further rock magnetic and chronostratigraphic results on reversal excursions during the last 50 ka as derived from northern high latitudes and discrepancies in precise ams 14c dating summary a total of five sediment cores from three sites the arctic ocean the fram strait and the greenland sea yielded evidence for geomagnetic reversal excursions and associated strong lows in relative palaeointensity during oxygen isotope stages 2 and 3 a general similarity of the obtained relative palaeointensity curves to reference data can be observed however in the very detail results from this high resolution study differ from published records in a way that the prominent laschamp excursion is clearly characterized by a significant field recovery when reaching the steepest negative inclinations whereas only the n r and r n transitions are associated with the lowest values two subsequent excursions also reach nearly reversed inclinations but without any field recovery at that state a total of 41 accelerator mass spectrometry ams 14 c ages appeared to allow a better age determination of these three directional excursions and related relative palaeointensity variations however although the three sites yielded more or less consistent chronological as well as palaeomagnetic results a comparison to another site ps2644 in the iceland sea revealed significant divergences in the ages of the geomagnetic field excursions of up to 4 ka even on basis of uncalibrated ams 14 c ages this shift to older 14 c ages cannot be explained by a time transgressive character of the excursions because the distance between the sites is small when compared with the size of and the distance to the geodynamo in the earth s outer core the most likely explanation is a difference of reservoir ages and or mixing with old 14 c depleted co2 from glacier ice expelled

comparison of site classification from vs30 vs10 and hvsr in italy the aim of this work is to verify if 1 the average of shear wave velocity from the surface to 30 m depth vs30 is a good proxy of site amplification in a country with complex geology like italy 2 the grouping of vs30 in different soil classes and relevant spectra in the italian seismic codes is adequate and 3 shallow shear wave profiles vs10 could be a more economical tool for site classification to answer the first two questions we examined 40 estimates of vs30 derived from microzonation projects we performed in italy 27 obtained with downhole measurements plus 13 ve locity profiles obtained with surface techniques in all the sites we installed seismic instrumentation to record earthquakes and to estimate site response using horizontal to vertical spectral ratios hvsr the comparison between hvsr and vs30 showed in about one third of the sites that vs30 is not a good proxy of observed amplification effects if the site does not have a monotonically increasing velocity profile the reason vs30 does not provide satisfactory estimates in italy is linked to peculiar geological settings that are widespread in the country we then compared the observed ampli fications from earthquake hvsr data with the ones provided by the italian seismic code noting a substantial underestimation by the code a somewhat unsettling situa tion because hvsr is usually considered to be a lower bound for amplification es timated with other techniques finally we studied 45 vs downhole profiles to 30 m depth performed also at sites where earthquake recordings are not available on this data set we noticed that vs10 could predict site classification with the same performances of vs30 we consider al ternative soil classification schemes that include soil frequency besides the velocity profile in this two parameter approach vs10 could be substituted for vs30

nonlinearity liquefaction and velocity variation of soft soil layers in port island kobe during the hyogo ken nanbu earthquake clear nonlinear behavior is analyzed from the acceleration records of the 1995 hyogo ken nanbu earthquake at port island kobe from four triaxial in struments placed at four different depths the surficial effects during strong ground motions were compared with those during weak motions before and after the main shock we used a spectral ratio technique and a nonlinear inversion for velocity structure to analyze the data from the spectral analysis we observed a large variation of the spectral ratios between the surface and different depths during the strong ground motions and during the liquefied state the spectral ratios after the mainshock i e after the liquefied state are different from those before the mainshock the peak frequencies in the spectral ratios after the mainshock are shifted to lower fre quencies with respect to those in the spectral ratios before the mainshock we in verted the s wave velocities using a genetic algorithm technique to determine the velocity structure before during and after the mainshock the s wave velocity struc ture before and after the mainshock was found to be different specifically the s wave velocity of the second layer 5 m to 16 m depth after the mainshock was 20 lower than before our analysis shows that the liquefied state remains at least 3 hr after the mainshock but no more than 24 hr the rigidity of the soil decreased close to zero when liquefaction happened and later increases gradually following a trend that resembles a consolidation curve the strong influence of nonlinearity during the mainshock yielded a big reduction of the horizontal surface ground motions so that the observed horizontal peak acceleration was only about 25 of the peak acceler ation expected from the linear theory however the nonlinear effects in the vertical peak acceleration were not significant

sedimentary records of relative paleointensity of the geomagnetic field theory and practice sediments have proved irresistible targets for attempts at determining the relative variations in the earth s magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution the assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short no dependence on magnetic field was expected nonetheless a number of experiments showed that sedimentary magnetizations varied in accordance with the field albeit not always in a simple linear fashion experiments in which the sediments were stirred in the presence of a field to simulate bioturbation showed a reasonably linear relationship with the applied field and these results spurred the hope that variations in the earth s magnetic field might indeed be recoverable from appropriate sedimentary sequences examination of existing paleointensity data sets allows a few general conclusions to be drawn it appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the earth s magnetic field the dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences moreover when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences there is considerable agreement among the various records when viewed over timescales of hundreds to thousands of years relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another suggesting that they are dominated by nondipole field behavior when viewed over timescales of a few tens of thousands to hundreds of thousands of years however the records show coherence over large distances at least thousands of kilometers and may reflect changes in the dipole field on the basis of a sequence spanning the brunhes and matuyama chrons the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years but these oscillations are not clear in the record prior to about 300 ka thus they are probably not an inherent feature in the geomagnetic field and the correspondence of the period of oscillation to that of obliquity is probably coincidence

magnetostratigraphic data from late quaternary sediments from the yermak plateau arctic ocean evidence for four geomagnetic polarity events within the last 170 ka of the brunhes chron summary palaeomagnetic investigations of two sediment cores recovered from rv polarstern near the eastern slope of the yermak plateau sites ps 1533 and ps 2212 reveal convincing evidence for four polarity events of the earth s magnetic field during the last 170 ka a comprehensive rock magnetic study of the sediments proved that fine grained magnetite is the principal carrier of the remanent magnetization no changes in magneto mineralogy across the polarity transitions in the sediments investigated were found calcareous nannofossil biostratigraphy ams 14c accelerated mass spectrometry and oxygen isotope data and 10be and 230th stratigraphies yielded age ranges of 24 29 ka for the mono lake event 34 43 ka for the laschamp event 72 86 ka for the norwegian greenland sea event and 118 128 ka for the blake event two reverse polarity samples at the base of core ps 2212 3 kal are interpreted as the termination of the biwa i event 171 181 ka the events exhibit full inversion of inclination in both cores the data suggest that the transition process of the earth s magnetic field during such polarity events requires some 1 ka

