correlation of changes in gravity elevation and strain in southern california measurements made once or twice a year from 1977 through 1982 show large correlated changes in gravity elevation and strain in several southern california networks precise gravity surveys indicate changes of as much as 25 microgals between surveys 6 months apart repeated surveys show that annual elevation changes as large as 100 millimeters occur along baselines 40 to 100 kilometers long laser ranging surveys reveal coherent changes in areal strain of 1 to 2 parts per million occurred over much of southern california during 1978 and 1979 although the precision of these measuring systems has been questioned the rather good agreement among them suggests that the observed changes reflect true crustal deformation

volcano monitoring by microgravity and energy budget analysis microgravity monitoring of active volcanoes can provide evidence of sub surface mass and or density changes that precede eruptions if in addition precursory increases in thermal emissions are observed an integrated mechanistic model for volcanic activity may be developed with the potential for forecasting eruptions crater lake volcanoes provide an interesting target for such studies since thermal output can be monitored simply through lake water calotimetry here we summarize 10 years of gravity and thermal data from poas volcano costa rica mass energy balance calculations demonstrate that in the steady state the large thermal inertia of the crater lake acts as a buffer to short term changes in the energy input from the cooling magma feeder pipe since february 1986 it is postulated that there has been gradual emplacement of a shallow magma intrusion associated with vesiculation and gas loss to the surface this follows from unambiguous gravity increases constrained by elevation control that are coincident in time with a period of long term increased energy input to the crater lake progressive reduction of the lake volume by evaporation seepage culminated in an april 1989 ash eruption providing a good documented record of combined gravity and thermodynamic precursors to volcanic activity

the global geodynamic effect of the macquarie ridge earthquake besides generating seismic waves which eventually dissipate an earthquake also generates a static displacement field everywhere within the earth this global displacement field rearranges the earth s mass distribution causing the earth s rotational properties and gravitational field to change the size of these changes depends in general upon the size of the earthquake the macquarie ridge earthquake of may 23 1989 is considered to be the largest earthquake to have occurred since the 1977 sumba and tonga events as such the coseismic effect of this earthquake upon the earth s length of day polar motion and low degree harmonic coefficients of the gravitational field are computed it is found that this earthquake should have caused the length of day to decrease by 0 06 mu sec the position of the mean rotation pole to shift 0 11 milli arcsec towards 323 degree e longitude and selected degree l 2 5 gravitational field coefficients to change by about 1 part in 10 sup 13 these changes are all smaller than can be detected by current observational techniques however changes of this size could perhaps be detected in the future with the implementation of proposed improvements to the techniques of monitoring the earth s rotation and especially for the low degree gravitational field coefficients with the placement ofmore gps receivers onboard orbiting spacecraft less

integration of modern and past pollen accumulation rate par records across the arctic tree line a method for more precise vegetation reconstructions abstract in order to assess the potential role of the boreal forest in the early to mid holocene global dynamics it is critical that the tree line reconstructions are based on precise palaeorecords pollen accumulation rate par calculations provide a technique for estimating past plant population densities independently for each species when considering tree lines an important concept is the threshold par value for the absence presence and dominance of the most common tree species such threshold values are often based on modern lake sediment core top par values in northern fennoscandia records obtained through pollen monitoring now add an important element to these studies as they allow more precise definitions of the temporal and spatial limits of par values here we compare modern par values based on pollen monitoring data with sediment core top data to evaluate the robustness of the threshold values for the major tree species of finnish lapland we apply these threshold values to two fossil par records from this tree line region and as an example of the potential of the approach map the vegetation patterns in the area at 7000 cal yr bp the results indicate that the extent of the mid holocene tree line advance in northern fennoscandia was substantially less than climate vegetation model simulations have predicted despite their value and potential in vegetation reconstructions the use of par values requires careful validation of the data we stress the importance of basin size of precise and reliable sediment dating age depth modelling and assessment of the sedimentary processes in each study basin as prerequisites for par based studies

