three dimensional dynamic simulation of the 1992 landers earthquake the 1992 landers california earthquake magnitude 7 3 was modeled as the propagation of a spontaneous rupture controlled by a realistic prestress distribution with the use of a three dimensional finite difference method the dynamic rupture reproduces the general slip pattern used to compute the initial stress distribution and generates near fault ground motions at the surface similar to observations the simulated rupture propagates on the fault along a complex path with highly variable speed and rise time changing the magnitude and pattern of the stress significantly the method provides the framework to estimate earthquake rupture parameters from recorded seismic and geodetic data

evidence of multiple halogen sources in scapolites from iron oxide copper gold iocg deposits and regional nacl metasomatic alteration norrbotten county sweden abstract scapolites from barren regional na cl metasomatic assemblages rm iron oxide copper gold deposits iocg scapolite altered metabasic rocks iocg m and from iocg proximal alteration na skarns iocg ps from norrbotten county in northern sweden have been analysed for halogen content and cl stable isotope composition the aim of the study was to constrain the source of halogens within alteration assemblages and to investigate the possible fractionation of cl isotopes between scapolite and the hydrothermal fluid scapolite separates were analysed for cl br and major oxide concentrations using electron probe micro analysis epma and micro x ray fluorescence xrfm spectrometry chlorine was extracted from the scapolite separates via pyrohydrolysis and then analysed for their stable cl isotope compositions by isotope ratio mass spectrometry irms all samples of scapolite investigated in this study are marialitic in composition one of the scapolites from the gruvberget deposit iocg ps had a cl br molar ratio of 2363 which is the highest amongst all scapolites reported in the literature to date cl br molar ratios lower than seawater 650 were identified in two iocg ps scapolite samples cl br 554 and 271 as well as in two iocg m scapolites cl br 393 and 565 three rm scapolites had cl br molar ratios very close to or slightly higher than seawater values 639 to 770 samples with cl br molar ratios less than seawater are inferred to have halogens derived from evaporative residual brines whereas samples with molar ratios higher than seawater may have halogens derived from fluids that have dissolved halite and or are from magmatic systems considering the wide variation of the cl br molar ratios in the iocg ps and iocg m scapolites compared to the restricted composition of the regional alteration rm it is proposed that the hydrothermal fluids interacted with several different protoliths to generate the iocg alteration rm alteration scapolites had 37 cl values from 0 1 to 0 3 two iocg m scapolites had values of 0 2 and iocg ps scapolites had 37 cl values from 0 1 to 1 0 using a previously published 37 cl value from fluid inclusion leachates 1 7 from the iocg m mineralisation at pahtohavare and the 37 cl value of co existing scapolite measured in this study 0 2 an empirical fluid scapolite fractionation factor was calculated to be 1 9 this large fractionation factor is not supported by previous predictions for monovalent chlorides and assuming equilibrium indicates that 37 cl was preferentially accommodated in the a site of the scapolite structure this indicates that either the stable cl isotope partitioning between the cacl 2 rich brine and the scapolite may differ from currently available estimates for nacl brines in equilibrium with silicate minerals or that the scapolite and brine are not in isotopic equilibrium overall the data in this study suggest that halogens in early scapolites were derived from residual brines and halite during metamorphism of evaporites linked to the rm alteration later in the history of the norrbotten district components of the rm alteration were recycled and mixed during magmatic and local metasomatic events to varying extents resulting in the brines associated with iocg alteration

the chemistry of geothermal waters in iceland ii mineral equilibria and independent variables controlling water compositions abstract the major element chemistry of icelandic geothermal waters is predictable provided two parameters are known this follows from an attainment of or a close approach to an overall chemical equilibrium in the geothermal systems at temperatures as low as 50 c it is considered that the geothermal system composition temperature and kinetic factors determine which alteration minerals form the system composition is not so much fixed by rock composition as by the rate of leaching of the various constituents from the fresh rock and the composition of inflowing water the water chemistry is determined by the system composition and the external variables acting on the system they include temperature and the mobility of chloride pressure which theoretically should be regarded as an external variable has insignificant effect on water compositions in the range 1 200 bars occurring in the geothermal systems

variability of near field ground motion from dynamic earthquake rupture simulations this study investigates near field ground motion variability due to dynamic rupture models with heterogeneity in the initial shear stress ground velocity seismograms are synthesized by convolving the time histories of slip velocity obtained from spontaneous dynamic rupture models with green s functions of the medium calculated with a discrete wavenumber finite element method peak ground velocity pgv estimated on the synthetics generally matches well with an empirically derived attenuation relation whereas spectral acceleration sa shows only an acceptable match at periods longer than 1 sec using the geometric mean to average the two orthogonal components leads to a systematic bias for the synthetics in particular at the stations closest to the fault this bias is avoided by using measures of ground motion that are independent of the sensor orientation the contribution from stress heterogeneity to the overall ground motion variability is found to be strongest close to the fault and in the backward directivity region of unilaterally propagating ruptures in general the intraevent variability originating from the radiation pattern and the effect of directivity is on the same order or larger than the interevent variability the interevent ground motion variability itself is dominated by the hypocenter station configuration and is influenced only to a lesser extent by the differences in the dynamic rupture process due to the stress heterogeneity in our modeling approach the hypocenter location is not picked arbitrarily but is determined to be mechanically consistent with the stress heterogeneity through a procedure emulating tectonic stress loading of the fault and nucleation compared to the peak ground motion recorded during the 2004 parkfield california earthquake our simulated seismograms show enhanced spatial correlation that may be attributed to the simplicity of the assumed crustal model or to an incomplete representation of the spatial heterogeneity of dynamic rupture parameters nevertheless the intraevent pgv variability in the near fault region determined for the parkfield dataset is of the same order of magnitude as for our simulations

