review of magnetic and electric field effects near active faults and volcanoes in the u s a abstract synchronized measurements of geomagnetic field have been recorded along 800 km of the san andreas fault and in the long valley caldera since 1974 and during eruptions on mount st helens since 1980 for shorter periods of time continuous measurements of geoelectric field measurements have been made on mount st helens and near the san andreas fault where moderate seismicity and fault slip frequently occurs significant tectonic and volcanic events for which nearby magnetic and electric field data have been obtained include 1 two moderate earthquakes m l 5 8 for which magnetometers were close enough to expect observable signals about three source lengths 2 one moderate earthquake m s 7 3 for which magnetometers were installed as massive fluid outflow occurred during the post seismic phase 3 numerous fault creep events and moderate seismicity 4 a major explosive volcanic eruption and numerous minor extrusive eruptions and 5 an episode of aseismic uplift for one of the two earthquakes with m l 5 8 seismomagnetic effects of 1 3 and 0 3 nt were observed for this event magnetometers were optimally located near the epicenter and the observations obtained are consistent with simple seismomagnetic models of the event similar models for the other event indicate that the expected seismomagnetic effects are below the signal resolution of the nearest magnetometer precursive tectonomagnetic effects were recorded on two independent instruments at distances of 30 and 50 km from a m l 5 2 earthquake longer term changes were recorded in one region in southern california where a moderate m l 5 9 earthquake has since occurred surface observations of fault creep events have no associated magnetic or electrical signature above the present measurement precision 0 25 nt and 0 01 respectively and are consistent with near surface fault failure models of these events longer term creep is sometimes associated with corresponding longer term magnetic field perturbations correlated changes in gravity magnetic field areal strain and uplift occurred during episodes of aseismic deformation in southern california primarily between 1979 and 1983 because the relationships between these parameters agrees with those calculated from simple deformation and tectonomagnetic models the preferred explanation appeals to short term strain episodes independently detected in each data set an unknown source of meteorologically generated noise in the strain gravity and uplift data and an unknown but correlated disturbance in the absolute magnetic data might also explain the data no clear observations of seismoelectric or tectonoelectric effects have yet been reported the eruption of mount st helens generated large oscillatory fields and 9 2 nt offset on the only surviving magnetometer a large scale traveling magnetic disturbance passed through the san andreas array from 1 to 2 h after the eruption subsequent extrusive eruptions generated small precursory magnetic changes in some cases these data are consistent with a simple volcanomagnetic model magneto gas dynamic effects and a blast excited traveling ionospheric disturbance traveling ionospheric disturbances tids also generated by earthquake related atmospheric pressure waves may explain many electromagnetic disturbances apparently associated with earthquakes local near fault magnetic field transients rarely exceed a few nt at periods of a few minutes and longer

postglacial land uplift patterns of south sweden and the baltic sea region comparison of the land uplift pattern for the last 10 300 years shown by studies of raised shorelines of the baltic ice lake with the pattern of present day land uplift of fennoscandia shows that significant regional changes of uplift pattern have taken place some of these changes seem to be related to a halt in ice retreat during the younger dryas cold stage it is also probable that some observed anomalies in the present uplift are not of glacio isostatic origin but are possibly related to structures in the lower lithosphere and upper mantle or large scale tectonics

the 2004 parkfield earthquake test of the electromagnetic precursor hypothesis 1 a controversy has existed for 30 years concerning the possibility of earthquake prediction using electromagnetic precursors long term electromagnetic monitoring prior to during and after the m6 0 earthquake at parkfield california on 28 september 2004 now provides a definitive test of this hypothesis during the earthquake our instruments recorded clearly documented electrical signals from an earthquake impulsive changes of up to 2 5 mv at an electrode located 250 m from the rupture zone followed by a transient decay lasting at least 3 1 4 hours similar signals 1 5 mv were recorded with the only two m 5 aftershocks but their transient decays lasted for only 17 min these signals are unambiguously a result of the earthquake and can be useful in studies of fluid flow in faults a major current topic in earthquake physics patterns in the distribution of transient voltages differ from static stress changes but are consistent with a small coseismic pressure drop on the fault and fluid flow inward signals at a distance of 1 km from the fault are less than 1 mv and are statistically no different from zero in our analysis transient voltages for the aftershocks have opposite polarity from those recorded with the main shock and are again maximal close to the fault suggesting subsequent outward flow from the fault finally there is no evidence of any precursory signal strongly calling into question previously suggested signals preceding smaller or more distant earthquakes

