a multivariate mixing model for identifying sediment source from magnetic measurements a sequential method for quantitative identification of sediment source components based on magnetic measurements has been developed and tested for sediments from the rhode river maryland simulated mixing tests and multiple regression were employed to establish numerical relationships between source component proportions and the magnetic measurements of mixtures on the basis of these multivariate mixing models source components of three estuarine sediment cores were estimated by linear programming the results strongly support the previous studies on this catchment which indicated a dramatic change in sediment source some 150 to 200 yr ago quantitative calculations are more useful and informative than purely qualitative descriptions

stressing of locked patches along a creeping fault an analysis is presented of the stressing of locked patches along a fault zone which is creeping elsewhere the model consists of strike slip faulted elastic lithospheric plates loaded in a manner equivalent to imposition of a remotely uniform tectonic shear stress the fault zone has an inhomogeneous strength distribution both depth wise and along strike it is modeled as being composed of locked patches and freely slipping parts treated as cracks the solution is given with the use of the line spring model which analyzes the problem by thickness averaged plane stress theory for lithospheric plates which slip along a discontinuity cut at the plate boundary as a boundary condition thickness averaged stress and slip at each local section along the cut was related to one another by the result of an antiplane strain analysis of slip for the crack or crack pair which describes the slipping and locked depth ranges at that section the analysis indicates that the slip distribution along creeping parts of the fault as well as the stress distribution along locked patches depends strongly on the geometry of these zones the model is used to examine stress concentrations associated with a slip deficient seismic gap along strike and to study the effect of local irregularities in the margin of a locked region it is also used to simulate slip and stressing processes associated with the creeping portion of the san andreas fault in central california between the presently locked zones of the great 1906 and 1857 ruptures and to constrain the nature of an apparently locked zone at the southeastern end which ruptures in characteristic parkfield earthquakes near fault creep and broadscale displacement data along the fault since the 1966 parkfield earthquake and inferences from seismicity distributions are used limitations of the modeling procedure at short spatial wavelengths prohibit an accurate description of the parkfield locked patch but results suggest that it may be localized and occupy a small fraction of area of the normal seismogenic zone an effective remote stressing rate of order 0 3 10 6 shear modulus yr is inferred together with a less well constrained 30 to 40 km lithospheric thickness for consistency with the displacement data results enable estimates of stress accumulation along the locked 1857 rupture zone and the build up of fracture energy release capability of order 107 j m2 in 150 yr at its lower margin

magnetism of loess palaeosol sequences recent developments abstract investigation of the global loess record primarily as a palaeoclimatic archive continues unabated one important aspect of this research effort is the determination of the magnetic properties of the loess itself and of the palaeosols developed within it in earlier work these have provided a record of major long term past climatic changes however recent studies are now revealing sub milankovitch signals with teleconnections from the chinese loess plateau clp to the greenland and antarctic ice caps as well as to climate related events in sediments recovered from the north atlantic these high resolution profiles cover to the last 130 000 years however progress is also being made in extending the record down below the loess sensu stricto of the clp into the so called red clay so that the aeolian record can now be carried back beyond 7 ma bp several authors have attempted to obtain quantitative estimates of palaeoprecipitation by comparing magnetic susceptibility of palaeosols with values for modern soils although the broad picture is qualitatively clear wetter interglacials dryer glacials the actual quantitative values provided by the various techniques differ significantly in the classic chinese sites palaeosols are always magnetically enhanced due to in situ pedogenic processes however evidence is now emerging to indicate that this is not the only magnetoclimatological model in cases such as siberia where wind vigour dominates over pedogenesis the opposite pattern is found with magnetic susceptibility minima in palaeosols this also occurs in situations where the sediments are water logged for significant amounts of time leading to gleying and concomitant loss of magnetic minerals the use of magnetic remanence as a geomagnetic polarity clock requires caution early interpretations were acceptable for establishing broad chronological control however recent scrutiny suggests that remanence lock in may be delayed by as much as 20 000 years furthermore synthetic magnetostratigraphies modelled on the basis of realistic sediment characteristics indicate that there are limits to the amount of geomagnetic detail likely to be resolved this seems to be the case for the blake event which manifests itself in a very variable way at different sites in the clp

mudflat distal fan and shallow lake sedimentation upper vallesian turolian in the tianshui basin central china evidence against the late miocene eolian loess the tianshui basin in central china contains a thick sedimentary sequence 1400 m of continental deposits aragonian to villafranchian miocene pliocene in age intense himalayan movements around the paleogene miocene boundary triggered the uplift of mountain ranges around the tianshui basin providing the deposition site for continental sediments the sedimentary infill of the basin consists of four stratigraphic units i to iv this paper focuses on unit ii most of the accommodation space was occupied by unit i so during the sedimentation of unit ii the morphology of the basin was relatively flat promoting the development of wide distal fan mudflat areas and wide shallow lakes deposits include red mudstones pedogenic and groundwater calcretes reworked calcrete deposits sheet floods fluvial channels rippled sandstones siltstones ooidal peloidal packstones palustrine limestones bioturbated marls and intraclastic limestones marls the characteristics and organization of the deposits indicate the gradual transition from alluvial to lake environments within the distal fan mudflat the deposition of reworked calcrete clasts as one of the most striking facies of the basin may be attributed to substantial recycling of calcrete levels and red clays at the lake margins the presence of ooids and palustrine limestones suggests the possibility of ramplike margins within different energy settings the origin of some of the deposits of the basin s qa i section is under discussion and their consideration as eolian has been recently proposed however we believe this possibility is precluded by the characteristics of the deposits our proposal has important implications for revising the interpretation of miocene paleoclimatic conditions in central asia

partly pedogenic origin of magnetic variations in chinese loess quaternary sequences of interbedded aeolian silts loess and buried soils palaeosols potentially provide one of the best terrestrial records of past climates1 5 magnetic susceptibility variations in loess and palaeosol sequences from china are strongly correlated with climate induced fluctuations of oxygen isotope ratios in deep sea sediments2 4 6 8 as an explanation of this correlation it has been suggested that the recorded variations in magnetic susceptibility depend primarily on the degree of dilution by non magnetic bulk loess constituents of a uniform magnetic assemblage derived from remote but unknown sources6 8 it has also been suggested that such a model involving changes in the accumulation rate of loess superimposed on a constant flux of magnetic particles provides a relative geological timescale through the control of loess deposition by astronomically modulated climate fluctuations7 here we report a preliminary test of this model using rock magnetic properties especially sensitive to variations in magnetic grain size our results show that palaeosols are characterized by much finer magnetic grain size assemblages than are the intervening loess units this suggests that a simple model based on constant magnetic influx and dilution by variable amounts of non magnetic loess is inadequate our magnetic measurements establish the close comparability of the fine grained magnetic minerals in the palaeosol samples to those in contemporary soils and thus point to a partially pedogenic origin for the magnetic mineral assemblages in the palaeosols

stress transfer during three sequential moderate earthquakes along the central calaveras fault california seismic and geodetic information on three sequential moderate earthquakes the 1979 m5 9 coyote lake the 1984 m6 2 morgan hill and the 1988 m5 1 alum rock along the central calaveras fault is used to examine how tractions on fault segments are transferred from one fault segment to the other geodetic data obtained before and after each earthquake are inverted for slip at depth the perturbed stress field induced by each earthquake is calculated using 3 d elastic dislocation theory although only long wavelength features of the slip distributions are well resolved the model suggests that the distribution of slip is nonuniform the largest slips tend to occur in regions having relatively few aftershocks or near the hypocenter according to the model the coyote lake earthquake induced 0 1 1 bar 0 01 0 1 mpa shear stress across the northern part of the morgan hill segment and only about 0 1 bar at the hypocenter of the morgan hill mainshock a shear stress of 0 5 bar was induced by the morgan hill earthquake at the hypocenter of the alum rock mainshock the average stress drops for the coyote lake morgan hill and alum rock earthquakes are estimated to be 30 25 and 15 bars respectively thus the earthquake induced shear stress of 0 1 0 5 bar at the hypocenter of the next earthquake is only a few percent of the total stress drop and cannot be solely responsible for the occurrence of the sequential earthquakes however the induced shear stress can trigger the adjacent earthquakes in the sense that adjacent earthquakes would have occurred at a later time if there was no such coseismic stress transfer it is inferred that fault geometry may influence the local stress field thereby initiation and termination of the rupture but not the regional stress accumulation

rock magnetism and geochemistry of two plio pleistocene chinese loess palaeosol sequences implications for quantitative palaeoprecipitation reconstruction abstract stratigraphic and spatial variations in the magnetic properties of chinese loess are widely accepted as reflecting changes in the intensity of soil forming processes over time and space recently several workers have used statistical correlations between the magnetic susceptibility and related parameters of modern loess plateau soils and modern climate to generate transfer functions to reconstruct past temporal and spatial trends in precipitation here we try to assess the viability of this approach by conducting magnetic and geochemical measurements of two long 2 5 my loess palaeosol sequences from different locations on the modern precipitation gradient across the loess plateau one site duanjiapo is located on the southern humid margin of the plateau and the other site luochuan is located in a drier modern environment some 160 km further north our results indicate discrepancies between magnetic and geochemical indicators of weathering and soil forming intensity and that any attempt to predict palaeoprecipitation at duanjiapo using published approaches would be significantly in error overall our results indicate a complex response of magnetic mineral transformations to climate including the production of both ferrimagnetic and antiferromagnetic minerals within the same climatic interval together with the destruction of magnetic material within some of the palaeosols therefore we suggest considerable caution should be exercised in generating quantitative climatic reconstructions from a single magnetic parameter

