source inversion of intensity patterns of earthquakes a destructive shock in 1936 in northeast italy 1 we demonstrate that it is possible to retrieve geometric and kinematic information about the source of a destructive past earthquake by inverting its regional macroseismic intensity patterns in fact in the case study the inversion results agree with the seismological instrumental measurements of the 1930s and with neotectonic evidence this leads to the hope that more knowledge about preinstrumental events can be obtained a key toward improving the calculation of seismic hazard mostly in the old world after validating our technique on the 1987 whittier narrows california earthquake pettenati and sirovich 2003 gentile et al 2004 we achieve the present results by investigating an earthquake that happened in 1936 in northeast italy the automatic inversions were performed by using a simplified formula for body waves that radiate from a linear source the inversion shows two minima on the hypersurface of the minimum residuals calculated observed intensity at all sites in the multiparameter model space these two minimum variance source models resemble the two auxiliary planes of the same theoretical fault plane solution similar to that given by the standard use of the first p wave arrivals the present result encourages us to treat more cases and to explore new inversion techniques for quantitatively treating the intensity patterns of earthquakes because at least in some cases they carry geometric and kinematic information about their source

anelasticity and thermal structure of the oceanic upper mantle temperature calibration with heat flow data the thermal structure of the oceanic upper mantle is examined on the basis of seismic attenuation surface heat flow and mid ocean ridge basalt morb composition the comparison of seismic q with laboratory q determined at sufficiently high pressure and temperature allows us to constrain temperature in the earth since the inferred absolute temperature relies on knowledge of the frequency dependence of q if any we calibrate an unknown frequency effect on q by comparing the derived temperatures with geotherms calculated from surface heat flow and morb composition this comparison yields a correction factor found to be small for the geotherm derived from laboratory ultrasonic q examples of thermal structure are presented for the iceland plateau and the pacific ocean under the iceland plateau we expect partial melt for the asthenosphere only in the vicinity of the spreading ridge in asthenosphere older than 5 m y of the iceland plateau and older than 40 m y of the pacific ocean temperatures are below the dry peridotite solidus and no partial melt is expected these results are consistent with the thermal structure derived from a comparison of seismic and laboratory velocities it is shown that there is a good agreement between temperatures determined from seismic attenuation heat flow and morb chemistry since the laboratory measurements were made on a dry peridotite and we employ a dry peridotite solidus for determining the geotherm the consistency in temperatures derived from three independent data strongly suggests that the upper mantle remote from subduction zones is essentially dry it appears that the spreading process in mid ocean ridge causes asthenosphere material to ascend from greater depths where it was at higher temperature this gives rise to pressure release melting

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

a new regional hypocenter location method a new regional hypocenter location method is presented in this article to address location problems associated with sparse station distributions and lack of velocity information for an arbitrary slowness structure an unknown parameter slowness deviation is introduced to construct a more precise slowness model than a homogeneous slowness model by incorporating a genetic algorithm the new method yields reasonable solutions for epicenters and origin times both inside and outside the seismic network the synthetic data tests indicate that outside the network the new method gives excellent results for epicenter locations compared with geiger s method even when geiger s method is used with the true velocity structure the relocation of the loma prieta mainshock and 26 aftershocks occurring within the first 24 h after the mainshock by using the new hypocenter location method places the aftershocks an average of 2 29 km to the southwest of the locations published by the northern california earthquake data center the new locations are in better agreement with the geodetic measurements

crustal velocity structure in italy from analysis of regional seismic waveforms abstract in this article we use regional seismic waveforms recorded by the recently installed istituto nazionale di geofisica e vulcanologia ingv national network and the mediterranean very broadband seismographic network mednet stations to develop 1d crustal velocity models for the italian peninsula about 55 000 p wave and 35 000 s wave arrival times from 4727 events are used to derive average seismic parameters in the crust and uppermost mantle we define four regions according to geological constraints and recent travel time tomography results based on the average seismic parameters we combine broadband seismic waveforms and travel times of regional phases to model crustal structures for the four regions by applying the genetic algorithm our results indicate smooth velocity gradients with a depth beneath the apennines and a deep moho beneath the central alps green s functions from the regionalized 1d velocity models are used to determine source depths and focal mechanisms for 37 events with a magnitude larger than 3 5 by a grid search technique our results show that normal and strike slip faulting source mechanisms dominate the apenninic belt and that most thrust faulting events occur in the adriatic sea and the outer margin of the northern apennines

