aftershock decay productivity and stress rates in hawaii indicators of temperature and stress from magma sources 1 we examined dozens of aftershock sequences in hawaii in terms of gutenberg richter and modified omori law parameters we studied p the rate of aftershock decay ap the aftershock productivity defined as the observed divided by the expected number of aftershocks and c the time delay when aftershock rates begin to fall we found that for earthquakes shallower than 20 km p values 1 2 are near active magma centers we associate this high decay rate with higher temperatures and faster stress relaxation near magma reservoirs deep earthquakes near kilauea s inferred magma transport path show a range of p values suggesting the absence of a large deep magma reservoir aftershock productivity is 4 0 for flank earthquakes known to be triggered by intrusions but is normal 0 25 to 4 0 for isolated main shocks we infer that continuing post main shock stress from the intrusion adds to the main shock s stress step and causes higher ap high ap in other zones suggests less obvious intrusions and pulsing magma pressure near kilauea s feeding conduit we calculate stress rates and stress rate changes from pre main shock and aftershock rates stress rate increased after many intrusions but decreased after large m7 8 earthquakes stress rates are highest in the seismically active volcano flanks and lowest in areas far from volcanic centers we found sequences triggered by intrusions tend to have high ap high 0 10 day c values a stress rate increase and sometimes a peak in aftershock rate hours after the main shock we interpret these values as indicating continuing intrusive stress after the main shock

near axis seamount distribution and its relationship with the segmentation of the east pacific rise and northern pacific antarctic ridge 17 n 56 s abstract we used satellite derived gravity anomaly maps and bathymetry data to analyze the distribution of off axis seamounts on young crust on the flanks of the east pacific rise epr and northern pacific antarctic ridge par from 17 n to 56 s latitude we observed large scale variations in the distribution of the volcanoes which we attribute to variations of the regional magmatic budget of the underlying mantle two distinct settings for off axis volcanism were observed at the scale of second order segments the first one favors volcano growth near first or second order discontinuities the second type occurs preferentially near the centers of second order segments we infer that the distribution of seamounts is controlled both by the availability of melt under the flanks of the ridge and by the vulnerability of the young lithosphere near the northern epr and in some places near the par the off axis volcanism results mostly from fracturing of the crust near axial discontinuities probably due to the thermomechanical stresses that develop in the cooling lithosphere in the southern epr especially in the 14 19 s area the young lithosphere is probably much more vulnerable to off axis volcanism the seamount distribution seems to reflect variations in the off axis melt production at the scale of second order segments 150 km we infer that the difference in volcanic setting close to segment ends or close to segment centers between the northern and southern epr results from the difference in spreading rate and consequently the variation in rheology of the young lithosphere

the distribution of intraplate volcanism in the pacific ocean basin a spectral approach summary although the major linear island chains in the pacific ocean basin have been extensively studied relatively little attention has been given to the origin of the numerous seamounts and oceanic islands that are concentrated in the western pacific spectral analysis of shipboard gravity and bathymetry data provides a means for statistically estimating the distribution of this volcanism in space and time estimates of the power spectrum of the bathymetry fb and the average gravitational admittance z cross spectrum correlation of gravity and bathymetry were obtained as functions of age of the oceanic lithosphere by dividing the pacific plate into eight age regions a h the power spectrum of the bathymetry is indicative of the overall bathymetric roughness primarily volcanic in each region this roughness shows three gradations regions a d 140myr with many large volcanic features are very rough the average gravitational admittance is indicative of the degree of isostatic compensation of bathymetric features in each region and can be used to estimate the proportionh of total bathymetric power that was emplaced on relatively old lithosphere and the proportion that formed on young lithosphere at or near a mid ocean ridge crest the highest proportion of bathymetry that formed on old lithosphere occurs in region h fi 0 90 and the lowest proportion occurs in regions g and e fi 0 40 0 60 the power spectrum of the bathymetry and the gravitational admittance allow an estimate of the total bathymetric power pi ipb accounted for by intraplate volcanism intraplate power and the power pr 1 fi pb which is of on ridge origin ridge power at long wavelengths in regions e g the amount of on ridge volcanism exceeds the amount of intraplate volcanism pr pi indicating the presence of several large plateaus and rises which formed at ridge crests this suggests the

nature of and controls on volcanism in the ca 2 32 2 06ga pretoria group transvaal supergroup kaapvaal craton south africa abstract the time of deposition of the pretoria group between 2 32 and 2 06 ga on south africa s kaapvaal craton was characterized by the first major increase in atmospheric oxygen it was accompanied by the extrusion of significant thicknesses of volcanic deposits namely the bushy bend lavas of the timeball hill formation the hekpoort formation and the machadodorp volcanic member of the silverton formation marking the three major volcanic events within the transvaal supergroup which are thought to be precursors to the succeeding bushveld complex magmatism the bushy bend lava member of the timeball hill formation is characterized by subaqueous basaltic andesitic fissure eruptions with fumarolic activity and probably minor subaerial explosive eruptions in the hinterland of the transvaal supergroup basin the subaerial volcanism of the hekpoort formation appears to have been dominated by fissure eruptions with a preponderance of lava flows and locally important pyroclastic material intermittent hiatuses in volcanism were marked by local lacustrine shale deposition finally the machadodorp lava member can be interpreted as the deposits of several seamounts aligned along a fissure probably within an extensional environment the geodynamic control inferred for the evolution of the pretoria group basin encompasses two cycles of prerift uplift subsequent mechanical rifting and long lived thermal subsidence the limited extent of the bushy bend lavas in the south of the pretoria group depository attests to the likelihood that volcanism accompanying the first rifting event was short lived in contrast the second rifting cycle postulated for the pretoria group basin has a strong association with widespread and large scale volcanism of the hekpoort ongeluk flood basalts which may have been plume related the scale of the second cycle volcanism and its importance in influencing the upper part of the pretoria group basin fill is underlined by the machadodorp volcanism probably related to hot spot volcanism as the ongeluk hekpoort plume waned

on geoid heights and flexure of the lithosphere at seamounts the sea surface height has now been mapped to an accuracy of better than 1 m by using radar altimeters on board orbiting satellites the major influence on the mean sea surface height is the marine geoid which is an equipotential surface we have carried out preliminary studies of how oceanic volcanoes which rise above the ocean floor as isolated seamounts and oceanic islands or linear ridges contribute to the marine geoid simple one and two dimensional models have been constructed in which it is assumed that the oceanic lithosphere responds to volcanic loads as a thin elastic plate overlying a weak fluid substratum previous studies based on gravity and bathymetry data and uplift subsidence patterns show that the effective flexural rigidity of oceanic lithosphere and the equivalent elastic thickness te increase with the age of the lithosphere at the time of loading the models predict that isolated seamounts emplaced on relatively young lithosphere on or near a mid ocean ridge crest will be associated with relatively low amplitude geoid anomalies about 0 4 0 5 m km of height while seamounts formed on relatively old lithosphere on ridge flanks will be associated with much higher amplitude anomalies 1 4 1 5 m km studies of the seasat altimetric geoid prepared by nasa s jet propulsion laboratory support these model predictions geoid amplitudes are relatively low over the mid pacific mountains and line islands which formed on or near a mid ocean ridge crest and relatively high over the magellan seamounts and wake guyots which formed off ridge direct modeling of the altimetric geoid over these features is complicated however by the wide spacing of the satellite tracks which can exceed 100 km and poor bathymetric control beneath individual satellite tracks in regions where multibeam bathymetric surveys are available models can be constructed that fit the altimetric geoid to better than 1 m studies of geoid anomalies over the emperor seamount chain for example suggest that these seamounts formed on 20 30 ma old oceanic lithosphere while anomalies over the louisville ridge and the hawaiian ridge suggest that these seamounts formed on lithosphere older than about 80 ma this indicates that geoid anomalies over bathymetrie features depend not only on the crustal structure prior to loading and the overall shape and density of the features but also on their tectonic setting we examine the implications of these results to studies which attempt 1 to separate temporal variations of the sea surface due to oceanographic effects 2 to predict seamounts directly from satellite derived gravity and geoid data and 3 to isolate the geoid effects of deep processes occurring beneath the lithosphere such as those due to mantle convection

an estimate of the absolute stress tensor in kaoiki hawaii the stress tensor orientation was calculated by inversion of 81 fault plane solutions of m 3 5 0 6 earthquakes located in an area 6 by 16 km at depths of 7 2 km this crustal volume is situated on a straight line between the summits of the active volcanoes kilauea and mauna loa the orientation of the greatest principal stress was found to be near horizontal and in the line connecting the two volcanoes this is further evidence supporting the model that magmatic expansion within the feeding conduits of these volcanoes is the source of stress that causes earthquakes in southern hawaii these earthquakes are tectonic earthquakes in the sense that they occur in a brittle elastic crust at distances of a few to several tens of kilometers from the volcanoes and that they are not directly associated with the opening of cracks by intrusions we propose that this model leads to the corollary that the shear stresses responsible for earthquake failure in the shallow hawaiian crust are approximately 3 2 mpa and that the pore pressure of ground water p in the hypocentral volume must be under near lithostatic pressure this result is obtained by arguing that the greatest principal stress 1 is equal to the magma pressure in the volcanic feeder pipe but cannot exceed the least principal stress 3 by more than the tensile strength of the crust because otherwise hydrofracture would occur opening a crack against the least principal stress given the difference between the greatest and least principal stress the fact that the overburden at the 7 km hypocentral depth equals the intermediate principal stress and the ratio r 1 2 1 3 0 4 which is obtained in the inversion of fault plane solutions we estimate that 1 202 mpa 2 200 mpa 3 196 mpa 3 mpa and n 199 mpa with uncertainties on the order of several mpa it follows that for any coefficients of friction larger than 0 15 the pore fluid pressure necessary for faulting on the observed fault planes has to be larger than 90 of the lithostatic pressure these considerations suggest that many earthquakes represent fault ruptures under low ambient shear stresses in the order of the average observed stress drop of 3 mpa