links between bulk sediment particle size and magnetic grain size general observations and implications for chinese loess studies using a combination of particle size analysis magnetic measurements scanning electron microscopy and transmission electron microscopy imaging this study shows that in a wide range of depositional environments there is a strong link between particle size classes and magnetic response especially below the upper limit of stable single domain magnetic behaviour ferrimagnetic grain assemblages dominated by stable single domain magnetosomes regularly have peak susceptibility and remanence values in coarser grades than do those containing finer grained viscous and superparamagnetic secondary magnetic minerals formed during pedogenesis this effect is despite the fact that there is a one to two orders of magnitude size difference between the particle size boundaries at 1 or 2 m and key domain state transitions mostly below 0 05 m the implications of these results are explored using samples spanning 22 myr of loess accumulation on the chinese loess plateau the results from the loess sections complemented by data from low temperature magnetic experiments show that there are subtle distinctions in mean ferrimagnetic grain size between the pleistocene and miocene parts of the record thus allowing more refined rock magnetic interpretations of the fine grained ferrimagnetic mineral assemblages arising from the effects of weathering pedogenesis and possibly diagenesis in the sections studied

review of recent developments in mineral magnetism of the chinese loess mineral magnetism of the chinese loess has been investigated for more than 20 years although there is a consensus that the neoformation of fine grained maghemite particles in the superparamagnetic sp and single domain sd grain size regions accounts for the magnetic enhancement in the chinese paleosols quantitative retrieval of paleoclimatic signals in terms of rock magnetic proxies is still a subject of debate the ambiguities arise from the inherent complexities of magnetic proxies as well as the multiple factors that control the pedogenic processes therefore a better description of the magnetic assemblage including its mineralogy grain size distribution and stoichiometry of two distinct origins pedogenic and eolian can help us better understand mechanisms behind variations in magnetic proxies at different timescales in order to link them to the paleoclimatic processes this review focuses on recent developments in loess magnetism and carefully evaluates merits and limitations of rock magnetic proxies furthermore several currently unsolved problems are addressed

the record of changing hematite and goethite accumulation over the past 22 myr on the chinese loess plateau from magnetic measurements and diffuse reflectance spectroscopy 1 high field isothermal remanence hirm and diffuse reflectance spectroscopy drs wereusedto characterize hematite and goethite in a suite of bulkand particle sized fractioned samples from the chinese loess plateau representing the main changes in magnetic mineralogy over the past 22 myr the record of drs defined hematite largely reflects variations in the pedogenic fraction 8 mm makes the main contribution to goethite concentrations in the bulk samples pedogenic goethite concentrations generally vary inversely with those of hematite the lack of consistent correlations between hirm inferred mineral content and drs measurements casts doubt on the validity of using hirm to characterize the concentrations and relative proportions of hematite and goethite in sediments representing a long time span or a wide grain size distribution the progressive decline in the coercivity of the antiferromagnetic minerals in bulk samples throughout the whole period is mainly an expression of changes in the detrital fraction reflecting changes in the source region possibly involving increased aluminum substitution the good correlation between drs defined hematite and frequency dependent magnetic susceptibility in several chronologically defined subgroups of samples is consistent with the hypothesis that maghemite forms a transitional phase in a weathering sequence from ferrihydrite to hematite the results shed new light on using hirm to characterize hematite and goethite in sediments and on the history of weathering and climate change over the past 22 myr in the dust source regions and on the loess plateau

15 myr periodicity in the frequency of geomagnetic reversals since 100 myr the statistical properties of the magnetic polarity time scale have been examined recently1 2 for several time scales for some of these scales the time evolution of the frequency of reversals as seen through an 8 myr moving rectangular window have been determined in particular over the past 100 myr the mean reversal frequency appears to be dominated by a linearly increasing trend on which is superimposed a rhythmic fluctuation we have attempted to analyse the mathematical structure of the reversal frequency curve in detail and to determine whether the rhythmic fluctuation was indeed periodic by modelling the time evolution of the reversal frequency we find a dominant frequency corresponding to a period of 15 myr this result represents one of the most persistent long term periodicities ever reported for a geophysical phenomenon

stress diffusion along rupturing plate boundaries an analysis is made of viscoelastic lithosphere asthenosphere coupling in the time dependent redistribution of stress along plate boundaries or other seismic lineaments following great earthquakes the study is based on a generalization by rice of elsasser s model of stress diffusion in which general elastic plane stress deformations are allowed in lithospheric plates which are coupled in an elementary way to a maxwellian viscoelastic asthenosphere solutions are developed which describe the large scale quasistatic distribution of thickness averaged in the lithosphere at or near stationary or travelling rupture zones modeled here by either crack like zones of fixed stress drop or dislocation type slip zones sudden ruptures shed load onto the asthenosphere which is gradually transferred back to the lithosphere by a slow relaxation process the spatial and temporal characteristics of the predicted stress alterations suggest a significant role of lithosphere asthenosphere coupling effects in triggering interactions of great earthquakes patterns of prolonged aftershock activity and the breaking of barriers or gaps by time dependent stressing

earthquake source nucleation a physical model for short term precursors abstract ohnaka m 1992 earthquake source nucleation a physical model for short term precursors in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 149 178 this paper deals with a quasistatic leading to a quasidynamic at a later time nucleation process that precedes the earthquake dynamic rupture and models of the earthquake source nucleation are put forward based on physical principles a quasistatic to quasidynamic rupture nucleation process is an intrinsic part of the ensuing earthquake dynamic rupture in other words the nucleation process itself is an short term or immediate precursor that occurs in a localized zone during the earthquake nucleation premonitory slip proceeds in the localized nucleation zone and shear stress also decreases gradually in the zone since slip weakening occurs during the nucleation immediate foreshock activity is a part of the nucleation process leading to the mainshock dynamic rupture therefore hypocentral locations of foreshocks are necessarily restricted to lie near the mainshock hypocentre onset of the mainshock rupture whether or not foreshocks occur during the mainshock nucleation depends on how the rupture growth resistance varies on a local to small scale since patches of greater rupture growth resistance are considered to prevail on a local to small scale in the fault zone carrying immediate foreshocks would be one of the major characteristics of earthquakes that nucleate within the brittle seismogenic layer when a mainshock earthquake nucleates within the brittle seismogenic layer and its hypocentre is located near the base of the seismogenic layer immediate foreshocks for this mainshock are necessarily restricted to lie within a localized region shallower than hypocentral depth of the mainshock by contrast the nucleation process below the base of the seismogenic layer is aseismic in nature so that carrying no conspicuous foreshocks would be a common characteristic of interplate earthquakes that nucleate below the base of the brittle seismogenic layer the breakdown strength p the breakdown stress drop b and the critical slip displacement dc are indicative of the rupture growth resistance to estimate depth variations of these parameters the effects of the normal stress n and temperature t on those parameters are examined using available data so far published the combined effects of n and t on p in the brittle to semibrittle regime are found to be represented empirically by p n t po n 1 cosh 50 t 40 sinh 50 t exp 2000 t where po n 135 7 0 750 n and t is measured in k and n in mpa it is found that dc increases sharply with t but is insensitive to n above 300 c while dc depends on n but is insensitive to t below 300 c on the basis of these results variations in p dc and b at mid crustal depths are estimated for quartzo feldspathic rocks for a given geothermal gradient

biomarkers challenge early miocene loess and inferred asian desertification 1 fine grained miocene sediments from tianshui basin northeastern tibetan plateau have received intense attention recently because these sediments were identified as loess the presence of early miocene loess pushes the timing of initiation of inland asian desertification from 8 ma back to 22 ma however mudflat distal fan and shallow lake sediments of miocene have also been reported in tianshui basin consequently the origin of these fine grained miocene sediments in this area remains controversial here we investigate then alkane biomarker characteristics of neogene sediments from a north south transect of exposures within tianshui basin and compare these molecular distributions with those published quaternary loess to help resolve the disputed origin we found thatn c23 and n c25 alkanes sourced from either aquatic macrophytes or palustrine plants are ubiquitous in the miocene sediments from tianshui basin but are largely absent in quaternary loess this striking difference between n alkane distributions in the tianshui samples and the quaternary loess casts doubt on an eolian origin for the tianshui samples and challenges the hypothesis of an early miocene onset of asian interior desertification

geomagnetic secular variation impulses the secular variation of the earth s internal magnetic field changes on time scales of the order of a year such impulses occurred in 1969 and probably in 1913 they provide constraints on deep mantle conductivity which appears to be rather low and on the motions of the top of the fluid core which are probably dominated by westward drift present data favour the existence of upwelling of core material at the core mantle boundary the impulses can be correlated with extrema in the earth s rotation rate and interpreted in terms of electromagnetic core mantle coupling

precursors to the kalapana m 7 2 earthquake the kalapana hawaii earthquake of november 1975 had a rupture length of 40 to 50 km and was located on the south flank of the active volcano kilauea the source mechanism was dip slip normal faulting on a plane dipping 20 degrees to the se with the greatest principal stress oriented in that direction and accumulated by volcanic intrusions into kilauea s rifts the source area of the 1975 earthquake was subject to intensive geological and geophysical research for many years before this earthquake because of its proximity to the volcano we studied the distribution of epicenters for small earthquakes from 1962 to 1975 seven seismographs located within 3 km of the aftershock area had been in operation for 3 to 10 years before the mainshock and geodetic triangulations and trilaterations in the source area had been carried out repeatedly since 1914 we found that precursory changes occurred throughout most of the rupture area but in two distinctly different patterns in the larger outer anomalous area the seismicity rate was decreased by 50 during the 3 8 years before the mainshock in addition several geodetic lines indicated anomalous strain release during this time within two inner areas the seismicity remained high then increased shortly before the mainshock in one of the inner areas a p wave travel time delay of 0 2 s could be detected which began about 3 5 years before the mainshock within the other inner anomalous area geodetic strain was accumulating until the first half of 1975 when 3 5 10 4 strain 35 bars was released aseismically by contrast the outer anomalous volume was experiencing strain softening from 1970 1971 on we interpret these observations as indicating that strain softening by fault creep in the outer anomalous area transferred stress into two major asperities locked portions of the fault a velocity decrease and foreshocks were observed in one asperity and high stress accumulation in the other implying that dilatancy of the crust probably occurred our model is qualitatively supported by the independent evidence of strong motion records which show that the kalapana earthquake was a complex multiple rupture we conclude that the kalapana earthquake was preceded by a preparatory process which lasted 3 8 0 3 years and which had dimensions of 45 10 km covering approximately the aftershock area

