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

seismicity of the galapagos 95 5 w propagating rift an array of ocean bottom seismometers was deployed for 21 days about the 95 5 w galapagos propagating rift tip to obtain seismic evidence for the propagating rift hypothesis and to investigate the dynamic effects of rift propagation seismicity was fairly constant 10 17 events per day with no indication of swarm activity hypocenters were determined for 304 earthquakes the largest of which is estimated to be mb 2 3 with a seismic moment of 2 9 1020 dyn cm the cumulative seismic moment released by all local earthquakes is estimated to be 6 6 1020 dyn cm a b value of 0 77 0 08 was estimated for the region implying a stress regime dominated by a broad tectonic stress field the distribution of earthquakes is consistent with the propagating rift hypothesis most earthquakes occurred south of the propagating rift within the de steiguer deep seismicity within the transform zone is clustered in such a way as to suggest that two or three discrete fault segments may be active at the same time the seismicity observed along the propagating rift is diffuse earthquakes were observed at the propagating rift tip but not in front of it almost all earthquakes occur south of the northern pseudofault and its extension west of the rift tip that is earthquakes occur almost exclusively in lithosphere that has been or is being overtaken by the propagating rift six composite focal mechanisms are consistent with the propagating rift hypothesis first motions from earthquakes located on the propagating rift indicate extension whereas motions associated with the failing rift area indicate compression a compressional regime is also indicated west of the transform zone twelve earthquakes from the transform zone produce a poorly constrained focal mechanism consistent with right lateral strike slip motion along a fault plane that is roughly parallel to the isochrons of the transferred lithosphere

microearthquake characteristics and crustal velocity structure at 29 n on the mid atlantic ridge the architecture of a slow spreading segment we report the results of a microearthquake and seismic tomography experiment conducted along the southern half of the mid atlantic ridge segment at 29 n and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes the seismic velocity structure is obtained from two dimensional 2 d and three dimensional 3 d tomographic inversions of travel times from shots along an axial refraction line inversion solutions indicate that the velocity structure in the lower crust is heterogeneous with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high the thickness of the lower crust at the segment end is asymmetric across axis with thinner crust beneath the inside corner the indicated variations in crustal thickness are consistent with those inferred from mantle bouguer gravity anomalies the microearthquakes located along axis during the 41 day recording period cluster in three separate along axis regions 1 the southern segment end near 28 55 n 2 the central along axis topographic high at 29 11 n near and north of the broken spur hydrothermal vent field and 3 a region midway between beneath a volcano near 29 02 n the greatest level of microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset this pattern of off axis microearthquake activity and the cross axis asymmetry in crustal thickness at the segment end support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions anomalous focal mechanisms for microearthquakes beneath the along axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center in contrast may be the result of volcanic and hydrothermal processes such as magma movement or thermal stresses generated near cooling plutons a comparison of microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature whereas microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust along axis well resolved focal depths determined with a 3 d velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center these observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state

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

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

a diagnostic study of upper tropospheric cold lows over the western north pacific abstract four summers 1967 1969 1970 1971 of rawinsonde data from four western north pacific island stations guam midway johnston and wake were used to form a three dimensional composite of the subtropical upper tropospheric cold core lows commonly found in the mid pacific trough the basic meteorological parameters of wind height temperature and humidity were utilized to form an initial composite from the 117 lows identified during the period of study satellite mosaics were used to define a composite of horizontal cloud cover the basic composite was harmonically smoothed and dynamically adjusted to generate a consistent set of derived fields that emphasize the important large scale characteristics of the phenomenon north south and east west vertical cross sections through the center of the composite low are used to illustrate the prominent features of the zonal and meridional winds temperature and height anomalies relative vorticity divergence vertical motion and relative humidity field

tectonic and kinematic regime along the northern caribbean plate boundary new insights from broad band modeling of the may 25 1992 m s 6 9 cabo cruz cuba earthquake on may 25th 1992 an m s 6 9 earthquake occurred off the southwestern tip of cuba along the boundary between the caribbean and north american plates this earthquake was the largest to strike southern cuba since 1917 and the largest ever recorded in that region by global seismic networks it is therefore a key element for our understanding of the tectonic and kinematic regime along the northern caribbean plate boundary in order to test the previously proposed source parameters of the cabo cruz earthquake and to better constrain its focal mechanism we derived a new set of source parameters from unfiltered broad band teleseismic records we used a hybrid ray tracing method that allows us to take into account propagation effects of seismic waves in a realistic crustal model around the source our solution is consistent with the long period focal mechanism solution of virieux et al 1992 our solution also models the higher frequency crustal and water layer phases the primarily strike slip focal mechanism has a small thrust component its shows an east west trending nodal plane dipping 55 to the north that we interpret as the rupture plane since it corresponds to the geometry of the major active fault in that area the displacement on this plane is a left lateral strike slip combined with a small amount of southward thrust the result is in good agreement with the active tectonic structures observed along the oriente fault south of cuba the small thrust component demonstrates that contrary to prior belief the transpressive regime extends along this whole segment of the caribbean north american plate boundary together with historical seismicity it suggests that most of the stress accumulated by the caribbean north american plate motion is released seismically along the southern cuban margin during relatively few but large earthquakes

