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

variabilities of the equatorial electrojet in brazil and peru 1 this report presents seasonal and longitudinal variabilities of the equatorial electrojet in the east brazil sao luis 2 3 s 315 8 e 0 5 s dip latitude and west jicamarca peru 11 95 s 283 13 e 0 6 n dip latitude coasts of the continent of south america ground based magnetic field perturbation measurements h for solar maximum 2001 2002 and solar minimum 2006 2007 conditions from the two equatorial stations sao luis and jicamarca have been used for the study the h signal which is a measure of the strength of the equatorial electrojet is spectrally analyzed using wavelet analysis the results of our analysis show that 1 the equatorial electrojet has maxima around equinoxes in jicamarca peru but it has a prominent maximum during southern hemisphere summer centered about december january in sao luis brazil the observed seasonal behavior of the equatorial electrojet in sao luis is highly likely due to the large magnetic declination angle about 20 west there 2 the equatorial electrojet is stronger in the west coast jicamarca compared to the east coast sao luis irrespective of solar activity condition 3 the magnitude of the equatorial electrojet is more variable with season and solar cycle over sao luis than over jicamarca

distribution characteristics of topside ionospheric density irregularities equatorial versus midlatitude regions 1 the global distribution of the occurrence rate for density irregularities at 600 km topside ionosphere between 35 geographic latitudes has been studied with the rocsat data during moderate to high solar activity years of 1999 to 2004 the result indicates that the global occurrence distribution of the intermediate scale 0 1 to 50 km density irregularities can be grouped into two different populations one in the equatorial region and the other in the middle to subauroral latitude region the global seasonal longitudinal s l distribution of equatorial irregularities in the current report reproduces the result of mcclure et al 1998 obtained with the ae e observations of the mesoscale 50 to 1000 km plasma bubble structures during high solar activity years of 1978 to 1980 two solar cycles ago this implies that the density irregularities of different scales from multistage cascading process of the large scale 1000 km gravitational rayleigh taylor instability have manifested in same global s l distribution pattern furthermore global variation in seeding mechanism and growth condition of the instability process that results in major features in global irregularity pattern seems to persist for past 25 years in addition the current result further indicates that an upper latitudinal limit of the equatorial irregularity distribution is located at about 30 a different kind of midlatitude irregularity distribution starts to fill in from this dip latitude in other words the equatorial density irregularity inside a depleted flux tube can only rise on statistical average to an apex height of 2000 km different magnetic and solar variability effects as well as the local time dependence are noted for the occurrences of density irregularities in the equatorial region versus that at midlatitudes the occurrence frequency of equatorial density irregularities increases with solar flux intensity whereas the midlatitude density irregularity is more likely to occur during low solar activity period the equatorial density irregularities are more likely to occur during periods of low magnetic activity than during magnetic disturbed times on the other hand the occurrence of midlatitude density irregularities indicates little dependence on geomagnetic activity the local time distribution of equatorial irregularity peaks before midnight while the midlatitude irregularity indicates a plateau of high occurrence rate after midnight such opposite characteristics in the occurrence pattern between these two spatially separated distributions suggest that different instability mechanisms are operated in two different latitude regions for the occurrence of intermediate scale density irregularities

latitudinal and magnetic flux tube extension of the equatorial spread f irregularities a comparative study has been carried out of the spread f occurrence characteristics at the magnetic equatorial station fortaleza 4 s 38 w dip latitude 1 8 s and the low latitude station cachoeira paulista 22 s 45 w dip latitude 14 s located with a relatively small longitudinal difference in brazil the results show strong correlation between the occurrences of spread f events at the two stations for most of the time while all the spread f events over cachoeira paulista are accompained by such events over fortaleza the converse is not true also the spread f onset times over the equator are systematically earlier than those over the low latitude another interesting result is that while the spread f durations at the two stations are approximately linearly related an event over cachoeira paulista is observed only for those that have durations more than 4 5 hours over fortaleza possible implications of these results on the plasma bubble rise velocities in the equatorial ionosphere are discussed it seems possible also to predict the approximate duration of a spread f event over the equator by observations carried out at a low latitude station situated close to a common magnetic meridional plane

zonal meridional wind and disturbance dynamo electric field control of the low latitude ionosphere based on the sundial atlas 1 campaign we present an evaluation of global scale equatorial and low latitude ionospheric disturbances in response to the weak to moderate disturbed conditions that marked the sundial atlas1 twelve day campaign of march 22 to april 2 1992 ionosonde data from south american and indian asian longitudes are analyzed to examine first the climatological the average pattern of the critical f region parameters the layer peak density and height in comparison with the empirical international reference ionospheric model and then to characterize the day to day variabilities aiming at identifying potential causal contributions from either disturbance zonal and meridional winds or magnetospheric and disturbance dynamo electric fields included in this analysis are data from south american midlatitude locations which are used to determine meridional winds using an adaptation of the servo analysis technique in the field line integrated plasma flip model we have made an assessment of the causal mechanism of the day to day variabilities as arising from latitude dependent disturbance meridional winds and from electric fields produced by disturbance zonal winds and disturbance dynamo while the contribution from disturbance meridional winds decreases from middle to equatorial latitudes that of the electric fields maximizes around the equator in particular first time evidence based on ionosonde data is provided for a disturbance dynamo electric field in the equatorial ionosphere it is found that there are two time intervals of maximum ionospheric variability resulting from the weak to moderate magnetospheric disturbance conditions that prevailed during the campaign one near the evening postsunset hours and the other in the postmidnight sunrise hours over low latitudes at midlatitudes a broad maximum of the response occurs from premidnight to morning hours we provide a comparison of results for the south american and indian asian longitudes and a discussion of the competing roles of the disturbance zonal and meridional winds and magnetospheric and disturbance dynamo electric fields as a function of latitude

