mechanisms and depths of atlantic transform earthquakes we have studied 40 earthquakes on seven major transforms along the mid atlantic ridge the majority show the expected transform parallel strike slip motion on steeply dipping fault planes some anomalous mechanisms suggesting either unusually shallow dip or a component of dip slip motion occur near ridge transform intersections the vertical tectonism suggested by transform morphology may thus be restricted to intersections where thermal contrast across the transform is greatest and stress orientations can be oblique to both ridge and transform centroid depths determined by waveform inversion are limited to a shallow zone corresponding approximately to the area above the 400 c isotherm of the thermal structure expected from lithospheric cooling the depth extent of faulting predicted by estimating fault area from seismic moments extends approximately to the predicted 600 c isotherm assuming that the areas above the 400 c or 600 c isotherms fail seismically moment release for six of the transforms accounts for varying fractions of the slip predicted by plate motion suggesting that some aseismic slip occurs if the time period sampled is representative in contrast on the vema transform the inferred seismic slip is greater than predicted unless faulting extends to the 650 c isotherm alternatively the vema may have an unusually long recurrence time or be mechanically anomalous due to its complex history in contrast to transform events centroids for oceanic intraplate earthquakes extend to approximately the 750 c isotherm this result is intriguing if depths reflect lithospheric strength as a function of depth transforms have much higher strain rates than intraplate areas so if both had the same rheology the limiting isotherm for transforms should be higher alternatively if a weaker rheology were appropriate the higher strain rate should cancel the weakening and yield a limiting isotherm similar to that for intra plate regions transforms may thus be either weaker and or have higher temperatures than expected in the latter model the required high temperatures perhaps due to intrusion at depth predict measurable heat flow anomalies the second possibility unusual rheological weakness is reminiscent of the unexplained weakness of the san andreas fault where heat flow data exclude anomalously high temperatures

the active part of charlie gibbs fracture zone a study using sonar and other geophysical techniques a short survey with gloria side scan sonar and other geophysical instruments has revealed new information about charlie gibbs fracture zone between 29 and 36 w the traces of two transform faults have been clearly delineated they fit small circles about the pole of rotation with an rms error of only about 1 km but they do not always follow the deepest parts of the transform valleys the transforms are joined by a short spreading center at 31 45 w the median transverse ridge appears to have been produced by normal seafloor spreading at this center and bears identifiable vine matthews magnetic anomalies a transverse ridge along the eastern inactive part of the northern transform may be an intrusive feature considerable thicknesses of sediment appear to have been deposited in the northern transform valley from norwegian sea overflow water passing through the fracture zone but transverse ridges have prevented the sediment reaching the southern valley

transform seismicity at the intersection of the oceanographer fracture zone and the mid atlantic ridge a seismic array consisting of nine hawaii institute of geophysics hig ocean bottom seismometers obss was deployed at the eastern intersection of the oceanographer fracture zone ofz and the mid atlantic ridge mar the 12 day experiment was designed to relate low magnitude earthquakes to the structure and tectonics of the mar ofz intersection an average of 10 locatable events with duration based magnitudes between 1 0 and 2 0 were recorded per day excellent hypocentral locations of 112 events were obtained earthquake locations based on more than eight observations generally show 50 confidence volume constraints within obs location errors the earthquake locations cover a broad swath across the corner of the intersection zone magnitude weighted earthquake location likelihood maps suggest a decline in magnitudes near the intersection bathymetry low composite focal plane solutions suggest source mechanisms which indicate that the region is dominated by extensional tectonics alternative source solutions indicating translational movement are presented but are inconsistent with apparent bathymetric trends the transition from the diverging mar to translational ofz plate margin occurs in the context of reduced magma genesis and crustal thinning due to the influence of the adjacent older lithosphere the region may be described in terms of semirigid plate tectonics accompanying transform valley genesis

microearthquakes beneath median valley of mid atlantic ridge near 23 n tomography and tectonics data from a microearthquake experiment in the median valley of the mid atlantic ridge near 23 n in 1982 are used to measure earthquake source parameters to determine the laterally heterogeneous seismic velocity structure across the inner floor and to develop a kinematic tectonic model for this portion of the median valley fifty three microearthquakes occurred over a 10 day period beneath the median valley inner floor and eastern rift mountains twenty of 23 well located inner floor epicenters define a line of activity about 17 km long having a strike of n25 e and located near an along axis depression some 300 400 m deeper than surrounding regions earthquakes with well resolved hypocenters generally have focal depths of 4 8 km beneath the seafloor of both the inner floor and the rift mountains the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45 normal faulting mechanisms although poorly constrained are also indicated for the rift mountain microearthquakes seismic moments approximate fault dimensions and average stress drops for the largest events recorded are 1019 1020 dyn cm 200 400 m and 1 70 bars respectively a twodimensional tomographic inversion of p wave travel time residuals from microearthquakes and local shots indicates a well resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor p wave velocities at 1 5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions the most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column by this view the seismic velocity structure at the ridge axis evolves probably by the sealing of cracks and pores within the first few hundred thousand years of crustal accretion consideration of the detailed sea beam bathymetry in this region of the inner floor the characteristics of large earthquakes that the region has experienced during the past 25 years and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years