investigations of fennoscandian glacial isostatic adjustment using modern sea level records we present a new method for determining the rates from a network of regional tide gauge data the method involves using a model for the sea level observations annual averages of relative sea level that explicitly includes a term for the interannual sealevel variations which are assumed to be constant for all sites in the network the resulting simultaneous analysis of all the sea level observations yields sea level rates which are biased by the unknown temporal slope of the interannual variations by differencing sea level rate estimates with respect to one site site referenced sea level rates are calculated which are free from this bias the differencing procedure also removes any signal due to a common sea level rate such as might be associated with eustatic sea level rise the resulting site referenced sea level rates therefore in principle reflect only the relative vertical crustal motions of the tide gauge sites with respect to the geoid these site referenced sea level rates represent a potentially more accurate data set than have previously been used for sea level studies we have used these rates to investigate glacial isostatic adjustment gia in fennoscandia specifically we have used the site referenced sea level rates within the baltic sea to estimate adjustments to the ice history and earth model which are used to calculate vertical rates of adjustment due to gia a method never before used for fennoscandia the parameters selected were lithospheric thickness the viscosities of assumed isoviscous upper and lower mantles and a fennoscandian ice thickness scaling parameter we present the results of this estimation procedure for a specific test ice model we also find evidence that the sea level record for the site nearest the center of uplift furuogrund may be contaminated by systematic errors of unknown origin data from this site have previously been used to determine a rate for the maximum uplift that we find is perhaps 1 mm yr 1 or 10 too large

physical properties of the variations of the electric field of the earth preceding earthquakes ii determination of epicenter and magnitude abstract as reported in the preceding paper a transient change of the electric field of the earth seismic electric signal hereafter called ses appears many hours before an earthquake eq by measuring this change in a given direction and dividing it with a suitable relative effective resistivity one obtains a quantity that reflects the current density in this direction measurements in two directions e w and n s give the relative signal intensity jrel at the station under consideration by measuring jrel at a number of stations and considering that it attenuates according to a 1 r law the epicenter can be determined with an accuracy usually around 100 km once the epicenter has been determined the product jrel r can be evaluated so that the magnitude m can be estimated by resorting to an empirical log jrel r versus m plot the uncertainty of m is around 0 5 units following sobolev 1975 and for the statistics to be beyond any doubt predictions were officially documented before the eq occurrence for 23 earthquakes with a magnitude equal or greater than ms 5 0 two events were missed the present method is compared to other electrical methods used in china japan and soviet union a number of problems concerning the origin of the effect its directivity and the attenuation with distance remain open for further studies

monitoring resistivity changes prior to earthquakes in parkfield california with telluric arrays a telluric array has been designed to monitor long period changes of resistivity in parkfield prior to the next characteristic earthquake there the array straddles the san andreas fault and utilizes dipoles which range from 5 to 18 km long a method of data analysis using robust processing in the time domain and singular value decomposition yields daily estimates of telluric coefficients which are stable to within 1 the array has detected several fluctuations above the noise level of the measurements most of these fluctuations are uncorrelated with bursts of seismic activity changes in water levels anomalous strain and creep or magnetic storms however the largest fluctuation in 2 years coincided with a tensor strain anomaly and both of these preceded a m 3 7 earthquake by approximately 1 month this is the largest earthquake to occur in parkfield since the array was installed

monitoring resistivity change in parkfield california 1988 1995 continuous electric field measurements from 1988 1995 with a telluric array straddling the san andreas fault in parkfield california have been analyzed for changes in electrical resistivity associated with the four largest mb 4 0 local earthquakes and three larger but more distant california earthquakes 1989 loma prieta 1992 landers 1994 northridge treatment of the propagation of errors through the analysis process yields formal estimates of the uncertainties in the results and thus a definition of anomalous variations based on statistical confidence statistically significant variations of the telluric coefficients were associated with none of the four local earthquakes and none of the california earthquakes an obvious but mistaken conclusion from this disappointing result is that resistivity changes do not occur prior to earthquakes previous observations in both the field and laboratory have shown that they do occur a more likely explanation of the current result is that the telluric array is not sufficiently sensitive to resistivity changes associated with such small earthquakes the next parkfield earthquake while overdue should be of sufficient magnitude to test this explanation results from this experiment have shown the need for continuous redundant measurements in precursory monitoring studies