precursory surface deformation in great plate boundary earthquake sequences abstract we present an analysis of time dependent precursory source processes and associated ground surface deformation prior to the great earthquake rupture of a long seismic gap zone along a transform plate boundary this work is based on a theoretical model proposed by li and rice 1983 and reviewed briefly here in the model thickness averaged stress transmission in the lithosphere is analyzed by a generalized elsasser plane stress model which includes coupling to a viscoelastic asthenosphere upward progression of preseismic rupture at each section along strike is analyzed as quasi static extension in local antiplane strain of an elastic brittle crack this provides the necessary boundary condition in the context of the line spring procedure along strike for the elsasser plate model the approach to instability is simulated using what are thought to be typical material and tectonic parameters it is found that prior to a large earthquake the shear strain increases rapidly near the seismic gap zone but tends to diminish at moderate distances from the plate boundary such a strain reversal phenomenon although difficult to measure at present due to its small magnitude may be helpful in the interpretation of geodetic measurements and in the planning of geodetic network locations for the purpose of earthquake prediction the acceleration of slip with consequent acceleration of stress increase in the upper brittle crust predicted by the model may provide a plausible explanation for nonlinear features associated with seismicity preceding some major events see e g raleigh et al 1982 a precursor time t prec during which anomalous strain rate at the seismic gap exceeds twice its background level is found to be in the range of a couple of months to 5 yr depending on the tectonic loading rate t prec varies approximately as r 1 3 where r is a dimensionless loading rate parameter this precursor time may be associated with physical anomalities related to rapid stress increase such as ground tilting water table variation and radon emission which precede some large earthquakes consistent with observations the analysis predicts longer precursor times for larger earthquakes

spatial and temporal seismic energy release in the efate region central new hebrides island arc evidence for buckling a study of the spatial and temporal dismbutions of the seismicity in the central new hebrides island arc shows that the efate region including the interplate boundary and the uppa plate can be considered as a single block under stress r xied activation of very specific m nes within this block as indicated by seismic clusters is temporally correlated with larger than average energy release rates these relative high rates are associated with main shocks with magnitude ms mb 5 0 in the upper pan of the interplate boundary and indicate that slip at the plate boundary is not steady no correlation is found with main shocks located in the lower pars of the interplate boundary where slip occurs more freely we propose that these observations are consistent with a buckling process with a wavelength of about 170 km within the ovemding plate buckling or bending of the upper plate may be due to the impingement of a seamount on the subduction system elevating the stress level of the efate region which behaves like a single mechanical entity

magnetic enhancement is linked to and precedes hematite formation in aerobic soil 1 soil formation usually increases magnetic susceptibility most often by increasing the concentrations of magnetite and maghemite which are two ferrimagnetic iron oxides here we provide evidence that magnetic enhancement in aerobic soils not affected by detrital magnetic inputs or thermal transformation of other iron oxides is mostly due to the formation of maghemite which is later transformed into hematite the iron oxide that gives red color to soil we show that the maghemite hematite ratio is influenced by the particular environment and the degree of soil development so it constitutes an effective tool for paleoenvironmental and planetary studies

magnetotactic bacteria and their magnetofossils in sediments abstract living magnetotactic bacteria from freshwater environments show under natural and laboratory conditions a great variety of morphological appearances their magnetosomes vary in number shape and size one species of bacteria yields up to 1000 magnetosomes per cell individual particles reach a size of up to 200 nm as a rule they form elongated chains in bacteria which are held under laboratory conditions loop shaped arrangements and nearly unordered clusters are also found during the laboratory experiments different species of bacteria periodically dominated freshwater samples always exhibited a great variety of different magnetosomes on the other hand samples from marine environment clearly showed the preponderance of one type of particle these observations may reflect specific properties of the physico chemical milieu in which the bacteria grew at the end of our experiments north and south seeking bacteria co existed in roughly equal quantities quaternary unconsolidated sediments from the ammersee bavaria and the antarctic quaternary to tertiary deep sea sediments from the atlantic and pacific and jurassic limestones from the sonnwendgebirge tyrol austria were analyzed for fossil magnetosomes we found that fossil magnetosomes were morphologically similar to those from living bacteria but seemed to be corroded in some cases the observed differences in quantity size and shape of magnetosomes from various sediments may be influenced by variations in bacterial viability and particle stability at different sites but are not determined by geographical latitude

an estimate of average lower mantle conductivity by wavelet analysis of geomagnetic jerks it has recently been proposed that geomagnetic jerks observed at the earth s surface could be viewed as singularities in the time behavior of the geomagnetic field with a regularity of about 1 5 when wavelet analyzed such a signal should have suffered some distortion when diffusing from the core mantle boundary cmb through the conducting mantle assuming that the upper mantle is an insulator and given the electromagnetic time constant of the mantle we compute the distortion that a pure singularity introduced at the cmb suffers as it traverses the mantle we compute this distortion through its effects on the so called ridge functions extracted from the wavelet transform of the signal this distortion is very similar to the small but significant one that we observe in real data we therefore speculate that jerks must have been pure singularities at the base of the mantle and infer an average estimate for the mantle electromagnetic time constant from the way the signal is distorted by fitting the synthetic ridge functions to the experimental ones assuming for example a thickness of 2000 km for a uniform lower conducting mantle we find an electrical conductivity smaller than 10 s m 1 this value is in reasonable agreement with values derived from high pressure experiments for a silicate mantle

aseismic seismogenic coupling for a long strike slip fault summary a 2 d mechanical model for the elastic coupling between the upper seismogenic and lower aseismic parts of a long strike slip fault is presented the model spans the quasistatic and dynamic stages of the earthquake cycle by predicting the dynamic dislocation at the fault trace in terms of the quasistatic build up of average stress drop on the seismogenic zone aa toh w where w is the width of the seismogenic zone h is the thickness of the crust and to is the time dependent remote stress the stress drop aa z is predicted to be nearly uniform with depth z except near the base of the seismogenic zone where a stress concentration develops if coseismic slip is constrained to the crust then faults which are at least four times longer in the strike direction relative to the coseismic depth attain a mean dislocation which is independent of fault length the rise time may be as small as 6 s in the absence of coseismic movement beneath the seismogenic fault and increases to about 30 s for coseismic slip of the entire crust rapid post seismic creep on the lower aseismic fault is predicted by the model since the dislocation at the surface of the earth exceeds the pre seismic dislocation at the base of the plate the application of the model to contemporary small earthquakes near the southern segment of the san andreas fault leads to an interpretation that the 1857 seismic gap is locked at a depth of about 7 8 km by asperities which become unlocked by a maximum energy release rate of the order of lo7 j m this estimate for the maximum energy release rate is equal to that of tse dmowska rice 1985 and comparable to the estimate of li 1987 of 0 6 to 3 8 x lo7 j m 2 for a typicul san andreas great earthquake with a surface dislocation of 6m an average recurrence time of 150yr a relative plate velocity of 4cmyr and a stress rate of 0 1 bar yr the model predicts for w h 8 40 and 12 30 respectively a depth of coseismic slip of 12 and 30 km a stress drop of 75 and 38 bars and a critical energy release rate of 0 28 and 0 17 x lo7 j mp2 these lower energy release rates are more consistent with the estimate of rudnicki 1980 of 0 38 x lo7 j m and imply that a great earthquake may occur before the energy release rate reaches the maximum value of lo7 j m 2

magnetic minerals in calcic luvisols chromic developed in a warm mediterranean region of spain origin and paleoenvironmental significance abstract soil formation under aerobic conditions usually enhances magnetic signals mainly through neoformation of sub micron magnetite and or maghemite in order to better understand the nature and magnitude of the enhancement in this work we characterized the iron fe oxides and magnetic properties of two calcic luvisols chromic developed on calcareous orthoquartzites in a spanish region with a warm mediterranean climate weathering of the fe bearing minerals in the parent material mostly fe rich smectite had resulted in the neoformation of hematite and maghemite rather than goethite thus causing a substantial increase in magnetic susceptibility saturation magnetization m s and other magnetic signals the absolute frequency dependent magnetic susceptibility fd which is a typical proxy for the concentration of sub micron superparamagnetic pedogenic maghemite was strongly correlated with the concentration of hematite hm the estimated percent frequency dependent magnetic susceptibility fd was 15 7 this and various other magnetic indicators indicate that the grain size distribution gsd of the pedogenic maghemite is narrower around the superparamagnetic single domain threshold than is that of maghemite in soils and paleosols developed under other temperate climates e g the chinese loesss plateau where fd fd ratio fd the hematite goethite ratio and several indicators of the degree of weathering provides a better tool for paleoclimatic reconstructions than does the current use of alone