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

focused magma supply at the intersection of the cobb hotspot and the juan de fuca ridge 1 we examine the interplay between the cobb hotspot and the juan de fuca ridge by mapping the base of the crust at their intersection near axial volcano pmp traveltimes from an extensive active source seismic experiment are used in conjunction with prior crustal tomography to map crustal thickness a cross section along the ridge axis consists of two distinct trends in addition to 1 2 km of excess crust inferred from gravity along a 100 km section of the central ridge we find a narrow 20 40 km diameter root extending to 11 km directly beneath the volcano focused magma flux from the cobb hotspot 0 3 0 8 m3 s is inferred to be the cause of both the narrow root and the large magma reservoir at axial the rapid return to thinner crust away from the intersection marks a transition back to ridge dominated magmatism

shear strength of the great pacific fracture zones shipboard bathymetry and gravity data from 30 crossings of 6 great pacific fracture zones fzs the mendocino murray molokai clarion clipperton and udintsev are compared with the predictions of a model in which fzs are locked beyond the ridge transform intersection such that no vertical motion occurs on the fault in response to differential thermal subsidence at least some sections of all of these fzs except the molokai are consistent with this model and sustain shear stresses as high as 20 mpa however none of the fzs is locked along its entire length as inferred from observed shear stresses dropping below 75 of the value necessary to maintain a locked fault there is some suggestion that the unlocking may be related to excess volcanism

on the accuracy of digital bathymetric data the global 5 arcmin gridded topography data etopo 5 are based on contour maps rather than original soundings and have large errors the artificial statistical distribution generated by digitizing contours makes these data unsuitable for use in regression models for depth age variations their amplitude spectrum is bounded by a frequency 4 power law so that they should not be used when the gravity topography transfer function is important such as in studies of flexural isostasy and mantle convection i assess the accuracy of 14 491 069 digital ship soundings in 2253 cruise surveys collected between 1955 and 1992 in the lamont doherty earth observatory on line data base by analyzing 329 058 crossover errors coes at intersecting ship tracks five percent of cruises with internal coes yield root mean square coe amplitudes exceeding 500 m all of these have errors in digitizing two way travel time from analog precision depth recorder traces twenty eight cruises were found which had errors caused by misinterpretation of the nominal sound velocity used when travel times were reported as nominal depths two nominal sound velocities in common use differ by 2 5 an amount which is often undetectable producing uncertainties in depth of this magnitude ship data have been acquired at different rates over time with the peak of activity in the early 1970s although present technologies can yield very accurate data these are acquired at a rate which is small with respect to the total available data the cumulative median global coe has remained constant at 26 m since the late 1970s most recent data acquisition has been in the northern hemisphere oceans and the oldest and least accurate data are in the southern oceans where median coes are 100 250 m the majority of the data in the south pacific were acquired before the advent of satellite navigation

predicting bathymetry from geosat erm and shipborne profiles in the south atlantic ocean abstract portions of two geosat erm altimeter tracks and corresponding suborbital shipboard gravity and bathymetry profiles in the south atlantic ocean were analyzed one across the walvis ridge about 1100 km long and the other in the brazil basin about 2300 km long together these profiles sample those types of sea floor topography which dominate the gravity signature at wavelengths of 20 to 300 km the walvis ridge is a massive aseismic ridge and the brazil basin profile crosses both an old seamount emplaced at the time the crust was young and a very young mid plate volcano both profiles cross fracture zones after the gravity and bathymetry profiles were split into subprofiles various cross spectral characteristics could be determined by fft techniques analysis showed that observed admittance is not well constrained by either an airy type or flexural compensation models across the walvis ridge but those over the brazil basin can be readily explained by an airy type model with a mean crustal thickness of about 20 km a theoretical filter was then designed based on a priori geological knowledge and used to predict bathymetry from the high passed gravity geoid anomalies not surprisingly the predicted bathymetry shows more detailed and correct short wavelength 20 300 km features than those predicted from the historical data base as represented e g by the dbdb5 gridded bathymetric model for areas where historical shipboard bathymetry measurements are widely spaced longer than about 10 km for single beam data but where some regional geologic information is available such as the relative ages of mid plate volcanoes and crust bathymetry predicted from altimetric data can be used to upgrade regional bathymetrie data bases on which regional geologic geophysical understanding depends

a geometric technique for relocating hotspots and refining absolute plate motions an age independent geometric relationship is presented that links hotspots to the seamounts which they produce and so permits the use of undated seamounts to refine the motion of tectonic plates this technique has the potential to rigorously assess hotspot fixity and to locate extinct hotspots the present application of this method points to a recent change in pacific plate motion and suggests a relocation of the louisville hotspot to the hollister ridge south of the eltanin fracture zone

seismic hazard in hawaii high rate of large earthquakes and probabilistic ground motion maps the seismic hazard and earthquake occurrence rates in hawaii are locally as high as that near the most hazardous faults elsewhere in the united states we have generated maps of peak ground acceleration pga and spectral acceleration sa at 0 2 0 3 and 1 0 sec 5 critical damping at 2 and 10 exceedance probabilities in 50 years the highest hazard is on the south side of hawaii island as indicated by the m i 7 0 m s 7 2 and m i 7 9 earthquakes which occurred there since 1868 probabilistic values of horizontal pga 2 in 50 years on hawaii s south coast exceed 1 75 g because some large earthquake aftershock zones and the geometry of flank blocks slipping on subhorizontal decollement faults are known we use a combination of spatially uniform sources in active flank blocks and smoothed seismicity in other areas to model seismicity rates of earthquakes are derived from magnitude distributions of the modern 1959 1997 catalog of the hawaiian volcano observatory s seismic network supplemented by the historic 1868 1959 catalog modern magnitudes are m l measured on a wood anderson seismograph or m s historic magnitudes may add m l measured on a milne shaw or bosch omori seismograph or m i derived from calibrated areas of mm intensities active flank areas which by far account for the highest hazard are characterized by distributions with b slopes of about 1 0 below m 5 0 and about 0 6 above m 5 0 the kinked distribution means that large earthquake rates would be grossly underestimated by extrapolating small earthquake rates and that longer catalogs are essential for estimating or verifying the rates of large earthquakes flank earthquakes thus follow a semicharacteristic model which is a combination of background seismicity and an excess number of large earthquakes flank earthquakes are geometrically confined to rupture zones on the volcano flanks by barriers such as rift zones and the seaward edge of the volcano which may be expressed by a magnitude distribution similar to that including characteristic earthquakes the island chain northwest of hawaii island is seismically and volcanically much less active we model its seismic hazard with a combination of a linearly decaying ramp fit to the cataloged seismicity and spatially smoothed seismicity with a smoothing half width of 10 km we use a combination of up to four attenuation relations for each map because for either pga or sa there is no single relation that represents ground motion for all distance and magnitude ranges great slumps and landslides visible on the ocean floor correspond to catastrophes with effective energy magnitudes m e above 8 0 a crude estimate of their frequency suggests that the probabilistic earthquake hazard is at least an order of magnitude higher for flank earthquakes than that from submarine slumps

seamount statistics in the pacific ocean we apply the wide beam sampling technique of jordan et al 1983 to approximately 157 000 km of wide beam profiles to obtain seamount population statistics for eight regions in the eastern and southern pacific ocean population statistics derived from wide beam echograms are compared with seamount counts from sea beam swaths and with counts from bathymetric maps we find that the average number of seamounts with summit heights h h is well approximated by the exponential frequency size distribution h oe h the exponential model for seamount sizes characterized by the single scale parameter 1 is found to be superior to a power law self similar model which has no intrinsic scale in describing the average distribution of pacific seamounts and it appears to be valid over a size spectrum spanning 5 orders of magnitude in abundance large scale regional variations in seamount populations are documented we observe significant differences in seamount densities across the murray fracture zone in the north pacific and the eltanin fracture zone system in the south pacific the eltanin discontinuity is equally evident on both sides of the pacific antarctic ridge in the south pacific regions symmetrically disposed about the ridge axis have very similar seamount densities despite the large difference between pacific plate and antarctic plate absolute velocities evidently any differences in the shear flows at the base of the pacific and antarctic plates do not affect seamount emplacement systematic variations in o and are observed as a function of lithospheric age with the number of large seamounts increasing more rapidly than small seamounts these observations have been used to develop a simple model for seamount production under the assumptions that 1 an exponential size frequency distribution is maintained 2 production is steady state and 3 most small seamounts are formed on or near the ridge axis the limited data available from this study appear to be consistent with the model but they are insufficient to provide a rigorous test of the assumptions or determine accurately the model parameters however the data from the south pacific indicate that the off axis production of large seamounts probably accounts for the majority of seamounts with summit heights greater than 1000 m

seafloor tectonic fabric from satellite altimetry abstract ocean floor structures with horizontal scales of 10 to a few hundred kilometers and vertical scales of 100 m or more generate sea surface gravity anomalies observable with satellite altimetry prior to 1990 altimeter data resolved only tectonic lineaments some seamounts and some aspects of mid ocean ridge structure new altimeter data available since mid 1995 resolve 10 km scale structures over nearly all the world s oceans these data are the basis of new global bathymetric maps and have been interpreted as exhibiting complexities in the sea floor spreading process including ridge jumps propagating rifts and variations in magma supply this chapter reviews the satellite altimetry technique and its resolution of tectonic structures gives examples of intriguing tectonic phenomena and shows that structures as small as abyssal hills are partially resolved a new result obtained here is that the amplitude of the fine scale 10 80 km roughness of old ocean floor is spreading rate dependent i

resolution of the scripps noaa marine gravity field from satellite altimetry the july 1995 declassification of the entire geosat gm satellite altimeter data set enabled a joint scripps noaa effort to compute a new version 7 2 marine gravity field on a 2 minute grid this gravity field covers the world oceans between 72 n and 72 s and is derived from a combination of ers 1 and geosat gm and erm data an earlier noaa geosat only gravity field solution was confined to the southern latitudes because the 1992 declassification was limited to gm data south of 30 s a simple coherence analysis between accurately navigated ship gravity profiles and comparable gravity profiles obtained from the gravity grids reveals that the scripps noaa gravity field is coherent with ship gravity down to 23 30 km this slight increase in resolution over the previous noaa geosat only gravity field short wavelength resolution of 26 30 km implies that the increased spatial coverage provided by the ers i altimeter when combined with geosat improves the solution coherence analyses between satellite gravity and ship topography and ship gravity and ship topography show that even shorter wavelength gravity anomalies 13 km are present in sea surface measurements made by ship even so the scripps noaa marine gravity field does an excellent job of resolving most of the short wavelength gravity anomalies covering the world oceans

new global seamount census from altimetry derived gravity data summary recent revisions to the satellite derived vertical gravity gradient vgg data reveal more detail of the ocean bottom and have allowed us to develop a non linear inversion method to detect seamounts in vgg data we approximate vgg anomalies over seamounts as sums of individual partially overlapping elliptical polynomial functions which allows us to form a non linear inverse problem by fitting the polynomial model to the observations model parameters for a potential seamount include geographical location peak vgg amplitude major and minor axes of the elliptical base and the azimuth of the major axis the non linear inversion is very sensitive to the initial values for the location and amplitude hence they are constrained by the centre and amplitude of the uppermost contours obtained with a 1 eotvos contour interval with these initial conditions from contouring we execute a step wise and fully automated inversion and obtain optimal model estimates for potential seamounts these are statistically evaluated for significance using the akaike information criterion and f tests a logarithmic barrier technique is applied to ensure positivity of all seamount amplitudes after automatic and manual inspections of the model parameters we estimate actual heights and basal ellipses of the inspected potential seamounts directly from the predicted bathymetry grid in this study we find globally 24 643 potential seamounts h 0 1 km that are located away from continental margins 8458 potential seamounts are taller than 1 km although our global estimate is significantly lower than predictions from previous studies a first order reconciliation of the size frequency statistics obtained from those studies reveals that the previous counts are systematically overestimated because of the ambiguity of gravity signals due to small seamounts of h 1 km and the overlap with abyssal hills we estimate the global seamount census to lie in the 40 000 55 000 range the seamount data from this study are accessible from http www soest hawaii edu pt smts

implications of volcano and swell heights for thinning of the lithosphere by hotspots the heights of hotspot volcanoes are modeled by assuming isostatic equilibrium between the magma column and the adjacent lithosphere the depth to the level of compensation is assumed to be related to the thickness of the lithosphere after it has been reheated and thinned by a hotspot there is a square root relationship between volcano height and lithospheric age relationships between volcano height and lithospheric thickness and between reheated thickness and age of the lithosphere can be constrained within the limits imposed by uncertainties in lithospheric and magma densities if the reheated thickness determined from the isostatic model can be compared with lithospheric thickness determined from thermal models of the lithosphere then a relationship between the thickness of the lithosphere before and after reheating can be derived combining the upper limit on the reheated thickness age relationship with a theoretical expression derived by s t crough which relates swell height to lithospheric age reset thickness and physical constants yields a swell height age relationship that is in good agreement with empirical swell height data this agreement supports the assumption that the lithospheric thickness defined isostatically by volcano heights is closely related to the thermal thickness of the lithosphere a result that in turn supports the thermal origin of hotspot swells

mechanical response of the south flank of kilauea volcano hawaii to intrusive events along the rift systems abstract increased earthquake activity and compression of the south flank of kilauea volcano hawaii have been recognized by previous investigators to accompany rift intrusions we further detail the temporal and spatial changes in earthquake rates and ground strain along the south flank induced by six major rift intrusions which occurred between december 1971 and january 1981 the seismic response of the south flank to individual rift intrusions is immediate the increased rate of earthquake activity lasts from 1 to 4 weeks horizontal strain measurements indicate that compression of the south flank usually accompanies rift intrusions and eruptions emplacement of an intrusion at a depth greater than about 4 km such as the june 1982 southwest rift intrusion however results in a slight extension of the subaerial portion of the south flank horizontal strain measurements along the south flank are used to locate the january 1983 east rift intrusion which resulted in eruptive activity the intrusion is modeled as a vertical rectangular sheet with constant displacement perpendicular to the plane of the sheet this model suggests that the intrusive body that compressed the south flank in january 1983 extended from the surface to about 2 4 km depth and was aligned along a strike of n66 e the intrusion is approximately 11 km in length extended beyond the january 1983 eruptive fissures which are 8 km in length and is contained within the 14 km long region of shallow rift earthquakes