high resolution magnetostratigraphy of four sediment cores from the greenland sea ii rock magnetic and relative palaeointensity data summary high resolution magnetostratigraphic analysis of three sediment cores from the base of the volcanic seamount vesteris banken in the greenland basin and one from the jan mayen fracture zone revealed records of three pronounced geomagnetic events the mono lake excursion 28 27 ka the laschamp event 37 33 ka and the biwa i jamaica event 189 179 ka rock magnetic investigations measurements of magnetic susceptibility and arm and irm acquisition demagnetization experiments show that only fine grained titano magnetite 0 5 m is the dominant magnetic carrier mineral in the hemipelagic muds there is no evidence for high amounts of high coercivity minerals like haematite or goethite volcanic basaltic ash layers in sediments around vesteris banken seamount are characterized by extremely high values of magnetic susceptibility reaching 16 000 times 10 6 si as well as by low median destructive fields of the anhysteretic remanent magnetization mdfarm indicating magnetite of larger grain sizes 1 m besides this no significant variation in rock magnetic parameters across intervals of intermediate to reversed chrm inclinations are observed therefore these are interpreted as true records of geomagnetic field variations the recovered sediments characterized by only moderate fluctuations in concentration as well as grain size related rock magnetic parameters provided good conditions for the reconstruction of the relative palaeointensity of the geomagnetic field the recorded geomagnetic events are clearly linked to a drastic reduction in the relative palaeointensity of the field the lowermost values were obtained for the time of the polarity transition with a trend to higher relative intensities in the course of the polarity events event ages and the relative palaeointensity variations based on different methods derived from the greenland sea sediments investigated are in good agreement with data published in the literature

geomagnetic paleointensity for the last 100 kyr from the sub antarctic south atlantic a tool for inter hemispheric correlation abstract we report relative paleointensity proxy records from four piston cores collected near the agulhas ridge and meteor rise south atlantic the mean sedimentation rate of the cores varies from 24 cm kyr to 11 cm kyr the two cores with mean sedimentation rates over 20 cm kyr record positive remanence inclinations at 40 41 ka coeval with the laschamp event age models are based on oxygen isotope data from three of the cores augmented by radiocarbon ages from nearby core rc11 83 and by correlation of paleointensity records for the one core with no oxygen isotope data the relative paleointensity proxy records are the first from the south atlantic and from the high to mid latitude southern hemisphere prominent paleointensity lows at 40 ka and 65 ka as well as many other features can be correlated to paleointensity records of comparable resolution from the northern hemisphere the records are attributable in large part to the global scale field and therefore have potential for inter hemispheric correlation at a resolution difficult to achieve with isotope data alone

site response shallow shear wave velocity and wave propagation at the san jose california dense seismic array ground motion records from a 52 element dense seismic array near san jose california are analyzed to obtain site response shallow shear wave velocity and plane wave propagation characteristics the array located on the eastern side of the santa clara valley south of the san francisco bay is sited over the evergreen basin a 7 km deep depression with miocene and younger deposits site response values below 4 hz are up to a factor of 2 greater when larger regional records are included in the analysis due to strong surface wave development within the santa clara valley the pattern of site amplification is the same however with local or regional events site amplification increases away from the eastern edge of the santa clara valley reaching a maximum over the western edge of the evergreen basin where the pre cenozoic basement shallows rapidly amplification then decreases further to the west this pattern may be caused by lower shallow shear wave velocities and thicker quaternary deposits further from the edge of the santa clara valley and generation trapping of surface waves above the shallowing basement of the western evergreen basin shear wave velocities from the inversion of site response spectra based on smaller local earthquakes compare well with those obtained independently from our seismic reflection refraction measurements velocities from the inversion of site spectra that include larger regional records do not compare well with these measurements a mix of local and regional events however is appropriate for determination of site response to be used in seismic hazard evaluation since large damaging events would excite both body and surface waves with a wide range in ray parameters frequency wavenumber plane wave analysis is used to determine the backazimuth and apparent velocity of coherent phases at the array conventional high resolution and multiple signal characterization f k power spectra and stacked slowness power spectra are compared these spectra show surface waves generated scattered at the edges of the santa clara valley and possibly within the valley at the western edge of the evergreen basin

evidence for a geomagnetic excursion recorded in the sediments of lac st front france a link with the laschamp excursion anomalous paleomagnetic directions accompanied by low intensities of natural remanent magnetization are recorded at 17 m depth in the sediments cored from lac st front massif central france the rock magnetic properties and the parameters characterizing anisotropy of susceptibility of samples taken from the sediment horizons carrying these anomalous directions spanning some 30 40 cm are identical to those of samples taken from the other stratigraphic intervals carrying a record of normal geomagnetic secular variation therefore the sequence of anomalous directions is interpreted as a geomagnetic excursion furthermore it is concluded that this excursion occurred at a time when the paleointensity field was low the excursion is dated at about 33 14c 37 ka b p by correlating the susceptibility and paleomagnetic logs from lac st front with the corresponding logs from the nearby lac du bouchet sedimentary sequence the excursional directions in lac st front can be averaged out with a triangular window of 20 cm these smoothed excursional directions resemble the directional variations observed in the corresponding lac du bouchet interval this finding indicates that the absence of this excursion in lac du bouchet may be due to a larger smoothing of the geomagnetic signal

topographic slope as a proxy for seismic site conditions and amplification we describe a technique to derive first order site condition maps directly from topographic data for calibration we use global 30 arc sec topographic data and v s 30 measurements here v s 30 refers to the average shear velocity down to 30 m aggregated from several studies in the u s as well as in taiwan italy and australia v s 30 values are correlated against topographic slope to develop two sets of parameters for deriving v s 30 one for active tectonic regions where topographic relief is high and one for stable shields where topography is more subdued by taking the gradient of the topography and choosing ranges of slope that maximize the correlation with shallow shear velocity observations we can recover to first order many of the spatially varying features of sitecondition maps developed for california our site condition map for the low relief mississippi embayment also predicts the bulk of the v s 30 observations in that region despite rather low slope ranges we find that maps derived from the slope of the topography is often well correlated with other independently derived regional scale site condition maps but the latter maps vary in quality and continuity and subsequently also in their ability to match observed v s 30 measurements contained therein alternatively the slope based method provides a simple approach to uniform site condition mapping

