upper crustal seismic velocity structure and microearthquake depths at the endeavour segment juan de fuca ridge 1 we present the results of a study to invert microearthquake and explosive shot data from the endeavour segment of the intermediate spreading juan de fuca ridge the average isotropic p wave velocity structure derived from the shot data in the uppermost 1 5 km of the oceanic crust is characterized by an increase with age of 8 from the axis to at least 0 5 ma that is attributed to the sealing of layer 2a porosity by hydrothermal processes superimposed on this variation are axis parallel 2 km wide alternating bands of high and low velocity with a peak to peak variation of 5 12 high and low velocities away from the axis correspond to bathymetric trenches and ridges respectively and are likely due to variations in layer 2a thickness p wave azimuthal anisotropy is present in the data that is best fit with a model of 9 anisotropy at 750 m depth decreasing to 1 at 3 km depth and is likely due to the preferential alignment of vertical cracks and fissures in the along axis direction anisotropy and velocity heterogeneity are coupled anisotropy alone may explain the form but not the magnitude of the axis parallel bands there are strong trade offs between the hypocentral depths of microearthquakes and the p and s wave velocity structures changing the mean hypocentral depth by up to 0 5 km leads to only modest increases in the travel time rms but the resulting velocity models appear more feasible when the earthquakes are forced deeper than when they are forced shallower

a detailed seismic anisotropy study during the 2011 2012 unrest period in the santorini volcanic complex abstract the santorini volcanic complex svc is an area in the southern aegean greece which has been characterized by low seismicity rates for the last decades especially in the santorini caldera where they have been very low until 2010 this pattern changed completely in february 2011 when intense microseismic activity was initiated within the caldera during the manual analysis of the events the shear wave splitting phenomenon was observed revealing the existence of an anisotropic upper crust in the svc area a detailed anisotropy study has been conducted using 231 events within the shear wave window that fulfilled the selection criteria the polarization direction of the fast shear wave the time delay between the two split shear waves and the source polarization direction were calculated after visual inspection using both the polarigram and the hodogram representations this procedure applied for eight local stations resulted in the determination of 340 splitting parameters the obtained mean anisotropy directions are not homogeneous revealing a complex regime in the activated area nevertheless these results are explained by the ape model related to the stress sensitive behavior of fluid saturated microcracked rocks a detailed analysis of the temporal evolution of both the time delay and anisotropy direction was carried out the time delays measured in the band 1 window exhibit gradual increase and sudden drop that can be related to imminent bursts of seismicity as well as to the major m w 5 1 and 5 2 events which took place about 40 km sw of santorini on 26 and 27 january 2012 respectively on the other hand no significant temporal variations or 90 flips of the s fast polarization direction were observed

geology of a vigorous hydrothermal system on the endeavour segment juan de fuca ridge a high precision high resolution geologic map explicitly documents relationships between tectonic features and large steep sided sulfide sulfate silica deposits in the vigorously venting endeavour hydrothermal field near the northern end of the juan de fuca ridge water depth in the vent field varies from 2220 to 2200 m location of the most massive sulfide structures appears to be controlled by intersections of ridge parallel normal faults and other fracture fissure sets that trend oblique to and perpendicular to the overall structural fabric of the axial valley the fractured basaltic substrate is primarily composed of well weathered pillow and lobate flows as presently mapped the field is about 200 by 400 m on a side and contains at least 15 large 1000 m3 sulfide edifices and many tens of smaller commonly inactive sulfide structures the larger sulfide structures are also the most vigorously venting features in the field they are commonly more than 30 m in diameter and up to 20 m in height actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller maximum venting temperatures of 375 c are associated with the smaller structures in the southeastern portion of the field highest temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20 30 c lower hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture controlled black smoker activity near the top of the structures a different type of high temperature venting takes place from the upper sides of the structures in the form of overflow from fully exposed quiescent pools of buoyant 350 c vent water trapped beneath overhanging sulfide sulfate silica ledges or flanges these flanges are attached to the upper outer walls of the large sulfide edifices two types of diffuse venting in the endeavour field include a lower temperature 8 15 c output through colonies of large tubeworms and 25 50 c vent fluid that seems to percolate through the tops of overhanging flanges the large size and steep walled nature of the these structures evidently results from sustained venting in a mature hydrothermal system coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development

geodetic imaging reservoir monitoring using satellite interferometry summary fluid fluxes within subsurface reservoirs give rise to surface displacements particularly over periods of a year or more observations of such deformation provide a powerful tool for mapping fluid migration within the earth providing new insights into reservoir dynamics in this paper we use interferometric synthetic aperture radar insar range changes to infer subsurface fluid volume strain at the coso geothermal field furthermore we conduct a complete model assessment using an iterative approach to compute model parameter resolution and covariance matrices the method is a generalization of a lanczos based technique which allows us to include fairly general regularization such as roughness penalties we find that we can resolve quite detailed lateral variations in volume strain both within the reservoir depth range 0 4 2 5 km and below the geothermal production zone 2 5 5 0 km the fractional volume change in all three layers of the model exceeds the estimated model parameter uncertainty by a factor of two or more in the reservoir depth interval 0 4 2 5 km the predominant volume change is associated with northerly and westerly oriented faults and their intersections however below the geothermal production zone proper the depth range 2 5 5 0 km there is the suggestion that both north and northeast trending faults may act as conduits for fluid flow

geodetic imaging of thermal deformation in geothermal reservoirs production depletion and fault reactivation abstract we investigate thermally induced surface deformation in geothermal systems to define source mechanisms at depth we assess the mechanical process of subsurface deformation by assuming a spherically cooled fractured reservoir in an infinite medium and derive relations that define magnitudes of thermal contraction stress change and permeability evolution the magnitude of thermal deformation in typical geothermal system is larger than anticipated and suggests two different modalities of surface subsidence thermal contraction and fault reactivation here surface deformation vertical displacement surface tilt and horizontal strain induced by the two different modalities are assessed with mogi contraction and okada slip models and compared with instrumental sensitivity of high precision surface geodetic tools we show that 1 year of geothermal operation at 10 mw with a power plant conversion efficiency of 12 can yield 3 0 104 m3 of subsurface volume change for a reservoir at 2000 m depth this induces 1 7 mm of vertical surface displacement 800 nano radians of surface tilt and 900 nano strains of surface strain this result implies that typically observed magnitudes of surface subsidence order of cm year are naturally expected in massive 100 mw scale geothermal operations and observed surface subsidence may largely be the result of thermal contraction conversely thermal unloading can trigger fault reactivation analysis with an okada slip model shows these shear offsets on pre existing faults can also result in surface deformations of considerable magnitude our analysis of field operational data from various geothermal projects suggests that both thermal contraction and slow fault reactivation may contribute to the observed large surface deformation comparison of predicted deformation with instrumental sensitivity of high precision surface tools confirms that geodetic signals especially tilt and strain are indeed sufficiently large to describe reservoir evolution and to potentially deconvolve reservoir parameters of interest such as permeability

shear wave polarization alignments recorded above the kaoiki fault zone hawaii eighty four shear wave records from the seismometer station ain at kaoiki hawaii covering the years 1979 1983 were examined for the presence of shear wave splitting and possible precursory change associated with a mainshock in 1983 time delayed arrivals with different polarizations were detected suggestive of shear wave splitting and the polarizations of the first arriving shear waves were aligned in the direction ese wnw this direction is consistent with the anisotropy being caused by cracks aligned approximately perpendicular to the direction of least principal stress which is uniform over the ray path in an otherwise heterogeneous stress field no time dependent variation of the polarization of the leading split shear wave was observed investigation of temporal variation of time delays between split shear waves was not possible due to the presence of scattered arrivals

seismic attenuation structure beneath the juan de fuca ridge from tomographic inversion of amplitudes tomographic inversion of seismic amplitudes for attenuation structure has been applied to first arrival amplitudes from a seismic refraction profile across the juan de fuca ridge to further constrain a velocity structure previously obtained by travel time velocity tomography and to obtain a model for attenuation variations within the shallow crust amplitude analysis has identified an additional boundary defined by a decrease in vertical velocity gradient at 2 0 km subbottom which was not required by the travel time data this transition zone likely represents the oceanic layer 2 layer 3 boundary the analysis also identified zones of relatively high attenuation qe 20 100 at subbottom depths of 0 3 2 0 km beneath the ridge axis which correlate well with low velocity zones velocities reduced by 0 4 km s in the velocity model the subaxial zone is interpreted as a region of increased fracture porosity and or permeability associated with axial hydrothermal circulation within the extrusives and sheeted dike complex