kinematic framework of the cocos pacific plate boundary from 13 n to the orozco transform fault results from an extensive magnetic and seamarc ii survey during the summer of 1987 magnetic anomaly data were collected by surface ship as part of an extensive seamarc ii investigation of the east pacific rise epr from 13 n to the orozco transform the survey extended to either side of the rise axis onto seafloor at least 1 8 million years m y in age enabling the recent evolution of the structural and kinematic framework of the plate boundary to be studied in detail north of 13 50 n there has been a major perturbation in the evolution of the plate boundary swaths of lineaments that trend oblique to epr parallel topography form a north pointing v shaped discordant zone on the pacific and cocos plates that is broadly symmetric about the epr axis on the pacific plate a zone of discordant morphology 130 km long and between 6 and 14 km wide with a structural grain that is highly oblique to the present day spreading direction is observed on seafloor 0 9 1 8 m y in age a similar but more subtle feature of the same age is also present on the cocos plate these zones of discordant lineaments can be correlated with changes in the magnetic lineation pattern on the pacific plate the disturbed zone lies between anomalies j and 2 creating greater than normal distance between the anomalies on the cocos plate the disturbed zone is characterized by a distinct high amplitude northwestward trending magnetic anomaly the observed structural grain and the changes in the magnetic anomaly patterns associated with the disturbed zones are very similar to those observed at propagating ridges based on the magnetic anomalies a propagation rate of 10 8 cm yr in a n10 w direction is estimated for the past 1 8 m y a detailed examination of the structures developed within the disturbed zone on the pacifc plate indicates that the rift propagation in this area can best be explained by the model of wilson 1990 which involves cyclic rift failure with inward curvature of both rift tips plate adjustment to the propagation event is ongoing there is a pronounced change in the morphology of the rise axis with distance from the propagation event close to and for 100 km behind the propagator tip the epr crest is not well developed and is characterized by a series of low relief of 100 200 m ridges and troughs with a poorly defined neovolcanic zone further away 100 km from the propagator tip the rise crest is a single linear horst shaped ridge with a well developed narrow axial graben at the axis of the ridge regional morphologic and magnetic data suggest that this cycle of rift propagation may have begun approximately 2 5 million years before present m y b p near the eastern ridge transform intersection of the paleo o gorman transform and is continuing today at the orozco transform

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

crustal structure of the southwest indian ridge at the atlantis ii fracture zone the southwest indian ridge is a slow spreading end member of the mid ocean ridge system the deepest borehole penetrating the lower oceanic crust ocean drilling program hole 735b lies on the eastern transverse ridge of the atlantis ii fracture zone at 57 e a wide angle seismic survey in the vicinity of the borehole reveals a crustal structure that is highly heterogeneous to the east of atlantis bank on which hole 735b is located the crust consists of a 2 2 5 km thick high velocity gradient oceanic layer 2 and a 1 2 km thick low velocity gradient layer 3 the transform valley has a 2 5 3 km thick crust with anomalously low velocities interpreted to consist largely of highly serpentinized mantle rocks the seismically defined crust is thickest beneath the borehole where layer 2 is thinner and the lower crust is inferred to contain 2 3 km of partially serpentinized mantle the seismic velocity models are consistent with gravity data which show weak residual mantle bouguer anomalies because the regions of thinner crust have lower crustal densities stress variations deduced from mass balances between the transform valley floor and the adjacent transverse ridges are much larger than the likely threshold for lithospheric failure and therefore indicate that the relief is supported dynamically the variation of crustal thickness with spreading rate defined by data from the southwest indian ridge and elsewhere is consistent with models of melt generation in which the upwelling mantle is cooled by conductive heat loss at very slow spreading rates resulting in reduced melt generation under the spreading axis large segment scale variations in crustal thickness suggest subcrustal along axis migration of melt toward segment centers

projection of north pacific tropical upper tropospheric trough in cmip5 models implications for changes in tropical cyclone formation locations abstractthe strong westerly shear to the south flank of the tropical upper tropospheric trough tutt limits the eastward extension of tropical cyclone tc formation over the western north pacific wnp and thus the zonal shift of the tutt in warming scenarios has an important implication for the mean formation location of tcs the impact of global warming on the zonal shift of the tutt is investigated by using output from phase 5 of the coupled model intercomparison project cmip5 of 36 climate models in this study it is found that considerable spread exists in the zonal position orientation and intensity of the simulated climatologic tutt in the historical runs which is forced by observed conditions such as changes in atmospheric composition solar forcing and aerosols the large spread is closely related to the diversity in the simulated sst biases over the north pacific based on the 15 models with relatively high skill in their historical runs the near term 2016 35 projection shows no signi

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

temporal evolution of overlapping spreading centers at 16 20 n on the east pacific rise abstract a compilation of sea beam data on the right stepping overlapping spreading center osc at 16 20 n on the east pacific rise epr and in the surrounding region is presented sea beam data from the rise flanks at 16 20 n reveal three deep basins at least one of which is flanked by a curved ridge whose morphology is similar to that at the offset the widths of these offaxis basins range from 8 to 13 km and allow an estimate to be made of the sizes of the offsets at which they were formed it is proposed that they represent traces on the rise flank which provide a record of the development of the offset and a basis for prediction of its future evolution the repeated pattern of abandoned overlap basins and cuspate ridges supports the notion that oscs are episodic on a time scale of approximately 200 000 years and are cyclic in nature it also reveals an alternation of dominance between the southern and northern segments which have shifted the location of the discontinuity along the axis so as to create zig zag traces of basins on the ridge flanks

