late carboniferous to late permian carbon isotope stratigraphy a new record from post variscan carbonates from the southern alps austria and italy abstract an integrated study of the litho bio and isotope stratigraphy of carbonates in the southern alps was undertaken in order to better constrain 13 c variations during the late carboniferous to late permian the presented high resolution isotope curves are based on 1299 13 c carb and 396 13 c org analyses the carbon isotope record of diagenetically unaltered samples from the carnic alps austria and karavanke mountains slovenia shows generally high 13 c values but late carboniferous and early permian successions are affected by a diagenetic alteration as consequence of glacio eustatic sea level changes negative 13 c excursions are related to low stand deposits and caused by diagenetic processes during subaerial exposure the comparison with 13 c records from other parts of the world demonstrate that 13 c values are high in most unaltered samples an overall negative trend during the permian as recently published is not obvious and negative excursions related to changes in the carbon isotope composition of the global oceanic carbon pool cannot be confirmed except for the permian triassic boundary interval

ridge segmentation faulting and crustal thickness in the atlantic ocean the mid atlantic ridge mar between the kane and atlantis fracture zones consists of segments 20 85 km in length1 bull s eye patterns in the mantle bouguer gravity anomaly field2 centred on several segments associated with narrow rift valleys3 have been interpreted as centres of strong mantle upwelling and thick crust here i present a map of normal faults inferred from 5 104 km2of sea beam bathymetry4 along the mar between 28 n and the atlantis fracture zone faults are mapped both on and off ridge using a criterion that distinguishes them from volcanic topography abyssal hill lineations seem to form at the rift valley walls through the growth of the normal faults towards ridge segment ends these faults are more widely spaced with larger throws than those at segment centres and often define the boundaries of the gravity bull s eyes these two different faulting styles which probably reflect changes in lithospheric strength are preserved into the rift mountains large throw faults are highly correlated with the mantle bouguer highs suggesting that amagmatic extension on the large faults contributes to the crustal thinning inferred towards the segment ends

microearthquake characteristics and crustal vp vs structure at the mid atlantic ridge 35 n we report on the results of a microearthquake experiment conducted at the along axis bathymetric high of the slow spreading ridge segment near 35 n on the mid atlantic ridge mar of a total of 255 microearthquakes recorded during the 43 day experiment 31 were located near the oceanographer transform at the northern end of the segment 79 occurred at the nontransform offset at the southern end of the segment and 145 were at the segment center at the segment center earthquake epicenters lay within the median valley inner floor and formed a 12 km long trend paralleling a steep scarp bounding the western wall of the inner valley focal depths were 3 4 km below the seafloor most 80 of the focal mechanisms for 32 segment center earthquakes are consistent with normal faulting on faults approximately parallel to the axial trend from a joint inversion for hypocenters and p and s wave velocity structures we determined a horizontally averaged v p v s ratio that decreases from 2 9 in the shallowmost 300 m to 1 7 at 2 km depth and we interpret this decrease as indicating a decreasing contribution of thin cracks to fracture porosity with depth the maximum depth of seismicity 4 km is anomalously shallow compared with other mar segments at which microearthquake experiments have been carried out cross axis relief is also anomalously low for this segment s center and on the basis of this and other mar microearthquake experiments there appears to be a correlation between cross axis relief and maximum depth of seismicity from the correlation of cross axis relief and inferred crustal thickness we suggest a relationship between thick crust high crustal temperatures and low cross axis relief in qualitative agreement with thermomechanical models for the depth of the axial valley

progressive multiple event location pmel abstract this paper introduces a new method of using earthquake arrival times to simultaneously determine earthquake hypocenters and station corrections progressive multiple event location pmel forms a solution in three major steps 1 hypocenters are located by conventional single event methods using current station correction estimates 2 an annulled data set pavlis and booker 1980 is formed and accumulated using householder transformations and 3 station correction perturbations are calculated from a pseudoinverse solution of the annulled data set in practice pmel converges rapidly we analyze the non uniqueness of pmel estimates in terms of resolution and covariance matrices one of the most important features of pmel is the computer algorithm that we describe which requires only a modest amount of computer memory that is independent of the number of earthquakes to be processed consequently this algorithm can process a virtually unlimited number of earthquakes we apply pmel to a set of 8049 arrival times from 652 earthquakes and 4 explosions recorded by a network in the coso range inyo county california pmel converges after four adjustments to the station corrections the final rms misfit of 0 05 sec is remarkably good and the station correction estimates correlate strongly with surface geology

