evidence for denitrification regulated by pyrite oxidation in a heterogeneous porous groundwater system abstract denitrification is an important natural attenuation process that has been observed in many fissured and porous aquifers however an important factor limiting denitrification in aquatic systems is the microbial availability of electron donors pyrite as the most abundant sulfide mineral in nature represents one of the potential electron sources for denitrifiers to reduce nitrate but the reaction mechanisms coupling denitrification processes to pyrite oxidation are still questionable we utilized hydrochemical data and stable isotopes of nitrate and sulfate in groundwater isotope ratios of sulfur compounds in aquifer sediments and tritium based groundwater dating for assessing denitrification processes in a pyrite bearing porous groundwater system the oxic part of the aquifer with mean water transit times of approximately 60 years was characterized by nitrate concentrations of around 15 mg l and 15 n values were similar to those typical for nitrification in contrast in the anoxic part with mean water transit times of up to 100 years low nitrate concentrations accompanied by elevated 15 n values were observed furthermore isotope data of groundwater sulfate and sulfur compounds in the aquifer sediment suggest that pyrite oxidation is the dominant source of sulfate in the aquifer the trend of increasing 15 n values and decreasing nitrate concentrations in concert with depleted 34 s values of groundwater sulfate similar to 34 s values of pyrite fes 2 suggests that denitrification is coupled to pyrite oxidation particularly when water mean transit time is elevated

southern san andreas fault seismicity is consistent with the gutenberg richter magnitude frequency distribution the magnitudes of any collection of earthquakes nucleating in a region are generally observed to follow the gutenberg richter gr distribution on some major faults however paleoseismic rates are higher than a gr extrapolation from the modern rate of small earthquakes would predict this along with other observations led to the formulation of the characteristic earthquake hypothesis which holds that the rate of small to moderate earthquakes is permanently low on large faults relative to the large earthquake rate wesnousky et al 1983 schwartz and coppersmith 1984 we examine the rate difference between recent small to moderate earthquakes on the southern san andreas fault ssaf and the paleoseismic record hypothesizing that the discrepancy can be explained as a rate change in time rather than a deviation from gr statistics we find that with reasonable assumptions the rate changes necessary to bring the small and large earthquake rates into alignment agree with the size of rate changes seen in epidemic type aftershock sequence modeling where aftershock triggering of large earthquakes drives strong fluctuations in the seismicity rates for earthquakes of all magnitudes the necessary rate changes are also comparable to rate changes observed for other faults worldwide these results are consistent with paleoseismic observations of temporally clustered bursts of large earthquakes on the ssaf and the absence of m 7 earthquakes on the ssaf since 1857

seismic moment catalog of large shallow earthquakes 1900 to 1989 we compile a worldwide catalog of shallow depth 70 km and large ms 7 earthquakes recorded between 1900 and 1989 the catalog is shown to be complete and uniform at the 20 sec surface wave magnitude ms 7 0 we base our catalog on those of abe 1981 1984 and abe and noguchi 1983a b for events with ms 7 0 those catalogs however are not homogeneous in seismicity rates for the entire 90 year period we assume that global rates of seismicity are constant on a time scale of decades and most inhomogeneities arise from changes in instrumentation and or reporting we correct the magnitudes to produce a homogeneous catalog the catalog is accompanied by a reference list for all the events with seismic moment determined at periods longer than 20 sec using these seismic moments for great and giant earthquakes and a moment magnitude relationship for smaller events we produce a seismic moment catalog for large earthquakes from 1900 to 1989 the catalog is used to study the distribution of moment released worldwide although we assumed a constant rate of seismicity on a global basis the rate of moment release has not been constant for the 90 year period because the latter is dominated by the few largest earthquakes we find that the seismic moment released at subduction zones during this century constitutes 90 of all the moment released by large shallow earthquakes on a global basis the seismic moment released in the largest event that occurred during this century the 1960 southern chile earthquake represents about 30 to 45 of the total moment released from 1900 through 1989 a frequency size distribution of earthquakes with seismic moment yields an average slope b value that changes from 1 04 for magnitudes between 7 0 and 7 5 to b 1 51 for magnitudes between 7 6 and 8 0 this change in the b value is attributed to different scaling relationships between bounded large and unbounded small earthquakes thus the earthquake process does have a characteristic length scale that is set by the downdip width over which rupture in earthquakes can occur that width is typically greater for thrust events at subduction zones than for earthquakes along transform faults and other tectonic environments

long term influence of giant earthquakes backward empirical evidence and forward test abstract we investigate the capability of the strongest earthquakes to modify significantly the seismicity in a wide spatiotemporal window in particular we show that the strongest earthquakes of last century were probably able to influence the seismicity at large spatiotemporal distances extending their reach over thousands of kilometers and decades later we report statistically significant differences in worldwide seismicity before and after the occurrence of the strongest earthquakes of the last century whose perturbation is modeled by means of coseismic and postseismic stress variations this long term coupling has produced time variations in worldwide seismic activity that appear related to the physical coupling between the focal mechanism of source earthquakes and the tectonic setting of each zone these results could provide new important insights on seismic hazard assessment because they raise doubts on the validity of two paradigms that is the steadiness of the mainshock rate and the isolation of a seismic region from the surrounding areas finally in addition to this backward analysis we also provide a formal forward test by forecasting the effects on global seismicity of the recent sumatra andaman earthquakes this is maybe a unique chance to test the long term hypothesis with an independent dataset that avoids by definition any kind of often unconscious optimization of the results that is an unavoidable possibility in backward analyses

the seismic cycle in southern california precursor or response the seismicity rate m 3 0 in southern california shows two cycles with periods of high activity 90 events year from 1945 1952 and 1969 1992 and lower activity 60 70 events year from 1952 1969 and 1992 present abrupt drops in the seismicity rate occur after the 1952 kern county m7 5 and the 1992 landers m7 3 earthquakes the sudden increase in 1969 does not coincide with any major event but approximates the time needed to reaccumulate the seismic moment released in the 1952 earthquake this temporal correlation with the preceding earthquake suggests that the seismic cycle lower seismicity after a major earthquake and higher seismicity before the next major earthquake should be interpreted as a response to the first earthquake rather than a precursor to the second southern california is now at a rate of seismicity as low as it experienced in the 1950s and 1960s

distinguishing between random and nonrandom patterns in the energy release of great earthquakes romanowicz 1993 suggested that great earthquakes alternate in a predictable fashion between strike slip and dip slip mechanisms on a 20 30 year cycle we investigate whether or not the observed pattern of shifting predominance between strike slip and dip slip is capable of frequent reproduction by random simulation to accomplish this we randomly drew strike slip and dip slip events in the same proportion as in the historic record and determined how frequently a pattern of predominance similar to the historic record is generated out of 1000 simulations a pattern similar to the historic record was observed 183 times this throws considerable doubt on romanowicz s 1993 conslusion that a mechanism is at work producing the distribution she observed of great earthquakes the difficulty in coming to conclusions from data of the type used by romanowicz 1993 is explored along with the psychological literature on the difficulty of detecting random and nonrandom patterns from relatively small amounts of information

testing self organized criticality by induced seismicity we examine the hypothesis proposed in recent years by several authors that the crust is in a self organized critical soc state this hypothesis has been suggested on the basis of the observation of power law distributions such as the gutenberg richter law for earthquakes and the fault length distribution and of the fractal geometry of sets of earthquake epicenters and of fault patterns these self similar properties are shared by simplified models of the crust exhibiting a spontaneous organization toward a critical point characterized by similar scale invariant properties the term critical is here used in the sense of phase transitions such as the curie point in magnetism the usefulness of a hypothesis is measured by its predictive and explanatory power outside the range of observations that have helped defined it we thus explore how the soc concept can help in understanding the observed earthquake clustering on relatively narrow fault domains and the phenomenon of induced seismicity we review the major reported cases of induced seismicity in various parts of the world and find that both pore pressure changes p and mass transfers m leading to incremental deviatoric stresses of 1 mpa are sufficient to trigger seismic instabilities in the uppermost crust with magnitude ranging up to 7 0 in otherwise historically aseismic areas once triggered stress variations of at least 1 order of magnitude less but still larger than the 0 01 mpa tidal stress are enough to sustain seismic activity we argue that these observations are in accord with the soc hypothesis as they show that a significant fraction of the crust is not far from instability and can thus be made unstable by minute perturbations this property is shared by simplified models of soc not all perturbations however trigger seismic activity this is also compatible with the soc hypothesis which embodies naturally the existence of large heterogeneities in the stress field the induced seismicity is found to obey generally the gutenberg richter law up to a magnitude cutoff which correlates well with the width of the local seismogenic bed ranging in size from that of mine pillars for mining induced seismicity to the thickness of brittle sedimentary beds in the vicinity of dams or depleted hydrocarbon reservoirs in conclusion the properties of induced seismicity and their rationalization in terms of the soc concept provide further evidence that potential seismic hazards extend over a much larger area than that where earthquakes are frequent