formation of curved seafloor fabric by changes in rift propagation velocity and spreading rate application to the 95 5 w galapagos propagator the geometry of oceanic spreading centers can change by rift propagation in which a new ridge segment propagates and preempts a preexisting one as the locus of relative motion between the two plates shifts the dying and growing ridge segments are joined by a boundary usually a transform which migrates with time causing seafloor to be transferred from one plate to the other as a result originally ridge parallel isochrons in a zone between the growing and failed rifts can be reoriented to trends oblique to both the ridge and spreading direction an attractive feature of this model is that the reorientation occurs by strictly rigid plate tectonics since relative motion occurs only between points on different plates recent surveys of the galapagos propagating rift system at 95 w show seafloor fabric curving away from the growing rift and toward the failed rift these observations have been interpreted as requiring a zone of distributed simple shear between the two rifts such behavior would represent a local deviation from rigid plate tectonics since relative motion would be distributed over a finite region not part of either of the two plates here we explore possible rigid plate models for the formation of curved seafloor lineaments by rift propagation we find that such fabric can be generated by changes either in the velocity rate or direction of rift propagation or in the spreading rate during rift propagation the curvature of various tectonic elements is diagnostic of the different possibilities the geometry observed at the galapagos can result from either rift propagation acceleration or a spreading rate decrease during the last few hundred thousand years the reverse curvature could result from either deceleration of rift propagation or an increase in spreading rate propagator acceleration and spreading rate slowing which produce similar structural trends could in principle be distinguished using spreading rate data at galapagos however since either would have occurred within the present period of normal polarity both are consistent with the magnetic anomaly data we thus conclude that the data interpreted as requiring a shear zone are equally consistent with two distinct models based on rigid plate tectonics it may however be possible to discriminate between rigid and shear models using fabric observations at other propagators the shear model requires that the sense of curvature at galapagos be a ubiquitous feature of active propagators in contrast the rigid plate tectonic model allows for fabric with a variety of orientations including reverse curvature formed by either propagation deceleration or increasing spreading rate and linear fabric formed by constant propagation and spreading velocities

abrupt breakdown of the predictability of early season typhoon frequency at the beginning of the twenty first century maintenance of the anomalous anticyclone cyclone over the philippine sea following strong peak el nino la nina leads to a negative correlation between the pre winter october december el nino southern oscillation enso index and the tropical cyclone tc genesis frequency in the ensuing early tc season january june this negative lead lag correlation has established the predictability of the western north pacific wnp tc genesis frequency in the early season however little is known about whether this relationship remains steady to date especially in the context of evolving features of enso in recent decades here we show that the negative relationship experiences an abrupt breakdown around the early 2000s 1999 2006 which primarily occurs in the southeastern quadrant of the wnp during the breakdown period the anomalous anti cyclone shifts westward to the south china sea and an anomalous cyclonic circulation occupies the southeastern quadrant of the wnp causing enhanced tc formation we further reveal that the westward displacement of the wnp anticyclonic anomaly during 1999 2006 is induced by the westward shift of sea surface temperature sst and associated precipitation anomalies during the decaying phase of enso which increases cyclonic circulation and ascending motion while reduces vertical wind shear and thereby enhances tc formation over the southeastern wnp the results suggest that the frequent occurrence of decaying enso events with sst anomaly center over the central pacific cp accounts for the abrupt breakdown of predictability and the impacts of enso on tc formation in the early tc season are sensitive to the location of the maximum sst anomaly distinguishing eastern pacific ep and cp types of enso and or strong and weak enso events may help to improve the early season tc genesis forecast over the wnp the results also have important implications for understanding the future projection of tc changes under global warming scenarios

westward shift of western north pacific tropical cyclogenesis tropical cyclones tcs in the western north pacific or typhoons account for one third of all tcs in the world and the change of the mean tc genesis location can affect billions of people in pacific islands and asian countries the annual mean tc genesis longitude is generally controlled by the east west shift of the tropical upper tropospheric trough tutt a pronounced westward shift in the tutt is found in all of the available reanalysis data sets during 1979 2012 suppressing tc genesis in the eastern portion east of 145 e of the western north pacific basin due to the enhanced vertical wind shear associated with the tutt shift as a result the annual mean tc genesis longitude has significantly shifted westward since 1979 the westward shifting trends in the tutt and tc genesis are associated with the enhanced tropical tropospheric warming which is consistent with the response of the tropospheric temperature to global warming

microearthquake activity on the orozco fracture zone preliminary results from project rose we present preliminary hypocenter determinations for 52 earthquakes recorded by a large multi institutional network of ocean bottom seismometers and ocean bottom hydrophones in the orozco fracture zone in the eastern pacific during late february to mid march 1979 the network was deployed as part of the rivera ocean seismic experiment also known as project rose the orozco fracture zone is physiographically complex and the pattern of microearthquake hypocenters at least partly reflects this complexity all of the well located epicenters lie within the active transform fault segment of the fracture zone about half of the recorded earthquakes were aligned along a narrow trough that extends eastward from the northern rise crest intersection in the approximate direction of the cocos pacific relative plate motion these events appear to be characterized by strike slip faulting the second major group of activity occurred in the central portion of the transform fault the microearthquakes in this group do not display a preferred alignment parallel to the direction of spreading and several are not obviously associated with distinct topographic features hypocentral depth was well resolved for many of the earthquakes reported here nominal depths range from 0 to 17 km below the seafloor

rivera ocean seismic experiment rose overview the rivera ocean seismic experiment rose was designed as a combined sea and land seismic program to utilize both explosive and earthquakes to study a number of features of the structure and evolution of a mid ocean ridge a major oceanic fracture zone and the transition region between ocean and continent the primary region selected for the experiment included the rivera fracture zone the crest and eastern flank of the east pacific north of the rivera and adjacent areas of baja california and mainland mexico the experiment included 1 study of the east pacific rise south of the orozco fracture zone primarily using ocean bottom recording and explosive sources 2 a seismicity program at the orozco and 3 a land based program of recording natural events along the coastal region of mexico a considerable amount of useful data was obtained in each of the three subprograms in the marine parts of the experiment we were able to address a variety of problems including structure and evolution of young oceanic crust and mantle structure and dynamics of the east pacific rise seismicity of the orozco fracture zone and partitioning of energy transmission between the ocean volume and the crust lithosphere on land the fortuitousmore occurrence of the petatlan m7 6 earthquake of march 14 1979 permitted the acquisition of an excellent data set of foreshocks and aftershocks of this large event which provide new insight into the filling of a major seismic gap in the region this overview describes the scientific rationale and the design of the experiments along with some general results less

