earthquakes faulting and stress in the los angeles basin since 1920 fourteen moderate sized m l 4 9 6 4 earthquakes have been reported in the los angeles basin these events are associated with both mappable surficial faults and concealed faults beneath the basin sediments to determine the style of faulting and state of stress in the basin single event focal mechanisms for 244 earthquakes of m 2 5 that have occurred during 1977 1989 have been calculated fifty nine percent of the events are strike slip and are mostly located near two of the major northwest striking right lateral strike slip faults in the basin the newport inglewood fault and the palos verdes fault the 1988 pasadena and the 1988 upland earthquakes showed left lateral strike slip on northeast striking faults numerous small earthquakes in the eastern part of the basin show left lateral strike slip faulting and form a northeast trend near yorba linda thirty two percent of the events have reverse mechanisms and are distributed along two broad zones the first the elysian park fold and thrust belt coincides with anticlines along the eastern and northern flank of the los angeles basin extending into santa monica bay the second the torrance wilmington fold and thrust belt coincides with anticlines mapped on the southwest flank of the basin and extends from offshore newport beach to the northwest into santa monica bay oblique faulting that could be inferred by the merging of strike slip and compressional tectonics does not occur in the basin instead the coexistence of zones of thrusting and large strike slip faults in the basin suggests that the thrust and strike slip movements are mostly decoupled a few normal faulting mechanisms appear to be related to faulting orthogonal to the axes of plunging anticlines the trend of the maximum horizontal stress varies from n1 w to n31 e across the basin and consistently forms high angles with the fold axes the stress field that exists along the flanks of the basin has a vertical minimum stress axis this stress field and ongoing folding and thrusting suggest that tectonic deformation is concentrated along the flanks of the deep central basin today the deformation of the basin consists of uplift and crustal thickening and lateral block movement to accommodate the north south compression across the basin

possible resonance effect in the distribution of earthquake damage in mexico city the destruction caused by the earthquake of 19 september 1985 in mexico city shows three remarkable features1 first a concentration of damage in the former lake bed second a peculiar distribution of high and low damage areas alternating within a few city blocks and third a selectivity for buildings between five and fifteen storeys high while the last point is understood by civil engineers2 and the first confirms the relevance of the soft ground the second feature is the most puzzling to a physicist such a pattern evokes the idea of a standing wave with low and high damage areas corresponding to nodes and antinodes this suggests a resonance phenomenon of the waterlogged ground moving collectively from the navier equations of classical elasticity it is known3 that a transfer of the incoming seismic wave energy to longitudinal waves takes place if a sharp change from hard to soft ground exists solving then the two dimensional poisson equation for the longitudinal waves within the ancient lake boundaries we show that for the observed seismic wave period of 2 s a standing wave solution exists which explains qualitatively the damage distribution pattern

elastic finite difference modeling of the 1971 san fernando california earthquake finite difference seismograms calculated for the 1971 san fernando earthquake show strong effects due to lateral variation in sediment thickness in the san fernando valley and the los angeles basin using basin structure derived mostly from well logs and teleseismically determined source parameters two dimensional sh and p sv finite difference calculations reproduce the amplitude and duration of the strong motion velocities recorded across the los angeles and san fernando basins for the period range of 2 to 10 sec the edges of basins nearest the seismic source show ground motion amplification up to a factor of 3 over the case without the basin and tend to convert direct shear waves into love and rayleigh waves that travel within the basins

three dimensional simulations of ground motions in the san bernardino valley california for hypothetical earthquakes on the san andreas fault three dimensional finite difference simulations of elastic waves in the san bernardino valley were performed for two hypothetical earthquakes on the san andreas fault a point source with moment magnitude m5 and an extended rupture with m6 5 a method is presented for incorporating a source with arbitrary focal mechanism in the grid synthetics from the 3 d simulations are compared with those derived from 2 d vertical cross section and 1 d flat layered models the synthetic seismograms from the 3 d and 2 d simulations exhibit large surface waves produced by conversion of incident s waves at the edge of the basin seismograms from the flat layered model do not contain these converted surface waves and underestimate the duration of shaking the seismograms from the 3 d simulations have larger amplitude coda than do the seismograms from the 2 d case because of the presence of off azimuth surface wave arrivals in the 3 d simulations that are not included in the 2 d simulations snapshots of the wavefield of the 3 d simulation show that these off azimuth arrivals represent surface waves reflected from the edges of the basin the anelastic attenuation of the sediments is a key parameter controlling the overall duration of motion some of the coda energy at rock sites near the basin edges represents leakage of surface wave energy out of the basin for the m6 5 earthquake simulation the largest ground velocities occur where surface waves reflected from the edge of the basin interfere constructively with the trapped waves that follow the direct s wave maps of maximum ground velocity are produced for two directions of rupture propagation the largest velocities occur in localized portions of the basin the location of the largest velocities changes with the rupture propagation direction contours of maximum shaking are also dependent on asperity positions and radiation pattern

array analysis of the ground velocities and accelerations from the 1971 san fernando california earthquake profiles of ground velocity and acceleration displayed as a function of epicentral distance are analyzed for recordings of the 1971 san fernando earthquake three long profiles 50 km and three short profiles 2 km are studied although there is considerable variation in waveforms and peak amplitudes observed along the long profiles there are also many examples of coherent phases seen on adjacent stations there are striking differences in the amplitudes and durations of ground velocity observed at stations located on hard rock sites as opposed to stations located within the large sedimentary basins of the los angeles area furthermore the san fernando basin which is adjacent to the source area seems to respond quite differently from the los angeles basin which is about 30 km from the earthquake source area ground acceleration profiles however show that there is no corresponding change in the duration or amplitude of high frequency shaking with site characteristics we infer that the excitation of surface waves within sedimentary basins is the reason that large peak velocities and displacements are observed for soft sites the ground velocity waveforms are nearly identical along the three short profiles which are all located within the los angeles basin greater variation of waveforms and amplitudes are seen for ground acceleration along these short profiles although strong phase coherence is still observed

pollen analysis of postglacial sediments from eildun lake district of mackenzie n w t canada sediment from eildun lake located at 63 8 6 n 122 46 5 w in the northern boreal forest was cored to a depth of 268 cm and analyzed for pollen content pollen percentages and 14c dates suggest a

propagation and resonance of sh waves in the salt lake valley utah abstract the salt lake valley is an alluvium valley where seismic amplification measured at some sites can exceed a factor of 10 relative to hard rock sites adjacent to the basin it is important to understand the causes for these amplifications in order to predict which sites in the salt lake valley are most prone to strong ground shaking towards this end finite difference synthetic seismograms were used to successfully characterize key parameters controlling low frequency sh wave amplification in the salt lake valley plane wave source synthetic seismograms reveal that sh wave amplification in the frequency band 0 2 to 2 0 hz is primarily controlled by the two shallowest lithologic units the 0 25 to 1 0 km thick unconsolidated alluvial quaternary sediments v s 0 98 km sec and the underlying 0 5 to 1 0 km thick semi consolidated tertiary sediments v s 1 42 km sec the quaternary sediments tend to increase the coda duration while the semi consolidated tertiary sediments act as a wave guide to trap the low frequency seismic energy it is also found that the basin can cast a seismic shadow over adjacent bedrock sites inhibiting seismic wave propagation to sites adjacent to the basin calibration of valley site amplification to these bedrock sites may result in overestimation of basin site amplification finite difference synthetic seismogram modeling can clearly demonstrate the significant effects of deep basin lithology and geometry on low frequency site amplification observed across the salt lake valley

three dimensional finite difference modeling of the san andreas fault source parameterization and ground motion levels olsen et al 1995 recently simulated an mw 7 75 earthquake on the san andreas fault predicting long period t 2 5 sec ground velocities of 140 cm sec in the los angeles basin about 60 km from the fault these motions are much larger than estimates derived from empirical relations or other numerical simulations standard area magnitude relations predict that the 170 16 km fault used in the simulations would produce an mw 7 5 earthquake giving a moment of 2 0 1027 dyne cm which is 2 4 times smaller than the moment used by olsen et al 1995 further self similar scaling predicts a rise time of 3 sec for an mw 7 75 event and 2 2 sec for an mw 7 5 event the filtered impulse slip function used by olsen et al 1995 has an effective rise time of 1 6 sec yielding a response that is about 2 times larger than expected for periods less than 5 sec this combination of high seismic moment and short rise time along with the use of a uniform slip distribution leads to the extreme ground motion levels predicted by olsen et al 1995 to quantify the sensitivity of the long period ground motion response to source parameterization we have performed 3d finite difference fd simulations using various combinations of seismic moment source rise time and slip heterogeneity these calculations incorporate the same grid dimensions fault size and bandwidth employed by olsen et al 1995 with a moment of 2 0 1027 dyne cm a rise time of 2 sec and a smoothly heterogeneous slip distribution we simulate peak long period ground velocities of 155 cm sec in the near fault region and 40 cm sec in the los angeles basin these values are much closer to although still higher than empirical predictions a uniform slip distribution produces the largest peak motions both in the near fault region and in the los angeles basin whereas a rough asperity slip distribution noticeably reduces the maximum near fault ground velocities our results indicate that the accurate simulation of long period ground motions requires a realistic source parameterization including appropriate choices of seismic moment and rise time as well as the use of spatial and temporal variations in slip distribution

millimeter scale microrelief affecting runoff yield over microbiotic crust in the negev desert abstract the negative effect of microrelief upon runoff yield explained by the increase in water storage and the slowdown in flow velocity is a known phenomenon however research concerning surface roughness hitherto focused on decimeter or centimeter scale microrelief owing to the heterogeneous character of many natural plots the effect of centimeter scale microrelief was mainly shown under controlled sprinkling experiments whereas no data was yet made available concerning the possible effect of a millimeter scale microrelief upon runoff yield under natural field conditions when runoff yield was measured over crusted i e surfaces covered by 1 3 mm of cyanobacterial crust and scalped 2 6 m 2 plots at the hallamish dune field negev desert israel during 5 winter seasons 1990 1995 no runoff was generated from the scalped plots during the first year following scalping while meagre quantities were received during the second year nevertheless during the third season and onward runoff yield at the scalped plots was consistently and significantly higher than that of the crusted control plots and this was so albeit the fact that the crust chlorophyll content of the scalped plots during the third season was as low as half the amount of the crusted plots these findings appeared to contradict previous findings that showed the existence of a positive linear relationship between the crust chlorophyll content and runoff yield the apparent contradiction was explained by the differences in the plot microrelief indicating that minute differences even of a millimeter scale may greatly affect runoff yield and thus may compensate for lower crust chlorophyll content surface smoothness which characterizes many of the arid zones may thus explain at least partially the high runoff yield obtained in deserts