time domain observations of a slow precursor to the 1994 romanche transform earthquake low frequency spectral anomalies have indicated that some large earthquakes are preceded by extended episodes of smooth moment release but the reality of these slow precursors has been debated because they have not been directly observed in the time domain high gain seismograms from the 14 march 1994 romanche transform event moment magnitude m w 7 0 show a precursory ramp with a moment of 7 1018 newton meters beginning about 100 seconds before the arrival of the high frequency p waves this precursor was the initial phase of a slow component of slip that released nearly half of the total moment of the earthquake such behavior may be typical for large earthquakes on the oceanic ridge transform system

improving local earthquake locations using the l1 norm and waveform cross correlation application to the whittier narrows california aftershock sequence experiments with different earthquake location methods applied to aftershocks of the october 1 1987 whittier narrows earthquake in california ml 5 9 suggest that local event locations can be greatly improved through the use of the l1 norm station corrections and waveform cross correlation the whittier narrows sequence is a compact cluster of over 500 events at 12 to 18 km depth located within the dense station coverage of the southern california seismic network scsn a telemetered network of several hundred short period seismographs scsn travel time picks and waveforms obtained through the southern california earthquake center are examined for 589 earthquakes between 1981 and 1994 in the vicinity of the mainshock using a smoothed version of the standard southern california velocity model and the existing travel time picks improved location accuracy is obtained through use of the l1 norm rather than the conventional least squares l2 norm approach presumably due to the more robust response of the former to outliers in the data a large additional improvement results from the use of station terms to account for three dimensional velocity structure outside of the event cluster to achieve greater location accuracy waveforms for these events are resampled and low pass filtered and the p and s wave cross correlation functions are computed at each station for every event pair for those events with similar waveforms differential times may be obtained from the cross correlation functions these times are then combined with the travel time picks to invert for an adjusted set of picks that are more consistent than the original picks and include seismograms that were originally unpicked locations obtained from the adjusted picks show a further improvement in accuracy location uncertainties are estimated using a bootstrap technique in which events are relocated many times for sets of picks in which the travel time residuals at the best fitting location are used to randomly perturb each pick improvements in location accuracy are indicated by the reduced scatter in the residuals smaller estimated location errors and the increased tendency of the locations to cluster along well defined fault planes median standard errors for the final inversion are 150 m in horizontal location and 230 m in vertical location although the relative locations within localized clusters of similar events are better constrained seismicity cross sections resolve the shallow dipping fault plane associated with the mainshock and a steeply dipping fault plane associated with a ml 5 3 aftershock these fault planes appear to cross and activity began on the secondary fault plane prior to the large aftershock

comparison of mid oceanic earthquake epicentral differences of travel time centroid locations and those determined by autonomous underwater hydrophone arrays 1 mid oceanic interplate earthquakes are difficult to locate accurately because they normally occur far away from land based seismic stations use of water borne t waves recorded by autonomous underwater hydrophone auh arrays records an order of magnitude more highly accurate regional low seismicity along the north mid atlantic ridge than the international seismic centre isc even though the physical meaning of an auh locations is still not well known auh s small location errors are important for better constraining mid oceanic earthquakes comparison of such auh locations with those in isc and harvard centroid moment tensor cmt location catalog and relocated ones based on the high resolution bathymetry and teleseismic p phases is made in this study auh locations are used as a reference to compare the teleseismically determined locations for large earthquakes with known focal mechanisms we find that relocated locations agree with auh ones better than with isc we also note that the centroid vectors from relocated epicenters are usually larger than auh centroid vectors the relocated epicenters and auh locations lie in similar azimuthal directions to the associated cmt epicenters the larger relocated and auh centroid vectors than the error ellipses of auh cmt and relocated ones combined might be explained by the fault rupture process for smaller events relocated location confidence ellipses are usually large enough to cover auh locations and their error ellipses overall the highly accurate auh locations can be used to confirm the mid oceanic earthquake hypocenters and seismicity characteristics and for detail studies of the low level seismicity associated with the plate motions