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

initial shear wave particle motions and stress constraints at the anza seismic network summary we use focal plane solutions to constrain principal stress directions in the vicinity of six anza network stations which show evidence for shallow shear wave anisotropy in the vicinity of the anza seismic gap region of the san jacinto fault faulting near all stations is consistent with approximately n s maximum compressive stress five of these stations show nearly n s initial particle motion alignment consistent with the anisotropy stress relationship expected for stress aligned microcracks however one station knw has a well defined preferred initial shear wave polarization direction of n40 w although this polarization direction differs dramatically from the local maximum compressive stress direction it is consistent with the anisotropy expected due to a local alignment of anisotropic bedrock minerals particularly biotite thus anisotropy observed at this station most likely reflects a fixed palaeostrain alignment of anisotropic minerals and or microcracks and does not require a dependence on the current stress field

evaluation of anisotropy by shear wave splitting the polarizations of three component shear wavetrains carry unique information about the internal structure of the rock through which they pass specifically commonly observed shear wave splitting may contain information about the orientation of crack distributions this information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface even for nearly vertical incidence however shear wave splitting in synthetic three component vertical seismic profiles in some cases may be interpreted directly in terms of the direction of strike of vertical cracks because the human eye is not skilled at identifying the phase relationships between three component seismograms played out conventionally as parallel time series the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear wave splitting estimating the orientations of cracks an

earthquake prediction a new physical basis summary subcritical crack growth in the laboratory occurs slowly but progressively in solids subjected to low stresses at low strain rates the cracks tend to grow parallel to the maximum compressive stress so that when this stress is aligned over a large enough region the cracks will also be aligned and possess effective seismic anisotropy it is suggested that such subcritical crack growth produces extensive dilatancy anisotropy eda throughout earthquake preparation zones this process is a possible driving mechanism for earthquake precursors observed at substantial distances from impending focal zones and provides in the shear wave splitting which has been observed in several seismic regions a possible technique for monitoring the build up of stress before earthquakes

changes in static stress on southern california faults after the 1992 landers earthquake the magnitude 7 5 landers earthquake of 28 june 1992 was the largest earthquake to strike california in 40 years the slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults these changes in stress should in turn affect the likelihood of future earthquakes stress changes that load faults towards failure have been cited as the cause of small1 5 moderate6 and large7 earthquakes conversely those that relax neighbouring faults have been related to a decrease in seismicity5 here we use an elastic half space model8 to estimate the stress changes produced by the landers earthquake on selected southern california faults including the san andreas we find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4 5 and that the largest changes 1 10 bar occurring on part of the san bernardino segment of the san andreas fault may have decreased the time to the next magnitude 8 earthquake by about 14 years

azimuth determination of and from horizontal ocean bottom seismic sensors a number of factors must be considered when using particle motion information to determine azimuths to source or azimuths of horizontal geophone axes on or below the ocean floor these factors include anisotropy unmatched sensor response incorrect instrument location tilted sensors or dipping structure and poor coupling accurate determination of azimuths requires identification and understanding of these factors and their effects on the data

observation and modelling of fault zone fracture seismic anisotropy i p sv and sh travel times summary three component vsp borehole seismograms taken in the vicinity of an active normal fault in california show strong systematic shear wave splitting that increases with proximity to the fault using cerven s method of characteristics for ray tracing in anisotropic heterogeneous media and hudson s formulation of elastic constants for media bearing aligned fractures we have fitted a suite of p sv and sh hanging wall and foot wall travel times with a simple model of aligned fractures flanking the fault zone the dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault the increase in fracture density is non uniform power law or gaussian with respect to distance to the fault although the hanging wall and the foot wall rock are petrologically the same unit the fracture halo is more intense and extensive in the hanging wall than in the foot wall upon approach to the fault plane the fracture density or fracture density gradient becomes too great for the seismic response to be computed by hudson cerven procedures the maximum fracture density that can be modelled is about 0 08 within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low velocity waveguide we observe production of trapped waves within the confines of the intense fracture interval

monitoring and modeling land subsidence at the cerro prieto geothermal field baja california mexico using sar interferometry images derived from repeat pass spaceborne interferometric synthetic aperture radar insar systems offer the possibility of mapping surface deformation of small spatial extent and monitoring its spatio temporal evolution a slow local subsidence has been detected at the cerro prieto geothermal field from images acquired by the european space agency remote sensing satellites ers 1 2 between 1993 and 1997 although agricultural activity in the area limited the investigation interferometric monitoring revealed that the ground deformation is associated with the withdrawal of geothermal fluid and agreed with the leveling data modeling of the subsidence was carried out assuming elastic deformation in a half space from simple point sources of which five were necessary to reproduce the fringe patterns observed on the interferograms the depths and locations of three of the sources are compatible with the location of the known reservoir the study improves prior knowledge of the displacement field and of the mecanisms involved in the subsidence phenomenon

shear wave anisotropy of active tectonic regions via automated s wave polarization analysis abstract using an automated aspect ratio method to evaluate shear wave anisotropy from earthquakes recorded on arrays of three component digital seismographs over the bucaramanga nest of colombia the east rift zone of hawaii and the phlegraean fields uplift area of italy a shear wave velocity anisotropy of 4 7 is observed the anisotropy is attributed to tectonic processes acting either to align anisotropic minerals or to align stress related cracks at most fixed seismographs the polarization of the leading s phase is consistent from event to event independent of earthquake location on a regional basis the polarization may vary from station to station but a regional trend is apparent for events from the bucaramanga nest colombia a small seismogenic volume at 160 km depth in a subducted plate shear wave splitting averages 0 5 0 1 s for more than 100 events the polarization direction of the fast shear wave is 355 n 15 parallel to the strike of the benioff zone at the phlegraean fields in italy which represent an active quaternary caldera the average shear wave splitting observed from 51 events is 0 2 s corresponding to a minimum velocity anisotropy of 7 the strike of the inferred maximum compressive stress is consistent with values obtained from fault plane solutions in hawaii the polarization of the leading shear wave tends to parallel the strike of the east rift zone average velocity anisotropy is about 5 if one assumes that the entire travel path contributes to the observed splitting however station variations in fast shear wave polarization direction imply that some of the splitting is due to near station effects

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

estimates of orientations of stress and strain tensors based on fault plane solutions in the epicentral area of the great hawaiian earthquake of 1868 the 95 best quality fault plane solutions for earthquakes in a 30 by 10 km area near the epicenter of the 1868 hawaiian earthquake m about 7 9 were used for estimating the orientation of the stress and strain tensors most of the earthquakes had magnitudes of ml 3 5 0 6 with nine events in the range 4 2 ml 5 5 although this area hilea is capable of great earthquakes it seldom produces events in the range 5 5 ml 6 5 we found that the directions of the slip and the principal strains are much more uniform in hilea than in the neighboring regions of kaoiki and south kona not varying more than 15 the directions of the stress tensor are poorly constrained because of the uniformity of fault plane solutions therefore one cannot be certain that the stress tensor is uniform in this area but we assume that it is because the strain and slip directions are the stress and strain tensor directions differ significantly from those in the kaoiki and south kona areas the slip directions as well as the plane defined by e1 and e3 are oriented approximately perpendicular to the upper part of mauna loa s southwest rift suggesting that magma intrusions there at a distance of 20 km probably provide the stress for the seismic activity in the hilea area we propose that a tectonic boundary exists near latitude 19 19 separating the kaoiki and hilea areas the stress and strain tensor orientations are significantly different north and south of this boundary and virtually no strike slip earthquakes occur in the hilea area whereas they are the majority of solutions in the kaoiki area the fault plane solutions in hilea have a near horizontal and a near vertical nodal plane in other areas of hawaii the evidence suggests that the near horizontal plane is a zone of weakness provided by buried oceanic sediment which allows slip of the upper crust away from the volcanic centers toward the coasts here however the stress inversion does not lead to a clear choice of preferred fault plane thus it may be that both nodal planes do act as fault planes in this area and that the rupture process in large earthquakes of the 1868 type may be complex involving decollement steeply dipping normal faulting and even strike slip faulting