hypocentral relocation using clustering along planes constraints implications for fault geometry summary hypocentre location is an ill posed inverse problem even assuming that the velocity model is known because different sets of hypocentre locations may satisfy the fitting criterion we present a regularized hypocentre inversion in which the constraints of spatial proximity of the hypocentres to target planes are used this constraint introduces the geological bias that earthquakes might occur along fault planes here the target planes may be either 1 planes specified by the interpreter or 2 planes fitting groups of events we assume also that initial estimates of hypocentres and origin times are available then the initial hypocentre estimates origin times and target planes are used as input to an inversion problem to relocate the hypocentres so that the maximum possible clustering of events along the given planes is attained matching the observed traveltimes we use l1 norm for data fitting l2 norm for the plane proximity criterion and a polytope algorithm to minimize the functional results from synthetic and real data indicate that the plane proximity constraint allows for hypocentre relocation presenting a high degree of clustering along planes the real data example is an intraplate earthquake sequence in ne brazil our methodology defined the geometry and strike of fault segments close to known geology and focal mechanism data in addition the new method indicates that the fault is characterized by a splay geometry in its southern end and that more than three fault segments are necessary to explain the hypocentre distribution

earthquake location genetic algorithms for teleseisms kennen b l n and sambndge m s 1992 earthquake location genetic algorithms for teleselsms phys earth planet inter 75 103 110 the location of earthquakes requires the estimation of the spatial and temporal components of the hypocentre this can he achieved by a direct mmlmtsatlon of a measure of the misfit between observed and calculated travel times and also slownesses and azimuths if array data are available an efficient means of carrying out this optlmlsatlon procedure is to make use of genetic algorithms this technique is based on the use of many estimates of the hypocentre location at once and the properties of the cluster of estimated locations in four dimensions are exploited m the course of the optlmtsatlon process each estimate of the hypocentral location is represented on a local discrete grid by a bit string and successive iterations generate new bit strings and hence location estimates by operations based on biological analogues these operations are the replication of the best fining bit strings the cross over of reformation between pairs of bit strings and the mutation of indwldual bits in a string the non local character of the information on the misfit function carried in the cloud of hypocentral estimates is usually sufficient to prevent the location being trapped in local minima of the misfit surface convergence to the global misfit minimum can be achieved with a very limited sampling of the original spatml and temporal grid no derivatives of the seismic phase information are reqmred and so the technique is easily generahsed to three dimensional velocity models and can be used with any statable measure of the quahty of an earthquake location by the choice of the misfit criterion between observed and calculated quantities

new observations of gulf of gdansk seismic events abstract seismicity of northern poland and of the adjacent area of the baltic sea has been considered weak if any historical reports mention just a couple of seismic events felt in the region while observatory seismology has recorded just a single swarm of seismic events in the gulf of gdansk the lack of information is caused not only by the weak seismicity of the area but also by the lack of seismological observatories in the region this situation is changing with the installment of a new seismological station suwalki in northeastern poland and two temporary very broadband stations of the geofon network one in northwestern poland and the other on oeland island sweden just months after the installations we are able to detect and locate numerous seismic events in the gulf of gdansk this does not necessarily mean that gulf of gdansk is a seismically active area as there are indications that the events may be caused by human activity one of the objectives of this paper was to obtain reliable location solutions with the best possible location error control since only a few stations are in operation in the nearby area and little is known about the velocity structure the bayesian inversion method has been used to locate event epicenters this method allows to minimize location errors caused by insufficient knowledge of local crust velocity structure and non gaussian statistics of measurement modeling errors the method being fully nonlinear does not introduce instabilities typical for inverse approaches which uses linearization

