source parameters of the 2014 mw 6 1 south napa earthquake estimated from the sentinel 1a cosmo skymed and gps data abstract using the combination of two insar and one gps data sets we present the detailed source model of the 2014 mw 6 1 south napa earthquake the biggest tremor to hit the san francisco bay area since the 1989 mw 6 9 loma prieta earthquake the insar data are from the sentinel 1a s1a and cosmo skymed cs satellites and gps data are provided by nevada geodetic laboratory we firstly obtain the complete coseismic deformation fields of this event and estimate the insar data errors then using the s1a data to construct the fault geometry one main and two short parallel sub faults which haven t been identified by field investigation as expected the geometry is in good agreement with the aftershock distribution by inverting the insar and gps data we derive a three segment slip and rake models our model indicates that this event was a right lateral strike slip earthquake with a slight reverse component in the west napa fault as we estimated the fault is 30 km long and more than 80 of the seismic moment was released at the center of the fault segment where the slip reached its maximum up to 1 m we also find that our geodetic moment magnitude is 2 07 1018 nm corresponding to mw 6 18 larger than that of usgs mw 6 0 and gcmt mw 6 1 this difference may partly be explained by our insar data including about one week s postseismic deformation and aftershocks the results also demonstrate high snr and great ability of the newly launched sentinel 1a in earthquake study furthermore this study suggests that this earthquake has potential to trigger nearby faults especially the green valley fault where the coulomb stress was imparted by the 2014 south napa earthquake

stress sensitivity of fault seismicity a comparison between limited offset oblique and major strike slip faults we present a new three dimensional inventory of the southern san francisco bay area faults and use it to calculate stress applied principally by the 1989 m 7 1 loma prieta earthquake and to compare fault seismicity rates before and after 1989 the major high angle right lateral faults exhibit a different response to the stress change than do minor oblique right lateral thrust faults seismicity on oblique slip faults in the southern santa clara valley thrust belt increased where the faults were unclamped the strong dependence of seismicity change on normal stress change implies a high coefficient of static friction in contrast we observe that faults with significant offset 50 100 km behave differently microseismicity on the hayward fault diminished where right lateral shear stress was reduced and where it was unclamped by the loma prieta earthquake we observe a similar response on the san andreas fault zone in southern california after the landers earthquake sequence additionally the offshore san gregorio fault shows a seismicity rate increase where right lateral oblique shear stress was increased by the loma prieta earthquake despite also being clamped by it these responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones we can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes whereas in more limited offset faults fluids could rapidly escape the difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes since many of these events occur both inside and outside major fault zones

the stress shadow problem in physics based aftershock forecasting does incorporation of secondary stress changes help main shocks are calculated to cast stress shadows across broad areas where aftershocks occur thus a key problem with stress based operational forecasts is that they can badly underestimate aftershock occurrence in the shadows we examine the performance of two physics based earthquake forecast models coulomb rate state crs based on coulomb stress changes and a rate and state friction law for their predictive power on the 1989 mw 6 9 loma prieta aftershock sequence the crs 1 model considers the stress perturbations associated with the main shock rupture only whereas crs 2 uses an updated stress field with stresses imparted by m 3 5 aftershocks including secondary triggering effects slightly improves predictability but physics based models still underestimate aftershock rates in locations of initial negative stress changes furthermore crs 2 does not explain aftershock occurrence where secondary stress changes enhance the initial stress shadow predicting earthquake occurrence in calculated stress shadow zones remains a challenge for stress based forecasts and additional triggering mechanisms must be invoked

quantitative earthquake forecasts resulting from static stress triggering 1 in recent years the triggering of earthquakes has been discussed controversially with respect to the underlying mechanisms and the capability to evaluate the resulting seismic hazard apart from static stress interactions other mechanisms including dynamic stress transfer have been proposed to be part of a complex triggering process exploiting the theoretical relation between long term earthquake rates and stressing rate we demonstrate that static stress changes resulting from an earthquake rupture allow us to predict quantitatively the aftershock activity without tuning specific model parameters these forecasts are found to be in excellent agreement with all first order characteristics of aftershocks in particular 1 the total number 2 the power law distance decay 3 the scaling of the productivity with the main shock magnitude 4 the foreshock probability and 5 the empirical bath law providing the maximum aftershock magnitude which supports the conclusion that static stress transfer is the major mechanism of earthquake triggering

