evidence for inner core rotation from possible changes with time in pkp coda 1 inner core grazing pkp waves from seven french nuclear tests on mururoa island recorded at norsar appear to change over a decade we compared filtered and stacked signals from the explosions which took place from 1977 to 1987 and were separated by less than 10 km the pkp energy 3 to 10 s after the first arrivals differs greatly between the norsar subarrays and evolves over time the small time shifts of coda arrivals a few tenths of a second are qualitatively consistent with shifts predicted for point scatterers in an inner core that rotates in the range 0 05 to 0 10 per year there are also visible changes in the envelope of the pkp energy although the alternative of systematic shifts in source location cannot be ruled out the apparent time evolution of pkp supports claims of slow inner core rotation song and richards 1996 creager 1997 laske and masters 1999 vidale et al 2000 and offers a powerful new tool for its assessment

is the rotation real earth9s interior consists of a mantle liquid core and at the very center a solid inner core made of iron one of the inner core9s most peculiar properties is its anisotropy seismic waves travel slightly faster if they are parallel to earth9s rotation axis in her perspective souriau describes recent seismic studies that suggest the inner core rotates faster than earth9s mantle and comments on the current dilemma in interpreting the experimental findings

support for differential inner core superrotation from earthquakes in alaska recorded at south pole station observational evidence for differential inner core rotation has recently been reported the most convincing evidence has been restricted to one ray path from earthquakes in the south sandwich islands to stations in alaska here we show that the differential travel times between pkp df and pkp bc for a new ray path from earthquakes in southern alaska to the station at the south pole have increased by about 0 6 s over 37 years providing support for a differential inner core rotation we apply a newly developed technique to invert simultaneously for the inner core structure and the rotation rate the rotation rate determined from the new pathway is about 0 6 yr faster than the mantle compatible with the estimates of the rate from earthquakes in the south sandwich islands to stations in alaska we also address some criticisms concerning the reported detection of the differential inner core rotation including biases from event mislocations biases from mantle heterogeneity and uncertainty in the tilt of the inner core anisotropy axis we show that although these potential biases would affect the determination of the rotation rate they are unlikely to account for the observed time dependence in the bc df times we found no evidence for a correlation between bc df residuals and event magnitudes as suggested previously

seismic velocity and attenuation structures in the top 400 km of the earth s inner core along equatorial paths 1 we study seismic velocity and attenuation structures in the top 400 km of the earth s inner core based on modeling of differential traveltimes amplitude ratios and waveforms of the pkikp pkikp phases observed at the epicentral distance range of 120 141 and the pkpbc pkikp phases observed at the distance range of 146 160 along equatorial paths our data are selected from the seismograms recorded in the global seismographic network from 1990 to 2001 and many regional seismic networks the observed pkikp pkikp and pkpbc pkikp phases exhibit distinctive east west hemispheric patterns 1 at the distance ranges of 131 141 and 146 151 pkikp phases arrive about 0 3 s earlier than the theoretical arrivals based on the preliminary reference earth model prem for the pkikp phases sampling the eastern hemisphere 40 e 180 e of the inner core and about 0 4 s later for those sampling the western hemisphere 180 w 40 e at the distance range of 151 160 pkikp phases arrive about 0 7 s earlier than the predicted arrivals based on prem for those sampling the eastern hemisphere and about 0 1 s later for those sampling the western hemisphere 2 amplitude ratios of the pkikp pkikp phases at the distance range of 131 141 and of the pkikp pkpbc phases at the distance range of 146 151 are in general smaller for the pkikp phases sampling the eastern hemisphere than for those sampling the western hemisphere at distances greater than 151 the pkikp pkpbc amplitude ratios become indistinguishable for the two hemispheres these observations can be best explained by two different types of seismic velocity and attenuation models along equatorial paths one for each hemisphere in the top 400 km of the inner core for the eastern hemisphere the velocity structure has a velocity increase of 0 748 km s across the inner core boundary icb a small velocity gradient of 0 0042 km s 100 km in the top 235 km followed by a steeper velocity gradient of 0 1 km s 100 km extending from 235 km to 375 km and a velocity gradient of 0 01 km s 100 km in the deeper portion of the inner core the attenuation structure has an average q value of 300 in the top 300 km and an average q value of 600 in the deeper portion of the inner core for the western hemisphere the velocity structure has a velocity increase of 0 645 km s across the icb and a velocity gradient of 0 049 km s 100 km in the top 375 km the attenuation structure has an average q value of 600 in the top 375 km of the inner core our results suggest that the inner core hemispheric variations in velocity extend deeper than 375 km below the icb and the top 235 km of the inner core in the eastern hemisphere is anomalous compared to the rest of the inner core in having a small velocity gradient high velocity and high attenuation