global postseismic deformation deep earthquakes we study the global viscoelastic deformations associated with a shear dislocation on a fault embedded in a viscoelastic mantle to address this problem we extend the theory of the quasi static deformations of a maxwell spherical n layer incompressible earth previously limited to the modeling of the effects of earthquakes occurring only within the elastic lithosphere permanent postseismic deformations which can be generated by lithospheric sources cannot be sustained if the source region is viscoelastic however the transient response of the mantle strongly depends on the viscosity of the source region we use the technique developed here to investigate thoroughly the quasi static surface deformations induced by seismic events of variable depth we show that owing to the combined effect of sphericity and viscoelastic mantle relaxation the surface displacements do not systematically decrease as the depth of the source increases instead with increasing source depth we predict an increasing efficiency of mantle relaxation in triggering postseismic deformations of large size most of our results are based on a simple four layer model with a 100 km thick lithosphere upper and lower mantle separated by the 670 km discontinuity and a fluid core the last part of this work is devoted to a study of the effects of a low viscosity asthenosphere on the rates of deformation detected at the earth s surface the findings reported here may be useful for the interpretation of the transient motions of the earth s surface in response to deep focus earthquakes

can great quakes extend their reach seismologybig earthquakes can trigger other quakes thousands of kilometers away and decades later according to calculations presented on page 1245 1 of this issue of science the researchers simulated how stress travels through deep viscous rock and found that the great earthquakes that struck the far north pacific in the 1950s and 60s could have set off a slow moving stress wave that triggered a pulse of seismic activity in california in the 1980s 1 http www sciencemag org cgi content short 280 5367 1245

age of lucy and the first family single crystal 40ar 39ar dating of the denen dora and lower kada hadar members of the hadar formation ethiopia single crystal laser fusion 40ar 39ar analyses on previously undatable tephra deposits provide the first reliable dates for the important hominid fossils popularly called lucy and the first family here dated to 3 18 and 3 20 ma respectively these results help to establish an age framework for hominid and biostratigraphic evolution for the fossil rich hadar formation the abundance of well preserved vertebrate fossils at hadar can be attributed in part to an unusually high sediment accumulation rate of approximately 80 cm ka which might be due to increased tectonic subsidence of the hadar basin between 3 4 and 3 2 ma the new dates also provide supportive evidence that the boundary ages for the kaena and mammoth subchrons of the geomagnetic polarity time scale need to be increased by as much as 4 to 5

global post seismic deformation summary we quantify the effects of post seismic deformation on the radial and horizontal components of the displacement in the near and far field of strike and dip slip point dislocations these sources are embedded in the elastic top layer of a spherical self gravitating stratified viscoelastic earth within the scheme of the normal mode technique we derive the explicit analytical expression of the fundamental matrix for the toroidal component of the field equations this component is propagated together with its spheroidal counterpart from the core mantle boundary to the earth s surface viscosity stratification at 670km depth influences the radial and horizontal deformation accompanying viscoelastic relaxation in the mantle over time scales of 103 104 yr both in the near field ranging from 100 to 500 km and in the far field from 103 to 5 x 103 km if the upper mantle is differentiated into a low viscosity zone beneath the lithosphere and a normal upper mantle faster relaxation is obtained for an asthenospheric viscosity of 1020 pa s we obtain for a strike slip dislocation and a seismic moment of 1022 n m characteristic of an average large earthquake horizontal rates of 1 4 mm yr 1 in the near field and 0 05 0 4 mm yr 1 in the far field these values are maintained over time scales of 10 103 yr larger rates with shorter duration are obtained if the viscosity is reduced in the low viscosity channel as expected strike slip dislocations are the most effective in driving horizontal deformation in the far field in comparison with dip slip ones it is noteworthy that horizontal velocities are maintained longer in the far field in comparison with radial ones which is not surprising since momentum is propagated in far regions essentially in the horizontal direction radial deformation is generally lower in the far field vlbi techniques with a precision of a few parts per billion over distances of 103 km can detect global post seismic deformation induced by large earthquakes our results affect the interpretation of the transfer of stress and seismic activity among different plate boundaries

oceanic broad multifault transform plate boundaries oceanic transform plate boundaries consist of a single narrow a few kilometers wide strike slip seismic zone offsetting two mid ocean ridge segments however we define here a new class of oceanic transform boundaries with broad complex multifault zones of deformation similar to some continental strike slip systems examples are the 750 km long 120 km wide andrew bain transform on the southwest indian ridge and the romanche transform where the mid atlantic ridge is offset by a lens shaped 900 km long 100 km wide sliver of deformed lithosphere bound by two major transform valleys one of the valleys is seismically highly active and constitutes the present day principal transform boundary however strike slip seismic events also occur in the second valley and elsewhere in the deformed zone some of these events may be triggered by earthquakes from the principal boundary numerical modeling predicts the development of wide multiple transform boundaries when the age offset is above a threshold value of 30 m y i e in extra long 500 km slow slip transforms multiple boundaries develop so that strike slip ruptures avoid very thick and strong lithosphere

oxygen and sulfur isotope geochemistry of acidic groundwater discharge in british columbia yukon and district of mackenzie canada the paint pots in kootenay national park british columbia appear to derive the fe zn pb and contents of their water from sulfide mineralization in lower and middle cambrian carbonates the fe zn ni and contents of groundwater discharging into a tributary of engineer creek yukon are likely derived from sulfide mineralization in devonian or ordovician black shales exposed in the area the high fe and contents of a natrojarosite deposit northeast of fort norman northwest territories are probably derived from pyritiferous cretaceous shales in that area isotope analyses of water and of dissolved and precipitated sulfur species from these three sites where acidic heavy metal bearing groundwater is being discharged revealed that between 38 and 74 of the oxygen used in the subsurface oxidation of metal sulfides is supplied by h2o molecules rather than by molecular dissolved oxygen the available data also suggest that lower percentages of water oxygen in the secondary sulfates reflect increasing

earthquakes and relative sealevel changes 1 using a spherical model of postseismic deformation for the first time we have computed the global contribution of large earthquakes to the relative sealevel variations in the twentieth century we have found that great earthquakes have the overall tendency to produce a sealevel rise and that they affect the measurements taken at those tide gauge sites that are commonly employed to obtain global estimates of sealevel rise though on a global scale most of the signal is associated with thrust events on a regional scale the effects of great transcurrent earthquakes cannot be neglected depending on the viscosity of the asthenosphere the contribution of earthquakes to the long term sealevel changes amounts to at least 0 1 mm yr thus the climate driven long term sealevel changes deduced by tide gauge observations may be slightly but not negligibly overestimated

coseismic and post seismic signatures of the sumatra 2004 december and 2005 march earthquakes in grace satellite gravity s u m m a r y the grace satellite mission has been measuring the earth s gravity field and its temporal variations since 2002 april although these variations are mainly due to mass transfer within the geofluid envelops they also result from mass displacements associated with phenomena including glacial isostatic adjustment and earthquakes however these last contributions are difficult to isolate because of the presence of noise and of geofluid signals and because of grace s coarse spatial resolution 400 km half wavelength in this paper we show that a wavelet analysis on the sphere helps to retrieve earthquake signatures from grace geoid products using a wavelet analysis of grace geoids products we show that the geoid variations caused by the 2004 december m w 9 2 and 2005 march m w 8 7 sumatra earthquakes can be detected at grace resolution the 2004 december earthquake produced a strong coseismic decrease of the gravity field in the andaman sea followed by relaxation in the area affected by both the andaman 2004 and the nias 2005 earthquakes we find two characteristic timescales for the relaxation with a fast variation occurring in the vicinity of the central andaman ridge we discuss our coseismic observations in terms of density changes of crustal and upper mantle rocks and of the vertical displacements in the andaman sea we interpret the post seismic signal in terms of the viscoelastic response of the earth s mantle the transient component of the relaxation may indicate the presence of hot viscous material beneath the active central andaman basin

isotopic evidence for plio pleistocene environmental change at gona ethiopia abstract a 4 5 ma record of fluvial and lacustrine deposits is well exposed at gona in the afar depression of ethiopia we use isotopic values of pedogenic carbonate and fossil teeth to reconstruct plio pleistocene environmental change at gona an increase in 13c values of pedogenic carbonates since 4 5 ma points to a shift from woodlands to grassy woodlands in the early pliocene 10 4 to 3 9 vpdb to more open but still mixed environments in the late pleistocene 3 0 to 1 4 vpdb this pattern is also seen in isotopic records elsewhere in east africa however at 1 5 ma the higher proportion of c4 grasses at gona is largely a result of a local facies shift to more water limited environments the wide range of 13c values of pedogenic carbonate within single stratigraphic levels indicates a mosaic of vegetation for all time intervals at gona that depends on depositional environment elements of this mosaic are reflected in 13c values of both modern plants and soil organic matter and plio pleistocene soil carbonate indicating higher amounts of c4 grasses with greater distance from a river channel in both the modern and ancient awash river systems 18o values of pedogenic carbonates increase up section from 11 9 in the early pliocene to 6 4 vpdb in the late pleistocene the wide range of 18o values in paleovertisol carbonates from all stratigraphic levels probably reflects short term climate changes and periods of strong evaporation throughout the record based on the comparison between 18o values of plio pleistocene pedogenic carbonates and modern waters we estimate that there has been a 6 5 increase in mean annual 18o values of meteoric water since 4 5 ma 18o values of pedogenic carbonate from other east african records indicate a similar shift increasing aridity and fluctuations in the timing and source of rainfall are likely responsible for the changes in 18o values of east african pedogenic carbonates through the plio pleistocene