local gravity anomalies produced by dislocation sources rundle 1978 and walsh and rice 1979 have shown that the change in the vertical component of gravity is proportional to uplift for a spherical source of dilatation and for slip on an infinitely long dip slip fault in the first case no free air gravity anomaly is produced and in the second case no bouguer gravity anomaly gravity anomalies due to other dislocation sources in three dimensions are as follows for strike slip faulting the ratio of the gravity change to uplift depends upon position however the gravity change contours are roughly similar to those corresponding to a zero free air gravity anomaly nor is the ratio constant for dip slip faulting except for the two special cases of dip slip on a vertical fault and horizontal slip on a horizontal fault neither of which produce a bouguer anomaly the bouguer anomaly produced by an open horizontal crack is the same as would be produced had the material within the crack been mined out without deforming the solid if the horizontal crack were filled with material of density equal to the host rock a good approximation to sill formation no bouguer anomaly is produced for cracks of other inclinations the ratio of gravity change to uplift is not constant thus dilatancy in general does not correspond to the absence of a free air anomaly as might be suggested by the special case of a spherical source of dilatation for two dimensional models a cylindrical source of dilatation produces no free air gravity anomaly dip slip faulting produces no bouguer anomaly and open cracks produce a bouguer anomaly equal to that which would be produced had the material within the crack been mined out without deforming the solid a two dimensional crack filled with material of density equal to that of the host rock would produce no bouguer anomaly jachens et al 1983 have reported temporal changes in gravity elevation and area strain along the san andreas fault in southern california such that the bouguer anomaly apparently remains unchanged and the uplift to strain ratio is about 100 km several dislocation mechanisms are proposed that fulfill these constraints but these mechanisms appear to be rather contrived and are not regarded as satisfactory explanations

hazard assessment during caldera unrest at the campi flegrei italy a contribution from gravity height gradients hazard assessment and risk mitigation at restless calderas is only possible with adequate geophysical monitoring we show here how detailed long term micro gravity and deformation surveys may contribute to hazard assessment at the campi flegrei caldera cfc in italy by evaluating gravity height change g h gradients obtained during ground inflation and deflation between 1981 and 2001 such gradients provide a framework from which to assess the likelihood and type of volcanic eruptions our new analysis of unrest at the cfc allows us to separate noise during the gravity survey from the signal of deep seated magmatic processes this facilitates identification of the dynamics within the magma reservoir beneath the cfc we found that magma replenishment during rapid uplift between 1982 and 1984 was insufficient probably by one to two orders of magnitude to trigger an eruption similar to the 1538 monte nuovo eruption the most recent volcanic eruption within the cfc furthermore our interpretation of g h gradients for the ongoing period of deflation since 1984 suggests that eruptive volcanic activity is not imminent short periods of minor inflation associated with large gravity changes since 1981 are interpreted to reflect noise which to some degree is probably due to sub surface mass density changes within shallow hydrothermal systems beneath the cfc indicating no risk of eruptive volcanic activity we propose that monitoring g h gradients at restless calderas is essential as a caldera develops from a state of unrest to a state where volcanic eruptions have to be anticipated adoption of this method for the several tens of restless calderas world wide will provide early warning of changes in or increase of activity at these supervolcanoes

strain on the san andreas fault near palmdale california rapid aseismic change frequently repeated strain measurements near palmdale california during the period from 1971 through 1980 indicate that in addition to a uniform accumulation of right lateral shear strain engineering shear 0 35 microradian per year across the san andreas fault a 1 microstrain contraction perpendicular to the fault that accumulated gradually during the interval 1974 through 1978 was aseismically released between february and november 1979 subsequently november 1979 to march 1980 about half of the contraction was recovered this sequence of strain changes can be explained in terms of south southwestward migration of a slip event consisting of the south southwestward movement of the upper crust on a horizontal detachment surface at a depth of 10 to 30 kilometers the large strain change in 1979 corresponds to the passage of the slip event beneath the san andreas fault