a hotspot model for iceland and the voring plateau the proximity of a hotspot to a spreading center may result in the channeling of atmosphere to the section of rise crest closest to the hotspot this produces more basalt and thicker crust at these locations thus forming a plateau over time the geometric constraints of such a model predict a unique orientation location and age progression for a plateau formed by this mechanism the hotspot will channel material to the closest part of the rise therefore the orientation of the plateau will differ from that of a hotspot track by the component of absolute motion perpendicular to the rise axis the plateau will be symmetric with respect to the location of the rise axis at the time of formation also the age progression of the plateau will be contemporaneous with the age of formation of the seafloor on either side because the plateau is seafloor just with thicker crust a set of reconstructions based upon magnetic isochrones and a fixed hotspot reference frame is presented for the norwegian greenland sea as a means of evaluating the model s predictions by locating the iceland hotspot reconstructing the relative positions of the greenland and european plates and then assuming material would be channeled from the hotspot to the closest section of the rise crest we can trace the tectonic evolution of the greenland faeroe and voring plateaus the model is able to locate the plateaus explain their orientations and predict an age progression that satisfies observed age determinations the analysis demonstrates that both plateaus could have been formed by the iceland hotspot with the greenland faeroe plateau being in effect a continuation of the voring plateau which was cut off due to transform motion between the northern and southern spreading centers

validation of the automatic nonlinear source inversion of the u s geological survey intensities of the whittier narrows 1987 earthquake in this work we validated the automatic inversion of the u s geological survey usgs intensity data set of the whittier narrows m l 5 9 1987 earthquake this validation of our new technique was obtained by comparing the source inversion results with the principal source parameters coming from instrumental measurements independent of our study to perform the inversion first we used a genetic algorithm ga with a population of 20 000 individuals i e sources second because the problem is bimodal we also used a niching genetic algorithm nga with four demes of 2 000 individuals this gave us almost the same results the site intensities were calculated by our kinematic kf function twelve source parameters were involved in the inversions the most sensitive of which are the epicentral coordinates and the fault plane solution two minimum variance models were determined by both the ga and the nga inversions 1 one east west trending dip slip source which is in agreement with that already known from instrumental measurements and 2 one almost coinciding with its auxiliary plane in the fault plane solution these findings almost coincide with those produced by the grid search inversion method but the ga nga inversion is much faster and does not need strong constraints this confirms that it is possible to get an approximate idea of the source of the studied earthquake also by automatically inverting the regional pattern of the usgs intensities this result encourages us to validate our inversion technique with more well documented earthquakes and to treat intensities of preinstrumental earthquakes which are the principal target of our work manuscript received 30 july 2003

multimode rayleigh wave attenuation and qβ in the crust of the barents shelf the multimode method of cheng and mitchell 1981 is used to study the attenuation properties of the crust throughout the barents shelf the sensitivity of the spectra to various source factors and to propagation factors which might characterize a shelf region are thoroughly studied for plane layered models the method is very sensitive to q in the upper crust and to q of sedimentary layers if those layers are thick and highly attenuating it is far less sensitive to q in the lower crust and to q the spectra for shallow strike slip earthquakes are very insensitive to changes in strike slip and dip angles of the fault throughout the entire period range of interest but are very sensitive to variations in focal depth except in the long period range of the fundamental mode and the short period range of the higher modes the fundamental mode but not the higher mode spectra for one path across the barents shelf are apparently distorted at short periods by lateral heterogeneities along the travel path simple two layer trial and error inversions yield a model of the barents shelf with an upper crustal q value of about 80 three layer inversions with a sedimentary layer however yield an upper crustal q of about 250 if the sediments are 1 km thick and are characterized by q values of 40 shear velocity models for the barents shelf show variations between about 3 0 and 3 6 km s in the upper crust along different paths the lower crust shows smaller regional differences but velocities vary between 3 5 and 4 0 km s through the depth range 25 35 km the variation of shear velocities in the upper mantle may correlate with those of the upper crust and vary regionally between about 4 3 and 4 8 km s

application of genetic algorithms to constrain near source velocity structure for the 1989 sichuan earthquakes abstract when a complex earthquake source process is studied using broadband body waveforms a common problem is finding an adequate layered velocity model for the near source structure tests with several existing crustal models are not sufficient or objective but an enumerative search scheme to search through a whole possible model space is not practical we study the feasibility of applying genetic algorithms ga to rapidly and effectively explore the model space to find an optimal model for the near source structure our data set consists of multi station broadband body waveforms of a simple event recorded at teleseismic distances the near source velocity structure is approximated by a layered model and the model parameters include the layer thicknesses and velocities as well as the source depth forming a multi parameter search space test results with synthetic data indicate that optimal models can be found using ga depending on the noise level when the noise level is low optimal models are well recovered with small uncertainty results with the data of the 1989 earthquakes of sichuan province china indicate that a three layer velocity model obtained with ga is in agreement with regional models and better explains the observed waveforms