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

secondary aftershocks and their importance for aftershock forecasting the potential locations of aftershocks which can be large and damaging are often forecast by calculating where the mainshock increased stress we find however that the mainshock induced stress field is often rapidly altered by aftershock induced stresses we find that the percentage of aftershocks that are secondary aftershocks or aftershocks triggered by previous aftershocks increases with time after the mainshock if we only consider aftershock sequences in which all aftershocks are smaller than the mainshock the percentage of aftershocks that are secondary also increases with mainshock magnitude using the california earthquake catalog and monte carlo trials we estimate that on average more than 50 of aftershocks produced 8 or more days after m 5 mainshocks and more than 50 of all aftershocks produced by m 7 mainshocks that have aftershock sequences lasting at least 15 days are triggered by previous aftershocks these results suggest that previous aftershock times and locations may be important predictors for new aftershocks we find that for four large aftershock sequences in california an updated forecast method using previous aftershock data and neglecting mainshock induced stress changes can outperform forecasts made by calculating the static coulomb stress change induced solely by the mainshock manuscript received 20 november 2002

seamount abundances and distributions in the southeast pacific abstract sea beam bathymetry was recorded for 17 277 km of ship track in the southeast pacific and has been analyzed for seamount population characteristics all the ship tracks are over the pacific plate and most fall along 7 lines parallel to the east pacific rise between 7 s and 22 s the lines fall into three age categories one line is over 0 5 2 ma crust three are over 5 10 ma crust and three are over 30 40 ma crust seamount locations were recorded diameters were manually estimated and heights were measured if the swath crossed the seamount center over 382 features were counted along the entire ship track with heights ranging from 50 m to over 2500 m with sampling most consistent at heights between 100 m and 1000 m height to radius ratios vary considerably suggesting that seamount shapes do not scale to a single parameter the observed variety of morphological forms demonstrates that there is a wide range of seamount shapes for small features the distribution of sizes for the population was approximated by exponential dependence and by power law dependence using methods developed by smith and jordan 17 the power law distribution overestimates abundances at the smallest size ranges and both distributions fail to predict abundances of large seamounts determined from wide beam data 17 size distributions were also determined for seamounts in each seafloor age category almost all small seamounts appear to be produced on crust younger than 0 5 2 ma while the number of larger seamounts increases to 5 10 ma for all size ranges observed here the number of seamounts at 5 10 ma and at 30 40 ma is roughly identical these observations suggest that most small seamounts are formed on very young thin lithosphere that permits the passage of small volumes of magma small volumes would cool in older thicker lithosphere before reaching the surface although larger magma bodies might not numerous small sources of melt must exist near the ridge crest to supply the small seamounts probably trapped remnants of the large scale upwelling

volcano spacing and plate rigidity in plane stresses which accompany the flexural deformation of the lithosphere under the load adjacent volcanoes may govern the spacing of volcanoes in hotspot provinces specifically compressive stresses in the vicinity of a volcano prevent new upwelling in this area forcing a new volcano to develop at a minimum distance that is equal to the distance in which the radial stresses change from compressional to tensile the inflection point if a volcano is modeled as a point load on a thin elastic plate then the distance to the inflection point is proportional to the thickness of the plate to the power of 3 4 compilation of volcano spacing in seven volcanic groups in east africa and seven volcanic groups of oceanic hotspots shows significant correlation with the elastic thickness of the plate and matches the calculated distance to the inflection point in contrast volcano spacing in island arcs and over subduction zones is fairly uniform and is much larger than predicted by the distance to the inflection point reflecting differences in the geometry of the source and the upwelling areas

intraplate volcanism of the western pacific new insights from geological and geophysical observations in the pigafetta basin understanding intraplate volcanism is a key to deciphering the earth s magmatic history one of the largest intraplate volcanic events occurred during the mid cretaceous roughly 75 to 125 ma in the western pacific to investigate the origin and effects of this volcanism on various earth systems we present the first comprehensive study of volcanism in the pigafetta basin using seismic surveys magnetic and gravity modeling and ocean drilling program drill core and well log data from site 801 our results show that intraplate volcanism in the pigafetta basin coincides with the rest of the western pacific seamount provinces supporting the previously suggested plumelets scenario for the origin of intraplate volcanism during the mid cretaceous volcanic events we also discover that the late stage volcanism does not overprint the remanant magnetization acquired by the jurassic ocean crust in the pigafetta basin and hence marine magnetic anomalies recorded in the jurassic basement are preserved also the formerly identified rough smooth boundary rsb is indistinguishable from any other rough smooth topographic boundaries throughout the survey area suggesting that the rsb is unlikely to be a cretaceous sill jurassic basement boundary lastly the apparent ages and spatial distribution of volcanic features suggests a dynamic history of hydrothermal circulation in the pigafetta basin indicating that hydrothermal circulation was ongoing well past 100 ma this article is protected by copyright all rights reserved

properties of aftershock sequences in southern california the temporal behavior of 39 aftershock sequences in southern california 1933 1988 was modeled by the modified omori relation minimum magnitudes for completeness of each sequence catalog were determined and the maximum likelihood estimates of the parameters k p and c with the standard errors on each were determined by the ogata algorithm the b value of each sequence was also calculated many of the active faults in the region both strike slip and thrust were sampled the p values were graded in terms of the size of the standard error relative to the p value itself most of the sequences were modeled well by the omori relation many of the sequences had p values close to the mean of the whole data set 1 11 0 25 but values significantly different from the mean as low as 0 7 and as high as 1 8 exist no correlation of p with either the b value of the sequence or the mainshock magnitude was found the results suggest a direct correlation of p values is with surface heat flow with high values in the salton trough high heat flow and one low value in the san bernardino mountains and on the edge of the ventura basin both low heat flow the large fraction of the sequences with p values near the mean are at locations where the heat flow is near the regional mean 74 mw m2 if the hypothesis that aftershock decay rate is controlled by temperature at depth is valid the effects of other factors such as heterogeneity of the fault zone properties are superimposed on the background rate determined by temperature surface heat flow is taken as an indicator of crustal temperature at hypocentral depths but the effects on heat flow of convective heat transport and variations in near surface thermal conductivity invalidate any simple association of local variations in heat flow with details of the subsurface temperature distribution the interpretation is that higher temperatures in the aftershock source volume caused shortened stress relaxation times in the fault zone materials leading to a faster decay rate higher p value in 1967 mogi proposed such a relation on the basis of japanese data seismic velocity distributions published for southern california generally support the hypothesis with low velocities higher temperatures corresponding to high p values and vice versa

aftershock occurrence rate decay properties for intermediate and deep earthquake sequences we perform a detailed analysis of seven aftershock sequences with intermediate or deep focus mainshocks occurring in the aleutian islands japan the pamir hindu kush phk tonga and bolivia for each sequence we determine the modified omori law mol p value and goodness of fit for all sequences we are able to find a time interval where the sequence conforms to mol decay with either c 0 or c variable the two sequences with deep focus mainshocks have slightly larger p values than the intermediate depth sequences despite the rarity of well defined intermediate or deep aftershock sequences neither the observed p values nor the overall sequence decay behavior significantly differ from that observed for shallow sequences

rapid deformation of kilauea volcano global positioning system measurements between 1990 and 1996 campaign global positioning system gps measurements from 1990 to 1996 are used to calculate surface displacement rates on kilauea volcano hawaii the gps data show that the south flank of the volcano which has generated several large earthquakes in the past 3 decades is displacing at up to 8 cm yr to the south southeast the summit and rift zones are subsiding with maximum subsidence rates of 8 cm yr observed a few kilometers south of the summit caldera elastic dislocation modeling of the gps data suggests that the active sources of deformation include deep rift opening along the upper east and east rift zone fault slip along a subhorizontal fault near the base of the volcano and deflation near the summit caldera a nonlinear optimization algorithm was used to explore the parameter space and to find the best fitting source geometry there is a broad range of model geometries that fit the data reasonably well however certain models can be ruled out including those that have shallow rift opening or shallow fault slip some offshore aseismic slip on a fault plane that dips between 258 north northwest and 88 south southeast is required best fitting slip and rift opening rates are 23 28 cm yr although rates as low as 10 cm yr are permitted by the data

estimating tectonic stress rate and state with landers aftershocks a new method of analyzing the location and timing of aftershocks that makes possible estimates of the rate at which faults in the area are being loaded with stress has been developed this new method is an application of a theory of aftershocks proposed by dieterich 1994 the stress step from the mainshock triggers the aftershocks which release energy stored by tectonic loading and the mainshock and the tectonic stressing rate is responsible for the background activity thus a comparison of the number of aftershocks with the rate of background activity as a function of stress step can provide an estimate of tectonic stress rate this method requires analysis of a catalog which includes background activity as well as aftershocks the method has been applied to the aftershocks of the 1992 landers california earthquake the tectonic stress rate estimated for the mojave block is 1 2 pa d which corresponds to a repeat time for large events of 14 000 years and suggests that much of the observed strain in the mojave is anelastic the southern san andreas fault system is found to have a stress fate of 100 pa d which corresponds to a repeat time of 270 years and is consistent with primarily elastic strain accommodation inversions for the best fitting background stress state are consistent with fits of focal mechanisms and support an effective friction coefficient near 0 6 fits to the aftershock decay provide estimates of the dieterich friction parameter ad ranging from 0 02 if lithostatic pore fluids are present to 0 0003 if they are not

reconciling late neogene pacific absolute and relative plate motion changes new models of pacific absolute plate motion relative to hot spots and models of relative plate motion involving the pacific plate all agree there was a significant change in the late neogene chron 3a 5 89 ma reflecting a more northerly absolute motion than previously determined as pacific absolute plate motion became slightly more northerly left stepping transform segments came under compression some left stepping segments became microplates with clockwise rotation others show clear evidence of compressional deformation conversely right stepping transforms came under tension and many developed intratransform spreading centers or show similar evidence for transform magmatism several large left stepping transform offsets represent portions of the nazca plate protruding into the pacific and as such act as obstacles to the more northerly pacific absolute plate motion we suggest these obstructions act to enhance the generally tensile equatorial pacific stress regime caused by distant slab pull as a consequence the greater french polynesia region has experienced diffuse volcanism that increased considerably following the chron 3a plate motion change we propose that since the hawaii emperor bend time the distant pacific slab pull and the friction between the large buoyant ontong java plateau and the northern margin of the australia plate have produced episodes of increased tensile stresses in the equatorial pacific we believe these stress excursions are responsible for much of the intraplate volcanism observed in a wide triangular region from samoa to the east pacific and pacific antarctic rises

