the nature of the arctic oscillation and diversity of the extreme surface weather anomalies it generates abstractthrough a cluster analysis of daily ncep ncar reanalysis data this study demonstrates that the arctic oscillation ao defined as the leading empirical orthogonal function eof of 250 hpa geopotential height anomalies is not a unique pattern but a continuum that can be well approximated by five discrete representative ao like patterns these ao like patterns grow simultaneously from disturbances in the north pacific the north atlantic and the arctic and both the feedback from the high frequency eddies in the north pacific and north atlantic and propagation of the low frequency wave trains from the north pacific across north america into the north atlantic play important roles in the pattern formation furthermore it is shown that the structures and frequencies of occurrence of the five ao like patterns are significantly modulated by el nino southern oscillation enso warm cold enso enhances the negative positive ao phase compared with enso neutral winters finally the surface weat

an upper dalradian microfossil reassessed an acritarch from the highest dalradian macduff slate of northeast scotland is confirmed as veryhachium cf lairdii its presence implies that the macduff slate is ordovician this conclusion is compatible with the view that there is stratigraphical and structural continuity between the dalradian and lower palaeozoic rocks in the vicinity of the highland border e g the lower cambrian keltie water grit formation but runs counter to other lines of evidence notably biotite cooling ages which suggest that the dalradian had undergone metamorphism by the late cambrian further investigations are needed to reproduce and confirm the biostrati graphical evidence for the age of the macduff slate

the linkage between the eastern pacific teleconnection pattern and convective heating over the tropical western pacific abstractthe eastern pacific ep pattern is a recently detected atmospheric teleconnection pattern that frequently occurs during late winter through analysis of daily era interim data and outgoing longwave radiation data for the period of 1979 2011 it is shown here that the formation of the ep is preceded by an anomalous tropical convection dipole with one extremum located over the eastern indian ocean maritime continent and the other over the central pacific this is followed by the excitation of two quasi stationary rossby wave trains departing from the subtropics north of the region of anomalous convection one rossby wave train propagates eastward along the east asian jet from southern china toward the eastern pacific the second wave train propagates northward from east of japan toward eastern siberia and then turns southeastward to the gulf of alaska both wave trains are associated with wave activity flux convergence where the ep pattern develops the results from an examination of the e vecto

geochronological constraints on the magmatic metamorphic and thermal evolution of the connemara caledonides western ireland the connemara region of the western irish caledonides records the evolution of a short lived mid ordovician c 475 463 ma continental magmatic arc that intruded into dalradian metasedimentary rocks during regional ductile deformation new u pb geochronological data for intermediate intrusive phases and anatexis in southern connemara are consistent with the relative age succession suggested by field relationships calc alkaline plutons quartz diorites were emplaced into the dalradian sequence and into the connemara gabbros at about 467 2 ma coeval with anatexis which was manifested by a 467 2 ma granitic pegmatite and 468 2 ma leucosome formation in a metapelite metasomatic diopside rocks from west central connemara and a calcsilicate rock from southern connemara yield u pb titanite dates of c 462 2 ma and 463 2 ma respectively these dates postdate the metamorphic peak by at least five million years and indicate titanite crystallization at a late stage during the metamorphic history on the basis of u pb and 40 ar 39 ar geochronological data we conclude that the cooling history of southern connemara following anatexis was simple and brief at a minimum rate of 35 c ma 1 between 468 and 460 ma and 14 c ma 1 between 460 and 450 ma

garnet provenance studies juxtaposition of laurentian marginal terranes and timing of the grampian orogeny in scotland a study of the composition and zoning patterns of detrital garnets from ordovician greywackes from the northern belt of the southern uplands terrane of scotland reveals characteristics of the metamorphic sources very similar to the dalradian supergroup of the scottish grampian terrane the radiometric cooling and uplift ages of the dalradian metamorphic zones and the depositional ages of the southern uplands greywackes support the hypothesis for local provenance detrital metamorphic garnet identical to dalradian garnet has also been identified in the upper ordovican sandstones of the highland border complex these observations do not support proposals that the grampian midland valley and southern uplands terranes were exotic to each other in the late ordovician time these new results together with a review of published age dates clarify the late ordovician palaeogeography for this part of the laurentian margin the distances between the grampian midland valley and southern uplands terranes may have been similar to the present day it is concluded that large rivers flowed out of the uplifting mountainous grampian terrane and across the midland valley into a southern uplands trench during the late ordovician time the main orogeny i e mountain building in the grampian terrane was therefore post cambrian producing the first high mountains and resultant flysch in the caradoc

testing for the presence of a terrane boundary within neoproterozoic dalradian to cambrian siliceous turbidites at callander perthshire scotland the southern highland group dalradian and keltie water grit formation which includes the lower cambrian leny limestone form an inverted 1 4 km thick largely arenaceous sequence at callander the grits have the same detrital mineralogy throughout mainly quartz plagioclase an 1 3 muscovite and biotite chlorite formed from detrital biotite during low grade regional metamorphism t 40 ar 39 ar laser fusion dating of detrital muscovites yields an age spectrum with a peak at 1600 1800 ma in the dalradian rocks similar old ages occur in the keltie water grits but are diluted by ages of 507 886 ma we interpret these new data as showing that the rocks were most likely deposited as a single sequence possibly with a disconformity in neoproterozoic to early cambrian times before the onset of grampian orogenesis in the early palaeozoic no major structural or stratigraphical breaks have been identified and there is no direct evidence for the presence of two separate terranes

new evidence that the lower cambrian leny limestone at callander perthshire belongs to the dalradian supergroup and a reassessment of the exotic status of the highland border complex

