effects of slip slip rate and shear heating on the friction of granite the stability of fault slip is sensitive to the way in which frictional strength responds to changes in slip rate and in particular to the effective velocity dependence of steady state friction ss ln v this quantity can vary substantially with displacement temperature and slip rate to investigate the physical basis for this behavior and the possible influence of shear heating we slid initially bare granite surfaces in unconfined rotary shear to displacements of hundreds of millimeters at normal stresses n of 10 and 25 mpa and at room temperature we imposed step changes in slip rate within the range 10 2 to 103 5 m s and also monitored frictional heating with thermistors embedded in the granite the transient response of to slip rate steps was fit to a rate and state dependent friction law using two state variables to estimate the values of several parameters in the constitutive law the first 20 mm of slip shows rising friction and falling ss ln v further slip shows roughly constant friction ss ln v and parameter values suggesting that a steady state condition is reached on the fault surface at v 10 m s ss ln v 0 004 0 001 at higher rates the response is sensitive to normal stress at n 25 mpa granite shows a transition to effective velocity strengthening ss ln v 0 008 0 004 at the highest slip rates tested at 10 mpa granite shows a less dramatic change to ss ln v 0 at the highest rates the maximum temperature measured in the granite is 60 c at 25 mpa and 103 5 m s temperatures are in general agreement with a numerical model of heat conduction which assumes spatially homogeneous frictional heating over the sliding surface the simplest interpretation of our measurements of ss ln v is that the granite is inherently velocity weakening ss ln v 0 mimics velocity strengthening these results have implications for the frictional behavior of faults during earthquakes high slip rates may cause a switch to effective velocity strengthening which could limit peak coseismic slip rate and stress drop for fluid saturated faults strengthening by this mechanism may be partly or fully offset by weakening due to thermal pressurization of a poorly drained pore fluid

frictional resistance of faults during accelerating and decelerating earthquake slip the dynamic friction along faults controls earthquake ruptures in the crust but many previous studies have quantified this value only for constant slip rates experiments accounting for the more realistic condition of changing slip rates suggest that faults undergo a sequence of strengthening weakening and healing during acceleration and deceleration of slip the dynamic friction of faults during earthquake slip is a critical control on earthquake ruptures in the crust extrapolation of brittle crack theories to natural earthquakes1 2 3 has led to the commonly held view that fault friction reduces during rapid earthquake slip a process known as slip weakening1 high velocity gouge experiments4 5 and recent analyses of thermal pressurization6 7 and frictional melting8 support such a notion however these studies dealt with constant rates of slip along faults here we present the results of experiments aimed at understanding the frictional behaviour of fault zone materials under variable slip rates conditions that are more representative of natural earthquakes our results show that faults undergo a sequence of strengthening weakening and healing during acceleration and deceleration of slip such a sequence may be explained by the extrapolation of rate and state frictional behaviour9 10 at low slip velocities to more realistic slip rates but involving different physical mechanisms and a different scale the initial strengthening should impose a barrier for rupture growth into large earthquakes the healing on decelerating fault motion may lead to pulse like earthquake ruptures11 12 13 14 and static stress drops that are low in comparison with the dynamic stress changes15

the principal nonseasonal modes of variation of global sea surface temperature abstract thirty five million ship reports for the period 1949 79 have been used to construct global monthly mean sea surface temperature fields for 5 5 latitude longitude regions the monthly means have been subjected to eigenvector analysis to bring out the nonseasonal modes of variation the el nino mode is the most important and its eigenvector time series is identical to that found previously by weare et al for the pacific alone addition of data from the other oceans adds unexplained variance so that the percentage of the variance explained by the el nino mode is smaller in the global case than for the pacific alone the second most important eigenvector pattern shows evidence of a cooling trend for the north pacific and north atlantic in the 1964 79 period this is in agreement with previous work an zonal mean sea surface temperature by the authors with recent work on the north pacific by douglas et al and with a polar cap cooling in the lower troposphere for the same period reported by boer

climatology extremes in the indian ocean in contrast to the atlantic and pacific oceans the indian ocean has seemed to manifest little coherent behaviour in the variability of such parameters as water temperature and rainfall now however two studies identify a so called dipole mode in which the distribution of ocean temperature can be related to rainfall patterns in east africa and indonesia

atmospheric hydrospheric mechanisms of climate anomalies in the western equatorial indian ocean atmosphere ocean mechanisms of rainfall anomalies at the coast of eastern africa are studied using long term ship observations in the indian ocean surface current measurements subsurface casts upper air analyses by the european centre for medium range weather forecasts rain gauge series in eastern africa and india and an index of the southern oscillation so the high so phase being defined by anomalously high low pressure at tahiti darwin the causalities of precipitation anomalies at the coast of eastern africa differ for the two rainy seasons centered on april may and october november and only the latter is strongly related to the so in the high so phase april may pressure is low over the entire indian ocean domain whereas in october november pressure is high in the west and low in the east concomitantly surface waters are anomalously cold in the west and strong westerlies sweep the equatorial zone of the indian ocean eastern african rainfall anomalies are related to the so through a combination of cooperative mechanisms that function most effectively in the boreal autumn rainy season of eastern africa 1 equatorial westerly winds are conducive to lower tropospheric divergence over equatorial east africa and in the high so phase these are accelerated especially in october november owing to the anomalous eastward pressure gradient 2 the equatorial westerly winds drive the eastward equatorial jet in the upper hydrosphere which entails cold water upwelling in the western extremity of the basin where sea surface temperature further hydrostatically affects the zonal pressure gradient and thus feeds back into the equatorial westerly winds 3 in addition cold water anomalies in the western indian ocean most pronounced in october november during the high so phase also suppress convection 4 in the high so phase the indian summer monsoon tends to be strong leaving behind an anomalously cold western indian ocean which in turn feeds into mechanisms 1 to 3 the eastward equatorial jet thus has a role to play in feedback mechanisms contributing to the anomalies of the boreal autumn rains at the coast of eastern africa