locating regional seismic events with global optimization based on interval arithmetic more than 1000 seismic events in northern europe at distances of up to 400 km from the detecting network are located using an optimization method in which the global minimum of the traveltime function residuals is searched for using an interval arithmetic ia method epicentres are determined using p waves detected by the finnish national seismic network up to 15 stations were used in the analysis the ia results coincide with locations provided by the university of helsinki bulletins with a median location bias of 7 6 km a second data set of 59 explosions in the siilinjarvi mine in central finland was examined in detail because the locations of the explosions were known exactly in this case the median difference of ia locations was 3 8 km from the average location of mine explosions while all 59 events were located within 9 km of the true epicentres the corresponding median error of the university of helsinki locations was smaller 3 2 km but some helsinki locations were well over 10 km from the mine the convergence towards the global optimum using interval arithmetic was fast when compared with the conventional least squares approaches for epicentre determinations

upper mantle q structure beneath the east pacific rise a model of q as a function of depth has been determined for the upper mantle beneath the east pacific rise the model is appropriate for shear waves with periods from roughly 15 to 30 s the amplitudes of multibounce sh phases which propagate within the upper mantle were used to constrain the q structure the multibounce phase amplitudes were measured with respect to the s wave amplitude for each seismogram the data consist of s ss sss and ssss waves for distances from 30 to 80 the sources are earthquakes which occurred on transform faults associated with the east pacific rise and the seismograms were recorded by world wide standard seismograph network and canadian seismic network stations in western north america mantle beneath oceanic crust less than 15 ma and tectonically active continental crust is sampled since the s wave data primarily sample the deep mantle the resulting upper mantle q model depends on the q structure assumed for the lower mantle we assumed a constant q of 312 for the mantle beneath 650 km depth with this assumption the upper mantle phase amplitudes require a q of about 70 in the upper 150 km of the mantle from 150 to 400 km depth q increases to 180 the q in the transition zone from 400 to 650 km depth was found to be 180 significantly lower than the lower mantle q the resulting q model is consistent with several surface wave studies of mantle attenuation beneath the pacific however our results predict a qscs 210 which is significantly higher than many direct measurements of scs attenuation indicate furthermore the large range in measured qscs is inconsistent with lateral q variations being confined to the upper mantle if the region we have studied represents the low q extreme of upper mantle we conclude that large lateral variations in q exist in the lower mantle

attenuation of surface waves across the arabian peninsula abstract amplitude spectra of fundamental mode rayleigh and love waves are used to determine shear wave q q models for the upper crust of the arabian peninsula the observed spectra are compared to theoretical spectra computed for earthquakes with known fault plane solutions shear wave q in the upper crust can be obtained by trial and error modeling to fit the the slope of the amplitude spectra at short periods 5 30 s if the source time function is less than about 6 s and if the depth of the earthquake is known to within about 3 km shear velocity q values obtained in this way vary from about 60 along the margin of the red sea to 100 150 in the central part of the peninsula to 65 80 in the eastern folded region these values are unusually low for a stable region and suggest that observed anelasticity there is affected by tectonic and epeirogenic activity which has occurred over the past 30 m y the shear wave q models derived from surface waves at periods of 5 30 s closely predict reported values of lg q at 1 hz shear wave q therefore appears to be independent of frequency in this region at periods between about 1 and 30 s