global event location with full and sparse data sets using three dimensional models of mantle p wave velocity in order to improve on the accuracy of event locations at teleseismic distances it is necessary to adequately correct for lateral variations in structure along the ray paths either through deterministic model based corrections empirical path station corrections or a combination of both approaches in this paper we investigate the ability of current three dimensional models of mantle p wave velocity to accurately locate teleseismic events we test four recently published models two are parameterized in terms of relatively long wavelength spherical harmonic functions up to degree 12 and two are parameterized in terms of blocks of constant velocity which have a dimension of a few hundreds of km these models together with detailed crustal corrections are used to locate a set of 112 global test events consisting of both earthquakes and explosions with p wave travel time data compiled by the international seismological centre isc the results indicate that the supposedly higher resolution block models do not improve the accuracy of teleseismic event locations over the longer wavelength spherical harmonic models for some source locations the block models do not predict the range of observed travel time residuals as well as the longer wavelength models the accuracy of the locations largely varies randomly with geographic position although events in central asia are particularly well located we also tested the effect of reduced data sets on the locations multiple location iterations using 30 p wave travel times indicate that teleseismic events may be located within an area of 1000 km of the true location 66 of the time with only the model based corrections and increasing to 75 if calibration information is available if as few as 8 phases are available then this is possible only 50 of the time further refinement in models and or procedure such as the addition ofp phases azimuth data and consideration of p wave anisotropy may provide further improvement in the teleseismic location of small events

teleseismic location techniques and their application to earthquake clusters in the south central pacific abstract improved methods for single and multiple event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the south central pacific ocean bayesian statistical methods are used to incorporate a priori information about arrival time variance into the derivation of hypocenter confidence ellipsoids permitting a more realistic calculation of critical parameters in the case where the number of stations is small the diagonal elements of certain projection operators called data importances by minster et al 1974 are used to evaluate network balance the hypocentroid of an event cluster is defined to be the average location of events within the cluster and the deviations of individual hypocenters from the hypocentroid are called cluster vectors the problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result here termed the hypocentroidal decomposition theorem the algorithm based on this analysis appears to have many advantages over other published methods for multiple event location both in its efficient use of available information and its computational speed the application of this method to three clusters of shallow intraplate seismicity in the south central pacific designated regions a b and c demonstrates that the seismicity within each cluster is very localized the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9 6 and 12 km respectively estimates of the epicentroids are

a global analysis of mid ocean ridge axial topography summary current views of mid ocean ridges are strongly influenced by extensive mapping of the mid atlantic ridge and east pacific rise the global picture of the mid ocean ridge system particularly in the sparsely surveyed southern oceans is still based primarily on underway bathymetry profiles collected over the past 40 years this study presents a quantitative analysis of global mid ocean ridge morphology based on 156 of these underway bathymetric profiles thereby allowing commonly recognized features such as axial valleys and axial ridges to be compared on a global basis an empirical orthogonal function eof analysis is used to separate deterministic and stochastic components of axial morphology and to quantify the dependence of each on parameters such as spreading rate and axial depth it is found that approximately 50 per cent of the variance in axial morphology may be described as a linear combination of five independent symmetric and anti symmetric modes the remainder is considered stochastic maximum axial valley relief decreases with spreading rate for rates less than 80 mm yr 1 while axial ridge relief remains relatively constant for all rates greater than 50 mm yr 1 the stochastic component of the axial morphology referred to as bathymetric roughness also decreases with spreading rate for rates less than 80 mm yr 1 and remains relatively constant at higher rates although both axial valley relief and bathymetric roughness near the ridge axis show a similar spreading rate dependence they are weakly correlated at slow spreading centres the distinct differences in morphologic variability of fast and slow spreading ridges may result from the episodicity of magmatic heat input which controls the lithospheric rheology at slow spreading ridges these observations support the notion of a critical threshold separating two dynamically distinct modes of lithospheric accretion on mid ocean ridges