hybrid empirical ground motion prediction equations for eastern north america using nga models and updated seismological parameters in the field of earthquake engineering ground motion prediction models are frequently used to estimate the peak ground acceleration pga and the pseudos pectral acceleration psa in regions of the world where ground motion recordings are plentiful such as western north america wna the ground motion prediction equations are obtained using empirical methods in other regions such as eastern north america ena with insufficient ground motion data alternative methods must be used to develop ground motion prediction equations gmpes the hybrid empiri cal method is one such method used to develop ground motion prediction equations in areas with sparse ground motions this method employs the stochastic simulation method to adjust empirical gmpes developed for a region with abundant strong motion recordings in order to estimate strong motion parameters in a region with a sparse database the adjustments take into account differences in the earthquake source wave propagation and site response characteristics between the two regions in this study a hybrid empirical method is used to develop a new gmpe for ena using five new ground motion prediction models developed by the pacific earthquake engineering research center peer for wna a new ena gmpe is derived for a magnitude range of 5 to 8 and closest distances to the fault rupture up to 1000 km ground motion prediction equations are developed for the response spectra pseudoacceleration 5 damped and the pga for hard rock sites in ena the resulting ground motion prediction model developed in this study is compared with two ena ground motion models used in the 2008 national seismic hazard maps as well as with available observed data for ena

origine des substances dissoutes dans les eaux des sources thermales et des forages de la region asal ghoubbet republique de djibouti abstract from a chemical and sr isotopic study of hot fluids from the asal area republic of djibouti we conclude that the fluids result from basalt seawater interaction associated with variable evaporation some spring waters result from a mixing between geothermal water and seawater or asal lake water in contrast with previous studies dissolution from evaporitic rocks appears unimportant the negative boron anomaly in the fluids from the boreholes suggests the presence of a vapour dominated subsystem in the asal geothermal field

87sr 86sr ratios in hydrothermal waters and deposits from the east pacific rise at 21 n abstract 87 sr 86 sr ratios of three hydrothermal waters collected on the east pacific rise at 21 n define a mixing line between seawater and a hydrothermal end member at 0 7030 which is derived by seawater basalt interaction at ca 350 c and water rock ratio of about 1 5 sr concentrations are not affected in the process while mg uptake from seawater is almost complete up to2 3 of this hydrothermal component is involved in anhydrite precipitation while the sr isotopic ratio in sulfides chalcopyrite sphalerite cannot be distinguished from that of sulfate it is estimated that ca 1 10 10 moles of strontium are yearly cycled in the hydrothermal systems of mid oceanic ridges thereby affecting the 87 sr 86 sr budget of seawater mass balance between river runoff limestone precipitation and ridge basalt alteration suggests that the 87 sr 86 sr ratios of the river runoff are in the range 0 7097 0 7113 and are largely dominated by limestone alteration

hydrothermal activity in the northern tanganyika rift east africa abstract hydrothermal fluids from sub lacustrine and sub aerial sites were sampled in the north tanganyika rift during the 1991 tanganydro expedition two types of hydrothermal end member fluids a na hco 3 type and a na cl type have been identified within the sample set the na hco 3 hydrothermal fluids are emitted at the level of the sub lacustrine pemba and sub aerial kabezi and mashuzi sites which are located in the northernmost bujumbura sub basin the na cl hydrothermal fluid vents at the level of the sub lacustrine cape banza field on the southeastern edge of the ubwari ridge the two types of hydrothermal fluids display isotopic and chemical evidence of interaction with magmatic volatiles the magmatic effects are strongest in the fluid discharging on the pemba hydrothermal field where abundant co 2 bubbling accompanies fluid emission the 13 c values of the co 2 gas sampled at pemba and at cape banza indicate a magmatic origin the pemba fluid shows the highest alkalinity and boron content and the lowest ph of the hydrothermal end member fluids and it has equilibrated at depth at the highest temperature 214 7 c in addition the sulfide mineralizations associated with the pemba hydrothermal vents are enriched in as and tl which similarly to b can be transported as volatiles in magmatic fluids the northern tanganyika na hco 3 fluids are similar to the na hco 3 fluid of the geothermal system of lake bogoria in the eastern branch of the east african rift and they can be compared with the na hco 3 hydrothermal water that is inferred to flow into lake kivu the cape banza na cl fluids may derive from the mixing between meteoric water and a deep seated basement brine similar to the ones found in old continental shields such as the canadian shield the high equilibrium temperatures and the geochemistry of the north tanganyika rift hydrothermal fluids suggest that degassing magmatic bodies are present at depth in the area these magmatic bodies may be either the southern expression of the south kivu magmatism or a more recent activity not yet expressed as surface volcanism the hydrothermal sites are located along major active rift bounding faults which can be associated with deep and strong seismicity as in the case of the pemba field such deep fractures may have favoured the uprise of the magmatic bodies that are the source of heat and volatiles to the hydrothermal systems the circulation of hydrothermal fluids within the tanganyika lacustrine sedimentary pile could play an important role in the maturation of the organic matter enclosed in the lake sediments and the subsequent oil generation and migration

earthquake ruptures with strongly rate weakening friction and off fault plasticity part 1 planar faults abstract observations demonstrate that faults are fractal surfaces with deviations from planarity at all scales we study dynamic rupture propagation on self similar faults having root mean square rms height fluctuations of order 10 3 to 10 2 times the profile length our 2d plane strain models feature strongly rate weakening fault friction and off fault drucker prager viscoplasticity the latter bounds otherwise unreasonably large stress concentrations in the vicinity of bends our choice of a cohesionless yield function prevents tensile stress states and thus fault opening a consequence of strongly rate weakening friction is the existence of a critical background stress level above which self sustaining rupture propagation in the form of self healing slip pulses first becomes possible around this level at which natural faults are expected to operate ruptures become extremely sensitive to fault roughness and exhibit substantial fluctuations in rupture velocity except for shallow inclinations of the maximum compressive stress to the fault less than about 20 the fluctuations are anticorrelated with the local fault slope these accelerations and decelerations of the rupture together with naturally emerging slip heterogeneity excite waves of all wavelengths and result in ground acceleration spectra that are flat at high frequency consistent with observed strong motion records