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

earthquakes associated with the back arc opening in the eastern bismarck sea activity mechanisms and tectonics abstract to make clear the precise locations of earthquakes and their tectonic implications for the active back arc extension in the eastern bismarck sea we deployed a network of ocean bottom seismometers obss for a period of 26 29 days in late 1983 in an area that is the intersection of a previously postulated transform fault zone and spreading center we located the hypocenters of 200 earthquakes with high resolution using arrival times for p and s waves at five or more obs stations and using the station corrections their focal depths were estimated to be generally in the western survey area area 1 a linear zone of obs located earthquakes trending n65 w defines the locus of the major transform fault to within a linear zone in addition south of the two major seismic zones mentioned above we identified two isolated spots with microearthquakes shallower than 10 km the composite focal plane solution of the isolated earthquakes is similar to those teleseismically obtained for shallow events south of the linear seismic segments in the western bismarck sea suggesting a systematic pattern of the intraplate stress regime of the bismarck plate we finally suggest that the isolated intraplate events partly reflect the fundamental tectonic origin of the extension in the bismarck sea

microearthquakes on the endeavour segment of the juan de fuca ridge 1 we report the results of a 55 day microearthquake experiment on the endeavour segment of juan de fuca ridge the network covered a 5 km section of the ridge axis centered on the main vent field and extended 15 km off axis on the west flank the ridge axis and flanks were seismically active and 1750 earthquakes were located with a minimum of five travel time picks including at least one s wave over half the earthquakes occurred in swarms and a waveform cross correlation technique was used to obtain relative locations on the western flank the hypocenters for four swarms at midcrustal depths are compatible with steeply dipping fault planes that strike at 035 050 n and oblique to the abyssal hills focal mechanisms determined from p wave first motions and p s amplitude ratios are predominantly strike slip with north south compression and appear to be affected by the reorganization of the explorer plate earthquakes beneath the ridge axis are concentrated in a band of intense seismicity at 1 5 3 5 km depth to the north of high rise vent field the seismicity defines a plane striking parallel to the ridge axis and dipping east at 70 and the earthquakes appear to extend beneath an axial reflector previously imaged at 2 3 km depth farther south the hypocenters are not compatible with a single fault plane focal mechanisms are characterized by subhorizontal tension axes oriented in all directions except parallel to the ridge and suggest a stress field that is about equally influenced by ridge spreading and hydrothermal cooling

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

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

a 6 year time series of hydrothermal plumes over the cleft segment of the juan de fuca ridge the causal links between magmatic tectonic processes and hydrothermal venting along the mid ocean ridge axis will be difficult to discern without broadband observations of the temporal variability of hydrothermal activity i report here on temporal variability in the intensity and distribution of chronic and event plumes generated by continuous and episodic hydrothermal discharge at two vent fields on the cleft segment of the juan de fuca ridge hydrothermal temperature and light attenuation c anomalies were comprehensively mapped between 1 and 4 times per year during nine cruises from 1986 to 1991 intensity of and c in the chronic plume overlying the north cleft vent field showed more variability between years than within a given year both and c were exceptionally high in 1986 fell to moderate levels during 1988 to 1990 and declined to minimum values in 1991 the chronic plume over the south cleft vent field varied similarly superimposed on the chronic plumes were brief and intense releases of hydrothermal fluid that created event plumes distinguishable by their symmetry and exceptional rise height event plumes at the north cleft field ranged from the 1986 megaplume with a rise height exceeding 1000 m to smaller events in 1987 and 1989 with rise heights of 400 m chronic hydrothermal heat 660 277 1 mw and mass 204 86 1 g s 1 fluxes from the north cleft field were determined by assuming these fluxes were equivalent to the long term net advection of a mean cross section of the vertically integrated and c in the plume this 6 year history of hydrothermal plumes indicates that episodic discharge has been an important component of the recent hydrothermal flux and that the aggregate heat and mass fluxes declined significantly though not consistently between 1986 and 1991

global positioning system measurements of deformations associated with the 1987 superstition hills earthquake evidence for conjugate faulting large station displacements observed from imperial valley global positioning system gps campaigns are attributed to the november 24 1987 superstition hills earthquake sequence thirty sites from a 42 station gps network established in 1986 have been reoccupied during 1988 and or 1990 displacements at three sites within 3 km of the surface rupture approach 0 5 m eight additional stations within 20 km of the seismic zone are displaced at least 10 cm this is the first occurrence of a large earthquake ms 6 6 within a preexisting gps network best fitting uniform slip models of rectangular dislocations in an elastic half space indicate 130 8 cm right lateral displacement along the northwest trending superstition hills fault and 30 10 cm left lateral displacement along the conjugate northeast trending elmore ranch fault the geodetic moments are 9 4 1025 dyn cm and 2 3 1025 dyn cm for the superstition hills and elmore ranch faults respectively consistent with teleseismic source parameters the data also suggest that postseismic slip along the superstition hills fault is concentrated at shallow depths distributed slip solutions using singular value decomposition indicate near uniform displacement along the elmore ranch fault and concentrated slip to the northwest and southeast along the superstition hills fault a significant component of nonseismic secular displacement is observed across the imperial valley which is attributed to interseismic plate boundary deformation

