detailed observations of california foreshock sequences implications for the earthquake initiation process we find that foreshocks provide clear evidence for an extended nucleation process before some earthquakes in this study we examine in detail the evolution of six california foreshock sequences the 1986 mount lewis ml 5 5 the 1986 chalfant ml 6 4 the 1986 stone canyon ml 4 7 the 1990 upland ml 5 2 the 1992 joshua tree mw 6 1 and the 1992 landers mw 7 3 sequence typically uncertainties in hypocentral parameters are too large to establish the geometry of foreshock sequences and hence to understand their evolution however the similarity of location and focal mechanisms for the events in these sequences leads to similar foreshock waveforms that we cross correlate to obtain extremely accurate relative locations we use these results to identify small scale fault zone structures that could influence nucleation and to determine the stress evolution leading up to the mainshock in general these foreshock sequences are not compatible with a cascading failure nucleation model in which the foreshocks all occur on a single fault plane and trigger the mainshock by static stress transfer instead the foreshocks seem to concentrate near structural discontinuities in the fault and may themselves be a product of an aseismic nucleation process fault zone heterogeneity may also be important in controlling the number of foreshocks i e the stronger the heterogeneity the greater the number of foreshocks the size of the nucleation region as measured by the extent of the foreshock sequence appears to scale with mainshock moment in the same manner as determined independently by measurements of the seismic nucleation phase we also find evidence for slip localization as predicted by some models of earthquake nucleation

occurrence patterns of foreshocks to large earthquakes in the western united states observations of foreshocks preceding large earthquakes provide one of the few well documented cases of premonitory events that are clearly related to a subsequent earthquake unfortunately the apparent randomness of foreshock occurrence they precede some events and not others has severely hampered their use in reliable earthquake prediction understanding the factors that control foreshock occurrence is critical for determining how large earthquakes initiate and whether reliable short term prediction will ever be possible1 here we report the results of a comprehensive study of the occurrence patterns of foreshocks to large earthquakes in the western united states the incidence of foreshocks decreases with increasing depth of the mainshock and also depends on the mainshock slip orientation this pattern of occurrence may be explained by a decrease in small scale crustal heterogeneity with increasing depth and suggests that increasing normal stress both regional tectonic stress and lithostatic load inhibits the occurrence of foreshocks no relationship is observed between any aspect of foreshock occurrence and the magnitude of the subsequent mainshock suggesting that the eventual size of the mainshock may be independent of the earthquake nucleation process or that foreshocks are not part of this process

chronological discord between the last interglacial paleosol s1 and its parent material in the chinese loess plateau abstract an and porter geology 25 1997 603 reported six high dust influx events of millennial timescales recovered from the last interglacial paleosol s1 and correlated them to six cool events of millennial timescale in the north atlantic however the complexity of soil forming processes may have made the chronological correlation with the north atlantic records inadequate to examine the complexity of the s1 formation the s1 paleosol was traced laterally and identified based on the preserved characteristics observed in the field and analyzed in the laboratory our data show that from the northwest to the southeast the s1 paleosol gradually converges from three distinctive soil profiles into a single welded profile because the net rate of loess accumulation was attenuated to the southeast and pedogenic development intensified southeastward during the last interglacial three soil forming events within the s1 paleosol s1s1 s1s2 and s1s3 separated by two loess units s1l1 and s1l2 in the northwestern part of the loess plateau are stratigraphically coeval with a single soil profile in the southeastern margin of the loess plateau in the southeast the s1 paleosol developed into underlying older loess l2 e g at the lantian section the three paleosols s1s1 s1s2 and s1s3 are partially welded in the central part of the chinese loess plateau e g at the tianshui section where the lower portion of s1 paleosol developed in the underlying older loess unit l2 in the northwestern margin of the chinese loess plateau e g at the lanzhou section the preservation of the repeating soil loess sequence s1s1 s1l1 s1s2 s1l2 and s1s3 continuously documented the climatic events of the last interglacial our data also show that the magnetic signatures and particle size information are more or less acceptable climatic proxies only for the northwestern sections where the degree of pedogenesis was lower and the rate of eolian influx was greater during the last interglacial it appears that in all cases investigated the median grain size and the coarse fraction 63 m content define the upper and lower boundaries of the s1 paleosol reasonably well and can be used to estimate the time transgressive nature of the s1 paleosol relative to its parent material soil welding bioturbation and material translocation within the s1 soil profiles make it impossible to preserve the detailed and high resolution information of climate changes in those s1 profiles in the southeastern part including the popularly called central part of the chinese loess plateau

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

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

the contribution of loess magnetism in china to the retrieval of past global changes some problems abstract in the loess plateau of china the stratigraphical position of the brunhes matuyama boundary was not found in loess unit l8 at luochuan and jixian as is commonly accepted the interbedded palaeosols may be differentiated from the parent loess not only by their high magnetic susceptibility but also by their enhanced remanence properties these contrasts mainly result from concentration changes of the inherited magnetic minerals grain size changes and authigenic formation of magnetic minerals which are all linked to climate controlled soil formation processes

the age and duration of the matuyama brunhes transition from new k ar data from la palma canary islands and revisited 40ar 39ar ages abstract paleomagnetic investigations conducted on lava sequences from la palma canary islands yielded several independent records of the matuyama brunhes transition mbt seven k ar ages of flows sampled across the mbt suggest a duration between 0 and 11 kyr and provide a weighted mean age of 786 3 ka analytical uncertainty only for the transition this value is significantly older than the previously admitted age of 779 2 ka derived from volcanic or sedimentary sequences when the 1 uncertainty on the calibration standard used in the present study is considered our age for the transition recorded at la palma becomes 786 8 ka we have recalculated the 40ar 39ar determinations from the previous studies using recent revisited ages for 40ar 39ar standards this yields a global dataset of 23 k ar and 40ar 39ar determinations our best estimate for the mbt age derived from this compilation is 789 2 ka analytical error or 789 8 ka total error within this 8 kyr uncertainty which could be considered as a minimum estimate this age remains compatible with recent determinations made by the astronomical time scale however the present study suggests that better control of the lock in depth as well as better quantification of uncertainties involved in the tuning of 18o records to orbital forcing models are needed to improve further the astronomical polarity time scale derived from sedimentary records in addition absolute dating by k ar and 40ar 39ar will gain in accuracy when uncertainty determinations on standards will be significantly improved

evolution em signals before earthquakes in terms of mesomechanics and complexity abstract mhz to khz electromagnetic em anomalies have been detected worldwide over time intervals ranging from a few days to a few hours prior to near surface earthquakes eqs that have occurred on land with magnitude approximately six or larger the mhz radiation systematically appears earlier than the khz laboratory experiments on rock samples reveal a similar change in the frequency content during progressive deformation i e the emissions exhibit a frequency shift from mhz to khz just before failure of the samples herein we attempt to associate these consecutively emerging mhz and khz modes of the pre fracture em emission with successive distinct stages of the fracture preparation process and the associated fracture mechanisms we base our approach on synergetic principles of physical mesomechanics a universal principle of physical mesomechanics is that the plastic deformation of a solid is related to its loss of shear stability at the micro meso and macroscale levels global fracture is determined by the mechanism controlling the development of macroscale bands of localized deformation associated with meso and microscale relaxation processes the major result we obtain is that the shift from mhz to khz em activity may signal the transition of plastic flow localization from the mesoscale to the macroscale culminating in global fracture a complexity measure approximate entropy and a persistency anti persistency measure hurst exponent also give evidence of a two stage model of eq preparation process significant complexity decrease and accession of persistency can be confirmed in the strong khz em activity that emerges in the tail of the pre seismic em activity laboratory experiments on rock samples reveal similar symptoms of an imminent global instability