evaluating the compatibility of dynamic rupture based synthetic ground motion with empirical ground motion prediction equation we performed an assessment of a database of synthetic ground motion generated by a suite of dynamic rupture simulations to verify compatibility of the peak ground amplitudes with respect to empirical ground motion prediction equations gmpes the dynamic rupture model database developed by dalguer and mai 2011 is composed of 360 earthquake scenarios with moment magnitudes in the range of 5 5 7 for three mechanisms of faulting reverse normal and strike slip and for both buried faults and surface rupturing faults having depth and non depth dependent normal stresses initial shear stress distribution follows a von karman stochastic distribution overall we show quantitatively that the upper frequency limit of the suite of simulations is 1 0 hz up to this frequency the synthetic data are compatible with the empirical model which means that the residuals defined as the differences between observed and predicted ground motions fall in the range of of the empirical gmpe characteristics of the mean values along distance 10 45 km and period t 1 0 s including standard deviation of the synthetic response spectra and peak ground velocity are comparable to their counterpart empirical gmpe at very near source 1 2 km synthetics show supersaturation of the mean peak values which is different from the saturation features predicted by the gmpes the effect of source parameters such as stress drop peak slip velocity and rupture speed and of surface and buried rupture as well as hanging wall and footwall are highly sensitive to ground motion suggesting that these effects contribute to the variability of ground motion near the source and that inclusion of them in the source terms may contribute to reducing uncertainties in gmpes

ground motion prediction equations for eastern north america from a referenced empirical approach implications for epistemic uncertainty abstract i outline a referenced empirical approach to the development of ground motion prediction equations gmpes the technique is illustrated by using it to develop gmpes for eastern north america ena the approach combines the ena ground motion database with the empirical prediction equations of boore and atkinson 2008 for the reference region of western north america wna the referenced empirical approach provides gmpes for ena that are in agreement with regional ground motion observations while being constrained to follow the overall scaling behavior of ground motion that is observed in better instrumented active tectonic regions they are presented as an alternative to the commonly used stochastic ground motion relations for ena the motivation of the article is not to supplant stochastic gmpes but is rather to consider other approaches that might shed light on their epistemic uncertainty differences between the referenced empirical gmpes of this study and the stochastic gmpes of atkinson and boore 2006 along with inconsistencies between both of these studies and inferences based on intensity observations suggest that uncertainty in median enagmpes is about a factor of 1 5 2 for m 5 at distances from 10 to 70 km uncertainty is greater than a factor of 2 for large events m 7 at distances within 10 km of the source it may be that saturation effects not modeled in the stochastic predictions but inferred from observations in other regions cause overestimation of near source amplitudes from large events in atkinson and boore 2006 on the other hand these saturation effects cannot be directly verified in ena data differences in predictions according to the approach taken are also significant at distances from 40 to 150 km due to uncertainty in the shape of the attenuation function that will be realized in future earthquakes

fluid geochemistry of natural manifestations from the southern poroto rungwe hydrothermal system tanzania preliminary conceptual model abstract the south poroto rungwe geothermal field in the northern part of the malawi rift tanzania divides in two main areas the relatively high altitude northern area around the main ngozi rungwe tukuyu and kyejo volcanoes is characterised by cold and gas rich springs in contrast hot springs occur in the southern and low altitude area between the kyela and livingstone faults the isotopic signature of the almost stagnant cold springs of the northern district is clearly influenced by h2o co2 g exchange as evidenced from negative oxygen shifts in the order of few deltas permil in contrast the isotopic signature of waters discharged from the hot springs of the southern district is markedly less affected by the h2o co2 g interaction this evidence is interpreted as an effect of the large permanent outflow of these springs which supports the hypothesis of a regional scale recharge of the major thermal springs measurements of carbon isotope variations of the dissolved inorganic carbon of waters and co2 g from the northern and southern springs support a model of co2 g driven reactivity all over the investigated area our combined chemical and isotopic results show that the composition of hot springs is consistent with a mixing between i cold surface fresh sfw and ii deep hot mineralised dhmw water indicating that the deep originated fluids also supply most of the aqueous species dissolved in the surface waters used as local potable water based on geothermometric approaches the temperature of the deep hydrothermal system has been estimated to be higher than 110 c up to 185 c in agreement with the geological and thermal setting of the malawi rift basin geochemical data point to i a major upflow zone of geothermal fluids mixed with shallow meteoric waters in the southern part of the province and ii gas absorption phenomena in the small perched aquifers of the northern volcanic highlands