fine scale seismic structure of the shallow volcanic crust on the east pacific rise at 9 50 n upper crust in a 3 3 km field area centered on the east pacific rise at 9 50 0 n we detonated 18 explosive shots 18 sources in a circular pattern 1 5 km radius on the rise axis and recorded seismic arrivals with eight ocean bottom seismometers eight receivers we observed 30 40 hz compressional body waves from all shots 144 p waves and 1 3 hz stoneley interface waves on a subset of source receiver pairs 58 interface waves using a station correction inversion we find that roughly half of the variance in the p wave first arrival times results from lateral variations in the thickness of the surface low velocity layer slvl a layer of extremely porous lava and basalt breccia with an average p wave velocity of 2 2 km s 1 the slvl thickness increases from 20 m along the axial summit trough ast to 120 m at near axis lava depocenters which are not symmetric about the rise axis depocenters are located 0 5 km to the west and 1 5 km to the east of the rise axis tomographic inversion of the stoneley wave first arrivals reveals that shear velocities in the slvl covary with the layer thickness exhibiting a similar asymmetric pattern with shear velocities increasing from 320 m s 1 near the ast to 520 m s 1 at the near axis depocenters our analysis demonstrates that the seismic characteristics of the extrusive layer near the rise axis are related primarily to volcanic features and processes the thickness and velocity of the slvl are low on the axis and within channel networks that deliver lava flows away from the axis and then increase rapidly at the distal ends of the channels where the lavas are deposited we find that azimuthal anisotropy exerts only a weak influence on our p wave first arrival times which we model as weak 4 seismic azimuthal anisotropy in the upper dikes with a fast axis oriented n23 32 w we find no evidence for seismic azimuthal anisotropy in the extrusive layer index terms 3025 marine geology and geophysics marine seismics 0935 3035 marine geology and geophysics midocean ridge processes 7220 seismology oceanic crust

upper crustal vp vs ratios at the endeavour segment juan de fuca ridge from joint inversion of p and s traveltimes implications for hydrothermal circulation

volume strain within the geysers geothermal field during the 1970s and 1980s the geysers geothermal region was rapidly developed as a site of geothermal power production the likelihood that this could cause significant strain within the reservoir with corresponding surface displacements led to a series of deformation monitoring surveys in 1973 1975 1977 and 1980 the geysers region was surveyed using first order class i spirit leveling in 1994 1995 and 1996 many of the leveling control monuments were resurveyed using high precision global positioning system receivers the two survey methods are reconciled using the geoid96 geoid model the displacements are inverted to determine volume strain within the reservoir for the period 1980 1994 peak volume strains in excess of 5x10 sup 4 are imaged there is an excellent correlation between the observed changes in reservoir steam pressures and the imaged volume strain if reservoir pressure changes are inducing volume strain then the reservoir quasi static bulk modulus k must be 4 6x10 sup 9 pa however seismic velocities indicate a much stiffer reservoir with k 3 4x10 sup 10 pa this apparent discrepancy is shown to be consistent with predicted frequency dependence in k for fractured and water saturated rock inversion of surface deformation data therefore appears to be a powerful method for imaging pressure change withinmore the body of the reservoir correlation between induced seismicity at the geysers and volume strain is observed however earthquake distribution does not appear to have a simple relationship with volume strain rate c 1999 american geophysical union less

inversion of synthetic aperture radar interferograms for sourcesof production related subsidence at the dixie valley geothermalfield we used synthetic aperture radar interferograms to imageground subsidence that occurred over the dixie valley geothermal fieldduring different time intervals between 1992 and 1997 linear elasticinversion of the subsidence that occurred between april 1996 and march 1997 revealed that the dominant sources of deformation during this timeperiod were large changes in fluid volumes at shallow depths within thevalley fill above the reservoir the distributions of subsidence andsubsurface volume change support a model in which reduction in pressureand volume of hot water discharging into the valley fill from localizedupflow along the stillwater range frontal fault is caused by drawdownwithin the upflow zone resulting from geothermal production our resultsalso suggest that an additional source of fluid volume reduction in theshallow valley fill might be similar drawdown within piedmont faultzones shallow groundwater flow in the vicinity of the field appears tobe controlled on the nw by a mapped fault and to the sw by a lineament ofas yet unknown origin

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

origin of crustal anisotropy shear wave splitting studies in japan shear wave splitting manifested as leading shear wave polarization that is parallel alignment of leading shear wave particle motions from a variety of sources has been observed at a number of seismograph stations in japan detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source receiver azimuths the shear wave splitting can be attributed to crustal anisotropy this paper discusses the relation between leading shear wave polarization directions and tectonic features of japan to explain the observed shear wave splitting the author proposes that at least three phenomena should be taken into account stress induced microcracks primarily aligned in vertical or subvertical planes cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes and intrinsic rock anisotropy resulting from preferred orientation of minerals travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15 25 km this implies that the density of nonhorizontally aligned cracks or fractures below 15 25 km and into the upper mantle is muchmore smaller than that in the crust above 15 25 km less

inter ethnic social interactions in 16th century la florida sourcing pottery using siliceous microfossils abstract a case study is presented to test the notion that minority pottery types from 16th century contexts at the fountain of youth foy site in st augustine reflect population movements from the north that preceded major political reorganizations in the region petrographic methods are employed to trace the manufacturing origins of early historic period aboriginal pottery in northeast florida fragments of siliceous microfossils including sponge spicules opal phytoliths and most notably diatoms were identified in the matrix of some early historic period aboriginal pottery from foy as well as in some clay samples from the coastal region of northeast florida and southeast georgia diatom taxa are identified and their spatial distribution is assessed the distribution of microfossils supports the nonlocal manufacturing origins of some samples from st augustine and conform to expectations about the historic movement of certain aboriginal groups to the settlement

comparison of orientations of stress and strain tensors based on fault plane solutions in kaoiki hawaii the stress tensor orientation was estimated based on inversion from 238 first motion fault plane solutions of earthquakes with mostly m 3 5 0 6 located in the 10 km radius kaoiki crustal volume separate inversions for subvolumes containing 20 50 events yielded the same results in several adjacent volumes suggesting that the stress tensor is homogeneous in those parts of the kaoiki area and that the inversion results are stable and meaningful five spatial subsets of the data were found for which the orientation of at least one of the principal axes was different from that in the other sets by 20 80 and at confidence levels exceeding 95 the volcano summits of kilauea and mauna loa and their rift systems are identified as the source of stress in the kaoiki crust because the greatest principal stress points to kilauea and mauna loa in addition the strain tensor due to energy released by these 238 earthquakes was computed for the kaoiki area and several subvolumes of it by summing the moment tensors the moment tensor of each earthquake was constructed from the individual fault plane solutions and from an estimate of the scalar moment derived from the moment magnitude relationship a comparison of the directions of strain and stress tensors showed close agreement for subvolumes with predominantly strike slip faulting in these volumes the inversion process for stress directions led to misfits of approximately equal size for the conjugate near vertical nodal planes these observations are interpreted to show that in the strike slip regime of the upper part of the crust neither of the nodal planes is preferred for faulting rupture probably occurs along the nw and along the nh striking nodal planes in separate earthquakes subvolumes with more decollement faulting showed significant differences of 30 40 between the principal strain and stress directions in these volumes the near horizontal nodal planes showed noticeably smaller misfits in the inversion for the stress directions these facts are interpreted to indicate that the decollement plane is weak allowing slip on it even if the principal stresses are inclined at a large angle to it it is proposed that comparison of strain and stress tensor calculations may be able to differentiate between tectonic regimes uniform in strength no well developed fault plane and regimes in which a fault with low frictional strength dominates as a function of time significant rotations of the strain tensor by approximately 45 can be observed which seem to be related to the occurrences of kaoiki mainshocks during three background periods of about 7 years each the average strain tensor showed an orientation typical for predominant decollement faulting while two premainshock periods of 2 5 years each showed an orientation closer to strike slip faulting it is proposed that this pattern may be repeated before the next kaoiki mainshock the strain released seismically is less than the geodetically observed strain by approximately an order of magnitude stress strain fault plane solutions hawaii