mantle layering from scs reverberations 1 waveform inversion of zeroth order reverberations this is the first paper in a four part sequence that examines the nature of mantle layering required by the multiple scs phases and internal reflections observed within the reverberative interval of sh polarized seismograms here we present path averaged estimates of the two way travel times through the crust mantle scs and crust alone m the whole mantle quality factor qscs and the reflection coefficient at the m discontinuity r0 zm obtained by iterative waveform inversion of scs reverberations we show that qscs estimates derived from direct waveform inversion can be biased significantly if reverberations originating within the crust are ignored especially in continental regions where the crust is thick applying our technique which explicitly models crustal reverberations we substantially expand the geographical coverage of scs and qscs measurements available from previous studies the scs data correlate well with whole mantle travel times predicted by the harvard tomographic models both the scs data and the qscs data support the hypothesis that variations in velocity and attenuation structure between old continents and oceans persist to depths of several hundred kilometers in particular the value of qscs derived for paths crossing stable continent is very high 280 30 at 30 mhz compared with that for mature ocean basin 189 16 consistent with the thick plate model of cratons requiring that differences in thermal structure extend to several hundred kilometers the crustal thicknesses implied by the observations of path averaged m are consistent with those inferred from short range refraction data gravity measurements and surface topography the path averaged reflectivity of the m discontinuity is also tectonically correlated becoming brighter as the percentage of continental crust encountered along the path increases this result implies that on average the impedance at the base of the continental crust is significantly lower than at the base of the oceanic crust

partial melt below iceland a combined interpretation of seismic and conductivity data a combined interpretation of published seismic and electric data is carried out to impose constraints on the state of melting below central iceland these data indicate high electrical conductivity and low vp velocity regionally below 6 km s for the crust mantle transition region while the mantle below shows lower conductivity but still anomalous low velocity the velocity data are treated by utilizing gravity data to quantify the elastic modulus decrease due to partial melt a comprehensive set of theoretical models on elasticity anelasticity and electrical conductivity of partially molten rock is applied to this data set the geometry the fraction and the interconnection of the melt phase are varied systematically the conductivity models are based on published laboratory data on conductivity of solid and melt phases of rocks anisotropy and possible unexplored conductivity mechanisms near the melting temperature are not considered here by means of the melt models the data can be explained most easily as follows in a depth range roughly between 20 and 50 km only a small percentage of melt 2 at the most 5 if melt pockets are also present occurs in thin films possibly with a distribution of aspect ratios and a low degree of interconnection if in the crust mantle transition region zones of low p wave velocity 6 km s are correlated with those of high conductivity 0 08 s m melt fractions between 10 and 20 are likely and the degree of interconnection should be high thin films can be excluded as a dominant melt geometry in that region apart from improved constraints on melt geometry the results are in good agreement with published models based only on conductivity data however they disagree with published models based entirely on seismic data the discrepancy is now resolved by the combined interpretation

seismic wave attenuation and dispersion resulting from wave induced flow in porous rocks a review one major cause of elastic wave attenuation in heterogeneous porous media is wave induced flow of the pore fluid between heterogeneities of various scales it is believed that for frequencies below 1 khz the most important cause is the wave induced flow between mesoscopic inhomogeneities which are large compared with the typical individual pore size but small compared to the wavelength various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter scale mesoscopic heterogeneities laboratory and field measurements of seismic attenuation in fluid saturated rocks provide indications of the role of the wave induced flow signatures of wave induced flow include the frequency and saturation dependence of p wave attenuation and its associated velocity dispersion frequency dependent shear wave splitting and attenuation anisotropy during the last four decades numerous models for attenuation and velocity dispersion from wave induced flow have been developed with varying degrees of rigor and complexity these models can be categorized roughly into three groups according to their underlying theoretical framework the first group of models is based on biot s theory of poroelasticity the second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation another group of models is derived using the theory of viscoelasticity though all models predict attenuation and velocity dispersion typical for a relaxation process there exist differences that can be related to the type of disorder periodic random space dimension and to the way the local flow is incorporated the differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies in recent years some theoretical models of wave induced fluid flow have been validated numerically using finite difference finite element and reflectivity algorithms applied to biot s equations of poroelasticity application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data

thermal structure of the low velocity zone derived from laboratory and seismic investigations laboratory measurements of seismic velocities in upper mantle peridotite can be used to constrain the thermal state in the low velocity zone the measurements carried out at high temperature and high confining pressure can be extrapolated to upper mantle conditions we compare laboratory velocities with seismic observations and estimate temperatures and partial melt fractions in the low velocity zone the comparison gives 1310 c and 7 vol partial melt at 45 km depth in 0 5 my asthenosphere beneath the iceland plateau and 1450 c and 3 vol partial melt at 85 km depth in 0 20 my asthenosphere beneath the pacific ocean the laboratory results show that up to a 6 velocity drop in the low velocity zone relative to the lithospheric velocity does not necessarily require the existence of partial melt this implies that partial melt may not exist in the low velocity zone older than 5 my and 50 my in the iceland plateau and the pacific ocean respectively