on the separability of the north atlantic oscillation and arctic oscillation we address the issue of whether the arctic ao and north atlantic oscillations nao are inseparable forming an annular mode in the northern hemisphere atmospheric circulation this annular mode is the leading empirical orthogonal function of hemispheric sea level pressure slp data explaining the largest amount of its variability we examine whether the nao and ao are inseparable spatial modes of the atmospheric circulation using rotated principal component analysis rpca a methodology that identifies simple and unique patterns of spatial dataset variability rpca of the spring summer and autumn slp fields from 1946 1998 reveal nao and ao like patterns occurring as separate regional teleconnections forming the first and second principal components respectively the rpca based nao dipole pattern is like that observed in many prior studies while the ao like pattern exhibits high slp variability over the kara and laptev seas during winter however and in annual analyses a distinct ao like pattern is not obtained and the two patterns may be inseparable using commonly accepted rpca methods the rpca based ao like mode is significantly linked to north central siberian seasonal air temperatures and to the prevailing direction of motion of the underlying arctic ocean in summer suggesting that the non winter ao like pattern as a stand alone teleconnection separate from the nao contributes significantly to high latitude climate and ocean variability the winter nao ao inseparability is discussed as a possible effect of a shared winter storm track between the northeastern atlantic and the arctic

faulting apparently related to the 1994 northridge california earthquake and possible co seismic origin of surface cracks in potrero canyon los angeles county california apparent southward dipping reverse fault zones are imaged to depths of about 1 5 km beneath potrero canyon los angeles county california based on their orientation and projection to the surface we suggest that the imaged fault zones are extensions of the oak ridge fault geologic mapping by others and correlations with seismicity studies suggest that the oak ridge fault is the causative fault of the 17 january 1994 northridge earthquake northridge fault our seismically imaged faults may be among several faults that collectively comprise the northridge thrust fault system unusually strong shaking in potrero canyon during the northridge earthquake may have resulted from focusing of seismic energy or co seismic move ment along existing related shallow depth faults the strong shaking produced ground surface cracks and sand blows distributed along the length of the canyon seismic reflection and refraction images show that shallow depth faults may underlie some of the observed surface cracks the relationship between observed surface cracks and imaged faults indicates that some of the surface cracks may have devel oped from nontectonic alluvial movement but others may be fault related imme diately beneath the surface cracks p wave velocities are unusually low 400 m sec and there are velocity anomalies consistent with a seismic reflection image of shallow faulting to depths of at least 100 m on the basis of velocity data we suggest that unconsolidated soils 800 rrdsec extend to depths of about 15 to 20 m beneath our datum 25 m below ground surface the underlying rocks range in velocity from about 1000 to 5000 m sec in the upper 100 m this study illustrates the utility of high resolution seismic imaging in assessing local and regional seismic hazards

vendian and riphean rifting in nw scotland in the neoproterozoic nw scotland was located in a tract of extending continental lithosphere that evolved into the eastern passive margin of laurentia when iapetus opened in the vendian on the nw caledonian foreland major rift events are inferred in the riphean and vendian corresponding to the intra torridonian and sub cambrian unconformities respectively in this context current views that the moine and dalradian rocks of the scottish highlands were subjected to repeated orogeny during the same period become difficult to sustain we propose a tectonostratigraphic history for the two areas in which neoproterozoic orogeny plays no part

neonereites uniserialis from c 600 ma year old rocks in western scotland and the emergence of animals a chain of pellets neonereites uniserialis usually attributed to a coelomate or pseudocoelomate metazoan is described from the middle dalradian bonahaven formation of western scotland in sandstones not far above the port askaig tillite this trail lies more than 8 km below u pb zircon dated tayvallich lavas 595 4 ma and predates early deformation plus intrusion by granites at ben vuirich dated at 597 11 and 590 2 ma u pb zircon chronology for early metazoan evolution suggests that this is the earliest putative evidence for coelomates or pseudocoelomate discovered so far with a minimum interglacial age of c 600 ma bp it is from about this time that a fall takes place in the maximum 13 c of carbonates suggestive of a decline in the long term rate of carbon burial one explanation for this decline is the increasing impact of metazoan bioturbation through the late neoproterozoic to cambrian interval

teleconnections in the geopotential height field during the northern hemisphere winter abstract contemporaneous correlations between geopotential heights on a given pressure surface at widely separated points on earth referred to as teleconnections in this paper are studied in an attempt to identify and document recurrent spatial patterns which might be indicative of standing oscillations in the planetary waves during the northern hemisphere winter with time scales on the order of a month or longer a review of existing literature on the subject reveals the existence of at least four such patterns the north atlantic and north pacific oscillations identified by walker and bliss 1932 a zonally symmetric seesaw between sea level pressures in polar and temperature latitudes first noted by lorenz 1951 and what we will refer to as the pacific north american pattern which has been known to operational long range forecasters in this country since the 1950 s a data set consisting of nmc monthly mean sea level pressure and 500 mb height analyses for a 15 year period is used as a basis fo

synoptic analysis of the pacific north american teleconnection pattern in this study we use various diagnostic techniques to investigate the synoptic evolution of the pacific north american teleconnection pattern pna national center for environment prediction national center for atmospheric research reanalysis data are used these data cover the years 1948 2008 for the months of november march it is found that the positive pna is initiated by enhanced convection over the western tropical pacific and weakened convection over the tropical indian ocean the excitation of the negative pna exhibits opposite features for both phases the response to tropical convection excites a small amplitude pna about 8 12 days prior to the pattern attaining its maximum amplitude this is followed by slow steady growth for about 5 days after which driving by synoptic scale waves via their eddy vorticity flux together with stationary eddy advection lead to much more rapid growth and the establishment of the full pna for the positive pna the synoptic scale waves propagate eastward into the midlatitude northeastern pacific where they are observed to undergo cyclonic wave breaking for the negative pna the synoptic scale waves first amplify over the midlatitude northeastern pacific and then propagate equatorward into the subtropics where they undergo anticyclonic wave breaking once established for both phases the pna appears to be maintained through a positive feedback that involves a succession of wave breakings these results suggest that preconditioning may play an important role in the formation of the pna for the positive pna in its early development the strengthening and eastward extension of the subtropical jet result in an increase in the cyclonic shear and a decrease in the meridional potential vorticity gradient features that are known to favour cyclonic wave breaking for the negative pna opposite changes were observed for the background flow which favour equatorward wave propagation and anticyclonic wave breaking the role of optimal growth is also discussed our results also suggest that the pna is potentially predictable 1 2 weeks in advance copyright 2011 royal meteorological society