fanning of fracture zones and a three dimensional model of the mid atlantic ridge present day spreading directions as represented by the active transform fault sections of fracture zones in the central north atlantic do not fit a small circle pattern about a unique pole1 5 as recently confirmed by a gloria survey contrary to this pattern which is geometrically required if ocean floor spreading is to occur6 the active transform fault directions show a convergence towards the east3 because of the fanning of the present directions the african and american plates are caught in what might be likened to a dovetail construction to allow motion the african plate must shrink about 2 in a north south direction such a shrinkage might be accomplished by thermal contraction tension due to the horizontal component of this contraction has been proposed as the primary cause of fracture zones4 7 however horizontal thermal contraction alone cannot explain the recorded pattern of spreading segments and transform faults because it does not explain why transform faults converge to the east and diverge to the west for this an active driving mechanism is needed we show here that the gravitational drive ridge push caused by thermal contraction in the lithosphere formed along a sinusoidal mid ocean ridge delivers such a mechanism

migration of mid ocean ridge volcanic segments the propagation of small offset volcanic spreading centre segments along mid ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non transform trends generated during along ridge propagation and in linking the major element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid ocean ridges

morphology and tectonics of the kane transform from sea beam bathymetry data high resolution sea beam data have been used to construct an accurate regional scale bathymetric map of the kane transform a large offset slowly slipping 30 mm yr central north atlantic transform the kane transform trends about 100 nearly orthogonal to the regional trend of the adjacent mid atlantic ridge mar rift valley the transform valley walls are composed of an alternating series of ridges and troughs which parallel the regional trend of the mar the ridges narrow and deepen toward the transform valley over distances of 10 20 km and the intervening troughs widen into distinct closed contour depressions that appear to be relict nodal basins the morphologic elements found on either side of the transform valley exhibit along strike continuity and represent undisrupted displacement markers the swath of terrain sandwiched between these markers is only several kilometers wide and is made up of elongate basins and ridges that integrate to create a distinctive transform parallel grain this interval the transform tectonized zone ttz marks the time averaged location of strike slip tectonism as parcels of crust move from each intersection along the transform interface the ttz is widest 6 km near the eastern ridge transform intersection rti and narrows to a strand less than 3 km wide along the central and western portions of the transform along the eastern portion of the transform valley a narrow 3 km median ridge several hundred meters high can be traced for more than 40 km along the valley floor and a similar but shorter feature is also present near the western rti at both rtis the neovolcanic zone from the adjacent rift valley can be traced as a series of constructional edifices across the entire intersection area to the opposing transform valley wall at the eastern rti the neovolcanic zone wraps around the eastern edge of the nodal basin while at the western rti it appears to bisect a paired nodal basin located in the intersection area a line drawn connecting the ends of each neovolcanic zone lies within the narrow ttz and has a strike of 098 using the bathymetric map of the kane transform as a record of the time integrated tectonic evolution of this large slowly slipping ridge transform ridge plate boundary we have constructed a simple model for the evolution of rtis our model emphasizes the interplay between temporal variations in crustal production rates i e variations in the volume of basaltic melt production along oceanic spreading centers and extensional tectonism this model can explain the existence of nodal basins at rtis and relict nodal basins along transforms and fracture zones as well as the occurrence of regularly spaced ridge and trough topography on the ridge flanks

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

seismic velocity reduction and accelerated recovery due to earthquakes on the longmenshan fault various studies report on temporal changes of seismic velocities in the crust and attempt to relate the observations to changes of stress and material properties around faults although there are growing numbers of observations on coseismic velocity reductions generally there is a lack of detailed observations of the healing phases here we report on a pronounced coseismic reduction of velocities around two locked sections asperities of the longmenshan fault with a large slip during the 2008 mw 7 9 wenchuan earthquake and subsequent healing of the velocities the healing phase accelerated significantly at the southern asperity right after the nearby 2013 mw 6 6 lushan earthquake the results were obtained by joint inversions of travel time data at four different periods across the wenchuan and lushan earthquakes the rapid acceleration of healing in response to the lushan earthquake provides unique evidence for the high sensitivity of seismic velocities to stress changes we suggest that stress redistribution plays an important role in rebuilding fault strength after a fault ruptures the recovery of its strength may be accelerated by earthquakes that redistribute stress according to an analysis of temporal variation in crustal seismic velocity on the longmenshan fault in china