rare earth element distribution in lake tyrrell groundwaters victoria australia abstract the rare earth element ree geochemistry of the saline to hypersaline acidic brines of lake tyrrell victoria australia were analyzed in order to determine the relative importance of concentration and modification processes in the lake system several types of brines were identified on the basis of different ree patterns and concentrations regional groundwater flowing into lake tyrrell yields the highest concentrations of ree while a transitional water group is identified on the basis of lower ree concentrations a third water group the reflux brine is characterized by extremely low ree concentrations the ree patterns suggest that ree are introduced to the lake tyrrell system by the regional groundwater which has concentrated ree from the aquifer material in its acidic waters eventually ree are removed from the evolving water mass as higher levels of ph are reached the data of the study strongly suggest that ree are scavenged from solution by fe oxyhydroxide phases which precipitate as the ph of the system rises water rock interactions probably control ph in the lake system and therefore ultimately control the concentration of ree in solution in this system our results are directly correlated with studies of hydrothermal vents and geothermal solutions which have shown the concentration of ree in solution to be dominantly controlled by ph

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

connecting crustal seismicity and earthquake driven stress evolution in southern california tectonic stress in the crust evolves during a seismic cycle with slow stress accumulation over interseismic periods episodic stress steps at the time of earthquakes and transient stress readjustment during a postseismic period that may last months to years static stress transfer to surrounding faults has been well documented to alter regional seismicity rates over both short and long time scales while static stress transfer is instantaneous and long lived postseismic stress transfer driven by viscoelastic relaxation of the ductile lower crust and mantle leads to additional slowly varying stress perturbations both processes may be tested by comparing a decades long record of regional seismicity to predicted time dependent seismicity rates based on a stress evolution model that includes viscoelastic stress transfer here we explore crustal stress evolution arising from the seismic cycle in southern california from 1981 to 2014 using five m 6 5 source quakes the m7 3 1992 landers m6 5 1992 big bear m6 7 1994 big bear m7 1 1999 hector mine and m7 2 2010 el mayor cucapah earthquakes we relate the stress readjustment in the surrounding crust generated by each quake to regional seismicity using rate and state friction theory using a log likelihood approach we quantify the potential to trigger seismicity of both static and viscoelastic stress transfer finding that both processes have systematically shaped the spatial pattern of southern california seismicity since 1992

sources of dissolved salts in the central murray basin australia abstract large areas of the australian continent contain scattered saline lakes underlain by shallow saline groundwaters of regional extent and debated origin the normative salt composition of subsurface pore fluids extracted by squeezing cores collected during deep drilling at piangil west 2 in the central murray basin in southeastern australia and of surface and shallow subsurface brines produced by subaerial evaporation in the nearby lake tyrrell systems helps constrain interpretation of the origin of dissolved solutes in the groundwaters of this part of the continent although regional sedimentation in the murray basin has been dominantly continental except for a marine transgression in oligocene pliocene time most of the solutes in saline surface and subsurface waters in the central murray basin have a distinctly marine character some of the tyrrell waters to the southwest of piangil west 2 show the increase in nacl and decrease in sulfate salts expected with evaporative concentration and gypsum precipitation in an ephemeral saline lake or playa environment the salt norms for most of the subsurface saline waters at piangil west 2 are compatible with the dilution of variably fractionated marine bitterns slightly depleted in sodium salts similar to the more evolved brines at lake tyrrell which have recharged downward after evaporation at the surface and then dissolved a variable amount of gypsum at depth apparently over the last 0 5 ma significant quantities of marine salt have been blown into the murray basin as aerosols which have subsequently been leached into shallow regional groundwater systems basin wide and have been transported laterally into areas of large evaporative loss in the central part of the basin this origin for the solutes helps explain why the isotopic compositions of most of the subsurface saline waters at piangil west 2 have a strong meteoric signature whereas the dissolved salts in these waters appear similar to a marine assemblage

trace metal solubility in an anoxic fjord abstract intermittent anoxia in the saanich inlet water column provides an easily accessible marine o2 h2s interface to study the response of metals to both a steep redox gradient and the availability of reactive reduced sulfur species our study indicates a strong anoxic zone sink for copper and cadmium and the characteristically enhanced solubility of manganese and iron thiosulfate and sulfite are below detection limits 1 m and 0 1 m respectively and thus not important in metal complexation elemental sulfur concentrations are high at the oxic anoxic interface and throughout the anoxic zone indicating the potential for metal complexation by polysulfides a thermodynamic approach employing metal sulfide formation and class specific sulfidic ligand complexation to generate equilibrium profiles adequately describes the solubility of iron copper and cadmium the extension of this scheme to other transition and class b metals in other marine environments with redox fronts is suggested