urban geology of the toowoomba conurbation se queensland australia abstract lying 125 km west of brisbane on the escarpment of the great dividing range the conurbation of toowoomba is mainly floored by basalts and associated rocks of the late tertiary main range volcanics mrv land surfaces including lateritic profiles developed before during and after the eruption of the mrv subsequent latest tertiary and quaternary denudation has resulted in westward migration of the great divide with the development of the east facing escarpment the topography and soils developed on these rocks provide a range of surficial conditions which must be considered in urban planning and development located on the great divide the conurbation depends chiefly on pumping from surface reservoirs at lower levels for its water supply groundwater from aquifers in the mrv represents both a source of supply and a strategic resource the mrv provides blue metal and road base

chlorine 36 evidence for the mono lake event in the summit grip ice core abstract a distinct peak has been discovered in the 36 cl data from the grip ice core between the dansgaard oeschger d o events 6 and 7 at approximately 32 kyr bp this peak can be attributed to a minimum of the geomagnetic dipole field associated with the mono lake event since the 36 cl peak reflects a higher production rate of all cosmogenic radionuclides it has an impact on the 14 c dating of the last ice age furthermore it provides an additional time marker similar to a peak found earlier corresponding to the laschamp event at approximately 39 kyr bp

magnetostratigraphic results from impact crater lake el gygytgyn northeastern siberia a 300 kyr long high resolution terrestrial palaeoclimatic record from the arctic summary a 12 7 m long sedimentary record recovered from lake el gygytgyn located in a meteorite impact crater created 3 6 ma in late cretaceous igneous rocks on chukotka peninsula northeast siberia has been analysed for its palaeo and rock magnetic properties continuous high resolution 1 mm measurements of magnetic susceptibility yielded successions of pronounced lows and highs analyses of the rock magnetic properties by low and high temperature runs of magnetic susceptibility determination of hysteresis parameters as well as irm acquisition experiments yielded a dominance of psd pseudo single domain magnetite in intervals of high magnetic susceptibility whereas due to selective magnetite dissolution associated with anoxic lake water and or pore water conditions during times of enhanced deposition of organic matter haematite dominates within low susceptibility intervals in terms of mass percentage here however magnetic properties are still dominated by magnetite five ams accelerator mass spectromentry 14 c ages eight irsl infrared stimulated luminescense ages together with preliminary pollen data suggest that variations in magnetite content reflect climatic variability of the last 300 ka with low high susceptibilities representing cold warm climates this pattern caused by a complex system of deposition preservation or decomposition of organic matter and or magnetic minerals in turn can be correlated in detail to global climate archives such as the oxygen isotope records from greenland ice cores and marine sediments respectively thus the sedimentary sequence recovered from lake el gygytgyn represents the longest continuous terrestrial climate record now available from the arctic

enhanced cosmic ray production of 10be coincident with the mono lake and laschamp geomagnetic excursions the cosmogenic isotope 10 be total be and al were measured in partly varved sediments from the upper 50 m of core 480 leg 64 dsdp gulf of california the concentration of 10 be from 1 to 50 kyr is in general agreement with estimates of the geomagnetic dipole moment obtained from archaeomagnetic and marine core research 10 be anomalies were also found at 32 kyr and 43 kyr contemporaneous with the mono lake and laschamp excursions respectively the production of 10 be required to explain these anomalies is too high particularly for the mono lake excursion to be produced by a combination of decreased geomagnetic field and unprecedented long term solar activity we conclude that the cause is a change in the galactic cosmic ray flux consistent with a supernova event the coincidence with the two excursions remains a paradox

site classification for earthquake hazard and risk assessment in australia while site response phenomena are well recognized globally the influence of regolith properties on ground motion has not previously been investigated for much of australia in an effort to characterize the regolith in terms of its ability to modify earthquake ground shaking this validation study assesses the national site classification map of australia which has been developed for application in first order earthquake hazard and risk assessment in the absence of regolith thickness and fundamental period data site classes are assigned based on a method developed in california that uses the relationship between geological material and the shear wave velocity of the upper 30 m v s 30 the classification scheme is then adjusted to suit the australian geological environment including a correction for the presence of weathered in situ regolith commonly encountered in this stable tectonic setting the results are validated using geophysical and geotechnical data from a variety of quaternary sedimentary environments in the newcastle sydney and perth urban areas and from bedrock dominated environments at a range of sites across australia the results of these analyses demonstrate the utility but also highlight the limitations of applying a depth limited shear wave velocity method in site classification for seismic hazard assessment

paleomagnetic constraints on sedimentation ratesin the eastern arctic ocean abstract deep sea sediments recovered from six sites visited during the international arctic ocean expedition of 1991 were examined to determine sedimentation rates in the eastern arctic ocean basin the dearth of age diagnostic biogenic material in these sediments precludes the application of biostratigraphic methods but ages can be deduced using paleomagnetism in conjunction with measurements of radiocarbon and carbonate concentration although no one of these techniques gives an unambiguous determination of age the interpretation most consistent with these diverse data implies that sedimentation rates in the eastern arctic are in general a few centimeters per thousand years such estimates of sedimentation rate are an order of magnitude greater than those previously determined from many sediment cores taken from the canada basin however one site examined on the morris jesup rise shows a relatively low rate of sediment accumulation less than 0 6 cm 103yr suggesting that although higher than in the canada basin sedimentation rates in the eastern arctic can be highly variable