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

influence of climate regime shift on the interdecadal change in tropical cyclone activity over the pacific basin during the middle to late 1990s in this study a new interpretation is proposed for the abrupt decrease in tropical cyclone tc activity in the western north pacific wnp after the late 1990s we hypothesize that this abrupt change constitutes a part of the phenomenon of interdecadal change in tc activity in the pacific basin including the wnp western south pacific wsp and eastern north pacific our analysis revealed that the climate regime shift crs in the pacific during the middle to late 1990s resulted in a la nina like mean state which was responsible for the interdecadal change in tc activity in the late 1990s analyses of the tc genesis potential index and numerical experiments revealed that the decline in tc activity in both the wnp and wsp was primarily attributable to the increase of vertical wind shear in the central pacific due to the la nina like associated cold sea surface temperature sst conversely the la nina like associated warm sst in the western pacific produced anomalous vertical transport of water vapor increasing moisture levels in the mid troposphere and tc activity in the western wnp furthermore the crs modified the mean tc genesis position and shifted the steering flow to the west resulting in the increased frequency of tc landfalls in taiwan southeastern china and northern australia after the late 1990s

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

linear volcanoes along the pacific cocos plate boundary 9 n to the galapagos triple junction abstract a seabeam based reconnaissance of the 500 km of the east pacific rise crest between 7 n and 2 40 n shows that the axial ridge is segmented by four 4 13 km non transform offsets into an en echelon string of distinctively different linear volcanoes these axial volcanoes are oriented orthogonal to relative plate motion except where their overlapping ends veer 15 toward each other and where small intra volcano offsets of their crestal rift zones create abrupt kinks longitudinal gradients of the crestlines are less than 5 m km except where they plunge at rift zones overlapped ends and where they rise locally to small axial peaks transverse profiles vary from trapezoidal to triangular with a steep shield shaped cross section being most common conventional sounding data indicate that this pattern continues to the 140 km offset siqueiros transform fault system at 8 2 n within this fault system is a short spreadingcenter volcano contained in a rift valley that links two strike slip fault zones immediately to the north is the shallow 9 0 8 3 n axial volcano with unusual relief mapped by a deeply towed instrument package at the southern end of the plate boundary as the rise crest enters the region of the pacific cocos nazca triple junction the axial ridge narrows deepens and acquires a more irregular long profile south of 2 30 n the rise crest has a 15 km wide rift valley that contains multiple volcanic ridges with north south strikes structural hypotheses suggested or supported by these morphologic observations include a point source magma supply to the spreading center from mantle diapirs the along strike continuity of axial magma chambers on fast spreading rises even across small rift zone offsets and the importance of magma intrusion as well as eruption for building the axial ridge hypotheses inconsistent with the new data include magma supply and long distance dispersal from a few widely spaced plumes primary control of the topographic volcanic and tectonic characteristics of the rise crest by distance from transform faults and localization of triple junctions over major mantle upwellings

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

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

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

a seismic swarm and regional hydrothermal and hydrologic perturbations the northern endeavour segment february 2005 the february 2005 swarm at the overlapping spreading center osc on the northern end of the endeavour segment is the first swarm on the juan de fuca ridge recorded on a local seafloor seismic network the swarm included several larger earthquakes and caused triggered seismicity and a hydrothermal response in the endeavour vent fields as well as regional scale hydrologic pressure perturbations the spatial and temporal pattern of over 6000 earthquakes recorded during this seismic sequence is complex small magnitude events dominate and seismicity rates wax and wane indicating a magmatic process the main swarm initiates at the northern end of the endeavour ridge however most of the moment release including six strike slip events occurs in the southwest endeavour valley where the swarm epicenters generally migrate south the main swarm is contemporaneous with a hydrologic pressure response at four sealed seafloor boreholes 25 105 km away we infer that the seismic sequence is driven by a largely aseismic magma intrusion at the northern endeavour axis resulting stress changes trigger slip on tectonic faults and possibly dike propagation at the opposing limb of the endeavour osc in the southwest endeavour valley consistent with the eventual decapitation of the endeavour by the west valley segment furthermore 2 5 days after the start of the main swarm seismicity is triggered beneath the endeavour vent fields and temperature increases at a diffuse vent in the mothra field we infer that this delayed response is due to a hydrologic pressure pulse that diffuses away from the main magma intrusion

small scale models of oceanic transform zones previous studies about oceanic transform zones reveal major morphological and structural differences that cannot be explained by simple factors such as spreading rates or offsets in order to determine how kinematic and mechanic parameters control the deformation pattern we perform small scale modeling using sand and silicone putty as analog material of brittle and ductile layers of oceanic lithosphere respectively the three dimensional depth geometry of these layers follows the thermal structure of the ridge transform system the structure of the deformed transform zone is very sensitive to input parameters specially to the offset the obliquity of the deformed zone with the extension direction i e spreading direction in a natural system depends on the offset and on the brittle layer thickness for greater thickness or a shorter offset the transform domain widens and the components of extension become significant for smaller lithosphere thickness or larger offset the transform domain becomes narrow with steeper walls and mainly undergoes strike slip deformation such variability in possible controls on the offset length may help explain the structure complexity observed within the oceanic transform zones a comparison with 28 natural systems indicates the same relationship of the offset with the angle between the principal transform displacement zone ptdz and spreading direction a relationship between offset and width is clearly established for mid atlantic ridge mar transforms as on analog models a similar relationship is observed for the east pacific rise epr transforms but with greater variabilty this difference is interpreted as resulting either from greater complexity of transform zones on epr than on mar making it difficult to estimate some parameters from different mechanical behaviors between slow and fast spreading systems or from higher rheological complexity of natural systems than out model