spread f occurrence over a southern anomaly crest location in brazil during june solstice of solar minimum activity 1 we present a study of spread f occurrence over a location under the southern crest of the equatorial ionization anomaly cachoeira paulista 22 7 s 45 0 w mag lat 16 s dip angle 32 3 brazil during the last solar cycle which presented an extended solar minimum activity after analyzing hundreds of ionograms obtained with a digital ionosonde dgs 256 between 2001 and 2010 we verified high spread f occurrence around midnight postmidnight during june solstice southern hemisphere winter with a peak occurrence between 2006 and 2009 when the solar flux has reached very low values 70 sfu at the brazilian sector the occurrence of spread f is known to be associated with equatorial plasma bubbles epbs and is observed mainly between september and march on the other hand epbs are rarely observed over cachoeira paulista hereafter referred as cp during june solstice because of the weak magnitudes of the equatorial electric field prereversal enhancement an essential condition to the development of the large scale irregularities at the equatorial region despite the rarity of epbs we observed several strong spread f events occurring under low plasma densities conditions which can extend for several hours we have found evidences that the spread f events over cp during june solstices of low solar activity can be caused by ionospheric disturbances that are unrelated to equatorial processes finally we present a statistical analysis of the events and we suggest that traveling ionospheric disturbances could be one of the main sources of these nonequatorial spread f events

field study of spatial variability in unsaturated flow beneath and adjacent to playas we quantified unsaturated flow beneath playa and adjacent interplaya settings at a site in the southern high plains united states to resolve issues related to where and how water moves through the unsaturated zone this is the first study in which the data density 39 boreholes and the variety of techniques used physical chemical and isotopic were sufficient to quantify spatial variability in unsaturated flow water contents water potentials and tritium eoneentrations were much higher and chloride eoneentrations were much lower beneath playas than in interplaya settings which indicated that playas focus recharge these results refute previous hypotheses that playas act as evaporation pans or that recharge is restricted to the annular region around playas water fluxes estimated from environmental tracers ranged from 60 to 120 mm yr 1 beneath playas and were 0 1 mm yr 1 during the past 2000 5000 years beneath natural interplaya areas not subjected to ponding to evaluate the apparent inconsistency between high recharge rates and thick clay layers beneath playas we applied bromide and fd c blue dye to evaluate flow processes these applied tracer experiments showed preferential flow along roots and desiccation cracks through structured clays in the shallow subsurface in playas

dependence of the evening prereversal enhancement of the vertical plasma drift on geophysical parameters 1 the variability of the evening prereversal enhancement pre of the vertical plasma drift has been investigated by using the average velocity at a fixed local time range but this method does not provide a clear distinction between the pre occurrence frequency and magnitude in this study we examine the dependence of the occurrence frequency occurrence local time and magnitude of the pre on geophysical parameters by using the retrievals of the individual pre from the first republic of china satellite data our result shows that the seasonal longitudinal variation of the average vertical velocity in the evening that has been identified in previous studies is associated with both the pre occurrence frequency and pre magnitude the magnitude of the pre shows an increasing trend with an increase of the solar flux and with a decrease of the conjugate e region sunset time lag the seasonal longitudinal variation of the occurrence local time of the peak pre shows a dependence on the e region sunset time at the magnetic equator rather than on the magnetic declination or the sunset time lag in the conjugate e regions this observation indicates that the optimum time for the creation of the peak pre is when the sunset terminator at the e region height and the magnetic meridian intersect near the magnetic equator