temperature fields generated by the elastodynamic propagation of shear cracks in the earth w 1 the temperature maximum shifts toward the crack tip for faults having slip zone thickness of the order of centimeters or less the onset of thermally induced phenomena e g frictional melting thermal pressurization etc may occur at any point along the rupture depending on the degree of slip localization and rupture duration in the absence of significant increases in the pore fluid pressure localized fault slip may raise temperature by several hundred degrees sufficient to cause melting the onset of frictional melting may give rise to substantial increases in the effective fault strength due to an increase in the effective fault contact area and high viscosity of silicate melts near solidus the inferred transient increases in the dynamic friction viscous braking are consistent with results of high speed rock sliding experiments and might explain field observations of the fault wall rip out structures associated with pseudotachylites possible effects of viscous braking on the earthquake rupture dynamics include 1 delocalization of slip and increases in the effective fracture energy 2 transition from a crack like to a pulse like rupture propagation or 3 ultimate rupture arrest assuming that the pulse like ruptures heal by incipient fusion the seismologic observations can be used to place a lower bound on the dynamic fault friction this bound is found to be of the order of several megapascals essentially independent of the earthquake size further experimental and theoretical studies of melt rheology at high strain rates are needed to quantify the effects of melting on the dynamic fault strength index terms 3210 mathematical geophysics modeling 5104 physical properties of rocks fracture and flow 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling 8010 structural geology fractures and faults

zonal circulations over the equatorial indian ocean abstract the ncep ncar 1958 97 upper air dataset and surface observations have been analyzed for evidence of zonal vertical circulations along the indian ocean equator and their role in climatic variability the long term mean upper tropospheric circulation is dominated in boreal winter by divergent outflow from the southern indian ocean northwestward into southern asia and in summer from southern asia southwestward into the southern hemisphere in boreal autumn only divergent easterlies blow from indonesia along the equator into an upper tropospheric convergence band over east africa and only then a closed zonal vertical circulation cell materializes along the indian ocean equator between the centers of ascending motion over indonesia and of subsidence over equatorial east africa and featuring westerlies in the lower layers the boreal autumn zonal vertical circulation varies interannually a regime of intense circulation features accelerated equatorial surface westerlies enhanced subsidence and d

spatiotemporal structure of the east india coastal current from satellite altimetry we use a newly processed altimeter data set to present a hitherto unprecedented description of the spatiotemporal structure of the east india coastal current eicc the data set resolves timescales ranging from a few months to a few years and the high along track resolution yields the first description of the cross shore structure of the current the seasonal cycle dominates the variability but the nonannual timescales have similar energy levels all along the eicc path there are short lived intense intraseasonal bursts in contrast to the seasonal cycle the interannual and intraseasonal components are decorrelated along the coast and possible mechanisms for the decorrelation discontinuity in the flow are discussed in the cross shore direction the current is highly correlated at all timescales the eicc is trapped against the shelf with the current offshore flowing in the opposite direction at most locations the eicc appears as an inherently discontinuous flow taking the form of a few recirculating loops along the eicc path with a typical cross shore spatial scale of 150 200 km the loops are highly variable in direction at all timescales from intraseasonal to interannual this discontinuity of the eicc in space and time implies that the basic pathways and advective timescales for the interbasin exchange of water masses between the bay of bengal and the arabian sea are not robust when the full spatiotemporal variability of the eicc is considered

low frequency sea surface temperature and wind variations over the indian and pacific oceans the wind and temperature fields over the indian and pacific oceans have been analyzed to study the coupling between the two oceans on the time scale of the southern oscillation the analysis shows the propagation of zonal wind anomalies from the western pacific ocean toward the date line together with the displacement of a pool of warm water there is some indication of a zone of convergence over the equatorial indian ocean the results seem to rule out a direct propagation of a signal from the indian ocean toward the pacific ocean as the signal in the former ocean lags that in the latter

pseudotachylytes rarely generated rarely preserved or rarely reported pseudotachylyte is the only fault rock that is known to form exclusively at seismic slip rates so it is unique in preserving direct evidence of the dynamic processes in action during earthquakes it is commonly assumed that pseudotachylyte is rare and debate has centered on whether it is rarely generated or commonly generated but rarely preserved we present field and electron microscope observations of eight new pseudotachylytes from faults in the sierra nevada that have previously been the focus of many detailed studies of fault growth and mechanics these pseudotachylytes range from being abundant and easy to recognize in outcrop to being impossible to identify without microscope observations our data show that pseudotachylytes are much more common in the sierra nevada than has previously been reported we suggest that pseudotachylytes may be present within many fault zones but remain unreported primarily due to difficulty in identifying very thin or reworked pseudotachylytes in the field and therefore the use of these fault rocks to interpret dynamic earthquake processes must be revisited

a simple method for producing high quality porecasts of carbonate rocks abstract the study of porecasts is an important technique for evaluating the distribution and quantity of porosity in carbonate rocks this paper sets out a method by which high quality porecasts may be produced quickly and easily the procedure consists of impregnation firstly under vacuum then under pressure with low viscosity 6 10 3 pa s epoxy resin examination under sem reveals penetration of intragranular porosity at a submicron scale without disruption of the rock fabric

modeling the influence of epikarst evolution on karst aquifer genesis a time variant recharge boundary condition for joint karst epikarst development 1 the epikarst a zone of increased weathering near the land surface determines the distribution of recharge to a karst aquifer in both space and time it links climatic and near surface geological conditions with the karstification of a limestone aquifer defining both the hydraulic and the chemical boundary conditions for the development of the karst system realistic modeling of the epikarst is therefore a prerequisite for the simulation of karst aquifer genesis a conceptual model of the joint karst epikarst evolution is presented in this paper an epikarst module is developed and implemented in a numerical continuum discrete conduit flow model for karst genesis which accounts for the joint evolution of the epikarst and the main karstic conduit network under unconfined conditions the influence of epikarst genesis on the evolution of the underlying karst aquifer is investigated in four scenarios it is found that only the interaction of epikarst and initial heterogeneity in the underlying carbonate rock leads to the development of a dendritic cave system if no heterogeneity in the initial conduit network or in the recharge distribution is included maze type caves develop