temporal changes in the cumulative piecewise gradient of a variant of the gutenberg richter relationship and the imminence of extreme events abstract it is generally found that the relative frequencies of occurrence of earthquakes of different magnitudes in seismogenic zones have a power law distribution for a long term dataset the overall slope of this logarithmically transformed distribution is usually indicated by a best fit straight line and expressed as a b value this slope is stable and normally lies between 0 8 and 1 2 the actual value depending on the region examined and the threshold selected for data completeness the linearity of the distribution can be used to make statistical inferences about the potential for larger events over the long run and with appropriate reservations may even be extrapolated to magnitudes that are beyond recent experience we suggest the same information can also be viewed over shorter intervals in terms of an empirical piecewise distribution with relative frequencies of occurrence at adjacent magnitude steps controlling the local slope of the distribution an emergence through time of an excess number of lower magnitude earthquakes causes temporal changes to appear in the low end piecewise gradients of this distribution a marked excursion away from an overall uniform trend for the particular zone may be indicative of an imminent larger event on two separate occasions in 1982 and 1997 such temporal variations were seen in the magnitude distributions of sequences of events near tobago west indies and used to anticipate subsequent damaging mainshocks the recognition of temporal departures from overall linearity of the magnitude frequency relation in a suitable dataset may thus provide an evidential element that can contribute to earthquake forecasting this phenomenological approach was used in the analysis of the neic global dataset of earthquakes of magnitude 6 1 and bigger for the period 1973 2003 to explore its wider applicability trends in the piecewise gradients of the global data were interpreted as pointing to an imminent great earthquake perhaps exceeding magnitude 8 5 such an event did occur shortly afterwards in the form of the great sumatran earthquake of 2004 12 26 following that event global magnitude production continued to exhibit sharp imbalances in the lower magnitude bins indicating that another similar event was likely the second sumatran earthquake on 2005 03 28 satisfied that projection since that time a magnitude production imbalance persists in the global dataset suggesting the system could be poised to output an earthquake or earthquakes in the magnitude range 8 6 9 0 or even greater this contribution describes the piecewise gradient approach and examines its application to global earthquake data

controls of shear zone rheology and tectonic loading on postseismic creep 1 postseismic deformation is well documented in geodetic data collected in the aftermath of large earthquakes in the postseismic time interval gps is most sensitive to creep in the lower crust or upper mantle activated by earthquake generated stress perturbations in these regions deformation may be localized on an aseismic frictional surface or on a ductile shear zone these two hypotheses imply specific rheologies and therefore time dependence of postseismic creep hence postseismic creep constitutes a potential probe into the rheology of aseismic regions of the lithosphere i present a simple shear zone model of postseismic creep in which the rheology of the creeping element can be varied in the absence of tectonic loading during the postseismic time interval the displacement history of the shear zone obeying a power law rheology with stress exponent n follows an analytical relaxation curve parameterized by 1 n for a frictional surface postseismic creep follows the same relaxation law in the limit 1 n 0 a rough estimate of the apparent stress exponent can be obtained from continuous gps records application to data collected after the 1994 sanriku earthquake yields 1 n 0 1 which is consistent with dislocation creep mechanisms however the records of two other subduction zone events the 2001 peru event and the 1997 kronotski earthquake and a continental strike slip earthquake the 1999 izmit earthquake require negative 1 n rather than characterizing the shear zone rheology these negative exponents indicate that reloading of the shear zone by tectonic forces is important numerical simulations of postseismic deformation with nonnegligible reloading produce curves that are well fit by the generalized relaxation law with 1 n 0 although the actual stress exponent of the rheology is positive while this prevents rheology from being tightly constrained by the studied gps records it indicates that reloading is important in the postseismic time interval in other words the stress perturbation induced by an earthquake is comparable to the stress supported by ductile shear zones in the interseismic period

seismic cycles along the aleutian arc analysis of seismicity from 1957 through 1991 we catalog and relocate aleutian arc seismicity between 1957 and 1991 two great earthquakes ruptured the same 250 km long portion of the central aleutian arc the 1957 aleutian islands earthquake and the 1986 andreanof islands earthquake because accurate estimates of the moment distribution of the 1957 earthquake are not available the spatial distribution of aftershocks for each of these events is compared and tested against models describing the modes of occurrence of great subduction zone earthquakes earthquake relocations are based on p wave arrival times published in the international seismological summary the bureau central international seismologique and the international seismological centre bulletins and include corrections for the near source velocity structure associated with the down going slab magnitude estimates are extracted from bulletins and prior to 1964 are estimated by us from microfilmed records our catalog is complete above magnitude 5 5 aftershocks associated with the 1957 and 1986 earthquakes appear to occur in different areas east of the main shock epicenters aftershock locations are anticorrelated west of the main shock epicenter aftershocks of the 1986 earthquake tended to concentrate along the updip edge of aftershock clusters associated with the 1957 earthquake if we assume aftershocks rim the distribution of seismic moment release associated with each event these observations imply that the moment distribution of the 1986 earthquake was different from that of the 1957 earthquake this suggests that we should use caution in identifying mechanically strong portions of a fault asperities by simply mapping the moment distribution of a single great earthquake a fundamental tenet of the asperity model that rupture always occurs on the strongest portions of the fault with weaker portions rupturing either aseismically or dynamically as a result of rupture on a strong fault patch may in the case of the central aleutian arc not be correct thus observing the moment distribution from a single great earthquake may tell us little about what the distribution of moment release will look like during the next earthquake

dates and rates in ancient lakes 40ar 39ar evidence for an early cretaceous age for the jehol group northeast china the correlation of freshwater sediments in small fault bound basins in liaoning province northeast china known as the jehol or rehe group has been a subject of debate for many years with biochronological estimates ranging from late jurassic to the cretaceous periods we have applied the laser ar 40 ar 39 technique to volcanic intercalations and lacustrine sediments from the yixian formation at the base of the jehol group minerals and whole rock chips from the upper parts of the yixian formation give concordant ages with a mean of 121 1 0 2 ma 1 sigma ages for samples near the base of the yixian formation give 121 4 0 6 and 122 9 0 3 ma and appear to be synchronous or only slightly older than the top of the formation integrated ages of 122 ma for glaucony from the lacustrine sediments lying stratigraphically between the upper and lower parts of the yixian are in very good agreement with the absolute age framework provided by the volcanic units such disseminated facies of this clay show promise for directly dating lacustrine sediments all of the above ar 40 ar 39 dates provide an absolute calibration of the yixian formation and show that the whole formation was deposited entirely within early cretaceous time over an interval of no more than 2 3 ma

recalibration of the geomagnetic polarity timescale the quest for improved time control to estimate rates of geologic and paleoenvironmental processes is central to the earth sciences until the middle of this century geochronology was based solely on macrofossil evolution providing the largely unmodified definition of geologic stages in the last few decades with impetus from the deep sea drilling project dsdp and the ocean drilling program odp microfossil biostratigraphy has become increasingly important although new radiometric techniques have resulted in improved absolute age precision the direct correlation of absolute ages to biozonations and hence to geologic stage boundaries remains poor this is due to the difficulty of finding materials suitable for radiometric dating which can be directly correlated to biozonations the bridge between biozonations and absolute ages and the interpolation between absolute ages are best accomplished particularly for cenozoic and late mesozoic time through the geomagnetic polarity timescale gpts the template for late jurassic to quaternary geomagnetic polarity is derived from oceanic magnetic anomalies as demonstrated by heirtzler et al 1968 the gpts can be constructed using oceanic magnetic anomaly record s assuming constant seafloor spreading rate s and interpolating between available radiometric ages in the last quadrennium there have been outstanding developments in radiometric dating and astrochronology which have greatly improved the absolute age control of the gpts

postseismic deformation and stress diffusion due to viscoelasticity and comments on the modified elsasser model finite element techniques have been used to investigate the far field deformation and stress changes due to asthenospheric viscoelastic relaxation following a dip slip earthquake the diffusion of extensional stress toward the subduction zone following a thrust earthquake on land is qualitatively consistent with the modified elsasser model as proposed by rydelek and sacks 1988 1990 to explain the coupled occurrence of land and sea earthquakes near japan however the magnitude of the diffusing tensional signal is significantly smaller the nominal model consists of a partially faulted elastic lithosphere overlying a viscoelastic substrate other models consider thin channel flow rupturing of the entire elastic lithosphere and changes in the depth of faulting while some of these changes have significant impact on the magnitude and spatial features of the stress and deformation field the far field stress remains small numerical experiments demonstrate that the assumptions of the modified elsasser model accentuate uniaxial deformation when these assumptions are replicated in the finite element calculations reasonable agreement between the models is achieved