the origin of large or great thrust type earthquakes along subducting plate boundaries abstract seismicity deformation state of stress and abundance of fluids along subducting plate boundaries are reviewed and the origin of large or great thrust type earthquakes is discussed based on the recent experimental results on the slip behavior of halite and serpentine gouges shallow subducting plate boundaries above 20 25 km in depth are characterized by low seismicity low tectonic stress inter plate decoupling ductile deformation associated with the formation of metamorphic schistosity except at very shallow depths metamorphism suggesting solution processes on massive scale and presence of abundant h 2 o it is argued that these unique features are due to pressure solution processes to high fluid pressure to low strength and stable behavior of clayey sediments under wet environments and or to the deformation of soft unconsolidated sediments at very shallow depths the low seismicity in this zone is in marked contrast with major strike slip faults along which large earthquakes occur at depths shallower than 15 20 km it is emphasized that these unique features are expected only for restricted regions where there is constant supply of h 2 o due to progressive metamorphism or where fluids in the rocks are trapped and cannot escape to the surface large or great thrust type earthquakes in subduction zones initiate at depths of 30 50 km below the shallow decoupled zone in this focal depth range the supply of h 2 o during progressive metamorphism perhaps diminishes downwards the overriding and subducting plates are coupled and stick to each other during much of the inter seismic period and the resistance to slip or shear stress is presumably high it is suggested that these earthquakes begin to occur at a depth where the plate boundary zone becomes fairly dry deformation at these depths appears to be predominantly ductile so that the earthquakes cannot be regarded simply as a brittle phenomenon 1 creep instability i e instability associated with plastic deformation and 2 dehydration induced instability are the most likely mechanisms for initiating the earthquakes and both have some experimental support stick slip of halite gouge while undergoing ductile deformation primarily by intracrystalline gliding is described and discussed as a supporting evidence for 1 shear resistance of halite gouge increases with increasing confining pressure in stick slip regimes hence the observed stick slip may be a semi brittle phenomenon with respect to the pressure dependence of the shear resistance although the deformation texture cannot be distinguished from that formed by pressure insensitive flow serpentine gouge exhibits violent stick slip upon its decomposition under dry not wet environments supporting the mechanism 2 above exact mechanisms which lead to the unstable fault motion are poorly understood as yet but stick slip of both halite and serpentine gouges is recognized only when the slip rate dependence of friction is negative i e lower friction at faster slip rate consistent with the theoretical prediction of rice and ruina 1983 there is a possibility that the thrust type earthquakes can be explained essentially within the framework of fault constitutive laws developed by dieterich 1979 and ruina 1983

the controlling effect of viscous dissipation on magma flow in silicic conduits nearly all volcanic conduit models assume that flow is newtonian and isothermal such models predict that during high flux silicic eruptions gradients in pressure with depth increase upward as magma accelerates and becomes more viscous leading to extremely low pressure and fragmentation at a depth of kilometers below the surface in this paper i show that shear heating also known as viscous dissipation dramatically reduces the pressure gradient required for flow and concentrates shear in narrow zones along the conduit margin the reduction in friction may eliminate the zone of low pressure predicted by isothermal models and move the fragmentation level up to the surface

some characteristics of spread f at the magnetic equatorial station fortaleza spread f in ionograms of both the range and frequency spread types is studied for the magnetic equatorial station fortaleza geographic coordinates 38 w 4 s dip latitude 1 7 for a 1 year period during 1978 1979 satellite traces suggesting ionization structures with scale sizes of 100 200 km are found to be necessary precursors to the formation of the irregularities that give rise to range type spread f whereas frequency type spread f was observed under different circumstances striking differences are noted in the spread f characteristics and related parameters over fortaleza as compared with huancayo geographic coordinates 75 3 w 12 05 s dip latitude 0 6 and jicamarca geographic coordinates 76 8 w 11 95 s dip latitude 0 9 situated within only 38 in longitude west of fortaleza the postsunset maximum in h f in winter is delayed by about 1 hr with respect to other seasons over fortaleza as compared with huancayo where it occurs earlier in winter than in other seasons similar differences also exist in the prereversal peak near sunset in the vertical drift velocity vz between fortaleza and jicamarca the differences are attributed possibly to the large difference in the magnetic field declination and to other factors such as the difference in the magnetic field intensity and the relative separation between geographic and magnetic equators that exist between fortaleza and the other stations

global features of quiet time counterelectrojet observed by orsted 1 the present work investigates the reverse equatorial electrojet signature seen in the orsted satellite data during magnetically quiet conditions we found that whenever the satellite sees the reverse electrojet signature over the indian region it is always accompanied by the counterelectrojet cej at indian observatories and therefore the reverse electrojet signature at the satellite height can be considered a proxy for the cej observed on the ground then using satellite observations global features of the cej phenomenon are obtained the broad features of the phenomenon such as a seasonal variation of the cej occurrence frequency and also its anticorrelation with the normal electrojet strength are found to be in agreement with earlier ground based studies satellite based investigations have a distinct advantage over ground data for studying the global extent as well as the longitudinal variation of the cej occurrence it is observed that in general the longitudinal extent of the phenomenon is very narrow often restricted to less than 25 however the present analysis suggests that strong westward currents generated at a given universal time can result in cej at wider longitudinal locations the percentage of cej occurrence is found to be highest in the brazilian sector between 300 e and 330 e longitude it is observed that the cej is prone to occur where the magnetic declination is large highlighting the role of the geometry of the ambient geomagnetic field the present study emphasizes that the cej occurs more favorably when and where the strength of the normal equatorial electrojet is weak