calculation of synthetic seismograms in a laterally varying medium by the boundary element discrete wavenumber method abstract we investigate the propagation of lg waves in laterally varying crustal structures by numerical simulation the method of calculation is formulated in terms of boundary integral equations where the green9s functions are evaluated by wavenumber summation the approach is well suited to study the propagation of seismic waves in a layered medium where the interfaces are flat in some regions and irregular in others we investigate the effect of a crustal fault with vertical offset and study the case of a lateral change in crustal thickness the results show that the lg wave amplitude is only slightly affected by the presence of these heterogeneities they confirm the robustness of lg wave propagation in presence of lateral heterogeneities observed in other numerical simulations they show that large scale geometric features of the crust cannot account alone for the strong attenuation of lg waves observed in many regions the results also suggest a possible relation between the level of lg wave coda and the degree of roughness of the moho they further indicate the importance of backscattering and suggest a possible use of the backscattered wave field to map strong crustal heterogeneities

postglacial vegetation history of the mackenzie river basin abstract three radiocarbon dated cores from small lakes provide pollen records of postglacial vegetation change in the central mackenzie river basin these data are combined with previously published records to reconstruct vegetation development from central alberta to the mackenzie delta the paleoecological data demonstrate that a continuous corridor of herb and shrub dominated vegetation extended between beringia and the great plains of north america from deglaciaiton until about 10 000 yr b p the initial vegetation was replaced by spruce forests between 10 000 and 8500 yr b p significant vegetation change continued until approximately 500 yr b p when the muskeg dominated landscape typical of the present subarctic was established the initial herb and shrub vegetation was likely a reprise of conditions which existed during earlier pleistocene deglaciations these conditions may have provided a favorable environment for animals adapted to nonforested vegetation and promoted migration southward from beringia and northward from the great plains during the last deglaciation and earlier pleistocene deglaciations the sequence of postglacial vegetation change in the mackenzie basin likely reflects a combination of climatic change edaphic change and plant migration patterns and interactions

microbiotic crust control of runoff generation on sand dunes in the negev desert 1 the general belief that dune sands in arid regions are not likely to generate runoff and that runoff necessitates wet conditions is examined runoff generation was measured within an arid dune field in the western negev desert israel during 1990 1994 on 16 plots 1 2 6 6 m2 constructed along a continuum of crust covered crust the relationships between chlorophyll a and carbohydrates of the microbiotic crust and runoff yield were studied whereas chlorophyll a 15 45 mg m 2 and carbohydrates 4 9 17 6 g m 2 of the cyanobacterial crusts showed positive linear relationships with runoff coefficients a significant reduction in runoff yield was obtained once the moss dominated crust with 53 54 5 mg m 2 chlorophyll a and 28 5 28 9 g m 2 of carbohydrates which inhabited the wettest habitat within the dune field was also included in the analysis contrary to previous assumptions that an increase in runoff generation occurs on arid and semiarid sand dunes in wet climates wet climates may lead to the establishment of moss dominated crusts that in turn decrease runoff yield

seismic response of shallow alluvial valleys the use of simplified models a method is proposed to approximately compute the seismic response of shallow three dimensional 3d alluvial valleys it is based on the exact solution for a simple configuration namely the antiplane response of a rectangular deposit with a moving rigid base in such a solution which is used as a framework for our approximations the spectral signature of locally generated surface waves is explicit thus an approximate expression is constructed to separately account for one dimensional 1d response and lateral effects results are presented for two and three dimensional cases comparisons are provided with rigorous solutions as well our formulas allow for fast computations

dynamic processes acting on a longitudinal seif sand dune abstract field measurements were made on a longitudinal dune in the sinai desert in order to understand its morphology and dynamics the field measurements contradicted the wind structure indicated by the helicoidal flow theory rather it was found that winds coming from two basically different directions at different times and striking the dune obliquely were responsible for sand transport and erosion or deposition along the lee flank the essence of this mechanism is the deflection of the wind airflow on the lee flank of the dune to a direction parallel to the crest line the occurrence of erosion or deposition depends upon the angle of incidence between the wind and the crest line when this angle is 40 the velocity of the deflected wind drops and deposition takes place on the lee flank the angle of incidence in each wind storm is changed intermittently between 30 and 100 along the dune because the dune meanders and because of the sinuous outline of the crest line in this manner sand transport and erosion or deposition occurs along the lee flank depending on the angle of incidence between the wind and the crest line as a result of the deflection of the wind the dune elongates at an average rate of more than 1 m per month peaks and saddles along the crest line advance at an average rate of 0 7 m per month the lack of uniformity in the effects of the wind on both sides of the dune creates a lack of uniformity in the rate of erosion and deposition this can explain the formation of peaks along the crest line of the dune

the limited role of localized convective storms in runoff production in the western negev desert abstract high scale floods in the negev desert are usually linked to rains originating from the red sea trough rst characterized by high intensity rainspells while the effect of the rst storms was assessed in the southern central and eastern parts of the negev almost no information exists as to their possible impact in the western parts of the negev rain characteristics and its role in runoff production were studied during the years 1989 1994 in the nizzana research site in the western negev desert only 7 out of 49 rainstorms 1 mm originated from the rst and only two of these seven storms produced runoff while the rst induced runoff accounted for only 3 9 of the total runoff in nizzana it accounted for 83 5 100 of the total runoff of four major ephemeral streams in the eastern part of the negev it is suggested that the limited role of the rst in the western negev and its major role in flood formation in the eastern negev may contribute to stream capture of formerly westward flowing streams eastward into the dead sea a positive feedback mechanism is thus suggested whereas the topography may enhance atmospheric instability and hence high convective rainstorms the resultant floods may enhance rift widening which in turn will foster atmospheric instability

holocene migration of lodgepole pine pinus contorta var latifolia in southern yukon canada a palynological reconstruction n 25 profiles suggests that the northern extent of lodgepole pine pinus contorta var latifolia engelmann ex s watson occurred between 59 and 60 n latitude in

microbiotic crusts in the high equatorial andes and their influence on paramo soils although microbiotic crusts are known from many lowland deserts they have apparently never been reported from alpine areas this paper describes microbiotic crusts from an equatorial high andean location paramo de piedras blancas in venezuela the main objectives are to describe the basic characteristics of the crusts to examine the properties of the soils on which they occur and to discuss their possible geomorphic significance for paramo soils properties of crusts and of the soils beneath them were compared with those of adjacent bare soils at three sites between 4360 and 4550 m altitude measurements on unconfined soil compressive strength and infiltration rates were taken in the field crust specimens were also gathered and later examined and sectioned under a microscope to ascertain micromorphological details laboratory data included soil texture organic matter content color ph water storage at field capacity and stability of soil aggregates crusts formed discontinuous garland like patches parallel to contours or covered small round soil buds produced by frost sorting microbiotic crusts contained many plants including an unclassified hepatic genus marsupella dumort a moss grimmia longirostris hook and several lichen species crusts consisted of a thin organic layer 5 35 mm overlying mineral soil and showed three parallel bands with diffuse boundaries the outer layer had green reddish brown live plant shoots and stems with few fine mineral grains the middle layer had anatomically recognizable dark brown plant material and a greater content of mineral grains the light colored basal layer had little organic matter mixed with coarse mineral particles it also contained many small orthovughs vesicles and some larger arched chambers caused mainly by frost activity soils with a microbiotic crust were darker than bare soils organic matter content in crust areas was two to three times higher than in adjacent bare soil ph dropped 0 3 units below crusts soils with crust had finer texture than bare ground the content of fines in crusts was 1 5 times higher than in bare soils but gravel in these was four times higher than below cryptogams indices of particle concentration showed that grains 0 5 1 mm were more abundant in crusts finer soils in crusts may result from a combination of a prior random texture variation b cryptogam colonization of fine soil patches sorted by frost c interception and capture of aeolian dust by cryptogams especially by mosses and d reduction of fine particle erosion by crusts as coarse grains are lost mean particle size within crusts drops geomorphic effects of crusts were also significant a infiltration rates were 70 greater in crust than in noncrust soils 67 0 vs 40 4 mm min b crust soils stored more water at field capacity 48 7 88 4 than bare areas 20 5 42 c crust soils had a higher resistance to raindrop erosion than bare areas 47 5 drops 0 1 g vs 10 2 drops 0 1 g undisturbed crust specimens were even more effective in resisting drop impact surviving up to 1100 drops 0 1 g d cryptogams increased soil compressibility more than 200 from 54 g cm2 in bare soils to 118 1 g cm2 in crust areas all these geomorphic changes should result in lower rates of soil erosion in paramo areas with dense crusts

runoff generation from successive simulated rainfalls on a rocky semi arid mediterranean hillslope runoff generation in a mediterranean carbonate rock environment the judaean mountains west bank and israel was studied by a 2 day sprinkling experiment on an 18 x 10 m 2 plot during the first day three spells of 24 1 mm h 1 12 3 mm 27 8 mm h 1 12 3 mm and 14 4 mm h 1 12 5 mm were applied almost saturating the runoff plot on the second day 20 h later two spells of 18 7 mm h 1 34 3 mm and 35 2 mm h 1 25 4 mm followed surface runoff was a combination of infiltration excess runoff from rocky portions of the plot and saturation excess runoff from areas covered by soil soil saturation was accelerated by lateral runoff from adjacent rocky areas once the plot was saturated 80 90 of the applied rainfall became surface runoff significantly different concentrations of chloride sulphate and nitrate ions in the sprinkled water on two successive days served as tracers during the second day a two component hydrograph separation pointed to the importance of shallow reservoirs playing active parts in runoff generation about 14 of the flow collected during the second day originated from water applied during the first day during the second day of sprinkling both water sources obviously mixed in saturated soil reservoirs and contributed in variable percentages to surface flow it is hypothesized that at least during high magnitude rainfall the steep rocky slopes investigated might be regarded as flood generating zones rather than areas of pronounced recharge into the underlying yarkon taninim aquifer

significance of interdune deposits and bounding surfaces in aeolian dune sands bounding surfaces and interdune deposits provide keys for detailed interpretations of the development shape type wavelength and angle of climb of aeolian bedforms as well as overall sand sea conditions current alternate interpretations of bounding surfaces require very different but testable models for sand sea deposition two perpendicular traverses of jurassic entrada sandstone utah reveal relations among cross strata first order bounding surfaces and horizontal strata these field relations seem explicable only as the deposits of downwind migrating climbing enclosed interdune basins horizontal strata and dune bodies consisting of superimposed smaller crescentic dunes cross stratified deposits a 1 7 km traverse parallel to the palaeowind direction provides a time transgressive view showing continuous cosets of cross strata first order bounding surfaces and interdune deposits climbing downwind at an angle of a few tenths of a degree changes occur in the angle of climb cross strata structure and interdune deposits these reflect changes in depositional conditions through time a 1 5 km traverse perpendicular to the palaeowind direction provides a view at an instant in geological time showing first order bounding surfaces and interdune deposits forming flat laterally discontinuous lenticular bodies the distribution of interdune sedimentary structures in this traverse is very similar to that of some modern interdune basins such as those on padre island texas hierarchies of bounding surfaces in an aeolian deposit reflect the bedform development on an erg the presence of three orders of bounding surfaces indicates dune bodies consisting of smaller super imposed dunes the geometry of first order bounding surfaces is a reflection of the shape of the inter dune basins second order bounding surfaces originate by the migration of the superimposed dunes over the larger dune body and reflect individual dune shape and type third order bounding surfaces are reactivation surfaces showing stages in the advance of individual dunes the presence of only two orders of bounding surfaces indicates simple dunes modern and entrada interdune deposits show a wide variety of sediment types and structures reflecting deposition under wet damp and dry conditions interdune deposits are probably the best indicators of overall erg conditions and commonly show complex vertical sequences reflecting changes in specific depositional conditions