along strike variations in the electrical structure of the san andreas fault at parkfield california magnetotelluric exploration has been used to image along strike variations in the electrical resistivity structure of the san andreas fault at parkfield california a low resistivity wedge extending to a depth of several kilometers is continuous over a horizontal distance of 8 km the base of the wedge is coincident with the shallowest microearthquakes a change in the electrical and fluid connection of the san andreas fault with a low resistivity zone in the franciscan formation is observed along the parkfield segment

quaternary stratigraphy of the fladen area central north sea a multidisciplinary study based on detailed stratigraphic investigations on a 200 6m long core bgs borehole no 81 26 from the fladen ground area british sector core material from the sleipner field norwegian sector and shallow seismic profiles between the core sites the following conclusions are drawn 1 the north sea was glaciated sometime during th elaterpart of matuyama reversed period a complete glacial interglacial glacial cycle is recorded in these sediments 2 in a period of marine sedimentation in the middle pleistocene a transgression regression cycle under boreal arctic regime is recorded 3 the fladen area has subsided between 0 9 and 0 6 m ka through the later parts of the quaternary 4 a major glacial event dated at between 130 and 200 ka is recorded as a thick till unit in 81 26 this till which was deposited by ice moving from the southwest scotland probably represents a period when the scandinavian and british ice sheets coalesced in the north sea 5 based on the seismic data and the stratigraphy of the sleipner core an ice free open embayment dry land is favoured for the central north sea during the late weichselian 6 from the amino acid data it is shown that there has been an episodic style of sedimentation through the quaternary 7 of the investigated sediments which span the last 1 mill year ca 98 have been deposited under arctic to boreal arctic conditions

the moho depth distribution in fennoscandia and the regional tectonic evolution from archean to permian times abstract from a variety of seismic refraction and reflection profiling results a new detailed moho depth map for the fennoscandian part of the baltic shield has been compiled a complementary crystalline crustal thickness map was compiled for denmark the skagerrak sea offshore western norway and adjacent areas where sedimentary strata reach maximum thicknesses of 10 km and even more these maps are discussed in relation to tectonic evolution and crustal ages of fennoscandia seismic velocities bouguer gravity and seismicity areas deviating from the proposed age thickness relationships are the kola peninsula archean and the oslo rift permian the average crustal thickness in the former area is around 44 km while the latter area is associated with a moho elevation of 3 5 km maximum crustal thicknesses in excess of 50 km have been found in southwestern and central finland and correlate with a crust of proterozoic age the caledonides of western norway do not exhibit as elsewhere a crustal root sub moho p n and s n seismic velocities are relatively small in that area and bouguer gravity values are strongly negative intracrustal p velocity variations do not seem to reflect geological age or specific tectonic features although the seismicity is moderate in fennoscandia most of the earthquake activity and in particular the largest earthquakes are confined to areas where the crustal thinning is most pronounced

east european river runoff and black sea and caspian sea level changes as simulated within the paleoclimate modeling intercomparison project during the last late pleistocene and post glacial epochs the caspian sea fluctuated between regressive and transgressive stages the black sea experienced fluctuations too but their amplitude was smaller because they were mainly controlled by the world ocean due to water exchange through the bosporus strait in the water budget equation the change in storage over a time period is balanced by the sum of the inflows and outflows that occur during the time period they can be calculated from atmospheric general circulation models gcms a modeling initiative the palaeoclimate modelling intercomparison project pmip could help to resolve this question the pmip has focused on two slices of the past the mid holocene 6 ka calendar years before present bp and the last cold event of the late quaternary 21 ka calendar years bp the pmip gcm simulations indicate that changes in level are primarily influenced by changes of river runoff ocean precipitation and evaporation together play a minor role at 21 ka bp the estimated level lowering was 50 and 200 m for the caspian sea and black sea respectively this lends credit to the idea of the connection between deep regressive states of caspian sea and black sea and mature stages of late quaternary glacial cooling drying planetary events