pleistocene climates in china dated by magnetic susceptibility the magnetic susceptibility of loess and interbedded soils in central china varies with the degree of pedogenesis and serves as a proxy measure of climate the concentration of magnetic minerals in the sediment is inversely proportional to sedimentation rates throughout the brunhes epoch susceptibility measurements combined with the reversal stratigraphy provide a time scale that is independent of astronomic chronology on this scale the susceptibility record closely parallels the oxygen isotope fluctuations in deep sea sediments suggesting a close interdependence of the chinese dust falls the volume of land based ice and global climate

magnetism of chinese loess deposits summary the age of chinese loess deposits has long been disputed biostratigraphical and earlier magnetostratigraphical investigations placed the entire loess formation within the pleistocene and ascertained a maximum loess age of about 1 2 myr a new collection of nearly 500 samples from a natural outcrop and a borehole section near lochuan lat 35 8 n long 109 2 e shaanxi province has been dated by magnetic stratigraphy thermal cleaning of the natural remanent magnetization nrm removes a strong secondary component of viscous origin along the present geomagnetic field which resides largely in magnetite the characteristic nrm component is due to haematite which is thought to be of chemical origin rhythmical intensity variations of nrm and initial susceptibility depend on the loess lithology and may reflect climatic changes during loess deposition the palaeomagnetic results are consistent between the two sections and yield a clearly defined magnetic polarity zonation the brunhes matuyama boundary and the jaramillo subchron have been positively identified in both outcrops at exactly the same stratigraphic level the olduvai subchron has been found in the borehole section which records the entire loess sequence most probably the formation of chinese loess began shortly after the matuyama gauss polarity transition therefore a late pliocene age of about 2 4 myr is assigned to the oldest loess sediments measured

lower mantle conductivity anomalies estimated from geomagnetic jerks 1 the occurrence epochs and duration periods of the 1969 1978 and 1991 geomagnetic jerks are estimated for 65 geomagnetic observatories by applying a statistical time series model to monthly means of the geomagnetic eastward component it is found for every jerk that the occurrence epochs around south africa and south pacific ocean are delayed several years from those in other regions moreover it is found for the 1969 and 1978 jerks that the duration times around south africa which are defined as the lengths of the duration periods are longer than those in other regions the distribution of these anomalous regions of the jerks i e south africa and south pacific ocean may imply the existence of higher conductivities in the lower mantle beneath these regions which possibly relates to the fact that the velocity of a seismic s wave in the d layer beneath these regions is slower than that in other regions the mantle conductivities beneath south africa south pacific ocean and other regions are estimated through a numerical simulation the magnetic diffusion equation in the mantle is solved for an abrupt magnetic change generated at the core mantle boundary comparing the results obtained by the simulation with those obtained by the data analysis it is plausible that the mantle conductivities beneath south africa and south pacific ocean are higher than that beneath other regions although there is an ambiguity in the estimated magnitudes of the mantle conductivity

forecast model for large and great earthquakes in southern california an earthquake instability model is used to compute theoretical fault slip and ground surface deformation before possible future earthquakes on the san andreas fault in southern california the quasistatic model represents the earth s crust by an elastic half space and the san andreas imperial cerro prieto and san jacinto faults by flat rectangular surfaces whose strikes approximate the actual local strikes the seismogenic zone of the san andreas fault is represented by a brittle strain softening patch extending from the ground surface to 11 km depth and from parkfield to the salton sea 500 km the strength of the patch varies along strike and unstable failures of parts of the patch are earthquake analogs failed areas heal immediately after instability for the model to reproduce the lengths slips and times of earthquakes since 1080 a d reported by k sieh and others the patch must be divided into five sections along strike the sections have alternately low and high strength with the two strongest sections being roughly coincident with the major bends of the san andreas fault ten to twenty years before unstable slippage of patch areas portions of the patch bottom begin to fail stably aseismically the stable failure causes accelerating fault slip at depth and at the fault trace as well as anomalous changes in lengthening rates of trilateration lines in the model simulation the next two sections to fail unstably in the future are just north of the salton sea and just south of parkfield rupture lengths are about 100 km and 40 km at favorable locations precursory anomalies of fault slip and trilateration line length become large enough to be detected in field data a few years before each earthquake

magnetic susceptibility evidence of monsoon variation on the loess plateau of central china during the last 130 000 years the magnetic susceptibility of loess and paleosols in central china represents a proxy climate index closely related to past changes of precipitation and vegetation and thus to summer monsoon intensity time series of magnetic susceptibility constructed for three loess paleosol sequences in the southern part of the chinese loess plateau document the history of summer monsoon variation during the last 130 000 yr they correlate closely with the oxygen isotope record of stages 1 to 5 in deep sea sediments soils were forming during intervals of strong summer monsoon whereas loess units were deposited at times of reduced monsoon intensity the chinese loess paleosol sequence can thus be viewed as a proxy record of asian monsoon variability extending over the last 2 5 myr

environmental magnetism a practical guide j walden f oldfield j and smith eds technical guide no 6 quaternary research association london 1999 243 pp price 17 00 9 00 to qra members isbn 0 907780 42 3

the magnetic properties of loess and paleosol samples from the chinese loess plateau spanning the last 22 million years abstract magnetic measurements susceptibility isothermal and anhysteretic remanence and quotients derived from these are presented for a suite of loess and paleosol samples from sites on the chinese loess plateau the three sites xifeng dongwan and qinan qa i together span most of the last 22 million years the results provide the first multi parameter magnetic measurements from pre pleistocene loess paleosol sequences that began accumulation in the early miocene mass specific measurements have been corrected for carbonate concentrations which peak in the loess layers and range up to 57 all the magnetic properties recorded indicate variability on both orbital and supra orbital timescales the changing magnetic properties allow the record to be divided into a sequence of eight sample groups that broadly parallel the main supra orbital changes in magnetic susceptibility the pre pleistocene parts of the record include intervals mainly late miocene and pliocene during which loess as well as paleosol layers are characterized by high values for properties such as frequency dependent susceptibility fd usually associated with strong weathering most of the miocene record prior to 8 5 ma with the exception of a brief interval from ca 14 4 to 15 9 ma shows reduced values for indicators of pedogenic ferrimagnetic concentrations in both the loess and paleosol layers as well as a low amplitude of variability between loess and paleosol values the palaeoenvironmental implications of this part of the sequence are unclear since they hinge on the type of model of pedogenic magnetic mineral formation adopted high field remanence measurements indicate that the coercivity of the antiferromagnetic minerals represented declined progressively throughout the whole period in both the loess and the paleosol samples though there is no unambiguous evidence for long term diagenetic alteration of the magnetic properties on timescales beyond the orbital ones that drive the loess paleosol alternations the present results suggest caution in applying existing climofunctions based on magnetic susceptibility to deposits spanning the whole of the last 22 ma

early diagenesis in north atlantic abyssal plain sediments characterized by rock magnetic and geochemical indices processes of early diagenesis are investigated in a sequence of late quaternary turbiditic sediments from the madeira abyssal plain map west of the nw african continental margin in three sediment cores from this region solid phase geochemical indices of redox sensitive ion mobilization are compared with rock magnetic parameters as proxies for diagenesis characterization the mud dominated distal turbidites are emplaced mainly as a result of glacio eustatic changes in sea level and derive both from different sites on the nw african margin and from the flanks of the canary islands thus the turbidites are of variable composition especially in terms of carbonate detrital magnetic mineral and organic carbon content within organic rich 0 5 corg turbidite horizons remineralization of organic detritus proceeds in two stages the initial stage involves suboxic reductive diagenesis following depletion of pore water o2 while the later stage involves oxidative diagenesis associated with the slow descent of an oxidation front through the sediment due to diffusion of o2 from the overlying seawater rock magnetic parameters of these sediments when expressed on a carbonate free basis reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains has occurred within the organic rich turbidites during suboxic diagenesis normalized quotients of magnetic parameters and remanence coercivity profiles of selected samples also show that reductive diagenesis is a ferrimagnetic grain size selective process but it has a minimal effect on the canted antiferromagnetic fe3 oxides in the sediment such components if present therefore become relatively enriched in magnetic assemblages as the ferrimagnetic grains are progressively dissolved in all three cores studied however there is clear evidence for the presence of ultrafine ferrimagnetic iron oxide grains at depth within the suboxic zones of organic rich turbidites beneath both active and fossil oxidation fronts these grains are probably associated with populations of live magnetotactic bacteria which inhabit such organic rich horizons and which form a link in the chain of bacterially mediated reactions normally associated with suboxic diagenesis these results comply with previous observations made in studies of suboxic diagenesis in hemipelagic sediments and demonstrate the value of rapid and non destructive rock magnetic measurements for illuminating redoxomorphic processes of diagenesis involving transformation of iron phases in deep sea sediments

on the geomagnetic jerk of 1969 french and british scientists have published reports describing a sudden change in the geomagnetic secular acceleration which took place around 1969 they claimed that this change called an impulse or jerk took place in a period of a year or two and that the sources for the jerk were internal a recent paper by alldredge questioned their method of analysis and concluded that abrupt changes in the observed field which occur in a year or two do not originate in the core of the earth this paper questions alldredge s conclusion and provides further evidence in support of the reports of the french and british investigators the average centroid of the jerk is found to be 1969 5 evidence is presented that indicates that the centroid of the jerk differs for the various spherical harmonics so that the centroid of the degree one spherical harmonic is significantly later than the centroids for the next three harmonics new estimates for the spherical harmonic coefficients for the jerk and the prejerk and postjerk secular acceleration are given some possible further investigations are suggested