the use of earthquake rate changes as a stress meter at kilauea volcano stress changes in the earth s crust are generally estimated from model calculations that use near surface deformation as an observational constraint but the widespread correlation of changes of earthquake activity with stress has led to suggestions that stress changes might be calculated from earthquake occurrence rates obtained from seismicity catalogues although this possibility has considerable appeal because seismicity data are routinely collected and have good spatial and temporal resolution the method has not yet proven successful owing to the non linearity of earthquake rate changes with respect to both stress and time here however we present two methods for inverting earthquake rate data to infer stress changes using a formulation for the stress and time dependence of earthquake rates application of these methods at kilauea volcano in hawaii yields good agreement with independent estimates indicating that earthquake rates can provide a practical remote sensing stress meter

abundances distribution and sizes of volcanoes in the pacific ocean and implications for the origin of non hotspot volcanoes abstract in this paper i present data on the abundances sizes and crustal age for all volcanoes volcanic islands and seamounts which appear on published bathymetric charts of the pacific ocean these new data shed light on the origin of non hotspot volcanoes and are important in combination with data on the chemical compositions of seamounts and volcanic islands for estimates of the bulk composition of ocean crust these data also provide firm constraints on off ridge oceanic volcanism models results of this study show that the size frequency distribution of pacific volcanoes is poisson like and that the smallest volcanoes are much more abundant than large ones this study shows clearly that the most abundant volcanoes on the earth are the submerged oceanic volcanoes which comprise 5 25 of the oceanic volcanic layer on pacific crust of eocene age and younger the abundance of volcanoes number of volcanoes per unit area increases monotonically with increasing age assuming steady state the production rate of new off ridge volcanoes number of volcanoes per unit area per unit time is inversely proportional to the square root of the lithosphere age 1 on crust older than eocene the number of volcanoes per unit area of crust decreases monotonically with increasing age however the total volume of lava represented by these edifices increases with increasing age size frequency distributions of volcanoes on swaths of successively older crust indicate that these abundance patterns are partly due to the effect of sediment burial of small edifices on old pacific crust as well as the effect of increased lithosphere thickness on seamount size these general patterns are not appreciably changed by omitting from consideration known hotspot volcanoes 2 and volcanoes built at fossil constructional plate margins 3

deep ocean paleo seafloor erosion in the northwestern pacific identified by high resolution seismic images abstract the deposition and diagenesis of pelagic marine sediments together play important roles in global geochemical cycles by returning elements such as silica from the hydrosphere to the geosphere and stably sequestering them within deep ocean sedimentary sections around the world here we present new high resolution vertical resolution 2 5 m seismic reflection data from the northwestern pacific that we interpret to indicate a period of erosional activity that has returned silica to the hydrosphere and thus represents a departure from uniform sequestration by pelagic sediment deposition the interpreted erosional features are observed as a 200 to 300 m wide horizontal variation in reflection character within the 20 m interval overlying the regional chert porcellanite layer this reflection character variation is not consistent with uniform pelagic sedimentation and it has not previously been noted in the northwestern pacific where most seismic data are lower resolution erosion of seafloor sediments may have been caused by bottom current activity but the geometry of the observed features suggests that fluid expulsion erosion from diagenesis of primarily siliceous sediment played some role in driving this mass transfer this period of erosional activity occurred after this portion of the pacific plate left the equatorial zone of high biological productivity 75 ma and ended between approximately 35 and 55 ma regardless of the details of the process erosion of deep ocean seafloor sediments represents a return of silica and other chemicals from the sediment to the bottom water and if pervasive in the oceans these processes should be accounted for in models of global geochemical cycles

deformation of accretionary wedges in response to seamount subduction insights from sandbox experiments sandbox experiments using a two dimensional and a three dimensional approach are used to study the deformation of margins in response to seamount subduction successive mechanisms of deformation are activated during the subduction of conical seamounts first reactivation of the frontal thrusts and compaction of the accretionary wedge is observed then back thrusting and conjugate strike slip faulting develops above the leading slope of the subducted seamount the basal d6collement is deflected upward in the wake of the subducting high and a large shadow zone develops behind the seamount trailing slope consequently frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone when the main d6collement returns to its basal level in the wake of the seamount the margin records a rapid subsidence and a new accretionary wedge develops closing the margin reentrant the sediments underthrusted in the wake of the seamount into the shadow zone are underplated beneath the rear part of the accretionary wedge substantial shortening and thickening of the deformable seaward termination of the upper plate basement associated with basal erosion is observed seamount subduction induces significant material transfer within the accretionary wedge favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin the subduction and underplating of relatively undeformed water ladden sediments associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal ddcollement consequently significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount

buoyant decompression melting a possible mechanism for intraplate volcanism 1 volcanic activity away from plate boundaries occurs in a variety of settings linear age progressive volcanic chains have been explained as the manifestation of fixed hot spots possibly generated by plumes originating deep in the mantle however important examples of oceanic intraplate volcanism cannot be explained in this way including short lived chains and chains violating the predicted age distance behavior furthermore a significant fraction of volcanism does not occur in linear chains and observations suggest control by lithosphere structure in many cases in this study melting in mantle upwellings that result from the buoyancy associated with the melting itself is considered as a mechanism of intraplate volcanism numerical models of this buoyant decompression melting in a layer that is initially at its melting temperature over some depth range are used to determine the duration and amount of melting in upwellings which organize from an initially small random perturbation for a range of model parameters a predicted inverse correlation between the amount of melt produced and the duration of melting may be diagnostic of the buoyant melting process buoyant melting could occur in a number of geologic settings since the oceanic upper mantle may have previously melted beneath a spreading center and subsequently cooled spontaneous buoyant decompression melting may be possible only in regions where a large scale mantle upwelling can counteract conductive cooling keeping the mantle at its solidus temperature over some depth range where the mantle is at or sufficiently near its solidus buoyant decompression may spontaneously generate convective storms of melting this may explain the abundance of volcanism associated with the south pacific superswell where mantle is slightly cooler than its melting temperature buoyant melting may not occur spontaneously but may be triggered by some initial upwelling this may provide a physical explanation for volcanism that is controlled by lithosphere structure

statistical self similarity of hotspot seamount volumes modeled as self similar criticality the processes responsible for hotspot seamount formation are complex yet the cumulative frequency volume distribution of hotspot seamounts in the easter island salas y gomez chain esc is found to be well described by an upper truncated power law we develop a model for hotspot seamount formation where uniform energy input produces events initiated on a self similar distribution of critical cells we call this model self similar criticality ssc by allowing the spatial distribution of magma migration to be self similar the ssc model recreates the observed esc seamount volume distribution the ssc model may have broad applicability to other natural systems

spatial variability of seismicity parameters in aftershock zones the spatial variability of the b value of the frequency magnitude relationship and the decay rate of aftershocks as described by the p value of the modified omori law is investigated by using dense spatial grids we map out the distribution of b and p values within the landers northridge morgan hill and kobe aftershock sequences considerable spatial variability is found with b values of independent subvolumes ranging from 0 6 to 1 4 and p values ranging from 0 6 to 1 8 these systematic and statistically highly significant differences argue that it is an oversimplification to assign one single p and b value to an aftershock sequence that extends up to 100 km the spatial distribution of these two parameters is compared with the slip distribution during the mainshock suggesting that the areas of largest slip release correlate with high b value regions we hypothesize that the frictional heat created during the event may influence the p value distribution within an aftershock zone while applied shear stress crack density and pore pressure govern the frequency magnitude distribution by investigating the frequency magnitude distribution separately for preseismic and postseismic periods for the morgan hill mainshock we find that only the volume in the vicinity of the highest slip release shows a significant increase in the b value which decays to premainshock values within a year surrounding areas of the aftershock zone show an approximately constant b value with time because the aftershock hazard after a mainshock depends strongly on both the b and p value we propose that aftershock hazard assessment can be improved by taking into account the spatial distribution of the parameters

bathymetry of shatsky rise northwest pacific ocean implications for ocean plateau development at a triple junction oceanic plateaus are large igneous edifices thought to have been created by nascent mantle plumes but owing to sparse data their origins remain uncertain understanding plateau evolution is important because they are significant ocean features and may provide clues about mantle plume dynamics we constructed a bathymetry map of shatsky rise a large pacific plateau combining multibeam and wide beam echosounder data from 87 cruises and u s navy multibeam contours the rise consists of three large isolated volcanic edifices massifs surrounded by nearly normal lithosphere a linear volcanic ridge and a group of about 80 scattered seamounts massif flank slopes are typically gentle 1 5 and often parallel magnetic lineations or fracture zones the slope angles imply effusive volcanism similar to flood basalts whereas the rise shape suggests formation near the pacific izanagi farallon triple junction with modification of volcano flanks by spreading ridge tectonics edifice sizes and inferred ages imply a trend of decreasing volume and age from southwest to northeast furthermore gaps between massifs suggest episodic volcanism existing data are broadly consistent with the plume head hypothesis wherein the largest edifice formed by massive plume head eruptions the ridge formed from the plume tail and the two massifs in between represent a transition seamounts east of the rise are morphologically distinct which suggests that they may have been formed by a different source or mechanism than shatsky rise proper

earthquake hazard after a mainshock in california after a strong earthquake the possibility of the occurrence of either significant aftershocks or an even stronger mainshock is a continuing hazard that threatens the resumption of critical services and reoccupation of essential but partially damaged structures a stochastic parametric model allows determination of probabilities for aftershocks and larger mainshocks during intervals following the mainshock the probabilities depend strongly on the model parameters which are estimated with bayesian statistics from both the ongoing aftershock sequence and from a suite of historic california aftershock sequences probabilities for damaging aftershocks and greater mainshocks are typically well constrained after the first day of the sequence with accuracy increasing with time

shape analysis of pacific seamounts abstract shape statistics have been compiled from 85 profiles of well surveyed pacific seamounts in the height range 140 3800 m a flat topped cone was fit to each seamount s cross sectional profile maintaining the slopes of the sides as closely as possible on each profile a basal width d b a summit width d t and a maximum height h were measured the height to basal radius ratio is h is estimated by the ratio 2hd b and flatness f by the ratio d t d b slope angle arctan e is estimated from e 2h d b d t summit height and basal radius are found to be highly correlated r 0 93 the 85 point sample mean of the height to basal radius ratio is h 0 21 0 08 implying that a seamount s summit height is typically one fifth its basal radius despite the high correlation individual points show some scatter and there may be groupings into different morphological types for example all but one of the seamounts with summit heights above 1000 m have values of h that are larger than the sample mean the 85 point sample mean of flatness is f 0 31 0 18 data points show a large scatter with values of f varying between 0 a pointy cone and 0 69 a flat topped cone a histogram representation of flatness however indicates that certain values of f may be more common than others the histogram shows a bimodal distribution with maxima occurring at values of f in the ranges 0 10 0 20 and 0 35 0 50 moreover there is some evidence that the mean flatness decreases with summit height so that the preferred shape of a large sized seamount may be a pointy cone slope angle has an 85 point sample mean of 18 6 individual values of vary between 5 and 36 in addition to having a lower than average mean flatness seamounts with heights above 2600 m also have a lower than average mean slope angle 15 to determine which variables account for most of the observed variation in the seamount shapes a multivariate principal component analysis was performed on the data using five shape variables summit height basal radius summit radius flatness and slope the analysis indicates that most of the variation is described by two variables flatness and summit height