fluid circulation in the active emerged asal rift east africa djibouti inferred from self potential and telluric telluric prospecting abstract in 1996 self potential sp and telluric telluric tt surveys were performed in the asal rift in the republic of djibouti in order to study water circulation through the rift from ghoubbet strait to asal lake located 11 km northwest and at the altitude of 155 m below sea level sp surveys reveal two main positive anomalies several kilometres wide up to 150 mv in amplitude parallel to the rift and separated by a relatively large 1 km negative anomaly the northern anomaly is well centred above the active rift the central part of the anomaly is cut by a sharp and narrow negative anomaly 200 mv in amplitude 150 m width due to a major fault running from ghoubbet strait to asal lake the two positive anomalies are interpreted by ground seawater flowing toward asal lake and warmed up during their advance by the strong geothermal flux existing beneath the rift the negative anomalies are most probably due to large and rapid fluid transfer along faults and high permeable geologic beds bending toward asal lake the rapid flow would prevent any large heating of the ground water the largest negative anomaly which isolates the two positive ones gives new valuable information on the tectonics the extension of the northern positive anomaly to the southwest is stopped by a fault located about 1 km to the north of a normal fault considered until now as a major rift structure the 16 telluric telluric tt surveys point out three behaviours of the telluric field near ghoubbet strait and inland the telluric field is well polarised in a n90 100 e direction and the amplitude expresses a medium to a low resistivity beneath the recording sites this pattern is the signature of the coast effect and the regional polarisation of the field along normal faults bordering the rift the se nw polarisation seems to be controlled by the rift tectonics inside the rift the telluric field becomes weaker indicating a decay of the resistivity in the central part of the rift the telluric field amplitude becomes 1 3 of the generally recorded amplitude and the polarisation cannot be estimated it is corresponding to very low resistive areas in agreement with high positive sp values

modeling tsunamis from earthquake sources near gorringe bank southwest of portugal the azores gibraltar fracture zone with the huge bathymetric reliefs in the area southwest of portugal is believed to have been the source of large historic tsunami events this report describes simulations of tsunami generation and propagation from sources near the gorringe bank the well documented 1969 tsunami event is examined both with a ray tracing technique and with finite difference models based on various shallow water equations both methods show that the most likely source location is southeast of the gorringe bank near the epicenter location determined from seismic data the tsunami source is calculated by formulas given by okada 1985 for surface deformation of an elastic half space caused by faulting observed wave amplitude and travel time and values computed from an initial wave field according to okada 1985 formulas show acceptable agreement for most stations along the coast of portugal and spain however in order to explain a large primary wave with downward displacement observed on the coast of morocco an alternative source model with a larger area of downward displacement has been introduced this also leads to a better overall fit with observed travel time implications for disastrous events as the one in 1755 are also discussed linear hydrostatic shallow water models are used for most of the simulations but the importance of nonlinearity and dispersion is examined with the boussinesq equations the sensitivity of the solution to changes in the location and the strength of the source is discussed and a series of grid refinement studies are performed in order to assess the accuracy of the simulations

terashake2 spontaneous rupture simulations of mw 7 7 earthquakes on the southern san andreas fault abstract previous numerical simulations terashake1 of large m w 7 7 southern san andreas fault earthquakes predicted localized areas of strong amplification in the los angeles area associated with directivity and wave guide effects from northwestward propagating rupture scenarios the terashake1 source was derived from inversions of the 2002 m w 7 9 denali alaska earthquake that source was relatively smooth in its slip distribution and rupture characteristics owing both to resolution limits of the inversions and simplifications imposed by the kinematic parameterization new simulations terashake2 with a more complex source derived from spontaneous rupture modeling with small scale stress drop heterogeneity predict a similar spatial pattern of peak ground velocity pgv but with the pgv extremes decreased by factors of 2 3 relative to terashake1 the terashake2 source excites a less coherent wave field with reduced along strike directivity accompanied by streaks of elevated ground motion extending away from the fault trace the source complexity entails abrupt changes in the direction and speed of rupture correlated to changes in slip velocity amplitude and waveform features that might prove challenging to capture in a purely kinematic parameterization despite the reduced pgv extremes northwest rupturing terashake2 simulations still predict entrainment by basin structure of a strong directivity pulse with pgvs in los angeles and san gabriel basins that are much higher than predicted by empirical methods significant areas of those basins have predicted pgv above the 2 probability of exceedance poe level relative to current attenuation relationships even when the latter includes a site term to account for local sediment depth and wave guide focusing produces localized areas with pgv at roughly 0 1 0 2 poe about a factor of 4 5 above the median in contrast at rock sites in the 0 100 km distance range the median terashake2 pgvs are in very close agreement with the median empirical prediction and extremes nowhere reach the 2 poe level the rock site agreement lends credibility to some of our source modeling assumptions including overall stress drop level and the manner in which we assigned dynamic parameters to represent the mechanical weakness of near surface material future efforts should focus on validating and refining these findings assessing their probabilities of occurrence relative to alternative rupture scenarios for the southern san andreas fault and incorporating them into seismic hazard estimation for southern california

evaporitic carbonates in the pre salt of santos basin genesis and tectonic implications abstract since the first discoveries of oil accumulations in the carbonate reservoirs of the lower cretaceous barra velha formation of the santos basin their genesis has been the subject of intense discussion different depositional models have been proposed with some authors advocating a microbial origin while others an abiotic origin as travertines or travertine like rocks this search for a modern carbonate analogue for these unusual rocks may have disregarded an intriguing alternative an evaporative model supported by our petrographical and geochemical data evaporitic sediment patterns in endorheic basins in a depositional framework of brine drawdown are dominated by ephemeral salt pan hydrology bedded salt crusts form a stack of subaqueous aligned crystal beds at the sediment water interface salt grows diagenetically within saline and mud layers beneath the dry pan surface finally laminated sediments are deposited we observe a similar cyclic pattern in the pre salt core we described beds of shrub like crystals and diagenetic spherulites growing in magnesium clay matrix are followed by laminites the model we propose attempts to contribute solving the puzzle of pre salt carbonate deposition by integrating sedimentology and geochemistry with basin tectonics our observations and basin tectonics point to the exciting possibility that the sag phase carbonates of the santos campos basin were deposited as the initial transitional stage of a closed sub sea level evaporite basin whose rising salinities culminated in the deposition of halite and bittern salts carnallite sylvite tachyhydrite basinwide evaporites are singular events in the geological record they form when a giant sub sea level basin becomes a sump of continental marine and hydrothermal fluids producing unexpected brine compositions in unusual depositional settings present in ancient evaporitic basins but absent from most modern ones the novelty in this paper is that we apply to pre salt carbonate deposition the sedimentology hydrology and geochemistry of ancient evaporitic basins with this concept in mind we propose a geochemical model for the unusual pre salt carbonate precipitation of the santos basin regarding the walvis rio grande volcanic high as a topographic barrier during the aptian we suggest that the main sources of calcium to the barra velha paleolake were hydrothermal brines mostly from infiltrating seawater that reacted at depth with the basalts of the volcanic barrier hyperextension of the lithosphere with a shallow moho increased geothermal gradients to levels favorable for the generation of hydrothermal cacl2 brines a process facilitated by the already elevated ca ion and low sulfate concentration of the cretaceous ocean these cacl2 rich fluids would pass through the volcanic barrier into a sub sea level paleolake in the rift basin the high salinity paleolake would become saturated in calcium bicarbonate carbonate as internally drained nahco3 bearing alkaline waters formed by physical chemical weathering of penecontemporaneous basaltic rocks mix with the cacl2 bearing hot brines percolating through the barrier producing a hybrid brine 2nahco3 cacl2 ca hco3 2 2nacl such a depositional system integrating the geochemical depositional controls of sag phase carbonates with those of the south atlantic evaporites has important implications for unravelling the genesis of these exotic deposits