examination of systematic mislocation of south sandwich islands earthquakes using station pairs implications for inner core rotation 1 one of the most robust observations for the inner core rotation is that the differential pkp bc df traveltimes from south sandwich islands ssi earthquakes to college alaska and some other stations in alaska have increased systematically over the past 50 years the time shift in the differential time residuals is some 0 3 s over 30 years this temporal change is thought to result from a shift of the inner core structure from a superrotation of the inner core however the observation has been hotly debated and has been suggested to be an artifact of systematic earthquake mislocation here we examine this issue using three mutually independent approaches with a goal to quantify the amount of the systematic mislocation if any all three approaches involve differencing data between north stations at a similar azimuth as the ssi alaska azimuth and south stations at nearly the opposite azimuth mislocation along this azimuth has greatest impact on the pkp differential times in approach 1 we examine how the double difference of p traveltimes between a north station and a south station changes with time because the differential apparent slowness between the p traveltimes of the pairs is 8 to 16 times that of the differential bc df times mislocation would cause a time shift in the double difference an order of magnitude larger than that of bc df times in approach 2 we compare directly difference in p arrival times between two ssi events that are close by and between a north and a south stations which avoids using earthquake locations and earth models entirely in approach 3 we compare pkp traveltime residuals between a group of north and south stations because pkp traveltimes are generally not used for location in earthquake bulletins they provide an independent data set to check earthquake location errors the results from approaches 1 and 2 both of which use p data are generally consistent the mislocation is up to 3 6 4 2 km one standard deviation explaining less than 10 of the observed col temporal change the results from approach 3 using pkp data show a larger mislocation but with a larger error 14 4 10 8 km over 30 years it can explain about 30 22 of the observed col temporal change therefore inner core superrotation is still the best explanation for the observed temporal change in the differential traveltimes

global seismic body wave observations of temporal variations in the earth s inner core and implications for its differential rotation differential rotation of the earth s inner core has been predicted in some geodynamo models and seismic studies over the past 15 yr have resolved rotation rates up to 1 yr 1 most previous seismic body wave studies have focussed on south sandwich islands events recorded at station col in alaska here we present a globally extended study into temporal variations in the inner core over some 25 yr using pkpbc pkpdf traveltime residuals to test for differential rotation of the inner core displacement of inner core heterogeneities over time is sought we introduce a new method of space flattening to remove the effect of spatial variations on the time variations this allows for the use of both polar semi equatorial and equatorial geometries first we reanalyse polar paths from south sandwich islands events to stations col and ink in north america these stations yield a differential rotation of the inner core at a rate of 0 12 0 38 yr 1 in an eastward direction in agreement with previous studies however station dawy which has a very similar path through the inner core as col yields at best a westward differential rotation of the inner core thus dawy results are incompatible with the col ink inferred rotation secondly earthquakes in the aleutian islands region observed at bosa and lbtb in southern africa exhibit temporal variations that are incompatible with the south sandwich islands col ink inferred rotation rate thirdly kuril islands events recorded in south america at station bdf yield inconclusive results finally our final piece of evidence for the irreconcilability of differential inner core rotation with global data comes from using earthquakes in the vanuatu region recorded at bcao bgca in central africa an equatorial geometry these residuals resolve a westward inner core rotation at a rate of 0 14 yr 1 over the same time period that south sandwich islands events indicate an eastward rotation as any rigid body rotation should yield the same direction and rate independent of where the inner core is sampled our results allow us to reject previously reported inner core differential rotation rates of up to 0 1 0 5 yr 1 instead our results suggest that structure in either the inner or the outer core is varying with time over relatively short timescales and in ways that cannot be explained by and do not support a differentially rotating inner core

shear wave splitting in the appalachians and the urals a case for multilayered anisotropy observations of shear wave splitting in the northeastern u s appalachians and in the foredeep of the urals vary significantly with the back azimuth and incidence angle of the incoming phase these variations suggest two or more layers within the upper mantle with different anisotropic properties synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach relying on a subset of back azimuth and incidence angle may strongly bias the model inferred especially if the observations are averaged on the other hand the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the earth using a new error estimation technique for splitting we find that individual measurements from broadband data have errors of the order of 3 7 for the fast direction and 0 1 0 2 s for the delay of split shear waves the azimuthal variation of splitting parameters is broadly consistent throughout the appalachian terranes in the northeast united states especially for two long running stations in the northeast united states hrv harvard massachusetts and pal palisades new york observations can be separated into two distinct populations with mean fast axis azimuths of n60 e 4 and n119 e 2 delay values within each population range from near zero to 1 s azimuthal splitting variation for aru arti russia in the foredeep of uralian mountains is characterized by sharp transitions between different groups of observations using synthetic seismograms in simple structures we develop one dimensional anisotropic models under stations hrv and aru the model for hrv includes two layers of anisotropic material under an isotropic crust with fast axis azimuths n53 e and n115 e for the bottom and the top layers respectively the model for the upper mantle under aru includes a layer with a fast axis at n50 e atop a layer with fast axis azimuth n90 e our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under aru reported by levin and park 1997a our results suggest that both urals and appalachians possess a relict anisotropy in the tectosphere associated with past continental collision and accretion underlain by anisotropy with orientation similar to the local absolute plate motion suggesting an asthenospheric component to the signal