indian ocean warming of 1997 1998 the indian ocean underwent substantial changes in 1997 1998 the observations show not only the appearance of a dipole mode in the tropical region but also a persistent basin scale warming we present in this study an analysis of the basin scale sea surface temperature sst variations during 1997 1998 using satellite observations in situ temperature measurements and national centers for environmental prediction reanalyses we find that the indian sst anomaly peaks occurred at two periods i e november december january coinciding with the nino3 peak and the following april may june and were phase locked to the indian ocean seasonal cycle the changes of sst in the equatorial ocean were related to a coupled interaction between the atmosphere and the ocean oceanic upwelling in the east and downwelling in the west played a major role in giving rise to the sst anomalies associated with the dipole mode structure the upwelling off the coast of sumatra elevated the regional thermocline by more than 80 m in december 1997 on the other hand the changes of sst in the southern indian ocean were largely induced by the changes of local latent flux during boreal fall winter of 1997 the southeasterly trades were displaced and abnormally prolonged in their northernmost equatorial position this shifted the center of the trades toward the equator weakened the winds in the central southern ocean reduced the latent heat flux in the region and subsequently induced a surface warming the total change of the sst anomalies integrated over the two periods july december 1997 and january may 1998 were explained well by the same period latent flux integral in both intensity and pattern the cross basin upper ocean temperature sections show that the extratropical warming was rather uniformly distributed in the upper 60 m further supporting the role of mixed layer processes in the warming

dynamics of the surface wind field over the equatorial indian ocean the annual cycle and interannual variability of the equatorial westerlies over the indian ocean are examined in the context of the equations of motion using long term surface ship observations eastward pressure gradient along the equator may accelerate equatorial westerlies but the process is more intricate the south indian ocean trade winds can recurve in the southern hemisphere only with an eastward pressure gradient this is counteracted by the coriolis acceleration directed at right angles to the left of the motion with weak trades and strong eastward pressure gradient the flow can recurve at relatively high southern latitudes thus allowing a broad equatorial zone in which westerlies can develop in the annual cycle the southern indian ocean high and trade winds are closest to the equator and strongest in austral winter when however the eastward pressure gradient in the equatorial zone is also steepest accordingly the latitude of flow recurvature in the southern trade winds stays farthest away from the equator in boreal spring and autumn consequently the equatorial westerlies peak in these short transition seasons the boreal autumn equatorial westerlies are the surface manifestation of a powerful zonal circulation cell along the indian ocean equator the interannual variability of boreal autumn is characterized by weak westerlies accompanying a slack zonal pressure gradient and strong southern trade winds with recurvature near the equator such an ensemble of circulation departures is characteristic of abundant rainfall in east africa and deficient precipitation in indonesia copyright 2004 royal meteorological society

diagnosing the 2005 drought in equatorial east africa abstract equatorial east africa suffered severe drought during its 2005 short rains centered on october november the circulation mechanisms of such precipitation anomalies are examined using long term upper air and surface datasets and based on diagnostic findings from earlier empirical investigations the steep eastward pressure gradient is conducive to fast westerlies over the central equatorial indian ocean surface manifestation of a powerful zonal circulation cell with subsidence over east africa and ascending motion over indonesia with fast westerlies rainfall in east africa is deficient and they tend to be accompanied by anomalously cold waters in the northwestern and warm anomalies in the southeastern extremity of the equatorial indian ocean basin without any seesaw between these domains in october november 2005 pressure in the west was anomalously high entailing a steep eastward pressure gradient along the equator conducive to fast westerlies and further symptomatic of the zonal ci

empirical predictability study of october december east african rainfall relationships between september atmospheric dynamics over the global tropics and the east africa short rains over the domain 4 s 4 n 30 39 e were investigated to define a seasonal rainfall forecasting scheme from total and partial correlation analyses over the period 1968 97 three major and useful signals appear in september precursor components of the october december east west circulation cell appear over the equatorial indian ocean eio they were discovered through various indices of sea surface temperature sst zonal wind vertical velocity and moist static energy computed over the western central and eastern parts of the eio principal component analyses were applied to these indices to form two synthetic indices which discriminate ocean atmosphere variability in the eastern and western parts of the basin the indian monsoon dynamics control the ssts off the somalia coast which are known to play an important role in the variability of the short rains ridge trough systems develop off south africa which when persistent enough drive sst anomalies in the eastern tropical and equatorial atlantic these in turn are associated with anomalies in the middle levels over an equatorial band ranging from the guinea gulf to the western indian ocean the three september signals used as predictors in a multiple linear regression model explain 64 of the rainfall variance a comparable efficiency is also achieved with a linear discriminant analysis model copyright 2002 royal meteorological society

on the deterministic description of earthquakes 1 the quantitative estimate of earthquake damage due to ground shaking is of pivotal importance in geosciences and its knowledge should hopefully lead to the formulation of improved strategies for seismic hazard assessment numerical models of the processes occurring during seismogenic faulting represent a powerful tool to explore realistic scenarios that are often far from being fully reproduced in laboratory experiments because of intrinsic technical limitations in this paper we discuss the prominent role of the fault governing model which describes the behavior of the fault traction during a dynamic slip failure and accounts for the different and potentially competing chemical and physical dissipative mechanisms we show in a comprehensive sketch the large number of constitutive models adopted in dynamic modeling of seismic events and we emphasize their prominent features limitations and specific advantages in a quantitative comparison we show through numerical simulations that spontaneous dynamic ruptures obeying the idealized linear slip weakening sw equation and a more elaborated rate and state dependent friction law produce very similar results in terms of rupture times peaks slip velocity developed slip and stress drops provided that the frictional parameters are adequately comparable and more importantly that the fracture energy density is the same our numerical experiments also illustrate that the different models predict fault slip velocity time histories characterized by a similar frequency content a feeble predominance of high frequencies in the sw case emerges in the frequency ranges 0 3 1 and 11 50 hz these simulations clearly indicate that even forgiving the frequency band limitation it would be very difficult virtually impossible to discriminate between two different but energetically identical constitutive models on the basis of the seismograms recorded after a natural earthquake

indian ocean dipolelike variability in the csiro mark 3 coupled climate model abstract coupled ocean atmosphere variability in the tropical indian ocean is explored with a multicentury integration of the commonwealth scientific and industrial research organisation csiro mark 3 climate model which runs without flux adjustment despite the presence of some common deficiencies in this type of coupled model zonal dipolelike variability is produced during july through november the dominant mode of variability of sea surface temperature resembles the observed zonal dipole and has out of phase rainfall variations across the indian ocean basin which are as large as those associated with the model el nino southern oscillation enso in the positive dipole phase cold sst anomaly and suppressed rainfall south of the equator on the sumatra java coast drives an anticyclonic circulation anomaly that is consistent with the steady response gill model to a heat sink displaced south of the equator the northwest southeast tilting sumatra java coast results in cold sea surface temperature