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data

the plate tectonic approximation plate nonrigidity diffuse plate boundaries and global plate reconstructions the central assumption of plate tectonics that plate interiors are rigid remains a useful but uncertain approximation strain rates of stable plate interiors are bounded between 10 i12 10 i11 year i1 and 4 10 i10 year i1 the narrowness of all plate boundaries the other main assumption of plate tectonics as originally conceived is contradicted by many observations both in the continents and in the oceans some diffuse plate boundaries in both continents and oceans exceed dimensions of 1000 km on a side diffuse plate boundaries cover 15 of earth s surface the maximum speed of relative plate motion across any one diffuse plate boundary ranges from 2t o 15 mm year which is faster than some upper bounds on intraplate motion across stable plate interiors 2m m year i1 strain rates in diffuse plate boundaries can be as high as 10 i8 year i1 25 times higher than the upper bound on strain rates of stable plate interiors but 600 times lower than the lowest strain rates across typical narrow plate boundaries the poles of rotation of the plates flanking a diffuse oceanic plate boundary tend to be located in the diffuse boundary which is a consequence of the strong coupling across the boundary

frequent activity on vulcano italy spanning the last 80 ky new insights from the chemo stratigraphy of the brown tuffs abstract the brown tuffs bt are widespread reddish brown to grey ash rich pyroclastic deposits recognized in the stratigraphic sequences of the aeolian islands and capo millazzo peninsula sicily that span the last 80 ky they have very homogeneous lithological textural and sedimentological features which make it difficult to reliably correlate units on the islands to proximal units in the source areas here we carefully re interpret the stratigraphic profiles of the bt on vulcano and lipari where the deposits are thickest and present the most complete succession the investigation is based on a large dataset of major and minor element geochemistry of juvenile glass components for the majority of the recognized bt depositional units whilst also providing new radiocarbon ages the distinctive chemical groupings observed within the glass analyses both temporally and spatially allow us to fingerprint the three main stratigraphically defined macro units in which the bt succession can be sub divided using prominent tephra marker beds the ischia tephra monte epomeo green tuff 56 ky and monte guardia pyroclastics from lipari herein radiocarbon dated to 27 26 ky the lower 80 56 ky lbt intermediate 56 27 ky ibt and upper bt here dated at 24 6 ky ubt macro units display k series volcanic glasses ranging from basaltic trachy andesites through trachy andesites to more evolved trachytes all consistent with an origin on vulcano the ubt are clearly distinguished from the lower macro units by their higher sio2 trachy andesite to trachytic glasses which extend to noticeably lower tio2 cao and mgo contents these features make it possible to re define the geochemical evolutionary boundary between ibt and ubt as corresponding to the 24 ky spiaggia lunga scoria bed on vulcano which is stratigraphically higher and younger than the previous boundary marker monte guardia the glass compositions of the lbt ibt and ubt are used to 1 assess links to known proximal eruption units outcropping on vulcano 2 validate medial distal bt occurrences across the aeolian archipelago salina filicudi and panarea and on capo millazzo 3 confirm that the bt are responsible for distal volcanic ash layers preserved in central mediterranean marine sedimentary archives interestingly the glass compositions of the ubt are very similar to those of the punte nere unit the earliest pyroclastic products erupted from the currently active la fossa cone on vulcano indicating the corresponding magmatic system has likely erupted similar melts and products over the last 24 ky and thus extending its life cycle such information is crucial for evaluating the long term eruption scenarios underpinning hazard assessment of the la fossa caldera magmatic system