spatiotemporal patterns in the energy release of great earthquakes for the past 80 years the energy released in great strike slip and thrust earthquakes has occurred in alternating cycles of 20 to 30 years this pattern suggests that a global transfer mechanism from poloidal to toroidal components of tectonic plate motions is operating on time scales of several decades the increase in seismic activity in california in recent years may be related to an acceleration of global strike slip moment release as regions of shear deformation mature after being reached by stresses that have propagated away from regions of great subduction decoupling earthquakes in the 1960s

rupture process of the andreanof islands earthquake of may 7 1986 the andreanof islands earthquake of may 7 1986 ms 7 7 is the largest earthquake to have occurred along the aleutian arc in the past 20 yr source parameters of the mainshock three large aftershocks 6 1 ms 6 7 and a foreshock ms 6 0 are determined by inverting digitally recorded teleseismic waveforms and amplitudes long period p and sh waves are inverted simultaneously to determine the strike dip rake centroid depth source time function and seismic moment of each earthquake complexities in the rupture process of the mainshock are modeled using broadband 5 to 0 01 hz p wave displacement waveforms modeling the mainshock as a propagating line source the long period and broadband waveforms indicate that the locus of largest moment release propagated bilaterally along the arc with a well resolved velocity of 1 5 km sec the source mechanism is consistent with faulting on a low angle 23 1 thrust fault the fault s strike 260 10 and the rake of the slip vector on the fault 93 8 are less well resolved and trade off with each other acceptable centroid depths range between 10 and 25 km within this range of depths the misfit of the synthetic to the observed long period and broadband waveforms varies by only 6 per cent while moment estimates vary by as much as 64 per cent for a centroid depth of 20 km the estimated seismic moment is 1 1 1028 dyne cm 85 per cent of the moment estimated from surface wave analysis ninety per cent of the moment release was generated by three patches on the fault surface all located within 90 km of the nucleation point very little moment however was released near the epicenter of the mainshock the epicentral locations source mechanisms and depths of the foreshock and one of the aftershocks modeled by us are consistent with faulting on the same thrust plane as the mainshock the other two modeled aftershocks are located north of the island arc and have source depths of less than 15 km indicating that they occurred within the overriding plate the focal mechanisms of these events indicate almost pure strike slip faulting but the waveform data do not contain sufficient information to distinguish which of the couples of the double couple mechanism corresponds to the fault plane joint relocations of several of the aftershocks from one of these strike slip earthquakes suggests that right lateral strike slip faulting occurred on a northeasterly trending fault plane

gravitational viscoelastic postseismic relaxation on a layered spherical earth viscoelastic relaxation of a ductile asthenosphere underlying a purely elastic plate is a strong candidate process for explaining anomalous rates of crustal deformation observed following large earthquakes the nongravitational treatment of pollitz 1992 which is valid on a global scale and includes the effects of compressibility is here extended to permit the calculation of gravitational viscoelastic relaxation for a specified spherically layered viscoelastic rheology following an earthquake in an elastic layer the simple approximations we adopt make the resulting treatment particularly suitable for near field calculations for an asthenosphere with a maxwell rheology the effect of gravitational coupling is manifested only many relaxation times after the earthquake as obtained by previous investigators its effect is generally to speed up the long wavelength component of the relaxation process and attenuate the overall vertical displacement pattern several subtle features common to the relaxation behavior from several different fault types thrust rift and strike slip are identified the effect of gravitational coupling on horizontal displacements is consistent with flexure of the upper elastic plate driven by the corresponding effects on the vertical displacement stress diffusion away from the source region generally exhibits pulse like behavior which is dispersive in both space and time if the asthenosphere is confined to a relatively narrow channel then the dispersion branches governing relaxation are radically altered and stress diffusion effects far from the coseismic rupture zone exhibit a complicated time dependence reflecting the competing tendencies of toroidal and spheroidal mode relaxation

bering sea earthquake of february 21 1991 active faulting along the bering shelf edge on february 21 1991 an ms 6 8 shallow earthquake occurred in the eastern bering sea in an area which previously has shown no significant seismic activity the earthquake was located on the bering shelf close to the shelf edge near one of the elongate basins zhemchug basin that follow the outer shelf to better understand the causes of this unusual event we relocated the earthquake and its aftershocks determined its source mechanism and depth and examined multichannel seismic observations of structures near the epicenter the event was relocated from regional and teleseismic p and s arrival times using a nonlinear inverse technique as were the 19 previous well recorded bering sea earthquakes in global catalogs 1964 1987 the 1991 mainshock epicenter was located on the western flank of zhemchug basin the only four previous events with small 1 location errors relocated at the outer shelf near the pribilof islands the focal mechanism for the 1991 event determined by inversion of teleseismic broadband body waves and centroid moment tensor data shows oblique normal faulting with the t axis oriented north south one nodal plane dips steeply to the northeast and strikes parallel to the basin axis subparallel to faults that bound zhemchug basin this earthquake may be due to slip on one of these normal faults several of which are seen to offset young strata on a multichannel seismic line 30 km south of the earthquake it has previously been suggested that the large canyons that cut the bering shelf and the adjacent outer shelf basins are fault controlled but this event and its aftershocks provide the first strong evidence to support the hypothesis that these structures are currently active and produce earthquakes seismic activity along the bering shelf edge may provide a good explanation for felt reports of several large events in the 1800s that have so far been difficult to associate with interplate activity along the aleutian trench or with any other seismically active structure

increased correlation range of seismicity before large events manifested by earthquake chains earthquake chains are clusters of moderate size earthquakes which extend over large distances and are formed by statistically rare pairs of events that are close in space and time neighbors earthquake chains are supposed to be precursors of large earthquakes with lead times of a few months here we substantiate this hypothesis by mass testing it using a random earthquake catalog also we study stability under variation of parameters and some properties of the chains we found two invariant parameters they characterize the spatial and energy scales of earthquake correlation both parameters of the chains show good correlation with the magnitudes of the earthquakes they precede earthquake chains are known as the first stage of the earthquake prediction algorithm reverse tracing of precursors rtp now tested in forward prediction a discussion of the complete rtp algorithm is outside the scope of this paper but the results presented here are important to substantiate the rtp approach

18o incorporation into sulphate during the bacterial oxidation of sulphide minerals and the potential for oxygen isotope exchange between o2 h2o and oxidized sulphur intermediates abstract experiments were conducted using 18 o enriched oxygen or water in an attempt to determine the source of oxygen incorporated into the sulphate ions produced during the bacterial oxidation of a metal sulphide the initial results indicated that 18 o 2 was incorporated into water and sulphate in the presence of the metal sulphide and t ferrooxidans the incorporation of 18 o into h 2 o can occur via bacterial respiration and potentially also by oxygen isotope exchange subsequent experiments using sulphite oxidase to oxidize sulphite to sulphate in the presence of 18 o enriched oxygen or water indicate that oxygen isotope exchange between o 2 and water is not mediated by the sulphite oxidase system

viscosity of oceanic asthenosphere inferred from remote triggering of earthquakes a sequence of large interplate earthquakes from 1952 to 1965 along the aleutian arc and kurile kamchatka trench released accumulated stresses along nearly the entire northern portion of the pacific plate boundary the postseismic stress evolution across the northern pacific and arctic basins calculated from a viscoelastic coupling model with an asthenospheric viscosity of 5 x 10 17 pascal seconds is consistent with triggering of oceanic intraplate earthquakes temporal patterns in seismicity at remote plate boundaries and space based geodetic measurements of anomalous velocity over an area 7000 by 7000 kilometers square during the 30 year period after the sequence

asthenospheric viscosity inferred from correlated land sea earthquakes in north east japan the viscosity of the earth s mantle has been estimated from studies of post glacial rebound1 2 post seismic deformations of the ground following large earthquakes3 4 and aftershock sequences5 7 here we derive a value for the viscosity of the asthenosphere from a correlation found in the historical catalogue of subduction induced seismicity between the intraplate land and interplate sea earthquakes in north east japan the correlation persists since the time of reliably reported earthquakes in ad 1600 land events precede sea events by 36 yr with a mean distance between land sea pairs of 200 km because of the visocoelastic coupling of the lithosphere to the asthenosphere a plausible mechanism to explain the correlation is stress migration governed by the viscosity of the asthenosphere large land shocks generate diffuse like stress pulses which sweep past and unlock the thrust fault in the subduction zone thus triggering the sea events the correlation time and distance provide a measure of the speed of diffusion 5 6 km yr 1 and hence an estimate of the viscosity 7 1018 pa s