the interaction of viscous heating with grain size dependent rheology in the formation of localized slip zones the formation of localized shear zones is important for understanding many local and global processes in geodynamics we have developed a self consistent thermal mechanical model together with a rheology which depends on temperature strain rate and grain size distribution the grain size distribution has contributions from both dynamic recrystallization and grain growth processes and is governed locally by a nonlinear ordinary differential equation a one dimensional model with 104 points is employed to resolve all of the scales involving grain size and temperature we found that grain growth inhibits the development of shear zones and that there is a delicate interplay between viscous heating and grain growth process in determining whether narrow fault zones are developed quickly for realistic parameters of rheology and grain boundary processes for wet olivine the magnitude of the rate of grain growth is crucial to determine whether shear zones are stable or unstable at temperature t 1000 k or shear stress 100 mpa

the thermodynamics of deformed metamorphic rocks a review abstract the deformation of rocks is a disequilibrium and strongly non linear phenomenon with a number of interacting chemical thermal and microstructural processes operating simultaneously we review progress in this area over the past 30 years deforming chemically reacting systems are dissipative systems and hence are characterised by highly ordered structures that develop through cooperative processes once parameters such as critical strains strain rates fluid infiltration rates damage densities or temperatures are attained such criticality is the hallmark of deformed rocks at all length scales and is the basis for a diverse range of structures such as foliations and lineations produced by metamorphic differentiation rotation recrystallisation folding boudinage and micro to regional scale fracture systems criticality is identified with classical criticality and not self organised criticality the first and second laws of thermodynamics are used to show that such structural diversity arises from reaction diffusion deformation equations criticality of the system is associated with the stored energy becoming non convex and structures arise in order to minimise this non convex energy these structures are scale invariant and hence are characterised by fractal and minimal surface geometries thermodynamics is a powerful discipline to integrate seemingly unrelated processes in structural geology and produce an integrated approach to the subject that crosses all length scales

comparison of penetration electric fields created by the solar wind with jicamarca data using swage 1 swage solar wind acting on the geophysical environment calculates the global ionospheric electric field generated by high latitude electrodynamics drivers determined from the time shifted solar wind data measured at l1 by joining the hill siscoe polar cap potential model with the n c ionospheric potential solver of particular interest are the conditions under which the eastward equatorial penetration electric field near twilight contributes to the pre reversal enhancement pre in the present model it is found that a steeper terminator conductance gradient leads to a more pronounced pre the model is statistically consistent with the jicamarca vertical drift data at twilight during quiet times for eighty two days in the years 1998 2005 the model is also consistent with the jicamarca vertical drift data during the november 2004 magnetic superstorms dst 250 nt and highlights the importance of including the lt dependence of the ionospheric response in this comparison disturbance dynamo dd effects are also included comparison is much better using the conductance model with a shallower terminator gradient and indicates that the conductance lt profile was relatively unchanged throughout the storms

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

experimental studies in natural groundwater recharge dynamics the analysis of observed recharge events abstract the amounts and time distribution of groundwater recharge from precipitation over an approximately 19 month period were investigated at two instrumented sites in south central kansas precipitation and evapotranspiration sequences soil moisture profiles and storage changes water fluxes in the unsaturated zone and hydraulic gradients in the saturated zone at various depths soil temperatures water table hydrographs and water level changes in nearby wells clearly depict the recharge process antecedent moisture conditions and the thickness and nature of the unsaturated zone were found to be the major factors affecting recharge although the two instrumented sites are located in sand dune environments in areas characterized by shallow water table and subhumid continental climate a significant difference was observed in the estimated effective recharge the estimates ranged from less than 2 5 to approximately 154 mm at the two sites from february to june 1983 the main reasons for this large difference in recharge estimates were the greater thickness of the unsaturated zone and the lower moisture content in that zone resulting from lower precipitation and higher potential evapotranspiration for one of the sites effective recharge took place only during late winter and spring no summer or fall recharge was observed at either site during the observation period of this study

an investigation of ionospheric responses and disturbance thermospheric winds during magnetic storms over south american sector 1 this paper presents the results of a study of the ionospheric and thermospheric response to magnetic storm disturbances for four stations covering a wide range of latitude in the south american region the study is based on the f layer critical frequency fof2 and the peak height hmf2 from fortaleza fz which is equatorial cachoeira paulista cp which is low latitude concepcion con a midlatitude site and king george island kgi a middle to high latitude site equivalent neutral winds are extracted from the measured hmf2 by employing the field line interhemispheric plasma flip model the equatorial ionization anomaly eia can undergo enhancement due to magnetospheric disturbance electric field that penetrates to low latitudes during the growth phase of a storm substorm whereas eia inhibition occurs more often under disturbance dynamo dd electric field season dependent meridional transequatorial winds can significantly alter the eia response to disturbance electric fields large amplitudes of dd electric field is observed during evening and morning hours in equinoctial but not in winter months over the brazilian low latitude of large westward magnetic declination angle large equatorward meridional wind surges seem to be helped by the presence of disturbance zonal westward winds the low latitude disturbance can in some cases be traced to specific disturbance fronts originating promptly from auroral activity enhancements this study confirms earlier studies that show that the quiet time meridional wind increases with increase of latitude and that over midlatitude it shows decreases with increase of solar flux due to the effect of the ion drag force this study further shows that the disturbance meridional wind and the intensity of dd electric field increase with increasing intensity of high latitude energy input the negative ionospheric storm phase over middle latitude observed in equinox and not in winter is also intensified with the increasing intensity of high latitude energy input