the shaping of sydney by its urban geology abstract sydney s surface geology of hawkesbury sandstone capped with wianamatta shale has produced poor soils but good building material the topography derived from the subaerial erosion and extensive dissection of the permian triassic sedimentary strata of the sydney basin has been the strongest constraint on the city s growth steep sided valleys carved out of sandstone plateaux forced the direction of settlement along the flatter interfluves and in recent decades out onto the tectonic depression of the cumberland plain to the west holocene marine transgression flooded the eastern incisions of the eroded mass producing some of the few safe anchorages on the southeast australian coast as well as bedrock controlled zeta curved coastal beaches some of the geological disadvantages have turned into advantages over time the sterility of the skeletal sandstone soils made farming difficult but has allowed much of the native vegetation to be retained for conservation and recreation coastal quartz sand deposits produced equally infertile soils but provided pure sand fill for sydney s many attractive beaches along with industrial uses the depth of underlying coal deposits discouraged early mining within the city boundaries to the ultimate benefit of residential development while tectonic stability reduced complications for infrastructure engineering works

chronology and correlation of quaternary magnetostratigraphy and nannofossil biostratigraphy in norwegian greenland sea sediments paleomagnetic analyses of a series of sediment cores from the norwegian greenland sea document the existence of seven short lived geomagnetic events during the last about 500 000 years calcareous nannofossil biostratigraphy correlated to oxygen isotope chronostratigraphy provides a time framework to infer age and duration of these periods of extreme divergence of the earth s magnetic paleofield from the regular normal polarity configuration of the brunhes chron the geomagnetic events typically extend over 5 to 10 kyr two events are observed within the oxygen isotope stage 2 4 interval a short event is recorded at the stage 4 5 boundary and another just below the stage 5 6 boundary additional three geomagnetic events are identified in oxygen isotope stage 6 at the stage 9 10 boundary and near the top of oxygen isotope stage 13 respectively

magnetostratigraphy in three arctic ocean sediment cores arguments for geomagnetic excursions within oxygen isotope stage 2 3 two zones with shallow to steep negative palaeomagnetic directions have been detected within similar lithological units in three high latitude gravity cores from the arctic ocean coinciding azimuthal distributions of declinations relative to arbitrary oriented sub sampling surfaces are attributed to the systematic acquisition of magnetic components by frictional deformation of the unconsolidated sediment during the sub sampling procedure alternating field demagnetization to 50 mt was only partly successful in obtaining stable single component directions magnetic polarity zones are deduced by extrapolation of directional trends of inclinations during progressive af demagnetization high delta 18o values in sinistrally coiled n pachyderma range between 4 30 4 68 pdb mean values reflecting sediment accumulation during continental glaciation amino acid ratios of n pachyderma suggest ages less than approximately 60 000 years ago when seen in conjunction these observations suggest that the sediments were deposited within oxygen isotope stages 2 3 the reversed palaeomagnetic polarity zones are concluded to represent two short duration excursions of the geomagnetic field which occurred less than 60 000 years ago likely known candidates are the lake mungo 28 000 30 000 years ago and laschamp olby 35 000 40 000 years ago excursions all confined within oxygen isotope stage 3 24 000 59 000 years ago

the influence of vertical wind shear on moist thermals abstractalthough it is well established that vertical wind shear helps to organize and maintain convective systems there is a longstanding colloquial notion that it inhibits the development of dee

developing a map of geologically defined site condition categories for california consideration of site conditions is a vital step in analyzing and predicting earthquake ground motion the importance of amplification by soil conditions has long been recognized but though many seismic instrument sites have been characterized by their geologic conditions there has been no consistent simple classification applied to all sites as classification of sites by shear wave velocity has become more common the need to go back and provide a simple uniform classification for all stations has become apparent within the pacific earthquake engineering research center s next generation attenuation equation project developers of attenuation equations recognized the need to consider site conditions and asked that the california geological survey provide site conditions information for all stations that have recorded earthquake ground motion in california to provide these estimates we sorted the available shear wave velocity data by geologic unit generalized the geologic units and prepared a map so that we could use the extent of the map units to transfer the velocity characteristics from the sites where they were measured to sites on the same or similar materials this new map is different from the california geological survey preliminary site conditions map of california in that 19 geologically defined categories are used rather than national earthquake hazards reduction program categories although this map does not yet cover all of california when completed it may provide a basis for more precise consideration of site conditions in ground motion calculations

empirical attenuation of ground motion spectral amplitudes in southwestern western australia a dataset comprising some 389 strong and weak motion records for 69 events from the burakin 2001 2002 earthquake swarm and additional recent events is compiled to develop a regional ground motion model for the yilgarn craton southwestern western australia events range in size from moment magnitude 2 2 m 4 6 the decay of horizontal component spectral amplitudes can be approximated by a geometrical attenuation coefficient of r 1 0 within 80 km of the source the associated regional seismic quality factor can be expressed as q f 457 f 0 37 for frequencies 1 07 f 25 0 hz average corner frequencies for events with magnitude m 3 0 do not vary significantly with seismic moment m 0 indicating a steep distribution of m 0 versus corner frequency this causes anomalously low estimates of stress drop for smaller magnitude events m fourier spectral amplitudes corrected for geometric and anelastic attenuation were regressed with m to obtain quadratic attenuation coefficients modeled horizontal component displacement spectra fit the observed data well amplitude residuals predicted observed amplitudes are on average relatively small and do not vary significantly with hypocentral distance source spectra i e at r 1 km predicted from the regression parameters give self consistent amplitudes at low frequency f less than approximately 2 hz equivalent to predictive models from eastern north america ena for the same moment magnitude however our model predicts lower spectral amplitudes with increasing frequency as a consequence of the low stress drop events this is particularly apparent for the smaller magnitudes western australian source spectra begin to converge with ena models at increasing magnitudes if hypocentral distance is increased i e r 1 km the models begin to diverge at low frequencies owing to differences in geometrical attenuation coefficients the bulk of these data were recorded from an earthquake swarm with very shallow focal depths h

urban seismology northridge aftershocks recorded by multi scale arrays of portable digital seismographs we deployed portable digital seismographs in the san fernando valley sfv the los angeles basin lab and surrounding hills to record aftershocks of the 17 january 1994 northridge california earthquake the purpose of the deployment was to investigate factors relevant to seismic zonation in urban areas such as site amplification sedimentary basin effects and the variability of ground motion over short baselines we placed seismographs at 47 sites not all concurrently and recorded about 290 earthquakes with magnitudes up to 5 1 at five stations or more we deployed widely spaced stations for profiles across the san fernando valley as well as five dense arrays apertures of 200 to 500 m in areas of high damage such as the collapsed interstate 10 overpass sherman oaks and the collapsed parking garage at calstate northridge aftershock data analysis indicates a correlation of site amplification with mainshock damage we found several cases where the site amplification depended on the azimuth of the aftershock possibly indicating focusing from basin structures for the parking garage array we found large ground motion variabilities a factor of 2 over 200 m distances for sites on the same mapped soil unit array analysis of the aftershock seismograms demonstrates that sizable arrivals after the direct s waves consist of surface waves traveling from the same azimuth as that of the epicenter these surface waves increase the duration of motions and can have frequencies as high as about 4 hz for the events studied here we do not observe large arrivals reflected from the southern edge of the san fernando valley