interdecadal variations of the silk road pattern abstractbased on several reanalysis and observational datasets this study suggests that the silk road pattern srp a major teleconnection pattern stretching across eurasia in the boreal summer shows clear interdecadal variations that explain approximately 50 of its total variance the interdecadal srp features a strong barotropic wave train along the asian subtropical jet resembling its interannual counterpart additionally it features a second weak wave train over the northern part of eurasia leading to larger meridional scale than its interannual counterpart the interdecadal srp contributes approximately 40 of the summer surface air temperature s variance with little uncertainty and 10 20 of the summer precipitation s variance with greater uncertainty over large domains of eurasia the interdecadal srp shows two regime shifts in 1972 and 1997 the latter shift explains over 40 of the observed rainfall reduction over northeastern asia and over 40 of the observed warming over eastern europe

grampian and late grenville events recorded by mineral geochronology near a basement cover contact in north mayo ireland the palaeoproterozoic to early neoproterozoic annagh gneiss complex structurally underlies the dalradian sequence in north mayo ireland and has been proposed as the depositional basement to dalradian metasediments the annagh gneiss complex was deformed metamorphosed and migmatized during the grenville orogeny and later reworked under amphibolite facies conditions this paper focuses on the timing of the post grenville events and particularly on the possible presence of post grenville pre grampian deformation that could be attributed to the knoydartian orogeny seven u pb titanite analyses from annagh gneiss complex gneisses have a weighted mean 207 pb 206 pb age of 963 8 ma which dates cooling after the main grenville metamorphism locally a later phase of titanite growth at 943 8 ma post dates the last phase of grenville deformation the weak discordance of the titanite data suggests that post grenville events had little effect on the u pb system in titanite if the discordance was caused by a tectonic event this is likely to have occurred during the early ordovician grampian orogeny rather than in the neoproterozoic within the annagh gneiss complex cross cutting metadolerites provide a structural marker allowing post grenville deformation to be distinguished in contrast correlative metadolerites cutting the adjacent dalradian metasediments share all grampian deformation events affecting their host ar ar hornblende ages from the post grenville metadolerites indicate that reworking of the annagh gneiss complex and the first episodes of dalradian deformation occurred during the grampian orogeny in this part of ireland one sample yields a 475 4 ma ar ar plateau age which is interpreted to date grampian deformation younger ar ar hornblende and rb sr mica ages record post grampian cooling neither field nor isotopic evidence for the knoydartian orogeny has been found in this part of ireland

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

high precision u pb monazite geochronology of the c 806 ma grampian shear zone and the implications for the evolution of the central highlands of scotland two pegmatites and mylonitic rocks from the grampian shear zone yield u pb monazite ages of 806 3 ma 808 9 11 ma and 804 12 13 respectively these confirm that shearing and pegmatite crystallization was essentially synchronous lower intercepts of c 440 ma indicate disturbance during ordovician reworking these new data question the concept of a single latest proterozoic early palaeozoic orogeny in the grampian terrane and the significance of the great glen fault

evidence for a major neoproterozoic orogenic unconformity within the dalradian supergroup of nw ireland an intriguing and contradictory scenario has recently developed in the caledonides of the british isles concerning the ages of deposition and deformation of the neoproterozoic dalradian supergroup isotopic evidence although limited suggests that the lower parts of the sequence may have been deposited and undergone precambrian deformation i e pre 600 ma prior to deposition of the upper parts of the sequence i e post 600 ma given these existing constraints it is clear that a major break or breaks would be required in the intervening sequence to maintain a coherent tectonostratigraphy we present evidence for such an unconformity preserved at the base of the easdale subgroup in nw ireland reworked clasts that contain a pre existing tectonometamophic history are identified within a conglomerate that lies along this regionally recognized boundary the underlying sequence also exhibits pre existing deformational fabrics that display erosional truncation at the base of the conglomerates these relationships together with significant erosion and excision of the footwall sequence and extensive thickness and facies variations in the hanging wall units imply that a major tectonic unconformity exists within this succession as the conglomerate lies stratigraphically below precambrian c 600 ma lavas the structural fabrics contained within the clasts and the underlying sequence must also be of precambrian age and totally unrelated to the well established early palaeozoic caledonian orogenic deformation observed in the younger parts of the sequence the dalradian supergroup as defined may actually comprise at least two distinct tectonostratigraphic sequences

exhumation of a collisional orogen a perspective from the north american grenville province combined structural and geochronologic research in the southernmost portion of the contiguous grenville province of north america ontario and new york state show protracted periods of extension after the last episode of contraction the grenville province in this area is characterized by synorogenic extension at ca 1040 ma supported by u pb data on titanites and 40 ar 39 ar data on hornblendes followed by regional extension occurring along crustal scale shear zones between 945 and 780 ma as recorded by 40 ar 39 ar analysis of hornblende biotite and k feldspar by ca 780 ma the southern portion of the grenville province from ontario to the adirondack highlands underwent uplift as a uniform block tectonic hypotheses have invoked various driving mechanisms to explain the transition from compression to extension however such explanations are thus far geodynamically unconstrained numerical models indicate that mechanisms such as gravitational collapse and mantle delamination act over timescales that cannot explain a protracted 300 m y extensional history that is contemporaneous with ongoing uplift of the grenville province rather the presence of a plume upwelling underneath the laurentian margin combined with changes in regional stress directions permitted the observed uplift and extension in the grenville province during this time the uplift history while on a slightly different timescale from those of most plume models is similar to that seen in models of uplift and extension caused by the interaction of a plume with the base of the lithosphere some of the protracted extension likely reflects the contribution of farfield effects possibly caused by tectonic activity in other cratons within the rodinian supercontinent effectively changing the stress distributions in the grenville province of northeastern north america

characterization of an alluvial silt and clay deposit for monotonic cyclic and post cyclic behavior this paper presents a detailed characterization of the monotonic cyclic and post cyclic behavior of two strata within a recent holocene alluvial deposit of silty sand sandy silt silt and clay stratum a is composed predominantly of very soft clay and very loose silt with plasticity indices ranging from 5 to 27 whereas stratum b is composed predominantly of very loose silty sand and sandy silt with plasticity indices ranging from 0 to 10 characterization included in situ testing undisturbed soil sampling and laboratory testing and a field surcharge test section consolidation tests and monotonic cyclic and post cyclic direct simple shear tests were used to evaluate the effects of varying the consolidation stress consolidation stress history and initial static shear stress ratio the field and laboratory test data show distinct differences in behavior between the two soil strata which can be related to their different index test characteristics these results are compared with their respective