a three dimensional simulation of crustal deformation accompanied by subduction in the tokai region central japan abstract a great earthquake is expected to occur along the suruga trough where the philippine sea plate subducts beneath the eurasian plate we estimate crustal deformation prior to this hypothetical earthquake called the tokai earthquake based on a simulation model for plate subduction a three dimensional extension of the models by stuart 1988 and kato and hirasawa 1996 in this model the time evolution of a dislocation field in an elastic half space is governed by a quasi static interaction induced by the dislocation field itself as well as by a rate and state dependent friction law acting on the plate interface observed geophysical constraints taken into account are the shapes of the curved subducting plate interface the recurrence interval of past great earthquakes 90 150 years the average seismic coupling coefficient 0 5 the annual rate of level change 5 mm year between the two bench marks at hamaoka and kakegawa and the backslip distribution on the plate interface estimated from the result of gps observation the simulation shows the following the occurrence of great earthquakes is approximately periodic during interseismic times a strongly coupled region is formed in a seismic zone and it shrinks as the next great earthquake approaches the strongly coupled region is confined to an area where the plate inclination is gentle the rupture of the earthquake starts at a point close to the maximum shear stress acting on the plate boundary subsidence of hamaoka relative to kakegawa changes to uplift several years before the earthquake volumetric strain undergoes a change from compression to extension or vice versa at certain observation sites several years before the earthquake changes in volumetric strain accelerate as the occurrence of the earthquake impends the maximum strain change for 1 day observed on the ground surface just before the earthquake is 10 8 to 10 7 a value about one tenth of the value predicted by a related two dimensional model the pattern of shear stress accumulation and the location of the preslip are affected by the shape of the plate interface

equatorial ionospheric vertical plasma drift model over the brazilian region comparison between equatorial inospheric f region vertical plasma drift from satellite measurements fejer et al 1995 for the brazilian longitude sector and the drifts derived from ionosonde measurements around sunset shows significant differences on the prereversal peak behavior during solstices of high solar activity periods using ionosonde measurements around sunset and satellite measurements at other local times we constructed an ionospheric vertical plasma drift model that is representative of the equatorial region over the brazilian longitudes where the magnetic declination is around 20 the so derived drift model here called idm ionosonde drift model is used as an input to the sheffield university plasmasphere ionosphere model supim it is shown that the f layer heights given by supim with idm are in good agreement with ionosonde measurements over the brazilian longitudes and that idm better simulates the f layer heights than the averaged drifts given by the satellite drift model fejer et al 1995

on the fresh development of equatorial plasma bubbles around the midnight hours of june solstice using the 47 mhz equatorial atmosphere radar ear at kototabang indonesia the nocturnal evolution of equatorial plasma bubbles epbs were examined during the moderate solar activity years 2011 2012 while the evolution of epbs were mostly 86 confined to post sunset hours 1900 2100 lt during equinoxes in contrast the majority of epbs 71 in june solstice found evolve around the midnight hours 2200 0300 lt the mechanisms behind the fresh evolution of summer time midnight epbs were investigated for the first time through sami2 model simulations with a realistic input of background exb drift variation derived from cindi ivm on board c nofs satellite the term by term analysis of linear growth rate of rt instability indicates that the formation of high flux tube electron content height gradient kf steep vertical gradient region at higher altitudes is the key factor for the enhanced growth rate of rt instability the responsible factors are discussed in light of relatively weak westward zonal electric field in the presence of equatorward neutral wind and bottom side recombination around the midnight hours of june solstice the effects of neutral winds and weak westward electric fields on the uplift of equatorial f layer were examined separately using controlled sami2 simulations the results indicate that relatively larger linear growth rate is more likely to occur around midnight during june solstice because of relatively weak westward electric field than other local times in the presence of equatorward meridional wind

thermal mechanical effects of low temperature plasticity the peierls mechanism on the deformation of a viscoelastic shear zone we studied for the first time the effects of low temperature plasticity on the formation of shear zones a thermal mechanical model has been developed for describing the shear deformation of maxwell viscoelastic material with a rheology close to dry olivine we employed a one dimensional model with a half width of l deforming under a constant velocity u at the boundary and the spatially averaged strain rate u l was set to o 10 14 s 1 in addition to diffusion and power law creep we included deformation by low temperature plasticity called the peierls mechanism which is significant at low temperatures and has a strong exponential dependence on the stress when a sufficient magnitude of heat is generated by the rapid conversion from elastically stored energy into viscous dissipation thermal instability takes place and the deformation localizes in a narrow region by comparing the condition for thermal instability we found that the low temperature plasticity inhibits the development of thermal instability in shear zones in case of constant strain rate the peierls mechanism enhances deformation at a significantly lower stress compared to the rheology with solely diffusion creep and power law creep the enhanced deformation by low temperature plasticity produces lower amount of dissipative heating and thus stabilizes the shear zone comparing the stability between constant strain rate and constant stress boundary conditions we found that the peierls mechanism exerts an opposite destabilizing effect in the case of constant stress for dry olivine rheology and realistic magnitude of the strain rate the effect of low temperature plasticity is significant for temperatures between around 800 k and 1000 k this finding suggests that the low temperature plasticity may be crucial in determining the thermal mechanical stability in the shallow portion of slabs