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

lateral velocity variations in the orozco transform fault inferred from observed incident angles and azimuths of p waves summary body waves from earthquake and explosive sources which were recorded by a three component ocean bottom seismometer obs in the orozco transform fault show pronounced systematic deflections of incident angles and azimuths from those expected for a laterally homogeneous velocity structure after considering several phenomena which could affect the observations we conclude that the observed pattern of deflections in part reflects regional lateral velocity variations possible models explaining the observations are examined by tracing rays through three dimensional velocity structures a model in which the moho is elevated beneath a north south trending topographic high to the west of the obs can explain observations from sources to the north west and south and is compatible with seismic refraction results in the region east of the obs a low velocity region simulating a magma chamber results in very steep incident angles and large azimuthal deflections which qualitatively match those observed from earthquakes in the central transform region although these data alone do not provide irrefutable evidence for a magma chamber that the corrections to fault plane solutions and hypocentres implied by this model can explain major inconsistencies in the source parameters calculated relative to a laterally homogeneous velocity model supports its validity

seasonal modulations of different impacts of two types of enso events on tropical cyclone activity in the western north pacific the paper examines different impacts of eastern pacific warm cold epw epc and central pacific warm cold cpw cpc events on tropical cyclones tcs in the western north pacific wnp by considering the early season of april june amj the peak season of july september jas and the late season of october december ond during amj epw epc is associated with a significant increase of the tc genesis number in the southeastern southwestern sub region of the wnp but no class of el nino southern oscillation enso events shows a significant change in the tc lifetime and intensity during jas epw corresponds to an increase decrease of the tc genesis number in the southeastern northwestern sub region but cpw shows no significant change epc increases the tc genesis in the northwestern and northeastern sub regions and decreases the genesis in the southwestern sub region whereas cpc suppresses the genesis in the southeastern sub region both the lifetime and intensity of tcs are increased in epw but only a shortened lifetime is seen for cpc during ond epw reduces the tc genesis in the southwestern and northwestern sub regions whereas cpw enhances the genesis in the southeastern sub region over the south china sea cpw and cpc show a significant decrease and increase of the tc genesis respectively the tc lifetime is significantly longer in both epw and cpw and shorter in epc and tcs tend to be more less intense in epw cpc all of these variations are consistent with the development of enso related sst anomalies during different seasons and are supported by distributions of the genesis potential index a combination of large scale oceanic and atmospheric factors that affect tc activity tcs in the wnp mainly take the straight westward northwestward and recurving tracks during amj of epw years the tc steering flow patterns favor the recurving track and suppress the straight westward and northwestward tracks during jas epw is associated with the steering flows that are unfavorable for tcs to move northwestward or westward whereas cpw favors the northwestward track and suppresses the straight westward track the steering flow patterns during ond are similar to those during jas except that epc may increase the possibility of the northwestward track

axial seamount periodic tidal loading reveals stress dependence of the earthquake size distribution b value abstract earthquake size frequency distributions commonly follow a power law with the b value often used to quantify the relative proportion of small and large events laboratory experiments have found that the b value of microfractures decreases with increasing stress studies have inferred that this relationship also holds for earthquakes based on observations of earthquake b values varying systematically with faulting style depth and for subduction zone earthquakes plate age however these studies are limited by small sample sizes despite aggregating events over large regions which precludes the ability to control for other variables that might also affect earthquake b values such as rock heterogeneity and fault roughness our natural experiment in a unique seafloor laboratory on axial seamount involves analyzing the size frequency distribution of 60 000 microearthquakes which delineate a ring fault system in a 25 km3 block of crust that experiences periodic tidal loading of 20 kpa we find that above a threshold stress amplitude b value is inversely correlated with tidal stress the earthquake b value varies by 0 09 per kpa change in coulomb stress our results support the potential use of b values to estimate small stress variations in the earth s crust

termination of a 6 year ridge spreading event observed using a seafloor seismic network on the endeavour segment juan de fuca ridge 1 we present automatically determined epicenters and magnitudes for 36 523 earthquakes recorded along the endeavour segment between august 2003 and october 2006 using a local ocean bottom seismometer obs network the catalog is dominated by two swarm sequences in january and february 2005 in the vicinity of the endeavour overlapping spreading center which included earthquakes in west valley the northern portion of the endeavour segment southwest endeavour valley and the endeavour vent fields these swarms are attributed to volcanism including a dike intrusion on the northern endeavour in february 2005 and smaller diking events on the propagating tip of the west valley segment in both swarms the dike on the northern endeavour propagated to the south which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment following the swarms seismic activity on the endeavour segment decreased on average to 15 of pre swarm values and almost ceased at the segment ends we infer that a 6 year non eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate spreading stresses the inferred coupling between the 1999 and 2005 events the observation of extensive precursory activity prior to the 2005 swarms and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation hydraulic diffusion and magma withdrawal and replenishment