global postseismic stress diffusion and fault interaction at long distances abstract by means of a new theoretical model of global postseismic deformation we compute the time depending postseismic stress field associated with eight of the greatest events of the century on an area extending for almost half of the earth s surface we evaluate the stress transferred by these big earthquakes to all the seismogenic structures of the pacific belt that have generated earthquakes with m 5 in the last years we discuss the effect of this stress field on the state of the faults the distribution of favoured and not favoured events is not uniform the modeling suggests the existence of a physical connection among the patterns of release of seismic moment by the different plate margins of the pacific area and the possibility of a self organised geometrical configuration of the tectonic plates system

global plate velocities from the global positioning system we have analyzed 204 days of global positioning system gps data from the global gps network spanning january 1991 through march 1996 on the basis of these gps coordinate solutions we have estimated velocities for 38 sites mostly located on the interiors of the africa antarctica australia eurasia nazca north america pacific and south america plates the uncertainties of the horizontal velocity components range from 1 2 to 5 0 mm yr with the exception of sites on the pacific and nazca plates the gps velocities agree with absolute plate model predictions within 95 confidence for most of the sites in north america antarctica and eurasia the agreement is better than 2 mm yr we find no persuasive evidence for significant vertical motions 3 standard deviations except at four sites three of these four were sites constrained to geodetic reference frame velocities the gps velocities were then used to estimate angular velocities for eight tectonic plates absolute angular velocities derived from the gps data agree with the no net rotation nnr nuvel 1a model within 95 confidence except for the pacific plate our pole of rotation for the pacific plate lies 11 5 west of the nnr nuvel 1a pole with an angular speed 10 faster our relative angular velocities agree with nuvel 1a except for some involving the pacific plate while our pacific north america angular velocity differs significantly from nuvel 1a our model and nuvel 1a predict very small differences in relative motion along the pacific north america plate boundary itself our pacific australia and pacific eurasia angular velocities are significantly faster than nuvel 1a predicting more rapid convergence at these two plate boundaries along the east pacific rise our pacific nazca angular velocity agrees in both rate and azimuth with nuvel 1a

focal depths and mechanisms of large earthquakes on the arctic mid ocean ridge system as part of a global study of the source characteristics and tectonic implications of large earthquakes on mid ocean ridges we report on the focal depths and mechanisms of the six largest earthquakes that have occurred in the last 20 years on the arctic mid ocean ridge system for each earthquake we invert the long period p and sh waveforms to estimate the parameters of the best fitting point source including seismic moment centroid depth double couple source orientation and source time function three of the earthquakes occurred on the oceanic spreading center in the eurasian basin along ridge segments spreading at half rates of 4 6 mm yr these events have mechanisms very similar to those of ridge crest earthquakes on the mid atlantic ridge almost pure normal faulting on planes that dip at approximately 45 and strike parallel to the rift axis moments of 4 5 1024 dyn cm centroid depths of 1 2 km beneath the seafloor and water depths inferred from the predominant period of water column reverberations appropriate to epicentral locations within the median valley the remaining three earthquakes also characterized by normal faulting are associated with the continuation of the divergent plate boundary 2 3 mm yr half rate onto the continental shelf of the laptev sea where the crust becomes transitional in nature one of the largest known spreading center earthquakes august 25 1964 m0 1 1026 dyn cm occurred where the oceanic ridge intersects the outer edge of the continental slope waveform inversion for this event can resolve unilateral rupture from north to south landward along a fault at least 30 km in length the preferred centroid depth is 5 km beneath the seafloor in crustal material with an unusually low shear velocity but a centroid depth as great as 15 km cannot be ruled out two earthquakes beneath the continental shelf have significantly greater centroid depths 10 20 km than mid ocean ridge earthquakes indicating a thicker brittle regime and a cooler thermal structure than are typical of oceanic spreading centers the tectonic environment of these events is more representative of rifted continental lithosphere than of a mid ocean ridge

robust and exploratory analysis of active mesoscale tectonic zones in japan utilizing the nationwide gps array abstract a monitoring gps array recently developed in japan can yield nationwide maps of active inland tectonic zones atzs on a mesoscale approximately 70 to several hundred kilometers in lateral extent but it has been difficult to characterize atzs in japan as they are in fact operational on multiple scales and our efforts are often hindered by various irregularities in the data the key to overcoming these problems would be to gain an insight into the available data before any precise kinematic modeling is performed with indefinite assumptions in this study horizontal velocity fields deduced from the nationwide gps array were treated with a set of techniques in robust smoothing and exploratory data analysis that brought out exceptionally powerful mesoscale atzs and made them easier to characterize the resolved atzs were then retrospectively monitored to study their regional and temporal variations using a set of approx 840 observation stations about 30 km apart for a 4 year series of fixed observation time intervals 810 days each the smoothing operation involved three steps 1 imputation of the velocity fields for the purpose of anti aliasing 2 robust smoothing of the velocity fields with the median operative and 3 visualization of deformation rate distributions in several coordinate independent parameters and post filtering the geometrical resolvability of mesoscale atzs was confirmed by calibrating the smoothing scheme against synthetic tectonic boundary models before it was applied to the case study in japan atzs in japan which are essentially visible as systematic deviations in the velocity fields on the international terrestrial reference frame itrf and as strain rate anomalies were highlighted sharply along some known tectonic zones chains of active volcanoes and areas above low seismic velocity anomalies in the crust and upper mantle all of which generally paralleled the offshore trench axes the geometrical agreements among the mapped atzs and the physical anomalies in the crust are presumably due to their common structural weakness on the mesoscale in the four main islands of japan all but 30 40 of the strain rate anomalies persisted during the entire 6 years of the case study period while the rest sporadically appeared or disappeared in a period from several months to a few years the transient shifts in the deformation rates were remarkably synchronous with some nearby major tectonic episodes large earthquakes and slow events differential plate coupling strengths along the subduction zones can also be inferred from the persistent pattern of rotational strain rate anomalies forming clockwise and counterclockwise pairs along the pacific our empirical observations suggest that the first order features of interseismic crustal deformations in japan can be characterized as collateral processes behaving in response to fluctuations of the tectonic stresses on multiple scales likely influenced by changes of plate coupling strengths on the contiguous subduction faults

large earthquake occurrence affected by small stress changes abstract many observations suggest that relatively small changes in stress may trigger or inhibit earthquakes this result is surprising because the perturbing stress is only a small fraction of the stress drop from the affected earthquake we investigate this phenomenon by simulating the earthquake fault with a numerical model that is physically realistic a fault is not homogeneous and therefore the failure criteria will vary over the surface of the fault we model fault heterogeneity as simple coulomb failure on smaller but mostly uniform areas that comprise the fault as a result of multiple and repeated failures over these small areas the fault can have a very uneven stored stress we show that it is possible for small changes in confining stress of a few tenths of a bar to advance or retard the occurrence of large earthquakes by decades because such stress changes can be generated by nearby earthquakes and over much longer time scales by postseismic strain diffusion from large distant earthquakes estimates of the probabilities of earthquake occurrence must take into account these stress perturbations in order to be reliable

constraints on yield strength in the oceanic lithosphere derived from observations of flexure summary the wavelength and amplitude of outer rises seaward of sub duction zones and arches surrounding islands and seamounts are used to parameterize flexure profiles in terms of the moment and curvature at the first zero crossing the data show the clear age dependence in the mechanical thickness of the lithosphere up to 60 100myr saturation of moment at large curvature is interpreted in terms of a depth dependent yield strength for the lithosphere using relations adopted from laboratory experiments of rock deformation a comparison of theoretical curves with observed moments indicates that old oceanic lithosphere has no long term strength below about 40 km depth with no difference between 100 and 165 myr old crust moderate axial loading forces 200 mpa can explain most variations in the moment curvature observations except in the case of the kuril trench which appears anomalous given the age of the crust regional tension causes greater variability in moment as compared to regional compression because of the greater slope in the brittle failure envelope under tension the observations point to a lithosphere weaker than the prediction from experimental deformation of rocks of the possible weakening mechanisms elevated pore fluid pressure on faults does not predict the correct age dependence and is incompatible with earthquake focal mechanisms our favoured explanation is that the activation energy q appropriate for ductile flow at geological strain rates is lower than the values derived from laboratory extrapolations of dry olivine data taken at high temperatures if recent oceanic geotherms are reliable q in the lower lithosphere must be lower than 100kcal mol 1 the method used here is most appropriate for trench profiles with curvatures greater than 10 7m 1 for lower curvatures such as along seamount profiles small errors in the curvature estimate cause large changes in rheological parameters