high velocity frictional properties of gabbro high velocity friction experiments have been performed on a pair of hollow cylindrical specimens of gabbro initially at room temperature at slip rates from 7 5 mm s to 1 8 m s with total circumferential displacements of 125 to 174 m and at normal stresses to 5 mpa using a rotary shear high speed friction testing machine steady state friction increases slightly with increasing slip rate at slip rates to about 100 mm s velocity strengthening and it decreases markedly with increasing slip rate at higher velocities velocity weakening steady state friction in the velocity weakening regime is lower for the non melting case than the frictional melting case due perhaps to severe thermal fracturing a very large peak friction is always recognized upon the initiation of visible frictional melting presumably owing to the welding of fault surfaces upon the solidification of melt patches frictional properties thus change dramatically with increasing displacement at high velocities and such a non linear effect must be incorporated into the analysis of earthquake initiation processes

cosmic formosat 3 observations of equatorial f region irregularities in the saa longitude sector 1 the data of ionospheric electron density and s4 scintillation index from cosmic formosat 3 gps occultation soundings are used to study the morphology of the equatorial f region irregularity efi in the longitude sector 0 75 w where the south atlantic magnetic anomaly saa is located launched into low earth orbits on 14 april 2006 the six satellite constellation cosmic provides daily global measurements of the ionosphere at altitudes below 800 km the large data set enables us a statistically correlative study of electron density depletion and gps l1 scintillation which allows the global and altitudinal distributions of irregularity to be examined under solar minimum and geomagnetic quiet conditions postsunset efi scintillation events are found to concentrate in the saa longitude sector during northern winter months d months but nearly disappeared in the same longitude sector during the opposite season j months the d months average pattern of efi reveals that most of irregularities occurred in the bottom side f region 200 350 km altitude and at low magnetic latitudes adjacent to the locations of the maximum postsunset nmax and hmax this is consistent with the early postsunset irregularities that are generated in the region where plasma upward drifts have been greatly enhanced by examining the ut snapshots of the global distributions of electron density and scintillation events strong spatial correlations are found to exist among the sunset terminator the westward declined geo magnetic field strong scintillation event and density depletion in the saa longitude sector only during d months where the e region sunset terminator is most possibly parallel to the magnetic field direction at equatorial latitudes density depletion structures within this longitude sector were bounded by two postsunset ionization enhancement bands eia that extended over central saa and its conjugated region respectively such features provide further evidence that the enhanced conductivity gradients due to particle induced ionization in the vicinity of saa contribute additional electric field enhancements in the equatorial f region at the walls of the depletion region near the sunset lines prominent horizontal density gradients exist in both westward and southward directions due to sun setting and saa ionization enhancement respectively the combination of horizontal density gradients with antiparallel north eastward neutral winds during d months can contribute additional growth rate to the efi generation these cosmic observations not only confirm that the sunset equatorial electrodynamics plays a key role in controlling the seasonal and longitudinal occurrences of the quiet time efi but also reveal that seasonally dependent ionospheric responses to the energetic particle precipitation in saa can affect considerably the morphology of efi in the saa longitude sector

magnetic declination control of the equatorial f region dynamo electric field development and spread f we have carried out a comparative study of the evening prereversal enhancements in the equatorial f region vertical ionization drift velocities vz over fortaleza 4 s 38 w brazil and jicamarca 12 s 77 w peru two magnetic equatorial stations in the american zone the results show profound dissimilarities in the seasonal trends in the times and widths of the vz prereversal peak which reflect in the spread f characteristics as well at the two stations the dissimilarities are shown to be arising mainly from the difference in the magnetic field declination angles that causes differences in the conjugate e region sunset durations and hence in the f region polarization electric field development rates at the two stations

global distribution of density irregularities in the equatorial ionosphere we analyzed measurements of ion number density made by the retarding potential analyzer aboard the atmosphere explorer e ae e satellite which was in an approximately circular orbit at an altitude near 300 km in 1977 and later at an altitude near 400 km large scale 60 km density measurements in the high altitude regions show large depletions of bubble like structures which are confined to narrow local time longitude and magnetic latitude ranges while those in the low altitude regions show relatively small depletions which are broadly distributed in space for this reason we considered the altitude regions below 300 km and above 350 km and investigated the global distribution of irregularities using the rms deviation n n over a path length of 18 km as an indicator of overall irregularity intensity seasonal variations of irregularity occurrence probability are significant in the pacific regions while the occurrence probability is always high in the atlantic african regions and is always low in the indian regions we find that the high occurrence probability in the pacific regions is associated with isolated bubble structures while that near 0 longitude is produced by large depletions with bubble structures which are superimposed on a large scale wave like background considerations of longitude variations due to seeding mechanisms and due to f region winds and drifts are necessary to adequately explain the observations at low and high altitudes seeding effects are most obvious near 0 longitude while the most easily observed effect of the f region is the suppression of irregularity growth by interhemispheric neutral winds

