continuous high velocity aseismic zone beneath the izu bonin arc since 1904 eleven unusually deep earthquakes have been reported near the southern end of the izu bonin arc about 24 n 142 e these isolated events are separated from ordinary deep earthquakes in the wadati benioff w b zone of the izu bonin i b arc by a distance of about 300 km parallel to the trench axis they also are located 200 km closer to the trench than the natural extension of the shallower earthquakes above them using 3 d ray tracing a detailed analysis is performed of the difference in travel time residuals between the anomalous events and the deep events within the w b zone to the north this analysis yields higher reliability and resolution than can be obtained from ordinary tomographic imaging we find that these unusual events occurred in a high velocity zone which is connected to the northern w b zone of the i b arc the aseismic high velocity zone has a p velocity at least 3 faster than the surrounding mantle both our result and previous evidence for a horizontally lying slab west of the w b zone at 27 n e g okino et al 1989 are consistent with the existence of a prominent contortion of the subducted slab in the i b region

seismic velocity structure of the subducting pacific plate in the izu bonin region although numerous studies have been made on the contrast between subducting slabs and surrounding mantle little is known about the internal structure of the oceanic lithosphere after it descends into the mantle at subduction zones the velocity structure within the slab can be expected to be heterogeneous based on thermal and petrological considerations in this study observed travel time residual data from a spatially dense seismic network above deep earthquakes in the izu bonin region are compared with three dimensional ray tracing calculations the data are inconsistent with a homogeneous slab model and consistent with a heterogeneous slab model with regional velocity variations the residual data can be explained by a model that has a velocity gradient within the slab in the subducting pacific plate the velocity near the center of the slab is faster than that near the upper boundary and gradually decreases toward the bottom of the plate a model with a velocity decrease of 3 as predicted by a thermal profile explains the observed data

friction of drill bits under martian pressure 1 frictional behavior was investigated for two materials that are good candidates for mars drill bits diamond impregnated segments and polycrystalline diamond compacts pdc the bits were sliding against dry sandstone and basalt rocks under both earth and mars atmospheric pressures and also at temperatures ranging from subzero to over 400 c it was found that the friction coefficient dropped from approximately 0 16 to 0 1 as the pressure was lowered from the earth s pressure to mars pressure at room temperature this is thought to be a result of the loss of weakly bound water on the sliding surfaces holding the pressure at 5 torr and increasing the temperature to approximately 200 c caused a sudden increase in the friction coefficient by approximately 50 this is attributed to the loss of surface oxides if no indication of the bit temperature is available an increase in drilling torque could be misinterpreted as being caused by an increase in auger torque due to accumulation of cuttings rather than being the result of a loss of oxide layers due to elevated bit temperatures an increase in rotational speed to allow for clearing of cuttings would then cause greater frictional heating and would increase the drilling torque further therefore it would be advisable to monitor the bit temperature or if that is not possible to include pauses in drilling to allow the heat to dissipate higher friction would also accelerate the wear of the drill bit and in turn reduce the depth of the hole

finite amplitude convection through a phase boundary summary a finite amplitude numerical model of convective motions through a phase boundary is studied the phase change is assumed to occur over a finite vertical transition zone in the fluid and thus a continuously stratified model is considered the motions are driven by horizontal temperature gradients on the boundaries or super critical vertical stratification in the case of rayleigh instability the numerical results reproduce the neutral curve previously obtained in the analytic solution of the linearized problem when the parameters of the problem are chosen to simulate the olivine spinel transition in the upper mantle the following results are obtained 1 the vertical scale of motion is the entire depth of the fluid 2 the horizontal scale is not significantly changed from the case of a single phase fluid 3 the amplitude of the motion is not significantly changed due to buoyancy changes at the phase boundary being offset by corresponding sources and sinks of latent heat and 4 the phase boundary varies in depth by as much as 30 km when the vertical velocities are of the order of 10 1 cm yr 1

upper mantle compressional velocity structure beneath the west mediterranean basin p waveforms of regional crustal earthquakes have been modeled to obtain an upper mantle compressional velocity model for the west mediterranean basin data come from long period stations of the world wide standardized seismograph network and broadband stations located in the iberian peninsula synthetic waveforms have first been computed for published velocity models corresponding to different tectonic provinces and obtained with analogous techniques a model that strongly improves the fits to the data is then derived the proposed model is characterized by a 100 km thick lid overlaying a not very pronounced low velocity zone and a 3 discontinuity at 368 km where an increase of the velocity gradient also occurs these features could be explained either as a deflection of the olivine to spinel phase transition regionally detected at about 395 km resulting from the lower temperature produced by the subduction of the african plate or as being due to the presence below 300 km depth of a layer of silicate melt producing a strong reflection from its bottom and a more usual depth for the olivine spinel transition in both cases the occurrence of relatively low velocities beneath 300 km is interpreted as being caused by the presence of melt associated with the northward dipping subduction of the african plate