seismicity surveys with ocean bottom seismographs off western canada three arrays of ocean bottom seismographs have been deployed to study the seismicity at the northern end of the juan de fuca ridge system off western canada nearly 100 events were located with estimated accuracies generally better than 10 km all lying on or near the en echelon ridge transform fault plate boundaries as defined in this area by the magnetic anomalies the seafloor morphology and by other geophysical data the depths of 12 events were determined to lie between 2 and 6 km below the top of the crust the seismograms exhibit clear p and s wave arrivals along with phases that involve p to s and sometimes s to p conversion probably at the base of the sediments beneath the instruments the event magnitudes have been estimated from signal duration using four calibration events that were well recorded by a land station the magnitude estimates permit the determination of rough magnitude frequency of occurrence relations over the magnitude range of 1 to 3 that are in surprisingly good agreement with the recurrence relations for the area at larger magnitudes from 75 years of land station data the mean p wave velocity in the uppermost mantle from the earthquake datamore recorded by the sea floor arrays is 7 6 km s sup 1 and the mean v sub p v sub s ratio is 1 71 or a poisson s ratio of 0 24 less

contemporary crustal deformation around the southeast borderland of the tibetan plateau 1 we derive a detailed horizontal velocity field for the southeast borderland of the tibetan plateau using gps data collected from the crustal motion observation network of china between 1998 and 2004 our results reveal a complex deformation field that indicates that the crust is fragmented into tectonic blocks of various sizes separated by strike slip and transtensional faults most notably the regional deformation includes 10 11 mm yr left slip across the xianshuihe fault 7 mm yr left slip across the anninghe zemuhe xiaojiang fault zone 2 mm yr right slip across a shear zone trending northwest near the southern segment of the lancang river fault and 3 mm yr left slip across the lijiang fault deformation along the southern segment of the red river fault appears not significant at present time the region south and west of the xianshuihexiaojiang fault system whose eastward motion is resisted by the stable south china block to the east turns from eastward to southward motion with respect to south china resulting in clockwise rotation of its internal subblocks active deformation is detected across two previously unknown deformation zones one is located 150 km northwest of and in parallel with the longmenshan fault with 4 6 mm yr right slip and another is continued south southwestward from the xiaojiang fault abutting the red river fault with 7 mm yr left slip while both of these zones are seismically active the exact locations of faults responsible for such deformation are yet to be mapped by field geology comparing our gps results with predictions of various models proposed for tibetan plateau deformation we find that the relatively small sizes of the inferred microblocks and their rotation pattern lend support to a model with a mechanically weak lower crust experiencing distributed deformation underlying a stronger highly fragmented upper crust

ocean bottom seismograph observations on the charlie gibbs fracture zone summary two networks of four ocean bottom seismographs were deployed successfully in 1982 september on spreading centre sites in the region of the charlie gibbs fracture zone activity was observed beneath both networks and hypocentres for over 100 events have been determined the observations confirm the existence of seismic activity along a suspected short spreading centre near longitude 31 75 w linking the north and south transform valleys of the fracture zone a relatively thick 7 9 km seismogenic zone is seen beneath the axis which is in agreement with earlier microearthquake observations on the mid atlantic ridge

gloria investigations of oceanic fracture zones comparative study of the transform fault zone the tectonic patterns of some twenty spreading centre offsets have been determined using the long range sidescan sonar gloria and are reviewed in this paper offsets of less than about 20 km fail to produce transform faults but are accommodated by short sections of oblique spreading with overlapping spreading centres in the fast spreading case at slow slip rates such offsets can be associated with topographic fracture zones that are indistinguishable from those produced at true transform faults such features may account for a significant proportion of spreading centre offsets on slowly spreading mid ocean ridges offsets larger than about 30 km produce a true 9transform fault9 this comprises a 1 to 5 km wide band of individual faults that are parallel or subparallel to the spreading direction the band corresponds to the 9transform fault zone9 tfz of continental wrench faulting it includes the 9principal transform displacement zone9 ptdz and secondary structures such as riedel shears a narrow tfz is normally associated with a prominent ptdz which appears to be either a narrow furrow or a single scarp on the other hand wider zones often fail to show a clear ptdz on gloria both types are common on the slowly spreading mid atlantic ridge and may even occur at different points along the same large offset transform such as romanche although the narrow type appears to occur preferentially where the local ridge axis is oblique to the spreading direction narrow tfzs and prominent ptdzs appear to be more common on the fast spreading east pacific rise it is suggested that broader tfzs and less prominent ptdzs may form in response to the complicating effects of small components of compression across the transform fault perhaps as a result of departure of the transform fault trace from a true small circle such compression would be more easily supported in the thicker stronger lithosphere of slowly spreading ridges but could be relieved by tension due to the ridge push force where the ridge is oblique to the spreading direction