viscoelastic displacement and gravity changes due to point magmatic intrusions in a gravitational layered solid earth summary we present a method for the computation of time dependent geodetic and geophysical signatures deformation potential and gravity changes due to magmatic intrusions in a layered viscoelastic gravitational medium this work is an extension of a deformation model previously developed to compute effects due to volcanic loading in an elastic gravitational layered media the model assumes a planar earth geometry useful for near field problems and consists of welded elastic and viscoelastic layers overlying a viscoelastic half space every layer can either be considered elastic or viscoelastic the intrusion treated as a point source can be located at any depth in any of the layers or in the half space several examples of theoretical computations for different media are also presented we have found that in line with previous results obtained by other authors introducing viscoelastic properties in all or part of the medium can extend the effects displacements gravity changes etc considerably and therefore lower pressure increases are required to model given observed effects the viscoelastic effects seem to depend mainly on the rheological properties of the layer zone where the intrusion is located rather than on the rheology of the whole medium we apply our model to the 1982 1984 uplift episode at campi flegrei modelling simultaneously the observed vertical displacement and gravity changes the results clearly show that for a correct interpretation of observed effects it is necessary to include the gravitational field in the anelastic theoretical models this factor can change the value and pattern of time dependent deformation as well as the gravity changes explaining cases of displacement without noticeable gravity changes or vice versa cases with uplift and incremental gravity values and other cases the combination of displacement and gravity changes is found to be especially effective in constraining the possible characteristics of the magmatic intrusion as well as the rheology of the medium surrounding it

gravity changes as a precursor to volcanic eruption at poas volcano costa rica monitoring of gravity changes at active volcanoes has considerable potential for understanding magma chamber dynamics and for detecting eruptive precursors the technique essentially aims to identify changes in sub surface mass or density corresponding to departures from the free air gradient fag or bouguer corrected terrain corrected fag bcfag respectively1 2 such gravity changes of the order of 10 100 gal have been reported previously through eruptive periods3 6 here we present a ten year data set measured at poas volcano costa rica in which gravity increases in 1987 1989 relative to both the fag and the bcfag preceded the ash cloud eruptions of april may 1989 these results constitute the first unambiguous detection of gravity changes as a precursor to eruption and we interpret them in terms of the intrusion of an ascending relatively dense magma body

14c dating and palaeoenvironment of the historic little ice age glacier advance of nigardsbreen southwest norway moss and grass remains associated with a well developed in situ palaeosol buried beneath a moraine ridge in front of nigardsbreen jostedalsbreen ice cap southern norway have been 14c dated pollen preserved with the plant remains suggests the existence of an agricultural landscape prior to the deposition of the moraine the calibrated dates and the pollen spectra are in close accord with historically documented evidence for the timing and palaeoenvironment of the little ice age advance of the glacier considerable potential is indicated for estimating the maximum ages of moraine ridges and for palaeoenvironmental reconstruction where such documentary evidence does not exist

time variations in the earth s gravity field detectable with geopotential research mission intersatellite tracking the geopotential research mission grm is a proposed system of two artificial satellites in tandem drag free polar orbits of low altitude 160 km designed for a detailed global mapping of the earth s gravitational and magnetic fields the mission should provide a large amount of new information about the gravity field particularly over continents resolving spatial variations in the gravity field as short as 100 km half wavelength however grm should also perform a much more difficult task the detection of time variations in the gravity field among the effects which produce changes in the field that are large enough to be detected by the intersatellite millimeter wavelength 1 s precision grm sensor are the following 1 rebound from ancient deglaciation 2 very large dip slip earthquakes 3 detailed ocean tides and 4 the seasonal growth and decline of continental glacial fields in particular many nonzonal as well as zonal components affected by postglacial rebound should be detected in a 6 month grm mission repeat passes over the fault zones of earthquakes similar to alaska 1964 chile 1960 or sumbawa 1977 should reveal signal changes due to their earth movements the effect of ocean tides on the grm signal should be readily detectable over a broad band of frequencies even if all but 10 of the tides are correctly modeled continued geophysical interest in these now largely unobserved phenomena warrants extension of the grm mission to somewhat higher altitudes