near source attenuation of peak horizontal acceleration strong motion data recorded within 50 km of the rupture zone were used to study near source attenuation characteristics of horizontal peak ground acceleration for worldwide earthquakes of magnitudes 5 0 to 7 7 the data base consisted of 229 horizontal components of peak acceleration recorded from 27 earthquakes including the 15 october 1979 imperial valley earthquake these data were found to be adequately represented by the functional relationship pga a exp b m r c m d where pga represents the mean of the peak values scaled from the two horizontal components of each recording m is richter magnitude and r is distance from the fault rupture zone peak acceleration was found to be lognormally distributed with a standard error representing a 45 per cent increase in the median estimate the regression analysis statistically confirmed the results of earthquake simulation studies that have predicted peak acceleration to become independent of magnitude and distance in the near field an extensive sensitivity study showed that predictions based on our attenuation relationships are stable with respect to reasonable model and parameter variations an analysis of residuals was used to investigate the behavior of peak acceleration with respect to various earthquake site and recording parameters the more significant findings being 1 a similarity in the level of acceleration recorded on soil or rock 2 larger than average accelerations recorded at sites located on shallow soils or in areas of steep topography 3 larger than average accelerations associated with earthquakes having reverse fault mechanisms and 4 lower than average accelerations recorded in large embedded structures

spatial distribution of soil surface coverage on north and south facing hillslopes along a mediterranean to extreme arid climatic gradient four research sites were established along a climatic gradient in israel representing mediterranean xeric mediterranean arid and extreme arid climate conditions the cover percentage of eight soil surface components vegetation annual and perennial forms biogenic crust mosses lichens cyanobacteria and dwarf grass stones and bare soil patches were measured transects along hillslopes on each of two opposite aspects south and north facing slopes were established at each site altogether eight transects dominance and soil surface cover diversity indices and similarity coefficients between the transects were calculated high similarity values in soil surface cover were found between the south facing slopes located in the xeric mediterranean arid and extreme arid areas these hillslopes are dominated by stones and bare soil patches in contrast lower similarity values were found between the northern slope aspects dominated by biotic components vegetation and biogenic crusts the study along a climatic gradient together with the comparison between northern and southern aspects provide useful information that can be utilized for evaluation of changes in soil surface cover characteristics given various scenarios of future climate regimes in that region

locally modified surface winds on linear dunes as derived from directional raingauges the study deals with local airflow patterns induced by 12 16 m high partly vegetated dunes under various prevailing wind conditions detailed empirical data were derived by means of an unconventional use of directional raingauges the utility and accuracy of the method have been previously tested in a combined empirical and numerical study over ridges on a similar scale results are presented on the nature extent and intensity of locally modified patterns of surface flow and on their dependence on wind conditions and slope inclination results may serve to verify or refine existing perceptions or models of the flow affecting sand movement over dunes for example deflected surface wind flow and even stationary vortices were found to form under winds with resultant angles of incidence of only 5 10 relative to the axes of the dunes and on slopes inclined only 12 15 i e lower angles than often assumed in previous studies of dune morphology various effects of the resulting surface flow on the dune ecosystem are also referred to e g the distribution of settling dust and seed dispersal copyright 2002 john wiley sons ltd

rainfall runoff relationship over encrusted dune surfaces nizzana western negev israel a thin microbial crust covers the lower part of longitudinal dunes in the western negev where average annual rainfall is 95 mm in order to study the effect of the microbial crust on rainfall runoff relationships under natural rainfall conditions runoff plots equipped with pressure gauges were established on opposite north and south facing slopes that differ in their vegetal cover and crust properties the study covered four years 1990 94 the first two years were wet and the following two years relatively dry one to five flow events were recorded per year no correlation was found between runoff yield rain amount and rain intensity unlike many microbial crusts reported in the literature the microbial crust in the western negev is not hydrophobic infiltration rate is high under dry surface conditions and of the order of 9 12 mm h 1 when the crust is saturated the high final infiltration rate is explained by the occurrence of large pores that do not seal when the crust is saturated typical hydrographs have very steep rising and falling limbs pointing at a limited contributing area most flows last less than 10 min and runoff volumes collected are on the whole very small owing to differences in crust properties runoff is higher on north than on south facing slopes 1997 john wiley sons ltd

nonlinearity of ecogeomorphic processes along mediterranean arid transect the study was conducted along a climatic transect extending from the judaean mountains mean annual rainfall 700 mm annual mean temperature 17 c to the dead sea mean annual rainfall 100 mm annual mean temperature 23 c a high correlation was found between climatic conditions and factors and processes of the ecogeomorphological system the values of certain factors and processes such as soluble salt content and overland flow increase with increasing aridity whereas those of others such as soil organic matter content clay content aggregate size and stability and infiltration rate diminish with increasing aridity these trends express the long term effect of the prevailing mean climatic conditions in the research stations conditions that led to the development of various types of soil and of plant communities it was also found that for many of the factors and processes examined such as organic matter content salt concentration aggregate stability infiltration rate and overland flow yield the rates of change with respect to position along the transect are not linear and that a climatic threshold value exists in the zone with a mean annual rainfall of about 300 mm and in which the p t ratio is about 17 which creates a sharp boundary between arid and mediterranean ecogeomorphological systems while in arid systems erosion occurs mainly through physical mechanical processes under mediterranean systems it is governed mainly by chemical and biological processes the significance of the existence of a sharp boundary between the arid and the mediterranean zones is that even a relatively slight increase in the annual mean temperature and or a slight decrease in the mean annual rainfall is liable to shift the boundary towards the mediterranean zone and accelerate desertification processes

dense array recordings in the san bernardino valley of landers big bear aftershocks basin surface waves moho reflections and three dimensional simulations abstract fourteen geos seismic recorders were deployed in the san bernardino valley to study the propagation of short period t 1 to 3 sec surface waves and moho reflections three dense arrays were used to determine the direction and speed of propagation of arrivals in the seismograms the seismograms for a shallow d 1 km m 4 9 aftershock of the big bear earthquake exhibit a very long duration 60 sec of sustained shaking at periods of about 2 sec array analysis indicates that these late arrivals are dominated by surface waves traveling in various directions across the valley some energy is arriving from a direction 180 from the epicenter and was apparently reflected from the edge of the valley opposite the source a close in aftershock 25 km depth 7 km displays substantial short period surface waves at deep soil sites a three dimensional 3d finite difference simulation produces synthetic seismograms with durations similar to those of the observed records for this event indicating the importance of s wave to surface wave conversion near the edge of the basin flat layered models severely underpredict the duration and spectral amplification of this deep soil site i show an example where the coda wave amplitude ratio at 1 to 2 hz between a deep soil and a rock site does not equal the s wave amplitude ratio because of the presence of surface waves in the coda of the deep soil site for one of the events studied 90 km there are sizable phases that are critically reflected from the moho pmp and sms at one of the rock sites the sms phase has a more peaked spectrum that the direct s wave

spatial patterns of infiltration vary with disturbance in a shrub encroached woodland abstract woody plant encroachment is known to have substantial effects on a range of ecosystem processes research worldwide indicates that the area around shrubs and trees has higher levels of infiltration than the interspaces little is known however about the hydrological consequences of shrub removal on infiltration and how this might be influenced by grazing we examined the spatial patterns of infiltration across three treatments relating to shrub removal and grazing i undisturbed ungrazed unploughed ii grazed but unploughed and iii grazed and ploughed in general we found that disturbance was associated with a greater cover of bare soil but lower infiltrability our laboratory based measure of infiltration at the undisturbed site bare soil was patchy and localized with an autocorrelation range or connectivity of 1 4 m the autocorrelation range of infiltrability at this site a0 3 m was larger than would be predicted from the size of the shrub canopy and this was attributed to the presence of a well developed understorey layer and biological soil crust community at both grazed sites infiltration was confined to the immediate canopy area of the remaining shrubs a0 1 2 m in the unploughed grazed site additionally there was increasing connectivity of bare soil with disturbance up to 6 8 m at the ploughed grazed site with increasing disturbance resource rich shrub patches are likely to become more developed further reinforcing their growth and persistence at the expense of the bare interspaces our results indicate the importance of shrubs for maintaining landscape connectivity and the long term unsustainable practice of removal by ploughing which is likely to promote shrub dominance rather than suppression

observation of 1 to 5 second microtremors and their application to earthquake engineering part iii a two dimensional study of site effects in the san fernando valley abstract a two dimensional study of the influence of deep sediment on seismic ground motions was conducted by recording and analyzing long period microtremors in the san fernando valley california the recordings were made at 50 regularly distributed sites in the valley three reference sites were employed at the baserock outcrop around the valley so as to observe the time dependent characteristics of the microtremors 1 amplitudes in a period range of several seconds correlate with the thicknesses of the sedimentary layers 2 a site amplification effect which was evaluated in terms of sediment to rock spectral ratios is qualitatively consistent with available geological and strong motion data 3 the practical field observation procedure designed for this study the two dimensional approach can be easily used in other surveys for estimating the spatial characteristics of earthquake ground motions in the period range of several seconds

atmospheric dust in modern soil on aeolian sandstone colorado plateau usa variation with landscape position and contribution to potential plant nutrients rock derived nutrients in soils originate from both local bedrock and atmospheric dust including dust from fardistant sources distinction between fine particles derived from local bedrock and from dust provides better understanding of the landscape scale distribution and abundance of soil nutrients sandy surficial deposits over dominantly sandstone substrates covering vast upland areas of the central colorado plateau typically contain 5 40 silt plus clay depending on geomorphic setting and slope excluding drainages and depressions aeolian dust in these deposits is indicated by the presence of titanium bearing magnetite grains that are absent in the sedimentary rocks of the region thus contents of far traveled aeolian dust can be estimated from magnetic properties that primarily reflect magnetite content such as isothermal remanent magnetization irm isothermal remanent magnetization was measured on bulk sediment samples taken along two transects in surficial sediment down gentle slopes away from sandstone headwalls one transect was in undisturbed surficial sediment the other in a setting that was grazed by domestic livestock until 1974 calculation of far traveled dust contents of the surficial deposits is based on measurements of the magnetic properties of rock surficial deposits and modern dust using a binary mixing model at the undisturbed site irm based calculations show a systematic down slope increase in aeolian dust ranging from 2 to 18 of the surface soil mass similar to the down slope increase in total fines 18 39 of surface soil mass a combination of winnowing by wind during the past and down slope movement of sediment likely accounts for the modern distribution of aeolian dust and associated nutrients at the previously grazed site dust also increases down slope 5 11 in sediment with corresponding abundances of 13 25 fines estimates of the contributions of aeolian dust to the total soil nutrients range widely depending on assumptions about grain size partitioning of potential nutrients in weathered bedrock nevertheless aeolian dust is important for this setting contributing roughly 40 80 of the rock derived nutrient stocks p k na mn zn and