microearthquakes on the east pacific rise at 21 n and the rivera fracture zone during march and april of 1974 three ocean bottom seismometer obs capsules were deployed in arrays on the east pacific rise epr at 21 n for 8 days on the rivera fraction zone rfz for 9 days and at the epr rfz intersection for 12 days seismicity on the epr is characterized by extremely small events none large enough to trigger all of the instruments simultaneously a weak 24 hour periodicity in obs event trigger rate is observed this could be related to earth strain tides or a modulation in the trigger sensitivity due to a varying noise level on the rivera fracture zone an en echelon pattern of earthquakes with a maximum depth of 10 km was observed seismic activity at the junction of the epr and rfz becomes more diffuse and terminates abruptly at the epr the junction array was slightly offset from the active zone of seismicity and the depth of only one event is reliably determined at 5 4 2 km the s wave spectra of microearthquakes at the junction and on the rfz are computed and seismic moments and corner frequencies determined stress drops are between 0 1 and 10 bars and show no systematic difference at the two deployment sites larger earthquakes observed at teleseismic distances do show markedly lower stress drops when they originate near to spreading centers but these differences are not observed at the microearthquake level in this study it is suggested that this is due to a natural tendency for small earthquakes to have small stress drops and a lack of large earthquakes during the rfz deployment

review of oceanic transform activity and development oceanic transforms generally have a complex morphology and structure and they do not conform to the ideal concept of simple fractures that exactly follow small circles centered on euler poles of relative plate motions it is proposed that the special properties of transforms arise because they are inherently leaky i e new plate area forms along them as transforms have finite widths the adjacent ridge tips are at different distances from the euler poles and the plate boundaries along transforms are slightly oblique to the direction of plate motions such that new plate area forms along them however along transforms the normal process of seafloor spreading is interrupted and new plate area forms by a different process transforms are underlain by crust consisting of variable proportions of a material emplaced in the transform domains and b stretched ridge generated crust the transform structure is further modified by shearing deformation related to irregularities of lateral faults and because of propagation of adjacent ridges transform leakiness is not considered to result merely from changes of plate motions though such changes will influence transform structure but it reflects the segmented structure of mid oceanic ridges the persistence of transforms and their migration in the absolute reference frame with a component perpendicular to their strikes shows that the ridge structure is decoupled from flow in deeper levels thermal anomalies along transforms especially enhanced heat losses stabilize the segmented ridge structure

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

distinct tectonic activities in guangxi china due to large earthquakes on the longmenshan fault zone abstract it is of great importance to study active tectonics in guangxi china which is a region with relatively high level of seismic hazard in this work we attempt to apply the load unload response ratio lurr method to test stress state of tectonic field in guangxi and find obvious difference in the periods before and after earthquakes on the longmenshan lms fault zone the regions with higher stress accumulation indicated by lurr augmented significantly after the 2008 ms8 0 wenchuan earthquake and reduced gradually following the 2013 ms7 0 lushan earthquake the movement of lurr was along a northwestern trending strike slip fault the bama bobai fault showing the shear loading acting on the fault to evidence this viewpoint we construct gps velocity profile on this fault results show that the fault manifested left lateral motion after the wenchuan earthquake this movement which was never observed before the wenchuan earthquake faded away after the lushan earthquake we suggest that the detected changes both in lurr and gps velocity field were probably caused by stress redistribution on the lms fault zone due to occurrence of large earthquakes the results may imply the potential seismic hazard zone in guangxi

thermal contraction and the state of stress in the oceanic lithosphere nonuniform contraction of the oceanic lithosphere as it cools and thickens following its formation at the axis of a spreading center results in a complex three dimensional state of deviatoric stress which can be separated into two parts a thermal bending stress due to changes in the vertical temperature distribution and a thermal contraction stress due to lateral variations in the vertically averaged temperature we examine thermal contraction stresses due to temperature changes in a thin semi infinite rectangular plate bounded by the spreading center axis two stress free boundaries representing transforms or fracture zones define the plate width or ridge segment length l with the bottom of the elastic plate defined by a prescribed temperature the plate thickens as the square root of age as expected for a thermal boundary layer due to vertical conductive cooling above this prescribed temperature elastic stresses are assumed to relax quickly stresses are obtained by properly accounting for the rate of accumulation of the vertically averaged stress as initially stress free material is added to bottom of the cooling thickening plate the state of thermal contraction stress calculated from this model is characterized by large tensile stresses at the boundaries of the plate and relatively low stresses in the plate interior at the ridge axis ridge parallel tensile stresses are about 300 mpa the same as if the plate were not allowed to contract in this direction along the transform boundary the maximum transform parallel tensile stress occurs at a distance l 2 from the ridge transform intersection where its magnitude is comparable to the stress at the ridge axis the tangential stresses at the plate boundary decrease rapidly with distance from the boundary at a distance of l 4 from the ridge axis the ridge axis parallel stresses are one tenth of their ridge axis magnitude the stress magnitudes are independent of both spreading rate and ridge segment length a large transform parallel tensile stress may control the length of transform offsets thermal bending moments are influenced by the large thermal contraction stresses near the ridge axis however a short distance from the ridge these moments attain their free horizontal contraction values which previous studies have shown to cause observable bending of the plate and a geoid anomaly at fracture zones flexure due to thermal bending moments will concentrate bending stresses at a distance from the fracture zone determined by the flexural length of the plate thus providing a natural length scale controlling the spacing of transforms