short paper the depositional age of the dalradian supergroup u pb and sm nd isotopic studies of the tayvallich volcanics scotland zircons from a keratophyre associated with the tayvallich volcanics in the dalradian rocks of the sw scottish highlands have been dated by u pb methods yielding an age of 595 4 ma this age indicates that most or all of the dalradian is precambrian in age and that dalradian sedimentation may have lasted for about 200 million years the age also constrains the time interval between cessation of dalradian sedimentation and subsequent crustal thickening and regional metamorphism during the grampian orogeny sm nd isotopic data for the tayvallich volcanins and related metadolerite sills yield initial ond values of 2 to 4 which are thought to reffect the effects of melting of lithospheric mantle

current controversies in the caledonides this set of papers arose from a discussion meeting held in the department of geology applied geology university of glasgow on 9 10 september 1997 the aim of the meeting was to review the progress which had been made in resolving some of the more controversial aspects of the pre silurian caledonide geology of the british isles and scandinavia it was also held to mark the retirements of two well known caledonide geologists professors bernard leake and donald ramsay there were 34 talks and poster contributions and the meeting was attended by over 100 people we would like to thank our colleague tim dempster for helping to convene and organize the meeting it was fitting that the meeting should be held in the gregory building which was named after the glasgow professor who not only pioneered work on rift tectonics and regional uplift in africa but also wrote a perceptive book on dalradian geology gregory 1931 in this work gregory pointed out that his observations were made over a period of 25 years being made on recreational weekends and mainly in winter when the ground is less obscured by vegetation and the highland mountains most attractive an approach motivated by a spirit of enquiry and very productive but one which would be out of step with current research strategies we would like to dedicate this set of papers to mark piasecki who died on 13 june 1999 mark was the first person to draw attention to major problems concerning the relationship

multidecadal fluctuation of the wintertime arctic oscillation pattern and its implication abstractthe multidecadal fluctuations in the patterns and teleconnections of the winter mean arctic oscillation ao are investigated based on observational and reanalysis datasets results show that the atlantic center of the ao pattern remains unchanged throughout the period 1920 2010 whereas the pacific center of the ao is strong during 1920 59 and 1986 2010 and weak during 1960 85 consequently the link between the ao and the surface air temperature over western north america is strong during 1920 59 and 1986 2010 and weak during 1960 85 the time varying pacific center of the ao motivates a revisit to the nature of the ao from the perspective of decadal change it reveals that the north pacific mode npm and north atlantic oscillation nao are the inherent regional atmospheric modes over the north pacific and north atlantic respectively their patterns over the north pacific and north atlantic remain stable and change little with time during 1920 2010 the atlantic center of the ao always resemb

seismic response of liquefaction sites the authors of this technical paper focus on the seismic response of liquefaction sites by evaluating the transient shear strains and strong motion characteristics at treasure island during the 1989 loma prieta earthquake the strain levels are used to model soil and buried pipeline deformations observed in the marina district of san francisco during the same earthquake the authors found that transient strains play a critical role in lifeline performance based on the favorable comparisons of predicted and observed pipeline damage a simple model for liquefaction sites reveals that shear modulus reduction caused by increased pore pressure can amplify either acceleration or transient displacement depending on soil properties and deposit thickness liquefaction of thick loose sand deposits can attenuate accelerations while amplifying long period displacements the model offers a quantitative means of characterizing the phenomenon of ground oscillation that has been noted at liquefaction sites

north pacific north atlantic relationships under stratospheric control 1 based on a linear regression correlation analysis of monthly mean atmospheric sea level pressure slp data from the national centers for environmental prediction ncep reanalysis 1948 2000 we find a significant anticorrelation between pressure in the northern north atlantic and north pacific only if the stratospheric circulation is in the strong polar vortex regime but not when the vortex is weak since some general circulation models gcms e g european center hamburg echam4 are biased toward the strong vortex regime svr they tend to reproduce this anticorrelation already in the mean the pattern of the arctic oscillation ao is shown to be consistent with the mean surface pressure differences between the two stratospheric regimes the typical sw ne tilt of the node line of the north atlantic oscillation nao found with linear analyses i e with such that do not take the regime character of the stratospheric circulation into account in northern hemisphere winter is due to a superposition of correlation patterns based on physical processes working in the troposphere a strictly meridional dipole and the pattern resulting from planetary wave refraction in the strong vortex regime and those produced by the rapid transition from one stratospheric regime to the other with subsequent downward propagation of the signal this result underlines the necessity of the application of nonlinear statistics or the restriction of linear statistics to variations in the stable quasi linear environment of natural regimes

87sr 86sr chemostratigraphy of neoproterozoic dalradian limestones of scotland and ireland constraints on depositional ages and time scales new calcite 87 sr 86 sr data for 47 limestones from the metamorphosed and deformed neoproterozoic cambrian dalradian supergroup of scotland and ireland are used to identify secular trends in seawater 87 sr 86 sr through the dalradian succession and to constrain its depositional age dalradian limestones commonly have sr 1000 ppm indicating primary aragonite and marine diagenesis low mn mn sr 18 o and trace element data indicate that many 87 sr 86 sr ratios are unaltered since diagenesis despite greenschist to amphibolite facies metamorphism consistent with the documented behaviour of sr and o during metamorphic fluid rock interaction thus the 87 sr 86 sr data are interpreted largely to reflect 87 sr 86 sr of coeval seawater currently available data show that neoproterozoic seawater 87 sr 86 sr rose from c 0 7052 at 850 900 ma to c 0 7085 or higher in the latest neoproterozoic temporal changes at c 800 ma and c 600 ma bracket the range in 87 sr 86 sr values of calcite in grampian appin and lowest argyll group c 0 7064 0 7072 and middle and uppermost argyll group c 0 7082 0 7095 limestones consistent with a rise in seawater 87 sr 86 sr around 600 ma 87 sr 86 sr data are consistent with the sedimentary affinity of the islay subgroup with the underlying appin group and with a possible time interval between deposition of islay and easdale subgroup rocks they indicate that the dalradian as a whole is younger than c 800 ma