geodynamic estimates of the viscosity of the earth s inner core recent seismological studies1 2 have suggested that the inner core is rotating relative to the bulk of the earth a situation which according to numerical simulations3 may be sustained by convective flow in the liquid outer core on the other hand large gravitational forces due to the heterogeneous distribution of mass in the earth s mantle should be sufficient to keep the inner core aligned with the mantle4 here i show that the differential rotation of the inner core can be reconciled with these strong gravitational forces by allowing the shape of the inner core to adjust as it rotates so permitting an estimate of the effective viscosity of this innermost region of the earth the inferred rotation rate constrains the viscosity of the inner core to be less than 1016 pa s or greater than 1020 pa s as two different dynamical regimes are possible the viscosity estimates for these two regimes have very different implications for the origin of seismic anisotropy in the inner core

origin of temporal changes of inner core seismic waves abstract temporal changes of inner core ic seismic phases have been confirmed with high quality waveform doublets however the nature of the temporal changes is still controversial we investigated systematically the temporal changes of ic refracted pkikp and reflected pkikp waves with a large data set of waveform doublets we used non ic reference phase mainly skp which eliminated ambiguity where the temporal changes come from we found that the temporal changes have always started at refracted pkikp and the travel time changes correlate better with pkikp changes in reflected pkikp can be easily contaminated by the strong and time varying pkikp and coda wave trains and therefore are not reliable indicators for ic boundary changes combining with previous observations we conclude that the temporal changes come mostly if not all from the ic interior and ic surface changes as the sole source suggested previously can be ruled out the differential rotation of the ic shifting its heterogeneous uppermost structures is the simplest and most reasonable explanation for the origin of the time varying ic waves a rotation rate of about 0 05 0 1 per year with possible decadal fluctuation can reconcile all temporal change observations from body waves ic scattering and normal mode data

wide scale detection of earthquake waveform doublets and further evidence for inner core super rotation summary we report on more than 100 earthquake waveform doublets in five subduction zones including an earthquake nest in bucaramanga colombia each doublet is presumed to be a pair of earthquakes that repeat at essentially the same location these doublets are important for studying earthquake physics as well as temporal changes of the inner core particularly our observation from one south sandwich islands ssi doublet recorded at station ink in canada shows an inner core traveltime change of 0 1 s over 6 yr confirming the innercore differential motion occurring beneath central america observations from one aleutian islands doublet recorded at station bosa in south africa and from one kuril islands doublet recorded at station bdfb in brazil show an apparent inner core traveltime change of 0 1 s over 7 yr and 6 yr respectively providing evidence for the temporal change of inner core properties beneath central asia and canada respectively on the other hand observations from one tonga fiji solomon islands doublet recorded at station ptga in brazil and from one bucaramanga doublet recorded at station wrab in australia and station chto in thailand show no little temporal change no more than 0 005 syr 1 if any of inner core traveltimes for the three corresponding ray paths for which the path in the inner core is nearly parallel to the equatorial plane such a pattern of observations showing both presence and possible absence of inner core traveltime change can be explained by the geometry and relative directions of ray path lateral velocity gradient and inner core particle motion due to an eastward super rotation of a few tenths of a degree per year

systematic change of foraminiferal mg ca ratios across a strong salinity gradient abstract the mg ca ratio of foraminiferal calcite is an important proxy for estimating past ocean temperatures used in conjunction with 18 o of foraminiferal calcite it allows deconvolution of temperature and ice volume signals to infer past ocean temperatures and salinities assuming the relationship between seawater 18 o and salinity is known such work assumes that temperature is the only or at least the dominant environmental controller of foraminiferal mg ca the semi enclosed mediterranean sea where salinity varies from 36 to 40 psu over a seasonal temperature range of between only 5 c to 8 c provides a relevant setting to test this assumption outside the laboratory in this study planktonic foraminifera o universa g siphonifera g bulloides and g ruber white and pink were picked from 11 box core tops spanning the mediterranean salinity gradient and analysed for their trace element concentrations mg ca ratios are higher for the associated calcification temperatures than in other regions where calibrations have been conducted and correlate poorly with calcification temperature mg ca ratios are particularly high for samples from the eastern mediterranean where salinity is unusually high correlations of mg ca with the calcification salinity are statistically significant with mg ca changing by 15 59 per psu suggesting that salinity may act as a control on mg ca ratios in addition to the dominant temperature control we show that contamination by non carbonate material and diagenetic high mg carbonate overgrowths cannot account for the observed trend of increasing mg ca with salinity a relationship between mg ca and salinity is also suggested by re analysis of calibrations from open ocean settings these new mediterranean results are from a region with unusually high salinity but suggest that the effects of salinity on the mg ca palaeothermometer should be considered even in open ocean settings particularly where large salinity changes occurred in the past