gravity anomaly from faulting on a layered spherical earth with application to central japan abstract the gravity anomaly resulting from mass redistribution from an earthquake source point dislocation is re explored in a spherical earth geometry using a previously developed method of determining the green s functions for the static displacement field on a layered earth i obtain the corresponding green s functions for the perturbed gravitational potential and gravity anomaly in a spherical harmonic expansion the synthesized gravity anomaly includes the effects of changes in the intrinsic density distribution due to divergence of the displacement field and vertical displacement along level surfaces in density stratification displacement numerical examples of the bouguer gravity anomaly produced on a layered crustal model show that the contribution of intrinsic density changes dominates over the contribution of stratification displacement application to central japan yields a new image of a buried thrust structure suspected of having ruptured in the 1891 nobi earthquake

gravitational gradient changes following the 2004 december 26 sumatra andaman earthquake inferred from grace summary it has been demonstrated that the gravity recovery and climate experiment grace spaceborne gravimetry data are capable of observing coseismic gravity changes resulting from great earthquakes such as the 2004 december 26 sumatra andaman event mw 9 1 9 3 here we show for the first time that refined deformation signals from the 2004 december 26 sumatra andaman earthquake mw 9 1 9 3 together with the 2005 march 28 nias earthquake mw 8 6 can be revealed by deriving the full gravitational gradient tensor from grace monthly gravitational field the grace inferred coseismic gravitational gradient changes agree well with coseismic slip model predictions since the high frequency contents in gravitational field variation can be amplified by deriving the gravitational gradients the grace derived coseismic gravitational gradient changes clearly delineate the fault lines locate significant slips and better define the extent of the coseismic deformation

models of crustal deformation this review is principally concerned with theoretical models of crustal deformation developed during this quadrennium observational data as well as topics in lithospheric flexure and isostasy are covered in companion reviews in this volume progress has been made toward constructing both new inelastic and inhomogeneous elastic models of earthquake induced crustal deformation most of the inelastic models involve dislocations or green s functions in an elastic or anelastic lithosphere which overlies a maxwell viscoelastic asthenosphere

an approach to modeling present day deformation in southern california this paper presents an approach to modeling the complete time dependent deformation in southern california we make use of a variety of techniques to include the far field plate motion stress relaxation in the asthenosphere aseismic fault slip within the elastic lithosphere and the complex three dimensional nature of the faults within the southern california region among the conclusions of the work here are the following 1 the data exhibit a marginal preference for a relatively thin lithosphere in southern california 2 the most sensitive determinant for lithospheric thickness and viscosity of the asthenosphere is the rotation rate of the motion vectors toward or away from major faults 3 the major faults in southern california are slipping at depth at or near their average holocene rates 4 a shallow active decollement beneath the transverse ranges is apparently needed to realistically model the strain changes observed there and 5 this modeling technique offers the best approach for understanding the complex deformation field in southern california

surface pollen deposition on glacier forelands in southern norway ii spatial patterns across the jotunheimen jostedalsbreen region this paper presents the results of a study of surface pollen deposition on 26 glacier forelands in the jostedalsbreen jotunheimen region of southern norway numerical techniques including two way indicator species analysis twinspan detrended correspondence analysis dca and canonical correspondence analysis cca are used to identify distinct plant assemblages to examine the pollen vegetation relationship and to distinguish broad trends in the data the source of pollen is fundamental the majority of samples especially those from sparsely vegetated sites are dominated by arboreal pollen up to 90 of the total land pollen tlp sum most with a long distance source however indicator taxa notably salix and empetrum although present at low frequencies can hold the key to the true nature of the local vegetation indicator taxa produce strong correlations between their presence in the vegetation and representation in the pollen spectra at times essential to distinguish plant communities mult