seismicity and crustal structure in the orozco fracture zone project rose phase ii a total of 301 earthquakes were recorded in the vicinity of the orozco fracture zone by seven texas ocean bottom seismograph stations during the 2 week period of the rivera ocean seismic experiment rose phase ii using data from the entire rose array hypocenters of 50 earthquakes were determined these revealed two distinct zones of seismic activity within the fracture zone in addition to these earthquake families many very small events were detected by a station located very close to the spreading center of the east pacific rise the magnitudes of these earthquakes defined by their duration times were so small that most of them were recorded only at this station station 14 in continual or swarmlike occurrences the slope of the frequency magnitude distribution of these events is significantly larger than those of other earthquake groups detected during the experiment i e they appear to have an unusually high b value these results suggest that this new population of earthquakes may be associated with volcanic activity or stress release within highly fractured crustal material refraction studies in the fracture zone reveal the presence of a rather high velocity crustal layer 6 9 7 0 km s beneath the experiment zone the moho velocity and the crustal thickness are estimated at 7 8 km s and 6 2 km respectively

crustal structure and isostatic compensation near the kane fracture zone from topography and gravity measurements i spectral analysis approach summary six gravity and bathymetry profiles perpendicular to the kane fracture zone each more than 300 km long were gathered to study the variation in crustal structure in the vicinity of a major fracture zone and the gravitational edge effect at the contact between lithosphere of two different ages a spectral analysis of the gravity and bathymetric series as a function of wavelength shows that the gravitational edge effect is only significant at the longest wavelengths for remaining wavelengths the admittance the ratio of the amplitude of the gravity anomaly to the amplitude of the bathymetry is best explained by a model of isostasy in which topographic loads are partially supported by the flexural rigidity of an elastic plate about 6 km in thickness after subtracting the gravitational attraction of the bathymetry and its compensation substantial isostatic anomalies remain we interpret these anomalies as being caused by variations in crustal thickness which have little correlation with surface topography except at very long wavelengths the apparent crustal thickness varies by as much as a factor of 2 but there is no evidence indicating systematic thinning of the crust beneath the fracture zone our data do suggest that such density variations within the plate are also compensated by the isostatic response of an elastic plate but with very different effect from those at the surface this indicates that there are two different modes of crustal formation with different gravity and topographic signatures effusive volcanism which loads the surface of the elastic plate producing both topographic relief and coherent gravity anomalies and intrusive volcanism or underplating producing gravity anomalies but little topographic relief

microearthquake evidence for extension across the kane transform fault we report the results of a microearthquake experiment conducted in 1987 in the active transform portion of the kane fracture zone the kane is a slow slipping 25 mm yr large offset 150 km transform delineated by a pronounced transform valley the experiment site lies in the eastern half of the transform 40 km west of the eastern ridge transform intersection in a region of marked transform parallel topography the network consisted of 12 ocean bottom hydrophones and seismometers and extended for 25 km along the transform and 10 km in width spanning the transform valley and a narrow median ridge to the north hypocentral parameters were determined for 86 earthquakes epicenters within the network define a narrow transform parallel trend along the steep southern wall of the transform valley maximum focal depths increase from 6 to 9 km from east to west across the network significant activity was also detected outside the transform tectonized zone to the southeast this activity may be associated with the inside corner of the ridge transform intersection while to the north about 20 events occurred in 7 m y old crust in a region of ridge parallel bathymetry six focal mechanisms were obtained from p wave first motions for events within the transform valley the best constrained solutions for four earthquakes within the network show normal faulting on fault planes subparallel to the trend of the transform all the mechanisms indicate a tension axis perpendicular to the trend of the transform these results together with a significant historical record of large earthquakes near the experiment site lead us to conclude that the principal transform displacement zone was inactive during our experiment and that the activity we recorded is the result of extension in the adjacent lithosphere the observed focal mechanisms and the inference that the axis of least compressive stress is approximately perpendicular to the transform provide direct evidence that the transform fault is mechanically weak relative to the surrounding lithosphere potential sources of extension across the transform include thermal stress in the young oceanic lithosphere topographic loading a small component of plate divergence normal to the transform and northward motion of the asthenosphere relative to the surface plate