using waveform similarity to constrain earthquake locations abstract we demonstrate that the numerical value of the cross correlation between p or s waveforms commonly computed to determine differential travel times for earthquake relocation itself contains useful information about the relative location of the events this information can be used to supplement traditional travel time inversions especially when few stations are available and to investigate the heterogeneity that underlies the multiple scattering process

using earthquake doublets to study inner core rotation and seismicity catalog precision 1 we report our search for doublets in the south sandwich islands ssi region among 906 earthquakes from january 1982 to december 1998 event pairs with source separation less than 40 km which was calculated using pde locations were checked using vertical component short period and broadband digital records cross correlation coefficients of p or pkp waveforms were computed for similarity comparison whole waveforms of those event pairs with correlation coefficients larger than 0 8 were checked by eye by this process 17 doublets were found some of which form multiplets however only one doublet event pair 14 august 1995 and 28 march 1987 has clear pkp df phases on both records at college alaska col the pkp df travel time change measured by cross correlation is a decrease by 0 15 s with standard deviation 0 02 s over a time period of about 8 years this result strongly supports differential rotation of the earth s inner core relative to the mantle and the crust the estimated rotation rate ranges from 0 4 to 1 per year eastward the special case of a locked inner core can then be ruled out we also checked the precision of pde and isc locations using the 17 doublets results show that both catalogs have relatively high location precision standard deviations in latitude longitude and depth are less than 4 km 6 km and 10 km respectively supporting prior claims of detection of inner core rotation song and richards 1996

seismological evidence for differential rotation of the earth s inner core the travel times of seismic waves that traverse the earth s inner core show a small but systematic variation over the past three decades this variation is best explained by a rotation of the inner core that moves the symmetry axis of its known seismic anisotropy the inferred rotation rate is on the order of 1 per year faster than the daily rotation of the mantle and crust

clustering and periodic recurrence of microearthquakes on the san andreas fault at parkfield california the san andreas fault at parkfield california apparently late in an interval between repeating magnitude 6 earthquakes is yielding to tectonic loading partly by seismic slip concentrated in a relatively sparse distribution of small clusters 20 meter radius of microearthquakes within these clusters which account for 63 of the earthquakes in a 1987 92 study interval virtually identical small earthquakes occurred with a regularity that can be described by the statistical model used previously in forecasting large characteristic earthquakes sympathetic occurrence of microearthquakes in nearby clusters was observed within a range of about 200 meters at communication speeds of 10 to 100 centimeters per second the rate of earthquake occurrence particularly at depth increased significantly during the study period but the fraction of earthquakes that were cluster members decreased

constraints on relative earthquake locations from cross correlation of waveforms abstract cross correlations of filtered waveforms can be used to place strong constraints on relative locations of local earthquakes under the assumption that events producing very similar waveforms occur within a distance of one quarter of the shortest wavelength for which the similarity is observed tests using digital seismograms of mine blasts with known locations recorded by the university of utah seismic network support this quarter wavelength hypothesis application of this method can yield information on relative locations of earthquakes within source volumes of 1 km 3 or smaller which is seldom possible using arrival time data alone

repeating deep earthquakes evidence for fault reactivation at great depth we have identified three groups of deep earthquakes showing nearly identical waveforms in the tonga slab relocation with a cross correlation method shows that each cluster is composed of 10 to 30 earthquakes along a plane 10 to 30 kilometers in length some of the earthquakes are colocated demonstrating repeated rupture of the same fault and one pair of events shows identical rupture complexity suggesting that the temporal and spatial rupture pattern was repeated recurrence intervals show an inverse time distribution indicating a strong temporal control over fault reactivation runaway thermal shear instabilities may explain temporally clustered earthquakes with similar waveforms located along slip zones weakened by shear heating earthquake doublets that occur within a few hours are consistent with events recurring before the thermal energy of the initial rupture can diffuse away

inner core rotation rate from small scale heterogeneity and time varying travel times the time it takes seismic waves to propagate from south atlantic earthquakes through the inner core to station col in alaska has decreased systematically over the past 30 years travel times from three earthquakes in 1991 to an array of 37 seismometers in alaska suggest that lateral gradients in seismic wavespeeds are steep in this part of the inner core this combination of observations can be explained by postulating that the inner core is rotating 0 2 to 0 3 per year faster than the mantle

high precision relative locations of earthquakes at mount st helens washington several earthquake sequences have occurred at mount st helens as clusters of shallow events with nearly identical waveforms multiplets we present a technique that allows the relative relocation of events within a multiplet to at least an order of magnitude higher precision than is typically possible with traditional techniques a multiplet is studied in pairs of events called doublets each doublet is made up of a reference event and one other event from the multiplet the cross spectrum of a short window beginning just before the p arrival is used to compute differences in arrival times between seismograms from the same station for each doublet these time differences computed with a precision of 1 2 ms are then used to locate each earthquake relative to the reference event engineering explosions with well known locations are used to test the relocation technique explosions can be relocated with accuracy better than 20 m for events within 250 m of the reference explosion the relocation technique is applied to earthquakes preceding the september 10 1984 and the may june 1985 dome building eruptions of mount st helens forty events that occurred during a 12 hour period in september 1984 are located in a volume approximately 30 m in diameter beneath the lava dome the earthquakes following the extrusion do not occur in multiplets and appear to be distributed over a much larger volume in the may june 1985 eruption we recognize three multiplet sets that took place successively in time the multiplets are interpreted as repeated failure or slip within a relatively small volume due to high strain rates around the magma supply conduit