global equatorial plasma bubble occurrence during the 2015 st patrick s day storm an analysis of the occurrence of equatorial plasma bubbles epbs around the world during the 2015 st patrick s day geomagnetic storm is presented a network of 12 global positioning system receivers spanning from south america to southeast asia was used in addition to colocated vhf receivers at three stations and four nearby ionosondes the suppression of postsunset epbs was observed across most longitudes over 2 days the epb observations were compared to calculations of the linear rayleigh taylor growth rate using coupled thermosphere ionosphere modeling which successfully modeled the transition of favorable epb growth from postsunset to postmidnight hours during the storm the mechanisms behind the growth of postmidnight epbs during this storm were investigated while the latter stages of postmidnight epb growth were found to be dominated by disturbance dynamo effects the initial stages of postmidnight epb growth close to local midnight were found to be controlled by the higher altitudes of the plasma i e the gravity term modeling and observations revealed that during the storm the ionospheric plasma was redistributed to higher altitudes in the low latitude region which made the plasma more susceptible to rayleigh taylor growth prior to the dominance of the disturbance dynamo in the eventual generation of postmidnight epbs

geochemistry radiocarbon ages and paleorecharge conditions along a transect in the central high plains aquifer southwestern kansas usa abstract water samples from short screen monitoring wells installed along a 90 km transect in southwestern kansas were analyzed for major ions trace elements isotopes h b c n o s sr and dissolved gases he ne n2 ar o2 ch4 to evaluate the geochemistry radiocarbon ages and paleorecharge conditions in the unconfined central high plains aquifer the primary reactions controlling water chemistry were dedolomitization cation exchange feldspar weathering and o2 reduction and denitrification radiocarbon ages adjusted for c mass transfers ranged from 10 ka to reduce the average no3 concentration in recent recharge to the holocene background concentration

extreme precipitation drives groundwater recharge the northern high plains aquifer central united states 1950 2010 future extreme precipitation ep daily rainfall amount over certain thresholds is projected to increase with global climate change however its effect on groundwater recharge has not been fully explored this study specifically investigates the spatiotemporal dynamics of groundwater recharge and the effects of extreme precipitation daily rainfall amount over the 95th percentile which is tagged by ranking the percentiles in each season for a base period on groundwater recharge from 1950 to 2010 over the northern high plains nhp aquifer using the soil water balance model the results show that groundwater recharge significantly p 0 05 increased in the eastern nhp from 1950 to 2010 where the highest annual average groundwater recharge occurs compared to the central and the western nhp in the eastern nhp 45 1 of the annual precipitation fell as ep which contributed 56 8 of the annual total groundwater recharge in the western nhp 30 9 of the annual precipitation fell as extreme precipitation which contributed 62 5 of the annual total groundwater recharge in addition recharge by extreme precipitation mainly occurred in late spring and early summer before the maximum evapotranspiration rate which usually occurs in mid summer until late fall a dry site in the western nhp and a wet site in the eastern nhp were analysed to indicate how recharge responds to ep with different precipitation regimes the maximum daily recharge at the dry site exceeded the wet site when there was ep when precipitation fell as non extreme rainfall most recharge was less than 5 mm at both the dry and wet sites and the maximum recharge at the dry site became lower than the wet site this study shows that extreme precipitation plays a significant role in determining groundwater recharge 2016 the authors hydrological processes published by john wiley sons ltd