neoproterozoic environmental change recorded in the port askaig formation scotland climatic vs tectonic controls abstract the snowball earth hypothesis suggests that the neoproterozoic was characterized by several prolonged and severe global glaciations followed by very rapid climate change to hot house conditions the neoproterozoic port askaig formation of scotland consists of a thick succession of diamictite sandstone conglomerate and mudstone sedimentological and stratigraphic analysis of port askaig deposits exposed on the garvellach islands was carried out to establish the nature of neoproterozoic palaeoenvironmental change preserved in this thick succession particular emphasis was placed on identifying and distinguishing between climatic and tectonic controls on sedimentation port askaig formation diamictite units are attributed to deposition by sediment gravity flow processes or rainout of fine grained sediment and ice rafted debris in a glacially influenced marine setting associated facies record various depositional processes ranging from sediment gravity flows conglomerate massive sandstone and laminated mudstone to deposition under other unidirectional currents cross bedded and horizontally laminated sandstone the port askaig formation is also characterized by abundant soft sediment deformation features that occur at discrete intervals and are interpreted to record episodic seismic activity stratigraphic analysis of the port askaig formation on the garvellach islands reveals three phases of deposition phase i was dominated by sediment gravity flow processes and sedimentation was primarily tectonically controlled phase ii was a transitional phase characterized by continued tectonic instability an increased supply of sand to the basin and the preservation of current generated facies in the third and final phase of deposition the interbedded units of sandstone and diamictite are interpreted to reflect development of large sandy bedforms and ice margin fluctuations in a tectonically stable marine setting sedimentological and stratigraphic analysis of the port askaig formation demonstrates that tectonic activity had a significant influence on development of the lowermost parts of the succession climatic influences on sedimentation are difficult to identify during such phases of tectonic activity but are more easily discerned during episodes of tectonic quiescence e g phase iii of the port askaig formation the thick succession of diamictite interbedded with current deposited sandstone preserved within the port askaig formation is not consistent with deep freeze conditions proposed by the snowball earth hypothesis

a high precision u pb age for the ben vuirich granite implications for the evolution of the scottish dalradian supergroup high precision high accuracy u pb analyses of zircons from the ben vuirich granite in the scottish dalradian succession were performed on carefully picked high integrity abraded fractions of specific morphologies zircon needles which grew during crystallization give an age of 590 2 ma stubby prisms show inheritance with an upper concordia intercept of 1448 7 ma the age of intrusion at 590 ma contrasts strongly with a previous u pb zircon age of 514 6 7 ma determined on bulk fraction unabraded grains which it is argued must have suffered significant pb loss as the ben vuirich granite was emplaced between the regional d2 and d3 deformation events in the dalradian block the new data imply that all dalradian sedimentation was precambrian supposed lower cambrian fauna1 links between the dalradian succession and laurentia are thus discounted and the possibility that the dalradian supergroup has non laurentian affinities must be considered the early d1 d2 nappe forming grampian structures are also precambrian and may be associated with terrane docking along a tectonically active ocean margin later d3 structures and associated peak barrovian metamorphism suggest a subsequent thickening event possibly c 520 ma ago

the pacific center of action of the northern hemisphere annular mode real or artifact the leading empirical orthogonal function eof of the sea level pressure slp field referred to as the arctic oscillation ao or northern hemisphere annular mode nam consists of a dipole between the polar cap region and the surrounding zonal ring centered along 458n embedded within the outer ring are centers of action over the euro atlantic and pacific sectors in which slp fluctuates in phase that the observed slp fluctuations at these two centers of action are virtually uncorrelated raises the question of whether the pacific center in the annular mode could be an artifact of eof analysis it is argued that sea level pressure fluctuations at the pacific and euro atlantic centers of action of the ao nam would be more strongly correlated were it not for the fact that slp variability over the north pacific is dominated by a pattern in which fluctuations over the north atlantic and north pacific are inversely related evidence of the coexistence of such a pattern which resembles an augmented version of the pacific north american pattern is presented

interannual seesaw between the aleutian and icelandic lows part ii its significance in the interannual variability over the wintertime northern hemisphere abstract through analysis of observational data for the period of 1973 94 the late winter formation of an interannual seesaw between the surface aleutian and icelandic lows al and il respectively is shown to significantly impact the covariance structure of the leading mode of the interannual variability in the geopotential height field over the extratropical northern hemisphere the tropospheric leading mode for early winter november january is characterized by a polar midlatitude dipole over the euro atlantic sector with a high degree of the annularity coupled with the anomalous lower stratospheric polar vortex over the north pacific no significant anomalies are associated with this mode after the formation of the al il seesaw however the dipole no longer dominates in the upper tropospheric variability the dipole signature is masked in late winter february april by the predominant combined signature of the so called pacific north american pattern and a meridional dipole over the northweste

structural setting and geochronological significance of the west highland granitic gneiss a deformed early granite within proterozoic moine rocks of nw scotland a re investigation of the six major outcrops of granitic gneiss within the glenfinnan and loch eil divisions of the moine has shown that they are a suite of deformed and metamorphosed granite intrusions they record all the tectonic events recognized in their host metasediments while the absence of thermal aureoles suggests that their emplacement was synmetamorphic i e syn d 1 rather than pre d 1 their dominant gneissic fabric is of s 2 age but in low strain augen an earlier s 1 fabric is preserved which is cut by mp 1 leucocratic segregations contemporaneous with but compositionally distinct from those in the pelitic regional migmatites of the moine the deformed granites underwent local mp 2 remelting and have been widely intruded by later caledonian pegmatites the gneiss has a uniformly granitic sensu stricto bulk composition regardless of country rock lithology or location within the glenfinnan and loch eil divisions it has strongly s type chemical affinities being reduced and peraluminous with a high initial 87 sr 86 sr ratio it was probably produced at least in part by partial melting of moine like rocks at depth a published rb sr whole rock isochron of c 1028 ma probably dates intrusion of the ardgour body and confirms the reality of a 9grenvillian9 event in the glenfinnan and loch eil divisions of the moine