elastic finite difference modeling of strong motion in ashigara valley for the 1990 odawara japan earthquake abstract finite difference seismograms are calculated for the 1990 odawara earthquake to model transverse and radial components of strong motion data using a basin structure derived from a geotechnical survey and double couple solution determined from p first motion data and sh sv amplitude ratio two dimensional sh and p sv finite difference calculation along with correction for soft surface layers reproduces the observed spatial variation in amplitude and waveform of the transverse and radial records at the ashigara valley blind prediction sites for frequencies up to 2 hz comparison of the synthetic and the observed seismograms suggests that the observed spatial variation in the sh amplitude is attributed to the source radiation and amplification due to the soft surface layers while the sv amplitude variation is controlled by the amplification due to the sedimentary basin as well as the source and surface layer effects

water repellency and pore clogging at early successional stages of microbiotic crusts on inland dunes brandenburg ne germany abstract biological soil crusts play a key role for hydrological processes in many open landscapes they seal the topsoil and generate surface run off utilization of the mineral substrate at early stages of microbiotic crusts was investigated using scanning electron microscopy sem water repellency indices were measured using the ethanol water microinfiltrometer method and steady state water flow was determined on the dry crusts and after 300 600 1200 and 1800 s of wetting thus allowing to follow pore clogging through swelling of extracellular polymeric substances eps it was found that water repellency increases with initial crust development where filamentous cyanobacteria and filamentous green algae were dominating but decreases as coccal algae bryophytes and fungi associated with bryophytes emerged swelling of eps took place immediately after wetting and its influence on steady state water flow was most pronounced when filamentous cyanobacteria and algae partially filled in the matrix pores and enmeshed sand grains still leaving micropore channels available for free water infiltration but prior to appearance of coccal algae bryophytes and associated fungi which formed a dense cover on the surface it was concluded that a new phase of crust succession was accompanied by easier wettability but slower infiltration transition from hydrophobicity to pore clogging as ruling mechanisms causing water run off may occur during wetting of individual biological crusts but also during crust succession over time

effects of crust and shrub patches on runoff sedimentation and related nutrient c n redistribution in the desertified steppe zone of the tengger desert northern china runoff and erosion are critical geomorphic processes in arid ecosystems and they are strongly affected by vegetation patterns little is known about the influence of spot structured vegetation on runoff and erosion in temperate deserts especially the effects of vegetation patterns and human induced disturbances on runoff triggered nutrients redistribution are poorly understood a series of simulated rainfall experiments were conducted in the tengger desert northern china to examine 1 the effects of shrub patches on runoff and erosion and related nutrients redistribution on hillslopes 2 whether different amounts of water and nutrients are stored in the shrub patches compared to open inter patch areas and 3 how the runoff and erosion and related nutrients redistribution processes respond to human induced disturbances the results indicate that in undisturbed plots 53 of the applied water became runoff from crust patches and 55 of this was redistributed to the shrub patches water penetration was significantly deeper in shrub patches than in crust patches and more than 75 of the sediments 63 soil carbon 74 nitrogen and 45 73 dissolved nutrients triggered by runoff from crust patches were delivered to shrub patches the disturbance of crust patches tended to result in the uniform distribution of water over the whole slope and the reduction of flow of vital resources from the crust patches to the shrub patches disturbance of macrophytic patches significantly degraded the capacity for resource capture and retention and thereby decreased resources all of which are crucial for the maintenance of plant productivity and vegetation cover the understanding of the function of patterned vegetation in the study area suggests that it represents an aspect of the ecosystem that requires careful management

longitudinal dunes can move sideways longitudinal dunes occur in all major sand areas in the world their dominant mode of migration or extension is considered to be either prevailing or resultant wind parallel the dunes extending downwind via accretion of the terminal nose in the qaidam pendi in northwest china a series of active longitudinal dunes extend downwind at 5 10 m yr 1 internal sedimentary structures examined in the dunes however display beds dipping in one direction rather than two opposed directions as is expected analysis of aerial photographs confirms that these dunes migrate laterally up to three metres per year whilst maintaining a symmetrical longitudinal dune morphology

why deposits of longitudinal dunes are rarely recognized in the geologic record dunes that are morphologically of linear type many of which are probably of longitudinal type in a morphodynamic sense are common in modern deserts but their deposits are rarely identified in aeolian sandstones one reason for non recognition of such dunes is that they can migrate laterally when they are not exactly parallel to the long term sand transport direction thereby depositing cross strata that have unimodal cross bed dip directions and consequently resemble deposits of transverse dunes dune parallel components of sand transport can be recognized in ancient aeolian sands by examining compound crossbedding formed by small dunes that migrated across the lee slopes of large dunes and documenting that the small dunes migrated with a component in a preferred along crest direction over the large dunes

complex faulting deduced from broadband modeling of the 28 february 1990 upland earthquake ml 5 2 the 1990 upland earthquake was one of the first sizable local events to be recorded broadband at pasadena where the green s functions appropriate for the path are known from a previous study the synthetics developed in modeling the 1988 upland sequence were available for use in rapid assessment of the activity first motion studies from the caltech usgs array data gave two solutions for the 1990 main shock based on the choice of regional velocity models although these focal mechanisms differ by less than 5 in strike and 20 rake it proved possible to further constrain the solution using these derived green s functions and a three component waveform inversion scheme we obtain from long period waves a fault plane solution of 216 77 5 0 m 0 2 5 10 24 dyne cm depth 6 km and a source duration of 1 2 sec for which the orientation and source depth are in good agreement with the first motion results of hauksson and jones 1991 comparisons of the broadband displacement records with the high pass wood anderson simulations suggests the 1990 earthquake was a complicated event with a strong asperity at depth double point source models indicate that about 30 per cent of the moment was released from a 9 km deep asperity following the initial source by 0 0 to 0 5 sec our best fitting distributed fault model indicates that the timing of our point source results is feasible assuming a reasonable rupture velocity the rupture initiated at a depth of about 6 km and propagated downward on a 3 5 by 3 5 km length by width fault both the inversion of long period waves and the distributed fault modeling indicate that the main shock did not rupture the entire depth extent of the fault defined by the aftershock zone a relatively small asperity about 1 0 km 2 with a greater than 1 kbar stress drop controls the short period wood anderson waveforms this asperity appears to be located in a region where seismicity shows a bend in the fault plane

site amplification and attenuation of strong ground motion abstract at the mcgee creek california site three component strong motion accelerometers are located at depths of 166 m 35 m and 0 m the surface material is glacial moraine to a depth of 30 5 m overlying hornfels accelerations were recorded from two california earthquakes round valley m l 5 8 23 november 1984 18 08 utc and chalfant valley m l 6 4 21 july 1986 14 42 utc anti plane shear strains for the round valley and chalfant valley events are less than 1 8 10 5 and 1 2 10 5 respectively by separating out the sh components of acceleration we were able to determine the orientations of the downhole instruments peak sh accelerations at the surface were 5 7 times greater than those at 166 m in both earthquakes a constant phase velocity haskell thomson model was applied to generate synthetic sh seismograms at the surface using the accelerations recorded at 166 m in the frequency band 0 10 to 10 0 hz we compared the filtered synthetic records to the filtered surface data the onset of the sh pulse and the reflections from the interface at 30 5 m are clearly seen in both the synthetics and the recorded data the synthetic records closely match the data in both amplitude and phase the fit between the synthetic accelerograms and the data indicates that the seismic amplification at the surface is a result of the resonance of the surface layers and the contrast of the impedances shear stiffness of the near surface materials the impedance contrast of the materials does not account for all of the amplification of the peak accelerations at the surface the resonances of the layer system below 10 hz contribute the most to this amplification we find that a linear model predicts the soil response for frequencies up to 10 hz material damping is added to the soil layers of the model in the form of a quality factor q the damping moderates the strong resonance effects by allowing energy to radiate into the half space the quality factor q 10 is constant in this frequency range but does not accurately model the attenuation at frequencies above 10 hz

11 11 dune morphology and dynamics this chapter discusses the key types of sand dunes and how they can be classified and assesses the major controls on their development the processes of dune dynamics are reviewed leading to a discussion of the concept of equilibrium in solitary dune form recent advances in modeling are reviewed that show how interactions between dunes may result in equilibrium at the dune field scale rather than at the single dune scale

trampling of microphytic crusts on calcareous soils and its impact on erosion under rain impacted flow abstract twenty five intact soil monoliths dominated by microphytic crusts were tested for their susceptibility to erosion by raindrop impacted flow transport after subjecting them to simulated sheep trampling using a sheep s hoof five levels of trampling representing stocking rates ranging from nil to 1 6 sheep ha 1 were imposed on the monoliths an erosive stress was applied to each surface using a simulated rainfall intensity of 65 mm h 1 2 7 mm raindrops surface flow depths of 4 mm and 8 mm with a flow velocity of 25 mm s 1 there was a significant effect of trampling on erodibility p 0 003 and trampling explained 33 of the variability in erodibility only the severe trampling treatment had significantly higher erodibility compared with the other treatments compared to the control severe trampling resulted in a significant decline in the area of the surface covered by micro depressions and attached microphytic crust and concomitant increases in loose detached microphytic crusts coarse lag gravel and bare soil the results were supported by micromorphological examination which revealed that trampling and subsequent rainfall simulation resulted in considerable surface sealing and sorting of sediments loss of fine material and deposition of coarse material in micro depressions

pleistocene stratigraphy paleopedology and paleoecology of a multiple till sequence exposed on the little bear river western district of mackenzie n w t canada the little bear river section lies in a transition zone between mackenzie lowland and canyon ranges of mackenzie mountains within the transition zone the maximum extent of the laurentide ice sheet overlaps the former extent of montane glaciers that emanated from the higher parts of canyon ranges or from the still higher backbone ranges to the southwest five montane tills each with a paleosol developed in its upper part indicate five separate glaciations during each of which a valley glacier emanating from the headwaters of little bear river extended eastward into the transition zone the uppermost of the montane tills is overlain by boulder gravel containing rocks of canadian shield origin deposited by the laurentide ice sheet solum and b horizon depths red colours and lack of leaching and cryoturbation indicate that although each successive interglacial interval was cooler than the preceding one even the last of the intervals was warmer than the holocene climatic conditions during one of the int

late quaternary vegetation history of the central mackenzie mountains northwest territories canada the sedimentary and pollen records from three lakes in the tundra forest tundra and open forest of the central mackenzie mountains of canada are described although two of the lakes lie beyond the mapped limits of late quaternary glaciation their records only extend to 12 000 yr b p the third site lies within the limits of the late wisconsin laurentide advance and a basal radiocarbon date provides a minimum date of ca 11 500 yr b p for retreat of the northwestern margin of laurentide ice artemisia and salix dominated herb tundra occurred at all elevations from at least 12 000 yr b p until ca 10 200 yr b p when shrub tundra dominated by betula glandulosa expanded throughout the region populus balsamifera was also present by this time and probably grew up to and beyond the current treeline picea arrived in the central mackenzie mountains by 8500 yr b p and expanded to the elevation of the current treeline by 8000 yr b p vegetation composition became similar to that of the present by 6000 yr b p with the expansion of alnus at all elevations pollen accumulation rates and detrended correspondence analysis suggest picea population densities in the forest tundra were greater than present between ca 8000 yr b p and 5000 yr b p and have since declined steadily at the open forest site picea population density has not declined in the late holocene but the forest composition has shifted towards a greater affinity with higher elevation sites in general the changes apparent in the treeline vegetation of the mackenzie mountains during the postglacial are consistent with predicted changes in summer insolation based on the milankovitch theory