strong velocity weakening and powder lubrication of simulated carbonate faults at seismic slip rates 1 high velocity friction tests were conducted on solid and hollow cylinders of carrara calcite marble dolomite marble silicate bearing calcite marble and calcite gouge to investigate the strength of carbonate faults during seismic slip the experiments performed at normal stresses of 0 6 14 7 mpa slip rates of 0 03 1 60 m s and room temperature in a rotary shear friction testing machine yielded an extraordinarily low steady state friction coefficient 0 1 at slip rates of 1 1 1 2 m s the slip weakening distance of 4 28 m became shorter at higher normal stress or frictional work rate strong velocity weakening was observed not only in steady state but also in nonsteady state friction while the slip rate was changing thus slip deceleration was accompanied by fault strength recovery large rapid temperature rises in narrow shear localization zones less than a few micrometers induced carbonate decomposition such as the breakdown of calcite into aggregates of cao nanograins and co2 in carrara marble scanning electron microscope observation revealed that the shear localization zone in the highly porous decomposition product was a layer of scattered small grains mostly 1 m in diameter these microstructures and the measured high permeability 10 14 m2 of the decomposed marble indicate that the dominant weakening mechanism in our experiments was possibly powder lubrication powder rheology at high slip rates is not yet well understood but the frictional behavior of nanograins appears to be strongly velocity dependent if decarbonation occurs during seismic slip in natural carbonate faults powder lubrication may make the faults slippery even under fluid drained conditions

the influence of basin morphology on groundwater outflow twenty geomorphologic parameters which constitute a well balanced representation of the area length elevation form and shape aspects of watersheds as well as the texture of their drainage networks were considered on the basis of groundwater theory and the results of previous related studies eight of these parameters were identified as being relevant to the process of groundwater discharge values of the selected parameters were obtained for representative watersheds located in a section of the appalachian plateaus the data which were generated from u s geological survey topographic maps were then subjected to a factor analysis to assess the relative importance of the parameters with respect to their influence on groundwater outflow behavior the analytical results and their interpretation showed that among the parameters that are related to groundwater outflow total length of perennial streams average basin slope and drainage density are the ones that are most closely related to the process the influences of these parameters on groundwater outflow behavior are independent of each other thus in relating groundwater outflow characteristics to basin morphology only these parameters need be considered and the inclusion of additional parameters does not necessarily yield a better relationship and may result in redundancy the findings of the study are consistent with existing concepts regarding base flow or drought flow and have been confirmed by an independent study that investigated the process based on a more mechanistic approach

the application of resin impregnation to the three dimensional study of chalk pore geometry abstract asp art of a wider study of the hydraulic properties of upper cretaceous chalk from east yorkshire and the north sea a method of pore space casting is being employed thus with the aid of the scanning electron microscope it is possible to obtain by direct visual observation information about the shape and size of pores and grains the connectedness and roughness of pore channels the fluid flow in porous rocks a more detailed understanding of the nature of hydrocarbon of water bearing rock is becoming increasingly important in pursuance of studies of oil exploration oil exploitation and geotechnics in this paper a method of impregnation and casting the pore space in chalk is described together with some preliminary observations of chalk pore geometry

warming trend of the indian ocean sst and indian ocean dipole from 1880 to 2004 the state of the indian ocean dipole representing the sst anomaly difference between the western and southeastern regions of the ocean is investigated using historical sst reconstructions from 1880 to 2004 first the western and eastern poles of the sst based dipole mode index are analyzed separately both the western and eastern poles display warming trends over this period particularly after the 1950s the western pole tends to be anomalously colder than the eastern pole from 1880 to 1919 whereas in the interval 1950 2004 the sst anomalies over the western pole are comparable to those over the eastern pole though there are occasional outliers where the eastern pole is anomalously colder than the western pole the tendencies of the occurrences of positive and negative dipole events in september november show three distinct regimes during the period analyzed in 1880 1919 negative dipole events associated with la nina events occur more frequently than positive events in 1920 49 some weak positive events occur relatively independently of el nino events over the pacific the period of 1960 2004 is characterized by strong and frequent occurrences of positive events associated with el nino events

relative role of dynamic and thermodynamic processes in the development of the indian ocean dipole an ogcm diagnosis 1 the relative role of oceanic dynamics and surface heat fluxes in the initiation and development of the indian ocean dipole was investigated by analyzing results from an oceanic general circulation model the model was forced by observed surface wind stress and heat flux fields for 1958 1997 the results show that it was capable of reproducing observed dipole events over the tropical indian ocean the diagnosis of the mixed layer heat budget indicates that the sst anomaly ssta in the east pole is primarily induced by anomalous surface latent heat flux and vertical temperature advection whereas in the west pole it is mainly caused by meridional and vertical temperature advection anomalies in both regions shortwave radiation anomalies tend to damp the ssta the ocean rossby waves are essential in linking the anomalous wind and sst off sumatra and subsurface temperature variations in southwest indian ocean

what can physical source models tell us about the recurrence time of earthquakes abstract earthquake prediction no matter what the timescale has been and continues to be a contentious subject and it is indubitably a prominent challenge for modern seismology and earthquake physics indeed few natural events can have the catastrophic consequences of earthquakes earthquakes today account for about 60 of natural fatalities a physical description of an earthquake represents an amenable approach to the prediction but it suffers of some limitations basically due to the notorious ignorance about the initial state of a given fault and about the physical law controlling its traction evolution independent on those intrinsic epistemic limitations the concept of the earthquake recurrence based upon the idea of the cyclic or characteristic earthquake has been often invoked to describe and thus to predict subsequent instability events on a seismogenic structure in this paper by using the simplest analog fault model the one degree of freedom mass spring system we quantitatively show that the concepts of the recurrence time and the earthquake cycle have limitations even making them meaningless we will discuss in a compendious synopsis all the possible physical mechanisms which can dramatically affect the recurrence time our conclusions emphasize again that the competing mechanisms potentially occurring during faulting even in the simplest and idealized condition of an isolated fault can significantly complicate the regular cyclicity of earthquakes predicted by the analog fault system these conclusions can contribute to the debate about the role of the physical modeling of earthquakes in the contest of seismic hazard assessment