s and o isotopic character of dissolved sulphate in the cover rock aquifers of a zechstein salt dome abstract sulfur and o isotope analyses of dissolved so4 were used to constrain a hydrogeological model for the area overlying the gorleben rambow salt structure northern germany samples were collected from 80 wells screened at different depth intervals the study area consists of a set of two vertically stacked aquifer systems generally the isotope data show a good spatial correlation outlining well defined groundwater zones containing so4 of characteristic isotopic composition highly saline waters from deeper parts of the lower aquifer system are characterized by rather constant so4 isotopic compositions which are typical of permian zechstein evaporites 34s 9 6 11 9 18o 9 5 12 1 above this is a transition zone containing ground waters of intermediate salinity and slightly higher isotopic values average 34s 16 6 18o 15 3 the confined groundwater horizon on the top of the lower aquifer system below the low permeable hamburg clays is low in total dissolved solids and is characterized by an extreme 34s enrichment average 34s 39 1 18o 18 4 suggesting that bacterially mediated so4 reduction is a dominant geochemical process in this zone two areas of distinct isotopic composition can be identified in the shallow ground water horizons of the upper hydrogeological system sulfate in groundwaters adjacent to the river elbe and locknitz has a typical meteoric isotopic signature 34s 5 2 18o 8 2 whereas the central part of the area is characterized by more elevated isotopic ratios 34s 12 7 18o 15 6 the two major so4 pools in the area are represented by permian seawater so4 and a so4 of meteoric origin that has been mixed with so4 resulting from the oxidation of pyrite it is suggested that the s isotope compositions observed reflect the nature of the so4 source that have been modified to various extent by bacterial so4 reduction groundwaters with transitional salinity have resulted from mixing between brines and low mineralized waters affected by bacterial so4 reduction

paleoenvironments of the earliest stone toolmakers gona ethiopia fluvio lacustrine sediments of the hadar and busidima formations along the northern awash river ethiopia archive almost three million years 3 4 to 0 6 ma of human evolution including the earliest documented record of stone toolmaking at 2 5 2 6 ma this paper brings together sedimentologic and isotopic evidence for the paleoenvironmental context of early hominids from both formations but with particular emphasis on the setting for the early toolmakers the pre 2 92 ma record hadar formation is characterized by low gradient fl uvial paludal and lacustrine deposition in an undissected topography most analogous to reaches of the modern middle awash river near gewane the gona area experienced repeated deep dissection and aggradation by the awash river starting between 2 92 and ca 2 7 ma and continuing through the top of the record at 0 6 ma busidima formation each aggradational succession is 10 20 m in thickness and fi nes upward from wellrounded conglomerates at the base to capping paleosols at the top during this period the ancestral awash represented by these fi ning upward sequences was dominantly meandering and fl owed northeast as it does today smaller channels tributary to the axial awash system are also extensively exposed in the busidima formation compared to the axial system conglomerates the tributary channels transported fi ner less mature volcanic clasts mixed with abundant carbonate nodules reworked from adjacent badlands stone artifacts oldowan 2 6 2 0 ma at the oldest archaeological sites are only associated with the axial awash system in the bedded silts or capping paleosols of the fi ning upward sequences the implements were made from rounded cobbles from the channels but manufacture and use of the tools was always away from the channel bars on the nearby sandy banks and silt dominated fl oodplains archaeological sites higher in the record acheulian 1 7 ma occur in similar axial river contexts as well as along tributary channels further removed from artifact raw material sources mature paleosols in the hadar and busidima formations are mostly pale to darkbrown vertisols typifi ed by abundant clay slickensides pseudo anticlinal and vertical fracturing and carbonate nodules such calcic vertisols are common in the region today demonstrating that the paleoclimate over the past 3 4 m y has been semi arid and strongly seasonal carbon isotopic results from pedogenic carbonates in the vertisols allow reconstruction of the proportion of c 3 plants trees and shrubs to c 4 plants grasses through time the 13

application of the authigenic 10be 9be dating method to continental sediments reconstruction of the mio pleistocene sedimentary sequence in the early hominid fossiliferous areas of the northern chad basin the concentrations of atmospheric cosmogenic 10be normalized to the solubilized fraction of its stable isotope 9be have been measured in the authigenic phase leached from silicated continental sediments deposited since the upper miocene in the northern chad basin this method is validated by the systematic congruence with the biochronological estimations based on the fossil mammal evolutive degree of faunal assemblages the fifty five authigenic 10be 9be ages obtained along 12 logs distributed along two west east cross sections that encompass best representative mio pliocene outcrops including paleontological sites show a systematic stratigraphic decrease when considering all studied sedimentary facies extending from the pleistocene up to 8 ma and allow performing geologic correlations otherwise impossible in the studied area the resulting global sequence evidences and temporally specifies the succession of the main paleoenvironments that have developed in this region since the miocene under the special conditions encountered in the northern chad basin this study demonstrates that the authigenic 10be 9be ratio may be used as a dating tool of continental sedimentary deposits from 1 to 8 ma the half life of 10be theoretically allowing dating up to 14 ma it may have fundamental implications on important field research such as paleoclimatology and through the dating of fossiliferous deposits in paleontology and paleoanthropology

cosmogenic nuclide dating of sahelanthropus tchadensis and australopithecus bahrelghazali mio pliocene hominids from chad ages were determined at two hominid localities from the chad basin in the djurab desert northern chad in the koro toro fossiliferous area kt 12 locality 16 00 n 18 53 e was the site of discovery of australopithecus bahrelghazali abel and in the toros menalla fossiliferous area tm 266 locality 16 15 n 17 29 e was the site of discovery of sahelanthropus tchadensis toumai at both localities the evolutive degree of the associated fossil mammal assemblages allowed a biochronological estimation of the hominid remains early pliocene 3 3 5 ma at kt 12 and late miocene 7 ma at tm 266 atmospheric 10be a cosmogenic nuclide was used to quasicontinuously date these sedimentary units the authigenic 10be 9be dating of a pelite relic within the sedimentary level containing abel yields an age of 3 58 0 27 ma that points to the contemporaneity of australopithecus bahrelghazali abel with australopithecus afarensis lucy the 28 10be 9be ages obtained within the anthracotheriid unit containing toumai bracket by absolute dating the age of sahelanthropus tchadensis to lie between 6 8 and 7 2 ma this chronological constraint is an important cornerstone both for establishing the earliest stages of hominid evolution and for new calibrations of the molecular clock

postseismic stress diffusion in chile and south peru abstract by means of a spherical model of global postseismic deformation we perform an analysis of the postseismic stress diffusion process associated with the main seismic events which occurred in chile and south peru in the last century our aim is to ascertain if the viscoelastic relaxation of the mantle and asthenosphere following the earthquakes superimposed on the main mechanism of stress accumulation in the lithosphere that is to say the subduction process has in some way influenced the spatiotemporal pattern of seismic energy release in this region our investigations focus particularly on the areas where two important seismic gaps in an advanced mature state can be recognized we found that the evolution of the postseismic stress might give a satisfactory explanation for the anomalous duration of the seismic quiescence in the gap areas and also for the possible filling of one of these gaps

relative and absolute dating of land surfaces abstract the dating of land surfaces has long posed problems for geomorphologists relative methods stratigraphic geomorphic topographic are sound and convincing exhumed forms may complicate identification and relationships for both epigene and etch forms have been buried and exhumed but in tectonically undisturbed areas the higher surfaces are older than those preserved at lower levels also surfaces have an age range the relationship of surfaces with volcanic deposits old shorelines and genetically related sedimentary sequences provides sound ages and correlation with dated duricrusts and faults is also useful there are no temporal limits to relative dating for the methods are equally applicable to the dating say of proterozoic surfaces as of those of pleistocene age the disadvantage of such methods is that the necessary evidence is frequently either not preserved or not exposed the so called absolute physical numerical methods and especially those based on exposure age dating with in situ cosmogenic radionuclides are appealing because they produce direct numerical ages and appear to be widely applicable but there are severe temporal limitations and sampling problems complicate and may invalidate interpretation absolute age determinations must be consistent with the stratigraphic and geomorphologic settings the best results are obtained when physicists and earth scientists pool their knowledge and experience a background in local and regional geology is especially important

hydrochemical and stable isotopes h o s signatures in deep groundwaters of parana basin brazil this paper describes a hydrochemical major and trace elements and stable isotopes h o s study of rainwater and groundwater in a guarani aquifer system gas transect at sao paulo state brazil the brazilian code of mineral waters bcmw was adopted for the groundwater temperature classification allowing adequate insights in the hydrochemical data 2h and 18o data in circa 580 rainwater samples at sao paulo state have been selected showing a wide dispersion 18o from 21 5 to 4 9 v smow 2h from 162 to 43 2 v smow the 2h and 18o data in groundwater did not vary significantly fitting the regional meteoric water line and indicating that the groundwater origin is directly related to the meteoric water precipitation the 18o values in groundwater collected along the gas transect did not indicate a tendency towards more negative values according to the groundwater flow direction at sao paulo state as suggested elsewhere the 18oso4 and 18owater data indicated that the dissolved sulfate is obtained from previous sulfate dissolution the 34s and 18osulfate data of dissolved so4 2 suggested that the dissolution of evaporites and bacterial so4 2 reduction processes may be occurring at the gas the temperature redox potentials dissolved oxygen and organic matter content in the sediments provided an ideal environment for the 34s and 18osulfate changes along the gas transect accompanying the groundwater flow direction