a probabilistic seismic model for the european arctic 1 the development of three dimensional seismic models for the crust and upper mantle has traditionally focused on finding one model that provides the best fit to the data while observing some regularization constraints in contrast to this the inversion employed here fits the data in a probabilistic sense and thus provides a quantitative measure of model uncertainty our probabilistic model is based on two sources of information 1 prior information which is independent from the data and 2 different geophysical data sets including thickness constraints velocity profiles gravity data surface wave group velocities and regional body wave traveltimes we use a markov chain monte carlo mcmc algorithm to sample models from the prior distribution the set of plausible models and test them against the data to generate the posterior distribution the ensemble of models that fit the data with assigned uncertainties while being computationally more expensive such a probabilistic inversion provides a more complete picture of solution space and allows us to combine various data sets the complex geology of the european arctic encompassing oceanic crust continental shelf regions rift basins and old cratonic crust as well as the nonuniform coverage of the region by data with varying degrees of uncertainty makes it a challenging setting for any imaging technique and therefore an ideal environment for demonstrating the practical advantages of a probabilistic approach maps of depth to basement and depth to moho derived from the posterior distribution are in good agreement with previously published maps and interpretations of the regional tectonic setting the predicted uncertainties which are as important as the absolute values correlate well with the variations in data coverage and quality in the region a practical advantage of our probabilistic model is that it can provide estimates for the uncertainties of observables due to model uncertainties we will demonstrate how this can be used for the formulation of earthquake location algorithms that take model uncertainties into account when estimating location uncertainties

broadband observations of upper mantle seismic phases in northern australia and the attenuation structure in the upper mantle abstract sources in the earthquake belt through indonesia and new guinea recorded at a broadband guralp cmg3 seismometer at the warramunga array in the northern territory of australia give good coverage of propagation through the upper part of the mantle the midpoints of the propagation paths lie along the northern margin of the australian continent beyond 18 there is a significant difference in the frequency content of p and s waves the p waves remain high frequency but the s waves returned from the transition zone and below are of intermediate period 0 1 0 5 hz which would be difficult to record without a broadband instrument and a quiet site the later branches associated with the 410 and 660 km discontinuities are clearly seen in individual seismograms the s waves recorded on the radial sv and tangential sh components are of comparable quality because the hard rock recording site minimizes the influence of coupling to p on the radial component we interpret the observed difference in frequency content of p and s waves at transition zone distances as the effect of a layer under the seismically fast lid which possesses a large degree of shear dissipation we quantify the observation in terms of the decay rate of the spectral ratio of s and p waves that quantity can be interpreted in terms of path integrals of the difference in inverse q for s waves and p waves t s t p we analyse 22 seismograms from the wra broadband instrument and a further four from portable broadband instruments deployed near warramunga the measured slope of the s p spectral ratio is consistently small out to 18 epicentral distance where it increases dramatically assuming that shear dissipation dominates over bulk dissipation these measurements are consistent with an average quality factor for s waves in the lid of the order of q s 1400 q p 2800 on top of a highly attenuative asthenosphere of 200 km thickness with q s 100 q p 200 which is underlain by a transition zone with q s 600 q p 1200 the low q zone in the asthenosphere can equally well be modelled with a thinner layer with a lower q e g a layer of 100 km thickness with q s 60 frequency dependence of q may render the above estimates of q about 25 too large these results compare fairly well with published results from 0 3 hz body wave observations under the eurasian shield notable exceptions are that we obtain a somewhat lower value for q s in the asthenosphere and a much higher value in the lid the lid under northern australia is unusually

locating oceanic earthquakes the influence of regional models and location criteria summary earthquakes in oceanic areas are normally located using traveltime tables which are representative of continental paths since most seismic stations lie on continents it should therefore be possible to improve such locations by employing a set of traveltimes more appropriate to paths from oceanic events to continental stations a comparison has therefore been made between locations for a number of oceanic events using the recent iasp91 global traveltimes and the times for the pac91 model derived from observations of events in the pacific although there were often significant differences in the location estimates for the two models these were often no larger than the shifts induced by changing the misfit criterion used for determining the location for events in purely oceanic regions such as tonga and the marianas with little nearby continent the results from the pac91 model either provided a significantly better fit to the data or produced depth estimates in close accord with independent constraints e g centroid moment tensor locations in these cases the use of a specific set of oceanic traveltimes can be recommended however for marginal zones and island arcs the situation is less clear and it is probably best to employ the global traveltime set with the use of additional phases to improve depth estimates