co seismic geoelectric potential changes observed in japan co seismic signals of geoelectric potential have been observed for several earthquakes eqs in japan it has been argued that if there are precursory changes stronger signals should be at the time of eq our results prove that co seismic signals do exist but they start with the arrival of seismic waves and not at the origin time of eqs their amplitude does not scale with the dipole length both facts indicate that the changes are local effects of passing seismic waves and not due to electric signals emitted from the seismic source observed changes contain two components oscillatory and offset decay ones the former is synchronized with seismic waves whereas the latter with a rise time of a few seconds and a decay time of 0 1 1 hour is suspected to be related to eq induced subsurface hydrological change in both cases the origin of electric potential variations may be electrokinetic

the determination of rates of land uplift in norway it is the task and the responsibility of the geodetic section within the geographical survey of norway to establish and maintain the national horizontal and vertical geodetic primary control networks of that country provided that relative crustal movements take place within norway they will cause deformations to these networks there is no geodetic proof that any horizontal deformations of the surface layer of the crust take place in continental norway and there is no established geological evidence of such movements either in 1984 a small geodetic control network was established and measured for the first time across a possibly active fault it is not expected that horizontal movements will cause any serious problems in the maintenance of the horizontal network vertical movements of the ground however are causing us problems both in the establishment and in the maintenance of a national height system

isostatic topography and crustal depth corrections for the fennoscandian geoid abstract the medium wavelength geoid wavelengths the result of this study demonstrate that crustal thickness variations are important for the interpretation of the fennoscandian geoid or gravity however submoho deviatoric stresses might become very high about 200 mpa if the entire anomaly is assumed to be supported by lateral density variations at less than 80 km depth this implies that the pronounced negative anomaly is additionally maintained by dynamical flow inside the mantle

thermodynamics based constitutive framework for unsaturated soils 2 a basic triaxial model following the theoretical framework established in part 1 a basic elastoplastic unsaturated soil model is formulated the model is a combination of a soil skeleton model and an swcc soil water characteristic curve model for simplicity the model deals only with isotropic soils in triaxial compression space under a constant pore air pressure the paper shows that the central element in the model that differentiates itself from a saturated model is its dual plastic hardening which not only reflects the evolution of the internal structure of the soil skeleton but is also associated with the evolution of the solid fluid interactive structure for more flexible shear stress strain responses the non associated flow rule the alpha model by collins kelly based on a thermodynamic analysis is adopted the present model introduces six new material constants which can be calibrated following a systematic procedure the model successfully simulates the multiple series laboratory tests reported by sivakuma

the last interglacial glacial cycle in fennoscandia abstract the climate and glacier fluctuations during the last interglacial glacial cycle in fennoscandia seem to correspond roughly in time with the climate fluctuations recorded in france germany holland however the climate in fennoscandia was considerably colder and the coldest phases are recorded as periods with glacier fluctuations admittedly the exact age and amplitudes of some fluctuations are still much debated

land uplift and its implications on the geoid in fennoscandia abstract the rate of change of the geoid in fennoscandia has been determined on the assumption that the observed land uplift is accompanied by a viscous flow of mantle below the crust the model includes the deloading of water masses due to the uplift the result is compared with other numerical studies

continuous borehole strain and pore pressure in the near field of the 28 september 2004 m 6 0 parkfield california earthquake implications for nucleation fault response earthquake prediction and tremor near field observations of high precision borehole strain and pore pres sure show no indication of coherent accelerating strain or pore pressure during the weeks to seconds before the 28 september 2004 m 6 0 parkfield earthquake minor changes in strain rate did occur at a few sites during the last 24 hr before the earth quake but these changes are neither significant nor have the form expected for strain during slip coalescence initiating fault failure seconds before the event strain is stable at the 10 11 level final prerupture nucleation slip in the hypocentral region is constrained to have a moment less than 2 10 12 nm m 2 2 and a source size less than 30 m ground displacement data indicate similar constraints localized rupture nucleation and runaway precludes useful prediction of damaging earthquakes coseismic dynamic strains of about 10 microstrain peak to peak were superimposed on volumetric strain offsets of about 0 5 microstrain to the northwest of the epicenter and about 0 2 microstrain to the southeast of the epicenter consistent with right lateral slip observed strain and global positioning system gps offsets can be simply fit with 20 cm of slip between 4 and 10 km on a 20 km segment of the fault north of gold hill m 0 7 10 17 n m variable slip inversion models using gps data and seismic data indicate similar moments observed postseismic strain is 60 to 300 of the coseismic strain indicating incomplete release of accumulated strain no measurable change in fault zone compliance preceding or following the earthquake is indicated by stable earth tidal response no indications of strain change accompany nonvolcanic tremor events reported prior to and following the earthquake

