive bodies the origin of this structure and of the associated igneous activity is suggested to be related to trans tensional tectonics along a plio pleistocene regional strike slip fault system

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

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