constraints on 3 d stress in the crust from support of mid ocean ridge topography 1 the direction of crustal stresses acting at mid ocean ridges is well characterized but the magnitude of these stresses is poorly constrained we present a method by which the absolute magnitude of these stresses may be constrained using seafloor topography and gravity the topography is divided into a short wavelength portion created by rifting magmatism and transform faulting and a long wavelength portion associated with the cooling and subsidence of the oceanic lithosphere the short wavelength surface and moho topography are used to calculate the spatially varying 3 d stress tensor in the crust by assuming that in creating this topography the deviatoric stress reached the elastic plastic limiting stress the moho topography is constrained by short wavelength gravity variations under these assumptions an incompressible elastic material gives the smallest plastic failure stress associated with this topography this short wavelength topographic stress generally predicts the wrong style of earthquake focal mechanisms at ridges and transform faults however the addition of an in plane regional stress field is able to reconcile the combined crustal stress with both the ridge and transform focal mechanisms by adjusting the magnitude of the regional stress we determine a lower bound for in situ ridge perpendicular extension of 25 40 mpa along the slow spreading mid atlantic ridge 40 50 mpa along the ultra slow spreading ridges in the western indian ocean and 10 30 mpa along the fast spreading ridges of the southeastern indian and pacific oceans furthermore we constrain the magnitude of ridge parallel extension to be between 4 and 8 mpa in the atlantic ocean between 1 and 7 mpa in the western indian ocean and between 1 and 3 mpa in the southeastern indian and pacific oceans these observations suggest that a deep transform valley is an essential feature of the ridge transform spreading center

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

rupture characteristics of the 2003 mw 7 6 mid indian ocean earthquake implications for seismic properties of young oceanic lithosphere 1 analysis of broadband seismograms from the 15 july 2003 large earthquake m 7 6 in the central indian ocean reveals an unusual source process the source duration of longer than a minute is more than twice as long as expected from earthquake scaling relations yet 80 of the moment release occurred in two energetic asperities near the end of the rupture these two asperities were located in lithosphere with an age of 7 ma or greater a previous study has suggested that strike slip earthquakes in oceanic lithosphere having much longer than expected source durations also have a slow dissipative rupture process characterized by low radiated seismic energy and therefore low apparent stress we find no evidence for a slow rupture process to the 2003 earthquake instead the long duration appears to be due only to nucleation close to the actively spreading carlsberg ridge in lithosphere younger than 7 ma younger oceanic lithosphere may be able to generate small to moderate earthquakes but be unable to sustain slip in a large event due to steady release of strain in aseismic creep events large strike slip earthquakes within oceanic lithosphere may occur only in the central portions of long transform faults or in intraplate regions rupturing energetic asperities like those that failed in the mid indian ocean earthquake and leading to the observation that oceanic strike slip earthquakes have the largest apparent stresses among the global population of shallow earthquakes

the 2015 mw 7 1 earthquake on the charlie gibbs transform fault repeating earthquakes and multimodal slip on a slow oceanic transform the 2015 mw 7 1 earthquake on the charlie gibbs transform fault along the mid atlantic ridge is the latest in a series of seven large earthquakes since 1923 we propose that these earthquakes form a pair of quasi repeating sequences with the largest magnitudes and longest repeat times for such sequences observed to date we model teleseismic body waves and find that the 2015 earthquake ruptured a distinct segment of the transform from the previous 1998 earthquake the two events display similarities to earthquakes in 1974 and 1967 respectively we observe large oceanic transform earthquakes to exhibit characteristic slip behavior initiating with small slip near the ridge and propagating unilaterally to significant slip asperities nearer the center of the transform these slip distributions combined with apparent segmentation support multimode slip behavior with fault slip accommodated both seismically during large earthquakes and aseismically in between

the structure of mid ocean ridges recent research results on the structure of midocean ridges are reviewed the new view of ridge axis crustal structure obtained from high resolution seismology is reviewed emphasizing the variation of that structure with spreading rate and along axis at a given spreading rate recent results on upper mantle structure beneath ridges are examined including variations with seafloor age indications from anisotropy for directions of mantle flow and long wavelength along axis variations in structure and their implications for lateral heterogeneity in mantle temperature and composition