geomagnetic intensity and inclination variations at hawaii for the past 98 kyr from core soh 4 big island a new study and a comparison with existing contemporary data abstract reliable thellier thellier paleointensity determinations were obtained from 321 out of 766 examined samples from the upper 200 units 675 m of core soh 4 drilled in the kilauea volcano the main trend of the soh 4 record is a marked decrease from values higher than present in the last 4 kyr to values about 40 smaller from about 20 kyr to the bottom of the sampled interval three main lows coinciding with shallow or negative inclinations observed in the record were identified with an excursion previously documented in hawaii and the mono lake and laschamps events for the last 15 kyr the soh 4 intensities are largely in agreement with the worldwide distribution of volcanic data in the 15 75 kyr interval the general lack of volcanic data makes comparison with the soh 4 data difficult the soh 4 record is similar to a composite record of paleointensity largely based on a recent north atlantic sedimentary paleointensity stack napis 75 the soh 4 record is also generally consistent with a record of geomagnetic field intensity obtained from a study of 36 cl in the grip ice core calculated with the assumption that the production rate of 36 cl is controlled only by the geomagnetic field one pronounced discrepancy between the different records is observed around 50 kyr bp when the sedimentary and cosmogenic based records document high intensities which are not observed in soh 4 these high intensities are also absent in a recent record based on the analyses of sea floor magnetic anomalies the histogram of vadms obtained from soh 4 is different from that obtained from the sedimentary or 36 cl based records and is more skewed than the distribution given by kono and tanaka for the last 2 5 million years

the urban geology of darwin australia darwin on the north coast of the australian mainland lies only 12 south of the equator and experiences a pronounced wet dry tropical monsoonal climate this latitudinal position has strongly influenced the local geology for darwin is dominated by deeply weathered lateritic regolith formed on labile cretaceous marine sediments close to 2 billion years of geological history is lost from this immediate region because the largely horizontally bedded cretaceous strata unconformably overly folded proterozoic metasediments models of the landscape evolution of this region and indeed across much of northern australia have been based upon the incorrect interpretation of a type section displaying weathered cretaceous strata close to darwin city a recent reinterpretation of this type section has shown that the landscape here is a function of deep weathering and structural controls within the cretaceous strata rather than repeated cycles of uplift and pediplanation during the cainozoic the low relief of this landscape low rates of denudation and preservation of cainozoic deep weathering profiles and predominantly rocky shore have meant that quaternary sediments are restricted to shallow alluvial deposits in creek valleys and marine sediments forming beaches with limited dune and beach ridge development

a multiproxy approach to reconstruct the environmental changes along the eurasian continental margin over the last 150 000 years abstract sediment cores located along the eurasian continental margin arctic ocean have been studied to reconstruct the environmental changes in terms of waxing and waning of the barents kara sea ice sheets atlantic water inflow and sea ice distribution over the last 150 kyr the stratigraphy of the cores is based on stable oxygen isotopes ams 14 c and paleomagnetic data we studied variations in marine and terrigenous input by a multiproxy approach involving direct comparison of sedimentological and organo geochemical data extensive episodes of northern barents sea ice sheet growth during marine isotope stages mis 6 and 2 have been supported by at least subsurface atlantic water inflow moisture bearing storms low summer insolation and minimal calving of ice ice advance during mis 4 was probably restricted to the shallow shelf between mis 4 and mis 2 large ice sheet fluctuations correspond to contemporary laurentide surging events and indicate short term climatic changes in the arctic ocean as has been recorded in lower latitudes in contrast in low precipitation areas in eastern eurasia glacial activity was rather limited only distinct ice rafted debris ird input during termination ii and early mis 3 reflects severe glaciations on the northern severnaya semlya margin during mis 6 and mis 4 we conclude that 1 oscillations of ice sheets are less frequent along the eastern eurasian margin than in areas with continuous moisture supply like the western eurasian margins and that 2 major fluctuations of the kara sea ice sheet during the last 150 kyr apparently followed the major interglacial glacial mis 5 4 and mis 7 6 transitions rather than the precession 23 kyr and the tilt 41 kyr cyclicity of the earth s orbit as observed for the scandinavian sis and the svalbard ice sheets respectively mangerud j jansen e landvik j y 1996 late cenozoic history of the scandinavian and barents sea ice sheets in solheim a riis f elverhoi a faleide j j jensen l n cloetingh s eds impact of glaciations on basin evolution data and models from the norwegian margins and adjacent basins global and planetary chance special issue 12 pp 11 26 surface and or subsurface atlantic water masses coupled with seasonally ice free conditions penetrated continuously to at least the franz victoria trough during the last 150 kyr however sustained periods of open water were largely restricted to substages 5 5 5 1 and the holocene as indicated by distinct carbonate dissolution and higher accumulation of marine organic matter mom signals of periodic open water conditions along the northern margin of severnaya semlya are of less importance higher production of foraminifera probably due to atlantic water inflow occurred between 38 and 12 14 c kyr and corresponds to periodic atlantic water advection penetrating into the arctic ocean however marine organic proxies indicate a continuous decrease of surface water productivity from the western to the eastern eurasian continental margin due to a more extensive sea ice cover over the last 150 kyr