modelling flow and heat transfer through unsaturated chalk validation with experimental data from the ground surface to the aquifer abstract during the winter and spring of 2000 2001 large floods occurred in northern france somme river basin and southern england patcham area of brighton in valleys that are developed on chalk outcrops the floods durations were particularly long more than 3 months in the somme basin and caused significant damage in both countries to improve the understanding of groundwater flooding in chalk catchments an experimental site was set up in the hallue basin which is located in the somme river basin france unsaturated fractured chalk formation overlying the chalk aquifer was monitored to understand its reaction to long and heavy rainfall events when it reaches a near saturation state the water content and soil temperature were monitored to a depth of 8 m and the matrix pressure was monitored down to the water table 26 5 m below ground level the monitoring extended over a 2 5 year period 2006 2008 under natural conditions and during two periods when heavy artificial infiltration was induced the objective of the paper is to describe a vertical numerical flow model based on richards equation using these data that was developed to simulate infiltrating rainwater flow from the ground surface to the saturated aquifer the marthe computer code which models the unsaturated saturated continuum was adapted to reproduce the monitored high saturation periods composite constitutive functions hydraulic conductivity saturation and pressure saturation that integrate the increase in hydraulic conductivity near saturation and extra available porosity resulting from fractures were introduced into the code using these composite constitutive functions the model was able to accurately simulate the water contents and pressures at all depths over the entire monitored period including the infiltration tests the soil temperature was also accurately simulated at all depths except during the infiltrations tests which contributes to the model validation the model was used to calculate the aquifer recharge over a long period that included droughts and floods the calculated recharge is realistic as it makes it possible to simulate the corresponding monitored groundwater level data which increases confidence in the modelling approach

interaction of the monsoon and pacific trade wind system at interannual time scales part iii a partial anatomy of the southern oscillation abstract studies of surface wind fields sea surface temperature sst precipitation and sea level pressure in the tropical band extending 30 of the equator from africa to south america led to the following conclusions the southern oscillation el niflo and climatic variations in the monsoon system are all part of one global scale phenomenon the new information is that this phenomenon appears in the sea level pressure field to have a strong propagating component that appears first in the northern indian ocean and moves eastwardinto the eastern pacific similar propagation of information was found in the surface wind field and equatorial precipitation regimes these same conditions were amply demonstrated during the 1982 83 event and so it may be concluded that the evolution of that event bears many similarities to those in the historical record studies referred to in this paper in the surface wind field of the equatorial wave guide the large scale signal appears to take the form of a forced kelvin

recharge of the upper chalk aquifer at a site in hampshire england 1 water balance and unsaturated flow abstract measurements of the annual cycle of changes in water content and potential in the 40 m thick unsaturated zone of upper chalk at a site in southern england are described measured evaporation continued at the penman potential rate until the soil moisture deficit reached 175 mm this suggested that a root constant of 200 mm would be more appropriate than the much lower values generally used the field capacity concept could not be reconciled with the timing of renewed water recharge and the reasons for this are discussed the mechanisms of water flow through the unsaturated zone of the chalk are examined and it is shown that at this upper chalk site matrix flow is the dominant recharge mechanism

is the initial thermal state of a fault relevant to its dynamic behavior the prediction ofimpending earthquakes undoubtedly remains one of the most pursued goals of modern seismology within the framework of a deterministic description of earthquake faulting the initial state of the fault system and the choice of the governing model describing its rheological behavior play a fundamental role in the description of the earthquake recurrence in classical models of faulting this initial stateisbasicallydescribedbytheinitialshear stressdistribution priortothenextearth quakeevent andbytheinitialslidingvelocity inthispaper byassumingarate state andtemperature dependent rheology weinvestigatewhether theinitialthermalstate of the fault can also have a significant role in earthquake dynamics our numerical results clearly demonstrate that the initial temperature greatly influences the cosesimic slip and thus the earthquake magnitude the released stress and thus the radiated energy andtheintereventtime i e theearthquakerecurrence despitetheremainingissueson theconceptofearthquakecyclicity ourresultscancontributetothelivelydebateonthe deterministic hazard assessment illuminating that the temperature field also plays a fundamental role in earthquake dynamics not only because it controls possible phase changes and the chemical environment of the fault zone but also because it affects the response of a brittle fault and earthquake cycles

improved understanding of spatio temporal controls on regional scale groundwater flooding using hydrograph analysis and impulse response functions controls on the spatiotemporal extent of groundwater flooding are poorly understood despite the long duration of groundwater flood events and distinct social and economic impacts we developed a novel approach using statistical analysis of groundwater level hydrographs and impulse response functions irfs and applied it to the 2013 14 chalk groundwater flooding in the english lowlands we proposed a standardised index of groundwater flooding which we calculated for monthly groundwater levels for 26 boreholes in the chalk we grouped these standardised series using k means cluster analysis and cross correlated the cluster centroids with the standardised precipitation index spi accumulated over time intervals between 1 and 60 months this analysis reveals two spatially coherent groups of standardised hydrographs which responded to precipitation over different timescales we estimated irf models of the groundwater level response to effective precipitation for three boreholes in each group the irf models corroborate the spi analysis showing different response functions between the groups we applied identical effective precipitation inputs to each of the irf models and observed differences between the hydrographs from each group it is suggested this is due to the hydrogeological properties of the chalk and of overlying relatively low permeability superficial deposits recent unconsolidated sediments overlying the bedrock such as clays and tills which are extensive over one of the groups the overarching controls on groundwater flood response are concluded to be a complex combination of antecedent conditions rainfall and catchment hydrogeological properties these controls should be taken into consideration when anticipating and managing future groundwater flood events the approach presented is generic and parsimonious and can be easily applied where sufficient groundwater level and rainfall data are available

on the dynamics of droughts in northeast brazil observations theory and numerical experiments with a general circulation model abstract it is proposed that a possible mechanism for the occurrence of severe droughts over northeast brazil is the establishment of a thermally direct local circulation which has its ascending branch at about 10 n and its descending branch over northeast brazil and the adjoining oceanic region the driving for this anomalous circulation is provided by warming due to enhanced moist convection associated with warmer sea surface temperature anomalies over the northern tropical atlantic and cooling associated with colder sea surface temperature anomalies in the southern tropical atlantic the combined effects of thermally forced subsidence and reduced evaporation and moisture flux convergence produces severe drought conditions over northeast brazil we have examined the monthly mean sea surface temperature anomalies over the tropical atlantic and rainfall anomalies over two selected stations fortaleza 3 46 s 38 31 w and quixeramobim 5 12 s 39 18 w for 25 years 1948 72 it is found that the most sever