joint inversion for inner core rotation inner core anisotropy and mantle heterogeneity observational evidence for differential rotation of the inner core has recently been reported from time dependent travel times of the seismic inner core phase pkp df these analyses are hindered by potential biases from the mantle heterogeneity and systematic earthquake location errors which have raised doubts about the reality of the inferred differential rotation here we report time dependent pkp df pkp bc differential travel times of a total of 611 measurements obtained from 92 earthquakes in the south sandwich islands ssi over the past nearly half a century recorded at the college alaska station col and alaska seismic network asn the events are relocated using the joint hypercenter determination jhd technique dewey 1971 travel time residuals at col from ssi earthquakes over a period of 45 years and at three additional stations from ssi earthquakes over periods of about 22 years show steady increases with time the temporal trends are demonstrated to be statistically very significant with dense samples of the pathway at different time periods we use a joint inversion technique that allows us to separate the time dependent inner core structure from the time independent mantle biases surprisingly the inner core rotation seems resolvable even from digital data observed at asn in the 1990s spanning over 8 years alone in such a joint inversion the results provide strong support for a differential inner core rotation the inferred rotation rate ranges from 0 3 to 1 1 per year faster than the mantle when we take into account the uncertainty in the tilt of the anisotropy axis of the patch of the inner core that was sampled a westward inner core rotation can be ruled out

field characterization and data integration to define the hydraulic heterogeneity of a shallow granular aquifer at a sub watershed scale providing a sound basis for aquifer management or remediation requires that hydrogeological investigations carried out to understand groundwater flow and contaminant transport be based on representative data that capture the heterogeneous spatial distribution of aquifer hydraulic properties this paper describes a general workflow allowing the characterization of the heterogeneity of the hydraulic properties of granular aquifers at an intermediate scale of a few km2 the workflow involves characterization and data integration steps that were applied on a 12 km2 study area encompassing a decommissioned landfill emitting a leachate plume and its main surface water receptors the sediments composing the aquifer were deposited in a littoral sublittoral environment and show evidence of small scale transitional heterogeneities cone penetrometer tests cpt combined with soil moisture and electrical resistivity smr measurements were thus used to identify and characterize spatial heterogeneities in hydraulic properties over the study area site specific statistical relationships were needed to infer hydrofacies units and to estimate hydraulic properties from high resolution cpt smr soundings distributed all over the study area a learning machine approach was used due to the complex statistical relationships between colocated hydraulic and cpt smr data covering the full range of aquifer materials application of this workflow allowed the identification of hydrofacies units and the estimation of horizontal hydraulic conductivity vertical hydraulic conductivity and porosity over the study area the paper describes and discusses data acquisition and integration methodologies that can be adapted to different field situations while making the aquifer characterization process more time efficient and less labor intensive

pkp bc df travel time residuals and short scale heterogeneity in the deep earth differential pkp bc pkp df travel time residuals for quasi polar paths in the earth present large excursions from the trends predicted by simple models of inner core anisotropy we analyze a set of such paths classified according to specific source and station regions and show that observations are compatible with a constant anisotropy model of at most 1 5 in the outer 400 km of the inner core to explain local departures of up to 3 sec from the predictions of such a model strong lateral variations on scale lengths shorter than a few hundred km need to be invoked either in the core or in the mantle independent results of d modelling argue in favor of an important contribution from the latter

two decades of temporal change of earth s inner core boundary we report two decades of changing behavior of the earth s inner core boundary icb which provides the simplest explanation for the observed temporal change of the compressional seismic waves that are reflected from the icb pkikp and refracted in the inner core pkikp from earthquake doublets occurring in south sandwich islands between 1993 and 2013 in the early period before 2003 the icb is enlarged beneath the western coast of gabon republic of congo and southwest tanzania in the reflected points of the pkikp observed at seismic stations obn aak and aru while it experiences little change beneath zimbabwe or and kenya and beneath west angola or and north central african republic in the pkikp entry or and exit points of aak and aru observations respectively in the later period after 1998 the icb regions beneath the western coast of gabon republic of congo and southwest tanzania either shrink or remain unchanged and the temporal change migrates to beneath zimbabwe or and kenya and beneath west angola or and north central african republic with a decrease of inner core surface by 5 59 km between 1998 and 2009 beneath zimbabwe or kenya and by 1 73 km beneath west angola or north central african republic between 1998 and 2013 these results indicate that icb temporal change occurs in localized regions and is episodic rapidly migrating and alternately enlarged and shrunk