seismicity parameters preceding moderate to major earthquakes seismic events reported in the bulletins of the two large arrays lasa and norsar were merged with those from the neis bulletin for the period 1970 1977 using a lower cutoff of mb 5 8 510 main shocks within the p range of lasa or norsar were selected for this period and various seismicity trends prior to them were investigated a search for definite foreshocks based on a significantly short time delay to the main shock revealed that the true rate of foreshock occurrence was less than 20 foreshocks are almost exclusively associated with shallow h 100 km was demonstrated for shallow events 100 km this variable reveals numerous instances of clustering and spatial temporal seismic gaps with little tendency toward a uniformity of behavior prior to main shocks a statistical test of the validity of seismic precursors was performed for approximately 90 main shock regions which had sufficient seismicity using a five variable vector interevent time interevent distance magnitude epicentral distance to main shock and depth difference relative to main shock for each event in a precursory time window of 500 days before the main shock and for each event in a normal time window of 500 days before that the null hypothesis of equal vector means between the two groups was tested at 90 confidence level less than 30 of the main shock regions were thus found to exhibit precursory seismicity changes appendices are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 007 1 00 payment must accompany order

evolution of the rheic ocean the rheic ocean which separated laurussia from gondwana following the closure of iapetus is arguably the most important ocean of the palaeozoic its suture extends from mexico to turkey and its closure produced the climactic variscan alleghanian ouachita orogeny that assembled the supercontinent pangaea following protracted cambrian rifting that represented a continuum from neoproterozoic orogenic processes the rheic ocean opened in the early ordovician with the separation of several neoproterozoic arc terranes from the continental margin of northern gondwana separation occurred along the line of a former neoproterozoic suture following the onset of subduction in the outboard iapetus ocean the timing of rift drift transition and drive for subsequent spreading was likely governed by slab pull accounting for the rapid rate 8 10 cm yr at which the rheic ocean widened during the ordovician the ocean broadened at the expense of iapetus and attained its greatest width 4000 km in the silurian by which time baltica had sutured to laurentia and the neoproterozoic arc terranes had accreted to laurussia closing iapetus in the process closure of the rheic ocean began in the devonian and was facilitated by northward subduction beneath southern baltica and southward subduction beneath northwest gondwana closure was largely complete by the mississippian as gondwana and laurussia sutured to build pangaea north africa colliding with southern europe to create the variscan orogen in the devonian carboniferous and west africa and south america suturing to north america to form the alleghanian and ouachita orogens respectively during the carboniferous permian the rheic ocean consequently plays a dominant role in the basement geology of southern europe in the appalachian ouachita orogeny of north america and in the palaeozoic sedimentary structural and tectonothermal record from middle america to the middle east with its closure the ocean brought about the assembly of pangaea and brought the palaeozoic era to an end

a review of earthquake statistics fault and seismicity based models etas and bass there are two fundamentally different approaches to assessing the probabilistic risk of earthquake occurrence the first is fault based the statistical occurrence of earthquakes is determined for mapped faults the applicable models are renewal models in that a tectonic loading of faults is included the second approach is seismicity based the risk of future earthquakes is based on the past seismicity in the region these are also known as cluster models an example of a cluster model is the epidemic type aftershock sequence etas model in this paper we discuss an alternative branching aftershock sequence bass model in the bass model an initial or seed earthquake is specified the subsequent earthquakes are obtained from statistical distributions of magnitude time and location the magnitude scaling is based on a combination of the gutenberg richter scaling relation and the modified bath s law for the scaling relation of aftershock magnitudes relative to the magnitude of the main earthquake omori s law specifies the distribution of earthquake times and a modified form of omori s law specifies the distribution of earthquake locations unlike the etas model the bass model is fully self similar and is not sensitive to the low magnitude cutoff

a database for the matuyama brunhes magnetic reversal abstract we present a paleomagnetic database designated mbd97 comprising sedimentary and lava data for the matuyama brunhes reversal transition field using criteria regarding suitability of the rock samples measurement methods and temporal resolution of the data we consider 62 different studies of the matuyama brunhes reversal of these only 11 satisfy our selection criteria four lava sites one loess site three shallow marine sedimentary sites and three deep ocean sedimentary sites we discuss our reasons for accepting or disguarding each candidate data set we investigate the distribution of transitional virtual geomagnetic poles vgps they fall along american and asian australian longitudes the spatial and temporal complexity of the data indicate that the transitional field was almost certainly nonaxisymmetric and nondipolar the directional transition of the matuyama brunhes reversal took 2300 5000 years during which time the surface intensity fell to 10 20 of its pre and post transitional value we do not find any systematic bias due to site distribution or inclination shallowing although the errors in the data do vary over the duration of the reversal mbd97 is a small database but it has a fairly good geographic distribution and is suitable for studies of the matuyama brunhes transition nonetheless it is clear that further analysis of transitions will benefit from a continued program of data collection and analysis the data in mbd97 are listed

a new astronomical timescale for the loess deposits of northern china here we present a refined timescale for the entire sequence of quaternary chinese loess which relies upon the correlation of detailed monsoon records to the astronomical solution of laskar 1990 and the oceanic odp677 18o record of shackleton et al 1990 the chronological scheme considers in detail the relative structures of the palaeoclimatic and palaeomagnetic records to produce an accurate timescale that is consistent with the current understanding of loess depositional and post depositional processes analysis of this chronological framework demonstrates downward displacement of the palaeomagnetic horizons with respect to the climatic record

stacked 2 6 ma grain size record from the chinese loess based on five sections and correlation with the deep sea δ18o record 1 this study aims to establish a stacked climate record of the quaternary period from the chinese loess sequence and to address the forcing mechanisms for the regional climate history of the loess plateau by correlating the stacked record with a composite 18o record in deep sea sediments a total of 18 352 samples were obtained from five loess sections located at baoji lingtai jingchuan puxian and pingliang in the southern and middle loess plateau these yielded high resolution grain size records between section correlation of these shows that although small depositional hiatuses are present in places within a single section most parts of the sections display near continuous dust deposition throughout the quaternary the grain size records were tuned simultaneously to the theoretical variations in obliquity and precession of the earth s orbit the grain size records plotted on their orbital timescales were then averaged to form a stacked loess grain size time series termed the chiloparts record this resolves most of the orbital timescale paleoclimate events buried in the loess soil sequences of the southern and middle loess plateau and can be used as a regional archive of the pleistocene climate history in the loess plateau comparison of the chiloparts record with a composite marine 18o record shows that for the past 1 8 ma the loess paleosol record can be correlated almost cycle by cycle with the marine record several discrepancies in the climatic events between the two records have also been identified implying that regional forcing mechanisms may have played a part in the climatic evolution of the chinese loess plateau

loess stratigraphy in central china abstract loess sequences in central china provide a detailed record of climate changes during the last 2 5 million years at least 44 major shifts from glacial to interglacial conditions occurred during this time in china prolonged loess deposition lasting more than 40 millennia occurred eleven times with only four of these intervals in brunhes chron the climate oscillations took place with increased frequency between 2 4 and 0 5 ma compared to the last half million years major shifts toward cooler and drier climates occurred at about 2 4 ma 1 2 ma and 0 5 ma while less pronounced shifts are dated to 1 65 ma 0 8 ma and 0 2 ma three of the last four reversals of the earth s magnetic field whose lithostratigraphic position has been detail are recorded in a glacial deposit the low field magnetic susceptibility of loess and soils which reflects the concentration of ultrafine magnetite is related to the degree of pedogenesis and serves as a proxy measure of climate the accumulation rate of the magnetite was fairly constant throughout the last 1 2 million years and can serve as a measure of time susceptibility records from localities near xifeng and luochuan located 160 km apart are compared and used to define informal lithostratigraphic units in the loess plateau of china

are chinese loess deposits essentially continuous 1 we conducted a paleomagnetic examination of the last glacial loess of three representative profiles along an east west transect in the central loess plateau in order to assess the continuity of chinese loess the results show that the xifeng and luochuan profiles record the laschamp geomagnetic excursion but with different morphologies together with the published results from weinan southern plateau our results suggest that sedimentation of the last glacial loess in the central southern plateau was continuous at the time scale equivalent to the duration of the laschamp excursion 2 kyr but probably episodic at finer time scales 2 kyr no geomagnetic excursion was found at the yichuan profile near the yellow river valley where loess accumulation may be strongly affected by local environmental changes and thus may have been discontinuous both site location and time scale therefore need to be considered when considering continuity of chinese loess