joint inversion of observed gravity and gps baseline changes for the detection of the active fault segment at the red river fault zone summary methods for the determination of active fault segments by inversion of temporal gravity and gps global positioning system baseline changes are presented in this paper a numerical test is performed for estimating four types of fault movement models from simulated gravity changes with random noise results of the numerical test show clearly the possibility of detecting fault movement from observed gravity changes based on this work active segments of the red river fault zone rrfz are investigated by inversion of observed gravity changes from 1985 november to 1988 november and gps baseline changes from 1988 august to 1991 october both separately and jointly the results from the joint inversion indicate an active segment of the red river fault characterized by right lateral strike slip 1 0 0 2 m with a length of 232 3 km a width of 6 3 km a dip angle of 100 5 80 5 ne a depth of 10 1 km and a strike direction of 169 2 se these characteristics of the detected active segment at the rrfz agree with those of geological investigations and seismic activity in the area estimated horizontal displacements at the rrfz indicate that drastic movements are concentrated on the northern segment whereas the southern segment of the fault is relatively stable the estimated fault slip rate along strike at the red river fault is 8 11 mm yr 1 larger than the geological estimate of 6 mm yr 1 which implies that the rrfz is presently active the results obtained in this research support previous interpretations that the fault movements at the rrfz are mainly a result of the collision of the indian and the eurasian plates

absolute gravity measurements in california 1984 1989 repeated absolute gravity measurements have been made at 12 sites in california between 1984 3 and 1989 7 as determined in laboratory tests the instrument used has an estimated accuracy of 10 gal approximately 10 8 g the repeatability of the measurements is consistent with this accuracy assessment no gravity changes above the limits set by instrumental uncertainty and environmental noise are observed in california during this period the field observations provide upper limits on the rates of secular gravity changes which could be attributed to crustal deformation with a resolution corresponding to vertical displacement rates of 1 2 cm yr

inability of stratospheric sulfate aerosol injections to preserve the west antarctic ice sheet injection of sulfate aerosols into the stratosphere has the potential to reduce the climate impacts of global warming including sea level rise slr however changes in atmospheric and oceanic circulation that can significantly influence the rate of basal melting of antarctic marine ice shelves and the associated slr have not previously been considered here we use a fully coupled global climate model to investigate whether rapidly increasing stratospheric sulfate aerosol concentrations after a period of global warming could preserve antarctic ice sheets by cooling subsurface ocean temperatures we contrast this climate engineering method with an alternative strategy in which all greenhouse gases ghg are returned to preindustrial levels we find that the rapid addition of a stratospheric aerosol layer does not effectively counteract surface and upper level atmospheric circulation changes caused by increasing ghgs resulting in continued upwelling of warm water in proximity of ice shelves especially in the vicinity of the already unstable pine island glacier in west antarctica by contrast removal of ghgs restores the circulation yielding relatively cooler subsurface ocean temperatures to better preserve west antarctica

surface pollen deposition on glacier forelands in southern norway i local patterns of representation and source area at storbreen jotunheimen the relationship between vegetation and surface pollen deposition is examined at storbreen glacier foreland where a clear plant succession exists the aim is to determine whether the distinct plant communities present produce characteristic pollen assemblages the influence of environmental factors is also considered pollen assemblages from moss polsters collected from 22 paired sampling sites across the foreland are compared with local vegetation two way indicator species analysis and detrended correspondence analysis are employed to identify clusters and sequences initially in the vegetation data and subsequently in the pollen data sets vegetation and pollen data are compared simultaneously using canonical correspondence analysis three main plant communities are distinguished pioneer heath and snowbed broadly each community produces characteristic pollen assemblages boundaries between groups are not clear cut reflecting the mosaic of plant communities present recognition of distinct plant communities is hampered by the prevalence of long distance arboreal pollen and poor representation of entomophillously pollinated taxa late in the succession up to 78 of pollen could originate locally use of the non arboreal pollen sum significantly improves correspondence with vegetation the importance of indicator taxa is considered and both salix and empetrum are found to distinguish successfully early phases of succession from later phases strong correlations exist between the primary ordination axes of vegetation and pollen and with terrain age and altitude for example the correlation between altitude and total land pollen axis 1 is r 0 76 the surface data add new information to the interpretation of tree colonization in the area during the holocene