quantifying grain size distribution of pedogenic magnetic particles in chinese loess and its significance for pedogenesis quaternary glacial interglacial cycles have been imprinted on the chinese loess paleosol sequences through pedogenesis in order to accurately decode the paleoclimatic signals carried by these pedogenic particles it is essential to quantify the pedogenically produced magnetic particles in terms of mineralogy as well as grain size distribution gsd to date the gsd has not been accurately determined because of the dearth of available means for analyzing extremely fine grained nanometer scale pedogenic magnetic particles using low temperature techniques we systematically investigated the temperature dependency of fd defined as 1hz 10hz where 1hz and 10hz are ac magnetic susceptibility measured at 1 and 10 hz respectively from two characteristic loess profiles one located at the western chinese loess plateau and the other in the central plateau on the basis of neel theory for a shape anisotropy dominant grain and experimental analysis at low temperatures a quantitative gsd for pedogenic particles in chinese loess paleosols was constructed we found that the dominant magnetic grain size lies just above the superparamagnetic single domain threshold 20 25 nm and that the gsd is almost independent of the degree of pedogenesis this observation agrees well with other constraints from previous studies this new gsd model improves our understanding of the pedogenic processes in chinese loess enabling further explicit linkage of environmental magnetism to paleoclimate changes

two different r n geomagnetic reversals with identical vgp paths recorded at the same site we have studied two reverse normal r n geomagnetic reversals recorded at different stratigraphical levels of a single section of tortonian marine clays in western crete we report here that the virtual geomagnetic pole vgp paths associated with these two transitions which are separated in time by a period of 1 myr are identical although the possibility of an accidental coincidence cannot be rejected this result suggests that the basic mechanisms in the geodynamo leading to a reversal might persist unchanged over a long period of time moreover the two vgp paths are largely constrained in longitude along a mean great circle located 80 w of the site so that the transitional field cannot be correctly described in terms of pure axial symmetry

magnetostratigraphy of a late miocene pliocene loess soil sequence in the western loess plateau in china 1 eolian deposits of pliocene age have never been reported from the western loess plateau in china here a 73 7 m eolian sequence is dated using magnetostratigraphic method and micromammalian fossils the polarity zonation correlates with the geomagnetic polarity timescale gpts from chron 3br 1r to chron 2an 3n indicating an age from 7 10 to 3 52 ma bp the sequence is characterized by clear expression of eighty four pairs of loess and soil layers while individual soils are basically indefinable for the eolian deposits of the same age in the eastern loess plateau suggesting that the later has experienced other geological processes this sequence extends the upper limit of the previously reported miocene loess soil sequences at qinan into the pliocene about 3 52 ma bp

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

numerical models of the geodynamo and observational constraints 1 the past few years have seen the emergence of a large number of numerical simulations of the geodynamo in parallel both new and old geomagnetic archeomagnetic and paleomagnetic observations have been interpreted as actual geomagnetic features and used as constraints for dynamo models naturally model predictions should be tested against actual characteristics of the geomagnetic field despite huge differences sometimes in excess of a billion between the values of parameters used in the simulations and those estimated for the earth it is intriguing that many available simulations succeed in producing largely axial dipolar magnetic fields with weaker nondipolar structures in agreement with the first order characteristics of the geomagnetic field yet when considering finer characteristics there are significant differences and failures to actually produce a number of fundamental characteristic features in this presentation we first review numerical results obtained to date then we attempt to summarize which field characteristics derived from observational data sets can be considered robust on the basis of simple criteria used to evaluate the degree of confidence that can be placed in each datum we sort presumably characteristic geomagnetic features into three categories robust controversial and unlikely we conclude that numerical models should be illustrated with a number of key predictions averaged over at least 10 dipole diffusion times these predictions should be tested against the subset of robust observations only controversial observations should await additional confirmation

wind blown origin of dongwan late miocene pliocene dust sequence documented by land snail record in western chinese loess plateau recently the first late miocene pliocene eolian deposits from the western chinese loess plateau have been reported these deposits would complement the eolian record between quaternary loess paleosol sequences and miocene loess sequences if their wind blown origin could be documented however although several lines of evidence indicate the eolian origin of this sequence it is still necessary to address this issue through further efforts among these biological approaches are of particular value here we analyze the composition and preservation conditions of mollusk assemblages in the dongwan section our results show that all mollusk fossil individuals are preserved in perfect condition without indicating any reworking all identifiable mollusk species are terrestrial and are mostly identical to those identified in the quaternary loess paleosol sequences of the chi nese loess plateau generally the cold aridiphilous species are dominant in the loess layers while thermo humidiphilous species are mostly encountered in the paleosols the close similarity of the mollusk record in both the dongwan late miocene pliocene deposits and the quaternary loess sequence in the chinese loess plateau supports the supposition that the dongwan section is a typical loess paleosol sequence and is without any doubt of wind blown origin

magnetostratigraphy of an early middle miocene loess soil sequence in the western loess plateau of china 1 stratigraphy and magnetic susceptibility of the quaternary eolian deposits at different sites in china can be closely correlated confirming their spatial coherence and temporal continuity the extent to which these conclusions are true for the miocene loess soil sequences remains to be addressed here a 231 9 m eolian section qa iii is geomagnetically dated yielding an age span of 21 4 11 4 ma with the portion older than 19 6 ma being partially water reworked the magnetozones litholigic structure and magnetic susceptibility of the sequence are highly correlative with the qa i and qa ii sections 30 km west of qa iii these sequences demonstrate that the miocene loess deposits are also spatially correlative in stratigraphy present a near continuous sequence of paleoenvironment changes and that magnetic susceptibility is a useful tool for stratigraphic correlation the basal age of qa iii confirms again the onset of asian inland deserts and monsoon dominate climates by the early miocene

constitutive relations for fault slip and earthquake instabilities constitutive relations for fault slip are described and adopted as a basis for analyzing slip motion and its instability in the form of earthquakes on crustal faults the constitutive relations discussed include simple rate independent slip weakening models in which shear strength degrades with ongoing slip to a residual frictional strength and also more realistic but as yet less extensively applied slip rate and surface state dependent relations for the latter the state of the surface is characterized by one or more variables that evolve with ongoing slip seeking values consistent with the current slip rate models of crustal faults range from simple single degree of freedom spring slider systems to more complex continuous systems that incorporate nonuniform slip and locked patches on faults of depth dependent constitutive properties within elastic lithospheric plates that may be coupled to a viscoelastic asthenosphere

a geomagnetic jerk for the early 1990 s abstract records from geomagnetic observatories in europe are showing indications of another impulse in the third time derivative of the east component around 1991 these impulses otherwise known as jerks have occurred in the past at about 1925 1969 and 1978 and possibly also 1900 and 1913 they are believed to be internal in origin and have implications for the understanding of the fluid motions in the core which create them or for lower mantle conductivity or both

rock magnetic properties confirm the eolian origin of miocene sequences from the west of the chinese loess plateau abstract diagnostic rock magnetic characteristics of miocene sequences from the western part of the chinese loess plateau are used to confirm the eolian nature of sections the origin of which has been a matter of recent dispute alonso zarza et al 2009 2010 guo et al 2010 the key magnetic properties include i detailed correlation of both the orbitally timed and supra orbital variations in magnetic properties between widely separated sections ii the bi modal distribution of magnetic properties with respect to particle size reflecting in situ neoformation alongside the deposition of detrital minerals derived from the source areas iii the consistent presence in the clay fractions of fine ferrimagnetic grains close to the superparamagnetic stable single domain transition all within the size range characteristic of those formed by weathering and pedogenesis under at least partially aerobic conditions and iv absence of evidence for lacustrine indicators such as bacterial magnetosome assemblages sub oxic diagenesis or greigite formation

a global analysis of the 1991 geomagnetic jerk summary a recent examination of the geomagnetic annual mean values for the european magnetic observatories has shown the existence of a sudden change in the secular acceleration in about 1991 cafarella meloni 1995 macmillan 1996 using first differences of the y east geomagnetic field component mean values from 74 observatories the worldwide character of the 1991 impulse has been determined de michelis et al 1998 using data from 109 observatories widely distributed all over the world the structure of the secular variation for the x north and z vertical magnetic field intensities around 1990 was investigated and evidence of this most recent jerk was found external effects were removed from the annual mean data by comparing the long term variations of the geomagnetic field components at individual observatories with the long term variations of two geomagnetic indices aa and dst and of a solar index the wolf number r a careful analysis has been carried out on the amplitude of the external disturbance on its dependence on latitude and on the weights of the geomagnetic indices in the evaluation of the resulting external field the secular variation has been evaluated from the corrected annual means around 1990 the secular variation can be fitted at many observatories by two straight lines with a sudden and marked change in slope in this manner the jerk occurrence time and the intensity of the step in the second time derivative x y and z were computed maps of x y and z provide information on the worldwide intensity distribution of the examined event maps of the jerk occurrence time distributions are also given the mean jerk occurrence time is 1990 1 0 6 finally a spherical harmonic analysis was used to complete the quantitative description of this phenomenon in order to study the trend of the energy density spectrum as a function of the harmonic degree n

magnetic flux expulsion from the core as a possible cause of the unusually large acceleration of the north magnetic pole during the 1990s 1 the north magnetic pole nmp has been drifting in a north northwesterly direction since the 19th century both local surveys and geomagnetic models derived from observatory and satellite data show that the nmp suddenly accelerated during the 1990s its speed increased from about 15 km yr in 1989 to about 60 km yr in 2002 after which it started to decrease slightly using a green s function we show that this acceleration is mainly caused by a large negative secular variation change in the radial magnetic field at the core surface under the new siberian islands this change occurs in a region of the core surface where there is a pair of secular variation patches of opposite polarities which we suggest could be the signature of a so called polar magnetic upwelling of the type observed in some recent numerical dynamo simulations indeed a local analysis of the radial secular variation and magnetic field gradient suggests that the secular variation change under the new siberian islands is likely to be accompanied by a significant amount of magnetic diffusion in agreement with such a mechanism we thus hypothesize that the negative secular variation change under the new siberian islands that produced the nmp acceleration could result from a slowdown of the polar magnetic upwelling during the 1990s we finally note that the nmp drift speed is determined by such a combination of factors that it is at present not possible to forecast its future evolution