hydrologic framework of a sabkha along arabian gulf during the last 4 000 to 5 000 years sedimentary offlap and a relative fall in sea level have resulted in the development of broad gently seaward sloping about 1 3 000 planar areas or sabkhas along much of the south shore of the arabian persian gulf the depth of the water table below the sabkha surface increases with distance from the sea until it reaches 1 0 to 1 5 m after which it remains constant when the water table depth is in the range of 1 0 to 1 5 m the sabkha surface has reached a state of deflational equilibrium the water table beneath the sabkha always slopes seaward indicating that the direction of ground water flow is toward the sea the subsurface flow system beneath the sabkha is part of a regional seaward flowing groundwater regime in the seaward part of the sabkha a downward flow component is superimposed on the general seaward ground water flow pattern as a result of the infiltration and percolation of water from shallow bodies of seawater that are propelled inland over the sabkha surface by strong onshore winds farther inland where marine flooding does not occur the vertical flow component is directed upward in response to evaporative losses at the sabkha surface the net rate of evaporation from the central parts of the sabkha is on the order of 6 cm factor of 2 of ground water per year a regional ground water flow system which causes the water table level to rise as the sabkha progrades appears to be a requirement for the development and the preservation of broad sabkhas fluctuations in sea level during sabkha development influence the rate of progradation and the thickness of sedimentary facies the refluxing of sabkha brines beneath the lagoons in the abu dhabi area indicates a mechanism for preserving anhydrite during marine transgressions thus permitting the development of stacked sabkha successions which are common in the rock record

delayed seismicity rate changes controlled by static stress transfer on 15 june 2010 a m w5 7 earthquake occurred near ocotillo california in the yuha desert this event was the largest aftershock of the 4 april 2010 m w7 2 el mayor cucapah emc earthquake in this region the emc mainshock and subsequent ocotillo aftershock provide an opportunity to test the coulomb failure hypothesis cfs we explore the spatiotemporal correlation between seismicity rate changes and regions of positive and negative cfs change imparted by the ocotillo event based on simple cfs calculations we divide the yuha desert into three subregions one triggering zone and two stress shadow zones we find the nominal triggering zone displays immediate triggering one stress shadowed region experiences immediate quiescence and the other nominal stress shadow undergoes an immediate rate increase followed by a delayed shutdown we quantitatively model the spatiotemporal variation of earthquake rates by combining calculations of cfs change with the rate state earthquake rate formulation of dieterich 1994 assuming that each subregion contains a mixture of nucleation sources that experienced a cfs change of differing signs our modeling reproduces the observations including the observed delay in the stress shadow effect in the third region following the ocotillo aftershock the delayed shadow effect occurs because of intrinsic differences in the amplitude of the rate response to positive and negative stress changes and the time constants for return to background rates for the two populations we find that rate state models of time dependent earthquake rates are in good agreement with the observed rates and thus explain the complex spatiotemporal patterns of seismicity

geochemistry and hydrodynamics in the hutt and leeman evaporitic lagoons western australia a comparative study abstract hutt and leeman lagoons in the perth basin are shallow m evaporitic basins developed as a result of ponding in a pleistocene dune terrain the contemporary lagoons are playa surfaces in which sedimentation is related to distribution and interaction of hydrologic units groundwaters seawater and lagoonal brines precipitation is restricted to ephemeral halite in ponded waters and diagenetic emplacement of caco3 and caso4 minerals in playa sediments playa surfaces predominantly below sealevel fundamentally control the lagoonal circulation system a net influx of seawater impelled by a hydrostatic head of up to 4 m is seasonally balanced by evaporative reflux

formation of diagenetic dolomite in coastal sabkha along arabian persian gulf aragonitic intertidal sediments are being dolomitized in the subsurface supratidal environment of a sabkha along the south shore of the arabian persian gulf as a result of the percolation of wind driven marine derived brines the development of abundant diagenetic dolomite results from an optimum combination of high mmg mca ratio fluids a rapid flow rate which is related to the flooding frequency and sediment permeability and a shoreline configuration that maintains the proper conditions for a sufficient length of time strongly dolomitized areas are localized along the flanks of remnant channels and in the lee of islands that have been engulfed by the prograding sabkha the dolomitizing process takes about 1 000 to 1 500 years most diagenetic dolomite forms at the expense of primary aragonite according to the reaction mg 2caco3 arag rarr camg co3 2 ca dolomite forms under the following conditions a chloride concentration between 3 25 and 3 75 mcl kg b mmg mca greater than about 6 c ph between 6 3 and 6 9 d minimum pco2 of 10 2 to 10 3 atm e temperature between 25 and 40 c f saturation with respect to gypsum and g reducing environment in association with hydrogen sulfide other recent dolomite occurrences in the bahamas florida and bonaire are significantly different in terms of stratigraphy degree of lithification and evaporite mineral association from those in the arabian gulf