using shear wave amplitude patterns to detect metastable olivine in subducted slabs 1 olivine within the lowest temperature regions of subducting oceanic lithosphere may persist metastably to depths well below the equilibrium phase boundary for transformation to phase this may produce a tapering low seismic velocity wedge within the slab in the transition zone transformational faulting of such metastable olivine has been proposed as a possible explanation of deep earthquakes detecting seismic velocity structure associated with metastable olivine within a slab using seismic wave arrival time information alone has proven problematic motivated by previous two dimensional numerical calculations for upgoing and downgoing seismic waves a three dimensional gaussian beam method is used to assess whether the azimuthally varying teleseismic amplitude signature produced by a slab with an internal wedge of metastable olivine is distinguishable from that for a slab with equilibrium transitions five slab velocity structures are considered a thermal model with horizontal phase transitions at the same depths as in ambient mantle two models incorporating equilibrium transitions within the slab and two models with tongues of metastable olivine a depth and temperature dependent thermal conductivity term and the effect of the latent heat of transformation of olivine to phase are included in the thermal modeling three dimensional dynamic ray tracing and gaussian beam summation are performed for events located at different depths and lateral positions for each of the slab models for sources near 400 km depth the presence of low velocity metastable olivine produces relatively complex azimuthal shear wave amplitude patterns compared to thermal and equilibrium models comparison with observed amplitudes for four events in the kurile slab indicates that allowing for internal slab velocity complexity in the transition zone provides better agreement with observations than simple thermal and equilibrium slab models this indicates that careful modeling of teleseismic shear wave amplitude patterns may have the resolution to detect and characterize the presence of metastable olivine in deep slabs

waveform effects of a metastable olivine tongue in subducting slabs we constructed velocity models of subducting slabs with a kinetically depressed olivine and spinel transition and examined the effect that such structures would have on teleseismic p waveforms using a full wave finite difference method these two dimensional calculations yielded waveforms at a range of distances in the downdip direction the slab models included a wedge shaped low velocity metastable olivine tongue moto to a depth of 670 km as well as a plausible thermal anomaly one model further included a 10 km thick fast layer on the surface of the slab the principal effect of moto is to produce grazing reflections at wide angles off the phase boundary generating a secondary arrival 0 to 4 seconds after the initial arrival depending on the take off angle the amplitude and timing of this feature vary with the lateral location of the seismic source within the slab cross section careful analysis of waveforms from earthquakes with depths near 400 km simple sources and adequate station coverage in appropriate geometries will be required to resolve whether moto is present

p and s wave travel time inversions for subducting slab under the island arcs of the northwest pacific we have observed slablike high p and s velocity anomalies around the wadati benioff zone under island arcs of the northwest pacific through travel time tomographic inversions nineteen years of international seismological centre travel time residuals for events and stations in this large region are used analyses of resolution and noise show that the images are generally well resolved the images illustrate that slab anomalies are continuous along strike in most parts of the upper mantle of the region and become contorted and generally broadened with depth near the bottom of the upper mantle fingering of the slabs including segmenting and spreading is indicated the fast anomalies associated with the japan izu bonin and mariana subduction zones tend to flatten to subhorizontal at depth while downward spreading may occur under parts of the mariana and kurile arcs the fast anomalies below 700 km are not in the shape of a single coherent sheet the principal compressional axes of focal mechanisms in the region consistently follow the downdip direction of the high velocity slab even when it bends to subhorizontal at depth the depth at which compression begins to dominate the downdip stress regime in the slab apparently depends on bending of the slab and its dip slab fingering and intense deep seismicity probably are the consequence of the slab encountering a barrier of some form around the 670 km discontinuity

teleseismic p wave delays and modes of shortening the mantle lithosphere beneath south island new zealand a high speed zone in the mantle directly beneath the southern alps of new zealand is required by the recorded pattern of teleseismic p waves two parallel lines of 80 seismographs spaced at 2 km intervals recorded three earthquakes from the western pacific with epicentral distances of 52 53 and 78 azimuthal bearings were all within 15 degrees of the mean trends of the seismograph lines differences between measured delays and those predicted from the crustal structure reach 0 8 s along one line and 1 0 s along the other with the rays for the earliest arriving signals passing the depth of 120 km beneath the center of the island assuming these early arrivals are due to structure within the mantle shallower than 200 km they imply that the core of the high speed zone lies beneath the thickest crust which has been shortened by 100 km of convergence during the past 6 7 myr although the shape and position of the high speed body cannot be fixed uniquely a roughly symmetric body centered about a depth of 120 km 80 100 km wide with a depth extent of 100 km and with a maximum speed advance of 7 satisfies the observations the pattern of residuals does not fit with those predicted by simple models of intracontinental subduction in which crust and mantle lithosphere are detached and one slab of mantle lithosphere underthrusts the other rather the residuals favor thickening of mantle lithosphere by a more homogenous straining of it as if mantle lithosphere beneath continental crust behaved as a continuum an excess mass in the mantle is also required by the observed gravity anomalies once allowance is made for the seismically determined crustal thickness this high density mantle anomaly provides sufficient force per unit length to maintain the crustal root which is approximately twice as thick as that necessary to support the topography