magnetic enhancement and palaeoclimate study of a loess palaeosol couplet across the loess plateau of china summary the origin of the milankovitch palaeoclimatic signal so dramatically recorded in the chinese loess remains uncertain at least five hypotheses have been put forward to explain the well documented enhancement of magnetic susceptibility in palaeosols which formed when conditions were warmer and wetter we report here a magnetic study of a prominent loess soil couplet l4 s3 at three widely separated sites where present day mean annual rainfall and temperature range from 350mm to 700mm and from 6 c to 13 c the properties determined are laboratory remanences isothermal irm anhysteretic arm viscous vrm including viscous decay room temperature susceptibility and thermomagnetic behaviour at fields beyond 100 mt the irm spectra are all essentially identical both in shape and in absolute magnitude suggesting a uniform magnetic component widespread across the plateau subtracting the signal of this component isolates a second component the amount of which varies by an order of magnitude between sites the magnetic properties of this latter material are remarkably similar to those reported for bacterial magnetite in geological sediments and modern soils viscous decay of vrm takes place in two distinct stages one of which falls to zero in 10min we attribute magnetic enhancement which is the source of the palaeoclimatic signal to in situ pedogenetic formation of magnetite which we characterize in our interpretation as two separate forms of bacterial magnetofossils the most magnetic sample requires a mass fraction of about half of one per cent of this biogenic material

seismicity alert probabilities at parkfield california revisited for a decade the u s geological survey has used the parkfield earthquake prediction experiment scenario document to estimate the probability that earthquakes observed on the san andreas fault near parkfield will turn out to be foreshocks followed by the expected magnitude 6 mainshocks during this time we have learned much about the seismogenic process at parkfield about the long term probability of the parkfield mainshock and about the estimation of these types of probabilities the probabilities for potential foreshocks at parkfield are reexamined and revised in light of these advances as part of this process we have confirmed both the rate of foreshocks before strike slip earthquakes in the san andreas physiographic province and the uniform distribution of foreshocks with magnitude proposed by earlier studies compared to the earlier assessment these new estimates of the long term probability of the parkfield mainshock are lower our estimate of the rate of background seismicity is higher and we find that the assumption that foreshocks at parkfield occur in a unique way is not statistically significant at the 95 confidence level while the exact numbers vary depending on the assumptions that are made the new alert probabilities are lower than previously estimated considering the various assumptions and the statistical uncertainties in the input parameters we also compute a plausible range for the probabilities the range is large partly due to the extra knowledge that exists for the parkfield segment making us question the usefulness of these numbers

a way to synchronize models with seismic faults for earthquake forecasting insights from a simple stochastic model abstract numerical models are starting to be used for determining the future behaviour of seismic faults and fault networks their final goal would be to forecast future large earthquakes in order to use them for this task it is necessary to synchronize each model with the current status of the actual fault or fault network it simulates just as for example meteorologists synchronize their models with the atmosphere by incorporating current atmospheric data in them however lithospheric dynamics is largely unobservable important parameters cannot or can rarely be measured in nature earthquakes though provide indirect but measurable clues of the stress and strain status in the lithosphere which should be helpful for the synchronization of the models the rupture area is one of the measurable parameters of earthquakes here we explore how it can be used to at least synchronize fault models between themselves and forecast synthetic earthquakes our purpose here is to forecast synthetic earthquakes in a simple but stochastic random fault model by imposing the rupture area of the synthetic earthquakes of this model on other models the latter become partially synchronized with the first one we use these partially synchronized models to successfully forecast most of the largest earthquakes generated by the first model this forecasting strategy outperforms others that only take into account the earthquake series our results suggest that probably a good way to synchronize more detailed models with real faults is to force them to reproduce the sequence of previous earthquake ruptures on the faults this hypothesis could be tested in the future with more detailed models and actual seismic data

chemical magnetization in chinese loess the relationship between the intensity of characteristic remanent magnetization chrm and pedogenesis is examined in the luochuan section the results show a consistent link between the intensity values of remanent magnetization after every thermal demagnetization step and magnetic susceptiblity and other indexes of pedogenesis moreover the intensity of chrm is also much higher in paleosols than in loess these results suggest that chemical remanent magnetization crm is the main mode of nrm acquisition in paleosols and is linked to pedogenesis pedogenic processes overprint the magnetic signal to a certain depth in the underlying loess so the interpretation of the high resolution paleomagnetic records from paleosols requires a degree of caution

prediction probabilities from foreshocks when any earthquake occurs the possibility that it might be a foreshock increases the probability that a larger earthquake will occur nearby within the next few days clearly the probability of a very large earthquake ought to be higher if the candidate foreshock were on or near a fault capable of producing that very large mainshock especially if the fault is towards the end of its seismic cycle we derive an expression for the probability of a major earthquake characteristic to a particular fault segment given the occurrence of a potential foreshock near the fault to evaluate this expression we need 1 the rate of background seismic activity in the area 2 the long term probability of a large earthquake on the fault and 3 the rate at which foreshocks precede large earthquakes as a function of time magnitude and spatial location for this last function we assume the average properties of foreshocks to moderate earthquakes in california 1 the rate of mainshock occurrence after foreshocks decays roughly as t 1 so that most foreshocks are within three days of their mainshock 2 foreshocks and mainshocks occur within 10 km of each other and 3 the fraction of mainshocks with foreshocks increases linearly as the magnitude threshold for foreshocks decreases with 50 of the mainshocks having foreshocks with magnitudes within three units of the mainshock magnitude within three days we apply our results to the san andreas hayward san jacinto and imperial faults using the probabilities of large earthquakes from the report of the working group on california earthquake probabilities 1988 the magnitude of candidate event required to produce a 1 probability of a large earthquake on the san andreas fault within three days ranges from a high of 5 3 for the segment in san gorgonio pass to a low of 3 6 for the carrizo plain

the matuyama brunhes polarity reversal in four chinese loess records high fidelity recording of geomagnetic field behavior or a less than reliable chronostratigraphic marker abstract the paleomagnetic recording fidelity of chinese loess has been a matter of long term dispute to help shed light on the acquisition retention mechanisms of natural remanent magnetization in loess we report on high sampling resolution paleomagnetic constraints on the position of the matuyama brunhes boundary mbb in four typical loess sections in different marginal areas of the chinese loess plateau clp each of which has experienced different east asian winter summer monsoon strengths and pedogenic intensities detailed thermal demagnetization experiments for 1850 specimens reveal that none of the high accumulation rate loess with the weakest pedogenesis at caotan western clp the massive pedogenesis loess characterized by extensive development of thick calcareous nodule layers at jixian se clp and the moderately pedogenized loess at fanshan ne clp have preserved high fidelity geomagnetic records across the mbbs jiuzhoutai western clp one of the thickest loess profiles in the world contains the thickest and hitherto the most complex mbb that is too extensive to be regarded as rapid fluctuations of geomagnetic field during the m b reversal at jixian a predominance of pedogenesis induced chemical remanent magnetization overprinting has heavily obscured the original remanence the significant downward displacement of the mbb at fanshan and even its total absence at caotan together with extensive remagnetization of the coarse grained upper sandy loess layer l9 in both sections further suggest that the processes of remanence acquisition retention of chinese loess in the marginal areas of the clp are rather complicated thus interpretations of the fine structure of geomagnetic transitional field behavior and excursions especially in the matuyama chron should be regarded with caution

foreshock rates from aftershock abundance the question whether foreshocks behave like mainshocks that happen to have larger aftershocks is important for the understanding of earthquake nucleation and the modeling of earthquake clustering many models including the short term earthquake probability step model for forecasting short term earthquake hazard in california and the epidemic type aftershock sequence etas model for the description of earthquake clustering imply that all earthquakes of magnitude m including foreshocks can trigger earthquakes according to a magnitude dependent triggering ability 10 m the magnitudes of the triggered earthquakes are proportional to 10 bm according to the gutenberg richter relation for the magnitude frequency distribution and therefore can be larger than the magnitude of the initiating event however numerous studies observe a lower foreshock occurrence than predicted from aftershock models in this article we show that for homogeneous earthquake catalogs and a consistent definition of foreshocks and aftershocks foreshock rates can be predicted from aftershock characteristics for southern california the probability that the initiating event of an earthquake cluster in the magnitude range 2 0 4 5 is followed by at least one larger event within one day is around 4 this agrees well with corresponding data from the global centroid moment tensor cmt catalog where the probability that the initiating earthquake of a cluster is followed by at least one larger event within 30 days is around 3 these foreshock rates are significantly lower than previously reported and we discuss possible reasons for this