the timing and p t conditions of regional metamorphisni in the central highlands scotland the central highlands of the scottish caledonides are dominated by a sequence of poly deformed metasedimentary rocks largely belonging to the grampian and appin groups of the dalradian supergroup in the northern part of the area these are separated from a suite of variably gneissose to locally migmatitic rocks by a number of prominent mid crustal shear zones the early evolution of the central highlands is interpreted in terms of regional d1 d2 folding and n to nw directed thrusting accompanied by kyanite grade metamorphism p 7 0 8 0 kbar t 500 600 c the later stages of this event were characterized by continued ductile shearing along the pre existing shear zones under sillimanite grade metamorphic conditions p 4 7 5 9 kbar t 585 695 c the exact age of this progressive tectonothermal event remains uncertain however available geochronological data can be interpreted as indicating an entirely precambrian age c 840 800 ma this questions previously held views of an entirely caledonian age for deformation throughout the highlands

on the teleconnectivity of the arctic oscillation the term arctic oscillation ao has recently been introduced to describe the leading structure of slp variability over the northern hemisphere a key feature of the ao is its zonally symmetric appearance with a primary center of action over the arctic and opposing anomalies in midlatitudes does the ao s annular appearance result from significant temporal correlations between slp anomalies at distant longitudes the results presented indicate that the temporal coherence between the arctic and midlatitudes is strongest over the atlantic sector with weak correlations between the atlantic and pacific midlatitudes both on intraseasonal and interannual time scales during the past 50 yrs hence the annular character of the ao is more a reflection of the dominance of its arctic center of action than any coordinated behavior of the atlantic and pacific centers of action in the slp field the ao is nearly indistinguishable from the leading structure of variability in the atlantic sector e g the north atlantic oscillation their temporal correlation is 0 95 for monthly data

grenvillian extensional tectonics in northwest scotland new structural data indicate that eclogite bearing gneisses in the glenelg attadale inlier of the caledonian moine thrust nappe northwest scotland were involved in a major pre caledonian system of extensional top to the east ductile shear zones these shear zones coalesce upward to constitute a major extensional detachment below metasedimentary rocks of the moine supergroup both the eclogite bearing gneisses and the moinian metasedimentary rocks display evidence of extensional deformation but the latter contain structures indicative of a lower intensity of bulk finite shear strain and may have experienced only the later of two discrete phases of extension that we recognize in the inlier coplanar caledonian brittle ductile top to the west thrusting has resulted in only localized and limited reactivation timing of the extensional deformation is bracketed by a published sm nd age of 1 08 ga for eclogite equilibration and by a minimum age of deposition for the moine supergroup of 840 ma we propose that the eclogite bearing lower crust was exhumed as a result of collapse of the grenville orogen and that the evolution of the moinian basin was controlled by this extensional event

assessment of the liquefaction susceptibility of fine grained soils observations from recent earthquakes and the results of cyclic tests indicate that the chinese criteria are not reliable for determining the liquefaction susceptibility of fine grained soils fine grained soils that liquefied during the 1994 northridge 1999 kocaeli and 1999 chi chi earthquakes often did not meet the clay size criterion of the chinese criteria cyclic testing of a wide range of soils found to liquefy in adapazari during the kocaeli earthquake confirmed that these fine grained soils were susceptible to liquefaction it is not the amount of clay size particles in the soil rather it is the amount and type of clay minerals in the soil that best indicate liquefaction susceptibility thus plasticity index pi is a better indicator of liquefaction susceptibility loose soils with pi 0 85 were susceptible to liquefaction and loose soils with 12 0 8 were systematically more resistant to liquefaction soils with pi 18 tested at low effective confining stresses

arctic oscillation or north atlantic oscillation abstract the definition and interpretation of the arctic oscillation ao are examined and compared with those of the north atlantic oscillation nao it is shown that the nao reflects the correlations between the surface pressure variability at its centers of action whereas this is not the case for the ao the nao pattern can be identified in a physically consistent way in principal component analysis applied to various fields in the euro atlantic region a similar identification is found in the pacific region for the pacific north american pna pattern but no such identification is found here for the ao the ao does reflect the tendency for the zonal winds at 35 and 55 n to anticorrelate in both the atlantic and pacific regions associated with the nao and pna because climatological features in the two ocean basins are at different latitudes the zonally symmetric nature of the ao does not mean that it represents a simple modulation of the circumpolar flow an increase in the ao or nao implies stro

short paper the extent of grampian orogenic activity in the scottish highlands structures in atholl nappe grampian division rocks in the tayside region can be traced nw into the supposedly older central highland division in speyside and strath nairn where they are the earliest fabrics present grampian division central highland division contrasts appear to involve a gradational metamorphic change and the existence of the grampian slide supposed by previous workers to separate the two units has not been confirmed atholl nappe fabrics have been shown to be coeval with those of grampian orogenesis in the tay nappe where their age is constrained by the 590 2 ma ben vuirich granite grampian orogenic structures can thus be traced as the earliest structures present as far as the great glen fault a conclusion which may have a bearing on the significance of the fault as a terrane boundary central highlands the metamorphic rocks of the scottish central highlands fig 1 comprise units referred to the southern highland youngest argyll appin and grampian oldest groups while harris et al 1978 regarded the grampian group as the oldest dalradian group it is broadly equivalent to the grampian division moine gd of piasecki 1980 who also identified the central highland division moine chd which occupies several hundred square kilometres of inverness shire east and south of inverness and which forms three other small inliers in speyside not distinguished on fig 1 working largely in speyside and adjacent areas piasecki van breemen 1983 regarded the chd as lithostratigraphically broadly equivalent to moine rocks of the n highlands and