the geometric relationship between hot spots and seamounts implications for pacific hot spots abstract hot spots and the seamounts produced by them provide both geometric and temporal evidence for changes in absolute plate motion the main limitation in using hot spot produced seamounts in plate tectonic reconstructions arises from the multiple sources of error and ambiguity that plague radiometric age estimates in particular unless the hot spot has maintained a steady and voluminous flux rate over long periods of time the exact location of a hot spot which represents the zero age origin along the hot spot trail is poorly known here we discuss a unique geometric relationship between a hot spot and the seamounts produced by it that we recently have discovered i e hot spot produced seamounts have seafloor crustal flow lines that intersect at the hot spot location furthermore we obtain images of cumulative volcano amplitudes cva by convolving seamount shapes with their flow lines hot spots correspond to clear local maxima in this image and the amplitudes are proportional to cumulative hot spot flux this technique dubbed hot spotting allows us to determine hot spot locations based only on a set of seamount locations no age information is required we use the hot spotting technique to examine the pacific plate hot spots in general and the bowie and cobb hot spots in the gulf of alaska in particular we find that the hawaii louisville caroline cobb and bowie hot spots have clear representations in the cva images rurutu and or rarotonga are close to a large cva high while the other french polynesian hot spots in general exhibit a much more subdued and blurred expression we also conclude that the cobb hot spot plume may have been entrained by the juan de fuca ridge about 2 ma ago or alternatively is in a waning phase and cannot penetrate juan de fuca plate lithosphere the bowie hot spot appears to have encountered the ridge more recently

numerical modelling of space and time variations of seismic activity before major earthquakes summary the spatial and temporal variations of seismic activity before large earthquakes including precursory swarm earthquakes clustering pre seismic quiescence in the epicentral area doughnut patterns and foreshocks are numerically modelled by quasi static and dynamical rupture processes on three dimensional heterogeneous faults with various types of non uniform strength distributions the main physical parameters that could yield these precursory changes of seismicity patterns are the form of distributions of static and sliding frictional strengths and their spatial variations over the fault and also the rate of increase of tectonic stress for weakly heterogeneous faults a large scale main shock rupturing the entire fault surface occurs without any precursory indications if the fault has moderately to heavily non uniform strengths on a small scale gradually increasing activity tends to occur over a long period prior to a main event with no swarms or quiescence for extremely heterogeneous faults including a number of small to moderate size asperities with medium to high strengths a typical sequence appears with swarms quiescence foreshocks and a main shock event the temporal behaviour of these events seems to depend primarily on the form of distribution of the strengths if the fault includes a large size asperity with medium to high strengths in its central part a doughnut pattern of seismicity repeatedly appears with high activity and quiescence in the surrounding and central asperity regions if a belt like barrier region with extremely high strengths lies in the central part of the fault dividing the entire region into two sections large scale shocks take place successively in the adjacent unruptured regions at slightly different times and a main shock event rupturing the entire fault occurs after a long period of quiescence these results from numerical simulations could provide possible explanations of observed seismicity patterns prior to major earthquakes

bathymetric prediction from seasat altimeter data the linear response function technique is used to analyze two 1300 km tracks of seasat altimeter data and corresponding bathymetry in the musician seamounts region north of hawaii bathymetry and geoid height are highly correlated in the 50 to 300 km wavelength range a predictive filter is developed which can operate on seasat altimetry in poorly surveyed oceanic regions to indicate the presence of major bathymetrie anomalies modeling of the bathymetry geoid correlation in the musician region is attempted using the elastic plate model the flexural rigidity d of the plate is not well constrained by our data but appears to lie in the range 5 1021 n m 5 1022 n m at the time of loading since the musician seamounts and the crust on which they lie are both late cretaceous in age this value represents the effective flexural rigidity of very young lithosphere that was frozen in at the time of voicanism the modeling indicates that the general form of a predictive filter will strongly depend on various geologic parameters especially the effective flexural rigidity hence some a priori geologic constraints are necessary to estimate successfully the bathymetry from the altimeter data alternately if high quality bathymetry is available a crude estimate of the age of loading i e voicanism can be made from the altimeter data

evidence from the ad 2000 izu islands earthquake swarm that stressing rate governs seismicity magma intrusions and eruptions commonly produce abrupt changes in seismicity far from magma conduits that cannot be associated with the diffusion of pore fluids or heat such swarm seismicity also migrates with time and often exhibits a dog bone shaped distribution the largest earthquakes in swarms produce aftershocks that obey an omori type exponential temporal decay but the duration of the aftershock sequences is drastically reduced relative to normal earthquake activity here we use one of the most energetic swarms ever recorded to study the dependence of these properties on the stress imparted by a magma intrusion a 1 000 fold increase in seismicity rate and a 1 000 fold decrease in aftershock duration occurred during the two month long dyke intrusion we find that the seismicity rate is proportional to the calculated stressing rate and that the duration of aftershock sequences is inversely proportional to the stressing rate this behaviour is in accord with a laboratory based rate state constitutive law suggesting an explanation for the occurrence of earthquake swarms any sustained increase in stressing rate whether due to an intrusion extrusion or creep event should produce such seismological behaviour

origin of volcanism on the flanks of the pacific antarctic ridge between 41 30 s and 52 s 1 non hot spot intraplate volcanism is a common feature near the east pacific rise or pacific antarctic ridge volcanic ridges and seamount chains tens to hundreds of kilometers long are asymmetrically distributed about the ridge axis with most volcanic features occurring on the pacific plate their origins remain controversial we have analyzed off axis volcanic ridges near the pacific antarctic ridge from bathymetry backscatter gravity and geochemistry data of the pacantarctic 2 cruise k ar dating of samples dredged on these structures reveals a contrast of up to 3 ma between the volcanoes and the underlying crust the volcanic activity as suggested by the strong backscatter in sonar images appears to be limited to areas of seafloor younger than about 3 ma all surveyed ridges north of the menard transform fault tf show recent activity close to the ridge axis and are not affected by faults the off axis volcanic ridges south of the menard tf show recent volcanic flows in their center and are affected by n s extension two different types of volcanoes can be characterized conical flat topped ones and rough elongated ones associated with narrow e w trending volcanic ridges some of them showing strong backscattering on the em12 imagery from the morphology of the seamount chains and the ages of the lava samples we infer that the magma source for the volcanic ridges is related to the feeding of the ridge axis through three dimensional mantle convective circulation we suggest that a change in the relative plate motion since about 5 ma might have induced an offset of the mantle upwelling circulation under the ridge axis so that anomalously hot mantle rises under the pacific ridge flank the kinematic change is also likely responsible for the tectonic deformation in the young lithosphere south of menard tf

the relationship between the height of a volcano and the depth to its magma source zone a critical reexamination it is commonly assumed that hydrostatic pressure balance arguments can be used to establish a relationship between the maximum height to which a volcanic edifice is able to grow and the depth at which the partial melts providing its magma supply are formed such a relationship has been used to infer various aspects of the thermal and stress state of the lithosphere beneath volcanic constructs on earth mars io and venus we examine the assumptions behind this relationship which are that 1 a continuous pressure connection exists between source and summit 2 the pressure around the magma source is the local hydrostatic pressure dictated by the depth below the geoid and 3 the melt erupting at the summit has a net positive buoyancy and show that many of them require geologically unreasonable conditions we then critically assess the evidence cited in the literature for the relationship and find that there are other factors that may explain the observations we conclude that volcano heights on the terrestrial planets cannot be related in any simple way to lithospheric thickness or depth to the magma source zone and we review the range of other factors controlling volcano height

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

response to comment on mantle flow drives the subsidence of oceanic plates croon et al challenge our conclusion that sea floor depth variations are driven by the underlying mantle convection we point out that contrary to their claim our data analysis is pertinent and that the sea floor linear trend as the square root of the distance from the ridge is a robust observation the mechanism responsible for this trend is an asthenospheric flow faster than the overlying plate which shapes the lithosphere structure

the tectonic fabric of the ocean basins 1 we present a global community data set of fracture zones fzs discordant zones propagating ridges v shaped structures and extinct ridges digitized from vertical gravity gradient vgg maps we use a new semi automatic fz tracking program to test the precision of our hand digitized traces and find a mean absolute deviation of less than 3 4 km from the raw vgg minima that most clearly delineate each feature and less than 5 4 km from the fz location predicted by fitting model profiles to the vgg data that represent the morphology of the individual fzs these offsets are small considering gravity data only provide an approximation for the underlying basement morphology we further investigate the origin of non fz seafloor fabric by combining published abyssal hill heights computed from gravity anomalies with global half spreading rates a residual abyssal hill height grid with spreading rate effects removed combined with our interpreted tectonic fabric reveals several types of seafloor fabric distinct from typical abyssal hills where discordant zones do not overprint abyssal hill signals residual abyssal hill height anomalies correspond to seafloor that accreted near mantle thermal anomalies or zones of melt depletion our analysis reveals several areas where residual abyssal hill height anomalies reflect pseudo faults and extinct ridges associated with ridge propagation and or microplate formation in the southern pacific ocean

global distribution of seamounts from seasat profiles bathymetrie profiles and contour charts have been used to study the distribution of seamounts in the deep ocean basins but only a small fraction of the seafloor has been sampled by ships at the present exploration rate it will take several centuries to map significant portions of the seafloor topography satellite altimetry which maps the topography of the equipotential sea surface is a promising tool for studying the gravity fields of seamounts because all ocean basins can be sampled in a couple of years using a model of a gaussian shaped seamount loading a thin elastic lithosphere we develop a new technique for measuring basic characteristics of a seamount from a single satellite altimeter profile the model predicts that the seamount diameter is equal to the peak to trough distance along the vertical deflection profile and that the overall diameter of the signature reveals the age of the lithosphere when the seamount formed moreover the model suggests that these two measurements are relatively insensitive to the cross track location of the seamount we confirm these model predictions using seasat altimeter profiles crossing 14 well surveyed seamounts in the pacific we then apply the measurement technique to 26 106 million kilometers of seasat profiles resulting in a new global set of seamount locations approximately one quarter of the seamounts identified in seasat profiles were previously uncharted modeling suggests that there is no direct relationship between the size of a seamount and its signature in the geoid therefore the set of locations is not a straightforward sampling of the total seamount population but is weighted toward seamounts which are poorly compensated a preliminary analysis indicates considerable variations in population density and type across the oceans most notable among them are the absence of seamounts in the atlantic variations in population density across large age offset fracture zones in the pacific the prevalence of small signatures in the indian ocean and the existence of linear trends in the large seamounts of the west pacific

flow and mixing around a small seamount on kaena ridge hawaii abstract microstructure observations over a small seamount on the kaena ridge hawaii showed asymmetry in the along and across ridge directions the 400 m high seamount is on the southern edge of the ridge centered at 21 43 49 n 158 38 48 w 42 km northwest of oahu a 1 km resolution numerical simulation shows that the flow within the depth range of the seamount tends to be accelerated around the seamount rather than going up and over it the flow patterns however are more complicated than for an isolated seamount because of the influence of the 3000 m high kaena ridge comparison with the numerical simulations indicates that the across ridge asymmetry in which dissipation on the north northeastern side of the seamount was higher and more concentrated toward the bed than on the south southwestern side is consistent with an m2 tidal beam generated at the northern edge of the ridge the along ridge asymmetry with higher dissipation on the east southeastern flank than on the west northwestern fla