joint hypocenter relocation studies with sparse data sets a case history virginia earthquakes abstract this study considers the use of the joint location techniques for sets of virginia earthquakes recorded by different station distributions these input data sets consist of microearthquakes m b m b 4 6 that occurred in the same areas prior to network installation pre 1978 very different epicentral geometries both linear and diffuse and host geologic terranes one of folded and faulted sedimentary rocks and one of metamorphosed crystalline rocks are involved the main purpose of the investigation was to test if some characteristics of the spatial distribution of the original hypocenters might be due to bias resulting from the different networks used to locate the earthquakes and thereby not reflect the configurations of the source zones of the earthquakes specifically we relocated some 50 earthquakes 1959 to 1981 using both joint hypocenter determination jhd and a two stage location method jed se77 based on joint epicenter determination jhd with depth fixed in the computation seismic zones in southwestern giles county and central virginia were examined we did not find evidence that the use of hypoellipse with nonuniform networks has either generated spurious structure in the pattern of hypocenters or hidden a real structure the giles county lineation and the scattered distribution of sources in central virginia that appeared in the hypoellipse calculations also appear in the jhd and jed se77 calculations we were also not successful in convincingly improving the relative locations of regionally recorded earthquakes with respect to locally recorded events because too few of the local stations were installed when the regional events occurred in the central virginia zone however the joint location methods uncovered inconsistent arrival time data that had not been identified in the original hypoellipse computations on the whole the principal benefit derived from the joint location calculations was corroboration of the characteristics of the hypocenter distributions that had been inferred from the original individual locations

oceanic transform earthquakes with unusual mechanisms or locations relation to fault geometry and state of stress in the adjacent lithosphere on oceanic transforms most earthquakes are expected to occur on the principal transform displacement zone ptdz and to have strike slip mechanisms consistent with transform parallel motion we conducted a search for transform earthquakes departing from this pattern on the basis of source mechanisms and locations taken from the harvard centroid moment tensor catalogue and the bulletin of the international seismological centre respectively events with unusual mechanisms occur on several transforms we have determined the source mechanisms and centroid depths of 10 such earthquakes on the st paul s marathon owen heezen tharp menard and rivera transforms from inversions of long period body waveforms relative locations of earthquakes along these transforms have been determined with a multiple event relocation technique much of the anomalous earthquake activity on oceanic transforms is associated with complexities in the geometry of the ptdz or the presence of large structural features that may influence slip on the fault reverse faulting earthquakes occur at a compressional bend in the owen transform in the area of mount error and at the st paul s transform near st peter s and st paul s rocks a normal faulting earthquake on the heezen transform is located at the edge of a pull apart basin marking an extensional offset of the fault normal faulting earthquakes along the tharp menard and rivera transforms may also be related to extensional offsets some events with unusual mechanisms occur outside of the transform fault zone however and do not appear to be related to fault zone geometry for instance earthquakes with mechanisms indicating reverse faulting on ridge parallel fault planes are located near the ridge transform intersections of the st paul s and the marathon transforms possible additional contributors to the occurrence of anomalous earthquakes include recent changes in plate motion differential lithospheric cooling and the development of a zone of weakness along the fault zone but we do not find strong evidence to confirm the influence of these processes

locations of mid oceanic earthquakes constrained by seafloor bathymetry 1 earthquakes associated with the creation of new oceanic crust are difficult to locate precisely because they normally occur far away from seismic stations we assume that epicenters of these events can be constrained by seafloor topography and use the harvard centroid moment tensor cmt focal mechanism of an interplate earthquake to relate the event to either a transform fault a parallel in a coordinate system defined by the pole of rotation between two plates or a mid oceanic ridge a meridian using arrival times from the international seismological center isc bulletin we have relocated about 1500 globally distributed events for the time period 1976 1998 the isc locations can be as much as 70 km away from appropriate topographic features our new locations support that large interplate earthquakes occur on or very close to the primary bathymetric features for earthquakes with unknown mechanism or magnitude below the cmt threshold or predate the cmt catalog we use a simultaneous joint hypocenter determination procedure with respect to the already relocated master events jhdme we invert for earthquakes first for the romanche fracture zone rfz and then for a region extending from 30 n to 30 s seismicity agrees well with the main active ridge or transform however a small number of events might be related to the secondary bathymetry features the improved locations for mid oceanic earthquakes based on bathymetry could help improve our understanding of the oceanic crustal and deep earth structure seismic slip and budget constraint along plate boundaries and dynamics of plate tectonics