laboratory drilling under martian conditions yields unexpected results 1 current plans to drill boreholes in the surface of mars suppose that diamond drill bits will be used on drilling equipment developing drilling power of 100 w or less such drilling produces very fine cuttings at martian ambient pressure it has been shown that quite small gas flows are capable of clearing these cuttings from the hole for example the release of 1 l of gas compressed to 50 torr is capable of removing 25 g of cuttings from a borehole 50 mm in diameter and at least 250 mm deep generating such volumes of gas by compressing the martian atmosphere would not be difficult alternatively the required volume of gas could be provided by the sublimation of ice trapped in the pore spaces of soil or rock in the martian surface once heated by the friction of the drill bit the ice transforms into vapor and blows the cuttings out of the hole since the cuttings are in effect freeze dried they do not choke the drill bit or hole with the result that the drilling efficiency is much greater than under terrestrial pressures and temperatures

uniaxial compressive strengths of rocks drilled at gale crater mars measuring the physical properties of geological materials is important for understanding geologic history yet there has never been an instrument with the purpose of measuring mechanical properties of rocks sent to another planet the mars science laboratory msl rover employs the powder acquisition drill system pads which provides direct mechanical interaction with martian outcrops while the objective of the drill system is not to make scientific measurements the drill s performance is directly influenced by the mechanical properties of the rocks it drills into we have developed a methodology that uses the drill to indicate the uniaxial compressive strengths of rocks through comparison with performance of an identically assembled drill system in terrestrial samples of comparable sedimentary class during this investigation we utilize engineering data collected on mars to calculate the percussive energy needed to maintain a prescribed rate of penetration and correlate that to rock strength

seismic structure of the deep kurile subduction zone three dimensional modeling of p wave travel time residuals was performed to investigate the deep slab related velocity structure within the kurile subduction zone travel times of p waves from earthquakes at all depths within the kurile subduction zone to world wide standard seismograph network and canadian stations were measured instead of analyzing the travel time residuals of a single event relative travel time residuals of p waves to common stations have been analyzed and used in this study to isolate near source contributions from the total travel time residuals the effect of uncertainties in long wavelength heterogeneity far from the source in the lower mantle and the effect of uncertainties in the reference earth model have been significantly reduced by analyzing relative travel time residuals the effect of uncertainties in station corrections are eliminated by relative travel time residuals our measured p wave travel time data clearly indicate that high velocity anomalies exist in the lower mantle beneath the kurile subduction zone but they do not appear to be caused by simple slab continuation into the lower mantle the travel time modeling was accomplished using a fully three dimensional finite difference technique for wave propagation within the source region the modeling results show that high velocity anomalies extend several hundred kilometers into the lower mantle but they appear to be very complicated a large number of models consisting of simple slab continuation into the lower mantle were tested and the results show that none ofthese models was able to produce synthetics which match the observed p wave travel time data the slab related velocity anomalies in the upper mantle of the kurile subduction zone are generally consistent with the seismicity the high velocity anomalies in the lower mantle below the kurile subduction zone are broad and geometrically change from the southern to the northern kuriles below the southern kuriles the high velocity anomalies appear to be subhorizontal however beneath the central kuriles there are broad about 600 km wide high velocity anomalies on the continental side of the slab which extend several hundred kilometers into the lower mantle the high velocity anomalies found below the northern kuriles are broad and on both the continental and the ocean side of the slab and extend several hundred kilometers into the lower mantle

investigation of diamond impregnated drill bit wear while drilling under earth and mars conditions 1 experiments conducted on a dry and a water saturated rock under martian and earth atmospheric pressures revealed two different wear behaviors in diamond impregnated drill bits when the rock was saturated drilling under martian pressure caused the water in contact with the rotating bit to vaporize since the volumetric expansion of the liquid water or ice as it turned into a vapor was 170 000 the continuous flow of water vapor cleared the cuttings out of the hole thus the bit matrix was always exposed to abrasive wear by the rock cuttings and was continually wearing down and exposing new diamonds to the rock when the rock was dry an accumulation of rock cuttings protected the matrix from abrasive wear since fresh diamonds were not exposed in a timely manner the rate of penetration dropped both rock conditions namely dry or water saturated may exist on mars this adds to the complexity of the drill bit design as ideally a bit should penetrate the rock irrespective of whether it is dry or water saturated the fail safe bit would have a very soft matrix to always produce some rock penetration at the expense of potential excessive bit wear and shallower than anticipated hole depth