slip patterns and earthquake populations along different classes of faults in elastic solids numerical simulations of slip instabilities on a vertical strike slip fault in an elastic half space are performed for various models belonging to two different categories the first category consists of inherently discrete cellular fault models such are used to represent fault systems made of segments modeled by numerical cells that can fail independently of one another their quasi independence is assumed to provide an approximate representation of strong fault heterogeneity due to geometric or material property disorder that can arrest ruptures at segment boundaries the second category consists of models having a well defined continuum limit these involve a fault governed by rate and state dependent friction and are used to evaluate what types of property heterogeneity could lead to the quasi independent behavior of neighboring fault segments assumed in the first category the cases examined include models of a cellular fault subjected to various complex spatial distributions of static to kinetic strength drops and models incorporating rate and state dependent friction subjected to various spatial distributions of effective stress normal stress minus pore pressure the results indicate that gradual effective stress variations do not provide a sufficient mechanism for the generation of observed seismic response strong and abrupt fault heterogeneity as envisioned in the inherently discrete category is required for the generation of complex slip patterns and a wide spectrum of event sizes strong fault heterogeneity also facilitates the generation of rough rupture fronts capable of radiating high frequency seismic waves the large earthquakes in both categories of models occur on a quasi periodic basis the degree of periodicity increases with event size and decreases with model complexity however in all discrete segmented cases the models generate nonrepeating sequences of earthquakes and the nature of the large quasi periodic events is highly variable the results indicate that expectations for regular sequences of earthquakes and or simple repetitive precursory slip patterns are unrealistic the frequency size fs statistics of the small failure episodes simulated by the cellular fault models are approximately self similar with b 1 2 and ba 1 where b and ba are b values based on magnitude and rupture area respectively for failure episodes larger than a critical size however the simulated statistics are strongly enhanced with respect to self similar distributions defined by the small events this is due to the fact that the stress concentrated at the edge of a rupture expanding in an elastic solid grows with the rupture size when the fault properties e g geometric irregularities are characterized by a narrow range of size scales the scaling of stress concentrations with the size of the failure zone creates a critical rupture area terminating the self similar earthquake statistics in such systems events reaching the critical size become on the average unstoppable and they continue to grow to a size limited by a characteristic model dimension when however the system is characterized by a broad spectrum of size scales the above phenomena are suppressed and the range of apparent self similar fs statistics is broad and characterized by average b and ba values of about 1 the simulations indicate that power law extrapolations of low magnitude seismicity will often underestimate the rate of occurrence of moderate and large earthquakes the models establish connections between features of fs statistics of earthquakes range of self similar regimes local maxima and structural properties of faults dominant size scales of heterogeneities dimensions of coherent brittle zones the results suggest that observed fs statistics can be used to obtain information on crustal thickness and fault zone structure

comparison between thermal viscous coupling and frictional sliding abstract we investigated the similarity between thermal viscous coupling tvc and frictional sliding proposed by kameyama and kaneda pure appl geophys 159 2002 2011 we consider a one dimensional layer composed of viscous material which is sandwiched and sheared by two thick elastic layers the rate of viscous deformation depends on the temperature t c in the viscous layer as well as shear stress the temperature t c changes owing to heating by viscous dissipation and conductive cooling we carried out velocity stepping tests for the steady state deformation both numerically and analytically and compared the temporal evolution of small perturbations with that of the spring block model with rate and state dependent friction rsf we found that as is the case of frictional slip stability the manner of temporal evolution is classified into four regimes depending on whether it is stable or not and whether it is monotonous or oscillatory with time by further interpreting tvc in terms of general rsf theory by ruina j geophys res 88 1983 10359 we obtained the relations between the parameters appearing in the phenomenological rsf law and the nondimensional parameters which characterize the nature of tvc a further improvement of this approach might be important for estimating the actual values of frictional constitutive parameters at the deeper portion of seismogenic faults of interplate or inland earthquakes where a ductile deformation is expected to be significant

comparison between nighttime ionosonde incoherent scatter radar ae e satellite and hf doppler observations of f region vertical electrodynamic plasma drifts in the vicinity of the magnetic equator 1 nighttime f region vertical drifts were made using an ionosonde for the equatorial station ibadan 7 4 n 3 9 e 6 s dip from 1 year of data during 1957 1958 international geophysical year igy that corresponds to a period of solar maximum for undisturbed condition we compare the seasonal vertical drifts with measurements made by incoherent scatter radar ae e satellite and hf doppler for equatorial f region vertical drifts we find a comparable variability pattern during periods of high f layer heights during equinox and the december solstice and the opposite behavior occurs during june solstice the drifts are predominantly downward between 2000 and 0500 lt intervals ionosonde drifts are smaller in values by either a factor of two or three than other methods except for consistent june solstice ionosonde and satellite magnitudes the equinoctial average prereversal enhancements measured by the four techniques are roughly comparable about 36 m s and occur at the same local time 1900 lt for all the seasons the evening reversal times are similar apart from june solstice that exhibits large variations the morning reversal times are also in accord except for the equinoctial jicamarca drift our observations indicate that ionosonde drifts measurements are in better agreement with vertical drifts results at other equatorial stations

forecast model for great earthquakes at the nankai trough subduction zone an earthquake instability model is formulated for recurring great earthquakes at the nankai trough subduction zone in southwest japan the model is quasistatic two dimensional and has a displacement and velocity dependent constitutive law applied at the fault plane a constant rate of fault slip at depth represents forcing due to relative motion of the philippine sea and eurasian plates the model simulates fault slip and stress for all parts of repeated earthquake cycles including post inter pre and coseismic stages calculated ground uplift is in agreement with most of the main features of elevation changes observed before and after the m 8 1 1946 nankaido earthquake in model simulations accelerating fault slip has two time scales the first time scale is several years long and is interpreted as an intermediate term precursor the second time scale is a few days long and is interpreted as a short term precursor accelerating fault slip on both time scales causes anomalous elevation changes of the ground surface over the fault plane of 100 mm or less within 50 km of the fault trace

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1