multiple closely spaced transform faults in fast slipping fracture zones the 400 km long offset of the east pacific rise spreading axis between the quebrada fracture zone lat 3 5 s and the gofar fracture zone lat 4 6 s has been surveyed using the long range sidescan sonar gloria the offset is accomplished in three distinct fracture zones each of which contains up to four actively slipping transform faults separated by short spreading centers transform spacings range from 5 to 16 km each transform is associated with its own transverse valley but at quebrada fracture zone and to some extent at the other two these valleys merge into one large slightly oblique fracture zone valley the morphological trends of fracture zone valleys are therefore not necessarily reliable for estimating directions of relative plate movement multitransform fracture zones appear to be much commoner on the fast spreading east pacific rise than on slower spreading ridges the tectonic spreading fabric which comprises closely spaced normal faults formed near the spreading axes and striking subparallel to them is modified to a sigmoidal pattern between the closely spaced transforms even midway between transforms these faults remain oblique to the spreading direction the degree of obliquity increases as the transform separation decreases showing progressive rotation of the stress ellipse as transforms are approached

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

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

master event relocation of gorda block earthquakes implications for deformation we have used the master event technique to relocate 73 earthquakes in the northern half of the gorda block the revised epicenters indicate a systematic eastward bias of 30 km in epicenters as published in the isc bulletin the new seismicity pattern reflects a more symmetric distribution of earthquakes about the gorda rise and a reduced degree of seismicity within the gorda block although the level of intraplate activity has apparently been reduced a significant number of events 10 20 still appears to be related to intraplate deformation further relocation studies coupled with focal mechanism determinations are necessary to better constrain deformation models for this region

joint hypocentral location in media with lateral velocity variations and interpretation of the station corrections abstract the application of a new implementation of the joint determination of hypocenters origin times and station correction terms jhd technique to data from the arkansas swarm usa the campi flegrei italy and the nazca plate argentina throws new light onto the problem of earthquake location in all these cases good quality p and s wave arrival times were available the main results are as follows 1 large lateral velocity variations llvv are generally associated with relatively large station corrections these corrections in turn are usually much larger than the average of the station residuals determined when the same set of events is located using a single event location sel method 2 the locations determined by the jhd and sel methods may be substantially different 3 since the effect of llvv on jhd locations is not well established simple 3 d velocity models are constructed so that the jhd technique applied to synthetic data generated using these models produce station corrections similar to those observed 4 the analysis of the synthetic data also shows that the locations determined by the jhd method are much closer to the true locations than the locations determined with the sel method the use of the weighted average of the sel station residuals as station corrections does not give better located events although there is a large decrease in the r m s residual computed for each event therefore the use of r m s residuals to estimate the quality of event locations is not always warranted in view of these results it is possible to conclude that the jhd station corrections can be used to detect lateral velocity variations and to produce locations substantially less affected by these variations than the locations determined by the sel method furthermore under appropriate conditions the jhd station corrections can also be used in a forward modeling scheme to get a semiquantitative velocity model such a model may be the only information that can be obtained from a data set when the station and event distributions are not adequate for 3 d velocity inversion