estimated ground motions in los angeles due to mw 7 earthquake on the elysian thrust fault abstract the elysian thrust fault has been identified as a blind thrust fault davis et al 1989 hauksson and jones 1989 representing a potentially serious seismic hazard to the metropolitan los angeles and its neighboring areas we have simulated time histories peak ground accelerations and their uncertainties using a semi empirical method for a m w 7 0 earthquake on the elysian thrust fault using a flat layered crustal structure the accelerograms from the 4 october 1987 whittier narrows aftershock 10h 50m m l 5 3 are used to represent the source functions of each subfault on the fault surface to account for the velocity variation in the surface sediments we compared simulated ground motions using two separate shearwave velocities 0 6 km sec and 0 9 km sec respectively in the top layer of the crustal model the use of such a simple crustal model is validated by modeling the accelerograms recorded during the 1 october 1987 whittier narrows mainshock m l 6 0 the duration and the relative frequency content observed on these accelerograms are successfully modeled at some stations the simulated peak accelerations agree well with the observed values however at some sites the simulated and observed values differ by a factor of 2 to 3 the variation is attributed to a laterally varying crustal structure that is significant for small earthquakes additional site specific study is needed for an improved prediction of the observed peak accelerations for large magnitude earthquakes the near source peak ground acceleration appears to be controlled by the source receiver geometry relative to the fault as well as the location of asperities on the fault surface this is validated by simulating the peak acceleration data of the 1989 loma prieta earthquake recorded within 30 km of the source for our simulations on the elysian thrust fault an additional component is added to the uncertainty by analyzing several asperity models finally an analytical representation is given to the simulated average peak horizontal ground accelerations y r which is expressed by ln y r 5 38 0 085 2 09 0 0268 ln r 8 0 0 343 in the range of 7 5 r 35 km r is the closest distance to the seismogenic rupture zone this functional form predicts a rapid fall off rate for the simulated ground motions compared with the fall off rate predicted by the other published empirical attenuation relations we use the finite difference method to investigate the effects of irregular structure on ground motions resulting from point sources on the elysian park fault the response computed at several depths beneath the basin suggests that the response is dominated by the initial direct arrivals for sources located interior to the basin when the receivers are located at one end of the basin and the seismic sources are located at the opposite edge the characteristic features on the seismograms are the long durations caused by the trapped energy within the basin the level of trapped energy decreases as the depth of the source increases thus the fault model becomes important in determining the level of peak ground motions and shaking duration

erosion processes in a dune landscape along the dutch coast abstract the landscape of the dunes along the dutch coast is the result of the interaction of biological and geomorphological processes where the latter prevail new landforms are created blowouts by wind erosion and rills and accumulation footslopes by overland flow spatial extent and rate of the erosion processes have been assessed by mapping and by measuring it appears that dune sand is either sensitive to erosion by wind or erosion by water depending on the presence of organic material clean yellow sand is cohesionless and easily blown away but has no surface runoff grey sand containing organic matter on the other hand will resist wind erosion but it is waterrepellent and will be washed from slopes when rain follows a period of dry weather once the grey surface sand is removed from the surfaces by water erosion wind erosion begins in the underlying yellow sand present day geomorphological processes reshape the slopes of the original parabolic dunes wind is particularly active in summit areas where new blowouts are formed whereasoverland flow creates concave south exposed slopes the stepped surface of the north exposed slopes is due to mass movements following rabbit activity

late pleistocene aeolian and fluvial interactions in the development of the nizzana dune field negev desert israel the interdunal areas in the nizzana linear sand dune field contain both sandy and silty sediments a series of trenches was excavated across the interdunal corridor exposing stacked sequences of silty and sandy units which are locally restricted to palaeodepressions the silty units contain fining upward sequences and are interpreted as overbank deposits from the nahal nizzana thermoluminescence dating and identification of buried palaeosols indicates that the silt and clay layers were deposited over a period of several thousands of years in the late pleistocene the sands between the silt layers have been fluvially reworked and are not primary aeolian deposits the stacked sequences of fluvial deposits indicate that the palaeodepressions persisted in the landscape for a significant time attesting to long term stability of the interdunal areas it also suggests that the linear dunes themselves have not moved laterally during this time despite climatic changes and devegetation since the end of the late pleistocene the nahal nizzana has downcut and overbank deposition no longer occurs within the interdunal corridors the playa deposits today are positive relief features indicating that topographic inversion has occurred and that the interdunal areas are geomorphically active

the role of tephra covers on soil moisture conservation at haleakala s crater maui hawai i abstract the influence of tephra covers on soil water was studied in haleakala maui hawai i during two summers eight sites with tephra layers and silverswords argyroxiphium sandwicense dc were sampled at 2415 2755 m at each site eight paired sample sets were obtained in bare soils and under adjacent tephra at three depths tephra were sharply separated from underlying soils and showed prominent vertical stratification tephra clast size distribution was assessed by photosieving and on interstitial gravel samples stones included 45 6 cobbles 29 4 pebbles and 25 blocks moisture content increased with depth in both positions but soils below tephra had more water at all depths than exposed areas surface soils beneath tephra contained 83 more water than bare ground soils at 5 10 cm had 106 greater moisture under rocks but only 70 at 10 15 cm differences between plots were statistically significant p all soils were coarse with 20 gravel and 94 sand most fine material 0 063 mm was silt as clay content was negligible organic matter percentage was low 1 65 bulk density and porosity were associated with moisture variation both in tephra insulated and bare soils 80 of field moisture was statistically p

microclimate control upon sand microbiotic crusts western negev desert israel approximately 20 of the world s arid zones are covered by eolian sand parts of the sand are occupied by microbiotic crusts or have the potential of sustaining microbiotic crusts which may have a great impact upon geomorphological processes and the ecology of the ecosystem any attempt to understand the distribution of microbiotic crusts inevitably necessitates knowledge concerning the microclimate within these ecosystems and in particular moisture temperature and eolian input in the hallamish dune field in the western negev desert israel cyanobacterial crusts with a chlorophyll a content of 15 20 mg m 2 characterizes the south facing footslopes whereas a moss dominated crust with a chlorophyll a content of 50 60 mg m 2 covers the north facing footslopes since the entire dune field was re stabilized concurrently following the 1982 peace treaty with egypt it was hypothesized that physical conditions rather than time duration may account for the differences observed microclimatological data which included temperature rainfall runoff dew and fog surface moistness and eolian input were monitored the differences in crust type could not have been attributed to rainfall dewfall temperatures or eolian input although lower amounts of incident rain lower temperatures and lower eolian input characterized the moss dominated north facing footslope moss dominated crusts were also found in restricted areas of high temperatures and eolian input at the interface between the mobile and the encrusted dune section of the south facing aspect high variability in daytime moisture duration following rain was however monitored with surface moisture duration being approximately 2 5 times longer at the moss dominated habitat these differences are seen responsible for crust type distribution within the ecosystem which may affect in turn variable geomorphological processes

the 1971 san fernando earthquake a double event evidence is presented which suggest that the 1971 san fernando earthquake may have been a double event that occurred on two separate subparallel thrust faults it is postulated that the initial event took place at depth on the sierra madre fault zone which runs along the base of the san gabriel mountains rupture is postulated to have occurred from a depth of about 15 km to a depth of about 3 km a second event is thought to have initiated about 4 sec later on another steeply dipping thrust fault which is located about 4 km south of the sierra madre fault zone the surface trace of this fault coincides with the san fernando fault zone which was the principal fault associated with surface rupture it is postulated that rupture propagated from a depth of 8 km to the free surface the moments of the first and second events are approximately 0 7 10 26 dyne cm and 1 0 10 26 dyne cm respectively this model is found to explain the combined data sets of strong ground motions teleseismic p and s waveforms and static offsets better than previous models which consist of either a single fault plane or a plane having a dip angle which shallows with decreasing depth nevertheless many features of the observed motions remain unexplained and considerable uncertainty still exists regarding the faulting history of the san fernando earthquake

the dynamics of star dunes an example from the gran desierto mexico observations of patterns of erosion and deposition and surface wind velocity and direction on a 40 m high star dune in the gran desierto sand sea indicate that interactions between dune form and airflow as winds change direction seasonally play a major role in the formation of this dune type such interactions lead to deposition of sand in the central parts of the dune giving rise to its pyramidal shape as well as to some extension of the linear arms the major arms of the dune studied are oriented ne sw or transverse to summer sse and winter nnw winds an avalanche face up to 10 m high develops during the course of each season flow separation at the main crestline gives rise to a wide zone of lee side secondary flow which moves sand along the base of the avalanche face towards the central part of the dune where it is deposited as wind ripples migrate into zones of locally reduced flow velocity reattachment of the separated flow occurs on the lower part of the n or s arms parallel to the flow spring westerly winds move sand obliquely up the s and n arms of the dune and outwards on the e arm large scale flow separation and diversion are replaced by the development of strong helical eddies in the immediate lee of the main crestline which move sand along avalanche faces and into zones of lower flow velocity at the end of dune arms formation of star dunes in the gran desierto follows a sequence in which crescentic dunes migrating into areas of opposed winds first develop a reversing crestal ridge convergent leeside secondary flows are developed which result in the formation of linear elements parallel to each major wind direction and the concentration of sand in the central part of the dune examples of star dunes at different stages of their development can be documented

sediment yield on a devastated hill in southern china effects of microbiotic crust on surface erosion process in southern china most of the forests consist of pinus massoniana but they are usually poor thin and deteriorated due to human intervention such as cutting removal of litter for fuel and over grazing therefore exposed bare soil is found in many devastated hills as a consequence both the direct runoff percentage and the sediment yield are very high in order to understand the surface erosion processes in this environment we measured sediment yield and surface erosion depth in terms of catchment scale and carried out in situ sprinkling experiments focusing on the spatial variability of surface conditions and sediment sources the results of sediment yield measurements at the end of headwater catchment suggest the following characteristic 1 the surface water is capable of transporting more material than is supplied by detachment that is the sediment discharge process is detachment limited in situ sprinkling experiments indicate that 2 in the lower part of the hillslope where the surface is covered by microbiotic crust the ratio of surface runoff to rainfall is high but sediment yield is small 3 in the upper part of the hillslope where there is no microbiotic crust the ratio of surface runoff to rainfall is low but sediment yield is large measurements of surface erosion depths indicate that 4 microbiotic crusts provide a high degree of protection against soil erosion even though the surface runoff ratio is large at the lower hillslope consequently the surface erosion depth is almost zero at the lower part of hillslope 5 the main source of sediment was the upper part of hillslope based on these indications it can be concluded that when the sediment discharge process is detachment limited the effect of the surface crust and or the vegetation cover on the sediment discharge process is great that is the spatial variation of the surface crust and or the vegetation cover controls the source of sediment