the hypocentral resolution of microearthquake surveys carried out at sea summary the ability to locate the hypocentres of earthquakes occurring along ridge crests and fracture zones accurately is a prerequisite to solving several problems associated with seafloor tectonics and oceanic crustal formation such resolution has rarely been achieved in the past often because the seismic networks deployed were inadequate for the task we demonstrate that sea surface receivers are not useful in such studies and that the minimum acceptable size for a seafloor network is five receivers of which at least one must be capable of detecting shear waves unambiguously

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

structure and seismotectonics of the vema fracture zone atlantic ocean the structure and seismo tectonics of the vema fracture zone and the adjacent regions of its transverse ridge to the south and the mid atlantic ridge spreading centre to the north is discussed based on an analysis of seismic refraction profiles micro earthquake activity and gravity anomalies crustal structures were determined by analysis of wide angle seismic data from explosive and airgun sources recorded on arrays of ocean bottom seismometers micro earthquakes were recorded by six instruments for a period of 14 days in an array to the north of the ridge transform intersection immature crust similar to that reported by fowler 1976 from the famous area lies beneath the spreading centre the lower crust thins and normal mantle appears as the fracture zone is approached the thinnest crust lies beneath the northern end of the topographic deep at the fracture zone intersection this is also the locus of current microearthquake activity at sub bottom depths of 3 4 km which a composite fault plane solution suggests may originate on normal faults that parallel the trend of the local topography on the eastern wall of the median valley within the central region of the fracture zone valley the crust thickens but still retains an anomalous velocity structure seismic layer 3 is absent and crustal velocities are low this is probably the consequence of attenuated crustal accretion of faulting and cracking and of hydrothermal alteration the pattern of residual gravity anomalies after correction for topography for sediment thickness and for mantle temperature variations is consistent with the crustal structure defined by the seismic experiments the gravity measurements indicate that there is a broad thinning of the crust over 20 40 km in the vicinity of the fracture zone on which are superimposed localized regions of low and high densities to the south of the fracture zone the crust thickens and assumes a normal velocity structure including a mature seismic layer 3 the seismic moho is warped upwards under the transverse ridge suggesting a tectonic origin from buckling rather than uplift by serpentinization

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

a microearthquake survey at the junction of the east pacific rise and the wilkes 9 s fracture zone summary an ocean bottom seismograph survey of the junction of the east pacific rise and the wilkes fracture zone detected only three microearthquakes beneath the rise crest during seven days of recording in contrast during the same period 41 events were detected on the fracture zone all at distances greater than 10 km from the junction these results suggest that near the rise crest the thin crust can support sufficient stress only to generate infrequent small earthquakes and that most faulting may take place by aseismic slip at 10 km from the rise axis part of the crest has become competent enough to support stress resulting in earthquakes probably at depths of up to 5 km below the sea bed gear 5 waves on the seismometer records indicate that a magma chamber if it exists near the junction is less than 10 km across

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

experimental sytectonic dehydration of serpentinite under conditions of controlled pore water pressure we have studied the mechanical behavior of serpentinite under conditions favouring dehydration to olivine talc water at temperatures ranging between 300 and 600 and at total pressures from 100 to 300 mpa constant stress stress relaxation and constant displacement rate testing techniques permitted a wide range of deformation rates and paths to be accessed effective confining pressure was held constant during the dehydration process by means of a controlled pore water pressure system stress supported by the samples was almost independent of deformation rate at temperatures below those required for the onset of the breakdown reaction at 500 and 600 c however the strength was linearly dependent on the inverse of the deformation rate and independent of effective confining pressure over the range investigated microstructural studies revealed that the weakening was associated with sliding in narrow shear zones containing ultrafine grained 0 1 m olivine produced in the breakdown reaction the enhanced deformability is interpreted in terms of flow of the olivine in these shear zones by diffusion accommodated grain boundary sliding these experimental results suggest that metamorphic transformations affect the deformability of rocks and demonstrate enhancement of deformability through the transient existence of fine grained reaction products the results may also be applicable to the mechanics of deformation processes in oceanic transform faults and give some indication of the degree of weakening that might be expected from grain size reduction by dynamic recrystallization in plastically deforming rocks