physical models of earthquake instability and precursory processes this paper selectively reviews physical models of earthquake instability in these models instability arises as a result of interaction of a fault constitutive relation with deformation of the surrounding material that occurs in response to remote tectonic loading in contrast to kinematic models in which the fault slip is imposed it is calculated in physical models and consequently these models are essential for understanding precursory processes some kind of weakening behavior for the fault constitutive relation is required to produce an instability analogous to an earthquake two commonly employed idealizations discussed here are rate independent slip weakening and rate state dependent friction when these constitutive models are employed on surfaces embedded in elastic half spaces or layers possibly coupled to a viscoelastic substrate the results are capable of simulating realistically some aspects of earthquake occurrence common to all models is the prediction that earthquake instability is preceded by precursory slip which produces a departure of surface strain rate from the background level near the epicenter of a moderate to large earthquake the magnitude of this departure appears to be well within the range of current geodetic measurement accuracy and its duration is of the order of months to years however details depend on a variety of factors including the modelling of the constitutive relation near peak stress coupling of elastic crust to the asthenosphere and coupling of deformation with pore fluid diffusion

about the potential of wells to reflect stress variations within inhomogeneous crust abstract kumpel h j 1992 about the potential of wells to reflect stress variations within inhomogeneous crust in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 317 336 numerous pre co and post seismic well level changes that are believed to reflect stress variations in the crust have been observed in various parts of the world so far analysis of these data has not favoured a distinct model of the processes that could be involved the benefit from the signals is in fact limited because understanding of the underlying physical mechanisms is poor the paper discusses the impact that inhomogeneities in the brittle crust can have on the observations in particular the following problems are considered 1 could the well level signals be induced by the invasion of pressurized pore fluid at some place 2 could the hydrological conditions close to the observation well modulate any remote pore pressure variation 3 which features of the crust are most likely to dominate the process of stress transfer in the crust 4 how should a stress sensitive well be designed the results can be summarized as follows 1 stress diffusion due to the invasion of pressurized pore fluid can lead to significant well level changes only when the source region is at closer distance than a few kilometres probably less than a few hundred metres from the well 2 local inhomogeneities in the vicinity of the observation well may strongly influence the well s response to remote pressure changes even sign reversal of the anomaly may be encountered 3 there is increasing evidence that the wide range of stiffness values of natural rocks from 100 mpa or less till 100 gpa manifested in a fractal distribution of fault like weak zones within relatively stiff formations dominates stress transfer in the brittle crust uncertainty about the effective rheology for time scales from 10 4 to 103 years limits the inversion of many geophysical observations at present 4 of various configurations a well tapping an extensive hydraulic fracture could act best as a sensitive stress sensor provided that the fracture volume is isolated from the pore space of the host rock however experimental evidence of the practicability of such configuration is still due the weakness of conventional wells as stress sensors results from the fact that the process of fluid flow often lacks a sufficient control on attenuation mechanisms for low frequency well level variations regardless of whether the wells end in stiff or weak formations to a large extent the presented findings are compiled from publications of investigators working in various fields

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

particle size separation and evidence for pedogenesis in samples from the chinese loess plateau spanning the past 22 m y a comparison between particle size distributions determined by laser diffraction and the pipette method confi rms previous studies that point to a variable underestimate of the clay fraction using laser diffraction relative to the values obtained by pipette analysis in order to yield four particle sized fractions 8 m for further analysis the pipette method was used on loess and paleosol samples from sites on the chinese loess plateau spanning the past 22 m y in all cases a subsidiary mode in the 8 m fraction provides a basis for evaluating the relative contributions of pedogenic and detrital components pretreatment using acetic rather than hydrochloric acid for carbonate removal results in no signifi cant loss of ferrimagnetic minerals and allows magnetic characterization of the separate fractions this confi rms the isolation of almost all the fi nest pedogenic ferrimagnetic grains in the clay fraction and their negligible contribution to the magnetic properties of the coarse fractions in all the samples from early miocene time onward because similar conclusions may be drawn from previous studies of late pleistocene samples from the east central and extreme western parts of the chinese loess plateau we conclude that this approach provides a basis for separately characterizing eolian deposition and pedogenesis throughout the region and for the whole time span of loess accumulation

stiffness method for anticipating earthquakes in strain softening earthquake models the single variable r kf ks measures the nearness to earthquake instability kf and ks are stiffnesses of the fault zone and elastic surroundings instability occurs when increasing r reaches unity for a simple two dimensional antiplane model r is a function of d e du and h where e is shear strain at the ground surface and u is remote displacement observed at a distance h from the fault r is independent of the fault zone constitutive law crustal rigidity and shear stress magnitude

a time dependent model of the earth s magnetic field and its secular variation for the period 1980 2000 1 this study presents an investigation and description of the secular variation of the earth s magnetic field between 1980 and 2000 a time dependent model c3fm continuous covariant constrained endpoints field model of the main field and its secular variation between 1980 and 2000 is developed with gauss coefficients expanded in time on a basis of cubic b splines this model is constrained to fit field models from high quality vector measurements of magsat in 1980 and orsted in 2000 and to fit both magnetic observatory and repeat station secular variation estimates for the period in between these secular variation estimates first time derivatives are derived from observatories monthly or annual means and repeat station data in order to reduce the contributions of crustal noise annual and semiannual variation on average the model input consists secular variation estimates of the x y and z components at 130 locations per month treatment of covariance between the different components allows a higher temporal sensitivity of the model due to the exclusion of some external field variation the model is computed up to degree and order 15 the model is a useful extension of the hitherto existing time dependent description of the secular variation gufm which describes the secular variation until 1990 it reveals a short term secular variation on subdecadal timescale and has a higher spatial resolution than previously resolved the model is also valuable to test the frozen flux hypothesis and to link features of the radial field at the core mantle boundary to the geodynamo

correlation of himalayan exhumation rates and asian monsoon intensity although most data suggest that the india eurasia continental collision began 45 55 myr ago the architecture of the himalayan tibetan orogen is dominated by deformational structures developed in the neogene period 23 myr ago the stratigraphic record and thermochronometric data indicate that erosion of the himalaya intensified as this constructional phase began and reached a peak around 15 myr ago it remained high until 10 5 myr ago and subsequently slowed gradually to 3 5 myr ago but then began to increase once again in the late pliocene and pleistocene epochs here we present weathering records from the south china sea bay of bengal and arabian sea that permit asian monsoon climate to be reconstructed back to the earliest neogene these indicate a correlation between the rate of himalayan exhumation as inferred from published thermochronometric data and monsoon intensity over the past 23 myr we interpret this correlation as indicating dynamic coupling between neogene climate and both erosion and deformation in the himalaya although the india eurasia collision initiated 50 myr ago major deformation and exhumation of the himalaya did not begin until the early neogene 23 myr ago this coincides with the increased intensity of the asian monsoons as indicated by weathering records from the south china sea bay of bengal and arabian sea and hints at a dynamic coupling between climate and both erosion and deformation in the himalaya

establishing fire incidence in temperate soils using magnetic measurements abstract natural unburnt topsoil samples from the cotswolds region of england and samples from archaeological contexts in the uk and hungary some of which are known to have been affected by burning have been used to identify a distinctive magnetic signature linked to the effects of fire on magnetic properties a combination of bulk low field lf and frequency dependent fd magnetic susceptibility and normalized anhysteretic remanent magnetization arm measurements have been used on both unburnt and experimentally burnt samples from the cotswolds and on the archaeological samples complementary determinations were made for both sets of samples of the maximum potential susceptibility max and hence percentage fractional conversion conv which provides independent evidence of burning the results show that the ferrimagnetic mineral assemblages produced as a result of fire have a significantly finer grain size than those arising from weathering and soil formation alone burning thus gives rise to a distinctive envelope of values on a bilogarithmic plot of arm fd versus arm lf such a diagram provides a convenient basis for identifying magnetic signatures in soils archaeological materials and palaeosols that have arisen mainly through burning

sources of fine grained magnetic minerals in sediments a problem revisited progress in the discrimination of fine grained magnetic minerals in sediments and sediment sources using measurements of magnetic susceptibility at two frequencies and of magnetic remanence generated by a combination of direct and alternating currents is summarized the former measurements yield values for low field and frequency dependent susceptibility lf and fd respectively the latter generate an anhysteretic remanent magnetisation arm that when normalized to the dc bias field can be represented as the susceptibility of arm arm using quotients derived from lf fd and arm to generate bi logarithmic plots it is possible to discriminate between assemblages dominated by magnetic minerals formed through pedogenesis and those arising from the presence of magnetotactic bacteria that use chains of magnetic particles to seek optimum environments in surface sediments sediments to which both types of magnetic mineral contribute occupy positions between the envelopes of values typical of soil d