possible heterogeneity of the earth s core deduced from pkikp travel times the core of the earth is usually described by spherically symmetrical velocity models the core is made of two main spherical layers the fluid outer core with a radius close to 3 480 km and a p velocity increasing with depth from 8 km s 1 to 10 3 km s 1 and the solid inner core with a radius of 1 220 km and a p velocity close to 11 km s 1 refs 1 2 station residuals of the seismic core phase pkikp have been computed for 400 seismological observatories worldwide using 5 yr of the international seismological centre isc bulletins pkikp travel times can be corrected for upper mantle propagation by subtracting p delays thus pkikp p residuals are a measurement of the average vertical travel times in the lower mantle and in the core of the earth beneath seismic stations a spherical harmonic development of pkikp p delays up to degree 4 explains 58 of the variance in the data pkikp p exhibit a latitudinal dependence polar stations tend to be faster than equatorial stations we show here that this pattern may reflect a departure from spherical symmetry in the p velocity distribution in the vicinity of the inner core boundary of the earth

a practical biodegradation scale for use in reservoir geochemical studies of biodegraded oils existing scales widely used to describe the extent of biodegradation of petroleum have insufficient resolution to usefully characterize many heavy oil and bitumen occurrences including the volumetrically dominant heavily and severely biodegraded oil accumulations in the foreland basins of western canada and venezuela in these and other deposits existing classifications or descriptions of the biodegradation level may vary only slightly yet oil may vary in viscosity by orders of magnitude the manco biodegradation scale proposed here is based on integrating the extent of degradation of various members of compound classes not included in previous biodegradation scales they include alkyl aromatic and alkyl thiophenic compounds that show variable extent of alteration in samples degraded to uniform levels on standard scales but which may show variation in local degradation systematics related to biodegradation mechanisms and extent of oil mixing the manco scale uses a combination of a consideration of the extent of alteration within a compound class together with a consideration of biodegradation across a range of compound classes it can be reliably used as a basis for interpreting geochemical changes in heavily biodegraded oil suites and can also be used to differentiate biodegraded oil samples likely to be amenable to cold production from those requiring production strategies such as steam or chemical flooding as with other biodegradation scales the scale may also provide evidence for the influx of later higher quality oil into a reservoir fluid that had been previously biodegraded

earth s core and the geodynamo earth s magnetic field is generated by fluid motion in the liquid iron core details of how this occurs are now emerging from numerical simulations that achieve a self sustaining magnetic field early results predict a dominant dipole field outside the core and some models even reproduce magnetic reversals the simulations also show how different patterns of flow can produce similar external fields efforts to distinguish between the various possibilities appeal to observations of the time dependent behavior of the field important constraints will come from geological records of the magnetic field in the past

precursors to a magnitude 4 8 earthquake in the virgin islands spatial clustering of small earthquakes anomalous focal mechanisms and earthquake doublets abstract waveforms of small earthquakes that preceded a magnitude 4 8 earthquake in the virgin islands were studied using digital recordings from a local seismic network during the 10 months prior to the main shock a spatial cluster of five events 1 4 m 2 5 occurred within 3 km of the impending shock variations in the p wave first motions at one station as well as p sv amplitude ratios indicate that the focal mechanisms of the events within the cluster changed 2 months before the main shock the focal mechanism of the later events and of two foreshocks that occurred the day of the main event differed from the typical focal mechanism observed for the area the main event and most of its aftershocks exhibited the normal set of first motions recorded for events in this region the cluster events and the foreshocks occurred in pairs each pair with very similar waveforms cross correlations of these waveforms reveal that the earthquakes in each pair are located within about 110 m of each other each pair can be interpreted as an earthquake doublet that is two similarly sized events with adjacent rupture zones that are related in time the occurrence of doublets is unusual for events in this region and may be indicative of high stress in the vicinity of the impending shock no significant differences were observed between the spectra of the cluster events the foreshocks or the aftershocks

inner core anisotropy in three dimensions the purpose of this paper is the investigation of cylindrical anisotropy in the inner core based on the travel time anomalies of the pkikp phase we use the arrival times reported in the international seismological centre bulletins for years 1964 1990 we select only earthquakes which have a good azimuthal coverage and a sufficiently large number of reporting stations the earthquakes are relocated using corrections for lateral heterogeneity computed for our most recent three dimensional mantle model we use a total of 313 422 observations of the df branch of pkp travel time anomalies within the epicentral distance ranges of 120 140 and 150 180 reported by 2335 stations for 26 377 earthquakes we process the data using an averaging procedure which we call cylindrical anisotropy stacking that enhances the effects of anisotropy but is expected to suppress those due to lateral heterogeneity and random errors the processed residuals show a remarkably consistent pattern this confirms the dominance of the cylindrical elastic anisotropy in the inner core with approximately a constant axis of symmetry this axis of symmetry is found to be tilted 10 5 1 from the earth s rotation axis in the direction 160 e 5 in the northern hemisphere in this new coordinate system we determine a four layer axisymmetric model of transverse anisotropy with each layer approximately 300 km thick the model shows that the anisotropy is strongest 3 within the innermost part of the core the travel time anomalies show significant 1 5 s longitudinal variations even when the tilt of the axis of symmetry is considered there is a substantial increase in the amplitude of the longitudinal variations which are dominated by the second and fourth harmonics for rays with bottoming depths exceeding 400 km below the inner core boundary the measurable tilt of the axis of symmetry and the presence of significant nonaxisymmetric signal may provide important clues with respect to the possible causes of anisotropy the increase in the anisotropy in the innermost part of the core departs from earlier inferences that anisotropy may be limited to the outermost 200 300 km of the earth s inner core