late pleistocene geomagnetic excursion in icelandic lavas confirmation of the laschamp excursion in 1980 kristjansson and gudmundsson 1 reported a late glacial geomagnetic excursion in three hills in the reykjanes peninsula iceland with shallow negative inclinations and westerly declinations they named it the skalamaelifell excursion more extensive field work has identified the same excursional paleomagnetic direction declination 258 inclination 15 at four additional outcrops in a 10 10 km area in the reykjanes peninsula the excursion lavas are olivine tholeiites with similar petrography and chemical compositions paleointensity determinations by the thellier method average 4 2 0 2 t for 8 samples more than an order of magnitude weaker than the present geomagnetic field in iceland together these results suggest extrusion of the excursion lavas in a very brief span of time probably less than a few hundred years kar dating of the excursion lavas gives a mean age for 19 determinations of 42 9 7 8ka 2 compilation of thirty kar ages of the laschamp and olby flows by three laboratories yield a new age for the laschamp excursion in france of 46 6 2 4ka 2 the age of the excursion in southwestern iceland is statistically indistinguishable from the laschamp excursion at the 95 confidence level and both have very low paleointensities therefore we suggest that the laschamp and olby flows in france and the skalamaelifell units of iceland recorded essentially the same geomagnetic excursion differences in the virtual paleomagnetic poles vgps of these excursions may be due to 1 the probable non dipole character of the geomagnetic field during the excursion 2 rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in france and iceland and or 3 crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations

the initiation of northern hemisphere glaciation for well over a century scientists have speculated that variations in the atmospheric concentration of radiatively important trace gases such as carbon dioxide could control the earth s climate e g arrhenius 1 896 recent observations of significant glacial interglacial variation in atmo spheric co2 in ice cores put this idea on a firm observational as well as theoretical footing e g barnola et ai1 987 in this century as atmospheric co2 levels creep steadily upwards in response to the burning of fossil fuels and forests the specter of manmade global climate change looms before us ipcc 1990 a possibility that has heightened interest in the natural variability of climate in the past in particular the 3 c warming pre dicted for the next century has stimulated interest in past warm climates such as the middle pliocene around 3 0 million years ma ago an interval that has often been invoked as a greenhouse world budyko et al 1985 in this paper the climate transition from the warm mid pliocene around 3 2 ma to the onset of northern hemisphere ice ages around 2 4 ma is examined evidence for the initiation of significant northern hemisphere glaciation is examined as well as how this event affected climate around the globe while the cause of individual glacial interglacial oscillations is tied to milankovitch variations in the earth s orbit around the sun e g imbrie et ai 1 992 these insolation changes cannot account for the long term cooling trend which culminated in northern hemisphere glaciation in the final section of this paper mechanisms of long term climate change

variability of site response in the los angeles urban area this article addresses the variability of site response in the los angeles area and possible structural causes for the observations aftershock records from 231 sites in the san fernando and los angeles basins and the surrounding mountains are used in this study spectral ratios taken with respect to a low amplitude reference site are used to document the variation in site amplification in the frequency range 2 to 6 hz both spatially and with backazimuth to the source at higher frequencies 6 to 10 hz spectral ratios are shown to have greater spatial variability interstation spectral ratios are used to measure the standard deviation among source as a function of station separation an increase in the variation in ground motion is shown to take place at a station separation of 1 km relative site response estimates between nearby stations are used to demonstrate that preferred directions of motion can exist even in areas with no surface topographic effects significant variations in site response exist over short baselines up to a factor of 2 over 200 m that are not explained by differences in surficial geology or shallow shear wave velocity a variety of investigative approaches is used including spectral ratios arrival time variations 1d and 2d waveform modeling and comparison with seismic reflection lines to determine the most likely causes of these observations a correlation is demonstrated between late arrival times of p and s waves and larger site amplification in sherman oaks and northridge this observation in conjunction with waveform modeling and seismic reflection profiles is used to infer that sedimentary structures in the upper 1 to 2 km and topography on the sediment basement interface play an important role in determining site amplification these structures in the form of folds and buried basins focus energy in spatially restricted areas at the surface comparison of displacement waveforms at nearby stations having disparate site amplifications complemented by known shallow shear wave velocities at selected sites is used to support the argument that these structures in some cases can be the dominant factor in the modification of local ground motions

detailed study on the anisotropy of magnetic susceptibility of arctic marine sediments summary five sediment cores from three different areas the jan mayen fracture zone in the greenland sea the fram strait and the makarov basin in the arctic ocean have been subjected to a detailed analysis of the magnetic fabric by analysing their anisotropy of magnetic susceptibility ams as a supplementary parameter to already existing palaeo and rock magnetic data sets intervals of reversed inclinations documented within the five cores can be interpreted as geomagnetic excursions because all the investigated sediments are characterized by an undisturbed magnetic fabric as expected for layered sediments that is an oblate anisotropy ellipsoid with its short principal axis oriented vertically excursional directions are also not associated with significant changes in rock magnetic parameters however cores from the arctic ocean in particular exhibit a strong cyclicity in their anisotropy parameters here sand rich layers are characterized by a low anisotropy whereas clay rich layers yielded highest anisotropy degrees of up to 9 per cent indicating an ams mainly controlled by the matrix properties of the sediment a simple three axis determination of the anisotropy of anhysteretic remanent magnetization aarm on one of these cores surprisingly yielded an even higher degree of anisotropy of the ferrimagnetic fraction of up to 18 per cent i e twice as high as determined for the susceptibility which is also influenced by the paramagnetic fraction this implies a strong contribution of this sediment fraction to the overall magnetic susceptibility of the investigated arctic ocean deposits

geomagnetic events and relative palaeointensity variations during the past 300 ka as recorded in kolbeinsey ridge sediments iceland sea indication for a strongly variable geomagnetic field magnetostratigraphic analyses of five sediment cores recovered from the kolbeinsey ridge area revealed consistent records of several geomagnetic events linked with low relative palaeointensities within the past 300 ka interpretation of various rock magnetic parameters clearly rule out the possibility that the recorded non normal polarity directions are linked to a deviating magnetomineralogical fraction or a distorted magnetic fabric therefore these directions are interpreted as true recordings of geomagnetic field variations hysteresis parameters and thermomagnetic measurements revealed pure pseudo single domain psd magnetite with curie temperatures of 580 c as the dominant remanence carrier mineral due to the homogeneity of the rock magnetic parameters the sediments are also suitable for relative palaeointensity determinations oxygen isotope stratigraphies for two of the cores provided a time frame to estimate the age ranges for the recorded geomagnetic field variations the obtained record of ten events of up to 12 ka duration during the past 300 ka and the related record of relative palaeointensity indicate a fairly dynamic character of the earth s magnetic field which is in clear contrast to published polarity time scales