seismic anisotropy in the guerrero gap mexico preliminary results three component seismograms of low magnitude earthquakes md 3 recorded at the central station tetitlan of the seismic network located in the guerrero gap mexico show evidence of shear wave splitting the data set contains several months of activity from the installation of the network in 1987 until late 1989 even though the sampling rate is only 75 samples sec and the signal to noise ratio is not very large it was possible to estimate in most cases the polarization direction of the first shear wave as well as shear wave splitting time delays most polarizations show a direction which correlates with the main compressive stress in the area due to subduction of the cocos plate beneath the north american plate two groups of hypocenters can be distinguished a shallow group up to 25 km in depth and a deeper group from 32 to 45 km as there is no difference in the polarization direction pattern of both groups and the time delays do not increase with depth the shear wave splitting is interpreted in terms of an anisotropic layer above 25 km in depth filtering the seismograms to enhance the peak amplitude frequency band does not change the observed polarization directions but helps identify shear wave splitting

a decade of shear wave splitting in the earth s crust what does it mean what use can we make of it and what should we do next it is 10 years since shear wave splitting throught to be diagnostic of some form of seismic anisotropy was first positively identified in the earth s crust from the beginning it was argued that the splitting was probably associated with the presence of stress aligned cracks inclusions in the crust and that this would provide the opportunity for monitoring the in situ geometry of cracks and stress in a variety of different circumstances and in a variety of different applications

the use of diatom analysis in archaeology a review abstract although diatom analysis has been used in archaeological science for over 40 years it is still an under used technique in this paper the basis of the technique is explained and the literature on diatoms in archaeology is reviewed diatoms are good indicators of a range of water quality variables but emphasis here reflecting their dominant use so far in archaeology is placed on their value as indicators of water salinity a number of case studies are described from different parts of europe demonstrating the use of the technique in the location of prehistoric settlement sites along former shorelines in reconstructing salinity and tidal regimes in estuaries and in the location of clay sources for pot manufacture the potential of diatom analysis in contributing to questions of pottery typology is also indicated although diatom taxonomy remains very difficult and specialist analysis will always be required it is stressed that when there is an archaeological need to ask questions relating to the presence absence level pollution productivity acidity or salinity of a waterbody on or near to an excavation diatom analysis should be considered

comment on quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction by richard c aster peter m shearer and jon berger

distribution of attenuation in the kaoiki hawaii source volume estimated by inversion of p wave spectra a new method to simultaneously invert for q structure and source parameters was used on a set of 635 microearthquakes 0 9 m 2 0 in the kaoiki area of southern hawaii approximately 2800 signals were analyzed which had been recorded by 6 short period vertical seismographs at epicentral distances of a few to 10 km the hypocentral depths ranged from o to 14 km with the bulk of the sources in the 7 5 10 5 km range the hypothesis to be tested was that the source volume of the m 6 6 kaoiki main shock of november 16 1983 may be heterogeneous in attenuation distribution we assumed that the observed p wave displacement spectra could be modelled by a source spectrum with an 2 high frequency decay a single layer resonance filler to account for local site resonances and whole path attenuation along the ray path in a next step the attenuation factor q was constrained by tomographically reconstructing the three dimensional q structure for the source region and using it as starting model for a nonlinear inversion of the corner frequency the seismic moment m0 and a new q value this process was iterated until the results changed less than 0 1 and were accepted as final the average q was approximately constant and very low 105 q 115 between o and 7 km depth it increased to approximately 160 between 9 and 11 km in the depth ranges were the spatial coverage was good 3 9 km the q values in the nw part of the 1983 aftershock volume were larger by approximately 10 15 compared to those in the se part this 12 contrast correlates with other evidence for heterogeneity the nw region shows high coda q no surface faulting medium seismicity rate and medium precursory quiescence while the se region shows low coda q pervasive faulting high seismicity rate strong precursory quiescence and possibly low b values we conclude that the kaoiki source volume is strongly heterogeneous with the contrasting density of cracks controlling the difference in crustal parameters the new simultaneous inversion technique is well suited to derive detailed q structure and source parameter information from microeaithquake spectra recorded at close epicentral distances

synthetic aperture radar interferometry to measure earth s surface topography and its deformation synthetic aperture radar interferometry insar from earth orbiting spacecraft provides a new tool to map global topography and deformation of the earth s surface radar images taken from slightly different viewing directions allow the construction of digital elevation models of meter scale accuracy these data sets aid in the analysis and interpretation of tectonic and volcanic landscapes if the earth s surface deformed between two radar image acquisitions a map of the surface dis placement with tens of meters resolution and subcentimeter accuracy can be con structed this review gives a basic overview of insar for earth scientists and presents a selection of geologic applications that demonstrate the unique capabilities of insar for mapping the topography and deformation of the earth

ground ruptures of the 1974 and 1983 kaoiki earthquakes mauna loa volcano hawaii the november 30 1974 ml 5 5 and november 16 1983 ml 6 6 earthquakes generated left stepping en echelon ground cracks within the kaoiki seismic zone on the southeast flank of mauna loa volcano hawaii the general trend of the ruptures n48 55 e parallels a nodal plane of the main shocks focal mechanisms the ruptures themselves consist of short predominantly extension cracks which are up to 20 m long and strike roughly e w 30 50 clockwise from the overall trend of the zones some of the cracks are linked by secondary fractures and rubble breccia to form left stepping crack arrays which are themselves linked to form longer en echelon systems of ground rupture geologic maps and field observations indicate that these features emerge from an underlying strike slip fault and they form a fracture process zone above its tip the maximum displacement measured across cracks in the 1983 rupture zone is 0 5 m trilateration data however suggests that the overall shear displacement was about 1 5 m at depth elastic solutions indicate that a region of significant tensile stress can exist above buried strike slip faults we suggest that these stresses generated the extensional ground cracks and that shear displacements were transmitted to the earth s surface by subsequent growth and linkage of these cracks into the observed arrays we infer that the crack arrays accommodate increased displacement with depth and they merge downward into the parent strike slip fault at an estimated 1 2 km depth where strike slip displacement was probably more or less continuous along the 7 km length of the rupture in the kaoiki region only three major ground ruptures traverse a series of basaltic lava flows that date back 1500 years this suggests that the recent 10 year periodicity of moderate magnitude kaoiki strike slip events may not have extended far into the past the tectonic significance of strike slip faulting on mauna loa volcano remains enigmatic

shear wave splitting in the anza seismic gap southern california temporal variations as possible precursors shear wave splitting above small earthquakes in the anza seismic gap on the san jacinto fault southern california displays the effective anisotropy of vertical fluid filled microcracks the delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence these changes can be modeled by increasing the aspect ratio bowing of stressoriented microcracks which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid filled microcracks this is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake

an investigation of seismic anisotropy due to cracks in the upper oceanic crust at 45 n mid atlantic ridge summary a detailed seismic refraction survey using three 3 component ocean bottom seismographs over young 1 1 3 4 ma old mid atlantic ridge at 45 n recorded compressional wave velocity anisotropy in the upper crust of about 5 per cent and associated particle motion deviations of up to 8 the magnitudes and azimuths of these anisotropic variations suggest that they are generated by sets of subparallel near vertical water filled cracks in layer 2 the directions of the cracks cannot be determined uniquely but they lie between the seafloor spreading and the ridge axis directions in the range 30 60 with respect to the ridge axis we explain this unexpected crack orientation by the occurrence of cracking and fracturing induced in the upper crust as it moves away from the spreading axis the principal horizontal stresses responsible for this fracturing are parallel and perpendicular to the ridge axis the apparently negligible seismic influence exerted by ridge parallel sets of shallow cracks observed at the ridge axis is explained by their infilling by sediment and hydrothermal precipitates and by their closure under ridge normal compression we find a normal oceanic crustal velocity structure that can be explained by decreasing numbers of open cracks with increasing depth through the upper 2 km of the crust lateral variability across the 25 km square survey is limited in magnitude typically 0 05 s variations in delay times and the observed travel times are consistent with a model of the crust in which the isovelocity surfaces at depth are a subdued copy of the basement topography