mineralogy fluid inclusions and stable isotope study of epithermal au ag bi te mineralization from the se afar rift djibouti abstract in this paper the mineralogy fluid inclusions and stable isotopes of hydrothermal veins from the se part of the afar rift djibouti were studied mineralization occurs as gold silver in quartz carbonate veins and is mainly associated with felsic volcanism the morphologies and textures of quartz show crustiform colloform banding massive and breccias microthermometric measurements were made on quartz hosted two phases liquid vapor inclusions mean homogenization temperature ranges from 150 c to 340 c and ice melting temperatures range from 0 2 c to 1 6 c indicating that inclusion solutions contain 0 35 to 2 7 eq wt nacl furthermore 18o and 13c values from calcite range from 3 7 to 26 6 and 7 5 to 0 3 respectively the presence of platy calcite and adularia are consistent with boiling conditions our study shows that the deposit of precious metal occurred at around 250 c and is linked to a fluid generated by the mixture of magmatic fluid hydrothermal fluid and seawater

correlation of earthquake source parameters inferred from dynamic rupture simulations 1 we analyzed 315 dynamic strike slip rupture models computed up to 5 0 hz to get a quantitative understanding of the correlation and amplitude distributions of parameters describing the earthquake source such as slip and rupture velocity to account for the epistemic uncertainty of the problem we constructed a database of dynamic ruptures computed by ourselves and other authors this database contains ruptures computed using different models of initial stress peak stress and critical slip weakening distance using the set of computed ruptures we constructed probability density functions pdfs for the amplitude distributions of the source parameters and for the correlation between the source parameters we tried to extract parameter pairs that showed a small variability in the spatial correlation given the large epistemic uncertainty in the input we only analyzed the areas on the fault with subshear propagation speed the principal findings are as follows 1 final slip amplitude does not show correlation with the local rupture velocity 2 final slip amplitude correlates well with risetime 3 rupture velocity correlates well with peak slip rate and the duration of the impulsive part of the slip rate function 4 the pdf of rupture velocity risetime and peak slip rate depends on the distance from the nucleation zone 5 fracture energy is not the single controlling factor for the rupture velocity the slope of the linear slip weakening curve has a significant effect on the rupture velocity 6 the crack length length that is slipping at a given time decreases with the distance from the nucleation zone

sulfur and strontium isotopic study of epithermal mineralization a case study from the se afar rift djibouti epithermal mineralization was recently described in synrift volcanic rocks from the se afar rift in djibouti to infer fluid sources in this system coupled sulfur and strontium isotopic analyses were performed on mineralized samples from hydrothermal veins and stockwork zones mineralization includes gypsum and chalcedony and or quartz carbonate containing gold and sulfides sulfur isotopic compositions of sulfides and sulfates were determined using multicollector inductively coupled plasma mass spectrometer mc icpms laser ablation mc icpms and continuous flow elemental analyzer isotope ratio mass spectrometry while strontium isotopes were determined by thermal ionisation mass spectrometry tims methods sulfur isotopic composition of sulfides mainly pyrite varies from 9 2 to 6 6 these values are classically reported for hydrothermal sulfides resulting from the mixing of variable s sources such as seawater sulfate and magmatic source in addition individual pyrite grains within the same sample show both positive and negative 34s values suggesting multi phase mineralization processes coupled sulfur and strontium isotopic compositions of seven sulfate samples hosted in i gypsum mound ii evaporites iii stockwork veins in sediments or in volcanic rocks were also investigated to characterize the sources of sr and s the 34s values of the sulfate ranged from 0 8 to 14 3 while the 87sr 86sr ranged from 0 70389 to 0 70639 consistent with variable mixing ratios of sr and s derived from volcanic sources and seawater derived sulfate this study shows that although precious metal deposition is related to low sulfidation epithermal environment both acidic fluid of magmatic and saline origins contributed to the hydrothermal system in the se afar rift