three dimensional seismic structure beneath southwest japan the subducting philippine sea plate abstract a three dimensional seismic structure beneath southwest japan has been investigated in detail by applying an inversion method this analysis clearly reveals the present and possibly the past subduction of the philippine sea plate beneath southwest japan beneath the izu peninsula the philippine sea plate does not appear to subduct but collides against the continental asian plate here and its thickness appears thinnest at most about 30 km from the tokai to the shikoku regions the high velocity zone corresponds well to the zone of subcrustal earthquakes down to a depth of about 60 70 km indicating that the subduction of the present philippine sea plate does not reach below this depth here however a small high velocity portion appears at depths between 100 km and 150 km beneath a part of the kinki region and probably extends to the chugoku region where no corresponding seismic activities exist these high velocities might be an indication of the remnants of the past subduction of the philippine sea plate in the kyushu region the high velocity zone continues down to a depth of about 150 km or even to 200 km in the southern part corresponding to a steeply inclined seismic zone the philippine sea plate in this region appears to be thicker than in regions east of the kyushu palau ridge it was also found that there exist narrow low velocity zones intervening within the philippine sea plate where two aseismic ridges the kyushu palau and shichito iwojima ridges extend northwards and the nankai trough shows warping these low velocity zones give support to the buoyant subduction hypothesis it also appears that lateral heterogeneities existing within and around the subducting plate are closely related to the mode of rupture of large earthquakes occurring around there

velocity structure in the izu bonin seismic zone and the depth of the olivine spinel phase transition in the slab in this study a method is presented for determining the in situ elastic wave velocities of slabs simultaneously with the relative relocations of earthquakes the method is an extended arrival time difference atd scheme that incorporates a realistic model several assumptions are made about the characteristics of ray paths in and around the slab in order to take advantage of the realism of the model without completely sacrificing the simplicity inherent in the atd technique the errors introduced by these approximate rays are estimated by comparing them with more exact rays calculated with a three dimensional ray tracing routine in general the comparisons suggest that these errors are usually much less than 0 1 s the inverse method is tested on the same set of hypothetical data the results of these tests show that the parameters that influence the arrival time differences the most are the amplitude of the slowness anomaly within the slab and the azimuths and takeoff angles for rays leaving the master event the arrival time differences are relatively insensitive to the other parameters describing the slab strike dip half width and the position of the master event which implies that the scheme is fairly insensitive to the particular choice of analytic function to describe the slowness variation within the slab convergence of the inverse routine is rapid in the tests usually less than five iterations are required before a stable solution is reached it appears that coupling between solutions for slab parameters and ray directions is insignificant unlike the standard atd approach solutions for hypocenters and velocities are not directly coupled in this scheme although well located events are required to insure that the hypocenters are not unduly biased by variations in structure once the velocity structure of the slab is determined that information can be used routinely by this scheme to relocate less well recorded events this method was applied to arrival time data from earthquakes located between 200 and 600 km depth in the izu bonin subduction zone structure in the 200 300 km depth range could not be resolved and results in the 500 600 km depth range proved to be too sensitive to noise data from events between 27 n and 31 n show that p wave velocities within the slab from 400 to 500 km depth are 4 6 higher than those of the ambient mantle between 300 and 400 km depth the slab structure varies regionally north of about 32 5 n the p wave velocity within the slab is 3 4 higher than ambient from 180 to 375 km depth in contrast the velocities south of 32 5 n increase from 6 7 higher than ambient in the 375 410 km depth range to 8 10 higher in the 325 375 km depth range and appear to be 9 higher than ambient in the 270 325 km depth range the determination of the high velocities in this region between 325 and 375 km depth proved to be independent of the selection of data and of the choice of starting model above 325 km depth however the velocity determinations are somewhat dependent on the choice of starting model as a result of this dependence no upper bound on the depth of the high velocities could be resolved a plausible explanation for the exceptionally high p wave velocities within the slab south of 32 5 n is that the olivine spinel phase change boundary is elevated in this part of the slab while north of 32 5 n it is not significantly elevated it is not known if the phase change is elevated south of 31 n but the velocities between 400 and 500 km depth in this region imply that it is not depressed the high velocity zone south of 32 5 n lies in an area that marks the beginning of a seismic gap increasing in breadth to the south between deep and shallow seismicity the dip of the slab also becomes gradually steeper to the south of this region in contrast the lower velocities north of 32 5 n occur in a region where the seismicity is fairly continuous and is aligned along a shallow dipping plane if the seismic gap south of 32 5 n represents a region of high velocity then the increasingly vertical dip of the slab in the south could be due to a substantial gravitational force provided by the excess mass extending above the ambient olivine spinel phase boundary the decreasing width of the gap to the north suggests that the evolution of the izu bonin seismic zone has been governed by a propagating instability from the south