performance of localization algorithms for teleseismic mid ocean ridge earthquakes the 1999 gakkel ridge earthquake swarm and its geological interpretation in 1999 an unusual earthquake swarm at the 85 e 85 n volcanic centre on the ultraslow spreading gakkel ridge arctic ocean was detected teleseismically the swarm lasted over 9 months and counted 252 events with mb 3 1 it represents the strongest and largest ever recorded mid ocean ridge earthquake swarm and it occurred at a site where spreading rates are only about 10 mm yr 1 we relocated the earthquake swarm comparing the performance of three different localization algorithms 1 the absolute least squares routine hyposat 2 the absolute probabilistic routine nonlinloc and 3 the relative least squares routine mlocate the epicentres as calculated by each algorithm mostly did not agree within their error ellipses thus the choice of location algorithm proved more critical than for example the choice of a local velocity model we compiled a set of well localized events which closely agree in at least two routines mostly mlocate and nonlinloc we conclude that the earthquake swarm of 1999 was related to a spreading episode and shows a complex interplay of tectonic and magmatic events our geological interpretation revealed three phases in swarm activity in the first phase from january 17 up to february 1 fracturing of the crust took place either as a result of or enabling magmatic intrusion seismicity in the second phase from february 2 to april 6 expanded along and across axis it showed signs of magmatic interaction but a clear dyke migration pattern is absent at the beginning of the third phase a distinct break in the event rate suggested a change in the physical process either an adjustment of the stress field to the new regime or a transition to an effusive stage

on the robustness of seismic moment tensor inversions for mid ocean earthquakes the azores archipelago source models of mid oceanic earthquakes are often based only on far field teleseismic data the uncertainties of all source parameters are rarely quantified which restricts our understanding of how these events slip and how oceanic lithosphere is formed here we perform moment tensor inversions for five mw 4 6 5 9 earthquakes that occurred in the azores archipelago near the mid atlantic ridge in 2013 2016 taking advantage of the recently expanded seismic network in the region we assess moment tensor uncertainties due to data and earth model variability as well as the robustness of teleseismic versus local data inversions we find that for the events studied i existing 1 d earth models of the region based on receiver function data lead to a slightly improved data fit of local data compared to a widely used regional model based on active seismic surveys and ii using different 1 d earth models in the local data inversions leads to a variability in the retrieved source parameters of 15 30 in fault strike 5 20 in dip and 20 60 in rake depending on the earthquake s magnitude and location we study in detail the mw 5 9 2013 april 30 povoacao basin earthquake using 1 d and 3 d waveform modelling for which reported values of strike dip and rake in earthquake catalogues differ by 60 35 and 80 we find that our moment tensor solutions show a lower variability than in the catalogues and exhibit a persistent non double couple component of 40 60 per cent which is not due to a volumetric change we suggest that it is potentially due to geometrically complex faulting in the povoacao basin notably curved faults we find that the retrieved moment tensor solutions depend strongly on the earthquake s location if an accurate location is used joint inversions of local and teleseismic data can help to stabilize moment tensor solutions of oceanic earthquakes and reduce parameter trade offs compared to inversions of local data alone

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

the seismic moment budget of slowly spreading ridges mid ocean ridges with well developed median valleys are commonly the sites of large normal faulting earthquakes1 2 a recent global study3 6 of the centroid depths and source mechanisms of such earthquakes has yielded two important conclusions first that all the largest mid ocean ridge earthquakes occur beneath the inner floor of the median valley and second that the maximum thickness of the seismogenic layer increases with decreasing spreading rate these results lend support to the hypothesis that the median valley arises from the necking of a strong brittle lithosphere under horizontal extension7 9 here we compare the global rate of seismic moment release along slowly spreading mid ocean ridges with the rate predicted if widening of the median valley occurs entirely by seismogenic extension of the brittle lithosphere beneath the inner floor we find that earthquakes account for no more than about 10 20 of the rate of plate separation across the central median valley

crustal structure of north atlantic fracture zones seismic studies have established that large offset transforms along the slow spreading mid atlantic ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust seismically fracture zone crust in the north atlantic is extremely heterogeneous in both thickness and internal structure it is frequently quite thin 1 2 km thick and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3 a more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the mid atlantic ridge the geological nature of the seismically anomalous crust found within atlantic fracture zones and how this crust forms are still controversial one interpretation consistent with available seismic observations is that the crust within north atlantic fracture zones consists of a thin intensely fractured and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust the existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large scale detachment faults that form preferentially in the cold brittle lithosphere near the ends of segments