zum auf und ab des meeresspiegels in skandinavien langer streit um eustasie oder isostasie the phenomenon of the rise of the scandinavian shield during the holocene and the concomitant fall in level of the baltic sea has been investigated for centuries already in medieval times there were reports about the coastlines of the gulf of bothnia that are full of relevant observations during the eighteenth century scientists such as celsius and linnaeus collected observations such as these the result was that the search for the possible explanations of this rise and fall phenomenon intensified the generally favoured explanation was that there was an active sinking of sea level in the baltic rather than an active rising of the land surface in fennoscandia this was because water was seen as mobile in contrast to a terra firma the relevant discussion was often emotional and here we try to illustrate it using material from the geologenarchiv freiburg von hoff von buch and goethe no more than a few decades later it became obvious by the theory of ice age that both the sea level and the land could be mobile eustatic sea level changes glacial isostasy additionally of course plate tectonics had some influence norway is situated at the western end of the eurasian plate and is part of a passive continental margin there are still open research problems many of which can be addressed using modern methods of satellite based geophysics and geodesy some other aspects as the permanent uplift trend of scandinavia since the cambrium or the rhythmic to and fro of magma in the upper mantle during the pleistocene are mentioned

autochthonous block fields in southern norway implications for the geometry thickness and isostatic loading of the late weichselian scandinavian ice sheet the consistent geographical and altitudinal distribution of autochthonous block fields mantle of bedrock weathered in situ and trimlines in southern norway suggests a multi domed and asymmetric late weichselian ice sheet low gradient ice sheet profiles in the southern baltic region in the north sea and along the outer fjord areas of southern norway are best explained by movement of ice on a bed of deforming sediment although water lubricated sliding or a combination of the two may not be excluded the ice thickness distribution of the late weichselian scandinavian ice sheet is not in correspondence with the modern uplift pattern of fennoscandia early holocene crustal rebound was apparently determined by an exponential glacio isostatic rise later however crustal movements appear to have been dominated by large scale tectonic uplift of the fennoscandian shield centred on the gulf of bothnia the region of maximum lithosphere thickness

late quaternary lake level record from northern eurasia lake records from northern eurasia show regionally coherent patterns of changes during the late quaternary lakes peripheral to the scandinavian ice sheet were lower than those today but lakes in the mediterranean zone were high at the glacial maximum reflecting the dominance of glacial anticyclonic conditions in northern europe and a southward shift of the westerlies the influence of the glacial anticyclonic circulation attenuated through the late glacial period and the westerlies gradually shifted northward such that drier conditions south of the ice sheet were confined to a progressively narrower zone and the mediterranean became drier the early holocene shows a gradual shift to conditions wetter than present in central asia associated with the expanded asian monsoon and in the mediterranean in response to local monsoon type circulation there is no evidence of mid continental aridity in northern eurasia during the mid holocene in contrast the circum baltic region was drier reflecting the increased incidence of blocking anticyclones centered on scandinavia in summer there is a gradual transition to modern conditions after ca 5000 yr b p although these broad scale patterns are interrupted by shorter term fluctuations the long term trends in lake behavior show a clear response to changes in insolation and glaciation