source properties of dynamic rupture pulses with off fault plasticity large dynamic stresses near earthquake rupture fronts may induce an inelastic response of the surrounding materials leading to increased energy absorption that may affect dynamic rupture we systematically investigate the effects of off fault plastic energy dissipation in 2 d in plane dynamic rupture simulations under velocity and state dependent friction with severe weakening at high slip velocity we find that plasticity does not alter the nature of the transitions between different rupture styles decaying versus growing pulse like versus crack like and subshear versus supershear ruptures but increases their required background stress and nucleation size we systematically quantify the effect of amplitude and orientation of background shear stresses on the asymptotic properties of self similar pulse like ruptures peak slip rate rupture speed healing front speed slip gradient and the relative contribution of plastic strain to seismic moment peak slip velocity and rupture speed remain bounded from fracture mechanics arguments we derive a nonlinear relation between their limiting values appropriate also for crack like and supershear ruptures at low background stress plasticity turns self similar pulses into steady state pulses for which plastic strain contributes significantly to the seismic moment we find that the closeness to failure of the background stress state is an adequate predictor of rupture speed for relatively slow events our proposed relations between state of stress and earthquake source properties in the presence of off fault plasticity may contribute to the improved interpretation of earthquake observations and to pseudodynamic source modeling for ground motion prediction

a composite source model for computing realistic synthetic strong ground motions a composite source model is presented for convolution with synthetic green s functions in order to synthesize strong ground motions due to a complex rupture process of a large earthquake subevents with a power law distribution of sizes are located randomly on the fault each subevent radiates a displacement pulse with the shape of a brune s pulse in the far field at a time determined by a constant rupture velocity propagating from the hypocenter thus all the input parameters have a physical basis we simulate strong ground motions for event station pairs that correspond to records obtained in mexico by the guerrero accelerograph network the synthetic accelerations velocities and displacements have realistic amplitudes durations and fourier spectra

geochemical study of the sakalol harralol geothermal field republic of djibouti evidences of a low enthalpy aquifer between manda inakir and asal rift settings abstract eighty six sodium bicarbonate to sodium chloride hot springs and four water wells in the tadjourah region of djibouti were investigated for major minor b br f sr li chemistry and isotope composition of water and dissolved components 87sr 86sr 11b 10b 13c 12c and 14c of dic 34s 32s and 18o 16o of sulfate the deep saline na cl reservoir at 143 c shows affinity with the shallow geothermal water from the active asal rift asal water is a diluted and recycled seawater component with the major cation composition obliterated by equilibration with stratoid basalt locally the deep reservoir is differentiated in term of recharge and re equilibration with rocks and mixing in particular two spring groups reveal contributions from evaporites typical of the passive graben setting of the afar a model on 34s 32s and 18o 16o demonstrates the isotope imprint of magmatic so2 disproportionation on dissolved and solid sulfate whose values probably persists in a sedimentary environment without trace of seawater on the other hand a seawater signature modified by mixing and secondary fractionation effects is partially maintained according to the boron isotope composition up to 27 4 temperature estimation in low enthalpy geothermal reservoirs is notoriously difficult especially where mixing with fluids of differing genesis and or conduction cooling take place from a geothermometric point of view the multi method approach followed in this study up to date theoretical and thermodynamic equations ad hoc silica geothermometers inferred from local rocks checking of the results on a 18o sulfate water vs temperature diagram provides some insights and perspectives on how to tackle the problem

dynamic rupture models of earthquakes on the bartlett springs fault northern california the bartlett springs fault bsf the easternmost branch of the northern san andreas fault system creeps along much of its length geodetic data for the bsf are sparse and surface creep rates are generally poorly constrained the two existing geodetic slip rate inversions resolve at least one locked patch within the creeping zones we use the 3 d finite element code faultmod to conduct dynamic rupture models based on both geodetic inversions in order to determine the ability of rupture to propagate into the creeping regions as well as to assess possible magnitudes for bsf ruptures for both sets of models we find that the distribution of aseismic creep limits the extent of coseismic rupture due to the contrast in frictional properties between the locked and creeping regions

use of two new na li geothermometric relationships for geothermal fluids in volcanic environments abstract this work has made it possible to obtain two new na li geothermometric relationships in addition to the three already known fouillac and michard 1981 kharaka et al 1982 and confirms that the na li geothermometer unlike the na k na k ca k mg and silica geothermometers or the isotope 18o h2o so4 geothermometer also depends on the fluid salinity and the nature of the reservoir rocks reacting with the geothermal water one of the relationships concerns the fluids derived from seawater basalt interaction processes existing in emerged rifts such as those of iceland reykjanes svartsengi and seltjarnarnes geothermal fields and djibouti asal ghoubbet and obock geothermal areas or in numerous oceanic ridges and rises mid atlantic and middle valley ridges east pacific rise etc the best adapted na li relationship for geothermal fluids discharged from emerged rifts between 0 and 365 c is t k 920 log n a l i 1 105 r 2 0 994 n 27 where na and li are the aqueous concentrations of these elements given in mol l the other na li relationship was determined using dilute waters collected from wells located in different high temperature 200 325 c volcanic geothermal areas of iceland krafla namafjall nesjavellir and hveragerdi this relationship can be expressed as follows t k 2002 log na li 1 322 r2 0 967 n 17 these two relationships give estimations of temperature with an uncertainty close to 20 c the second na li relationship was also successfully applied to ht dilute geothermal waters from the east african rift ethiopia kenya some case studies in the literature and thermodynamic considerations suggest that the na li ratios for this type of fluids could be controlled by full equilibrium reactions involving a mineral assemblage constituting at least albite k feldspar quartz and clay minerals such as kaolinite illite or muscovite and li micas unlike the na li ratios no thermometric relationship using li isotopes could be determined for this type of water however it was noticed that 7li values higher than 16 are always associated with low to medium temperature waters