global distribution of seamounts from ship track bathymetry data the distribution of submarine volcanoes or seamounts reflects melting within the earth and how the magma generated ascends through the overlying lithosphere globally 60 latitude we use bathymetry data acquired along 39 5 106 km of ship tracks to find 201 055 probable seamounts an order of magnitude more than previous counts across a wider height range 0 1 1 km implying that 24 000 60 remain to be discovered

seismic imaging of a convergent continental margin and plateau in the central andes andean continental research project 1996 ancorp 96 1 a 400 km long seismic reflection profile andean continental research project 1996 ancorp 96 and integrated geophysical experiments wide angle seismology passive seismology gravity and magnetotelluric depth sounding across the central andes 21 s observed subduction of the nazca plate under the south american continent an east dipping reflector nazca reflector is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km we interpret parts of the nazca reflector as a fluid trap located at the front of recent hydration and shearing of the mantle the fluids being supplied by dehydration of the oceanic plate patches of bright quebrada blanca bright spot to more diffuse reflectivity underlie the plateau domain at 15 30 km depth this reflectivity is associated with a low velocity zone p to s wave conversions the upper limits of high conductivity and high vp vs ratios and to the occurrence of neogene volcanic rocks at surface we interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during neogene shortening and plateau growth the imaging properties of the continental moho beneath the andes indicate a broad transitional character of the crust mantle boundary owing to active processes like hydration of mantle rocks in the cooler parts of the plate margin system magmatic underplating and intraplating under and into the lowermost crust mechanical instability at moho etc hence all first order features appear to be related to fluid assisted processes in a subduction setting

state of stress faulting and eruption characteristics of large volcanoes on mars the formation of a large volcano loads the underlying lithospheric plate and can lead to lithospheric flexure and faulting in turn lithospheric deformation affects the stress field beneath and within the volcanic edifice and can influence magma transport modeling the interaction of these processes is crucial to an understanding of the history of eruption characteristics and tectonic deformation of large volcanoes we develop models of time dependent stress and deformation for the tharsis volcanoes on mars by means of a finite element code we calculate stresses and displacements due to a volcano shaped load emplaced on an elastic plate overlying a viscoelastic mantle models variously incorporate growth of the volcanic load with time and a detachment between volcano and lithosphere the models illustrate the manner in which time dependent stresses induced by lithospheric plate flexure beneath the volcanic load may affect eruption histories and the derived stress fields can be related to tectonic features on and surrounding martian volcanoes as a result of flexure there are three regions where stresses become sufficiently large to cause failure by faulting according to the mohr coulomb criterion at the surface of the plate just outward of the volcano near the base of the elastic lithosphere beneath the center of the volcano and on the upper flanks of the volcano early in its growth history normal faulting is the dominant mode of failure predicted for the first region consistent with circumferential graben observed around the tharsis montes and with the scarp at the base of olympus mons interpreted as a large offset listric normal fault normal faulting mostly radially oriented is predicted for the second region failure in the third region is predicted to consist of thrust faulting circumferentially oriented on the upper and middle flanks and radially oriented on the lower flanks in models simulating a growing volcano this portion of the edifice is subsequently covered by later units which exhibit lower stresses and are not predicted to fail this volume of early failure remains the most highly stressed area in the edifice concentric terraces interpreted by some workers as thrust faults on the upper flanks of olympus mons may correspond to the predicted circumferential thrust features if the most recent increments of volcano growth were relatively large or in the presence of local material property or stress field variations for volcanoes detached from the plate predicted failure in the edifice takes the form of radial normal faulting near the volcano base the addition of a local extensional stress arising from the regional topographic slope yields a pattern of predicted faulting which closely matches that observed on the tharsis montes including the development of radial rifts on the lower volcano flanks to the northeast and southwest and the asymmetric formation of circumferential flank graben this stress state is also consistent with an interpretation of the aureole deposits of olympus mons as the result of gravity sliding along a basal detachment our models also suggest an explanation for the lack of strike slip features predicted by previously published flexural models around the tharsis volcanoes for a given load increment the first mode of near surface failure for most of the area immediately outward of the load is circumferential normal faulting and graben formation as the volcano grows and the flexural response to the increasing load proceeds the predicted failure mode in a portion of this annular region surrounding the volcano changes to strike slip faulting because normal faulting has been predicted to have taken place earlier however it is likely that release of later stresses will occur by reactivation and growth of these normal faults and graben rather than by the formation of new strike slip faults

geology tectonic settings and iron ore metallogenesis associated with submarine volcanism in china an overview abstract submarine volcanogenic iron oxide svio deposits are one of the most important sources of high grade iron ores in china the spatial distribution of the deposits is controlled by the tectonic settings including arc back arc and rift environments with the svio deposits mostly concentrated in the western part of china namely the southwestern yangtze craton western and eastern tianshan and altay orogens and the kaladawan iron ore district in the eastern part of the altyn tagh region the chinese svio deposits range in age from paleoproterozoic to mesozoic and were formed during two main metallogenic epochs in the proterozoic and paleozoic more than 70 of the svio deposits formed in the paleozoic with three important svio metallogenic provinces recognized in the altay eastern and western tianshan orogens these svio deposits are hosted in lithofacies that are related to submarine magmatism such as lavas and associated pyroclastic and volcaniclastic sedimentary rocks the iron orebodies are hosted in different volcanic lithofacies with different features moreover the different volcanic lithofacies in which the fe ores are hosted also provide information as to their spatial relationship ranging from distal to proximal to the eruption center or vent many of these deposits are characterized by well developed skarns and could be interpreted either by a distal position of the ore system in question and or exposed igneous rocks or active magma chamber or a relationship to early metamorphism and continuous alteration at relatively high temperature followed by retrograde alteration as temperatures decline geological and geochemical evidence suggests that these deposits were formed as a result of submarine magmatic activity including subaqueous volcanic eruptions associated volcano sedimentary lithofacies and related post magmatic hydrothermal activity iron oxide ore probably formed the hydrothermal fluids which generated the skarns could be a mixture of evolved magma derived water and convecting sea water driven by the heat from the shallow active magma chamber whereas volcano sedimentary deposits could be formed by the fallout of the ore bearing materials to the sea floor emanating from submarine eruption columns or fractional precipitation of iron which had been introduced locally into the bottom water by volcanic origin hydrothermal solutions and by leaching from the relatively iron rich volcanic rocks the formation of these various styles of fe ore deposits is controlled by several key factors such as magma differentiation lithofacies of host rocks temperature and chemical compositions of hydrothermal fluids as well as the depth of sea water in combination with their geological characteristics geodynamic mechanisms and metallogenesis we propose a genetic model in which the origin of these deposits can be related to the space time evolution of the submarine volcanism and their relationship to volcanic lithofacies variation such as central proximal and distal environments of ore formation

distribution of axial lava domes along a superfast overlapping spreading center 27 32 s on the east pacific rise 1 deep towed dsl 120 bathymetric data are used to investigate the pattern of lava dome formation along a superfast spreading portion of the southern east pacific rise epr including the overlapping limbs of a giant 120 120 km propagator near 29 s along the 670 km of the axis surveyed 1172 small domes were identified using a closed contour algorithm their abundance defined by spatial density is well correlated with the along axis relief of the ridge crest where the western and eastern limbs plunge toward the overlap zone densities are high 3 6 km 2 however where the axial depth profile is shallow and flat densities are comparably low 0 4 km 2 volcanic domes within the low abundance areas are characterized by lower ratios of height to basal radius 0 15 versus 0 22 smaller maximum heights 18 versus 40 m and a larger relative percentage of small versus large mounds the zone of high dome abundance encompasses the overlapping limbs of the rift and extends more than 100 km to the north and south beyond the overlap zone domes form dominantly during low effusion rate point source eruptions which suggests that discontinuous melt lenses underlie the ridge axis proximal to the overlapper conversely fissure fed sheet flows dominate along the more distal segments implying the presence of a more continuous axial magma lens throughout the survey area dome abundance increases systematically near second order segment boundaries within the high abundance zone some third order offsets also correlate with increased dome production but local peaks in abundance are not tied exclusively to higher order ridge offsets where dome abundance is low domes are clustered tightly near second order offsets and there is no increase in dome abundance near third order segment boundaries

deep submarine pyroclastic eruptions theory and predicted landforms and deposits submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows however that significant pyroclastic activity can occur even at depths in excess of 3000 m in order to document and illustrate the full range of submarine eruption styles we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions 1 no gas exsolution 2 gas exsolution but no magma disruption 3 gas exsolution magma disruption and hawaiian style fountaining 4 volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse 5 magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity and 6 volatile content builds up in the top of a dike leading to vulcanian eruptions we also examine the role of bulk interaction steam explosivity and contact surface steam explosivity as processes contributing to volcaniclastic formation in these environments we concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities the most likely circumstance is the quiet effusion of magma with minor gas exsolution and the production of somewhat vesicular pillow lavas or sheet flows depending on effusion rate the amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption the most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity due to the localized concentration of volatiles in magma that has a low rise rate magmatic gas collects by bubble coalescence and ascends in large isolated bubbles which disrupt the magma surface in the vent producing localized blocks bombs and pyroclastic deposits another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions these deposits being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater should be easily distinguishable from strombolian and hawaiian eruptions however we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles in this case a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian style activity the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile induced hawaiian eruption specifically typical pyroclast sizes might be smaller fountain heights may exceed those expected for the purely magmatic hawaiian case cooling of descending pyroclasts would be more efficient leading to different types of proximal deposits and runout distances for density flows would be greater potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer in addition flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles and thus extremely poor in vesicles relative to typical flows associated with hawaiian style eruptions in the primary magmatic gas case we examine several cases of reported submarine volcaniclastic deposits found at depths as great as 3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid ocean ridges furthermore the latter stages of development of volcanic edifices seamounts formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth but also to 1 the presence of a summit magma reservoir which favors the buildup of magmatic foams enhancing hawaiian style activity and episodic dike emplacement which favors strombolian style eruptions and 2 the common occurrence of alkalic basalts the co2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts these models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles

single link cluster analysis of earthquake aftershocks decay laws and regional variations using single link cluster analysis the authors investigate how various properties of aftershock sequences depend on their tectonic regime and focal depth for international seismological centre earthquakes of m sub b ge 4 8 they find that earthquakes deeper than 70 km have the fewest and smallest aftershock sequences even after accounting for differences in detectability and maximum magnitude they find that ridge transform earthquakes have smaller aftershock sequences that shallow subduction zone earthquakes among different subduction zones they find that zones with high moment release rates possess larger aftershock sequences comparing ridge transform zones they find those with slower spreading rates possess larger aftershock sequences by transposing origin times of several different aftershock sequences as if all had main shocks occurring at time zero they are able to study the properties of aftershock sequences which individually have too few aftershocks to study by other means secondary events determined by single link cluster analysis follow a modified omori s power law decay for time separations of 0 1 day to 20 days from the parent event with p values ranging from 0 539 plus minus 0 022 intermediate and deep focus earthquakes to 0 928 plus minus 0 024 ridge transform earthquakes they find that earthquake foreshocks and multiplets also follow a modified omori s law atmore greater times from the main shock the decay is steeper than a power law decay more like an exponential decay aftershocks in the adak catalog m sub b ge 2 0 show a marked decrease in activity between 40 and 50 km depth they speculate that the observed differences in number of aftershocks and p values may be caused by variations in fault heterogeneity or in fluid pressures less