new insights on long distance fault interaction abstract we investigated the plausibility of remote fault triggering on a global scale as a result of postseismic stress transfer by large earthquakes previous studies have shown that the postseismic stress field generated by eight of the largest events that have occurred in the pacific area promoted the rupture of 53 6 of all the m 5 events recorded in the last century in the circumpacific ring we tried to assess the statistical significance of this result by performing a set of new statistical simulations involving very intensive computational tasks to this aim we implemented a new numerical procedure based on parallel codes we found that our simulations did not show strong statistical evidence but there was a clear indication that as we applied more realistic geometrical constraints to the synthetic distributions we tended to reproduce more closely the observed quantities these results support the hypothesis of some kind of physical connection of the configuration of a plate margin and its activity with those of all the other plate margins more than the possibility of remote fault interaction in the classical sense

patterns of seismic release in the southern california region southern california experiences earthquakes on the san andreas system of vertical right lateral predominantly strike slip faults and on a second system of faults that includes thrusts oblique slip left lateral and other faults pattern recognition and cluster analysis are used to analyze the catalog of earthquakes with magnitudes 5 5 from 1915 to 1994 we use pattern recognition to find a suite of traits that would characterize each of these two systems and distinguish them from each other both pattern recognition and cluster analysis show that epochs of seismic release occur in which one or the other system is the predominant form of earthquake activity for the past 2 decades the second system has been the active one small changes in the direction of plate movements could account for this phenomenon seismic release on the san andreas system is preceded by episodes of activity in the great basin or in the gulf of california presumably these episodes would represent extension in the former region and spreading and slip on transform faults in the latter

postseismic deformation in chile constraints on the asthenospheric viscosity by means of a spherical earth model with maxwell rheology an analysis of the postseismic deformation field due to the viscoelastic relaxation of the mantle and asthenosphere following the great 1960 chile earthquake is performed due to the exceptional energy release of that earthquake geomorphological evidences of postseismic deformation are available in a broad area surrounding the rupture zone the comparison of the synthetic results with the available data allows to put some constraints on the viscosity of the asthenosphere beneath the chilean region values between 8 1019 and 1020pa s are compatible with the available postseismic deformation field data

a reappraisal of seafloor spreading lineations in the gulf of california implications for the transfer of baja california to the pacific plate and estimates of pacific north america motion an analysis of seafloor spreading lineations in the southern gulf of california demonstrates that divergence of the baja peninsula from north america since 3 6 ma has been significantly slower than pacific north america pa na motion estimated solely from closures of global plate circuits this discrepancy remains unchanged or increases with modifications to the plate circuits and kinematic data used to predict pa na motion suggesting that it arises from the probably incorrect assumption that baja california has been rigidly coupled to the pacific plate since 3 6 ma a 15 increase in seafloor spreading rates in the southern gulf after 1 ma could have been caused by a transfer of motion along faults west of baja california to the modern day plate boundary or alternatively could indicate a recent speedup in pa na motion both interpretations imply that pa na motion is now 4 2 mm yr 1 2 faster than predicted by the 3 16 myr average nuvel 1a model

dislocation recovery in olivine under deep upper mantle conditions implications for creep and diffusion the kinetics of dislocation recovery in olivine have been studied up to 10 gpa and 1500 c using a multianvil apparatus dislocations in the 010 100 slip system were generated by high temperature deformation at room pressure and then the specimens were statically annealed at high pressures and temperatures the rate of dislocation recovery was measured as a function of pressure at controlled oxygen fugacity and oxide activity under anhydrous conditions microstructural observations indicate that dislocation recovery occurs through the climb of edge dislocations and hence is controlled by diffusion a small activation volume v 6 1 cm3 mol was obtained which is interpreted to be due to an interstitial mechanism of diffusion for the relevant transport limiting species oxygen or silicon combined with the previous observation of a small activation volume for mg fe diffusion we conclude that ionic diffusion in the olivine lattice in general has small activation volumes and therefore will not vary significantly with depth in the deep portions of the upper mantle we have estimated the relative importance of dislocation and diffusion creep in the upper mantle using the present result and the published results on creep parameters such as stress exponents grain size exponents and activation volume for dislocation creep the results suggest that the dominant creep mechanism may change from dislocation creep in the shallow upper mantle to diffusion creep in the deep upper mantle

new age constraints on the timing of volcanism and tectonism in the northern main ethiopian rift southern afar transition zone ethiopia abstract forty new k ar and 40 ar 39 ar isotopic ages from the northern main ethiopian rift mer southern afar transition zone provide insights into the volcano tectonic evolution of this portion of the east african rift system the earliest evidence of volcanic activity in this region is manifest as 24 23 ma pre rift flood basalts transition zone flood basalt activity renewed at approximately 10 ma and preceded the initiation of modern rift margin development bimodal basalt rhyolite volcanism in the southern afar rift floor began at approximately 7 ma and continued into recent times in contrast post subsidence volcanic activity in the northern mer is dominated by mio pliocene silicic products from centers now covered by quaternary volcanic and sedimentary lithologies unlike other parts of the mer mio pliocene silicic volcanism in the mer afar transition zone is closely associated with fissural basaltic products the presence of pliocene age ignimbrites on the plateaus bounding the northern mer whose sources are found in the present rift indicates that subsidence of this region was gradual and that it attained its present physiography with steep escarpments only in the plio pleistocene large 7 5 ma silicic centers along the southern afar and northeastern mer margins apparently formed along an e w oriented regional structural feature parallel to the already established southern escarpment of the afar the addis ababa rift embayment and the growth of 4 5 3 ma silicic centers in the addis ababa area are attributed to the formation of a major cross rift structure and its intersection with the same regional e w structural trend this study illustrates the episodic nature of rift development and volcanic activity in the mer afar transition zone and the link between this activity and regional structural and tectonic features

global postseismic rebound of a viscoelastic earth theory for finite faults and application to the 1964 alaska earthquake a spherically symmetric earth model with viscoelastic rheology is used to study the postseismic rebound associated with finite lithospheric dislocations we perform a systematic study of surface deformations due to sources characterized by two and three dimensional faults modeled by a linear and planar distribution of point sources our approach is based on the normal mode technique for a layered earth with linear viscoelastic rheology and allows for a self consistent description of the time evolution of postseismic displacements due to strike and dip slip faults as a case study we compare the predicted horizontal displacements due to the 1964 alaska earthquake with very long baseline interferometry vlbi baselines changes observed in this region in the period 1984 1989 although subduction and postglacial isostatic adjustment are expected to contribute significantly to present day velocities our results show that the postseismic rebound due to the 1964 alaska earthquake plays a relevant role in the changes of several vlbi baselines the last section is devoted to the analysis of the evolution of other north american baselines as suggested by recent investigations based on forward approaches isostatic readjustment of the earth s crust in response to the melting of the last pleistocene ice sheets is a major contributor to baseline variations in this area our results indicate that a correct interpretation of vlbi baseline changes in north america should account also for the effects of postseismic deformation

impact of global seismicity on sea level change assessment 1 we analyze the effect of seismic activity on sea level variations by computing the time dependent vertical crustal movement and geoid change due to coseismic deformations and postseismic relaxation effects seismic activity can affect both the absolute sea level changing the earth s gravity field and hence the geoid height and the relative sea level rsl i e the radial distance between seafloor and geoid level by using comprehensive seismic catalogs we assess the net effect of seismicity on tidal relative sea level measurements as well as on the global oceanic surfaces and we obtain an estimate of absolute sea level variations of seismic origin we modified the approach adopted in our previous analysis considering the issue of water volume conservation by applying the sealevel equation and we improved our computational methods enabling us to evaluate the effect of an extremely large number of earthquakes on large grids covering the whole oceanic surface these new potentialities allow us to perform more detailed investigations and to discover a quantitative explanation for the overall tendency of earthquakes to produce a positive global relative sea level variation our results confirm the finding of a previous analysis that on a global scale most of the signal is associated with a few giant thrust events and that rsl estimates obtained using tide gauge data can be sensibly affected by the seismically driven sea level signal the recent measures of sea level obtained by satellite altimetry show a wide regional variation of sea level trends over the oceanic surface with the largest deviations from the mean trend occurring in tectonically active regions while our estimates of average absolute sea level variations turn out to be orders of magnitude smaller than the satellite measured variations we can still argue that the mass redistribution associated with aseismic tectonic processes may contribute to the observed regional variability of sea level variations a detailed study of these tectonic contributions is important to acquire a complete understanding of the global sea level variations and will be the subject of future investigations