structural and stratigraphic relationships across the continuation of the highland boundary fault in western ireland t he relationship between the dalradian supergroup and the highland border complex in scotland has remained contentious for over a century in western ireland the contact between the dalradian supergroup and the clew bay complex a correlative of the highland border complex is superbly exposed on the island of achill beg on the north mayo coast the unfossiliferous south achillbegsuccessionhasbeentraditionally assignedtotheclewbaycomplex andthisinterpretation is supported by a combination of sm nd model age data heavy mineral analysis and lithostratigraphic correlation tdm ages range from 1 99 2 66 ga mean 2 28 ga n 6 detailed structural mapping shows that both the dalradian and the clew bay complex share the same structural history a d1 high strain event is common to both units and is associated with the development of tectonic slides the d2 event is responsible for the formation of crustal scale nappes in both units beds are consistently downward facing on the s2 foliation later dextral shearing d3 resulted in the tilting of the originally recumbent s facing d2 nappes into this downward facing orientation rb sr and 40 ar 39 arradiometricdatingofmuscoviteconfirmsthatbothunitsweredeformedcontemporaneously as the s2 nappe fabric in each is dated at c 460 ma this middle ordovician age for deformation of the clew bay complex is highly significant not least because published microfossil data suggest a silurian age

the magnitude 6 7 northridge california earthquake of january 17 1994 the most costly american earthquake since 1906 struck los angeles on 17 january 1994 the magnitude 6 7 northridge earthquake resulted from more than 3 meters of reverse slip on a 1 5 kilometer long south dipping thrust fault that raised the santa susana mountains by as much as 70 centimeters the fault appears to be truncated by the fault that broke in the 1971 san fernando earthquake at a depth of 8 kilometers of these two events the northridge earthquake caused many times more damage primarily because its causative fault is directly under the city many types of structures were damaged but the fracture of welds in steel frame buildings was the greatest surprise the northridge earthquake emphasizes the hazard posed to los angeles by concealed thrust faults and the potential for strong ground shaking in moderate earthquakes

ground settlement in simi valley following the northridge earthquake significant differential settlements were observed on a flat undeveloped generally silty but highly stratified alluvial site in eastern simi valley calif due to groundshaking from the 1994 northridge earthquake the observed settlements were due to both liquefaction of soil beneath the ground water table and dry shaking of loose soils above it a nearby strong motion recording station provided highly reliable ground motion data for analysis extensive cone penetration tests cpt along with occasional seismic cones scpt flat plate dilatometer tests dmt and soil sampling were performed previously developed methods were adopted for converting the cpt data to equivalent standard penetration test blow count spt n values for establishing liquefaction potential and predicting settlement site specific calibration of cpt data was performed to establish a correlation between cpt readings and the percentage of fines contours of predicted ground settlement agreed very well with the location and the observed differential settlements along a ground fissure that developed shortly after the earthquake

are the north atlantic oscillation and the northern annular mode distinguishable this study addresses the question of whether persistent events of the north atlantic oscillation nao and the northern annular mode nam teleconnection patterns are distinguishable from each other standard daily index time series are used to specify the amplitude of the nao and nam patterns the above question is examined with composites of sea level pressure and 300 and 40 hpa streamfunction along with tests of field significance a null hypothesis is specified that the nao and nam persistent events are indistinguishable this null hypothesis is evaluated by calculating the difference between time averaged nao and nam composites it is found that the null hypothesis cannot be rejected even at the 80 confidence level the wave breaking characteristics during the nam life cycle are also examined both the positive and negative nam phases yield the same wave breaking properties as those for the nao the results suggest that not only are the nao and nam persistent events indistinguishable but that the nao nam events are neither confined to the north atlantic nor are they annular

reconsideration of the true versus apparent arctic oscillation abstract the physical reality of the arctic oscillation ao or northern annular mode is considered the data used are mainly the monthly mean sea level pressure slp a schematic figure is first presented to illustrate the relationship between the north atlantic oscillation nao pacific north american oscillation pna system and the ao negative correlation mode between the atlantic and the pacific ao ncm system although the nao pna apparent ao ncm and true ao ncm systems give rise to the same eofs the probability density functions for the time coefficients of the two leading modes are different therefore the discrimination of the two systems is possible several pieces of evidence indicate that in the real world the nao pna and the ao ncm are located on almost the same plane in phase space this means that the nao pna and ao ncm systems have the same variations on the plane in common implying that when the nao pna system is real the ao ncm is unlikely to be real simple independent compon

a pre d2 age for the 590 ma ben vuirich granite in the dalradian of scotland regional mapping demonstrates that s2 is the main fabric in the central highland dalradian and that it is this fabric not s3 as previously thought which overprints the 590 ma ben vuirich granite the granite appears to have been emplaced following a tectonic event d1 in the dalradian rocks for a finely spaced penetrative fabric seen in newly discovered hornfelses from the granite aureole is overgrown by contact metamorphic cordierite and andalusite porphyroblasts now both altered which predate the d2 metamorphism the ben vuirich granite cuts the neoproterozoic dalradian sequence of the central highlands of scotland fig 1 inset and was first recognized as an older foliated granite by barrow et al 1905 later workers bradbury et al 1976 bradbury 1978 concluded that the granite was intruded between d2 and d3 in the dalradian rocks harris et al 1976 and for many years an u pb zircon age of 514 7 ma from the intrusion pankhurst pidgeon 1976 provided a valuable datum point in the evolution of the scottish caledonides however dalradian chronology has recently undergone a radical revision following the determination of a precise u pb age of 590 2 ma for the emplacement of the ben vuirich granite rogers et al 1989 in accepting the interpretation of bradbury et al 1976 that the regional penetrative fabric which affects the country rocks and is continuous with that in the granite is of d3 age rogers et al were also guided by the conclusions of harte et al 1984 and dempster harte

behavior of a fine grained soil during the loma prieta earthquake results of an investigation into the behavior of a fine grained clayey soil at moss landing during the 1989 loma prieta earthquake are presented a deposit of this soil underlies portions of the mo