shear wave polarization anisotropy observed in a rift zone in japan abstract we have found polarization anisotropy in shear waves from small earthquakes both in the upper crust and in the uppermost mantle beneath a rift zone in central kyushu japan we have recognized two different directions in the polarization of the faster shear waves about e w directions at three stations located in the rift zone and about n s at two stations possibly located outside and close to the edge of the rift zone the former case e w is consistent with a hypothesis that shear wave splitting is caused by vertical cracks or fractures striking in the e w direction which have formed under the present tectonic stress inside a rift zone extending in the n s direction with the maximum horizontal compressive stress striking in the direction about e w and the minimum compressive stress vertical in contrast the latter case n s is inconsistent with the above hypothesis this inconsistency in the polarization direction may be due to a complicated pattern of crack distribution around the stations outside the rift zone alternatively it may result from a complicated stress regime within the crust close to the edge of the rift zone arrival time differences between the faster and slower split shear waves from the upper crustal earthquakes are less than about 0 12 s while those from the upper mantle events are less than about 0 25 s the relatively small arrival time differences from the upper mantle events with respect to their hypocentral distances could result from a small crack density in the lower crust and upper mantle compared with that in the upper crust

crustal shear wave anisotropy in southern hawaii spatial and temporal analysis split shear wave arrivals are analyzed in seismograms from local earthquakes in southern hawaii recorded at five temporary arrays and one permanent network station we identify split shear wave arrivals by their orthogonally polarized pulses linear particle motions and similar waveforms and estimate the delay time for the slow shear wave arrival s 2 using a waveform cross correlation method consistent leading shear wave polarizations were measured at the majority of our stations comparison of observed and predicted shear wave polarizations confirms that the former are due to anisotropy rather than earthquake source mechanism agreement between fast shear wave s 1 polarizations and independently estimated directions of the maximum horizontal compressive stress h for the ainapo and punaluu gulch arrays leads us to conclude that the predominant source of the observed anisotropy for these two areas is stress aligned cracks consistent with the extensive dilatancy anisotropy eda hypothesis two distinct s 1 polarization directions were observed over distances less than 1 km for the bird park and south flank arrays s 1 polarizations parallel to the ne striking kaoiki pali fault system for the bird park array combined with a nonhorizontal maximum principal stress 1 for the south flank region suggest stress induced cracks aligned by nearby faulting as a source for the observed anisotropy large station to station variations in s 1 polarization and the relationship between delay time and event depth for arrays in the kaoiki and south flank regions provide evidence for anisotropy that is predominantly shallow rather than pervasive average delay times for the five arrays vary from about 100 to 230 ms with standard deviations of the order of 30 ms estimated anisotropic velocity variations and crack densities exceeding 10 indicate that the upper crust of southern hawaii is highly fractured a search for possible temporal changes in delay time associated with the 1983 kaoiki main shock m l 6 6 at a station near the epicenter finds no evidence for change

a detailed study of the cobb offset of the juan de fuca ridge evolution of a propagating rift the cobb offset on the northern portion of the juan de fuca ridge has been identified as the tip of a northward propagating rift hey and wilson 1982 map compilations of magnetic and seismic data from four new cruises define the present locus of spreading and volcanism on the two ridge segments abutting the offset and permit detailed modeling of the recent evolution within this transform zone the axis of recent spreading on the southern ridge segment bends from the normal ridge trend n20 e to a n s trend north of 47 15 n the spreading axis on the northern ridge segment generally defines a n20 e trend except at the southern terminus where the spreading center is offset slightly to the east the two spreading centers overlap by about 33 km in the offset vicinity and there is evidence of recent volcanism on both segments present ridge axis morphology exhibits a transitional sequence from a symmetrical axial high along the more normal portions of each ridge segment to a grabenlike depression as the tip is approached the magnetic anomaly patterns observed in the cobb offset vicinity are not consistent with the patterns predicted by models of continuous northward propagation the magnetic anomaly patterns of the brunhes epoch require an event of rapid northward propagation about 0 7 m y b p followed by a more gradual southward propagation in the middle brunhes epoch most recently the spreading center on the southern ridge has extended northward to its present configuration prior to the brunhes epoch modeling of the magnetic anomaly patterns does not indicate a unique solution however net propagation has been northward we present alternative models for the period beginning 1 7 m y b p in the first model the cobb offset has evolved by a series of northward and southward events of propagation with net advance to the north in the second model stable asymmetric spreading from overlapping ridge segments has evolved into a transform fault offset by gradual decay of the northern ridge both models indicate that propagation has been discontinuous and relatively rapid compared to spreading rate and that realignment of ridge segments near the propagator tip has been an important component

behaviour of shear waves in rocks with two sets of parallel cracks summary the behaviour of shear wave polarizations and shear wave splitting observed at the surface suggesting propagation through parallel vertical cracks has been the stimulus for many recent investigations both in earthquake and exploration seismology cracks in surface outcrops however frequently display multiple sets of parallel vertical intersecting cracks this paper examines seismic shear wave propagation in media with two sets of parallel vertical cracks biplanar cracks to determine whether the behaviour of shear waves can distinguish between the effects of multiple crack sets and the effects of single sets of parallel cracks monoplanar cracks this study shows that the difference between the overall patterns of polarizations of biplanar and monoplanar systems of vertical cracks within the shear wave window in many circumstances is marginal and unlikely to be easily recognized in the field we conclude that it is frequently not possible from analysis of surface observations of shear wave polarizations alone to distinguish between the effects of biplanar sets of parallel vertical cracks and those of a single parallel set the difference can usually be recognized if an accurate estimate of both polarizations and time delays between the split shear waves is available over a wide range of azimuths and angles of incidence within the shear wave window however in areas with complex fracture and stress systems time delays may be much harder to estimate than the polarization angles of the leading split shear waves and it may not be easy to distinguish from seismic data alone the difference betwekn parallel and multiplanar sets of vertical fractures

evidence for a sea level lowstand between 4500 and 2400 years b p on the southeast coast of the united states abstract distribution of dated archaeological sites dredged pottery sherds 14c dates of submerged inplace stumps and the presence of numerous buried trees indicate a sea level lowstand of at least 3 m msl between 3000 and 2400 years before present b p on the georgia coast

seismic anisotropy in the shallow crust of the loma prieta segment of the san andreas fault system we study seismic anisotropy beneath the santa cruz mountains using digital recordings of s waves from 60 aftershocks of the 1989 loma prieta earthquake m 7 0 recorded at 18 local stations three component particle motion covariance matrix decomposition combined with a cross correlation method is used to estimate the polarization direction of the fast shear wave arrival and the delay time between the split shear waves results from this procedure are compared with those obtained through visual inspection of horizontal particle motions inclusion of the vertical component in the particle motion analysis allows the recognition and elimination of s to p converted phases from our study uncertainties in the polarization of the fast shear wave and in the delay time between arrival of the fast and slow waves are estimated to be about 20 and 10 ms respectively about half of the s wave records examined show clear evidence of shear wave splitting two persistent polarization directions of fast shear wave arrivals are observed regardless of the focal mechanism and hypocenter location of the individual events 52 of the polarizations are parallel to the strike of the san andreas fault nw se and 24 are parallel to the probable direction of local maximum horizontal compressive stress ne sw the fault parallel fast polarization direction dominates at 10 stations and may result from mineral or fracture alignment caused by shearing along the plate boundary the fast polarization direction perpendicular to the local axes of minimum horizontal compressive stress dominates at one station farthest from the san andreas fault and is consistent with the hypothesis of extensive dilatancy anisotropy eda where parallel alignment of fluid filled fractures produces the anisotropy the predominance of fault parallel fast shear wave polarizations indicates that the use of shear wave splitting results to deduce the orientation of local axes of principal stress may not always yield accurate results the splitting time between fast and slow shear waves varies from 10 to 125 ms and shows no systematic relationship with either hypocentral distance or focal depth this suggests that the seismic anisotropy is no deeper than 2 km the depth of the shallowest earthquakes analyzed for an anisotropic zone 2 km thick the average velocity anisotropy is about 5 in our study region