anelasticity of the iceland plateau from surface wave analysis we study the anelasticity of the asthenosphere beneath the youngest seafloor of the iceland plateau region using spectral analyses of surface waves the epicenters and station geometry naturally define a fan shooting experiment with pure path propagation in regions with different seafloor ages both rayleigh and love waves are used in this study to help constrain the source parameters needed for a reliable assessment of q the inversion results indicate an approximately 130 km thick layer of q of about 7 for the asthenosphere of the youngest region of 0 5 ma for the region of age 5 10 ma q is about 50 and increases to over a hundred for the oldest zone 10 ma these attenuation values when combined with laboratory experiments indicate a partial melt of 3 in the asthenosphere near the spreading ridge

errors in hypocenter location picking model and magnitude dependence abstract the location procedures of seismic events are influenced by two major classes of errors the error in picking individual seismic phases and modeling error due to the departure of the real earth from the reference model used in the location both classes of error influence the estimate of location and it is difficult to separate them the role of picking errors can be assessed by a nonlinear analysis using a monte carlo procedure arrivals times are perturbed with random numbers drawn from a normal distribution and the event is relocated using these perturbed arrival times by repeating the procedure many times a cluster of locations is obtained which can be used to investigate the effects of picking errors on the hypocenter this analysis is insensitive to velocity model errors as these are fixed for a given combination of stations and phases some care must be exercised when analysing multidimensional distributions in two dimensional slices because of a projection effect the modeling error due to the influence of lateral heterogeneity in the earth is examined by comparing the locations of the same event using different combinations of phases and network geometries which reinforces the need to use arrivals other than p for accurate depth resolution the sensitivity of p arrivals to changes in depth are swamped by model errors and inclusion of depth sensitive phases such as pp is highly recommended the effect of picking errors on location is found to be much smaller than the mislocation caused by neglecting lateral heterogeneity when only p arrivals are used consequently the monte carlo analysis which is primarily aimed at picking errors only is most appropriate when multiple phases have been used to more accurately constrain the hypocenter especially for the depth component altering the type of phase data used in the location plays a similar role in changing the network geometry in that both are mechanisms that influence the nature of the constraint on the hypocenter by relocating events with network geometries corresponding to the different magnitudes it is found that the location of the event can be affected significantly by the magnitude and when using robust statistics to describe earthquake residuals the mislocation can occur in a systematic manner the effect is marked in regions with significant lateral variations in seismic velocities for example low magnitude events in the flores sea are found to be dragged toward australia as a result of the fast paths to australian stations relative to the iasp 91 reference velocity model

qp melting temperature relation in peridotite at high pressure and temperature attenuation mechanism and implications for the mechanical properties of the upper mantle the anelastic properties of compressional waves in a peridotite have been determined in the laboratory at sufficiently high temperatures to 1280 c and pressures to 0 73 gpa to warrant comparison with seismic measurements of the earth a substantial decrease of qp is observed at temperatures well below the onset of partial melting qp systematically increases with increasing pressure over the entire temperature range of major significance is the finding that qp is dependent on the ratio of the temperatures to the melting solidus temperature i e qp depends on the homologous temperature the pressure dependence of qp appears through the pressure dependence of the solidus of the peridotite within the uncertainties of measurement of both qp and the phase diagram it appears that melting and high temperature anelastic properties have a common origin in peridotite the homologous temperature dependence of qp suggests that we may estimate the temperature and pressure dependence of qp for peridotites of different compositions and possibly even for hydrous peridotites if solidus temperatures are known as a function of pressure a far easier measurement than elastic and anelastic properties the activation volume of qp is greatly reduced at high pressure since the slope of solidus versus pressure rapidly decreases with increasing pressure pressure dependence of seismic velocity and melt fraction in peridotite also appears to be related to the homologous temperature the qp homologous temperature relation suggests a connection between qp and the properties of the grain boundaries that is the major loss of seismic energy occurs at the grain boundaries grain boundary relaxation or high temperature background attenuation is a possible mechanism for the grain boundary damping no frequency dependence of qp is resolved 0 0 2 in qp f over the pressure temperature and frequency ranges of the measurement the present results and the model of grain boundary relaxation suggest that an appropriate choice of grain size may give an ultrasonic q that is applicable to the earth experimentally determined anelastic properties of a peridotite are critical for modeling mechanical properties of the upper mantle implications of the results are as follows 1 seismic data commonly interpreted as indicating a partially molten asthenosphere may instead reflect a hot solid asthenosphere at 90 100 of the solidus temperature 2 partial melting may not produce any abrupt change of seismic velocity and q rather elastic and anelastic properties of the upper mantle will change gradually at the boundary where the geotherm crosses the solidus 3 there may be no sharp mechanical boundary between the lithosphere and the asthenosphere