dynamics of magma storage in the summit reservoir of kilauea volcano hawaii an eruption of kilauea volcano hawaii normally results in a decrease in the internal pressure of a summit magma reservoir as it partly drains the interior of the volcano responses to the decrease in pressure 1 by elastic expansion of remaining magma and gas 2 additional exsolution of gas dissolved in the magma and 3 by contraction of the volcanic edifice this process is modeled here using gravity geodetic and observational data for recent events at kilauea during magma removal the ratio of pressure change in the magma reservoir to the volume of associated summit subsidence ranges from less than 0 22 pa m3 to more than 0 43 pa m3 lower ratios occur during large subsidences because of a large effective volume of the reservoir small events may involve source volumes of only 2 km3 in contrast a volume of near 13 km3 is associated with larger events because the boundary between plastic and elastic behavior may migrate outward with increased time and strain the aggregate compressibility of kilauea s reservoir is significantly increased by volumetric changes of the co2 gas phase by bulk compression and solution with pressure this effect is more pronounced for relatively shallow sources because the bulk modulus of gas is proportional to pressure the average reservoir co2 weight fraction may be near 0 0005 to 0 0009 co2 appears to accumulate in the upper portion of the reservoir where the concentration reaches 0 0030 weight fraction gradual long term subsidences are different from the short term deflations associated with specific eruptions during long term subsidence no net change in mass or pressure occurs within the summit reservoir rifting of the summit area which may tend to shorten the reservoir as it widens or slow degassing and contraction of stored magma may cause long term subsidence

current deformation and the width of the seismogenic zone of the northern cascadia subduction thrust evidence has been obtained for the accumulation of elastic strain across the northern cascadia subduction zone that may be released in a future very large subduction thrust earthquake vertical and horizontal strain rates across the southern vancouver island region have been determined through 1 long term trends in tide gauge data 2 changes in repeated accurate leveling surveys 3 changes in repeated high accuracy gravity profiles and 4 horizontal shortening observed in repeated precise positioning surveys the outer coast is uplifting at a rate of a few millimeters per year decreasing landward and shortening is occurring across the 100 km wide coastal region at a rate of about 0 1 microstrain per year mm km 1yr 1 the results are compared with the distribution of strain accumulation predicted from elastic dislocation and viscoelastic models for a subduction thrust fault the location of the fault as used in the models is well defined by multichannel seismic reflection and other geophysical data most of the observed current deformation can be explained by interseismic strain accumulation associated with the subduction thrust of southern vancouver island and northern washington provided the locked portion is restricted to a 60 km wide band offshore beneath the continental shelf and slope this conclusion also results from modeling the coseismic subsidence on the outer coast of vancouver island about 300 years ago deduced from paleoseismicity data the unusually narrow downdip extent of the subduction thrust seismogenic zone that extends little if at all beneath the coast is a consequence of high temperatures associated with the young age of the subducted oceanic lithosphere and the thick blanket of insulating sediments the high temperatures limit brittle seismogenic behavior downdip to where the thrust fault is at a depth of less than 15 km the distance from the seismic portion of the megathrust limits the estimated ground motion at the major centers of vancouver and victoria from this source the narrow width may also limit the earthquake size however events of magnitude well over 8 are possible