shear wave splitting observations in the northern los angeles basin southern california abstract shear wave splitting with 20 to 120 msec time separation is observed on 3 component seismograms from earthquakes occurring at depths of 5 to 18 km in the crystalline basement beneath the los angeles basin shallow events that occur in the basement at the base of the 5 km thick sedimentary section exhibit little splitting while deeper events show progressively greater splitting with depth the polarization direction of the fast shear wave is n s independent of the azimuth of ray path between the event and station we interpret that the shear wave splitting occurs mainly in the crystalline basement and is the result of vertical crustal microcracks aligned in n s direction under the assumption that the alignment of microcracks is due to the subsurface stress regime the results from the shear wave splitting data yield a maximum principal stress direction of n s 15 at depth beneath the northern los angeles basin consistent with results from geological mapping and fault plane solutions ray trace modeling indicates that the observations of shear wave splitting can be explained in terms of an anisotropic basement containing vertical microcracks aligned in the n s direction and with a crack density of 0 04 overlain by a weakly anisotropic sedimentary section with a crack density less than 0 02 during the last 4 yr three major earthquakes occurred in the los angeles basin area they are the m 4 6 and m 4 4 montebello double earthquakes of 1989 the m 5 5 upland earthquake of 1990 and the m 5 8 sierra madre earthquake of 1991 with epicenters of 10 55 and 40 km away from the recorder site scs located in the northern los angeles basin respectively we did not observe significant variations of shear wave splitting at scs before and after the upland and sierra madre earthquakes however we found fairly rapid variations of shear wave splitting following the montebello events the time separation between the split shear waves sharply decreased from 70 msec to less than 20 msec after the mainshock and then returned to the average level gradually this rapid decrease in shear wave splitting may be a temporal variation or a result of location variations of the montebello events that were located approximately where the line singularity is expected

seismic structure and crustal magmatism at the mid atlantic ridge 35 n we have determined the three dimensional p wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the mid atlantic ridge mar near 35 n the best fitting one dimensional model is 0 8 km s lower in velocity at a given depth than for fast spreading ridges while the isotropic three dimensional velocity structure shows a strong lateral heterogeneity up to 1 4 km s peak to peak variation at 1 km depth in both the along and cross axis directions an anomalous region with velocities as much as 0 8 km s lower than average and 3 5 km in lateral extent is located beneath a near axis seamount we interpret this feature as a region of near solidus temperatures with possibly a small degree of partial melt from the variation of travel time residuals with azimuth we determine a best fitting p wave anisotropy model that has 4 anisotropy from 500 m to 1 km depth 2 anisotropy in the depth range 1 1 5 km and no anisotropy at greater depth this finding is consistent with vertical cracks aligned parallel to the axial valley we find no evidence for the shallow high velocity ridge parallel feature commonly observed at the axis of fast spreading ridges and attributed to high velocity dikes beneath a thin extrusive layer the absence of this anomaly at the mar implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25 000 years an unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes which may account for the generally lower velocities beneath the mar compared with the east pacific rise and a lateral variation in upper crustal lithology

woodland period ceramic provenance and the exchange of swift creek complicated stamped vessels in the southeastern united states results of instrumental neutron activation analysis inaa of middle and late woodland pottery n 313 and clay n 22 samples from northeastern florida and southeastern georgia are presented assemblages in this region include swift creek complicated stamped pottery that preserves unique evidence of social interactions through the inimitable qualities of designs stamped into vessel surfaces archaeologists have proffered various hypotheses to explain movement of ceramic vessels or the carved wooden paddles used in the manufacture of these vessels this study tests these hypotheses and indicates that nonlocal vessels particularly complicated stamped ones were deposited almost exclusively in mortuary contexts a pattern that requires new explanations for the role of pottery in social interactions these data are being integrated with a larger project that aims to reveal the social processes that were tied to the manufacture use and distribution of pottery

observations of shear wave splitting on the southeast flank of mauna loa volcano hawaii we use a combination of particle motion covariance matrix decomposition particle motion plots and cross correlation to estimate the polarization and split times of shear wave arrivals from local earthquakes in the kaoiki and hilea regions of southern hawaii consistent leading shear wave polarizations and identifiable fast and slow shear wave arrivals were frequently observed average delay times for the four arrays vary from 100 to 230 ms the leading shear wave polarizations at stations in the kaoiki region agree with the direction of the local maximum horizontal compressive stress h determined from focal mechanism studies and field observations of ground rupture for the bird park bp array in the kaoiki region the relationship between delay time and event depth shows evidence for both dominant shallow and weaker pervasive anisotropy the correlation between delays and event depth is much weaker for the ainapo an array than for bp however a smaller slope is apparent conservative estimates of the crack densities for the bp and an arrays are e 12 and e 6 respectively in the hilea region the leading shear wave polarization for the punaluu gulch pg array differs by approximately 30 from the direction of h determined from focal mechanism studies while the waihaka gulch wg array shows no predominant polarization direction no correlation between delay time and event depth was observed for either of the two hilea arrays

anisotropy in the oceanic lithosphere theory and observations from the ngendei seismic refraction experiment in the south west pacific summary p wave travel time data from a seismic refraction experiment on the 1983 ngendei expedition to the south pacific indicate anisotropy at two levels in the oceanic lithosphere in the upper mantle p wave velocities vary between 8 0 and 8 5 km s 1 with the fast direction at n30 e crustal anisotropy within layer 2 is characterized by azimuthal p wave velocity differences of about 0 2 0 4 km s 1 with the fast direction at n120 e orthogonal to the upper mantle anisotropy the observed anisotropy is consistent with a model in which aligned olivine crystals cause anisotropy in the upper mantle and aligned cracks within layer 2 cause anisotropy in the crust we examine a model of p wave propagation from a point source in an isotropic layer above an anisotropic half space and show that under these conditions anisotropy can cause seismic ray paths to deviate from the vertical plane connecting source and receiver this deviation has a negligible effect on seismic travel times but must be taken into account in modelling p wave polarization anomalies p wave polarization anomalies within the anisotropic half space are the sum of the particle motion deviation from the ray path and ray path deviation from the source receiver azimuth and are typically range dependent p wave polarization anomalies will also occur in the overlying isotropic layer purely as a result of the ray path deviation with greater deviations at long ranges a simple model of upper mantle anisotropy 6kms 1 crust above 8 0 8 6 km s 1 anisotropic mantle produces surface polarization anomalies of 1 5 3 5 in practice p wave polarization anomalies are difficult to observe because they are small compared to observed scatter in polarization data and are sensitive to differences in horizontal seismometer gain levels we examine the implications of anisotropic ray path deviations on travel times and p wave polarizations for the ngendei data as well as some previous marine seismic refraction experiments

diatoms as indicators of provenance in finnish sub neolithic pottery abstract the clays of finland originated from glacial deposits formed during the deglaciation of the weichselian ice sheet and during the different stages of the baltic sea the analysis of the fossil diatom flora is of central importance in the study of its developmental history the diatoms in the clay indicate its sedimentary environment which can be linked to the stages of the baltic sea the siliceous valves of diatoms tolerate heating up to 1400 c and can consequently be identified in ceramics fired at lower temperatures several diatom analyses have been made in finnish sub neolithic comb ware pottery in order to determinate the origin of their clay materials according to the results of these studies prehistoric potteries used clays deposited during the yoldia and the ancylus stages the criteria used in selecting raw materials were identical to those of modern ceramic industries further details as to provenance of clay materials have been illustrated in connection with research on pottery from the aland islands in the example from kymi south eastern finland changes in the composition of the diatom flora have been found to correlate with changes in pottery decoration styles