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

geotherms in the pacific ocean from laboratory and seismic attenuation studies seismic methods have been used extensively to measure attenuation coefficients in various regions of the earth attenuation is inversely related to the quality factor q which may be measured directly a low velocity and high attenuation zone in the upper mantle is commonly reported1 3 until now there has been no direct means to derive thermal structures from such seismic investigations recently q values have been determined in the laboratory for a dry peridotite an upper mantle rock for pressures and temperatures of up to 0 73 gpa and 1 280 c respectively4 the results indicate a quantifiable relation between q and temperature here we use these results to derive geotherms from q measurements for a region beneath the pacific ocean the geotherms are consistent with temperatures obtained from heat flow observations5 6 and imply that the oceanic upper mantle is rather dry in asthenos phere older than 40 myr geotherms for this region are sub solidus and no partial melt is expected

crustal q structure in the united states from multi mode surface waves determination of q structures are made using a new method which matches observed surface wave spectra with theoretical spectra computed for earthquakes with known fault plane solutions the importance of the method lies in its ability to utilize relatively short paths between a single source and receiver both of which are preferably located within a single geological province the method has been applied to study the regions of the eastern united states the colorado plateau and the basin and range province q models have been obtained through trial and error procedures and with an inversion scheme developed for that purpose models for each region obtained from these two procedures are in good agreement q values in the upper crust as obtained from inversions are 275 for the eastern united states 160 for the colorado plateau and 85 for the basin and range province q values in the lower crust although not well determined must be much higher than upper crustal values in all three regions the conclusions are consistent with those of previous work in which q values in the upper crust of a broad region of the western united states are lower than those of the eastern united states these new results show that substantial variations of q occur in different regions of the western united states the upper crustal q values show an inverse relationship to heat flow values

waveform inversion of surface wave data test of a new tool for systematic investigation of upper mantle structures summary in most tomographic inversion of surface wave data the long period seismograms are first interpreted in terms of dispersion and or attenuation curves before performing an inversion in terms of laterally and depth varying properties an alternative to this approach is to perform a direct waveform inversion or as in cara lev que 1987 to use another set of secondary observables built up from the seismograms in this paper we systematically test with actual rayleigh wave records this latter technique by considering laterally homogeneous models we use for this purpose a set of recent long period digital records from the geoscope station dumont d urville antarctica for surficial events in the south indian ocean and the southeast pacific ocean and for intermediate depth events in the vanuatu and kermadec trenches in addition to the obvious advantages of being able to deal with situations where overtones are present in the seismogram it is found that the waveform inversion procedure allows us to reach a better depth resolution than in classical inversion of dispersion curves even when only the fundamental mode is present in the seismogram quite good resolution is obtained to depth as large as 300km for s velocity when using surficial events located at a few thousand kilometres from the station while classical surface wave studies do not allow us to resolve s velocity at depth larger than 150 km for these events when intermediate depth events are used at distances of about 5000 km the presence of overtones in the signal allows us to get resolution to depth as large as 600 km for s velocity poorer resolution is obtained in both situations for the attenuation factor s velocity appears furthermore to be more robust than attenuation at depth where good resolution is achieved due to the great sensitivity of surface wave amplitude to departure from the assumption of lateral homogeneity more sophisticated direct modelling would be required to get more confidence in the inverted attenuation models