geomagnetic disturbance fields an analysis of observatory monthly means summary this work quantifies the extent to which disturbance phenomena contribute to the observed geomagnetic field on the time scale of months to years a deterministic approach was adopted in which geomagnetic monthly means were analysed in the time domain by presupposing the time dependence of the external and induced field variations which we assume to vary as the aa geomagnetic index we arrived at an estimate of the relative amplitude of the disturbance to each component of the field at a number of observatories in all some 58000 geomagnetic monthly means from 59 observatories operating this century were analysed special attention was paid to the selection and validation of the data as otherwise the numerical computations would have been unduly compromised the results consist of 177 estimates of the relative amplitude of disturbance one for each of three orthogonal components north x east y and vertically down 2 at each observatory the disturbance to the x component was found to be consistently negative over the whole of the earth s surface with an intensification in auroral regions the disturbances to the z component were found to be smaller than that for x except in high latitudes mean disturbances to the y component were smaller still results were in general consistent with the dipole field of the magnetospheric ring current aligned with though of opposite polarity to the earth s main field typical amplitudes of the mean disturbance field from month to month were of the order 10nt the results can be used to estimate the variation of the disturbance field subtracting this from both monthly and annual means yields an improved estimate of the field originating in the core and its secular variation some examples are presented

a local intermittency measure lim approach to the detection of geomagnetic jerks abstract in the present work we investigate the temporal distributions of the geomagnetic jerks occurring in the last three decades of the 20th century using a new method of analysis based on the wavelet transform the local intermittency measure lim this method which allows us to single out intermittent structures within a given time series was applied to geomagnetic monthly means time series of the eastward magnetic field component y of 44 worldwide observatories we study the bimodal distribution of the jerk occurrence in time for the 1978 and 1991 events and point out a local regional jerk in south africa and in the pacific ocean which happened around 1986 the worldwide character of the 1999 geomagnetic jerk is also investigated integrating the lim analysis with the traditional straight line fit

forecasting southern california earthquakes since 1978 and 1979 california has had a significantly higher frequency of moderate to large earthquakes than in the preceding 25 years in the past such periods have also been associated with major destructive earthquakes of magnitude 7 or greater and the annual probability of occurrence of such an event is now 13 percent in california the increase in seismicity is associated with a marked deviation in the pattern of strain accumulation a correlation that is physically plausible although great earthquakes magnitude greater than 7 5 are too infrequent to have clear associations with any pattern of seismicity that is now observed the san andreas fault in southern california has accumulated sufficient potential displacement since the last rupture in 1857 to generate a great earthquake along part or all of its length

crustal deformation in great california earthquake cycles periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model a depth l h extending downward from the earth s surface at a transform boundary between uniform elastic lithospheric plates of thickness h is locked between earthquakes it slips an amount consistent with remote plate velocity vpl after each lapse of earthquake cycle time tcy lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress the plates are coupled at their base to a maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions compatible with plate velocity are transmitted to the surface plates the coupling is described approximately through a generalized elsasser model we argue that the model gives a more realistic physical description of tectonic loading including the time dependence of deep slip and crustal stress buildup throughout the earthquake cycle than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone parameters of the model are chosen to fit seismic and geologic constraints and the apparent time dependence of surface strain rates along presently locked traces of the 1857 and 1906 san andreas ruptures we fix vpl 35 mm yr tcy 160 years and l 9 11 km based on earthquake nucleation depths the geodetic data are then found to be described reasonably within the context of a model that is locally uniform along strike and symmetric about a single san andreas fault strand by lithosphere thickness h 20 30 km and elsasser relaxation time tr 10 16 years we therefore suggest that the asthenosphere appropriate to describe crustal deformation on the earthquake cycle time scale lies in the lower crust and perhaps crust mantle transition zone and has an effective viscosity between about 2 1018 and 1019 pa s depending on the thickness assigned to the asthenospheric layer predictions based on the chosen set of parameters are also consistent with data on variations of contemporary surface strain and displacement rates as a function of distance from the 1857 and 1906 rupture traces although the fit is degraded by asymmetry relative to the fault and by slip on adjacent fault strands

asperity models for earthquakes a quasi static numerical method was used to simulate the failure of a strong stuck asperity on an otherwise creeping fault plane the numerical model produced the same slip distribution as analytical asperity models which for constant loading rate produced repeating events having a period t that scales with moment m 0 as t propto m 0 1 6 the scaling relation observed by nadeau and johnson 1998 at parkfield when the asperity is a composite of smaller hard unit asperities we still find t propto m 0 1 6 but the constant of proportionality depends on the density of unit asperities within the cluster since only clusters with a fixed asperity density follow the observed t propto m 0 1 6 scaling this result rules out the fractal spatial distribution cantor dust of unit asperities at parkfield suggested by sammis et al 1999 for solid asperities the average stress drop on the asperity is on the order of 100 mpa but the average stress drop over the entire rupture area is significantly lower equivalent to that estimated from spectral analysis however the stress drop for asperity models is not independent of seismic moment but decreases with earthquake size the energy release rate for asperity events is estimated to be mathcal g gtrsim 10 7 j m 2 near the upper limit of estimates by li 1987 using parameters from large events on the san andreas fault

preseismic rupture progression and great earthquake instabilities at plate boundaries we present a procedure for modeling the initially quasi static upward progression of a zone of slip from some depth in the lithosphere toward the earth s surface along a transform plate margin culminating in a great crustal earthquake stress transmission in the lithosphere is analyzed with a generalized elsasser model in which elastic lithospheric plates undergo plane stress deformation and are coupled by an elementary foundation model to a maxwellian viscoelastic asthenosphere upward progression of rupture over a finite length of plate boundary corresponding to a seismic gap along strike is analyzed by a method based on the line spring concept whereby a two dimensional antiplane analysis of the upward progression provides the relation between lithospheric thickness averaged stress and slip used as a boundary condition in the generalized elsasser plate model the formulation results in a nonlinear integral equation for the rupture progression as a function of time and distance along strike a simpler approximate single degree of freedom analysis procedure is described and shown to lead to instability results that can be formulated in terms of the slip softening slope at the boundary falling below the elastic unloading stiffness of the surroundings the results also indicate a delay of ultimate seismic instability due to the stiffer short versus long time asthenospheric response and predict a final period of self driven creep toward instability the procedures for prediction of rupture progression and instability are illustrated in detail for an elastic brittle crack model of slip zone advance and parameters of the model are chosen consistently with great earthquake slips and stress drops for example an effective crack fracture energy of the order 4 106 j m2 at the peak 7 to 10 km below surface of a gaussian bell shaped distribution of fracture energy with depth with variance of the order 5 km simulating strength build up in a seismogenic layer leads to prediction of nominal seismic stress drops of 30 to 60 bars and slips of 2 to 5 m in great strike slip earthquake ruptures breaking 100 to 400 km along strike precursory surface straining in the self driven stage is predicted to proceed at a distinctly higher rate over time intervals beginning 3 to 10 months before such an earthquake this interval being greater for longer distances along strike over which the preseismic upward rupture progression takes place

was the 1970 geomagnetic jerk of internal or external origin annual mean data from worldwide magnetic observatories show that there was a jerk1 step change in the second time derivative in the geomagnetic field which took place over an interval of 2 yr around 1970 if such a short lived phenomenon originated in the earth s core and were still detectable after passing through the mantle to the surface this would imply a much lower conductivity for the lower mantle than had been believed previously here we show by an objective test that most of the jerk has an internal origin

geochemical characteristics of the miocene eolian deposits in china their provenance and climate implications 1 in the loess plateau in northern china the quaternary loess soil sequences the hipparion red earth of eolian origin red clay and the miocene loess soil sequences constitute a near continuous terrestrial record of paleoclimates for the past 22 ma in this study miocene loess and paleosol samples from qinan qa i were analyzed for their major trace and rare earth element chemistry and compared with the pliopleistocene samples from xifeng with emphasis on their provenance and paleoclimatic implications the results show similar geochemical signatures for the eolian deposits of different ages and they are also comparable to the average composition of the upper continental crust these suggest that the dust materials were all derived from well mixed sedimentary protoliths which had undergone numerous upper crustal recycling processes they also support the notion of broadly similar source areas and dust transporting trajectories for different periods since the early neogene the slightly higher k2o fe2o3 and mgo concentrations and loss on ignition values and the lower na2o content in the miocene loess samples compared to their quaternary counterparts are attributable to the finer grain size of the miocene loess associated with weaker dust carrying winds in comparison with some loess in europe and america with less extensive sources eolian deposits from northern china show higher cs and lower zr and hf content this is attributable to the sorting processes from remoter sources during transportation and could be regarded as an indication of the desert origin of the loess deposits miocene paleosol samples show higher chemical index of alteration values and lower cao mgo and na2o concentrations than does the intervening loess indicating stronger weathering of the paleosols however the moderate chemical weathering of the paleosol samples indicates a constant semiarid and subhumid climatic range in northern china since the early miocene components 9791 words 8 figures 3 tables

stress transfer and nonlinear stress accumulation at subduction type plate boundaries application to the aleutians a study of stress accumulation in seismic gaps and of stress transfer along linear plate boundaries is presented time dependent reloading of plate boundaries following seismic ruptures is modeled by a modified elsasser model of a coupled lithosphere asthenosphere plate system this model is applied to study a series of large earthquakes in the aleutian islands and the alaska peninsula in 1938 1965 it is found that the rat island earthquake and the 1948 earthquake in the central aleutians are likely to have been triggered by adjacent ruptures in the sense that their occurrence would have come at a later time had their neighboring segments not been ruptured stresses in the unalaska gap and the shumagin gap are at a relatively high level and these segments of the plate boundary may be expected to rupture in the near future in general in the ten years about 16 of the earthquake cycle for the aleutians following an earthquake the stress recovery in the rupture zone is highly nonlinear resulting in a much more rapid stress accumulation than the linear case even at a later stage of an earthquake cycle adjacent ruptures can cause an acceleration of loading rate in addition to the coseismic stress jump a good example is the influence of the 1964 alaska earthquake on the 1938 rupture zone a general conclusion of this work is that long term earthquake prediction models must take into account the nonlinear stress accumulation behavior in seismic gaps also we have shown the interaction of adjacent plate boundary segments which suggests that some large earthquakes may have been triggered by nearby ruptures