what is a reference site many methods for estimating site response compare ground motions at sites of interest to a nearby rock site that is considered a reference motion the critical assumption in these methods is that the surface rock site record reference is equivalent to the input motion at the base of the soil layers data collected in this study show that surface rock sites can have a site response of their own which could lead to an underestimation of the seismic hazard when these sites are used as reference sites data were collected from local and regional earthquakes on digital recorders both at the surface and in boreholes at two rock sites and one basin site in the san jacinto mountains southern california the two rock sites keenwild and pinon flat are located on granitic bedrock of the southern california peninsular ranges batholith the basin site garner valley is an ancestral lake bed with watersaturated sediments on top of a section of decomposed granite which overlies the competent bedrock ground motion is recorded simultaneously at the surface and in the bedrock at all three sites when the surface rock sites are used as the reference site i e the surface rock motion is used as the input to the basin the computed amplification underestimates the actual amplification at the basin site for frequencies above 2 to 5 hz this underestimation by a factor of 2 to 4 depending on frequency and site results from the rock sites having a site response of their own above the 2 to 5 hz frequencies the near surface weathering and cracking of the bedrock affects the recorded ground motions at frequencies of engineering interest even at sites that appear to be located on competent crystalline rock the bedrock borehole ground motion can be used as the reference motion but the effect of the downgoing wave field and the resulting destructive interference must be considered this destructive interference may produce pseudo resonances in the spectral amplification estimates if one is careful the bedrock borehole ground motion can be considered a good reference site for seismic hazard analysis even at distances as large as 20 km from the soil site

relative geomagnetic paleointensity and δ18o at odp site 983 gardar drift north atlantic since 350 ka abstract at odp site 983 relative geomagnetic paleointensity and planktic and benthic 18o records have been acquired for the last 350 kyr the mean sedimentation rate in this interval is 11 3 cm kyr magnetic properties and hysteresis ratios indicate that pseudo single domain magnetite is the remanence carrier volume susceptibility k anhysteretic arm and isothermal irm remanence values vary by a factor of 3 4 well within the criteria usually cited for paleointensity studies natural remanent magnetization nrm is normalized by arm and irm to acquire the paleointensity proxy arithmetic means of nrm arm and nrm irm calculated for five demagnetization steps in the 25 45 mt range constitute the relative paleointensity estimates some paleointensity lows particularly those at 40 120 and 188 ka are associated with directional excursions of the field especially the event at 188 ka referred to here as the iceland basin event that constitutes a short lived polarity reversal for the last 200 kyr the records can be correlated with other high resolution paleointensity records such as those from the labrador sea mediterranean somali basin and sulu sea implying that the millennial scale features are globally synchronous a labeling system for paleointensity features is proposed that ties prominent highs and lows to oxygen isotope stages

nonlinear site response in kushiro during the 1994 hokkaido toho oki earthquake the 1994 hokkaido toho oki earthquake of m w 8 2 and its aftershocks were used to estimate nonlinear site response in kushiro hokkaido japan the records from the mainshock and 10 aftershocks observed at 14 sediment sites in kushiro and aks a reference rock site were used in this study the sediment sites were located in a limited area within a radius of 3 km and some of them were located in the areas where liquefaction damage was observed at backfilled or reclaimed land amplification factors of peak ground acceleration and spectral ratios between sediment and rock sites for strong motion were compared with those for weak motions to investigate nonlinear site response at several sites the sediment to rock ratios of peak ground acceleration for strong motion differed by more than 50 from those for weak motions the fundamental peak frequencies of the spectral ratios became 10 to 20 percent lower during the mainshock at tbs and kkp sites on the reclaimed ground the strong to weak spectral amplification ratios at the high frequency range 2 to 20 hz significantly decreased to 0 4 during the mainshock however those on the natural ground were 0 6 to 0 9 using a moving time window we observed the drop in the strong to weak amplification ratios during the s wave main shaking the ratios tended to rise gradually in the later part of the records both the extent of the falling and the time for recovery in spectral amplification ratios depended on the degree of nonlinearity our analyses showed that nonlinearity was observed at the sites in kushiro during the mainshock and that the degree of nonlinearity varied greatly from site to site even in a limited area this variation indicated that the soil response at each site was greatly affected by the surface geology in the kushiro area

spectral amplification in a sediment filled valley exhibiting clear basin edge induced waves various site response estimates are presented for a linear array deployed in the coachella valley california during the 1992 landers big bear aftershock sequence this systematic comparison is unique in that the response of the site is clearly dominated by basin edge induced waves average sediment to bedrock spectral ratios for long s wave windows which include the basin edge induced waves exhibit amplification factors as high as 18 below 7 hz the deep basin structure which gives rise to the obvious multi dimensional effects produces a fundamental resonant peak that shifts between basin sites from 0 23 to 0 48 hz as the depth to bedrock changes above 0 6 hz where the largest amplifications occur the response is remarkably similar between sites and appears to be dominated by a near surface layer that is relatively uniform across the valley the apparent fundamental resonant frequency of this layer is between 0 8 and 1 hz sediment to bedrock spectral ratios computed using shorter windows that exclude the basin edge induced waves imply that the multi dimensional effects are significant only below 4 hz where they increase amplifications by an approximate factor of 2 spectral ratios computed using coda windows taken at twice the s wave travel time exhibit amplifications that are an average factor of 1 7 greater between 1 and 4 hz than those of the s wave estimates this discrepancy does not improve by taking coda windows later at four times the s wave travel time horizontal to vertical component s wave spectral ratios do not agree with the sediment to bedrock ratios however they do exhibit a clear peak at the fundamental resonant frequency of the deep basin structure sediment to bedrock spectral ratios of ambient seismic noise are also inconsistent with the s wave estimates however horizontal to vertical noise ratios exhibit clear peaks near the fundamental resonant frequencies of both the deep basin structure below 0 6 hz and the suspected near surface layer between 0 8 and 1 hz therefore ambient noise data appear to provide valuable constraints on the basin structure ongoing efforts involve multi dimensional modeling of the observed basin edge induced phases and resonant frequencies