propagation of drought through groundwater a new approach using linear reservoir theory the effect of drought on groundwater heads and discharge is often complex and poorly understood therefore the propagation of a drought from groundwater recharge to discharge and the influence of aquifer characteristics on the propagation was analysed by tracking a drought in recharge through a linear reservoir the recharge was defined as a sinusoid function with a period of 1 year the decrease in recharge owing to drought was simulated by multiplying the recharge during 1 year with a drought fraction between 0 and 1 which represents a decrease in the recharge of 100 to 0 respectively the droughts were identified using the threshold level approach with a threshold that is constant in time for this case analytical formulations were derived which express the drought duration and deficit in the groundwater discharge in terms of the decrease in recharge the reservoir coefficient that characterizes aquifer properties and the height of the threshold level the results showed that the delay in the groundwater system caused a shift of the main part of the decrease in recharge from the high flow to the low flow period this resulted in an increase in drought deficit for discharge compared with the drought deficit for recharge also the development of multiyear droughts caused an increase in drought deficit the attenuation in the groundwater system caused a decrease in drought deficit in most cases the net effect of these processes was an increase of drought deficit as a result of the propagation through groundwater only for small droughts the deficit decreased from recharge to discharge the amount of increase or decrease depends on the reservoir coefficient and the severity of the drought under most conditions a maximum in the drought deficit occurred for a reservoir coefficient of around 200 days copyright c 2003 john wiley sons ltd

the seasonal development of an sst anomaly in the indian ocean and its relationship to enso abstract the seasonal development of the sea surface temperature sst anomaly in the indian ocean is investigated in relation to el nino southern oscillation enso using noaa optimally interpolated sst and ncep reanalysis data the result shows that the onset season of el nino affects the seasonal development of surface wind anomalies over the equatorial eastern indian ocean eeio these surface wind anomalies in turn determine whether the sst anomaly in the eeio evolves into the eastern pole of the dipole pattern in years when the dipole pattern develops surface zonal wind anomalies over the eeio switch from westerly to easterly in spring as la nina switches to el nino the seasonal zonal wind over the eeio also switches from westerly to easterly in spring and the anomalous wind strengthens seasonal wind from winter to summer stronger winds and resultant thermal forcings produce the negative sst anomaly in the eeio in winter and its amplitude increases in summer the sst anomaly becomes the ea

nonstationary enso precipitation teleconnection over the equatorial indian ocean documented in a coral from the chagos archipelago d 18 oc o f aporites coral from the chagos archipelago 71 e 5 s only after the mid 1970s a strong enso signal emerges in the d 18 oc during the analyzed period 1950 1994 in the 1970s the increasing sea surface temperature sst shifted the mean sst closer to the deep convection threshold at about 28 5 c enso related sst variability largely controls the deep convection and precipitation in the central equatorial indian ocean ceio when the sst is at this critical level the anomalies in the precipitation induce changes in the isotopic composition of the surface ocean waters the precipitation signal amplifies the sst signal in the coral d 18 oc and raises the correlation to enso the presented results have important implications for the reconstruction of enso indices from corals within the indian ocean citation timm o m pfeiffer and w c dullo 2005 nonstationary ensoprecipitation teleconnection over the equatorial indian ocean documented in a coral from the chagos archipelago geophys res lett 32 l02701 doi 10 1029 2004gl021738

mechanisms of water storage and flow in the unsaturated zone of the chalk aquifer abstract it is known that the volumes of water draining from some chalk catchments in recessions are significantly greater than can be explained by gravity drainage from fissure porosity the most likely source of this water is slow drainage from the matric porosity above the water table this paper suggests that the location of this storage is in the irregularities on the fissure surfaces studies of resin impregnated blocks of chalk failed to reveal the presence of hydrologically significant microfissures or macropores that could be the source of additional drainage ultrasonic measurements at varying values of effective stress show no indication of changes in the acoustic transmission properties that could be attributed to closure of microfissures drainage and mercury intrusion measurements indicate that there is significant storage on block surfaces the drainage measurements confirm that the storage is of the order of magnitude about 0 3 needed to explain the slow drainage this additional storage explains why the water table is slow to respond after recharge events and why the chalk is resilient to drought the presence of storage in the unsaturated zone implies that the position of the water table may not be an accurate indicator of groundwater resources the concept of filling and draining of irregularities on fissure surfaces also leads to a new model for the generation of fissure flow in the unsaturated zone of the chalk this suggests that fissure flow can be generated at any depth in the profile and in a sequence of uniform vertical permeability is likely to originate near the water table rather than high in the unsaturated zone

dynamic seismic ruptures on melting fault zones 1 we present a physical model that describes the behavior of spontaneous earthquake ruptures dynamically propagating on a fault zone and that accounts for the presence of frictional melt produced by the sliding surfaces first we analytically derive the solution for the temperature evolution inside the melt layer which generalizes previous approximations then we incorporate such a solution into a numerical code for the solution of the elastodynamic problem when a melt layer is formed the linear slip weakening law initially governing the fault and relying on the coulomb friction is no longer valid therefore we introduce on the fault a linearly viscous rheology with a temperature dependent dynamic viscosity we explore through numerical simulations the resulting behavior of the traction evolution in the cohesive zone before and after the transition from coulomb friction and viscous rheology the predictions of our model are in general agreement with the data from exhumed faults we also find that the fault after undergoing the breakdown stress drop controlled by the slip weakening constitutive equation experiences a second traction drop controlled by the exponential weakening of fault resistance due to the viscous rheology this further drop enhances the instability of the fault increasing the rupture speeds the peaks in fault slip velocity and the fracture energy density