subsurface seepage of seawater across a barrier a source of water and salt to peripheral salt basins many of the great phanerozoic salt basins share a common paleogeography in which a deep lake or sea is separated from the ocean by a narrow barrier examples include the aptian evaporites of the south atlantic rift basins and the messinian salt deposits of the mediterranean sea and the north caspian depression marine transgressions over the barrier have been proposed as the origin for saline conditions in these basins we test an alternate hypothesis that subsurface seepage of seawater through the barrier was a significant source of water influencing both water level and water composition our model permits flux of water and salt into and out of the basin via 1 seepage through the barrier 2 evaporation from the water surface 3 rainfall and or rivers and 4 incorporation into basin sediments our investigation focuses on three settings a modern analog in the gregory rift of northern kenya the messinian mediterranean basin and the early cretaceous aptian salt basins of the south atlantic we test our model through a numerical simulation applying reasonable values for the dimensions of the barrier differences in water level and hydraulic conductivities to calculate flow rates through the barrier modeling results indicate that flux of seawater through a barrier to a peripheral basin ranges from the dominant component of lake water to insignificant depending on the climate size of the peripheral basin and hydraulic conductivity of the barrier the flux of salt is significant over the full range of modeled hydraulic conductivities producing brines in the peripheral lake and or sea within 0 5 m y in most cases this demonstrates that seepage through the barrier can account for lithologic and organic geochemical evidence for saline water conditions and that marine transgressions across the barrier are not necessary to explain the apparent high salinities

ground motion simulation and validation of the 2008 chino hills california earthquake using different velocity models abstract we present a set of ground motion simulations of the 2008 m w 5 4 chino hills earthquake using different seismic velocity models available for southern california the simulations are tailored for a maximum frequency of 4 hz and a minimum shear wave velocity of 200 m s we use the community models developed by the southern california earthquake center cvm s and cvm h in the case of cvm h we use two available alternatives with and without considering the geotechnical layers gtl we compare the simulation results obtained with the models against each other and validate the synthetics with recordings obtained at more than 300 stations the comparisons are used to gain insight into the sensitivity of the results to differences in the models and to make conclusions about the strengths of each model s description of the crustal structure and the gtl in the region the validation is done using a goodness of fit gof criterion we find the gof values show a better match at frequencies below 1 hz within the basins and below 0 5 hz in general the models however do not converge at frequencies lower than 0 5 hz and the areas where each model leads to better results do not always coincide the synthetics start to deviate significantly from observations at frequencies above 1 and 2 hz although very good agreement with data can still be found at individual locations even at the higher frequencies cvm s yields the best results at frequencies up to 0 25 hz our results reveal significant sensitivity of the ground motion to the differences in the structural representation of basins and shallow layers in the models this indicates the models need to be cross referenced we close by offering suggestions on the type of improvements that could be adopted in the future

the source of halogens in geothermal fluids from the taupo volcanic zone north island new zealand abstract forty seven samples of deep geothermal well fluids and hot springs from 14 geothermal sites in the taupo volcanic zone tvz north island new zealand were analyzed for cl br and li concentrations and 37cl 7li 18o and d values the main purpose of this study was to identify the sources of salinity in the tvz fluids and to assess the processes that control stable chlorine isotopic fractionation in the geothermal waters the samples were obtained from deep wells in the mokai ohaaki wairakei rotokawa kawerau ngatamariki and tauhara geothermal fields and from the hot springs at rotorua waiotapu lake taupo waimangu waikite orakei korako and tokaanu two main meteoric recharge areas were identified in the central tvz one located to the ne with 18o and d values of 5 5 and 35 respectively and the other to the sw with a 18o value of 6 7 and d value of 43 water rock interactions in most of the geothermal fluids resulted in a 18o shift to more positive values up to 1 for mokai and over 3 for kawerau the 7li values range from 2 9 to 2 0 and suggest that the deep geothermal fluids have interacted with rocks with low 7li values the highest 7li values 1 4 in the reservoir fluids were found at kawerau where the low quartz content of the greywacke may result in the depletion of 6li the 37cl values of the samples ranged from 1 0 to 0 8 the 37cl values of 0 2 0 1 and 0 0 were measured in samples from orakei korako waiotapu and waimangu respectively and these are interpreted to have been affected by surface evaporation and distillation processes excluding these data most waters with high cl br ratios had positive 37cl values the cl br molar ratios in well samples ranged from 659 to 1664 and the 37cl values from 1 0 to 0 8 the hot springs had cl br molar ratios ranging from 858 to 1611and 37cl values from 0 8 to 0 7 overall the cl br molar ratios of the fluids are comparable with published data on basaltic magmas and these magmas are inferred to be the main source of halogens in the geothermal fluids however the geothermal fluids can be separated into two main groups one associated with rhyolitic magmas characterized by positive 37cl values and high cl br molar ratios and other related to andesitic magmas with negative 37cl values and lower cl br molar ratios we suggest therefore that the isotopic fractionation of the stable cl isotopes is related to the difference in water contents of andesitic and rhyolitic magmas and that the fluid phase is enriched in 37cl during magmatic evolution there is no evidence of isotopic fractionation during the transit of the geothermal fluids from the reservoir to the surface but evaporation and distillation processes at the surface can affect the 37cl values

the transition of dynamic rupture styles in elastic media under velocity weakening friction although kinematic earthquake source inversions show dominantly pulse like subshear rupture behavior seismological observations laboratory experiments and theoretical models indicate that earthquakes can operate with different rupture styles either as pulses or cracks that propagate at subshear or supershear speeds the determination of rupture style and speed has important implications for ground motions and may inform about the state of stress and strength of active fault zones we conduct 2d in plane dynamic rupture simulations with a spectral element method to investigate the diversity of rupture styles on faults governed by velocity and state dependent friction with dramatic velocity weakening at high slip rate our rupture models are governed by uniform initial stresses and are artificially initiated we identify the conditions that lead to different rupture styles by investigating the transitions between decaying steady state and growing pulses cracks sub shear and super shear ruptures as a function of background stress nucleation size and characteristic velocity at the onset of severe weakening our models show that small changes of background stress or nucleation size may lead to dramatic changes of rupture style we characterize the asymptotic properties of steady state and self similar pulses as a function of background stress we show that an earthquake may not be restricted to a single rupture style but that complex rupture patterns may emerge that consist of multiple rupture fronts possibly involving different styles and back propagating fronts we also demonstrate the possibility of a super shear transition for pulse like ruptures finally we draw connections between our findings and recent seismological observations