a review of the history of the baltic sea 13 0 8 0 ka bp abstract the dynamic history of the baltic sea during the millennia following the last deglaciation has been one of the main topics for many generations of quaternary geologists around the baltic sea based on the present state of knowledge and certain hypotheses a model for the development during the baltic ice lake yoldia sea and ancylus lake stages are presented the oresund strait was the threshold of the initial stage of the baltic ice lake and was eroded down to bedrock as a result of the isostatic uplift the emerging threshold forced the baltic ice lake to rise above sea level at ca 12 0 ka bp this gradual up damming ended at ca 11 2 ka bp when the large glacial lake was lowered 5 10m by possible subglacial drainage at mt billingen a glacial readvance during the younger dryas ended a ca 400 year long land bridge between sweden and the continent by blocking the former drainage route once again the baltic ice lake had to rise above sea level to be drained over the oresund strait at ca 10 5 ka bp a gradual ice recession began and 200 years later when the receding ice sheet could not withhold the up dammed water masses the final drainage of the baltic ice lake occurred which rapidly lowered the water level within the baltic basin with ca 25 m and ended the baltic ice lake stage the next stage the yoldia sea was characterized by a complex pattern of relative sea level changes with large differences within the baltic basin a very restricted baltic sea in the south and thus an extensive landbridge at ca 9 9 ka bp a short 100 200 years saline ingression is seen in the baltic sediment records from stockholm in the north to the southernmost parts of the yoldia sea the gradually shallower strait in south central sweden the narke strait ended the saline influence 300 400 years later the two remaining outlets west of lake vanern gota alv and otteid steinselva had become too shallow to swallow the outflowing yoldia sea waters at this stage the baltic lake vanern level could thus not fall in pace with the relative sea level fall outside the outlet areas this was the isolation of lake vanern and the baltic from the sea and the beginning of the ancylus lake since the water depth of the larger gota alv outlet had to be more or less maintained the ancylus lake level had to rise in pace with the uplift of the outlet region this caused a significant and rapid transgression 10 30 m in ca 300 years in the southern baltic which flooded large areas with recently immigrated pine forests since the oresund strait during the yoldia sea stage had been uplifted more than southerly areas the ancylus transgression ended with a new drainage pathway at ca 9 2 ka bp through the darss sill area in the southern baltic and northwards through the store belt strait this so called dana river had cut off the former large land bridge the quaternary deposits of this new threshold were eroded and a fairly rapid regression set in at ca 9 0 ka bp the dana river had eroded down to the sea level which isolated lake vanern from the ancylus lake and created a land bridge between southern sweden and northwards ca 800 years later when the sea level rose above the oresund threshold marine water could enter the baltic and at ca 8 0 ka bp brackish water characterized the southern baltic this marks the end of the ancylus lake

characteristics of electromagnetic noise in the ioannina region greece a possible origin for so called seismic electric signal ses the study of electromagnetic noise in the ioannina region nw greece shows the existence of long transient electric signals duration 10 s without measurable correlated magnetic signals as is the case for seismic electric signal ses observed in the van network of short term earthquake prediction our signals have an artificial origin and are emitted by different kinds of transmitters implanted in the ioannina region this origin can be clearly ascertained from simultaneous observation at stations in direct sight of the transmitters of four electromagnetic components two electric and two magnetic in a broad band of frequencies 10 3 10 hz relying on these observations it is proposed that the april 18 and 19 1995 electric signals interpreted by the van group as ses precursors to the may 13 kozani earthquake are due to some transmitter located far to the north of the ioa station of the van network the transmitters of this area would be of digital type as is increasingly common in radio telecommunication networks we believe that careful studies of electromagnetic noise in a broad range of frequencies should precede lengthy discussions of the statistical significance of recorded signals

reply to comment on seismomagnetic effects from the long awaited 28 september 2004 m 6 0 parkfield earthquake by m j s johnston y sasai g d egbert and r j mueller by p varotsos and s uyeda the comment by varotsos and uyeda 2008 vu hereafter does not have much to do with our article which reports electromagnetic data and their implications prior to during and following the 2004 m 6 parkfield earthquake eq in fact our article did not include any extensive discussion of the possible flaws in the seismic electric signal ses approach to eq prediction the four main points vu discuss in their comment are from a summary sentence in the introduction that is preceded by the phrase controversy about these ses results exists because 1 2 4 where 1 4 are the specific points that vu list we respond to each of these points in the following discussion a final comment made by vu concerns our observation that ses type signals are not seen in the data that we have we could have provided more detailed evidence about the lack of ses in the months preceding the earthquake but see little point in including long data plots that show nothing note that all of these data are freely available from the northern california earthquake data center ncedc www ncedc org last accessed october 2007 in our article we presented null results from sites above the parkfield eq rupture without extrapolating to the conclusion that sess do not exist anywhere in fact park et al 2007 report additional data from 10 20 km lines over the entire region that also show no evidence for ses type signals preceding the parkfield eq such signals were also not seen in 16 yr of previous monitoring with these lines park 1991 1997 furthermore no signals were seen in 2 yr of data on 10 km lines both along and within the fault and orthogonal to the fault a little farther to the north johnston 1989 to suggest that all of these observations resulted from an