diagenesis of magnetic minerals in the recent sediments of a eutrophic lake magnetic mineral assemblages in lake sediments are usulally interpreted as reflecting changing catchment erosion magnetic and chemical assemblages in a small eutrophic lake lough augher northern ireland suggest diagenetic dissolution of fine grained magnetite however probably as a result of changing redox conditions associated with eutrophication water column results show internal cycling of mn and fe from the sediments into the armxic hypolimnion calculations suggest that late summer hypolimnetic fe concentrations are controlled by formation of aulhigenic vivianite sedimentary pore water concentrations of total soluble fe and p suggest a similar process in the sediments the loss of mn from the lake by diffusion from the sediment and washout at overturn is indicated by mn budget calculations and by it marked decrease in its sedimentary concentration reflecting onset of eutrophication as indicated by diatom biostratigraph y the changes in the sediment mn profile are coincident with and in some cases identical to changes in several rnagnctic parameters the changes in magnetic stratigraphy suggest a coarsening of the magnetic grain size and reduction in the total concentration agreement with chemical profiles indicative of changing rcdox suggest that this coarsening results from dissolution of fine grained magnetite catchment stability together with other stratigraphic profiles of eutrophication and changing redox supports the conclusion that the magnetic profiles are reflecting in situ diagenetic changes rather than altered catchment processes with the data available it is not possible to identify the actual clissolution process involved or to determine whether it is chemically or bacterially controlled

magnetic properties of modern soils and quaternary loessic paleosols paleoclimatic implications the magnetic properties of paleosols developed in quaternary sequences of loess have been used for stratigraphic definition correlation with other terrestrial and deep sea sequences and paleoclimatic paleorainfall reconstruction in some loess paleosol sequences including those of the chinese loess plateau tajikistan and the czech republic maxima in magnetic susceptibility values correspond with the paleosol horizons and minima with the least weathered loess layers in other loess soil sequences including those of siberia alaska and argentina the relationship is completely opposite with susceptibility minima associated with the most developed paleosols to account for these opposite relationships the respective roles of 1 magnetic enhancement and 2 magnetic depletion and or dilution in determining soil magnetic properties are investigated for a range of modern soil types most magnetic enhancement is seen in the upper horizons of well drained cambisols absence or loss of magnetic iron oxides is apparent in acid podsol profiles and waterlogged soils for the cambisol profiles significant correlation is found between susceptibility and organic carbon cation exchange capacity and clay content the mineralogy morphology and grain size of soil magnetic carriers extracted from three modern enhanced soils and a paleosol from the chinese loess plateau are also identified by independent petrographic means microscopy and x ray diffraction of magnetic extracts magnetite and maghemite of ultrafine grain size from 0 4 to 0 001 i m are the major contributors to the magnetically enhanced soils weathering can concentrate detrital magnetic grains especially in the fine silt size fractions of soils the magnetic data from the modern soils indicate that interpretation of paleosol magnetic properties must be done on a site specific basis taking into account the possibilities of pedogenic enhancement pedogenic dilution or depletion and allochthonous inputs of magnetic minerals excessively arid wet or acid soils are unable to form significant amounts of pedogenic ferrimagnets well drained intermittently wet dry soils with reasonable buffering capacity and a substrate source of fe show most magnetic enhancement for those soils which favour magnetic enhancement processes correlation has been found between the maximum value of the pedogenic susceptibility and the annual rainfall the almost unique pedogenic system in the chinese loess plateau where variation in soil forming factors other than climate is reduced to a global minimum allows use of the paleo susceptibility values as proxy paleorainfall values to identify the mechanism of any link between magnetic properties for example susceptibility and climate change the mineralogy and grain size of soil magnetic carriers requires detailed investigation given the prospect of 1 quantitative paleorainfall records from continental loess records 2 predictions of present and future climate change and 3 the value of paleoclimate data for testing of numerical climate models such magnetic investigations can contribute significantly to our understanding of past and future environmental change

the magnetic properties of particle sized samples from the luo chuan loess section evidence for pedogenesis abstract measurements of magnetic susceptibility and of laboratory remanences have been carried out on particle size fractions of loess and palaeosol samples from luo chuan china they show that the sub micron clay fraction of the palaeosol is greatly enriched in fine grained

what do the hirm and s ratio really measure in environmental magnetism the hard isothermal remanent magnetization hirm and the s ratio are widely used in environmental magnetism to quantify the absolute and relative concentrations respectively of antiferromagnetic minerals hematite and goethite in mineral mixtures we demonstrate that synthetic al substituted hematite and goethite exhibit a wide range of coercivities which significantly influences the hirm and s ratio these parameters are therefore not necessarily straightforward indicators of the absolute and relative concentrations of hematite goethite to circumvent this problem we propose a new parameter the l ratio which is the ratio of two remanences after alternating field af demagnetization of an irm imparted in a 1 t field with a peak af of 100 mt and 300 mt irmaf 300mt irmaf 100mt these parameters are easily measured using modern vibrating sample or alternating gradient magnetometers changes in hirm only reflect changes in the absolute concentration of hematite and or goethite if the l ratio is relatively constant conversely l ratio fluctuations indicate variable coercivities that possibly reflect changes in the source of hematite goethite corresponding hirm and s ratio variations should be interpreted with caution in such cases the l ratio can be determined using equivalent terms depending on available instrumentation and measurement protocols for example the hirm is equivalent to irmaf 300mt likewise 0 5 sirm irm 100mt where irm 100mt represents the remanent magnetization obtained by first saturating the sample in a high field and then applying a back field of 100 mt is equivalent to irmaf 100mt the hirm 0 5 sirm irm 100mt ratio is therefore a suitable substitute for the l ratio when measurements are made with a long core magnetometer the newly proposed l ratio is straightforward to measure on a wide range of instruments and can provide significant new insights and reduce ambiguities associated with interpretation of two widely used parameters in environmental magnetism the hirm and s ratio

toward the discrimination of fine grained ferrimagnets by magnetic measurements in lake and near shore marine sediments the magnetic properties of low field susceptibility frequency dependent susceptibility fd and susceptibility of anhysteretic remanent magnetisation arm are used to define sets of natural soil and sediment samples within which by the criteria of king et al 1982 and maher 1988 the modal ferrimagnetic grain diameter is less than 0 07 0 1 m within this sample set two distinct groups with low and high values respectively for the quotients arm and arm fd are defined the first group includes sediment samples from sites where published studies propose a detrital origin for the fine grained ferrimagnetic content where catchment samples are available for comparison they fall within the same envelope as do clays from palaeosol samples within chinese loess this envelope of low quotient values also overlaps with the values for the fine grained synthetic magnetite samples within maher s 1988 new mt series the high quotient envelope of values includes sediments from the adriatic sea and clays from saltmarsh and shallow water marine sediments in nw britain in these cases no catchment source is postulated for the fine grained magnetite the size range of magnetite in this set appears to be almost exclusively stable single domain ssd and a bacterial origin seems likely sample sets from estuarine environments between river inflow sites and the open sea show intermediate values which in the case of the potomac are ordered by distance down river although at this stage magnetic measurements alone cannot discriminate between bacterial and fine grained detrital ferrimagnets in sediments they hold out some promise of doing so provided the distinctions proposed here can be substantiated by transmission electron microscopy

extending comprehensive models of the earth s magnetic field with orsted and champ data summary a new model of the quiet time near earth magnetic field has been derived using a comprehensive approach which includes not only pogo and magsat satellite data but also data from the orsted and champ satellites the resulting model shows great improvement over its predecessors in terms of completeness of sources time span and noise reduction in parameters with its well separated fields and extended time domain of 1960 to mid 2002 the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997 because all sources are coestimated in a comprehensive approach intriguing north south features typically filtered out with other methods are being discovered in the lithospheric representation of the model such as the s atlantic spreading ridge and andean subduction zone lineations in addition this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection the f region currents through which the satellites pass are now treated as lying within meridional planes as opposed to being purely radial results are consistent with those found previously for magsat but an analysis at orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite besides the model a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset this has proven very useful in diagnosing the cause of peculiar artefacts in the magsat vector data which seem to suggest the presence of a small misalignment bias in the vector magnetometer

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

a geomagnetic jerk for the end of the 20th century abstract the series of magnetic measurements at some european observatories give some hint of a new geomagnetic jerk around 1999 the geomagnetic impulses would present the remarkable and intriguing property to occur with a frequency of one per decade in the last third of the 20th century the geomagnetic jerks have been proposed to be indicators which anticipate the changes in the earth s rotation rate if this statement is of general validity the 1992 jerk should be followed by an acceleration of the earth rotation which would take place now and along the same lines we might predict a new deceleration in less than 10 yr

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

worldwide character of the 1991 geomagnetic jerk a recent examination of the geomagnetic element annual mean values for the european magnetic observatories has shown the existence of a sudden change in the secular acceleration at about 1991 in this work the space time character of the 1991 impulse is investigated and its worldwide character is evidenced an evaluation of the amplitude spatial distribution is given for the y component analysing data relative to 74 magnetic observatories the external effects were removed from the annual mean data and successively the y secular variation has been evaluated from the corrected annual means moreover the worldwide character of the intensity of the step in the second time derivative is compared with the character of the previous well known jerk events of 1969 and 1978