rupture history of 2014 mw 6 0 south napa earthquake inferred from near fault strong motion data and its impact to the practice of ground strong motion prediction the spatiotemporal rupture history of the 2014 south napa earthquake is constrained using near fault strong motion records an aggressive source parameterization with 1372 subfaults is adopted to match the signals in the transverse components up to 4 hz the result reveals that the rupture of the napa earthquake initiated at a depth of 9 84 km and propagated mainly to north northwest nnw and updip on a 13 km long fault patch a gradual increase in average rise time when the rupture propagates to shallower depth is observed however it is the rupture of a small mw 4 9 isolated and high stress drop fault patch that excited the largest ground acceleration at stations south of the epicenter such fine scale rupture heterogeneity shall be considered during seismic hazard analysis

magma genesis in an ongoing rifting zone the tadjoura gulf afar area basalts from the tadjoura gulf afar area northeastern africa have been analyzed for chemical and isotopic sr nd o compositions in order to understand processes of magma genesis in relation to the active rifting in this region we use these data to characterize the mantle source of the basalts and to examine the potential role of lithospheric mantle in their genesis the isotopic compositions of sr nd and o show considerable variation with 87sr86sr 0 703 0 707 nd 8 1 to 0 5 and 18o 4 6 to 8 4 combined with major and trace element data these isotopic results suggest that the basalts were essentially produced by partial melting of the mantle sources comparable to those of southern red sea basalts namely both depleted mantle and somewhat enriched plume sources are responsible for the generation of the magmas the continental lithospheric mantle seems to have played only a very minor role in the petrogenesis prior to the opening of the tadjoura gulf and the asal rift some lavas penetrated through continental crust resulting in variable degrees of crustal contamination at present the crust of the tadjoura gulf and the asal rift is dominantly basaltic some lavas of the recent series show very unusual chemical and isotopic characteristics high tath ratios positive eu anomalies and low 18o values these features can be best explained by a contamination process involving assimilation of hydrothermally altered gabbros by some basaltic magmas

hydrothermal hydrocarbon gases 2 application in the east african rift system hydrocarbon gases are ubiquitous in the hydrothermal systems of the east african rift system ears though often found at very low concentrations in the volcanic eastern branch as compared to the sedimentary western branch study of the chemical and isotopic compositions of these hydrocarbons from sites in kenya ethiopia djibouti and uganda reveals considerable homogeneity over hundreds of km of the various rift units consideration of c and he isotopic evidence points to a predominantly crustal thermogenic origin for the hydrocarbons there being no evidence of mantle inputs in either the morb or hotspot sectors of the ears temperature information from geothermal wells has been utilised to investigate the relationship between reservoir temperatures and ratios of ch4 to c2h6 the general c1 c2 geothermometric relationship proposed in part 1 of this study holds reasonably well and is shown to give results equal to or better than the inorganic gas geothermometers presently in use both in the wellfields and undeveloped high enthalpy geothermal areas results from low enthalpy hot spring systems are less well correlated with apparent deep temperatures but consistent with data from similar systems elsewhere in the world

rupture dynamics and ground motion from 3 d rough fault simulations 1 we perform three dimensional 3 d numerical calculations of dynamic rupture along non planar faults to study the effects of fault roughness on rupture propagation and resultant ground motion the fault roughness model follows a self similar fractal distribution over length scales spanning three orders of magnitude from 102 to 105 m the fault is governed by a strongly rate weakening friction and the bulk material is subject to drucker prager viscoplasticity fault roughness promotes the development of self healing rupture pulses and a heterogeneous distribution of fault slip at the free surface and at depth the inelastic deformation generated by the large dynamic stress near rupture fronts occurs in a narrow volume around the fault with heterogeneous thickness correlated to local roughness slopes inelastic deformation near the free surface however is induced by the stress waves originated from dynamic rupture at depth and spreads to large distances 10 km away from the fault the present simulations model seismic wave excitation up to 10 hz with rupture lengths of 100 km permitting comparisons with empirical studies of ground motion intensity measures of engineering interest characteristics of site averaged synthetic response spectra including the distance and period dependence of the median values absolute level and intra event standard deviation are comparable to appropriate empirical estimates throughout the period range 0 1 3 0 s this class of model may provide a viable representation of the ground motion excitation process over a wide frequency range in a large spatial domain with potential applications to the numerical prediction of source and path specific effects on earthquake ground motion

earthquake source characteristics from dynamic rupture with constrained stochastic fault stress one of the challenging tasks in predicting near source ground motion for future earthquakes is to anticipate the spatiotemporal evolution of the rupture process the final size of an event but also its temporal properties propagation velocity slip velocity depend on the distribution of shear stress on the fault plane though these incipient stresses are not known for future earthquakes they might be sufficiently well characterized in a stochastic sense we examine the evolution of dynamic rupture in numerical models of a fault subjected to heterogeneous stress fields with varying statistical properties by exploring the parameter space of the stochastic stress characterization for a large number of random realizations we relate generalized properties of the resulting events to the stochastic stress parameters the nucleation zone of the simulated earthquake ruptures in general has a complex shape but its average size is found to be independent of the stress field parameterization and is determined only by the material parameters and the friction law furthermore we observe a sharp transition in event size from small to system wide events governed mainly by the standard deviation of the stress field a simplified model based on fracture mechanics is able to explain this transition finally we find that the macroscopic rupture parameters e g moment moment rate seismic energy of our catalog of model quakes are generally consistent with observational data