seismic cycles and earthquake predictability on east pacific rise transform faults the concept of a seismic cycle where the stress on a fault repeatedly builds up over a long period of time and then is rapidly released in a large earthquake influences studies of both the basic physics of faulting and applied research aimed at estimating earthquake hazards this hypothesis suggests that large earthquakes might be quasi periodic and that the probability of a particular portion of a fault rupturing twice in quick succession should be low however this basic hypothesis has been difficult to verify owing to the long repeat times of the largest earthquakes on most faults east pacific rise epr transform faults are an advantageous location to evaluate the seismic cycle hypothesis owing to their fast slip rates and the moderate size m w 6 of their largest earthquakes using surface wave based determinations of the relative separations between earthquake centroids i document 16 pairs of m w 5 5 events that had overlapping ruptures the distribution of interevent times for these pairs is tightly clustered around 5 yr with a coefficient of variation 0 2 indicating that quasi periodicity may be prevalent for the largest events on these faults moreover i find no pairs of overlapping m w 5 5 6 2 earthquakes separated by less than 50 cm of elapsed plate motion indicating that the two basic features of the seismic cycle hypothesis are evident in the timing of large epr transform mainshocks i have also confirmed earlier results demonstrating a high degree of short term predictability of epr mainshocks by combining teleseismic and hydroacoustic earthquake catalogs thus there appears to be a high degree of both short and long term predictability on epr transforms

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

variations of mid pacific trough and their relations to the asian pacific north american climate roles of tropical sea surface temperature and arctic sea ice abstractthe mid pacific trough mpt occurring in the upper troposphere during boreal summer acts as an atmospheric bridge connecting the climate variations over asia the pacific and north america the first second mode of empirical orthogonal function analysis of the mpt which accounts for 20 3 13 4 of the total variance reflects a change in its intensity on the southwestern northeastern portion of the trough both modes are significantly correlated with the variability of tropical pacific sea surface temperature sst moreover the first mode is affected by atlantic sst via planetary waves that originate from the north atlantic and propagate eastward across the eurasian continent and the second mode is influenced by the arctic sea ice near the bering strait by triggering an equatorward wave train over the northeast pacific a stronger mpt shown in the first mode is significantly linked to drier and warmer conditions in the yangtze river basin southern japan and the northern united state

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

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

morphotectonic fabric of the orozco transform fault results from a sea beam investigation we have used data collected with the sea beam system to produce a detailed regional bathymetric map of the medium to fast slipping 90 mm yr orozco transform fault the morphotectonic fabric of the 90 50 km transform domain is spatially heterogeneous and is composed of relatively shallow areas 3200 m the plate boundary geometry is clearly identified morphologically and is characterized by a series of linked basins and troughs that approximate a zed pattern in plan view east west trending basins define the northern and southern boundaries of the transform domain and these troughs are thought to mark the location of strike slip tectonism a central north south trending ridge with an axial valley called here the mid orozco spreading center is thought to represent a short 30 km ridge segment that links two transform faults morphotectonic relationships developed along the southeastern flank of the survey area suggest that this plate boundary geometry has recently become unstable and is presently undergoing a transformation as the tip of the east pacific rise axis propagates northward across the transform domain bypassing the mid orozco spreading center if the southern ridge axis continues to propagate northward it will link with the aseismic extension of the northern strike slip strand and the present day plate boundary will evolve into a simple ridge transform ridge r t r geometry with an offset of approximately 70 km

sr nd and pb isotopic and chemical compositions of central deccan traps lavas and relation to southwestern deccan stratigraphy abstract sr nd and pb isotopic compositions and major and trace element abundances of lavas in five central deccan traps sections are generally very similar to those of lavas in the southwestern deccan western ghats escarpment the combination of strong isotopic and chemical similarities between lavas of the two regions indicates that they shared a closely similar petrogenesis our results indicate that unlike many lavas in the northeastern deccan most of the lavas studied in the central deccan sections can be grouped into different stratigraphic members and chemical types cts belonging to four formations thakurvadi bhimashankar khandala and poladpur present in the type sections of the western ghats bhimashankar and thakurvadi type lavas are found in a section near outram whereas only khandala type lavas make up the mhaishmal ellora and ajanta sections to the east and only poladpur type lavas are present in the easternmost section at lonar this west east sequence is the same as that seen going upward in the western ghats stratigraphy except that bushe type lavas which are located stratigraphically between the khandala and poladpur formations in the western ghats have not been found in this study overall our results indicate that the khandala and poladpur formations extend over west east distances of more than 300 km and some individual members e g the dhak dongar may extend over as much as 500 km the regional dip from igatpuri in the southwest to lonar in the east central deccan is less than 0 5 to the east which is similar to the southward regional dip in the southwestern deccan near horizontal flows in the central and southwestern deccan areas suggest that post deccan tectonic activity has had limited effects on the studied area large volumes of basaltic lavas derived from eruptive centers in the western deccan may have covered both the central and southwestern deccan areas

seismic constraints on caldera dynamics from the 2015 axial seamount eruption seismic observations in volcanically active calderas are challenging a new cabled observatory atop axial seamount on the juan de fuca ridge allows unprecedented real time monitoring of a submarine caldera beginning on 24 april 2015 the seismic network captured an eruption that culminated in explosive acoustic signals where lava erupted on the seafloor extensive seismic activity preceding the eruption shows that inflation is accommodated by the reactivation of an outward dipping caldera ring fault with strong tidal triggering indicating a critically stressed system the ring fault accommodated deflation during the eruption and provided a pathway for a dike that propagated south and north beneath the caldera s east wall once north of the caldera the eruption stepped westward and a dike propagated along the extensional north rift