modeling three dimensional site response effects in the marina district basin san francisco california abstract loma prieta aftershock data recorded in the marina district of san francisco clearly show a large amplification in ground motion and a much longer duration of shaking relative to the response recorded at nearby rock sites for frequencies between 1 and 10 hz some of the amplification can be explained by the difference in impedance between the marina sites and the rock sites however 1d models underpredict the amplification in the range 1 to 3 hz and cannot account for the increased durations the available geologic information within the marina district indicates lateral variations in sediment thickness and composition our previous experience with modeling wave propagation in these types of basin environments has shown that these structures can greatly amplify and prolong the duration of the signals observed at sites within the basin in this study the seismic response of the marina basin is modeled with a 3d finite difference wave field simulation technique results from the numerical simulations at the basin sites indicate that the large amplitude arrivals following the direct s wave are surface waves traveling at relatively slow apparent velocities in general the surface waves contain the strongest recorded motions and produce the long duration of shaking observed at these sites in addition spectral ground motions from the 3d basin models do fairly well in reproducing the observed spectral amplification near 1 hz that is characteristic of the basin sites the simulation results suggest that focusing effects due to the geometry of the basin structure itself have a significant impact in determining the resonant periods at these basin sites

spatial and temporal variability of water percolation and movement in a system of longitudinal dunes western negev israel subsurface lateral flow has previously been identified in dune areas where the average annual rainfall exceeds 200 mm very little is known about subsurface flow in sandy deserts with less than 100 mm of rainfall per year the present study deals with the water regime in a system of longitudinal dunes in the negev desert sixteen boreholes were dug down to a depth of 6 m across a sandy ridge and the adjacent corridor soil moisture and water movement were monitored with a neutron probe during two consecutive years the first year had been relatively wet and in the following year the rainfall was slightly lower than the long term average the data obtained show that in an average year water percolation is limited to 60 cm with no lateral water movement deeper percolation in the range 180 400 cm occurs in response to rainy spells with about 100 mm of cumulative rain within two months the process involves subsurface lateral flow and water movement by the piston effect the lateral flow on the flanks of the dune is explained by differences in hydraulic conductivities within the sloping layers 1997 by john wiley sons ltd

late quaternary eolian dust in surficial deposits of a colorado plateau grassland controls on distribution and ecologic effects abstract in a semi arid upland setting on the colorado plateau that is underlain by nutrient poor paleozoic eolian sandstone alternating episodes of dune activity and soil formation during the late pleistocene and holocene have produced dominantly sandy deposits that support grass and shrub communities these deposits also contain eolian dust especially in paleosols eolian dust in these deposits is indicated by several mineralogic and chemical disparities with local bedrock but it is most readily shown by the abundance of titaniferous magnetite in the sandy deposits that is absent in local bedrock magnetite and some potential plant nutrients especially p k na mn and zn covary positively with depth 3 4 m in dune crest and dune swale settings magnetite abundance also correlates strongly and positively with abundances of other elements e g ti li as th la and sc that are geochemically stable in these environments soil property variations with depth can be ascribed to three primary factors 1 shifts in local geomorphic setting 2 accumulation of relatively high amounts of atmospheric mineral dust inputs during periods of land surface stability and 3 variations in dust flux and composition that are likely related to changes in dust source regions shifts in geomorphic setting are revealed by large variations in soil texture and are also expressed by changes in soil chemical and magnetic properties variable dust inputs are indicated by both changes in dust flux and changes in relations among magnetic chemical and textural properties the largest of these changes is found in sediment that spans late pleistocene to early holocene time increased dust inputs to the central colorado plateau during this period may have been related to desiccation and shrinkage of large lakes from about 12 to 8 ka in western north america that exposed vast surfaces capable of emitting dust soil properties that result from variable dust accumulation and redistribution in these surficial deposits during the late quaternary are important to modern ecosystem dynamics because some plants today utilize nutrients deposited as long ago as about 12 15 ky and because variations in fine grained silt sediment including eolian dust influence soil moisture capacity

carbon cycling of biological soil crusts mirrors ecological maturity along a central european inland dune catena abstract photosynthesis and respiration of biological soil crusts bscs sampled along a mobile inland dune catena were determined to evaluate the applicability of odum s p r ratio determined under controlled conditions for estimation of ecosystem maturity the theory is that in the early stages of ecological succession the total photosynthesis p exceeds the rate of community respiration r so that the p r ratio is 1 and that p r approaches 1 as succession occurs in the special case of organic pollution the p r ratio is typically

runoff induced sediment yield over dune slopes in the negev desert 1 quantity and variability although extensive data exist on runoff erosion and rates for non sandy hillslopes data for arid dune slopes are scarce owing to the widespread perception that the high infiltrability of sand will reduce runoff however runoff is generated on sandy dunes in the hallamish dune field western negev desert israel p 95 mm due to the presence of a thin usually 1 3 mm microbiotic crust the runoff in turn produces erosion sediment yield was measured on ten plots 140 1640 m2 on the north and south facing slopes of longitudinal dunes two plots facing north and two facing south were subdivided into three subplots the subplots represented the crest of the active dune devoid of crust the extensively crusted footslope of the dune and the midslope section characterized by a patchy crust the remaining plots extended the full length of the dune slope no runoff and consequently no water eroded sediments were obtained from the crest subplots devoid of crust however runoff and sediment were obtained from the mid and footslope crusted subplots sediment yield from the footslope subplots was much higher than from the midslopes despite the higher sediment concentration that characterized the midslope subplots the mean annual sediment yield at the hallamish dune field was 432 g per metre width and was associated with high average annual concentrations of 32 g l 1 the data indicate that owing to the presence of a thin microbiotic crust runoff and water erosion may occur even within arid sandy dune fields copyright 2001 john wiley sons ltd

the mechanics and geological implications of dust transport and deposition in deserts with particular reference to loess formation and dune sand diagenesis in the northern negev israel

runoff generation in a sandy area the nizzana sands western negev israel this paper presents the results of sprinkling experiments conducted over the lower part of vegetated and crusted linear dunes as well as over flat playa surfaces that appear in the interdune corridors data obtained show that these two surface units respond quickly to rainstorms runoff generation can be expected for any storm exceeding 2 3 mm and runoff coefficients are high when the topsoil algal crust 1 to 2 mm thick is removed from the surface of the dune infiltration increases drastically and eliminates any possibility of runoff generation under present day rainfall conditions this data may be of great help in the understanding of the geomorphology and sedimentary sequence of the corridors separting linear dunes

differential water distribution over dune slopes as affected by slope position and microbiotic crust negev desert israel runoff is one of the main water sources responsible for water redistribution within a given ecosystem water redistribution is especially important in arid regions and may be of great importance on sandy dunes where the likelihood of runoff is low owing to the high infiltration rates of sand redistribution of water may significantly affect plant and animal distribution and may explain vegetation patterns within an ecosystem runoff yield over sandy dune slopes in the western negev desert was measured under natural conditions during 1990 1994 the magnitude of runoff yield on different slope sections and on north and south exposures was established the results demonstrate that while slope position controlled the microbiotic crust cover crust cover and crust biomass controlled the amounts of runoff obtained whereas no runoff was measured on the upper dune sections devoid of crust only meagre quantities were measured on the midslope sections characterized by discontinuous crust cover substantially larger amounts were however obtained at the bottoms of the slopes characterized by continuous crust cover north facing slopes usually characterized by a chlorophyll a content of 29 41 mg m 2 yielded on average 3 2 times more runoff than south facing footslopes characterized by a 17 mg m 2 chlorophyll a content whereas microbiotic crust was found to be responsible for runoff generation additional water supply owing to runoff may also explain the occurrence of a high biomass crust and the dense vegetation belt at the dune interdune interface of the northern exposure where runoff tends to collect thus whereas crust may reduce infiltration in certain habitats runoff generated by crust may also be responsible for the promotion of crust growth in other habitats runoff may also be used to promote vegetation growth at the dune footslopes the possibility of using runoff to facilitate agroforestry is discussed copyright 1999 john wiley sons ltd

active sand seas and the formation of desert loess abstract loess is a widespread eolian deposit dominated by coarse silt and very fine sand quartz grains 20 80 m that serve as an important archive of information on quaternary climate change despite the intensive research during the past century on desert loess formation its origin remains poorly understood and is still a fundamental problem in sedimentology the ongoing debate is focused on the first stage of loess formation the production of coarse silt quartz grains although laboratory experiments indicate the potential of dunes with their abundant quartz sand grains as a primary source for generating coarse silt grains this concept has been generally rejected because field based evidence of abrasion from sand grains eolian abrasion is scarce here we have adopted a global view of the sand sea loess association and examine in detail several well known late pleistocene loess regions in different subtropical deserts africa the middle east and arabia this analysis demonstrates that all these loess regions are located downwind of adjacent sand seas together with evidence of mineralogical similarity between the loess and the sand dunes and the contemporaneous deposition of the loess and movement of the dunes these observations suggest that sand seas are an important source for desert loess because there is only limited silt storage in sand dunes we postulate that the silt grains comprising most of the loess are not reworked from the dunes but are generated probably through active eolian abrasion of the sand grains under past climates characterized by intensified winds as a result the role of sand dunes and eolian abrasion in the formation of desert loess may be more important than previously thought

the seismic response of the los angeles basin california using strong motion data recorded in the los angeles region from the 1992 mw 7 3 landers earthquake we have tested the accuracy of existing three dimensional 3d velocity models on the simulation of long period 2 sec ground motions in the los angeles basin and surrounding san fernando and san gabriel valleys first the overall pattern and degree of long period excitation of the basins were identified in the observations within the los angeles basin the recorded amplitudes are about three to four times larger than at sites outside the basins amplitudes within the san fernando and san gabriel valleys are nearly a factor of 3 greater than surrounding bedrock sites then using a 3d finite difference numerical modeling approach we analyzed how variations in 3d earth structure affect simulated waveforms amplitudes and the fit to the observed patterns of amplification significant differences exist in the 3d velocity models of southern california that we tested magistrale et al 1996 graves 1996a hauksson and haase 1997 major differences in the models include the velocity of the assumed background models the depth of the los angeles basin and the depth location and geometry of smaller basins the largest disparities in the response of the models are seen for the san fernando valley and the deepest portion of the los angeles basin these arise in large part from variations in the structure of the basins particularly the effective depth extent which is mainly due to alternative assumptions about the nature of the basin sediment fill the general ground motion characteristics are matched by the 3d model simulations validating the use of 3d modeling with geologically based velocity structure models however significant shortcomings exist in the overall patterns of amplification and the duration of the long period response the successes and limitations of the models for reproducing the recorded ground motions as discussed provide the basis and direction for necessary improvements to earth structure models whether geologically or tomographically derived the differences in the response of the earth models tested also translate to variable success in the ability to successfully model the data and add uncertainty to estimates of the basin response given input scenario earthquake source models