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

anomalously thin crust in oceanic fracture zones new seismic constraints from the kane fracture zone three detailed and carefully positioned seismic refraction experiments have recently been carried out along the kane fracture zone near 24 n 44 w in order to place better geophysical constraints on the extent and origin of the anomalously thin crust reported from parts of this and other large atlantic fracture zones these three experiments when combined with earlier studies result in reversed structural control along about a 300 km long segment of the kane fracture zone extending from the ridge transform intersection out onto crust about 25 m y old an analysis of refraction lines obtained along the fracture zone trough by using both travel time modeling and delay time function techniques reveals large variations in crustal thickness and or velocity however along most of the eastern fracture zone trough upper mantle type velocities are found at depths of only 2 3 km below the seafloor less than half the typical depth to moho in the ocean basins this anomalous fracture zone crust is generally characterized by unusually low compressional wave velocities relatively high velocity gradients throughout most of the thinned crustal section about 1 2 s 1 and a distinct crust mantle boundary the very thin crust appears to be confined to the deepest parts of the kane fracture zone trough 10 km wide although a more gradual crustal thinning may extend up to several tens of kilometers from the fracture zone a specially designed delay time experiment at the intersection of the kane fracture zone and the mid atlantic ridge also revealed the presence of a thinned crustal section of a relatively uniform thickness about 3 km except beneath the nodal basin where the crust may be less than 1 km thick the anomalous crust in the intersection area appears to extend at least part way down the median valley we interpret this anomalous fracture zone crust as a primary accretionary feature that forms as a result of a restricted magma supply near ridge transform intersections although tectonic processes may also play a part at least locally in thinning the crust our results support a ribbon model of the gross seismic structure of the ocean basins in which bands of relatively homogeneous so called normal crust 50 100 km wide extend away from spreading centers each formed at a discrete ridge crest segment and each separated by the thin crust underlying the fracture zone trough

crustal structure of the mid atlantic ridge at 23 n from seismic refraction studies an explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120 km long section of the mid atlantic ridge mar median valley at latitude 23 n immediately south of its intersection with the kane fracture zone the data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques these solutions are combined using two dimensional ray tracing to produce a single model defining the major along axis structural changes occurring beneath this section of the median valley with the exception of slightly low layer 3 velocities the crust beneath several tens of kilometers of the mar median valley has all the characteristics of simple mature ocean crust including a well defined moho transition zone 8 km s upper mantle velocities and a total crustal thickness of 6 7 km no evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along axis we infer from these observations that the basic seismic characteristics of the ocean crust are frozen in in a time period less than that separating the major volcanic injection events 10 000 50 000 years by hydrothermal circulation penetrating to the base of the crust along the 100 km long ridge segment studied here two major along axis structural changes occur the first is a 10 to 15 km wide along axis zone of lowered velocities in the lower crust centered beneath a major along axis topographic high this is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities the second occurs abruptly at latitude 23 15 n and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly north of this boundary the 30 km long ridge segment that abuts the kane fracture zone has a crustal thickness of 4 5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust interpretation of the cause of this structural change is uncertain but may be related to local tectonic events perhaps a recent 10 to 20 km ridge jump to the east

microearthquakes beneath the median valley of the mid atlantic ridge near 23 n hypocenters and focal mechanisms we report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the mid atlantic ridge near 23 n during a 3 week period in early 1982 the network consisted of seven ocean bottom hydrophones and three three component ocean bottom seismometers the instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence the hypocentral parameters of the 26 largest microearthquakes are reported 18 of these events have epicenters arid focal depths which are resolvable to within 1 km formal error at the 95 confidence level microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30 or more these solutions are similar to the mechanisms of nearby large earthquakes microearthquakes also occur beneath the steep eastern inner rift mountains the rift mountain earthquakes have nominal focal depths of 5 7 km and epicenters as distant as 10 15 km from the center of the median valley but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure we interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7 8 km we infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment

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

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

recent faulting and microearthquakes at the intersection of the vema fracture zone and the mid atlantic ridge new air gun reflection profiles 3 5 khz reflection profiles and microearthquake data recorded by an array of ocean bottom seismographs form the basis for this study of the transition from a spreading center to a major transform fault disturbances of the thick normally flat lying turbidite deposits provide indications of recent vertical motions at the western intersection of the fracture zone with the median valley there is a depression in the sediments that represents the southerly extension of the median valley into the fracture zone valley the depression is terminated abruptly on the south by the active transform fault which acts as a locus for vertical as well as horizontal displacement flat lying undisturbed sediments terminate abruptly at the fault the western boundary of the depression is much broader and is characterized by a series of slumplike steps to the west there is little or no evidence for uplift or tilting of sediments which might indicate vertical recovery of the crust as it spreads away from the depression this suggests that uplift and recovery out of the depression is episodic in nature and has been inactive over the last million years along the western boundary to the east there is clear evidence of uplift and tilting of sedimentary layers a basement ridge emerging from the sediments is currently being uplifted and rotated in a manner analogous to processes responsible for the creation and cancellation of median valley relief the transition between the spreading center and the transform fault appears to take place within 1 2 km the width of the transform fault just east of the depression is less than a kilometer microearthquakes were located and displayed by new methods that directly account for nonlinearities associated with small arrays microearthquakes located by three or more ocean bottom seismometers show that the greatest seismic activity occurs along the eastern walls of the median valley at the basement ridge in the eastern portion of the depression and in the crestal mountains very little activity is associated with the western edge of the transform depression and the trace of the transform fault