comment on the existence of an inner core super rotation questioned by teleseismic doublets by georges poupinet annie souriau and olivier coutant abstract poupinet et al j geophys res 89 1984 5719 presented a new methodology for testing whether the pkp travel times observed at college alaska station from south sandwich islands earthquakes indicate inner core rotation they applied the method to seven events and concluded that there is no evidence for pkp df times increasing systematically with event dates and therefore no reason to postulate that the inner core rotates at a different rate than the mantle in a reanalysis of the same seven events i find that their results are systematically biased by the choice of radial earth models and by incorrectly assuming that one of the phases had reversed polarity i also question their choice of events but upon correcting for these biases conclude that these seven events are consistent with the previous model of inner core rotation

the effect of d on pkp ab df travel time residuals and possible implications for inner core structure abstract the hypothesis that the deep inner core is anisotropic is based on pkp travel time observations at large distances and relies on a small number of very anomalous measurements for paths quasi parallel to the earth s rotation axis here we analyze a global dataset of pkp ab df travel times residuals and discuss their significant dispersion 2 s and coherent large scale patterns we show that the trends observed for quasi equatorial paths are consistent with predictions from recent tomographic mantle models when the latter are modified to account for strong heterogeneity at the base of the mantle under the pacific ocean and africa as documented in several recent studies likewise for polar paths we show that a large part of the signal could be explained by deep mantle structure the effects of complex structure in the deep mantle on pkp ab df travel times should be carefully considered in order to reliably estimate the anisotropic structure of the central part of the inner core

inner core rotation a test at the worldwide scale abstract in order to determine the angular velocity of a possible inner core differential rotation with respect to the mantle we analyse travel times of waves sampling the inner core at the worldwide scale over a period of about 30 years for situations in which the inner core heterogeneities are dominant with respect to the mantle heterogeneities travel times of summary rays i e bunches of seismic rays with the same geometry must have smaller mean standard deviations when computed in the inner core reference frame than when computed in the mantle reference frame thus we build summary rays by applying a backward rotation of the inner core at angular velocity in order to sum residuals which sample the same inner core region at different times the mean standard deviation of the summary ray residuals computed for different values of should exhibit a minimum for the effective value this method has been applied to absolute pkp df residuals in considering only polar paths for which inner core heterogeneities are dominant with respect to mantle heterogeneities the data used come from a high quality file derived by engdahl et al engdahl e r van der hilst r buland r 1998 global teleseismic earthquake relocation with improved travel times and procedures for depth determination bull seismol soc am 88 722 743 from the catalogs of the international seismological centre isc for the period 1964 1995 the minimum standard deviation is obtained for the null rotation rate a similar analysis on the absolute pkp bc residuals which correspond to paths that do not sample the inner core shows that mantle heterogeneities are not at the origin of the observed minimum the same method has also been applied to a set of more than 1200 worldwide distributed pkp bc pkp df differential travel time residuals hand picked by ourselves or gathered by different authors for specific core studies here again the minimum standard deviation is obtained for the null rotation implying a synchronous rotation of the inner core with the mantle however the limited number of very old data and the geographic distribution of the paths prevent us from obtaining a resolution better than 1 year in particular a small super rotation with 1 year cannot be ruled out with the presently available data the problem of the level of the inner core heterogeneities compared to that of the mantle which is another possible limitation of the method is also discussed

inner core differential motion confirmed by earthquake waveform doublets we analyzed 18 high quality waveform doublets with time separations of up to 35 years in the south sandwich islands region for which the seismic signals have traversed the inner core as pkp df the doublets show a consistent temporal change of travel times at up to 58 stations in and near alaska and they show a dissimilarity of pkp df coda using waveform doublets avoids artifacts of earthquake mislocations and contamination from small scale heterogeneities our results confirm that earth s inner core is rotating faster than the mantle and crust at about 0 3 degrees to 0 5 degrees per year