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

evidence for the kamikatsura and santa rosa excursions recorded in eolian deposits from the southern chinese loess plateau 1 a detailed paleomagnetic investigation of a loess paleosol sequence in baoji southern chinese loess plateau reveals two distinct geomagnetic excursions accompanied by inferred relative paleointensity minima in the late matuyama chron rock magnetic experiments demonstrate that the sediments of the excursion intervals in loess unit l9 have the same magnetic properties as those from surrounding normal and reversed polarity intervals assuming constant sedimentation rate between the jaramillo matuyama and matuyama brunhes magnetic polarity boundaries the midpoint ages and durations for these two excursions are estimated at 0 89 ma and 16 kyr and 0 92 ma and 7 kyr respectively in addition these two events are separated by an interval with fully reversed polarity that had a duration of 16 kyr these estimates are supported by paleoclimatologically inferred age constraints the two excursions are probably correlated with the kamikatsura and santa rosa excursions respectively

sedimentation and diagenesis of chinese loess implications for the preservation of continuous high resolution climate records chinese loess has been extensively utilized to produce continuous and high resolution climate records of the late cenozoic such work assumes uninterrupted loess deposition and limited diagenesis here closely spaced optically stimulated luminescence osl dates are used to characterize the holocene and late pleistocene sedimentation histories of three sites across a nw se transect of the chinese loess plateau the results suggest that sedimentation is episodic at subglacial interglacial time scales with rates rapidly varying within units and between sites unconformities noneolian deposition and mixing of sediments also appear to be common existing understanding of loess deposition therefore requires reexamination while previous reconstructions of rapid climate change not dated using absolute methods should be regarded with caution loess deposits may still yield detailed climate records from specific high sedimentation rate strata and evidence for rapid climate change may yet be obtainable by targeting these units through absolute dating the rapid changes in sedimentation presented here indicate the east asian monsoon has the capacity to vary on millennial scales

geomagnetic episodes of the last 1 2 myr recorded in chinese loess 1 the matuyama brunhes m b boundary termination of jaramillo normal subchron j m as well as three young geomagnetic excursions has been revealed in the weinan loess section zhu et al 1994 1998 1999 recently we carried out further paleomagnetic directional studies on the same section and recovered the onset of the jaramillo normal subchron m j and four geomagnetic excursions two in the brunhes and two in the late matuyama the fact that all seven excursions found in this section are in loess units and expected excursions in paleosols are not found suggests that short lived geomagnetic signals could be only recorded in loess units and may have been fully smoothed or wiped out in paleosol units by strong pedogenesis constrained by a new astronomical timescale hld00 ages of the geomagnetic excursions and reversal boundaries have been estimated we tentatively correlated these excursions with previous results indicating that a large lock in model may not be correct for the whole loess profile although some apparently big positive age discrepancies were observed for the main boundaries

interpretation of earthquake epicenter and cmt centroid locations in terms of rupture length and direction abstract a comparison is made between hypocenter and centroid estimates of shallow earthquakes to investigate whether the difference between these two quantities contains useful information on fault rupture length and direction an initial comparison of international seismological centre isc and harvard centroid moment tensor cmt catalog locations suggests that for earthquakes below mw 6 5 the difference in location is dominated by location errors whilst larger earthquakes appear to show the effect of increasing spatial rupture however for this data set combined location errors greater than 25 km are observed for a majority of earthquakes imposing a limit on interpretation we use these constraints to investigate this approach using a consistent earth model in the re analysis of eight previously well studied events new hypocenter estimates based on teleseismic p wave arrivals from the isc bulletin and a 3 d velocity model of the mantle s p12 wm13 are compared with new centroid locations obtained using the same 3 d model and waveform data from international deployment of accelerometers ida global digital seismic network gdsn and re calibrated high gain long period hglp stations the distance between our new hypocenter and centroid locations is shown to provide reliable information on both the rupture length and direction the rupture lengths and azimuths generally agree with the most recent previous results from other methods and provide an absolute estimate of the location of the rupture

earthquake hazard after a mainshock in california after a strong earthquake the possibility of the occurrence of either significant aftershocks or an even stronger mainshock is a continuing hazard that threatens the resumption of critical services and reoccupation of essential but partially damaged structures a stochastic parametric model allows determination of probabilities for aftershocks and larger mainshocks during intervals following the mainshock the probabilities depend strongly on the model parameters which are estimated with bayesian statistics from both the ongoing aftershock sequence and from a suite of historic california aftershock sequences probabilities for damaging aftershocks and greater mainshocks are typically well constrained after the first day of the sequence with accuracy increasing with time

the last two geomagnetic polarity reversals recorded in high deposition rate sediment drifts high resolution records of geomagnetic polarity transitions are rare owing to the difficulty of preserving in the geological record details of a process with a duration of only a few thousand years lavas can record the instantaneous field on cooling but lava records are themselves discontinuous and therefore polarity transition fields are seldom captured sedimentary records are more continuous but typical deep sea sedimentation rates of a few cm kyr 1are too low to record transition fields adequately here we report transition records from north atlantic drift deposits with high sedimentation rates 12 cm kyr 1 at the ocean drilling program sites 983 and 984 virtual geomagnetic polar vgp paths are similar within and between sites for the last two geomagnetic reversals attesting to a memory effect spanning back to back reversals the complex vgp paths feature two vgp clusters one located in northeastern asia and the other in the south atlantic ocean off south america and provide a link between the longitudinally constrained transitional vgps often recorded in sediments1 2 which represent highly smoothed renditions of transition fields and the scattered transitional vgps3 and vgp clusters4 observed in lava records

timing and lock in effect of the laschamp geomagnetic excursion in chinese loess 1 the laschamp geomagnetic excursion lge at 41 ka is a critical age control for hemispherical comparison of paleoclimate records from various kinds of geological archives e g ice core marine and lake sediments the timing of the lge in chinese loess however remains poorly constrained due to the lack of a reliable chronology and the complex acquisition processes of natural remanent magnetization here we systematically compare the paleomagnetic results of three optically stimulated luminescence dated loess sequences on the chinese loess plateau our results indicate that the timing of the lge in the northwestern loess plateau is slightly older than the absolute radiometric age determination and the timing inferred from greenland ice core 10be flux and marine i e relative paleointensity and authogenic 10be 9be stack records but younger than the counterparts in the central loess plateau we attribute the different timing of the lge in the three loess sections to a progressive southeastward increase in the lock in depth caused by the combined effects of postdepositional processes e g surface mixing chemical weathering and lock in delay on the remanence acquisition process we conclude that caution is needed to use the lge in chinese loess as a reliable tie event for high resolution chronological correlation to marine and ice core records only if the potential lock in effect can be precisely determined