the influence of olivine metastability on the dynamics of subduction abstract both seismological observations and mineral physics experiments suggest that old and cold subducting slabs might contain a wedge of metastable olivine mo dynamically consistent models of subducting slabs in which the kinetics of the olivine to spinel wadsleyite or ringwoodite transition is treated in a simplified way are carried out to evaluate the buoyancy effect of mo on the subduction velocity assuming slab thicknesses according to the cooling half space model we find that mo forms in slabs with ages greater than approximately 70 myr and that a significant slowing down of subduction velocity occurs for ages greater than 100 myr because a decrease in the subduction velocity for old lithospheric ages is not observed our models suggest that the amount of mo in slabs is less than approximately 5000 km2 in cross section and that for old lithosphere the cooling half space model is not applicable

seismological evidence for metastable olivine inside a subducting slab the nature and location of the olivine spinel phase transition inside subducting slabs differ greatly from the situation in the surrounding mantle due to the different temperature distribution inside the slabs two models have been proposed for this phase transition one in which the location of the phase boundary between olivine and modified phase spinel is determined by equilibrium thermodynamics1 and the other including a metastable olivine phase which persists to a depth of 550 km ref 2 the location of the olivine spinel transition in the slab may be relevant to the generation of deep earthquakes3 5 and to the buoyancy forces driving subduction6 here we use travel time residuals from deep earthquakes recorded by the dense seismograph network in japan to investigate the configuration of the olivine spinel phase boundary inside the subducting pacific plate theoretical travel time residuals for the equilibrium model do not fit the observed residuals whereas those for the metastable model do implying the presence of metastable olivine inside the subducting slab

simultaneous travel time inversion for earthquake location and subduction zone structure in the central aleutian islands a combined location and velocity inversion technique is applied to travel time data from 151 well recorded central aleutian earthquakes the data include p and s wave arrivals at stations of a local network and p pp and sp wave arrivals at teleseismic stations after correction for crustal structure at the reflection points the depth phases pp and sp provide important constraints on subduction zone structure not ordinarily resolved by other types of data the structure is assumed to vary only across the arc and in depth it is parameterized with cubic splines by specifying the velocity at each point of a gridded arc cross section to avoid ray tracing it is assumed that the velocity part of the problem is linear preliminary results show that deep earthquakes apparently occur within a narrow downdip zone near the upper surface of the descending slab in contrast with previous studies which have located them within the presumably stronger and colder inner core of the slab a slab thickness of 80 100 km and a downdip length of about 400 km well below the deepest seismic activity are indicated the slab is characterized by seismic velocities as much as 11 higher than the surrounding mantle in its upper portions and 4 6 higher at depth a sharp velocity gradient and lower velocities occur directly beneath the volcanic arc near the top of the slab the slab anomaly appears to spread out and fall off very slowly with depth these are probably not real effects but consequences of the omission of three dimensional ray tracing

an anomalous low velocity region above the deep earthquakes in the japan subduction zone p wave travel time residuals for local intermediate and deep focus earthquakes are studied to infer a velocity model of the asthenosphere beneath the southern okhotsk sea and the western japan sea tectonically both regions belong to plate subduction zones large positive travel time anomalies are observed at stations above the deepest earthquakes consideration of source path and receiver effects leads to a conclusion that within the asthenosphere there is a lower velocity region below about 300 km depth with velocities about 5 10 below the jeffreys bullen velocity model

rock abrasion tool mars exploration rover mission 1 the rock abrasion tool rat is an integral part of the athena science payload serving primarily as the geologist s rock hammer the rat will expose fresh surfaces of martian rocks to other instruments on the payload the rat also brushes dust and debris from an excavated hole or unaltered rocks to accomplish these tasks autonomously the rat a sophisticated 3 axis precision controlled device was designed data products derived from rat telemetry enable the rat to also act as an important rock physical properties science instrument the returned rat grinding and penetration rate data will be inverted and compared to a rock library on the earth the design is also very compact and lightweight the rat is contained within a cylinder 128 mm long and 85 mm in diameter and has a mass of 687 g