distribution of large pacific seamounts from geosat ers 1 implications for the history of intraplate volcanism we characterize the seamount distribution on the pacific plate using the gridded vertical gravity gradient vgg or geoid curvature derived from geosat and ers 1 satellite altimetry the vgg amplifies short wavelength information and suppresses longer wavelength components making it suitable for seamount detection purposes furthermore the vgg over seamounts has a much more pronounced zero crossing than that of the free air anomaly faa the distance to the zero crossing can be used as a proxy for seamount radius after removing a regional field obtained by robust median filtering we identify seamount amplitudes and locations from local maxima in the vgg grid the radius of a seamount is more difficult to estimate since seamounts tend to cluster and overprint each other s signals individual seamounts are modeled as gaussian axisymmetric objects loading an elastic lithosphere the vgg over such features can be approximated by a simple analytical expression which we use to determine the zero crossing distances for overlapping seamounts by using the vgg the maximum amplitude and distance to the zero crossing become largely independent of the elastic plate thickness and infill density we do forward modeling of gaussian seamounts and their gravimetric response and create a look up table that relates seamount faa in milligals vgg in eotvos and zero crossing distance in kilometers to actual height and radius in kilometers the frequency size distribution of these predicted seamount heights follows a power law for heights between 2 and 8 km the seamount density number of seamounts per area is greatest in the central pacific we confirm earlier results suggesting that the majority of large seamounts are located in the western region of the pacific plate on older crust as crustal age increases so does seamount density peaking on 100 130 m y crust supporting suggestions of high magmatism in the cretaceous we demonstrate that there may be an empirical relationship between the seamount vgg amplitude and the age of the lithosphere at the time of seamount formation and invert this relationship to predict seamount ages from vgg amplitudes these pseudo ages have large uncertainties but nevertheless may be used to investigate temporal fluctuations in pacific intraplate volcanism our results indicate that seamount intraplate volcanism attained a maximum level in the mid cretaceous to late cretaceous about 70 120 ma apparently contemporaneous with the formation of large oceanic plateaus in the pacific

geology and petrology of mahukona volcano hawaii the submarine mahukona volcano west of the island of hawaii is located on the loa loci line between kahoolawe and hualalai volcanoes the west rift zone ridge of the volcano extends across a drowned coral reef at about 1150 m and a major slope break at about 1340 m both of which represent former shoreines the summit of the volcano apparently reached to about 250 m above sea level now at 1100 m depth did was surmounted by a roughly circular caldera a econd rift zone probably extended toward the east or sutheast but is completely covered by younger lavas from the adjacent subaerial volcanoes samples were vecovered from nine dredges and four submersible lives using subsidence rates and the compositions of flows which drape the dated shoreline terraces we infer that the voluminous phase of tholeiitic shield growth ended about 470 ka but tholeiitic eruptions continued until at least 435 ka basalt transitional between tholeiitic and alkalic basalt erupted at the end of tholeiitic volcanism but no postshield alkalic stage volcanism occurred the summit of the volcano apparently subcided below sea level between 435 and 365 ka the tholeiitic lavas recovered are compositionally diverse

deep magma transport at kilauea volcano hawaii the shallow part of kilauea s magma system is conceptually well understood long period and short period brittle failure earthquake swarms outline a near vertical magma transport path beneath kilauea s summit to 20 km depth a gravity high centered above the magma transport path demonstrates that kilauea s shallow magma system established early in the volcano s history has remained fixed in place low seismicity at 4 7 km outlines a storage region from which magma is supplied for eruptions and intrusions brittle failure earthquake swarms shallower than 5 km beneath the rift zones accompany dike emplacement sparse earthquakes extend to a decollement at 10 12 km along which the south flank of kilauea is sliding seaward this zone below 5 km can sustain aseismic magma transport consistent with recent tomographic studies long period earthquake clusters deeper than 40 km occur parallel to and offshore of kilauea s south coast defining the deepest seismic response to magma transport from the hawaiian hot spot a path connecting the shallow and deep long period earthquakes is defined by mainshock aftershock locations of brittle failure earthquakes unique to kilauea whose hypocenters are deeper than 25 km with magnitudes from 4 4 to 5 2 separation of deep and shallow long period clusters occurs as the shallow plumbing moves with the volcanic edifice while the deep plumbing is centered over the hotspot recent gps data agrees with the volcano propagation vector from kauai to maui suggesting that pacific plate motion azimuth 293 58 and rate of 7 4 cm yr has been constant over kilauea s lifetime however volcano propagation on the island of hawaii azimuth 3258 rate 13 cm yr requires southwesterly migration of the locus of melting within the broad hotspot deep long period earthquakes lie west of the extrapolated position of kilauea backward in time along a plate motion vector requiring southwesterly migration of kilauea s magma source assumed ages of 0 4 my for kilauea and 0 8 my for mauna loa are consistent with this model younger ages would apply if kilauea began its growth south of the locus of maximum melting as is true for loihi seamount we conclude that kilauea is fed from below the eastern end of the zone of deep long period earthquakes magma transport is vertical below 30 km then sub horizontal following the oceanic mantle boundary separating plagioclase and spinel peridotite then near vertical beneath kilauea s summit the migration of the melting region within the hotspot and kilauea s sampling of different sources within the melting region can explain 1 the long term geochemical separation of

mantle fault zone beneath kilauea volcano hawaii relocations and focal mechanism analyses of deep earthquakes 13 kilometers at kilauea volcano demonstrate that seismicity is focused on an active fault zone at 30 kilometer depth with seaward slip on a low angle plane and other smaller distinct fault zones the earthquakes we have analyzed predominantly reflect tectonic faultingin the brittle lithosphere rather than magma movement associated with volcanic activity the tectonic earthquakes may be induced on preexisting faults by stresses of magmatic origin although background stresses from volcano loading and lithospheric flexure may also contribute

morphological characteristics and emplacement mechanism of the seamounts in the central indian ocean basin abstract the morphotectonic features of the central indian ocean basin ciob provide information regarding the development of the basin multibeam mapping of the ciob reveals presence of abundant isolated seamounts and seamount chains sub parallel to each other and major fracture zones along 73 e 79 e and 75 45 e morphological analyses were carried out for 200 seamounts that occur either as isolated edifies or along eight sub parallel chains the identified eight parallel seamount chains that trend almost north south and reflecting the absolute motion of the indian plate probably originated from the ancient propagative fractures inspite of the differences in their height the seamounts of these eight chains are morphologically correlatable in the study area the seamounts are clustered north and south of 12 s latitude interestingly in the area north of 12 s area ii 9 12 s the seamounts are distinctly smaller 400 m height whereas the area south of 12 s area i 12 15 s has a mixed population of seamounts the normalized abundance of the ciob seamount is 976 seamounts 10 6 km 2 but on a finer scale this value varies from 500 to 1600 seamounts 10 6 km 2 which is less than the seamount concentrations of the pacific and atlantic oceans 9000 to 16 000 seamounts 10 6 km 2 three categories of seamounts are present in the ciob e g 1 single peaked 2 multi peaked and 3 composite the study indicate that single peaked seamounts are dominant 89 while multi peaked is less 8 and composite ones are rare 3 in the ciob the progressive northward movement of the indian continent caused collision between india and asia at around 62 ma ago a majority of the near axis originated seamounts in the ciob seemed to have formed as a consequence of the temporally widespread cretaceous 65 ma to late eocene

formation of submarine flat topped volcanic cones in hawai i high resolution bathymetric mapping has shown that submarine flat topped volcanic cones morphologically similar to ones on the deep sea floor and near mid ocean ridges are common on or near submarine rift zones of kilauea kohala or mauna kea mahukona and haleakala volcanoes four flat topped cones on kohala were explored and sampled with the pisces v submersible in october 1998 samples show that flat topped cones on rift zones are constructed of tholeiitic basalt erupted during the shield stage similarly shaped flat topped cones on the northwest submarine flank of ni ihau are apparently formed of alkalic basalt erupted during the rejuvenated stage submarine postshield stage eruptions on hilo ridge mahukona hana ridge and offshore ni ihau form pointed cones of alkalic basalt and hawaiite the shield stage flat topped cones have steep 25 sides remarkably flat horizontal tops basal diameters of 1 3 km and heights 300 m the flat tops commonly have either a low mound or a deep crater in the center the rejuvenated stage flat topped cones have the same shape with steep sides and flat horizontal tops but are much larger with basal diameters up to 5 5 km and heights commonly greater than 200 m the flat tops have a central low mound shallow crater or levees that surrounded lava ponds as large as 1 km across most of the rejuvenated stage flat topped cones formed on slopes 10 and formed adjacent semicircular steps down the flank of ni ihau rather than circular structures all the flat topped cones appear to be monogenetic and formed during steady effusive eruptions lasting years to decades these and other submarine volcanic cones of similar size and shape apparently form as continuously overflowing submarine lava ponds a lava pond surrounded by a levee forms above a sea floor vent as lava continues to flow into the pond the lava flow surface rises and overflows the lowest point on the levee forming elongate pillow lava flows that simultaneously build the rim outward and upward but also dam and fill in the low point on the rim the process repeats at the new lowest point forming a circular structure with a flat horizontal top and steep pillowed margins there is a delicate balance between lava heat supply to the pond and cooling and thickening of the floating crust factors that facilitate construction of such landforms include effusive eruption of lava with low volatile contents moderate to high confining pressure at moderate to great ocean depth long lived steady eruption years to decades moderate effusion rates probably ca 0 1 km3 year and low but not necessarily flat slopes with higher effusion rates sheet flows flood the slope with lower effusion rates pillow mounds form hawaiian shield stage eruptions begin as fissure eruptions if the eruption is too brief it will not consolidate activity at a point and fissure fed flows will form a pond with irregular levees the pond will solidify between eruptive pulses if the eruption is not steady lava that is too volatile rich or that is erupted in too shallow water will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones as occurs during the post shield stage the steady effusion of lava on land constructs lava shields which are probably the subaerial analogs to submarine flat topped cones but formed under different cooling conditions

density and size distribution of seamounts in the eastern pacific inferred from wide beam sounding data theoretical expressions relating the distribution of apparent seamount heights per unit length of ship track to the distribution of actual seamount heights per unit area of seafloor are derived for both narrow beam and wide beam echo sounders the shapes of seamounts are approximated as truncated right circular cones of constant slope e height to radius ratio and flatness f i e a seamount of basal radius r has a height h r a flat top of radius fr and a slope angle of arctan e the set of all seamounts in a given physiographic province is assumed to be a poissonian ensemble specified by a function v r the average number of seamounts per unit area for which r r a simple choice for the size distribution of seamounts and one shown to be consistent with data in the eastern pacific is to specify that it decays exponentially with r v r vo exp r in the case of a profile of length l taken in water of depth d with a wide beam echo sounder half beam width the shallow slope deep water approximation e 1 e d yields that the expected number of seamounts with apparent heights h h is n h l 2 1 ed 2 fh lv0e h we apply this sampling theory to apparent height data measured from 44 020 km of wide beam profiles on pacific crust of post mesozoic age although the spatial distribution of seamounts in the eastern pacific is certainly not poissonian the abundance statistics do not involve spatial correlations and we show they are robust with respect to deviations from the poisson model provided the statistics are interpreted as averages over the area actually sampled by the track lines the observed falloff of apparent heights in the eastern pacific is very nearly exponential justifying our use of an exponential model of seamount size distribution the shape parameters are estimated from multibeam surveys of pacific seamounts an analysis of 30 features in the height range 200 4000 m yields 0 21 0 011 and f 0 21 0 028 using these parameters we obtain vo 4 0 1 1 10 9 m 2 and 6 3 0 8 10 4 m 1 these values imply that per 106 km2 in the eastern pacific there are 1600 400 seamounts with summit heights greater than 300 m and 200 60 greater than 1000 m integrated over the entire size distribution seamounts occupy about 6 of the seafloor area and constitute about 0 4 of the oceanic crustal volume our estimates of seamount abundance are appreciably greater than those of most other authors a discrepancy we attribute primarily to the sampling bias of previous studies which generally used uncorrected map counts this hypothesis is corroborated by a comparison of our statistics with map count data for seamounts of heights exceeding 2000 m compiled for the eastern central pacific from the scripps charts above 3000 m the map counts are consistent with the predictions of the exponential model but below this height they are biased to low values