site response models for charleston south carolina and vicinity developed from shallow geotechnical investigations the study models the response of near surface materials in charleston south carolina and the adjacent area geotechnical investigations at 281 locations were made available by local engineering firms the data used for dynamic site response analysis were derived from shear wave velocity measurements at 52 loca tions site response was quantified as the ratio of surface motion to hypothetical hard rock basement outcrop motion scenario earthquake motions were developed with the stochastic model acceleration response ratios for 5 critical oscillator damping were computed for 12 frequencies ranging from 0 1 to 30 hz and for peak ground acceleration two features determine the general nature of site response in the study area the impedance contrast between mesozoic basement and cretaceous sediments and the shallow impedance contrast between quaternary and tertiary sediments average s wave velocities in the quaternary are relatively uniform and range from 150 to 250 m sec they are not strongly correlated with surface geology the velocities of the immediately underlying tertiary sediments range from 300 to 500 m sec because of the uniformity of velocity in the quaternary depth to the quaternary tertiary contact appears to be the most important variable leading to differences in calculated site response this surface is irregular and varies in depth from near surface at inland sites to approximately 30 m at sites near the coast as a consequence estimated site response in the frequency band 1 10 hz varies by as much as a factor of 3 site response at frequencies less than 1 hz is dominated by the first few resonant har monics of the entire sedimentary section with fundamental frequency near 0 2 hz

comparisons between vs30 and spectral response for 30 sites in newcastle australia from collocated seismic cone penetrometer active and passive source vs data although the time averaged shear wave velocity down to 30 m depth v s 30 can be a proxy for estimating earthquake ground motion amplification significant controversy exists about its limitations when used as a single parameter for the prediction of amplification to examine this question in absence of relevant strong motion records we use a range of different methods to measure the shear wave velocity profiles and the resulting theoretical site amplification factors afs for 30 sites in the newcastle area australia in a series of blind comparison studies the multimethod approach used here combines past seismic cone penetrometer and spectral analysis of surface wave data with newly acquired horizontal to vertical spectral ratio passive source surface wave spatial autocorrelation spac refraction microtremor remi and multichannel analysis of surface wave data the various measurement techniques predicted a range of different afs the spac and remi techniques have the smallest overall deviation from the median af for the majority of sites we show that v s 30 can be related to spectral response above a period t of 0 5 s but not necessarily with the maximum amplification according to the modeling done based on the measured shear wave velocity profiles both v s 30 and af values are influenced by the velocity ratio between bedrock and overlying sediments and the presence of surficial thin low velocity layers 0 5 s do the amplification curves consistently show higher values for soft site classes and lower for hard classes

australia s large earthquakes and recent fault scarps abstract nineteen large earthquakes up to magnitude ms 6 8 have occurred within continental australia this century and at least seven recent prehistoric fault scarps have been discovered the magnitude of the earthquake causing each of these prehistoric fault scarps has been estimated from the observed fault length and maximum scarp height and none exeeds magnitude 7 0 their occurrence demonstrates that the australian plate is undergoing significant internal deformation but as yet there is no model to explain their cause nor the likely site of the next large earthquake the absence of earthquake epicentres on the current tectonic map of australia is a regrettable omission

a multi tiered earthquake hazard model for australia abstract earthquakes result from tectonic processes and their distribution is strongly influenced by large scale geology and the tectonic stress field however earthquake hazard estimates particularly ground motion recurrence have traditionally been computed using source models based primarily on instrumental and historical seismicity in areas of low to moderate seismicity such as australia large earthquakes commonly occur in areas which have experienced little or no recent activity making it difficult to develop source models based solely on seismicity the seismotectonic model developed for australia that is presented here aus5 is based on geology geophysics tectonics and seismicity the model was developed using a number of tiers of information so that new information can easily be incorporated the information used includes but is not limited to tectonic provinces basins and ranges gravity magnetic topography and seismicity all on a regional scale on a local scale for a site specific earthquake hazard study active faulting can be incorporated to provide fault source zones an earthquake hazard map showing peak ground acceleration with a 10 chance of exceedance in 50 years for southeastern australia using the geologically defined seismotectonic model aus5 is presented as an indication of how the model performs

magnetostratigraphic results from the eastern arctic ocean ams 14c ages and relative palaeointensity data of the mono lake and laschamp geomagnetic reversal excursions summary a new high resolution magnetostratigraphic record from the eastern arctic ocean has yielded further evidence for the existence of the laschamp excursion 37 35 ka the mono lake excursion 27 25 5 ka and possibly another very short excursion 22 ka inferred from steep negative inclinations ages are based on nine ams accelerator mass spectrometry 14c dates oxygen isotope stratigraphy and correlation with odp site 983 estimates of relative palaeointensity variations for the time interval from 80 to 10 ka have revealed that the documented geomagnetic excursions are linked to large fluctuations of the relative palaeointensity the lowest values were obtained for the two excursions and the normal reversed n r and reversed normal r n transitions of the laschamp polarity excursion which itself is characterized by a slight increase of relative palaeointensity during its reversed state the results are in general agreement with palaeointensity studies from other regions indicating that these fluctuations could be global phenomena and that the geomagnetic field of the brunhes chron was very variable in amplitude as well as in geometry the new result is one of the rare records comprising large directional as well as large relative palaeointensity variations

cool climate lateritic and bauxitic weathering it is commonly held that laterites and bauxites form under tropical climates although some workers have suggested lateritic weathering was not representative of tropical climates the assertion is still almost universally made early tertiary 56 34 ma basalts are widespread in the monaro region of new south wales they overlie and enclose fluvial and lacustrine sediments hyaloclastites and tuffs and the upper parts of many flows are bauxitized the basalts are up to about 200 m thick and in one section comprise 22 flows eight tuffs and hyaloclastites two dolerites and nine interflow bauxitic profiles this region in the eastern australian highlands was tectonically stable and had probably been at its present elevation for some 50 100 ma immediately before the beginning of volcanism and during the extrusion and bauxitization of the earlier flows the region was at 57 5 s latitude climates were wet cool to cold and thermally seasonal and the region was well forested paleomagnetic studies sh

predictions of shear wave velocities in southern california using surface geology a new model of the average shear wave velocity in the uppermost 30 m has been generated by extrapolation of discrete velocity profiles using surface geology at several scales statistical methods have been applied to create a map that is no more complicated than is supported by the velocity data several geologic units with similar responses are grouped together the resulting map is simpler than previous ones and yet fits the observed velocity profiles better than earlier more complicated maps analysis within a geographic information system will permit updates and modification of the map as new velocity data are added