topography of the transition zone seismic discontinuities phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities nominally at 410 520 and 660 km depth thermodynamic principles govern phase transformations making them sensitive to changes in the mantle s ambient conditions through the thermodynamic response changes in temperature or composition shift the transformation to a different pressure creating topography on a level discontinuity with this use as an exploratory tool for the mantle in mind we review the basic seismological and phase equilibrium concepts underlying the detection and interpretation of the seismic wave arrivals associated with the transition zone discontinuities reviewing the evidence for and against the phase transition model we conclude that it is viable and describe how discontinuities have been and can be used to probe the physical and chemical state of the transition zone

amplitude distribution of seismic waves for laterally heterogeneous structures including a subducting slab summary we demonstrate the effects that the subducting plate 3 d velocity and q structures of the upper mantle beneath japan can have on the amplitudes of seismic waves observed above the inhomogeneous structure and we explore the possibility of finding the fine velocity and q structures by means of the amplitude distribution the methods used for the computation of the seismograms are the ray tracing and also gaussian beams which give more accurate amplitudes at caustics and shadow zones first we construct a slab velocity model by calculating the temperature distribution within and around the subducting slab and investigate the effects of relative source location within the slab possible detachment of shallow and deep parts of the slab and elevation and depression of the 400 km discontinuity within the slab the possibility of detecting the location of the leading edge of the subducted slab is also examined by calculating the amplitude distribution of incident plane waves the distribution of the ray amplitudes is also calculated for a velocity model including the pacific plate subducting beneath japan we finally adopt a 3 d velocity and q structure model including the subducting pacific plate and philippine sea plate in the uppermost mantle beneath japan obtained from previous studies the calculations for incident plane waves reveal that there may be large variations of the amplitudes on the surface it is found that the maximum amplitude distribution of waveforms calculated by the gaussian beam method for most of the above cases gives a smoothed pattern of the corresponding ray amplitudes and that the amplitudes for low frequency waves are smoother than those of high frequencies from all these calculations we find that the amplitude distributions are very sensitive to the velocity heterogeneities suggesting that the amplitude distribution may be used to obtain the source location within the slab and fine velocity and q structures of the upper mantle near the subduction zone

mapping of p wave slab anomalies beneath the tonga kermadec and new hebrides arcs abstract p wave velocity anomalies of several per cent fast relative to the jeffreys bullen jb model are seen around subduction zones under the tonga kermadec and new hebrides trenches from inversion of international seismological centre isc travel time residuals from intermediate and deep focus earthquakes mantle heterogeneities outside the model are corrected using previous velocity models ray paths inside the model region are traced allowing two dimensional velocity variations the rays of similar source and receiver locations are combined into summary rays and the hypocenters are relocated as part of the inversion iterations the resulting images are interpreted in conjunction with resolution and error estimations the slab like anomalies are generally continuous in places of high seismicity and decrease in amplitude with depth most coherent slab anomalies are surrounded by large slow patches between the surface and 350 km depth the fast slab beneath the tonga arc appears to end at 550 km depth before the seismicity ends but there is a fast band at 750 1000 km depths below the deepest tongan earthquakes the fast slab like anomaly beneath the kermadec arc tends to flatten to subhorizontal near the bottom of upper mantle and it occurs aseismically near new zealand down to 500 550 km depth interpretations in the lower mantle in many places such as kermadec and new hebrides arcs however are hampered by poor resolution

surviving the limits to life at the surface of mars the harshness of the environment at its surface severely limits the survival of unprotected terrestrial organisms on mars however the martian geologic record is consistent with an earlier history with much different climatic conditions more conducive to life followed by an extended climate change which led to the currently inhospitable state such a transition is ideal for evolutionary adaptation to new conditions because it provides both the forcing function and the extensive time for progressive specializations it is shown that against each of the current limits to life organics deprivation restricted amounts and physical forms of h2o limited availability of other key nutrients ionizing radiation insult and strongly oxidizing chemical species there are plausible countermeasure solutions most but not all of these mechanisms already exist as known examples in terrestrial biota a combination of these adaptations could conceivably allow highly tailored organisms either active or dormant to survive at or very near the surface of mars even in carefully preserved regolith samples returned to earth microorganisms could be difficult to detect without specific biochemical probes and or purposeful biological amplification by selective enhancement of the supply of nutrients optimized incubation parameters and highly sensitive detection of metabolic activity