topography of earth s inner core boundary from high quality waveform doublets summary precise determination of the topography of a major internal boundary of the earth is difficult because of the trade off with the unknown velocity structure above it however the discoveries of the inner core ic rotation and high quality teleseismic waveform doublets make the precise mapping of the topography of the inner core boundary icb possible here we examine ic refracted pkp df and reflected pkp cd waves recorded at the yellowknife array and other global stations from 14 high quality doublets among a large collection of doublets in s sandwich islands that are recently discovered our results show clear evidence for spatial and temporal variations of ic reflections in traveltimes and in waveforms if the time separation dt between the two members of the doublet is less than 3 yr the ic arrivals show little temporal change in traveltimes or waveforms if dt is greater than about 6 yr some doublets show large variations but some others do not the icb regions beneath atlantic and indian oceans show little temporal change the regions that show large variations are beneath africa and the central america which coincide with large seismic anomalies at the core mantle boundary cmb inside these two icb regions there are fine scale km scale variations the largest temporal changes of ic reflections are about 0 10 to 0 14 s corresponding to a topographic variation of up to about 3 7 to 5 2 km the results suggest icb topography of a few kilometres on fine to regional scales the geographical coincidence of the icb and cmb anomalies may suggest strong thermal coupling of the mantle and the core dynamic models include a bumpy icb rotating with the ic itself or a transient slurry boundary containing a mixture of molten materials and solidified patches of iron crystals which is rapidly modified by the turbulence at the base of the convecting outer core if the former is the main cause of the observed temporal change of the topography the ic rotation is required to be an oscillation whose half period has an upper limit of a few hundred years

fine scale heterogeneity in the earth s inner core the seismological properties of the earth s inner core have become of particular interest as we understand more about its composition and thermal state1 2 observations of anisotropy and velocity heterogeneity in the inner core are beginning to reveal how it has grown and whether it convects3 4 the attenuation of seismic waves in the inner core is strong and studies of seismic body waves5 6 have found that this high attenuation is consistent with either scattering or intrinsic attenuation5 the outermost portion of the inner core has been inferred to possess layering and to be less anisotropic than at greater depths7 8 9 10 here we present observations of seismic waves scattered in the inner core which follow the expected arrival time of the body wave reflection from the inner core boundary the amplitude of these scattered waves can be explained by stiffness variations of 1 2 with a scale length of 2 kilometres across the outermost 300 km of the inner core these variations might be caused by variations in composition by pods of partial melt in a mostly solid matrix or by variations in the orientation or strength of seismic anisotropy

the existence of an inner core super rotation questioned by teleseismic doublets abstract the first evidence for a differential rotation of the earth s inner core with respect to the mantle comes from the detection of a variation over 30 years of the differential travel time anomalies of seismic core phases song x richards p g 1996 seismological evidence for differential rotation of the earth s inner core nature 382 221 224 the comparison of teleseismic doublets provides a powerful means to measure such differential travel times and to test their temporal variation contrary to direct measurements of differential travel times this technique does not require to have an accurate location of the seismic events this method is first checked on examples of doublets for which no differential travel time is expected these tests allow us to point out the difficulties such as polarity reversals which may be encountered in the use of the doublet method it is then applied to deep tonga events recorded in france and to the south sandwich islands events recorded at station college col alaska which led song and richards to propose an inner core rotation for the first path we find no time variation in agreement with previous studies for the path from south sandwich island to col the doublet analysis shows that no significant temporal change in pkp travel time can be detected within the resolution of the method which is about 0 05 s the variation of the residuals observed by song and richards about 0 3 s over 30 years is in a large extent ascribable to hypocenter mislocations consequently present seismological observations do not detect the inner core differential rotation because if it exists it is smaller than about 0 2 year the present detection capacity of seismology

structure of the inner core inferred from observations of its spheroidal shear modes spheroidal modes of elastic gravitational oscillation that have an energy partition dominated by shear energy in the inner core are called core modes recordings from the ida gravimeter network of an earthquake in the tonga region on june 22 1977 have provided the first unambiguous observations of core modes they conclusively show that at low frequencies the inner core has a very high q about 3500 they also tightly constrain the ratio of the inner core radius to the mean shear velocity to be 345s with an uncertainty of 0 5

new seismological constraints on differential rotation of the inner core from novaya zemlya events recorded at drv antarctica summary novaya zemlya nuclear test records at the seismic station drv antarctica are analysed in order to obtain further constraints on a possible diverential rotation of the inner core with respect to the mantle these data allow the sampling of the inner core along a nearly polar path in very stable conditions over more than two decades from 1966 to 1990 the pkp bc pkp df traveltime residuals which reflect the innercore anisotropy and or heterogeneities sampled along the path exhibit a great stability through time a computation of the residuals that are expected for various diverential rotation rates and the same rotation axis as the mantle has been performed using the worldwide residual catalogue of engdahl et al 1997 for summary rays that include the time as an additional parameter in data stacking comparison of data and predictions shows that an eastward diverential rotation with a rate as large as 3 yr 1 as suggested by some authors is not possible but an eastward rotation at 1 yr 1 or lower cannot be rejected

the shuffling rotation of the earth s inner core revealed by earthquake doublets earth s inner core rotates at a different rate than the mantle and discrepancies exist between rotation rates derived from geophysical observations and geodynamical simulations an inverse analysis of seismic data from repeating earthquakes over the past 50 years suggests that the rotation rate of the inner core fluctuates on decadal timescales