a recipe for simulating the interannual variability of the asian summer monsoon and its relation with enso this study investigates how accurately the interannual variability over the indian ocean basin and the relationship between the indian summer monsoon and the el nino southern oscillation enso can be simulated by different modelling strategies with a hierarchy of models from an atmospherical general circulation model agcm forced by observed sst to a coupled model with the ocean component limited to the tropical pacific and indian oceans the role of heat fluxes and of interactive coupling is analyzed whenever sea surface temperature anomalies in the indian basin are created by the coupled model the inverse relationship between the enso index and the indian summer monsoon rainfall is recovered and it is preserved if the atmospherical model is forced by the ssts created by the coupled model if the ocean model domain is limited to the indian ocean changes in the walker circulation over the pacific during el nino years induce a decrease of rainfall over the indian subcontinent however the observed correlation between enso and the indian ocean zonal mode iozm is not properly modelled and the two indices are not significantly correlated independently on season whenever the ocean domain extends to the pacific and enso can impact both the atmospheric circulation and the ocean subsurface in the equatorial eastern indian ocean modelled precipitation patterns associated both to enso and to the iozm closely resemble the observations

interannual displacements of convection and surface circulation over the equatorial indian ocean the interannual variability of surface observations in the equatorial indian ocean is investigated from 23 years of ship reports 1954 1976 in october and november during certain years the monthly analyses show strong wind anomalies in the eastern and central equatorial indian ocean simultaneously cloud cover anomalies of opposite signs are observed in the eastern indian ocean and in the central indian ocean a linear analysis of heat induced circulation suggests that the surface wind anomalies are forced by anomalies of rainfall this circulation is probably dissipative with time scales in the range two to five days the reasons for the maintenance of anomalous circulations are not well known it is noted that during most el nino events in the pacific ocean anomalies are observed in the indian ocean however there are other years with the same anomalous pattern 1961 for example during the years with anomalously little cloudiness in the eastern indian ocean sea surface temperatures lower than normal are found in the eastern indian ocean this suggests that coupled air sea dynamics must be taken into account to understand the interannual variability

geomorphic controls on regional base flow the subsurface hydrological response plays an important role in the hydrology of humid regions in particular the physical relationship between base flow dynamics and the fluctuations in spatially averaged water table depth as described by the groundwater rating curve determine to a significant extent the nature of statistical persistence of hydrological anomalies in the unconfined aquifers level and river flow in this paper we propose that the scale and shape of the groundwater rating curve reflect some of the geomorphological characteristics of the region such as relief drainage density and the hypsometric distribution of the elevation field these connections between geomorphology and hydrology of river basins are investigated using a simple model of unconfined groundwater flow applied to synthetic basins as well as observed basins from illinois

mechanisms of climate anomalies in the equatorial indian ocean 1 building on earlier work on the interannual variability of the boreal autumn equatorial westerlies ueq over the indian ocean and concomitant rainfall anomalies at the coast of east africa and in indonesia the inherent circulation mechanisms are here explored further from long term surface and upper air data fast ueq and deficient east african rainfall come with positive sea level pressure p and negative sea surface temperature t departures in a domain w at the northwestern extremity and opposite departures in a domain e at the southeastern extremity of the equatorial indian ocean however there is no seesaw between w and e in either p or t and no indication of local forcing of t on p the large scale pressure field in particular the zonal pressure gradient along the equator and the south indian ocean pressure and southern tradewinds control the evolution of ueq fast ueq steepens the zonal temperature gradient thus tightening the inverse relationships between the zonal gradients of pressure and temperature the rainfall anomalies associated with the interannual variability of ueq surface manifestation of a zonal circulation cell along the indian ocean equator are favored by the kinematic and thermodynamic conditions in w and e thus with fast ueq the domain w features departure lower tropospheric divergence and subsidence and favored by the cold t and subsidence reduced precipitable water all conducive to deficient precipitation by contrast e has departure lower tropospheric convergence and ascending motion and favored by the warm t and ascending motion enhanced precipitable water in conjunction conducive to abundant rainfall the interannual variability of the boreal autumn equatorial westerlies dominated as it is by the large scale pressure field is crucial in the climate dynamics of the equatorial indian ocean region this leads to the question what controls the pressure pattern over the indian ocean basin

potential predictability of the sea surface temperature forced equatorial east african short rains interannual variability in the 20th century in this article the predictability of the 20th century sea surface temperature sst forced east african short rains variability is analyzed using observational data and ensembles of long atmospheric general circulation model agcm simulations to our knowledge such an analysis for the whole 20th century using a series of agcm ensemble simulations is carried out here for the first time the physical mechanisms that govern the influence of sst on east african short rains in the model are also investigated it is found that there is substantial skill in reproducing the east african short rains variability given that the ssts are known consistent with previous recent studies it is found that the indian ocean and in particular the western pole of the indian ocean dipole iod play a dominant role for the prediction skill whereas ssts outside the indian ocean play a minor role the physical mechanism for the influence of the western indian ocean on east african rainfall in the model is consistent with previous findings and consists of a gill type response to a warm cold anomaly that induces a westerly easterly low level flow anomaly over equatorial africa and leads to moisture flux convergence divergence over east africa on the other hand a positive el nino southern oscillation enso anomaly leads to a spatially non coherent reducing effect over parts of east africa but the relationship is not strong enough to provide any predictive skill in our model the east african short rains prediction skill is also analyzed within a model derived potential predictability framework and it is shown that the actual prediction skill is broadly consistent with the model potential prediction skill low frequency variations of the prediction skill are mostly related to ssts outside the indian ocean region and are likely due to an increased interference of enso with the indian ocean influence on east african short rains after the mid 1970s climate shift

an efficient mechanism to avert frictional melts during seismic ruptures abstract we consider the spatio temporal evolution of temperature due to frictional heating caused by the spontaneous propagation of 3 d dynamic seismic ruptures on planar faults in our numerical experiments which characterize typical crustal earthquakes we assume that fault friction is controlled by different linear and nonlinear slip dependent friction laws in this paper we confirm that a necessary condition to prevent melting is to have a nearly complete breakdown stress drop our simulations which employ a nonlinear slip dependent governing equation recently inferred from laboratory experiments by sone and shimamoto 2009 reproduce such a dramatic fault weakening and represent a plausible explanation for the prevention of melting during earthquake ruptures we also demonstrate that low friction alone although necessary is not a sufficient condition to avert melts the linear or classical slip weakening sw law would produce melting even assuming the same lengthscales and frictional levels to avoid melting with linear sw law we have to impose a specific value of the sw distance this reveals the prominent role of the time evolution of traction within the cohesive zone where the stress release is realized and of the value of the fracture energy density