morphometric variability within the axial zone of the southern juan de fuca ridge interpretation from sea marc ii sea marc i and deep sea photography the morphometric characteristics of the axial regions of oceanic spreading centers are determined by 1 the type of volcanic flows 2 the relation between primary volcanic relief on a scale of a few meters to tens of meters and degree of sediment cover and 3 the extent of surficial expression and timing of tectonic disruption of the young oceanic crust even within a single continuous linear spreading ridge segment with relatively uniform axial valley dimensions over a distance of 50 or more kilometers such as along the southern juan de fuca ridge the changes in morphometric characteristics along axis within the youngest crust indicate distinct variation in tectonic and volcanic activity over short distances within short time periods an integrated analysis of sea marc i sea marc ii and photographic data for the southernmost continuous segment of the juan de fuca ridge shows that generalizations about tectonic and volcanic processes at spreading ridges must consider both the temporal scale of processes as well as the physical scales of observations if predictive models are to be successful comparison of the morphometric expression within the major hydrothermal vent area and the rest of the southernmost ridge segment suggests that the mapped distribution of hydrothermal vents may reflect the extent of survey effort rather than uniqueness of geologic setting

land subsidence caused by the east mesa geothermal field california observed using sar interferometry interferometric combination of pairs of synthetic aperture radar sar images acquired by the ers 1 satellite maps the deformation field associated with the activity of the east mesa geothermal plant located in southern california sar interferometry is applied to this flat area without the need of a digital terrain model several combinations are used to ascertain the nature of the phenomenon short term interferograms reveal surface phase changes on agricultural fields similar to what had been observed previously with seasat radar data long term 2 years interferograms allow the study of land subsidence and improve prior knowledge of the displacement field and agree with existing sparse levelling data this example illustrates the power of the interferometric technique for deriving accurate industrial intelligence as well as its potential for legal action in cases involving environmental damages

evidence for shallow and pervasive seismic anisotropy in the wellington region new zealand the shear waves of local earthquakes were recorded during a 5 month deployment of seven three component digital seismographs on the wellington peninsula new zealand the seismographs were spaced an average of less than 5 km apart and over 300 local earthquakes were recorded with phase arrivals within the shear wave window a significant number 37 of the earthquakes recorded showed clear evidence of shear wave splitting identifiable fast and slow shear wave arrivals with similar pulse shapes consistent polarization directions at particular stations were also observed even when poor signal to noise ratio or scattering meant that no slow shear wave arrival could be identified however there were large station to station differences in the polarization directions correcting for the observed shear wave splitting improved the fit between the measured shear wave polarizations and those calculated assuming a double couple focal mechanism the cause of the observed shear wave splitting is therefore most likely to be seismic anisotropy large station to station differences in the polarization alignments ranging from 61 24 to 137 18 suggest that most anisotropy is confined to the top 2 3 km of the crust however there is evidence from one station for a small amount of pervasive anisotropy if such a trend existed on the other stations it could not be identified because of the large scatter in the data points the measured delay times between split shear waves vary from 0 02 to 0 22 s with a mean value of 0 1 0 06 s this translates to a near surface shear wave velocity anisotropy of about 10 with up to 2 pervasive anisotropy possible throughout the crust this data set indicates that extensive dilatancy anisotropy cannot be the sole cause of crustal seismic anisotropy and that foliations in the rock fabric and the fracture zones of active faults may also be important there is no evidence for temporal change in the shear wave splitting parameters during the period of the experiment

seismic anisotropy of the shallow crust at the juan de fuca ridge microearthquake data recorded on four ocean bottom seismometers are used to study shear wave splitting on the endeavour segment of the juan de fuca ridge the covariance matrix decomposition method is used to determine the sensor orientation from explosive shot data and to estimate the anisotropy parameters for 238 earthquake records at three of the four sites the results show a remarkably consistent fast direction parallel to the ridge axis the time delays between the fast and the slow waves range from 40 to 200 ms with an average of 90 ms they are not clearly related to earthquake range focal depth or source receiver azimuth the splitting of the shear waves is interpreted as an effect of structural anisotropy due to the presence of ridge parallel cracks in the shallow crust if we assume that anisotropy is concentrated in the upper 1 2 km the splitting times require a high crack density of 0 1

seismicity and tremor in a submarine hydrothermal field the northern juan de fuca ridge we have made the first multi year deployments of a small number of ocean bottom seismometers on a mid ocean ridge we located our instruments in and around a vigorous hydrothermal vent field on the endeavour segment of the juan de fuca ridge the deployments have revealed a consistent pattern of activity comprising several types of events most abundant are swarms of extremely small microearthquakes that we suggest are associated with cracking of the oceanic crust we also observe local tectonic earthquakes that occur about four times per day preliminary hypocentral locations place these tectonic events away from the ridge axis and outside the hydrothermal field the seismicity on the endeavour segment is remarkably different from the southern juan de fuca ridge where little if any tectonic activity is occurring we speculate that the endeavour and southern juan de fuca ridge are in two different phases of their long term evolution the latter has only rare activity that is only beginning to focus hydrothermal circulation along faults in contrast the endeavour segment is in an active tectonic phase and the faults have had time to develop robust hydrothermal circulation resulting in large hydrothermal vent deposits

seismic anisotropy in the upper oceanic crust seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at deep sea drilling project dsdp site 504b on dsdp leg 92 particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis there are four diagnostic patterns 1 along symmetry axes where vertically polarized shear waves sv are generated but horizontally polarized shear waves sh are not generated the particle motions are purely vertical 2 for azimuths at which both sv and sh are generated and the sh velocity is significantly faster than sv a cruciform pattern with horizontal first motion is observed 3 for azimuths at which both are generated and the sv velocity is significantly faster than sh a cruciform pattern with vertical first motion is observed and 4 for azimuths at which both are generated and sv and sh velocities are similar elliptical particle motions are observed the shear wave particle motions and compressional wave travel times from a 2 km radius circle are consistent with an anisotropic model with hexagonal symmetry the compressional wave velocity has a two theta azimuthal variation between 4 0 and 5 0 km s the symmetry axis is horizontal with an azimuth of n20 w 10 the spreading direction at the site 6 m y age is north south the observed seismic anisotropy is most probably caused by the preferred orientation of large scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting

shear wave splitting and crustal anisotropy at the mid atlantic ridge 35 n 1 shear wave splitting observed in microearthquake data at the axis of the mid atlantic ridge near 35 n has a fast polarization direction that is parallel to the trend of the axial valley the time delays between fast and slow s wave arrivals range from 35 to 180 ms with an average of 90 ms and show no relationship with ray path length source to receiver azimuth or receiver location the anisotropy is attributed to a shallow distribution of vertical fluid filled cracks aligned parallel to the trend of the axial valley joint modeling of the shear wave anisotropy and coincident p wave anisotropy results using recent theoretical models for the elasticity of a porous medium with aligned cracks suggests that the crack distribution that causes the observed p wave anisotropy can account for at most 10 ms of the shear wave delay most of the shear wave delay thus likely accrues within the shallowmost 500 m seismic layer 2a and the percent s wave anisotropy within this highly fissured layer is 8 30 isolated fluid filled cracks at 500 m to 3 km depth that are too thin or too shallow to be detected by the p wave experiment may also contribute to the shear wave delays the joint analysis of p and s wave anisotropy is an important approach for constraining the crack distributions in the upper oceanic crust and is especially suited for seismically active hydrothermal systems at slow and intermediate spreading mid ocean ridges