in situ remagnetization experiments of loess on the chinese loess plateau evidence for localized post depositional remanent magnetization 1 inconsistent stratigraphic positions of reversal boundaries and spurious paleomagnetic variations associated with polarity reversals recorded by loess paleosols on the chinese loess plateau clp have in general been attributed to inferred delayed acquisition of magnetization and high frequency variations of the geomagnetic field respectively these conclusions are partly based on an incomplete knowledge of the magnetization acquisition process of loess recent laboratory experiments which demonstrate that dry deposited loess acquires a stable remanent magnetization after its first wetting precipitation resulted in significant inclination errors that are not generally encountered in chinese loess laboratory experiments only remotely mimic natural conditions and we have therefore performed in situ experiments at two sites on the clp separated by some 650 km to elucidate whether variations in natural conditions can impose secondary remanent magnetization components in loess l1 the experiments were performed by relocating blocks volume 5 10 3 m3 of last interglacial loess after being turned upside down inside loess exposures inverted blocks were retrieved and analyzed after 8 and 17 months respectively thermal demagnetization reveals that partial remagnetization has occurred within different parts of the loess blocks this secondary magnetization which is characterized by blocking temperatures that overlap with those of the primary component to above 450 c cannot be attributed to a viscous remanent magnetization and is therefore proposed to represent local and partial realignment of magnetic grains we tentatively propose that physical unlocking and realignment of magnetic grains may be assisted by pressure release caused by differential thermal expansion of the different mineral grains in loess localized remagnetization may account for a number of reported spurious paleomagnetic results from clp

statistics between mainshocks and foreshocks in italy and southern california 1 the most used and accepted models for daily forecasts are based on short term space and time earthquake clustering for occurrence rates and on the gutenberg richter law for the frequency magnitude these models have been demonstrated to produce reliable prospective space time magnitude forecasts during an aftershock sequence but their skill in forecasting mainshocks is still under discussion this paper studies the foreshock statistics of the italian and californian seismicity in two ways i we compare the foreshock activity observed in real seismic catalogs and in synthetic catalogs derived from a pure epidemic type aftershock sequence etas model ii we analyze the triggering capability of earthquakes using different etas parameterizations in order to check whether large events are triggered in the same way as regular earthquakes the results indicate that the foreshock activity observed in the real catalogs is compatible with what is expected by the etas model moreover we find that the empirical foreshock rates have an intrinsic variability due to limited sampling that may explain most of the differences found so far in different seismic catalogs

age and palaeoclimatic significance of the loess of lanzhou north china extending in a broad arc that abuts the sandy non gobi deserts the loess plateau of northern china1 3 is one of the most massive accumulations of loess in the world the loess sequence is typically characterized by an alternation of silty or sandy loess with more clay rich palaeosols these alternations in conjunction with their enclosed faunas and distinctive mineralogies have been interpreted as reflecting pleistocene glacial interglacial cycles3 8 because it holds implications for the climatic and anthropological history of china9 10 the definition of a reliable chronological framework for loess deposition is of great interest recent palaeomagnetic studies4 5 have indicated that loess deposition in shaanxi province commenced 2 4 myr ago to assess the synchrony of loess accumulation across the loess plateau we have dated a 330 m thick loess sequence near lanzhou gansu province the magnetostrati graphical results reported here indicate that the base of this loess succession dates from 1 3 my r ago this young age in comparison to the shaanxi sequence is attributed to uplift along the northern fringe of the tibetan plateau11 that precluded early pleistocene loess preservation in this mountainous region palaeosols in the basal loess occur on average once every 25 kyr suggesting that climates conducive to soil forming events may have been modulated by orbital precession in the early pleistocene

an improved comparison of chinese loess with deep sea δ18o record over the interval 1 6 2 6 ma 1 based on a refined orbital timescale for the lower portion of pleistocene loess paleosol sequence at lingtai section we present an improved comparison of chinese loess with deep sea 18o record over the interval 1 6 2 6 ma unlike previous land ocean comparison our result suggests that each paleosol unit from s25 to s32 is approximately including two interglacial oxygen isotope stages the rationality of our revised comparison is supported by the well constrained ages of geomagnetic reversal boundaries in chinese loess and the almost cycle to cycle correlation between the grain size and deep sea 18o record spectral analyses demonstrate that variations of the magnetic susceptibility and grain size in the interval 1 6 2 6 ma were dominated by a prominent 410 ka periodicity implying a possible response of the east asian monsoon evolution to orbital eccentricity forcing

age and duration of the matuyama brunhes geomagnetic polarity reversal from 40ar39ar incremental heating analyses of lavas abstract constraints on the timing of geomagnetic polarity reversals have come mainly from k ar or more recently 40 ar 39 ar age determinations of lavas or their k rich phenocrysts that erupted prior or subsequent to particular geomagnetic events we have obtained 40 ar 39 ar isochron ages using incremental heating techniques on groundmass separates phenocryst poor whole rock samples or plagioclase from eight basaltic to andesitic lavas that erupted during the matuyama brunhes m b polarity transition at four geographically dispersed sites these eight lavas range from 784 6 7 1 ka to 770 8 5 2 ka 1 errors the weighted mean 778 7 1 9 ka gives a high precision age that is remarkably consistent with revised astronomical age estimates for the m b polarity transition 6 12 13 despite uncertainties in absolute calibration of 40 ar 39 ar ages relative to mineral standards used as neutron fluence monitors our age determinations are consistent with five other 40 ar 39 ar studies focused on the m b transition these results confirm that the earlier k ar based geomagnetic polarity time scale underestimated the age of the m b reversal by about 6 none of the eight isochron ages are distinguishable from one another at the 95 confidence level however we are tantalizingly close to testing for the duration of this reversal one lava at the base of a sequence of transitionally magnetized flows in chile and the uppermost lava in a similar sequence on maui are only just indistinguishable in age at the 95 confidence level and preserve different magnetic orientations we suggest that the 12 kyr difference in age represents an upper limit for the duration of the reversal and is similar to the period of low magnetic field intensity associated with records of the m b reversal from deep sea sediment cores together with the short duration 2 kyr of the directional reversal observed in several different marine sediment sections our data suggest that reversal of the field s direction could have occurred at slightly different times depending on the position of the recording site

testing earthquake prediction methods the west pacific short term forecast of earthquakes with magnitude mwhrv 5 8 in his paper kossobokov investigates the e ciency of our short term forecast for two western paciflc regions although we agree with the basic results of his evaluation that the forecast statistics is much better than a random guess we have reservations about his deflnition of earthquake prediction some of his tests and his interpretation of the test results we distinguish between deterministic earthquake predictions and statistical forecasts we argue that some techniques used by kossobokov may not be appropriate for testing our forecasts and discuss other testing methods based on the likelihood function we demonstrate that kossobokov s null hypothesis may be biased and this bias can in uence some of his conclusions we show that contrary to kossobokov s statement our algorithm predicts mainshocks when they are preceded by foreshocks

astronomical calibration of loess paleosol deposits at luochuan central chinese loess plateau abstract the 140 m loess paleosol profile at luochuan in the central chinese loess plateau was sampled at 5 cm intervals in loess units and at 3 cm in paleosol units in order to obtain a high resolution climatic record covering the past 2 5 million years all samples were measured for magnetic susceptibility which is regarded as a good proxy index of the east asian summer monsoon strength on the basis of the astronomical theory of pleistocene climatic change an age model of the luochuan loess paleosol sequence was developed by tuning the magnetic susceptibility record to time series of insolation changes the results show that the ages of the boundaries between the malan and lishi and lishi and wucheng loess formations are 71 and 1320 kyr bp respectively the onset of loess accumulation is at 2470 kyr bp our age model was tested by comparing the orbitally derived ages with absolute age determinations of magnetic reversals and cross spectrum analyzing with solar radiation variations for summer at 65 n these indicate that the calibration provides a reliable time scale for the luochuan loess paleosol deposit