global distribution of seamounts inferred from gridded geosat ers 1 altimetry using a forward modeling approach based on an axisymmetric gaussian seamount i characterize the global seamount distribution by locating circular maxima in the gridded vertical gravity gradient field derived from altimetry collected by the geosat and ers 1 satellite missions the global seamount distribution is long tailed and resembles a power law distribution for seamounts in the height range 2 7 km smaller seamounts are not well isolated by my technique nor are they well resolved in the gridded data several factors are likely to influence the height of volcanic seamounts such as melt availability magma driving pressure and plate thickness the observed relationship between seamount gravimetric amplitudes and the age of the underlying seafloor implies that there is an upper limit on seamount heights whether a seamount will reach that height depends most likely on supply side factors such as melt availability and magma driving pressure but the limiting height itself seems more likely to be controlled by the strength of the oceanic plate specifically compressional stresses directly beneath the seamount as a consequence of the lithosphere s flexural response to loading may eventually exceed the magma driving pressure and prevent magma from reaching the surface thus limiting the growth of the seamount because oceanic plate strength primarily is controlled by plate age the limit on seamount height is inferred to be a simple function of plate age at the time of seamount emplacement using analytical solutions i present a simple flexural model that predicts the observed global height age relationship

sizes and ages of seamounts using remote sensing implications for intraplate volcanism satellite altimetry was used to identify and characterize pacific intraplate seamounts the gravimetric amplitudes of seamounts appear to be related to the age difference between the sea floor and seamounts by inverting this relation pseudo ages can be obtained for undated seamounts these pseudo ages imply that excursions in seamount volcanism generally correlate with times of formation of large oceanic plateaus

factors influencing the locations of hot spots determined by the hot spotting technique seafloor beneath hot spot produced seamounts have crustal flow lines that intersect at the hot spot locations provided there is more than one stage rotation and that hot spots are stationary convolving seamount shapes with their associated flow lines yields an image of cumulative volcano amplitude cva we call the technique of correlating local cva maxima with hot spot locations hot spotting this technique is geometrical and requires no age information for seamounts or seafloor we analyze synthetic and actual seamount data and demonstrate that factors such as across trail scatter in seamount locations inaccurate stage poles and migration of hot spots all exert strong influences on the locations and appearances of cva maxima the interpretation of raw cva images is therefore not straightforward

the marquesas archipelagic apron seismic stratigraphy and implications for volcano growth mass wasting and crustal underplating multichannel seismic lines sonobuoy and gravity data across the marquesas islands are used to study volcano growth island mass wasting and crustal underplating at island chains with overfilled moats the marquesas bathymetry reflects the changing thickness of the sedimentary infill rather than the basement topography the moat contains two major regions of differing seismic stratigraphy 1 the moat edges where a unit of continuous layered reflectors is present containing minor lenses of chaotic diffractors and 2 the central moat where the deep moat basins are overfilled by an acoustically opaque unit of discontinuous reflectors of up to 2 km thickness in places capped by a ponded unit plate flexure models require broad underplating of the crust by low density crustal material at the marquesas islands to explain the depth to volcanic basement and gravity observations the seismic velocities and seismic stratigraphy as well as the general structure of the islands and surrounding seafloor indicate the apron is mostly debris from island mass wasting reflectors of the outermost moat generally onlap the flexural arch in the lower section and offlap and overfill it in the upper section in the central moat reflectors change shape from concave up in the lower section to convex in the upper section three dimensional diffusion models of sedimentation which incorporate a time dependent seafloor deflection from progressive island loading and vary sediment influx as islands are formed and mass waste suggest that three main factors make the moat stratigraphy at the marquesas different from hawaii 1 the marquesas moat is overfilled while the hawaiian moat is underfilled 2 sediment diffusivities are lower at the marquesas and 3 the marquesas islands are separated by deep sedimentary basins in contrast to hawaii where islands are separated by a shallow ridge the lower sediment diffusivity at the marquesas may reflect a larger proportion of blocky massive material in the central marquesas moat or alternatively a change in the dominant process of sediment transport while there is similar sediment supply for a given along moat distance at both the marquesas and hawaii the underfilled moat at hawaii is apparently a consequence of greater moat volumes due to the larger size of the hawaiian volcanoes and possibly variations in underplating that load the plate the difference in sediment edifice ratios is likely related to the larger eruption rates at hawaii and different styles of volcano construction between hawaii and the marquesas

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

elastic layer thickening with age of the oceanic lithosphere a tool for prediction of the age of volcanoes or oceanic crust summary to first order the estimated thickness te of the elastic part of the oceanic lithosphere increases linearly with the square root of the age of the lithosphere at the time of loading t in order to quantify this relationship in the particular case of volcanic loading a synthesis of te estimates reported both in this paper and in previous studies is conducted excluding the anomalously low estimates from the south central pacific the values are very consistent for the three main oceans and follow the empirical relationship te km 2 7 0 15 t ma the relationship is used to predict the age of volcanoes when the age of the crust is known for the trindade chain in the south atlantic ocean and to predict the age of the crust when the age of the volcanoes is known for the pacific plate east of the tonga trench the age estimates are in good agreement with the structural setting

toward a self consistent high resolution absolute plate motion model for the pacific the hot spot hypothesis postulates that linear volcanic trails form as lithospheric plates move relative to stationary or slowly moving plumes given geometry and ages from several trails one can reconstruct absolute plate motions apm that provide valuable information about past and present tectonism paleogeography and volcanism most apm models have been designed by fitting small circles to coeval volcanic chain segments and determining stage rotation poles opening angles and time intervals unlike relative plate motion rpm models such apm models suffer from oversimplicity self inconsistencies inadequate fits to data and lack of rigorous uncertainty estimates in addition they work only for fixed hot spots newer methods are now available that overcome many of these limitations we present a technique that provides high resolution apm models derived from stationary or moving hot spots given prescribed paths the simplest model assumes stationary hot spots and an example of such a model is presented observations of geometry and chronology on the pacific plate appear well explained by this type of model because it is a one plate model it does not discriminate between hot spot drift or true polar wander as explanations for inferred paleolatitudes from the emperor chain whether there was significant relative motion within the hot spots under the pacific plate during the last 70 m y is difficult to quantify given the paucity and geological uncertainty of age determinations evidence in support of plume drift appears limited to the period before the 47 ma hawaii emperor bend and apart from the direct paleolatitude determinations may have been somewhat exaggerated

effect of the galapagos hotspot on seafloor volcanism along the galapagos spreading center 90 9 97 6 w abstract studies along the mid atlantic ridge mar and east pacific rise epr show that seamount abundance is a strong function of spreading rate at the mar small axial seamounts are a dominant morphologic feature of the inner valley floor while at the epr seamounts are rarely observed within the neovolcanic zone the galapagos spreading center gsc provides an excellent location to test the influence of hotspot related magma supply variations on seamount formation at a relatively constant spreading rate in this study we use multibeam bathymetry data to examine the distribution of axial seamounts with distance from the hotspot along the gsc a numerical algorithm is developed to identify isolated volcanic edifices by searching bathymetry for closed concentric contours protruding above the surrounding seafloor seamount populations are fit with a maximum likelihood model to estimate the total number of seamounts per unit area and the characteristic seamount height the number of seamounts in the axial zone decreases significantly as the galapagos hotspot is approached varying from mar like abundances west of 95 5 w to epr like abundances east of 92 7 w the along axis variation in seamount density corresponds to a change in lava flow morphology from dominantly pillow flows in the west to a combination of pillows and sheet lobate flows in the east these changes in volcanic style suggest a transition from point source to fissure fed eruptions as magma supply increases we find no evidence for along axis variations in lava viscosity and therefore interpret the differences in seamount abundance and flow morphology to indicate an increase in effusion rates approaching the hotspot comparing seamount abundance with axial morphology crustal thickness and the presence and depth of an axial magma chamber amc we find that the transition from point source to fissure fed eruptions is most sensitive to the presence of a steady state amc in summary the correlation between seamount abundance lava morphology and magma supply at the gsc suggests that effusion rate is directly related to the presence of a steady state crustal magma chamber

the size distribution of pacific seamounts an analysis of wide beam sea beam and map count data in the eastern and southern pacific confirms the hypothesis that the average number of ordinary seamounts with summit heights h h can be approximated by the exponential frequency size distribution v h vo e h the exponential model characterized by the single scale parameter 1 is found to be superior to a power law self similar model the exponential model provides a good first order description of the summit height distribution over a very broad spectrum of seamount sizes from small cones h 3500 m the distribution parameters obtained from 157 000 km of wide beam profiles in the eastern and southern pacific ocean are vo 5 4 0 65 10 9m 2 and 3 5 0 21 10 3 m 1 yielding an average of 5400 650 seamounts per million square kilometers of which 170 17 are greater than one kilometer in height the exponential distribution provides a reference for investigating the populations of not so ordinary seamounts such as those on hotspot swells and near fracture zones and seamounts in other ocean basins if we assume that volcano height is determined by a hydraulic head proportional to the source depth of the magma column then our observations imply an approximately exponential distribution of source depths for reasonable values of magma and crustal densities a volcano with the characteristic height 1 285 m has an apparent source depth on the order of the crustal thickness

internal tide scattering at the line islands ridge 1 scattering of the m2 mode one internal tide from the line islands ridge is examined with a primitive equation numerical model model runs with baroclinic and barotropic forcing are performed to distinguish scattered from locally generated internal tides tpxo 5 tidal model sea surface elevations provide barotropic forcing while for the run with baroclinic forcing a mode one m2 energy flux of 1000 w m 1 is used to represent energy fluxes emanating from the hawaiian ridge scattering redistributes more energy flux from mode one than is locally generated in mode one for the higher modes scattering and generation contribute equally in terms of the overall energy flux spatial and modal distributions of energy density and flux show internal tide scattering dominates at hutchinson seamount while higher modes are generated locally at sculpin ridge hutchinson seamount s slopes are steeper over a greater continuous area than sculpin ridge scattered energy is found downstream of the steepest topographies similar to simulations with idealized gaussian ridges at the line islands ridge 37 of the incident mode one energy flux is lost because of scattering into modes 2 5 19 dissipation by the model s turbulence parameterization 15 and nonlinear transfer to the m4 internal tide 3 two topex ground tracks pass through the model domain roughly normal to the ridge topography and confirm the general features of the modal and spatial distribution found in the model in the topographically rough western pacific internal tide scattering may be a significant source of energy for mixing away from topography