crustal anisotropy from local observations of shear wave splitting in west bohemia czech republic abstract the results of a systematic study of shear wave splitting observed in the three component digital seismograms of about 400 local microearthquakes that occurred during the 1985 1986 west bohemian earthquake swarm are presented two shear phases polarized nearly perpendicularly in horizontal projection were observed at all five stations of the local seismic network for most of the events the polarization of the faster shear wave is aligned in the wnw ese direction regardless of the mechanism and hypocenter location of the individual events and coincides with the direction of the maximum horizontal compressive tectonic stress in the region these effects are interpreted in terms of effective anisotropy of the upper crust in the region the maximum delay time between the split shear waves is 0 15 sec and corresponds to 6 anisotropy directional variation of the delay time was observed at the nearest station epicentral distance 5 km this variation was compared with functions predicted by theoretical models of cracked media to resolve which model is the most appropriate for crustal anisotropy surprisingly no agreement could be found for hudson9s model of dry or water filled parallel cracks that is widely used by many authors a successful fit was obtained for schoenberg douma9s model of a medium with parallel fractures the fracture normal pointed n31 e and the optimum fracture parameters were e n 0 4 and e t 0 02 the theoretically calculated fractures are aligned parallel to the tectonic stress direction and do not correspond to any real tectonic lines in the region

deformation and seismicity in the coso geothermal area inyo county california observations and modeling using satellite radar interferometry interferometric synthetic aperture radar insar data collected in the coso geothermal area eastern california during 1993 1999 indicate ground subsidence over a 50 km 2 region that approximately coincides with the production area of the coso geothermal plant the maximum subsidence rate in the peak of the anomaly is 3 5 cm yr 1 and the average volumetric rate of subsidence is of the order of 10 6 m 3 yr 1 the radar interferograms reveal a complex deformation pattern with at least two irregular subsidence peaks in the northern part of the anomaly and a region of relative uplift on the south we invert the insar displacement data for the positions geometry and relative strengths of the deformation sources at depth using a nonlinear least squares minimization algorithm we use elastic solutions for a prolate uniformly pressurized spheroidal cavity in a semi infinite body as basis functions for our inversions source depths inferred from our simulations range from 1 to 3 km which corresponds to the production depths of the coso geothermal plant underpressures in the geothermal reservoir inferred from the inversion are of the order of 0 1 1 mpa except a few abnormally high underpressures that are apparently biased toward the small source dimensions analysis of the insar data covering consecutive time intervals indicates that the depths and or horizontal extent of the deformation sources may increase with time this increase presumably reflects increasing volumes of the subsurface reservoir affected by the geothermal exploitation we show that clusters of microearthquakes associated with the geothermal power operation may result from perturbations in the pore fluid pressure as well as normal and shear stresses caused by the deflation of the geothermal reservoir

profile of discontinuities beneath hawaii from s to p converted seismic waves we present new seismological evidence for the depth of depression of the volcano crust and crust mantle moho boundaries and the nature of the volcano crust interface beneath the southeast flank of mauna loa volcano hawaii from a profile of s to p converted waves from deep local earthquakes three component seismic array observations of these converted waves indicate the presence of a low velocity layer lvl at a depth of about 10 km corresponding to the base of the volcanic pile the thickness of the lvl is estimated to be approximately 1 km the lvl and the moho both deepen significantly from southwest to northeast beneath mauna loa s southeast flank the observed polarity reversal of the lvl conversion compared to the moho conversion strongly supports the existence of a buried sediment layer at the volcano ocean crust interface at a depth that is consistent with the depth distribution of thrust earthquakes our results conflict with models of lithosphere flexure at hawaii indicating a depression of the lithosphere greater than that predicted by flexure models the thickness of underlying oceanic crust is observed to be relatively constant so this discrepancy cannot be accounted for by crustal underplating alone

seismic fracture anisotropy in the earth s crust an overview fluid filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement if we are to comprehend the role of fractures and fluids in tectonic processes to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways we must understand how seismic waves interact with the fracture crack and microcrack structures within the rock mass a salient feature of fracture related rock structure is the effective seismic anisotropy of aligned fractures and microcracks with the advent of large volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10 100 km scale of crustal stress orientation and preseismic stress buildup through the 100 1000 m scale of fluid reservoir structure and characterization and the 1 10 scale of mining and geoengineering rock strain monitoring to millimeter and micrometer or possibly smaller fluid filled inclusions determining the elastic response of the intact rock mass to focus developments in seismic anisotropic studies of crustal fracture phenomena the american geophysical union and the society of exploration geophysicists sponsored a chapman conference on seismic anisotropy in the earth s crust held may 31 to june 4 1988 at the lawrence berkeley laboratory of the university of california this was the third international workshop on seismic anisotropy 3iwsa nineteen conference papers are presented in this special section of the journal of geophysical research

the effect of fluid compressibility and elastic rock properties on deformation of geothermal reservoirs a geothermal reservoir deforms when the extraction of pore fluid exceeds reservoir recharge causing a decrease in pore pressure the magnitude of this deformation is related to the amount of pore fluid that is extracted assuming compressible material properties in a homogeneous reservoir we derive an expression for the ratio of reservoir volume change per extracted fluid mass we show that this ratio depends on a number of parameters notably the compressibilities of reservoir rock and pore fluid we apply the obtained relationship to three different geothermal areas hellisheidi reykjanes and the geysers to illustrate under which circumstances the relation between reservoir deformation and the amount of extracted fluid is able to help us learn more about reservoir conditions we find that the fluid compressibility depending on whether the system is single phase or two phase may explain large differences in estimates of reservoir volume changes per mass of extracted fluid

using surface deformation to image reservoir dynamics the inversion of surface deformation data such as tilt displacement or strain provides a noninvasive method for monitoring subsurface volume change reservoir volume change is related directly to processes such as pressure variations induced by injection and withdrawal the inversion procedure is illustrated by an application to tiltmeter data from the hijiori test site in japan an inversion of surface tilt data allows us to image flow processes in a fractured granodiorite approximately 650 barrels of water injected 2 km below the surface produces a peak surface tilt of the order of 0 8 microradians we find that the pattern of volume change in the granodiorite is very asymmetrical elongated in a north northwesterly direction and the maximum volume change is offset by more than 0 7 km to the east of the pumping well the inversion of a suite of leveling data from the wilmington oil field in long beach california images large scale reservoir volume changes in 12 one to two year increments from 1

seismic anisotropy in the crust at the mid atlantic plate boundary in south west iceland summary seismograms of microearthquakes recorded on stations of the sil network in south western iceland exhibit strong shear wave splitting and are consistent with being caused by aligned parallel cracks in the uppermost crust splitting times of 0 1 0 3 s are observed with the larger values 0 2 0 3 s occurring beneath stations in the highly heated western volcanic zone and the smaller values 0 1 s in the younger less fractured south iceland seismic zone five of the stations have a fast shear wave polarization azimuth of n30 e n40 e7 parallel to the axis of the western volcanic zone i e the mid atlantic ridge in iceland and in the direction of maximum horizontal compression defined by fault dyke and fissure strikes and microearthquake focal mechanisms station gyg in the north western part of the network has the significantly different azimuth of n70 e this is parallel to one of the local strike slip fault trends and is probably due to a heated rock fabric caused by those faults

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

changes in shear wave splitting at anza near the time of the north palm springs earthquake changes in shear wave splitting are observed at knw before and after the m 6 north palm springs earthquake of july 8 1986 knw is a station of the anza seismic network monitoring the anza seismic gap on the san jacinto fault southern california the gradual increase in the delays between the split shear waves over 3 years at knw reported by peacock et al 1988 ended in june 1986 the further 2 years of observations analyzed here show that the behavior of the delays changed abruptly near the time of the north palm springs earthquake 33 km north of knw peacock et al demonstrated that the increase in delays could be simulated by increasing the aspect ratio of stress aligned fluid filled inclusions and speculated that this increase might be the result of a buildup of stress before an impending earthquake the new data appear to confirm this speculation but the temporal variations require a more complex interpretation although they still suggest that the changes in shear wave splitting are caused by earthquake induced stress changes to the fluid filled inclusions throughout the rockmass central to interpretation of temporal changes in shear wave splitting is the well established existence throughout at leastmore the uppermost 10 to 20 km of the crust of small fluid filled cracks microcracks and pores the existence of such inclusions introduces a compliant quality to otherwise stiff crustal rock the authors term these distributions of inclusions extensive dilatancy anisotropy or eda and the individual inclusions eda cracks because although they may include a wide range of shapes many of the seismic properties can be simulated by distributions of thin parallel cracks less