destructive strong ground motion in mexico city source path and site effects during great 1985 michoacan earthquake simultaneous consideration of source path and site effects on ground motion during the michoacan earthquake of 1985 allows us to draw coherent conclusions regarding the roles played for the disaster in mexico city by the rupture process the mode of propagation of the waves between the epicentral zone and mexico city and the local amplification in contrast to the horizontal component which showed dramatic amplification for the 2 to 3 sec motion at lake sediment sites we observe almost identical vertical displacement seismograms containing ripples with 2 to 3 sec period throughout the mexico city valley whether the recording site is on the lake sediments or on hard rock we therefore conclude that the 2 to 3 sec motion responsible for the destruction of mexico city was present in the incident field after performing a phase analysis we interpret the signal as the superposition of long period rayleigh waves and short period lg with a dominant period of about 3 sec the analysis of the teleseismic records indicates that the radiation of this event is enhanced for waves around the 3 sec period except in the case of stations for which an anomalous path effect is suspected the records present ripples appearing a few seconds after the beginning of the signal the characteristics of near fault records show that the rupture process consists of the growth of a smooth crack the numerical simulation indicates that the 3 sec period ripples can be explained by a series of changes of the rupture front velocity we examine two alternative source models associated with different crustal models to explain the characteristics of the vertical displacements recorded in mexico city our preferred model attributes the cause of the enhanced 3 sec motion to the irregularity in the rupture propagation in addition to the effect of the local conditions in mexico city this interpretation leads to a very coherent scenario of what happened from the start of the failure on the fault up to the destruction in mexico city this example illustrates the need to consider simultaneously source path and site effects in order to understand strong ground motions

zebra stripes in the atacama desert fossil evidence of overland flow abstract some hillslopes in the hyperarid region of the atacama desert in northern chile have surface clasts organized into distinct contour parallel bands separated by bare soil we call the bands zebra stripes due to the contrast between the darkly varnished clasts and the light colored salt rich soil gravel that comprises the zebra stripes is sorted such that the coarsest clasts are at the downslope front and fine progressively upslope how and when the zebra stripes formed are perplexing questions particularly in a region experiencing prolonged hyperaridity using googleearth satellite imagery and field observations we report the first quantitative and qualitative observations of zebra stripes in order to test hypotheses of the mechanisms and timing of their formation we consider soil shrink swell seismic shaking and overland flow as possible formation mechanisms and find that overland flow is the most likely based on cosmogenic 10 be concentrations in surface clasts salt deposition rates from the atmosphere and content in the soils we propose that the salt rich soils began accumulating 10 6 y ago and the zebra stripes formed 10 3 10 4 y at the latest the zebra stripe pattern has been preserved due to the self stabilization of the clasts within the stripes and the continued absence of life which would disturb the surface as seen at a wetter site to the south we conclude that the occurrence of zebra stripes is diagnostic of a set of distinct characteristics of local and or regional precipitation soil hillslope form and bedrock type

periodic ripple of coda envelope observed in northeastern japan abstract we have investigated the sh envelopes bandpass filtered and mean squared seismograms in the early coda of local and regional earthquakes observed at five stations in northeastern japan the most remarkable feature we have found is a periodic increase in coda amplitude with nearly a constant time interval of several seconds we call this phenomena ripple of coda envelope we examined four possible origins of periodic ripple an artifact due to inadequate data handling repeated source radiation site effect and path originated phase the possibility of artifact due to inadequate filtering or smoothing is low because the ripple is visible in both unfiltered seismograms and filtered traces with various filtering parameters repeated source radiation is not responsible for the ripple because the ripple has longer time duration than that of source time function of the investigated events and because the appearance of ripple at five stations is not synchronous local site amplification factors determined by the spectral ratio of horizontal to vertical motions suggest the contribution of surficial low velocity layers lvl to the periodic ripple the events with prominent ripple are predominantly distributed in shallow range off the pacific coast and intermediate depth range along the volcanic front of the japanese islands this trend is commonly observed for all the investigated stations having different site amplification factors this observation is suggestive of a path dependent origin such as trapped waves in the lvl in the upper part of the subducting slab thus the guided waves trapped in the lvl are the probable origin of the periodic ripple the lvl is either the surficial sedimentary layers under the recording site or the subducting oceanic crust

seismic waveform modeling in the los angeles basin the proximity of several recent earthquakes to the los angeles sedimentary basin provides an opportunity to isolate the effects of the basin on wave propagation the 4 october 1987 aftershock m l 5 3 of the whittier narrows sequence and the 28 june 1991 sierra madre mainshock m l 5 8 are on a similar azimuth to stations overlying the deepest part of the basin a distinctive feature of records from basin stations recording the 4 october after shock is the large amplitude of multiple s ss etc the multiples have up to twice the amplitude of the direct s phase on the tangential component at such a short range less than 25 km a horizontal seismic velocity gradient is needed to turn rays rapidly enough for large amplitude multiples to form a forward modeling approach is employed using finite difference numerical techniques that produce double couple point source solutions a model based on a recent geologic cross section constructed for the east edge of the los angeles basin generates more phases than are seen in the seismic records simpler models based on dipping layers with low shear velocities in the top few layers fit the data better the seismic velocity depth and dip of the layers are varied to fit the timing between the direct p the direct s and the first s multiple the timing and amplitude of the direct and first multiple s pulses are well modeled though the phase of the first multiple does not match the data including a steeply dipping west edge in the basin model has little effect on the synthetic waveforms except at distances near that basin edge the amplitude of ss is greatest in the deepest part of the basin where it is two times larger than direct s the coda duration increases from 8 sec to more than 20 sec from the ne to the sw the sierra madre mainshock occurred about 25 km to the ne of the whittier narrows sequence the model for whittier narrows was extended to this distance with a shallow basin between whittier and the sierra madre hypocenter to simulate the san gabriel sedimentary basin phases generated by the edge of the deep basin continue to dominate the synthetic waveforms but this model generates a lengthy coda this study shows that specific phases with frequencies up to 1 hz that have traveled through deep sedimentary basins can be explained by two dimensional seismic velocity models

a cross section of the los angeles area seismically active fold and thrust belt the 1987 whittier narrows earthquake and earthquake hazard retrodeformable cross sections across the los angeles area interpret the pliocene to quaternary deformation to be a developing basement involved fold and thrust belt the fold and thrust belt is seismically active as evidenced by the 1987 whittier narrows earthquake ml 5 9 and the 1971 san fernando earthquake mw 6 6 the structural geology of the los angeles area is dominated by three major compressional uplift trends 1 the palos verdes anticlinorium and western shelf 2 the santa monica mountains anticlinorium and 3 the verdugo mountains san rafael hills and the san gabriel mountains these trends result from major thrust ramps off a detachment s at 10 15 km depth thrusts of the verdugo mountains san rafael hills and the san gabriel mountains reach the surface the other two uplifts are associated with blind thrusts compressional seismicity is concentrated along these thrust ramps the 1987 whittier narrows earthquake probably occurred on the elysian park thrust which underlies the santa monica mountains anticlinorium the thrust interpretation accounts for the geometry of the anticlinorium the seismological characteristics of the earthquake and the geometry of coseismic uplift the earthquake and aftershocks occurred within a structurally complex narrow zone of miocene and pliocene northwest trending faults that cross the anticlinorium at a high angle these northwest trending faults are interpreted to be reactivated faults now behaving as tears in the elysian park thrust and not the result of active right lateral deformation extending into the whittier narrows area our analysis suggests the whittier narrows earthquake sequence occurred within a structurally weakened zone along the elysian park thrust we also suggest that the whittier fault is not an important quaternary structure and may not be seismogenic the regional cross section is a nonunique solution and other possible solutions are considered multiple solutions arise from the presence of two intersecting compressional belts in the los angeles area the transverse ranges and the northern peninsular ranges the belts may be due to one or more regional detachments and the northern peninsular ranges may be northeast or southwest vergent the deformed top of the crystalline basement along the regional cross section requires a minimum of 15 0 km of north south convergence between the palos verdes hills and the san andreas fault regardless of the structural solution restoration of our cross section solution requires 21 4 km of north south convergence on top of the crystalline basement including 6 4 km of slip continuing offshore to make structures of the continental borderland and 29 7 km of convergence on the basal detachment geologic relationships suggest major shortening started between early and late pliocene time 2 2 4 0 ma which yields a minimum convergence rate of 3 8 6 8 mm yr between the palos verdes hills and the san andreas fault convergence rates for our solution range from 5 4 to 13 5 mm yr between the edge of the continental borderland and the san andreas fault slip rate estimates for the elysian park thrust along the eastern portion of the santa monica mountains anticlinorium are 2 5 5 2 mm yr if the 1987 whittier narrows earthquake is the characteristic earthquake along the 170 km long anticlinorium then recurrence intervals are estimated to be 5 6 12 1 years this recurrence is higher than the historic seismic record along the santa monica mountains anticlinorium and suggests that 1 our slip rates are too high 2 some crustal shortening is taken up aseismically and or 3 earthquakes larger than the 1987 whittier narrows earthquake will occur along the anticlinorium but less frequently

impacts of soil crusts on soil physicochemical characteristics in different rainfall zones of the arid and semi arid desert regions of northern china soil crusts commonly form in arid and semi arid desert regions once shifting sand dunes stabilize the crusts affect soil properties such as texture and fertility however the role of crusts depends on the species composition their physiological activity and their stage of evolution and varies with precipitation and season in this study the soil properties of physical and biological crusts in different rainfall zones of the arid and semi arid desert regions of northern china were analyzed more developed crusts had finer soil texture the impact on chemical properties was greater in areas with higher precipitation contents of soil organic carbon soc total nutrients available n and exchangeable ions did not vary among crust types in a hyperarid region soc total n and p and available n increased with crust development at 150 400 mm rainfall soil crusts increased available p during the initial stage of development crust development increased available k at 300 400 mm precipitation but not at 250 mm precipitation caco3 changes were related to windblown dust and soil moisture in the crust electrical conductivity and total salinity varied in response to variation of soil nutrients and aeolian salt accumulation the ph did not vary greatly between crust types and sites crust development was accompanied by soil alkalization ca2 was more abundant than other cations crust development did not influence cl but increased hco3 and na during crust development so4 2 and k contents decreased and mg2 increased in regions with 250 mm precipitation the opposite changes occurred at 300 400 mm precipitation

pollen analysis of the 1973 ice core from devon island glacier canada abstract meltwater from a 299 m long ice core was filtered and analyzed for fossil pollen and spores pollen concentration was higher in the late holocene and interglacial intervals ca 7 liter 1 than in the early holocene and wisconsinan ca 1 2 liter 1 ones the late holocene and interglacial assemblages were dominated by alnus alder whereas the early holocene and wisconsinan ones were dominated by betula birch and artemisia sage during the holocene and probably the last interglaciation most of the pollen and spores were blown a minimum of 1000 km from low arctic shrub tundra and adjacent subarctic picea spruce forest these areas were dominated by the arctic air mass during the summer pollinating season during the wisconsinan early holocene glacier ice and arctic air were more widespread and pollen sources were more distant thus at this time relatively little pollen was incorporated into the ice the devon ice core data suggest that there should have been pollen in the continental ice sheet of wisconsin time when the ice sheet retreated this pollen would be carried by meltwater and redeposited with silt and clay together with contemporary pollen producing an ecologically anomalous assemblage