quantifying ultrafine pedogenic magnetic particles in chinese loess by monitoring viscous decay of superparamagnetism we propose a new approach for quantifying pedogenically produced ultrafine nanometer scale magnetic particles in chinese loess we have recorded the viscous decay t 100 s of imposed isothermal remanent magnetization irm assumed to be selectively carried by superparamagnetic particles jv application of viscosity measurements relaxometry to loess paleosol sequences from the se extremity of the chinese loess plateau reveals that jv aptly reflects contributions of viscous superparamagnetic sp particles to the total remanence carried by an assemblage of magnetic particles time decay of jv can be determined with a higher resolution than the conventionally used frequency dependent susceptibility fd for characterizing the relative abundance of sp particles the nearly logarithmic time decay of irm implies a rather broad grain size distribution of pedogenic particles in chinese loess that may range from a lower size limit of 10 nm to an upper limit of 40 nm with a prevalent grain size maximum of 21 25 nm numerical calculations of the time dependence of irm decay demonstrate that the actual contribution of fine grained pedogenic magnetite maghemite to the total remanence is at least twice as large as that estimated by both fd and our proposed jv parameter we therefore propose that determination of superparamagnetic contributions by the time dependence of irm viscosity is a useful technique for tracking pedogenic processes and hence paleoclimatic conditions of chinese loess

seismotectonics of the central denali fault alaska and the 2002 denali fault earthquake sequence in this article we analyze the spatial and temporal variations in the seismicity and stress state within the central denali fault system alaska before and during the 2002 denali fault earthquake sequence seismicity for 30 years prior to the 2002 earthquake sequence along the denali fault was very light with an average of four events with magnitude m l 3 per year we observe a significant increase in the seismicity rate prior to the m w 7 9 event of 3 november 2002 within its epicentral region starting about 8 months before its occurrence the majority of the aftershocks of the m w 7 9 event are located within the upper 11 km of the crust and form several persistent clusters with a few aseismic patches along the ruptured fault the most active aftershock source is associated with the epicentral region of the earthquake the overall b value of the aftershock sequence is 0 96 with the highest b values within the epicentral region we estimate that it will take 14 years for the seismicity rate to drop back to the background level the stress regime across the region varies in space and time the inferred stress regime prior to the 2002 sequence is predominately strike slip along the central part of the rupture zone the orientations of the least and intermediate stress axes are reversed after the 2002 earthquake sequence the maximum compressive stresses along the denali fault rotate clockwise by up to 35 the greatest rotations occur in the area of the rupture step over from the denali to the totschunda fault the inferred stress regime after the 2002 sequence reflects an interchanging thrusting and strike slip faulting along the ruptured fault the thrust faulting is concentrated in the epicentral region of the m w 7 9 event and along the rupture segments showing the largest surface offsets

foreshocks and time dependent earthquake hazard assessment in southern california the probability that an earthquake in southern california m 3 0 will be followed by an earthquake of larger magnitude within 5 days and 10 km i e will be a foreshock is 6 0 5 per cent 1 s d and is not significantly dependent on the magnitude of the possible foreshock between m 3 and m 5 the probability that an earthquake will be followed by an m 5 0 main shock however increases with magnitude of the foreshock from less than 1 per cent at m 3 to 6 5 2 5 per cent 1 s d at m 5 the main shock will most likely occur in the first hour after the foreshock and the probability that a main shock will occur decreases with elapsed time from the occurrence of the possible foreshock by approximately the inverse of time thus the occurrence of an earthquake of m 3 0 in southern california increases the earthquake hazard within a small space time window several orders of magnitude above the normal background level

magnetoclimatology teleconnection between the siberian loess record and north atlantic heinrich events new environmental magnetic data from loess and paleosol successions in outcrops in the upper reaches of the ob river drainage southern siberia track the major climatic variations over the last glacial interglacial cycle profiles of magnetic susceptibility and alternating deposition of loess and soil formation events correspond to oxygen isotope stages 1 5 the magnetic susceptibility data in association with the stratigraphic succession confirm that the wind vigor magnetoclimatological model is a viable alternative to the classic pedogenic model interpretation of magnetic susceptibility data from loess paleosol successions must therefore consider eolian dynamics available source materials and transport directions in addition to pedogenic processes rapid magnetic fluctuations are also observed these are identified for the first time in siberian records as the signature of the abrupt cold pulses responsible for the heinrich layers in north atlantic marine sediments the data thus form a component of climatic teleconnections across the northern hemisphere allowing correlations to be made among 1 siberian magnetic susceptibility stratigraphy 2 data recorded from other loess paleosol successions in china european russia europe and north america 3 north atlantic ice rafted detritus and 4 sea surface temperatures derived from molecular stratigraphy of marine sediments off the northwest coast of africa

single link cluster analysis of earthquake aftershocks decay laws and regional variations using single link cluster analysis the authors investigate how various properties of aftershock sequences depend on their tectonic regime and focal depth for international seismological centre earthquakes of m sub b ge 4 8 they find that earthquakes deeper than 70 km have the fewest and smallest aftershock sequences even after accounting for differences in detectability and maximum magnitude they find that ridge transform earthquakes have smaller aftershock sequences that shallow subduction zone earthquakes among different subduction zones they find that zones with high moment release rates possess larger aftershock sequences comparing ridge transform zones they find those with slower spreading rates possess larger aftershock sequences by transposing origin times of several different aftershock sequences as if all had main shocks occurring at time zero they are able to study the properties of aftershock sequences which individually have too few aftershocks to study by other means secondary events determined by single link cluster analysis follow a modified omori s power law decay for time separations of 0 1 day to 20 days from the parent event with p values ranging from 0 539 plus minus 0 022 intermediate and deep focus earthquakes to 0 928 plus minus 0 024 ridge transform earthquakes they find that earthquake foreshocks and multiplets also follow a modified omori s law atmore greater times from the main shock the decay is steeper than a power law decay more like an exponential decay aftershocks in the adak catalog m sub b ge 2 0 show a marked decrease in activity between 40 and 50 km depth they speculate that the observed differences in number of aftershocks and p values may be caused by variations in fault heterogeneity or in fluid pressures less

foreshocks 1966 1980 in the san andreas system california the spatial and temporal distributions of seismicity preceding moderate ml 5 0 main shocks in the san andreas fault system in california have been analyzed to recognize and characterize the patterns of foreshock occurrence of 20 main shocks in the san andreas system 7 or 35 per cent have been preceded by immediate foreshock sequences that included events within 1 day and 5 km of the main shocks a possible correlation of the rate of foreshock occurrence with type of faulting was found such that none of the four main shocks with reverse faulting had foreshocks while 44 per cent of the strike slip earthquakes had foreshocks some enhanced seismic activity was also observed at relatively large distances from the main shock 13 to 30 km 1 to 5 days before 40 per cent of the main shocks but this activity cannot be clearly distinguished from the background seismicity of the seven immediate foreshock sequences only two had the swarm like appearance of the class ii foreshocks defined by mogi the other foreshock sequences appear to be single events sometimes with their own aftershocks preceding the respective main shocks four of these sequences are spatially correlated with distinct physical discontinuities in their faults between the hypocenters of the foreshock and main shock and similar discontinuities may also be associated with the other sequences the durations of the foreshock sequences were found to decrease as the depths of the main shocks increase from 3 to 11 km which has been interpreted as a dependence on stress to account for this stress dependence of the duration and the presence of discontinuities a model for foreshocks occurrence is presented this model proposes that foreshocks may represent a process of delayed multiple rupture and that the delay between occurrence of foreshock and main shock might represent the time needed for static fatigue to break the stronger rock at the discontinuity in the fault

dependence of the duration of geomagnetic polarity reversals on site latitude an important constraint on the processes governing the geodynamo the flow in the outer core responsible for generating earth s magnetic field is the duration of geomagnetic polarity reversals that is how long it takes for earth s magnetic field to reverse1 it is generally accepted that earth s magnetic field strength drops to low levels during polarity reversals and the field direction progresses through a 180 change while the field is weak1 the time it takes for this process to happen however remains uncertain with estimates ranging from a few thousand up to 28 000 years here i present an analysis of the available sediment records of the four most recent polarity reversals these records yield an average estimate of about 7 000 years for the time it takes for the directional change to occur the variation about this mean duration is not random but instead varies with site latitude with shorter durations observed at low latitude sites and longer durations observed at mid to high latitude sites such variation of duration with site latitude is predicted by simple geometrical reversal models in which non dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction and provides a constraint on numerical dynamo models