concurrent changes to hadley circulation and the meridional distribution of tropical cyclones abstractpoleward trends in seasonal mean latitudes of tropical cyclones tcs have been identified in direct observations from 1980 to the present paleoclimate reconstructions also indicate poleward equatorward migrations over centennial millennial time scales hadley circulation hc is often both implicitly and explicitly invoked to provide dynamical linkages to these shifts although no direct analysis of concurrent changes in the recent period has been presented here the observational tc record 1981 2016 and era interim jra 55 and merra 2 are studied to examine potential relationships between the two a zonally asymmetric hc is defined by employing helmholtz theory for vector decomposition and this permits the derivation of novel hc diagnostics local to tc basins coherent variations in both long term linear trends and detrended interannual variability are found tc genesis and lifetime maximum intensity latitudes share trend sign and magnitude with shifts in local hc extent with rates being a

interannual shift of the tropical upper tropospheric trough and its influence on tropical cyclone formation over the western north pacific abstractthe east west migration of the tropical upper tropospheric trough tutt on the interannual time scale and its influence on tropical cyclone tc formation over the western north pacific wnp is investigated in this study climatologically the tutt can be identified from 100 to 400 hpa with a relative vorticity maximum between 150 and 200 hpa in addition to the strong westerly vertical wind shear in the south flank of the tutt this study shows that the cold core system is associated with low relative humidity and subsidence to the east of the trough axis as a result the tc formation is enhanced suppressed in the eastern portion of the wnp when the tutt shifts eastward westward on the interannual time scale the interannual tutt shift is closely associated with the sst anomalies in the central and eastern tropical pacific or enso phases the warm cold phase of enso corresponds to the eastward westward shift of the tutt the physical factors found to be responsible for the influence of

modulation of western north pacific tropical cyclone activity by the atlantic meridional mode this study examines the year to year modulation of the western north pacific wnp tropical cyclones tc activity by the atlantic meridional mode amm using both observations and the geophysical fluid dynamics laboratory forecast oriented low ocean resolution version of cm2 5 flor global coupled model 1 the positive negative amm phase suppresses enhances wnp tc activity in observations the anomalous occurrence of wnp tcs results mainly from changes in tc genesis in the southeastern part of the wnp 2 the observed responses of wnp tc activity to the amm are connected to the anomalous zonal vertical wind shear zvws caused by amm induced changes to the walker circulation during the positive amm phase the warming in the north atlantic induces strong descending flow in the tropical eastern and central pacific which intensifies the walker cell in the wnp the intensified walker cell is responsible for the suppressed enhanced tc genesis in the eastern western part of the wnp by strengthening weakening zvws 3 the observed wnptc amm linkage is examined by the long term control and idealized perturbations experiment with flor fa a suite of sensitivity experiments strongly corroborate the observed wnptc amm linkage and underlying physical mechanisms

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

crustal structure in the orozco transform zone seismic refraction data from lines of small shots and from individual large shots recorded during project rose are used to determine the crust and upper mantle velocity structure in the orozco transform zone the objective of this study is to provide constraints on the origin of a deep north south trending trough flanked by two shallow ridges these topographic features are incompatible with the present plate tectonic framework as defined by the regional spreading direction approximately east west and did not display any seismic activity during project rose travel times and amplitudes from reversed profile along one of the ridges reveal an anomalous crustal structure a velocity increase approximating a gradient of 0 5 s 1 extending through most of the crust and decreasing to 0 1 s 1 at the base and a total crustal thickness of about 5 4 km in contrast three profiles from topographically normal oceanic crust indicate a crustal structure typical of oceanic crust throughout the oceans a 1 5 km thick layer in which velocity increases from about 3 km s 1 to 6 7 km s 1 overlying a 4 5 km thick layer with a velocity of about 6 8 km s 1 the upper mantle velocity beneath both structures determined from travel times from large shots at ranges of 30 100 km is about 8 1 km s 1 these results are interpreted to indicate that the trough and flanking ridges represent an extinct spreading center that developed an axial valley as the spreading center progressively shifted to the present axis of the east pacific rise

influence of track changes on the poleward shift of lmi location of western north pacific tropical cyclones abstractthe annual mean latitude at which tropical cyclones tcs reach their lifetime maximum intensity lmi over the western north pacific ocean basin has shifted northward since the early 1980s

contribution of the interdecadal pacific oscillation to the recent abrupt decrease in tropical cyclone genesis frequency over the western north pacific since 1998 abstractprevious studies have documented an abrupt decrease of tropical cyclone tc genesis frequency over the western north pacific wnp since 1998 in this study results from an objective clus

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

seismicity at intersections of spreading centers and transform faults ocean bottom seismographs were used in a microearthquake monitoring experiment at the eastern junction of the oceanographer transform with the mid atlantic ridge at 35 n microearthquake activity at the junction occurred over a broad area 7 km these microearthquakes cut across the corner between the transform and median valley and are associated with fault scarps that form the inner walls on the west and north sides of the median and transform valleys at intersections of other major fracture zones 100 km offset and slow to moderate spreading centers microearthquake activity is also diffuse and cuts across the corner between the spreading center and transform fault the narrow zone of decoupling 1 km observed between spreading center and transform boundaries by detailed geological studies at the tamayo east pacific rise and vema mid atlantic ridge intersections suggest that the diffuse seismicity 20 to 30 km in width does not reflect a diffuse plate boundary at the transition from rift to transform valley instead the faulting probably reflects internal deformation of the corner by secondary faults off of the plate boundary

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