infiltration through three contrasting biological soil crusts in patterned landscapes in the negev israel abstract we examined the role of soil crusts in infiltration processes in three contrasting environments in the northern central and central western negev israel the removal of a thin cyanobacterial dominant crust from a sandy dune at nizzana in the central western negev and of a well developed lichen dominant and a cyanobacterial dominant crust from a loess covered hillslope at sayeret shaked in the northern negev resulted in three to fivefold increases in sorptivity and steady state infiltration under both ponding and tension the removal of a depositional crust colonised by cyanobacteria from a loess floodplain at sede zin in the central negev resulted in an increased infiltration under tension but had no significant effect under ponding we attribute the lack of effect under ponding to exposure of surface silts to water which resulted in the clogging of matrix pores and surface sealing the removal of the crusts in all three landscapes influences resource flows particularly the redistribution of runoff water which is essential for the maintenance of desert soil surface patterning it would also have marked effects on germination establishment and survival of vascular plants and soil biota leading ultimately to desertification

runoff induced sediment yield over dune slopes in the negev desert 2 texture carbonate and organic matter runoff induced sediments were collected in the hallamish dune field for four years 1990 1994 runoff and consequently water transported sediments were generated on the dunes owing to the presence of a thin microbiotic crust these sediments were analysed for their particle size distribution and carbonate content in addition the organic matter content was calculated by measuring the chlorophyll content within the runoff the results were compared to the slope parent material i e the crust and the underlying sand as well as to playa sediments which are scattered within the hallamish interdunal areas and which were previously hypothesized to originate from runoff induced sediments higher amounts of fines silt and clay and carbonate characterize the footslopes in comparison to the midslopes intermediate contents of fines 17 per cent and carbonate 8 per cent characterized the sediments in comparison to the fines 27 per cent and carbonate 15 per cent of the crust and to the fines 4 per cent and carbonate 4 per cent of the underlying sand the runoff induced fines and carbonate contents were significantly different from those of the playas suggesting that the playa flats do not originate from runoff induced sediments the sediments were enriched with organic matter organic matter which originates from the crust amounted to 0 3 0 4 per cent as compared to 0 1 per cent in the bare sand nevertheless the crust was found to be relatively resilient to water flow only 0 1 0 5 per cent of the crust was annually eroded off the slope by water with south facing crusts showing higher resilience than north facing crusts the data may thus assist in the evaluation of the crust s residence time copyright 2001 john wiley sons ltd

prediction of strong ground motion in the puget sound region the 1965 seattle earthquake spatial variations in strong ground motion in the 1965 seattle washington earthquake ms 6 5 can be accurately reproduced using three dimensional ray tracing techniques synthetic accelerograms calculated at nearly 1600 receivers spread over the puget sound region indicate that peak ground accelerations are significantly affected by subsurface structure as well as by near receiver soil properties the large synthetic accelerations computed for the harbor island downtown seattle area appear to be due to subsurface focusing of rays within a deep sedimentary basin and are consistent with the observed isoseismal distribution in extreme circumstances such as a bedrock site adjacent to a site situated upon a soft sedimentary lens peak accelerations may vary by nearly an order of magnitude predicted values of peak ground acceleration as a function of distance reproduce both the trend and scatter in data from subduction zone earthquakes of comparable depth in japan

contrasting hydrological response of coastal and desert biocrusts biocrusts abound in southern israel covering the hallamish dune field near nizzana niz in the negev mean annual precipitation of 95 mm and the coast of nizzanim nim near ashdod mean annual precipitation of 500 mm while the hydrological response of the niz crust to natural rain events was thoroughly investigated no data is available on the hydrological response of the nim crust runoff was monitored in runoff plots during the years 2005 2008 and in addition sprinkling experiments were carried out on nim and niz crusts for the evaluation of the possible factors that may control runoff initiation fine content of the parent material crust thickness compressional strength hydrophobicity surface microrelief organic matter biomass chlorophyll a and total carbohydrates and the crust s species composition of nim were studied and compared to that of niz the data showed that in comparison to the niz crust that readily generated runoff no runoff was produced by the nim crust this was so despite the fact that 1 microculeus vaginatus predominated in both crusts 2 the substantially higher rain intensities in nim 3 the greater thickness and higher chlorophyll content and 4 the lower microrelief at nim in comparison to niz the lack of runoff in nim was explained by its low amounts of exopolysaccharides that did not suffice to affectively clog the surface and in turn to facilitate runoff initiation the absence of runoff and its consequences on the nim ecosystem are discussed copyright 2012 john wiley sons ltd

the effect of wind and precipitation on vegetation and biogenic crust covers in the sde hallamish sand dunes vegetation and biogenic crust covers play an important role in sand dune stabilization yet there is a lack of high temporal and spatial resolution data on sand dune cover a field experiment aimed at measuring the dynamics of biogenic crust and vegetation in sand dunes was conducted at the sde hallamish sand dunes in the northwestern negev desert israel from july 2008 to august 2010 the climate of the sde hallamish sand dunes is arid the mean annual precipitation over the past 13 years is 61 mm and the dunes are linear and partially stable mainly due to the presence of biogenic crust and partially due to the presence of vegetation in july 2008 10 10 m plots on the four dune habitats crest interdune north slope and south slope were treated as follows i removal of vegetation and biogenic crust ii removal of biogenic crust only iii removal of vegetation only iv partial removal of biogenic crust and vegetation and v control plot the surface coverage of sand biogenic crust and vegetation was monitored on a monthly basis using a remote sensing technique especially developed for the sde hallamish sand dunes it was found that strong wind events with durations of several days accounted for the coverage changes in biogenic crust and vegetation the response to precipitation was much slower in addition no rehabilitation of biogenic crust and vegetation was observed within the experiment time period the changes in biogenic crust cover were not necessarily related to changes in dune dynamics since often an increase in biogenic crust cover is a result of wind erosion that exposes old crust that was buried under the sand wind hardly erodes biogenic crust at all due to its high durability to wind action the sde hallamish dunes seem to have become more active as a result of a prolonged drought during the past several years the field experiment reported here indicates that biogenic crust cover exhibits large seasonal variations that are not necessarily related to the growth of new crust but rather to the exposure of old buried crust

visualization of whole mantle propagation of seismic shear energy using normal mode summation correct representation of seismic waveforms propagating through the mantle from a 600 km deep earthquake is presented using graphic interpolation between synthetic seismograms computed across a grid of mantle depths and distances all torsional normal modes with periods above 12 s are summed to create 72 846 seismograms at depths between the surface and the core mantle boundary the resulting time snapshots show the manner by which seismic shear energy propagates around the core away from the source

modeling path effects in three dimensional basin structures path effects for seismic wave propagation within three dimensional 3 d basin structures are analyzed using a reciprocal source experiment in this experiment a numerical simulation is performed in which a point source is excited at a given location and then the wave field is propagated and recorded throughout a 3 d grid of points using the principle of reciprocity source and receiver locations are reversed this allows the modeling of path effects into a particular observation site for all possible source locations using only one simulation the numerical technique is based on the use of paraxial extrapolators and currently tracks only acoustic waves however the method is capable of handling arbitrary media variations thus effects due to focusing diffraction and the generation of multiple reflections and refractions are modeled quite well the application of this technique to model path effects for local earthquakes recorded at stations in the los angeles area of southern california indicates the strong influence of the 3 d crustal basins of this region on the propagation of seismic energy the modeling results show that the los angeles san fernando and san gabriel basins create strong patterns of focusing and defocusing for paths into these stations from various source locations these simulations correlate well with earthquake data recorded at both stations by comparing these calculations with earthquake data we can begin to evaluate the importance of these basin effects on observed patterns of strong ground motions

3d eight order elastic finite difference modeling of refraction and strong motion data from the coyote lake region california abstract 3d elastic modeling of vertical component explosive source refraction data is performed to constrain the shallow part of a velocity model followed by 3d modeling of three component strong motion data from a spatially and temporally extended earthquake source in that model modeling is by eighth order finite differencing on a staggered grid which automatically generates all body and surface waves including multiples and conversions application is to the 1981 usgs refraction survey and the 6 august 1979 coyote lake event in central california 3d modeling was able to reproduce the main observed features in both the refraction and earthquake data for the refraction data the synthetics explain local variations in first arrival travel times reverberations in the near surface sediments and surface waves for the strong motion data the initial arrival times and amplitudes are determined by the position and orientation of the initial rupture the shapes of the trailing edges of the pulses are modified by the actual source distribution changes of source position of 1 to 2 km are easily resolvable through changes in the position of nodal planes discontinuities in material properties e g layering produce a coda of multiples and converted waves lateral variations in velocities especially in the upper few kilometers change the energy distribution among the trace components by 3d refraction and so shift the apparent source directivity and position as well as contributing to the coda remaining differences between the observed and synthetic data are attributed to local seismometer coupling effects very local site structures and phenomena such as anelasticity and anisotropy which are not explicitly modeled

the potential roles of biological soil crusts in dryland hydrologic cycles biological soil crusts bscs are the dominant living cover in many drylands of the world they possess many features that can influence different aspects of local hydrologic cycles including soil porosity absorptivity roughness aggregate stability texture pore formation and water retention the influence of biological soil crusts on these factors depends on their internal and external structure which varies with climate soil and disturbance history this paper presents the different types of biological soil crusts discusses how crust type likely influences various aspects of the hydrologic cycle and reviews what is known and not known about the influence of biological crusts on sediment production and water infiltration versus runoff in various drylands around the world most studies examining the effect of biological soil crusts on local hydrology are done by comparing undisturbed sites with those recently disturbed by the researchers unfortunately this greatly complicates interpretation of the results applied disturbances alter many soil features such as soil texture roughness aggregate stability physical crusting porosity and bulk density in ways that would not necessarily be the same if crusts were not naturally present combined these studies show little agreement on how biological crusts affect water infiltration or runoff however when studies are separated by biological crust type and utilize naturally occurring differences among these types results indicate that biological crusts in hyperarid regions reduce infiltration and increase runoff have mixed effects in arid regions and increase infiltration and reduce runoff in semiarid cool and cold drylands however more studies are needed before broad generalizations can be made on how biological crusts affect infiltration and runoff we especially need studies that control for sub surface soil features such as bulk density micro and macropores and biological crust structure unlike the mixed effects of biological crusts on infiltration and runoff among regions almost all studies show that biological crusts reduce sediment production regardless of crust or dryland type copyright 2006 john wiley sons ltd

improved age estimates for the white river and bridge river tephras western canada new 14c ages date the eruptions that produced the white river and bridge river tephras two important holocene marker beds in western canada the 14c ages were obtained on trees in growth position buried in coarse tephra and a pyroclastic flow near the source vents the mean calendric age of the white river eruption based on four 14c ages is 1147 cal years bp calibrated years approximately equivalent to calendric years or ad 803 the 2 age range obtained from the two most precise 14c ages is 1014 1256 cal years bp or ad 694 936 the mean age of the bridge river eruption determined both from i the single most precise outer ring 14c age and ii the weighted mean of six outer ring 14c ages is 2360 cal years bp or 411 bc 2 age range 2349 2704 cal years bp or 755 400 bc