post depositional remanent magnetization lock in and the location of the matuyama brunhes geomagnetic reversal boundary in marine and chinese loess sequences article i nfo pdrm lock in loess paleosol bioturbation disturbs detrital magnetic particles after deposition so accurate recording of the ancient geomagnetic field in bioturbated sediments is widely attributed to acquisition of a post depositional remanent magnetization pdrm whereby the geomagnetic field exerts a torque on a magnetic particle and aligns it with the field after the final mixing event experienced by the particle the relationship between the matuyama brunhes boundary mbb and oxygen isotope age tie points in marine sediments has been widely used to determine the depth at which the paleomagnetic signal is locked in however such analyses can be badly affected by age discrepancies among different paleoclimatic proxies and by varying isotopic compositions of seawater in different locations and from the presence of different water masses at different depths at the same location it is therefore necessary to separately compare paleomagnetic data with respect to either benthic or planktonic foraminiferal oxygen isotope records for sites from the same water mass to avoid inadvertently introducing age differences to the analysis when the global data set is subjected to such a rigorous analysis few reliable data remain for the mbb using two complementary approaches we estimate that the mbb is on average shifted 20 cm below its true position in marine sediments this offset is the sum of the thickness of the bioturbated surface mixed layer which is possibly dominant and the pdrm lock in depth there is also controversy concerning observed differences in the position of the mbb relative to paleoclimatic proxies in marine sediments and chinese loess deposits for the chinese loess quartz grain size is insensitive to pedogenic alteration and is a useful parameter for determining the true position of the mbb with respect to paleoclimatic boundaries we conclude that the mbb occurs late in marine oxygen isotope stage 19 and in the upper part of chinese paleosol unit s8 rather than at the mid or lower part of loess unit l8 our results require adjustment of the generally accepted positions for the mbb and resolve a longstanding chronological conundrum for marine and chinese loess sequences

mineral magnetic record of the chinese loess and paleosols variations in the magnetic susceptiblity of the chinese loess and its interbedded paleosols correlate strikingly well with the deep sea oxygen isotope record low susceptibility values are found for the loess layers and high values are found for the soil horizons two interpretative models have been advanced to account for these magnetic variations both of these models discount any significant in situ formation of magnetite during soil forming periods instead they infer relative concentration of detrital or atmospheric magnetite at these times we critically examine the assumptions made in these models and on the basis of some new mineral magnetic data and fromalized magnetic flux calculations offer an alternative explanation of the loess magnetic record our model identifies pedogenic formation of magnetite as the major contributor to the high magnetic susceptibility of the paleosols the formation and preservation of this pedogenic magnetite is dependent on soil froming conditions and hence reflects the regional climate

focal mechanism dependence of static stress triggering of earthquakes abstract we perform a global statistical investigation into the problem of coulomb stress triggering of earthquakes by using the harvard cmt catalogue of shallow earthquakes from 1976 to 2001 we consider earthquake pairs i e pairs of successive earthquakes occurring near each other with similar focal mechanisms and address the problem of whether or not the change of the coulomb failure stress cfs produced by the first earthquake of the pair encourages the occurrence of the second one an interesting feature is that such a coulomb stress triggering phenomenon has an apparent focal mechanism dependence for thrust earthquakes a more evident triggering effect can be observed

second order moment of central california seismicity 1969 1982 the second order moment cross correlation function of earthquakes in the u s geological survey central california catalog between 1969 and 1982 was calculated with respect to a magnitude threshold m 4 0 over interevent distances up to 80 km and interevent times up to 320 days the statistical procedure results in a representation of the spatial temporal structure of the catalog associated with m 4 0 earthquakes and is capable of revealing patterns too weak to be detected in the space time distribution of seismicity for individual earthquake sequences a method is introduced for identifying aftershocks based on a physical two parameter model of the earthquake interaction process the results show that the aftershock process dominates the second order moment and may even obscure the statistical expression of a precursory process a concentration of foreshocks within 15 km and 3 days of m 4 0 main shocks exhibits an apparent migration toward the main shock loci with velocity 2 6 5 3 km d this concentration may be related to an observed tendency for m 4 0 events to cluster auto correlate over this interevent range with the identified aftershocks removed the residual catalog is poissonion in space and time when two m 4 0 earthquakes occur within 80 km and 40 days of each other aftershock productivity appears to be relatively enhanced in the earlier sequence this suggests that aftershock populations are not solely dependent on their main shocks and that unusually productive aftershock sequences may be predictors of future moderate earthquakes

foreshock occurrence rates before large earthquakes worldwide global rates of foreshock occurrence involving shallow m 6 and m 7 mainshocks and m 5 foreshocks were measured using earthquakes listed in the harvard cmt catalog for the period 1978 1996 these rates are similar to rates ones measured in previous worldwide and regional studies when they are normalized for the ranges of magnitude difference they each span the observed worldwide rates were compared to a generic model of earthquake clustering which is based on patterns of small and moderate aftershocks in california and were found to exceed the california model by a factor of approximately 2 significant differences in foreshock rate were found among subsets of earthquakes defined by their focal mechanism and tectonic region with the rate before thrust events higher and the rate before strike slip events lower than the worldwide average among the thrust events a large majority composed of events located in shallow subduction zones registered a high foreshock rate while a minority located in continental thrust belts measured a low rate these differences may explain why previous surveys have revealed low foreshock rates among thrust events in california especially southern california while the worldwide observations suggest the opposite california lacking an active subduction zone in most of its territory and including a region of mountain building thrusts in the south reflects the low rate apparently typical for continental thrusts while the worldwide observations dominated by shallow subduction zone events are foreshock rich

a test of foreshock occurrence in the central aleutian island arc abstract a statistical method for estimating the occurrence of foreshocks to larger earthquakes using local seismicity data is applied to the catalog of earthquakes from a local seismograph network near adak alaska foreshocks are defined in a restricted sense as earthquakes preceding larger earthquakes within a critical time calculated from the mean time between successive earthquakes in the volume surrounding the target event the threshold of completeness of the earthquake catalog is estimated as duration magnitude m d 2 2 a low magnitude cutoff on the catalog of m d 2 5 was used to minimize potential bias from variations in weather and instrumentation condition about 6 to 10 per cent of 51 main shocks of m b 4 5 in the adak thrust zone were preceded by detectable earthquakes satisfying this statistical definition for foreshocks as many as 14 percent of m b 5 0 earthquakes may have been preceded by detectable earthquakes satisfying this definition an estimate of foreshock occurrence using as background the seismicity rate in the 4 months preceding larger earthquakes indicates a slightly higher rate of occurrence the volume around the target events which showed foreshock activity is used to infer the size of a zone of preparation for larger earthquakes in the adak area the data are consistent with a zone of preparation whose size is a linear function of target event magnitude the size of the zone of preparation ranges from 20 to 45 km in radius for body wave magnitudes 4 0 to 5 7

comment on testing earthquake prediction methods the west pacific short term forecast of earthquakes with magnitude mwhrv 5 8 by v g kossobokov in his paper kossobokov investigates the efficiency of our short term forecast for two western pacific regions although we agree with the basic results of his evaluation that the forecast statistics is much better than a random guess we have reservations about his definition of earthquake prediction some of his tests and his interpretation of the test results we distinguish between deterministic earthquake predictions and statistical forecasts we argue that some techniques used by kossobokov may not be appropriate for testing our forecasts and discuss other testing methods based on the likelihood function we demonstrate that kossobokov s null hypothesis may be biased and this bias can influence some of his conclusions we show that contrary to kossobokov s statement our algorithm predicts mainshocks when they are preceded by foreshocks