the effects of metastable olivine α wedge in subducted slabs on theoretical seismic waveforms of deep earthquakes 1 assuming three kinds of materials 100 olivine mg0 89fe0 11 2sio4 pyrolite and piclogite as composition of the upper mantle we constructed pressure temperature dependent seismic velocity structure models in the upper mantle beneath a subduction zone for this purpose we employed values of physical properties of composite minerals newly determined by using a genetic algorithm ga and obtained the thermal structure beneath the subduction zone as a numerical solution of a two dimensional 2 d thermal fluid coupled problem furthermore we investigated the effects of a metastable olivine wedge in subducted slabs on theoretical seismic waveforms of deep earthquakes which propagate through the velocity structure by solving the p sv problem numerically the effects of center frequency of the excited waves at hypocenters focal mechanism location of hypocenter upper mantle compositions values of physical properties a dip angle of subducting slabs anelastic attenuation subducting velocity and depth of deep earthquakes on seismic waveforms were investigated in detail theoretical seismograms at the earth s surface indicate that coda waves with small amplitudes of duration time of 20 30 s appear following direct p and s waves for models with the metastable olivine wedge inside the slabs the coda waves can be explained by the excitation by reflections and conversions at a large seismic velocity discontinuity between the metastable olivine wedge and the surrounding slab for models with center frequency of 0 25 hz purer olivine composition a faster subducting velocity and focal depth of 500 km waves with larger amplitudes were excited at the discontinuity the results of this study may give important clues to detect the existence of the metastable olivine wedge from seismological observations

the p wavespeed structure in the mantle to 800 km depth below the philippines region geodynamic implications abstract p waves from earthquakes south of taiwan recorded by the bats seismic array and cwb seismic network were used define the p wavespeed structure between depths of 100 and 800 km below the philippines region the presence of a low wavespeed zone in the upper mantle is inferred although the details are unclear wavespeeds in the uppermost mantle are low as expected for seismic energy propagating within an oceanic plate the estimated depths of the 410 and 660 km discontinuities are 325 and 676 km respectively the unusually shallow depth of the upper discontinuity below and to the east of luzon is inferred by clearly resolving the travel time branch produced by refraction through the transition zone a possible explanation for the northern part of the region covered is that seismic energy reaches its maximum depth within or close to the cool subducted oceanic south china sea slab where subduction has been slow and relatively recent further south however the presence of a broken remnant of the south china sea slab formed during a period of shallower subduction is suggested at depths below 300 km to explain the broad extent of the elevated 410 km discontinuity the 660 km discontinuity is slightly deeper than usual implying that low temperatures persist to lower mantle depths the wavespeed gradients within the transition zone between depths of 450 and 610 km are higher than those predicted by both the pyrolite and piclogite models of the mantle possibly due to the presence of water in the transition zone

modeling the tonga slab can travel time data resolve a metastable olivine wedge we present the results of detailed modeling of the tonga slab with the goals of determining whether high resolution travel time data 1 can be fit by simple thermal and petrological slab models and 2 can resolve a metastable olivine wedge at depths greater than the equilibrium olivine spinel phase boundary we model arrival times recorded by a 1000 km line of 23 ocean bottom seismometers obs and island broadband seismic stations extending from the lau backarc basin across the tonga trench and onto the pacific plate the data consist of 388 local p wave travel times from 17 deep and 3 intermediate earthquakes recorded during the 3 month obs deployment in late 1994 we locate the events using both local and teleseismic arrival times and apply a relocation operator to the theoretical travel times to simulate the biases introduced in the data by locating the events with a reference earth model the modeling consists of grid searches using a three dimensional finite difference algorithm to compute local first arriving travel times for equilibrium and metastable p wave velocity models constructed from thermal mineralogical and morphological constraints the travel time anomalies are well fit by standard slab thermal models and p velocity temperature derivatives of 0 4 to 0 3 ms 1 c 1 forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low velocity region wedge velocity models provide a slightly better fit to the data than equilibrium models but f tests indicate the improvement is not significant at the 95 level our results suggest that providing direct seismological evidence of a wedge of metastable olivine in subduction zones will require either waveform modeling or the observation of later arriving phases created by the depressed phase boundary