constraints on magma chamber geometry at sierra negra volcano galapagos islands based on insar observations abstract we investigate the problem of estimating magma chamber geometry using insar observations of sierra negra volcano galapagos ascending and descending interferograms are combined to determine vertical and one horizontal component of displacement the ratio of maximum horizontal to vertical displacement suggests a sill like source spherical or stock like bodies are inconsistent with the data we estimate the geometry of the sill assuming a horizontal uniformly pressurized crack with unknown periphery and depth the sill is discretized into small elements that are either open and are subjected to the pressure boundary condition or remain closed we find the best fitting sill to be located beneath sierra negra s inner caldera at a depth of about 2 km using boundary element calculations we show that any magma chamber with a flat top coincident with the sill model fits the data equally well the data are insensitive to the sides and bottom of the magma chamber

deformation induced gravity changes in volcanic regions summary we analyse a new model to estimate the change in gravity and in elevation caused by volcanic eruption or inflation in the new model a permeable porous zone surrounds a spherical magma chamber magma may enter or leave the magma chamber and independently the permeable zone this arrangement simulates the flow processes inferred from field studies and provides a degree of freedom not available in the classic model proposed by mogi and used by many others since then we find in agreement with field data and previous analyses of the mogi model that the change in edifice volume is smaller than the volume of magma which produces it the local change in gravity is found to be proportional to the local elevation change the constant of proportionality being positive or negative depending on whether magma flow is directed into or out of the chamber or infiltrated zone

results from new gps and gravity monitoring networks at fernandina and sierra negra volcanoes galapagos 2000 2002 gps and gravity networks were established on sierra negra and fernandina volcanoes in 2000 and remeasured in 2001 and 2002 after a decade in which the caldera inflated by over 2 5 m sierra negra s caldera began to subside between 2000 and 2002 at a rate of about 9 cm a neither inflation nor deflation was accompanied by eruption simple elastic modeling indicates that the deformation at sierra negra can be attributed to volumetric contraction of a 2 1 km deep sill beneath the caldera caused by either movement of magma out of the sill or loss of bubbles through degassing precise gravity measurements support but do not prove the former mechanism fernandina volcano is in a phase of slow refilling of the summit reservoir after the 1995 eruption causing lateral expansion of the caldera rim by about 3 cm a our estimate for the depth of the shallowest part of the fernandina magma chamber is 1 to 2 km neither volcano shows evidence of flank spreading

a model of pollen source area for an entire lake surface a model of pollen deposition on the surface of an entire basin is developed to estimate pollen source area and results are compared with those for a point at the center of a basin i c prentice 1985 quaternary research 23 76 86 1988 vegetation history pp 17 42 kluwer academic this model is more appropriate for approximating the source area of pollen in lake sediment since mixing in lake water and focusing of sediment redistribute pollen originally deposited over the entire surface in general the pollen source radius for the entire basin surface is 10 30 smaller than the source radius for a point at the center the difference in the source radius is more profound for heavier pollen types such as spruce and sugar maple than for lighter types such as oak and ragweed the average pollen input to the entire surface is more strongly influenced by nearby pollen sources than pollen deposition at the center the pollen record from a lake may therefore provide different spatial resolution than the record from a bog of similar radius

contemporary plate motion and crustal deformation the measurement of relative plate velocities during the past few years is a signal accomplishment in earth science leading to refinement of the precepts of steady motion of plate interiors and cyclic deformation along plate margins regrettably deformation premonitory to an earthquake has yet to be detected with confidence delineation of the spatial and temporal buildup of strain between earthquakes however has put limits on models of the earthquake cycle a diverse set of fault structures and crustal rheologic conditions can explain the pattern of surface strain accumulation and release along strike slip faults in contrast the geometry of thrusts and normal faults revealed by earthquake deformation has been found to differ markedly from expectations geodetic observations have proved vital to monitor the ascent of magma through the earth s crust and to predict volcanic eruptions at the earth s surface episodic vertical and horizontal deformation in southern california remains a subject of dispute if anything it is less certain than it once seemed