the correction for the travel time effects of seafloor topography in the interpretation of marine seismic data problems associated with the application of topographic corrections to deep ocean marine seismic data are described synthetic travel time data are used to show that corrections for the delay time introduced by the water column are inferior to corrections for the true water path water path corrections place the shots at the ray entry points on the seafloor require no assumptions about the structure under investigation and maintain the correct distribution of data with range the water path correction can be most accurately applied during the stacking operation that transposes the observed data into intercept time slowness space there is then no requirement to make determinations of slowness and for experiments involving seafloor receiver and surface shots the data may be transposed to the seafloor with complete objectivity water path corrected stacks from different locations may then be compared in the sure knowledge that any observed differences are due to real structural changes and not to topographic correction errors or to invalid assumptions of lateral homogeneity

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

earthquakes in the orozco transform zone seismicity source mechanisms and tectonics as part of the rivera ocean seismic experiment a network of ocean bottom seismometers and hydrophones was deployed in order to determine the seismic characteristics of the orozco transform fault in the central eastern pacific we present hypocentral locations and source mechanisms for 70 earthquakes recorded by this network all epicenters are within the transform region of the orozco fracture zone and clearly delineate the active plate boundary about half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough that forms the northern boundary of the transform zone region 1 most focal depths for these events are very shallow within 4 km of the seafloor several well determined focal depths however are as great as 7 km no shallowing of seismic activity is observed as the rise transform intersection is approached to the contrary the deepest events are within 10 km of the intersection first motion polarities for most of the earthquakes in region 1 are compatible with right lateral strike slip faulting along a nearly vertical plane striking parallel to the spreading direction another zone of activity is observed in the central part of the transform region 2 the apparent horizontal and vertical distribution of activity in this region is more scattered than in the first and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism pronounced lateral variations in crustal velocity structure are indicated for the transform region from refraction data and measurements of wave propagation directions the effect of this lateral heterogeneity on hypocenters and fault plane solutions is evaluated by tracing rays through a three dimensional velocity grid while findings for events in region 1 are not significantly affected in region 2 epicentral mislocations of up to 10 km and azimuthal deflections of up to 45 may result from assuming a laterally homogeneous velocity structure when corrected for the effects of lateral heterogeneity the epicenters and fault plane solutions for earthquakes in region 2 are compatible with predominantly normal faulting along a topographic trough trending nw se the focal depths however are poorly constrained these results suggest an en echelon spreading center or leaky transform regime in the central transform region

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

location of instruments on the seafloor by joint adjustment of instrument and ship positions the problem of determining accurate locations for arrays of underwater instruments is similar to the problem of locating local earthquakes by utilizing a limited number of receivers but is complicated by the fact that estimated errors in longitude and latitude of a given receiver ship position are nonzero and are typically correlated estimates of ship positions at sea are commonly determined by fixes from the navy transit navigation satellite system this system calculates position error ellipses which vary in size and orientation from fix to fix deviations in the orientations of the principal axes of the error ellipses from north south and east west result in the introduction of off diagonal elements in the ship position variance matrix tying each ship fix to a stationary array of ocean bottom seismometers obs s using acoustic ranging allows averaging of fixes thus solving simultaneously for ship and obs positions puts tight constraints on the system as a whole and especially on the relative positions of the obs s in addition to ship positions the data required to locate the instruments are two way travel times between the ship and transponders on the instruments and initial depth estimates of obs s read from the bathymetry the parameters which we solve for are the locations of the instruments including depth and of the ship at each fix the forward problem calculating acoustic travel times from distances between ship fixes and obs s is solved by a ray shooting algorithm this system of equations is linearized and transformed to diagonalize the covariances it is then solved iteratively by inverting the data jointly using singular value decomposition and step length damping the solution includes the covariances of the parameters we show calculations of positions of four obs s for deployments during three experiments including the rivera oceanic seismic experiment rose typical fix error ellipses 1 are 300 m by 200 m and resulting instrument position uncertainties are approximately 100 m relative instrument positions are accurate to about 50 m relative errors can be decreased to 10 to 20 m by using simultaneous rather than sequential ranging