structure of the earth mantle and core the deep interior structure of the earth has been extensively analyzed using a wide variety of seismic phases and techniques during the last four years most studies have emphasized quantitative three dimensional mapping of the lateral velocity heterogeneity of the mantle and core these aspherical velocity variations are believed to be direct manifestations of thermal and compositional heterogeneity associated with convective processes a first generation of global models for the lateral velocity variations in the deep earth has been produced providing tantalizing images of large scale structures suggestive of a non steady state thermal convection system the upper 400 km of the mantle has the strongest lateral velocity variations of up to 10 for shear velocity surface wave analyses that do not require a priori regionalizations have demonstrated that there is a strong association between surface tectonic provinces and the uppermost mantle velocity variations thus the thermal and convective state of the upper 200 km of the mantle can be reliably interpreted in the context of plate tectonics the upper mantle models support the contention that continents have deep roots with differences in velocity structure from oceanic and active tectonic regions extending as deep as 400 km very long wavelength lateral velocity variations of a few percent have been detected in the transition zone at depths from 400 to 670 km as well as throughout the lower mantle these deep seated variations have little correspondence to surface tectonics and efforts to interpret their nature are just beginning the lowermost 200 km of the mantle d region has lateral velocity fluctuations comparable to those in the upper mantle and evidence has been presented for the presence of a sizable velocity discontinuity at the top of the d layer a combined thermal and compositional boundary layer roughly mirroring the lithosphere is a likely explanation for this anomalous zone the core mantle boundary appears to have significant 10 km long wavelength topography presumably sustained by dynamic stresses from deep mantle convection the inner core may have strong lateral heterogeneity or axially symmetric anisotropy suggesting a complex thermal and compositional state significant progress has been made in characterizing the frequency dependence of anelastic attenuation in the mantle in the short period body wave band models for teleseismic p wave attenuation operators have converged with t values of 0 7 1 0 s appropriate at 1 hz and t values of 0 4 0 6 s appropriate at 4 hz regional variations of attenuation are slowly being mapped out as well

shoemaker crater as the source of most ejecta blocks on the asteroid 433 eros the loose material regolith on the surfaces of asteroids is thought to represent ballistically emplaced ejecta from impacts but the identification of source craters and the detailed study of the regolith modification have been hampered by the limited spatial resolution and area coverage of the few asteroids imaged by spacecraft here we report the results of global mapping of the asteroid 433 eros from high resolution images obtained by the near shoemaker spacecraft based on the images and ejecta emplacement models we suggest that most large ejecta blocks on eros originate from a relatively young 7 6 km diameter crater a large fraction of the ejecta from impacts pre dating that crater has apparently been buried or eroded the images also show evidence for the action of a variety of sorting environments for regolith particles after they are deposited on the surface

subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab mantle interface seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested these studies employing waves reflected and converted at the slab mantle interface suggest that a velocity contrast of 5 10 occurs over a distance of 10 20 km to depths of at least 350 km the authors have investigated whether such sharp contrasts can result form thermal structure compositional differences phase changes or other effects such as preferred mineral orientation using a mantle and slab temperature model a petrologic model of mantle and slab mineralogy and a data base of elastic properties of mantle and slab phrases theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints at 9 6 gpa and 1000c conditions modeled at the slab mantle interface at approximately 280 km depth cooler temperatures in the slab interior produce at constant composition only approximately a 1 75 v sub p increase into the slab in any of the peridotitic and eclogitic mineralogies investigated variation in v sub p among the different bulk compositions is only approximately 0 5 these contrasts are substantially lower than those obtained from the seismological studies andmore indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts they propose that phase transformations in mantle and slab mineralogies notably elevation of the olivine spinel phase change normally occurring at 400 km depth are important factors in these sharp contrasts other contributing effects may include preferential mineral orientation near the slab surface and at shallower depths the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface less

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

the waveguide effect of metastable olivine in slabs we use a finite difference algorithm to compute p sv synthetic seismograms for deep earthquakes recorded at regional seismic stations located above a subducting slab we calculate synthetics for 2 d slab models in which the 410 km discontinuity is uplifted owing to the exothermic nature of the transition the equilibrium model and models in which the 410 km discontinuity is depressed owing to inhibited kinetics of the olivine transition the metastable model in the latter case the existence of the low velocity wedge of metastable olivine has a profound influence on the waveforms of deep focus earthquakes the wedge acts as a geometrical waveguide with the energy focusing effect apparent even after the wavefront has traveled the several hundred kilometers from the wedge boundary to the earth s surface the guided energy is most pronounced for receivers located near the surface projection of the deep slab but is observable over a range of 300 km and so may provide a valuable diagnostic for discriminating between equilibrium and metastable models of subduction a recent search for such guided energy using regionally recorded p and s waves from deep events in tonga yielded negative results

in situ evidence for an ancient aqueous environment at meridiani planum mars sedimentary rocks at eagle crater in meridiani planum are composed of fine grained siliciclastic materials derived from weathering of basaltic rocks sulfate minerals including magnesium sulfate and jarosite that constitute several tens of percent of the rock by weight and hematite cross stratification observed in rock outcrops indicates eolian and aqueous transport diagenetic features include hematite rich concretions and crystal mold vugs we interpret the rocks to be a mixture of chemical and siliciclastic sediments with a complex diagenetic history the environmental conditions that they record include episodic inundation by shallow surface water evaporation and desiccation the geologic record at meridiani planum suggests that conditions were suitable for biological activity for a period of time in martian history