delayed build up of arctic ice sheets during 400 000 year minima in insolation variability an east asian winter monsoon proxy record using grain size variations in chinese loess over the past 900 000 years shows that for up to 20 000 years after the interglacials at 400 000 year intervals the weak monsoon winds maintain a mild non glacial climate at high northern latitudes qingzhen hao and colleagues present a record of grain size variations in the chinese loess plateau a proxy of variations in the strength of the east asian winter monsoon and show that relatively high insolation at 400 000 year intervals held off the inception of the subsequent ice age for about 20 000 years relative to other glacial inceptions mechanistically the extension of warm conditions was probably linked to a weak siberian high pressure system associated with a delayed build up of northern ice and snow the authors speculate that these observations imply that arctic climate may remain in non glacial mode for more than 40 000 years even in the absence of anthropogenic greenhouse gases knowledge of the past variability of climate at high northern latitudes during astronomical analogues of the present interglacial1 may help to inform our understanding of future climate change unfortunately long term continuous records of ice sheet variability in the northern hemisphere only are scarce because records of benthic 18o content represent an integrated signal of changes in ice volume in both polar regions2 however variations in northern hemisphere ice sheets influence the siberian high3 an atmospheric pressure system so variations in the east asian winter monsoon eawm as recorded in the aeolian dust deposits on the chinese loess plateau can serve as a useful proxy of arctic climate variability before the ice core record begins here we present an eawm proxy record using grain size variations in two parallel loess sections representative of sequences across the whole of the chinese loess plateau over the past 900 000 years the results show that during periods of low eccentricity and precessional variability at approximately 400 000 year intervals the grain size inferred intensity of the eawm remains weak for up to 20 000 years after the end of the interglacial episode of high summer monsoon activity and strong pedogenesis in contrast there is a rapid increase in the eawm after the end of most other interglacials we conclude that for both the 400 000 year interglacials the weak eawm winds maintain a mild non glacial climate at high northern latitudes for much longer than expected from the conventional loess and marine oxygen isotope records during these times the less severe summer insolation minima at 65 n ref 4 would have suppressed ice and snow accumulation leading to a weak siberian high and consequently weak eawm winds

how does chinese loess become magnetized despite nearly three decades of paleomagnetic research on the extensive chinese loess deposits a convincing explanation has yet to be developed for how chinese loess becomes magnetized to address this problem we conducted re deposition experiments using weakly consolidated holocene chinese loess that was disaggregated in the laboratory and compare our results with published paleomagnetic data we simulated a depositional remanent magnetization drm associated with dry deposition of eolian sediments and a post depositional remanent magnetization pdrm in which the sediment was water saturated after deposition the simulated drm faithfully records the declination of the applied field but with systematic inclination flattening addition of minor water slightly improves recording of the applied field but inclination flattening persists reliable recording of the applied field occurs for pdrm simulation in water saturated sediment our synthesis of paleomagnetic data from chinese loess indicates that time averaged paleomagnetic directions are often indistinguishable from the expected geocentric axial dipole gad field but in many cases inclinations are shallower than for a gad field we conclude that the chinese loess is magnetized by a combination of drm and pdrm mechanisms with water content providing the dominant control on which mechanism aligns the detrital mineral fraction where pedogenesis causes neoformation of magnetic minerals an additional chemical remanent magnetization crm will occur the magnetization of chinese loess therefore appears to be controlled by a complex time varying combination of drm pdrm and crm mechanisms

variations in loess and palaeosol properties as indicators of palaeoclimatic gradients across the loess plateau of north china abstract new data are presented on micromorphology magnetic susceptibility and particle size characteristics of three soil sediment units at four locations along a wnw ese transect across the loess plateau of north china differences and spatial trends are used to assess the sensitivity of such data as proxies of regional palaeoclimatic gradients the steepest climatic gradient suggested by the magnetic susceptibility for the s1 palaeosol is at the eastern humid end of the traverse micromorphology and particle size analysis show that the loess units in the east are richer in clay than the best developed palaeosols in the semi arid west and that particle size in the west appears to provide a more sensitive record of climatically driven changes than magnetic susceptibility the use of surface and near surface soils as a datum for inferring palaeoprecipitation values is reviewed in the light of recent evidence of substantial degradation of such soils micromorphological and particle size analysis of closely spaced samples from loess palaeosol sequences is a sensitive addition to magnetic susceptibility stratigraphy and correlation

frequency of occurrence of moderate to great earthquakes in intracontinental regions implications for changes in stress earthquake prediction and hazards assessments we investigate the departure of shallow intracontinental earthquakes from the frequency magnitude relationship log n m a bm we consider seven large continental regions but exclude subduction zones in each active and stable areas are differentiated within active areas we separately categorize earthquakes on faults with high slip rates we use data sets from 1978 1994 and 1900 1994 which are complete for moment magnitudes mw 5 3 and 7 0 to calculate changes in b value with mw for all active intracontinental regions combined except asia b changes at 98 confidence from 0 90 to 2 1 at a corner magnitude mc of 6 9 to 7 0 the distribution for stable regions indicates a similar change values of mc are associated with downdip widths of rupture of about 23 to 27 km which are smaller than those for subduction zones in each case mc marks a transition from unbounded to bounded earthquakes in asia events from 1958 to 1994 also delineate a similar change near mc 7 0 earthquakes in asia of 7 0 mw 8 1 from 1900 to 1957 however define a b value of 1 0 and a moment release rate 5 6 times higher than that from 1958 to 1994 those anomalous values indicate that a large region of asia was close to failure i e near a self organized critical state during the earlier period we associate those changes with the giant himalayan earthquake in 1950 of mw 8 6 regional variations in seismic strain release rates correlate with the types of processes at adjacent plate boundaries extrapolating rates of activity of the past 95 years to mw 8 0 indicates that such events should be very rare in stable areas

can relative paleointensities be determined from the normalized magnetization of the wind blown loess of china although the wind blown loess and interbedded paleosol sequence in the loess plateau of china has been extensively studied and regarded as one of the most complete terrestrial records of both the geomagnetic field and climatic changes during the quaternary little attention has been paid to it as a source of relative paleointensity in this study we examine the malan loess l1 the last glacial sediments in the lingtai section to determine whether loess is able to reliably record the relative paleointensity toward that end both the conventional normalizing method as well as the pseudo thellier method tauxe et al 1995 were used in conjunction with the examination of the rock magnetic properties and natural remanent magnetization nrm rock magnetic properties of l1 show the uniformity in terms of magnetic mineralogy and grain size suggesting that they may be suitable for relative paleointensity studies normalized remanences derived from l1 show highs between both 72 66 ka and 62 51 ka and lows near 63 ka 42 ka and between 20 and 10 ka comparable results using different normalization parameters suggest that large amplitude variation of normalized magnetization may reflect the intensity variation of the geomagnetic field however comparisons of the normalized magnetization with contemporaneous marine records show some dissimilarities in particular the intensity low at 20 10 ka corresponding to the last glacial maximum is not compatible with the sint 800 composite record on the other hand spectral coherence analyses of middle and high frequency components of the normalized magnetizations suggest some climatic influence thus this apparent discrepancy may be explained by climatic changes in the loess plateau and the resulting effect on the nrm

10 be evidence for the matuyama brunhes geomagnetic reversal in the epica dome c ice core every few hundred thousand years earth s north and south magnetic poles swap places for the first time a record of one of these geomagnetic reversals has been found in an ice core because the strength of the field is very low during a reversal the intensity of cosmic rays in the earth s atmosphere is increased this leads to an increase in the quantity of 10be which like 14c is formed by cosmic rays in the atmosphere deposited in the ice it is this 10be enhancement in the ice that has been detected an ice core drilled at dome c antarctica is the oldest ice core so far retrieved1 on the basis of ice flow modelling and a comparison between the deuterium signal in the ice with climate records from marine sediment cores the ice at a depth of 3 190 m in the dome c core is believed to have been deposited around 800 000 years ago2 offering a rare opportunity to study climatic and environmental conditions over this time period however an independent determination of this age is important because the deuterium profile below a depth of 3 190 m depth does not show the expected correlation with the marine record2 here we present evidence for enhanced 10be deposition in the ice at 3 160 3 170 m which we interpret as a result of the low dipole field strength during the matuyama brunhes geomagnetic reversal which occurred about 780 000 years ago if correct this provides a crucial tie point between ice cores marine cores and a radiometric timescale