magnitude production imbalances and the present seismogenicity state of the san andreas fault system abstract seismicity of the san andreas fault system between 21 n 40 2 n and 105 w 125 w is subjected to the piecewise gradient analysis in order to assess the present magnitude production balance of that seismogenic system and its possible implications the analysis is based on a technique applied to a global seismicity dataset of earthquakes in the magnitude range 6 1 9 0 for the period 1973 2003 that suggests that temporal imbalances in the production of different magnitudes generated within a seismogenic system exhibiting departures from linearity in the gutenberg richter law may be symptomatic of an imminent significant magnitude event the approach uses a variant of the traditional b value plot which tracks through time any changes in piecewise gradients within subsets of the magnitude frequency statistics obvious offsets from the usual straight line fit are taken as an indication of a deficit in magnitude production that will eventually be supplied to restore linearity the timing of which can be informed by regular analysis of the relative production of different magnitude classes this study shows that in the recent past an emergent pattern of magnitude output imbalance seismicity acceleration and then relative quiescence has preceded all of the more significant events magnitude 6 5 within the defined san andreas fault system in 2010 two earthquakes of magnitude 6 5 occurred in quick succession which is a pattern not previously seen the overall piecewise gradient analysis technique suggests that the system appears poised to release an event of magnitude 7 5

seismicity trends and potential for large earthquakes in the alaska aleutian region the high likelihood of a gap filling thrust earthquake in the alaska subduction zone within this decade is indicated by two independent methods analysis of historic earthquake recurrence data and time to failure analysis applied to recent decades of instrumental data recent may 1993 earthquake activity in the shumagin islands gap is consistent with previous projections of increases in seismic release indicating that this segment along with the alaska peninsula segment is approaching failure based on this pattern of accelerating seismic release we project the occurrence of one or morem 7 3 earthquakes in the shumagin alaska peninsula region during 1994 1996 different segments of the alaska aleutian seismic zone behave differently in the decade or two preceding great earthquakes some showing acceleration of seismic release type a zones while others show deceleration type d zones the largest alaska aleutian earthquakes in 1957 1964 and 1965 originated in zones that exhibit type d behavior type a zones currently showing accelerating release are the shumagin alaska peninsula delarof and kommandorski segments time to failure analysis suggests that the large earthquakes could occur in these latter zones within the next few years

the world s oldest stone artefacts from gona ethiopia their implications for understanding stone technology and patterns of human evolution between 2 6 1 5 million years ago the systematic archaeological and geological survey and excavations at gona between 1992 1994 led to the discovery of well flaked stone artefacts which are currently the oldest known from anywhere in the world more than 3000 surface and excavated artefacts were recovered at 15 localities documented east and west of the kada gona river based on radioisotopic dating 40 ar 39 ar and magnetostratigraphy the artefacts are dated between 2 6 2 5 million years ago ma eg10 and eg12 from east gona are the most informative with the highest density providing the best opportunity for characterizing the oldest assemblages and for understanding the stone working capability of the earliest tool makers slightly younger artefact occurrences dated to 2 4 2 3 ma are known from hadar and omo in ethiopia and from lokalalei in kenya cut marked bones dated to 2 5 ma from bouri in ethiopia are now providing important clues on the function of these artefacts in addition australopithecus garhi known from contemporary deposits at bouri may be the best candidate responsible for the oldest artefacts surprisingly the makers of the gona artefacts had a sophisticated understanding of stone fracture mechanics and control similar to what is observed for oldowan assemblages dated between 2 0 1 5 ma this observation was corroborated by the recent archaeological discoveries made at lokalalei because of the similarities seen in the techniques of artefact manufacture during the late pliocene early pleistocene it is argued here that the stone assemblages dated between 2 6 1 5 ma group into the oldowan industry the similarity and simplicity of the artefacts from this time interval suggests a technological stasis in the oldowan 2000 academic press

the 1957 great aleutian earthquake the 9 march 1957 aleutian earthquake has been estimated as the third largest earthquake this century and has the longest aftershock zone of any earthquake ever recorded 1200 km however due to a lack of high quality seismic data the actual source parameters for this earthquake have been poorly determined we have examined all the available waveform data to determine the seismic moment rupture area and slip distribution these data include body surface and tsunami waves using body waves we have estimated the duration of significant moment release as 4 min from surface wave analysis we have determined that significant moment release occurred only in the western half of the aftershock zone and that the best estimate for the seismic moment is 50 100 1020 nm using the tsunami waveforms we estimated the source area of the 1957 tsunami by backward propagation the tsunami source area is smaller than the aftershock zone and is about 850 km long this does not include the unalaska island area in the eastern end of the aftershock zone making this area a possible seismic gap and a possible site of a future large or great earthquake we also inverted the tsunami waveforms for the slip distribution slip on the 1957 rupture zone was highest in the western half near the epicenter little slip occurred in the eastern half the moment is estimated as 88 1020 nm or m w 8 6 making it the seventh largest earthquake during the period 1900 to 1993 we also compare the 1957 earthquake to the 1986 andreanof islands earthquake which occurred within a segment of the 1957 rupture area the 1986 earthquake represents a rerupturing of the major 1957 asperity

multiscaling in earthquakes diffusion 1 earthquakes can be viewed as a relaxation process of the earth crust loaded with stresses mainly at the lithospheric plate boundaries when an earthquake occurs the stress in the crust shall be redistributed generating new earthquakes where and when the stress assume again a value greater than the slipping threshold this observation leads to study the earthquakes diffusion mechanism here we investigate the fluctuations of the average free walk between an earthquake and another evaluating the scaling of the distance between earthquakes with the waiting time we find scaling exponents varying with the order of the statistical moments and always less than 1 this implies strong anomalous diffusion that we interpret as due to different mechanisms of diffusion depending by the scale

interaction of the san jacinto and san andreas fault zones southern california triggered earthquake migration and coupled recurrence intervals two lines of evidence suggest that large earthquakes that occur on either the san jacinto fault zone sjfz or the san andreas fault zone safz may be triggered by large earthquakes that occur on the other first the great 1857 fort tejon earthquake in the safz seems to have triggered a progressive sequence of earthquakes in the sjfz these earthquakes occurred at times and locations that are consistent with triggering by a strain pulse that propagated southeastward at a rate of 1 7 kilometers per year along the sjfz after the 1857 earthquake second the similarity in average recurrence intervals in the sjfz about 150 years and in the mojave segment of the safz 132 years suggests that large earthquakes in the northern sjfz may stimulate the relatively frequent major earthquakes on the mojave segment analysis of historic earthquake occurrence in the sjfz suggests little likelihood of extended quiescence between earthquake sequences

post seismic relaxation and earthquake triggering in the southern adriatic region summary an attempt at quantifying post seismic relaxation triggered by decoupling earthquakes along the eastern thrusting border of the adriatic plate southern dinarides is carried out by finite element modelling with a model constituted by an elastic lithosphere riding on a viscous asthenosphere in particular it is investigated the possibility that the above phenomenon is responsible for the fact that in the last two centuries most major earthquakes in the southern dinarides ms 6 have been followed within a few years by intense mainly tensional earthquakes in southern italy i e the zone lying on the opposite margin of the adriatic plate this analysis has been applied to the last example of the supposed seismic interrelation i e the triggering 1979 april 15 montenegro event ms 6 7 and the presumably induced 1980 november 23 irpinia earthquake in the southern apennines ms 6 9 results indicate that the strain induced in the southern apennines by the triggering event has significant amplitude since it largely exceeds the effect of earth tides and the principal stress axes are consistent with those of southern apenninic earthquakes the order of magnitude of the time delay between the montenegro and irpinia events 1 6 yr could be explained by assuming that earthquake triggering is most probable when the highest values of the induced strain rate reach the southern apennines in particular this interpretation predicts the observed time delay when a model diffusivity of 400 m2 s 1 is assumed the constraints that this diffusivity value may pose on the structural and rheological features of the crust upper mantle system in the study area are discussed it is shown that the effects of the montenegro event on the present velocity field are comparable to though systematically lower than the velocities suggested by geodetic observations in the italian region this suggests that geodynamic interpretations of geodetic data given without taking into account possible transient effects on the kinematic pattern as those related to post seismic relaxation may be incorrect experiments carried out by tentatively simulating the presence of subducted lithosphere along the western margin of the adriatic plate as a lateral variation of diffusivity have shown that this structural feature may emphasize e w tensional strains in the southern apennines

plio pleistocene pumice floods in the ancestral rio grande southern rio grande rift usa abstract at least four times during the late pliocene and early pleistocene pyroclastic eruptions in the jemez volcanic field northern rio grande rift flooded the ancestral rio grande with gravel sized pumice following as much as 400 km of fluvial transport the pumice was deposited in beds 0 2 to 2 0 m thick in the camp rice formation of the southern rio grande rift a combination of reversal magnetostratigraphy and single crystal sanidine 40 ar 39 ar dating constrains the ages of pumice clast conglomerates at 3 1 2 0 1 6 and 1 3 ma the coarsest pumice beds cobbles boulders were deposited as antidune like bedforms in a fluvial channel and as a crevasse splay sheet granule and pebble sized pumice was deposited as dune bedforms in fluvial channels and as ripple bedforms on the floodplain the abundance of pumice clasts in the gravel fraction 60 100 suggests very rapid transport downriver probably in a few days or weeks the two older pumice clast conglomerates correlate with the puye formation in the jemez volcanic field whereas the younger two are coeval to the lower bandelier tuff and cerro toledo rhyolite