physical properties of the variations of the electric field of the earth preceding earthquakes i abstract in an earlier paper we described the physical properties of the preseismic transient variations in the electrotelluric field the present letter clarifies a few points that were not properly emphasized in our previous publications furthermore recent results like that of the destructive kalamata earthquake on september 13 1986 indicate that the duration of a preseismic electric signal may reach considerably high values lasting up to several hours

plausibility of long distance electrotelluric precursors to earthquakes a systematic analysis of the plausibility of different classes of models which could account for low frequency precursory electrotelluric signals observable at large distances from a large earthquake is carried out first we consider linear models for which either a stationary electrical field is generated by a current dipole in the source volume or a static deformation field results from a precursory strain in the source volume linearly inducing a streaming potential effect near the electrode in the former case only the conjunction of very favorable circumstances may lead to an electrical field larger than 10 mv km at more than 100 km with the constraint that no anomaly exceeds a few volts per kilometer in the source area it indeed requires a powerful electrical dipole current 104 a length 1 km which might be produced by a streaming potential effect associated with the connection of nonhydrostatic fluid reservoirs pore pressure difference up to 100 mpa a two dimensional channeling of the electrical currents from the source to the electrode area which would result from high conductivity horizontal layers and finally a local amplification factor of typically 100 near the electrodes which might be produced by a narrowing of the conductive channel for an elastic long distance interaction an electric field of 10 mv km at a distance 10 times the source length 100 km away from a m 6 earthquake requires a precursory deformation larger than the coseismic deformation if no strong local amplification is considered alternatively nonlinear models are proposed and discussed assuming that the crust is stuffed with small scale local instabilities fractures and nonhydrostatic fluid reservoirs among which a number are close to their instability threshold these instabilities may follow their own evolutionary process inducing sporadic fluid flow and hence streaming potential effect in the vicinity of the electrodes without any relation to a remote tectonic activity however these instabilities may be sensitive to very small changes in the stress field and hence be triggered by the elastic precursory deformation of a remote seismic source this requires that the precursory deformation near the instability is at least larger than the tidal strain 10 7 this necessary condition is met at a distance of up to 10 times the source dimension when the precursory strain source is only 10 of the coseismic deformation provided that the strain source is double the size of the seismic fault itself this model could account for the absence of self potential anomalies related to the earthquake itself as a large proportion of the available instabilities could have been swept away by the precursory strain furthermore with such a model a precursory strain in the source area could induce a detectable change in the background seismic activity over a vast area which may give a new interpretation of the times of increased probability observations for short distances the triggering of fluid instabilities by strain perturbation and the associated streaming potential effect may well be a promising model for explaining low frequency electric and magnetic precursors

strontium isotopic composition of modern and holocene mollusc shells as a palaeosalinity indicator for the baltic sea abstract the 87sr 86sr isotope ratio in subfossil carbonate mollusc shells from raised beach sediments is used as a palaeosalinity indicator for the baltic sea the main purpose is to demonstrate the applicability of the method in a formerly glaciated area using 87sr 86sr ratios found in modern shells mytilus edulis macoma baltica and radix balthica to elucidate the parameters and conditions to be used for palaeosalinity determinations based on subfossil shells proxy salinity data are calculated assuming that the sr concentration in baltic sea water is controlled by a two component conservative mixing between seawater and river water based on replicate determinations of the 87sr 86sr ratio in modern shells proxy salinity data can be quantified with a relative precision of better than 5 for salinities up to 10 comparison with measured present day salinities indicates that the accuracy is generally better than 5 for the proxy data with this precision and accuracy the present day salinity differences between the major baltic sub basins bothnian bay bothnian sea and baltic proper can be detected palaeosalinities were determined using subfossil shells m edulis and m baltica with calibrated 14c ages of 6770 3070 cal bp the shell deposits occur from 65 m a s l down to the present day sea level the palaeosalinities determined for the bothnian bay 4 8 10 3 and the bothnian sea 7 3 10 3 are in good agreement with earlier estimates of the maximum littorina sea stage palaeosalinity in these sub basins 8 10 at one locality two different shell species from the same shell deposit were dated m edulis and m baltica the shells differed in age by 460 70 14c years a possible indication that the ecological conditions at a locality may have been favourable to different faunas at different times