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

liquefaction and soil failure during 1994 northridge earthquake the 1994 northridge california earthquake caused widespread permanent ground deformation on the gently sloping alluvial fan surface of the san fernando valley the ground cracks and distributed deformation damaged both pipelines and surface structures to evaluate the mechanism of soil failure detailed subsurface investigations were conducted at four sites three sites are underlain by saturated sandy silts with low standard penetration test and cone penetration test values these soils are similar to those that liquefied during the 1971 san fernando earthquake and are shown by widely used empirical relationships to be susceptible to liquefaction the remaining site is underlain by saturated clay whose undrained shear strength is approximately half the value of the earthquake induced shear stress at this location this study demonstrates that the heterogeneous nature of alluvial fan sediments in combination with variations in the groundwater table can be responsible for complex patterns of permanent ground deformation it may also help to explain some of the spatial variability of strong ground motion observed during the 1994 earthquake

uplift patterns and orogenic evolution in the scottish dalradian rb sr and k ar mineral ages from the dalradian rocks of angus and perthshire indicate considerable diachronism of cooling following the caledonian regional metamorphism the oldest mineral ages 515 ma are similar to the independent estimates of peak metamorphic age and hence indicate very rapid cooling they occur in the highest grade samples of the angus region and are associated with localized uplift regional rapid uplift events at 460 440 ma and 410 390 ma may also be recognized and correlated with structural and igneous activity and metamorphic age dates within the scottish highlands thermal models show the importance of localized and discontinuous uplift in determining both the timing of metamorphism and the shape of the p t paths they suggest that the major diachronous metamorphism and granite production in the scottish highlands is depth controlled rather than representing major lateral diachronism of thickening or heat sources localized uplift may exercise considerable control on mineral facies variation within orogenic belts the local uplift in angus introduces hot high pressure tarfside nappe rocks into higher level tay nappe rocks thus the uniform block uplift models frequently used in the interpretation of metamorphic and structural phenomena in orogenic belts are an oversimplification of the real events

giant cross beds in the neoproterozoic port askaig formation scotland implications for snowball earth abstract the neoproterozoic port askaig formation is a thick succession of interbedded diamictite conglomerate sandstone and mudstone that records glacially influenced marine sedimentation in the extensional dalradian basin giant cross beds individual units up to 11 m thick are observed in sandstone at the base of member iii of the port askaig formation the cross bedded sandstones are closely associated with other sandstone diamictite and conglomerate facies and are characterized by both planar and trough shaped foreset laminae with an average maximum dip angle of 14 the cross bedding has compound internal structure and indicates a predominant paleocurrent direction towards the south the sandstone is medium grained well sorted and quartzitic the giant cross beds are interpreted to have formed as a result of the migration of large subaqueous simple and compound dunes in a marine setting based on sedimentological analysis of the sandstone and associated facies as well as the tectonic setting of the port askaig formation the giant dunes are thought to have formed under the influence of tidal currents and indicate open marine ice free depositional conditions the sedimentology of the cross bedded sandstone and its close association with diamictite and conglomerate indicate repeated ice margin advance and withdrawal with periods of both ice free and ice influenced conditions the environmental conditions recorded in these port askaig deposits appear inconsistent with deep freeze or rapid meltback associated with a snowball earth

the significance of the geal charn ossian steep belt in basin development in the central scottish highlands the geal charn ossian steep belt is a major composite synclinal structure cored by an upward facing appin group succession it is located at the margin of a west facing sedimentary basin where more than 8 km of grampian group sediment was deposited adjacent to an intrabasin structural high the high comprises mainly gneissose metasedimentary rocks the glen banchor succession which acted as the basement to the grampian and appin group rocks major unconformities are recognized at more than one level as the sedimentary basin successions onlapped onto the 9high9 the primary major upright folds and associated slides of the steep belt developed when considerable shortening was focused along the basin margin during the caledonian orogeny

the continuum of north pacific sea level pressure patterns intraseasonal interannual and interdecadal variability this study combines k means cluster analysis with linear unidimensional scaling to illustrate the spatial and temporal variability of the wintertime north pacific sea level pressure slp field daily wintertime slp data derived from the ncep ncar reanalysis are used to produce 16 slp anomaly patterns that represent a discretized approximation of the continuum of north pacific slp patterns this study adopts the continuum perspective for teleconnection patterns which provides a much simpler framework for understanding north pacific variability than the more commonly used discrete modal approach the primary focus of this research is to show that variability in the north pacific on intraseasonal interannual and interdecadal time scales can be understood in terms of changes in the frequency distribution of the cluster patterns that compose the continuum each of which has a time scale of about 10 days this analysis reveals 5 6 pacific north american like pna like patterns for each phase as well as dipoles and wave trains a self organizing map som analysis of coupled slp and outgoing longwave radiation data shows that many of these patterns are associated with convection in the tropical indo pacific region on intraseasonal time scales the frequency distribution of these patterns in particular the pna like patterns is

incidents of ground failure from the 1994 northridge earthquake abstract numerous incidents of ground failure from the 1994 northridge earthquake caused thousands of pipe breaks and damaged many structures in this article significant occurrences of ground failure in the san fernando valley simi valley santa clara river area central los angeles and a number of coastal areas are described and the likely culprit mechanisms are identified ground failure in many areas could be readily associated with liquefaction and or seismically induced compaction of young alluvial soils or fill materials previously known to have a high ground failure susceptibility however ground failure also occurred in areas not previously thought to be susceptible to such hazards taken together these incidents provide insight into a variety of ground failure phenomena that can affect urban areas during earthquakes

liquefaction susceptibility criteria for silts and clays new liquefaction susceptibility criteria for saturated silts and clays are presented that are based on the mechanics of their stress strain behavior and which provide improved guidance for selecting engineering procedures for estimating potential strains and strength loss during seismic loading monotonic and cyclic undrained loading test data for silts and clays show that they transition over a fairly narrow range of plasticity indices pi from soils that behave more fundamentally like sands sand like behavior to soils that behave more fundamentally like clays clay like behavior with the distinction having a direct correspondence to the type of engineering procedures that are best suited to evaluating their seismic behavior it is recommended that the term liquefaction be reserved for describing the development of significant strains or strength loss in fine grained soils exhibiting sand like behavior whereas the term cyclic softening failure be used to describe similar phenomena in fine grained soils exhibiting clay like behavior for practical purposes clay like behavior can be expected for fine grained soils that have pi 7 although a slightly lower transition point for soils with a cl ml classification perhaps pi 5 or 6 would be equally consistent with the available data issues related to the practical application of these criteria are discussed