thickness of the seismic slip zone this article reviews geologic and other evidence constraining the thickness of the principal slip zone psz that accommodates the bulk of coseismic shear displacement during an individual rupture event surface deformation from rupturing may occupy swaths tens of meters or more in width but trenches across active faults generally reveal that incremental slip is accommodated by a psz that is tens of centimeters or less in thickness geomorphic evidence coupled with the observations from trenching suggest a psz may stay well localized for distances of several kilometers through many rupture episodes mine exposures and exhumed fault zones demonstrate that pszs separating different lithologies within the fault core although contained within damage zones of variably fractured rock ranging up to hundreds of meters in thickness often comprise just a few centimeters of gouge ultracataclasite that have accommodated large finite displacements 1 km microstructural studies demonstrate incremental slip localized still further down to 1 10 mm as do other fault rock assemblages slickensides and slickenfibers fault veins of pseudotachylyte friction melt intravein septa in hydrothermal fault infills the accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone with extreme localization to less than 1 cm not uncommon however some distributed coseismic shear may also develop especially at rupture irregularities coseismic reduction of shear resistance from friction melting t 1000 c or from transient thermal pressurization of aqueous fluids t 100 c requires slip during moderate to large earthquakes u 1 m to be restricted to narrow zones respectively a few centimeters or tens of centimeters in thickness given the evidence for slip localization the apparent scarcity of pseudotachylyte suggests either that seismic friction melting is a rare phenomenon or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones

relationships among compressional wave attenuation porosity clay content and permeability in sandstones anelastic attenuation is the process by which rocks convert compressional waves into heat and thereby modify the amplitude and phase of the waves understanding the causes of compressional wave attenuation is important in the acquisition processing and interpretation of high resolution seismic data and in deducing the physical properties of rocks from seismic data we have measured the attenuation coefficients of compressional waves in 42 sandstones at a confining pressure of 40 mpa equivalent to a depth of burial of about 1 5 km in a frequency range from 0 5 to 1 5 mhz the compressional wave measurements were made using a pulse echo method in which the sample 5 cm diameter 1 8 cm to 3 5 cm long was sandwiched between perspex lucite buffer rods inside the high pressure rig the attenuation of the sample was estimated from the logarithmic spectral ratio of the signals corrected for beam spreading reflected from the top and base of the sample the results show that for these samples compressio

effect of frictional heating on pre seismic sliding a numerical simulation using a rate state and temperature dependent friction law summary using a simple spring block model and a rate and state dependent friction law incorporating the effect of temperature variation i perform a numerical simulation of aseismic sliding prior to unstable seismic faulting i compare the simulation result of pre seismic sliding in the case where the temperature effect due to frictional heating on frictional strength is taken into consideration heated fault with the result in the case where the temperature effect is neglected unheated fault assuming model parameters appropriate for the nucleation site of a large interplate earthquake it is found that an earthquake occurrence time is a little earlier on the heated fault than on the unheated fault this is because the frictional strength is weakened by temperature rise due to frictional heating during pre seismic sliding the amplitude of precursory crustal deformation caused by pre seismic sliding on the heated fault is estimated to be about 20 per cent smaller than that on the unheated fault

circulation mechanisms of kenya rainfall anomalies abstract expanding earlier studies on the boreal spring and autumn rainy seasons in equatorial east africa pending challenges on the mechanisms of rainfall variability are investigated eastward pressure gradient and slack south indian ocean trade winds allow surface equatorial westerlies in spring and autumn complementing that upper tropospheric easterlies are required for the development of a zonal vertical circulation cell along the indian ocean equator because of the summer warming and high stand of upper tropospheric topography over south asia strong upper tropospheric easterlies over the tropical northern and equatorial indian ocean persist from summer into autumn thus allowing the development of a zonal vertical circulation cell by contrast the winter cooling entails low stand of upper tropospheric topography in the north thus hindering easterlies over the equator consequently an equatorial zonal circulation cell does not develop in boreal spring the equatorial zonal circulation cell

temperature variations of constitutive parameters can significantly affect the fault dynamics abstract a rate and state dependent governing law with temperature dependent constitutive parameters is considered on the basis of laboratory inferences we model the whole seismic cycle of a homogeneous fault obeying to such a law by adopting a spring slider dashpot fault analog model we show that the variations of the parameter a accounting for the so called direct effect with the temperature cause the system to enter at high speeds in a conditionally stable regime and also in a velocity strengthening regime although we do not observe the complete cessation of slip we can see a severe reduction of the degree of the instability of the fault in particular the peaks of the sliding velocity are reduced as well as the developed temperature due to frictional sliding and the released stress during each instability event moreover the recurrence times are reduced of a factor of two with respect to a reference configuration where the canonical formulation of rate and state friction with temporally constant parameters is assumed the obtained results can help the interpretation of high velocities laboratory experiments and further illuminate the importance of the temperature in the context of seismic hazard assessment

thermochemical pressurization of faults during coseismic slip 1 during earthquakes frictional heating on the fault plane induces a temperature rise and thus a pore pressure rise which is known as thermal pressurization tp coseismic mineral dehydrations may occur because of this temperature increase and are included within the tp framework dehydrations are modeled as a source term for pore pressure because of the total volume change and as a sink term for temperature because they are endothermic the reaction occurs within the slipping zone when a threshold temperature ts is reached dehydration reaction kinetic is modeled using a first order reaction rate using energy and fluid mass conservation we derive analytically the equations of evolution of pore pressure temperature and reaction extent in the undrained adiabatic case using a constant reaction rate we investigate the values of the kinetic rate constant required to produce a significant effect which are much higher than laboratory data reported in the literature on clay serpentine and phyllosilicate dehydration we show however that such high values can be reached if the temperature dependency of the rate constant is taken into account next we include fluid and heat transport and use an arrhenius law to calculate the rate constant as a function of temperature the subsequent set of differential equations is then solved numerically the main effect of dehydration reactions is an increase of pore pressure and a stabilization of the temperature during slip we explore a wide range of parameters in order to determine in which cases dehydration can be considered as a nonnegligible process for high permeability rocks 10 18 m2 and when the amount of water that can be released is of the order of 10 dehydration is an important mechanism as it delays the onset of melting which would normally occur even within the tp framework if the onset temperature is low compared to the initial temperature t0 ts t0 150 c overpressure can occur if the reactions are highly endothermic and if their kinetic is fast enough frictional melting would not occur unless the dehydration reactions are completed within the slipping zone