postseismic strain following the 1989 loma prieta earthquake from gps and leveling measurements postseismic deformation in the 5 years following the 1989 loma prieta earthquake has been measured with the global positioning system and precise leveling postearthquake velocities at distances greater than 20 km from the coseismic rupture are not significantly different from those observed in the 20 years prior to the earthquake however velocities at stations within 20 km of the rupture exceed preearthquake rates and exhibit unanticipated contraction normal to the strike of the san andreas fault system a combination of forward modeling and nonlinear optimization suggests that the observed postseismic deformations were caused by aseismic oblique reverse slip averaging 2 9 cm yr on the san andreas fault and or the loma prieta rupture zone and 2 4 cm yr reverse slip along a buried fault within the foothills thrust belt the best fitting sources of postseismic deformation are all located at depths of less than 15 km we find no evidence for accelerated flow or shear below the loma prieta rupture in the first 5 years following the earthquake the inferred postseismic slip is likely to have been caused by the coseismic stress change updip of the 1989 rupture

toggling of seismicity by the 1997 kagoshima earthquake couplet a demonstration of time dependent stress transfer 1 two m 6 well recorded strike slip earthquakes struck just 4 km and 48 days apart in kagoshima prefecture japan in 1997 providing an opportunity to study earthquake interaction aftershocks are abundant where the coulomb stress is calculated to have been increased by the first event and they abruptly stop where the stress is dropped by the second event this ability of the main shocks to toggle seismicity on and off argues that static stress changes play a major role in exciting aftershocks whereas the dynamic coulomb stresses which should only promote seismicity appear to play a secondary role if true the net stress changes from a sequence of earthquakes might be expected to govern the subsequent seismicity distribution however adding the stress changes from the two kagoshima events does not fully capture the ensuing seismicity such as its rate change temporal decay or migration away from the ends of the ruptures we therefore implement a stress transfer model that incorporates rate state friction in which seismicity is treated as a sequence of independent nucleation events that are dependent on the fault slip slip rate and elapsed time since the last event the model reproduces the temporal response of seismicity to successive stress changes including toggling decay and aftershock migration nevertheless the match of observed to predicted seismicity is quite imperfect due perhaps to inadequate knowledge of several model parameters however to demonstrate the potential of this approach we build a probabilistic forecast of larger earthquakes on the expected rate of small aftershocks taking advantage of the large statistical sample the small shocks afford not surprisingly such probabilities are highly time and location dependent during the first decade after the main shocks the seismicity rate and the chance of successive large shocks are about an order of magnitude higher than the background rate and are concentrated exclusively in the stress triggering zones

near source modeling of the loma prieta earthquake evidence for heterogeneous slip and implications for earthquake hazard i have analyzed records from 20 strong motion instruments that recorded the 1989 loma prieta earthquake ml 6 9 to determine the spatial and temporal distribution of slip using a tomographic back projection technique i find that the earthquake ruptured bilaterally at approximately 80 of the local shear wave velocity over a distance of slightly more than 30 km from 13 km northwest to 20 km southeast of the hypocenter slip within the rupture zone was highly variable both in magnitude and direction slip at the hypocenter and immediately up dip was relatively low about 1 m there were two main areas of concentrated slip one centered 7 km northwest of the hypocenter at a depth of 14 km and the other centered 6 km southeast of the hypocenter at a depth of 12 km peak slip amplitude on these high slip patches exceeded 4 5 m a surprising aspect of the rupture model is that rake varies from being predominately strike slip to the southeast of the hypocenter to being predominately reverse slip to the northwest despite this variation in rake the average rake determined from the strong motion data is in agreement with that determined teleseismically there is a correlation between areas of high slip and areas of low aftershock activity a simple model to explain this observation is that the areas of high slip are also areas of high strength which slip only during infrequent large mainshocks other areas of the fault are weaker and may slip both seismically and aseismically most of the slip in the loma prieta earthquake occurred between 9 and 16 km depth on a structure that dips to the southwest and runs underneath the surface trace of the san andreas fault at a depth of 8 to 10 km this observation complicates the assessment of seismic hazard on the southern santa cruz mountains segment of the san andreas fault

the use of strontium isotopes in determining groundwater mixing and brine fingering in a playa spring zone lake tyrrell australia j hydrol 167 225 239 playas in arid and semi arid regions can be important regional groundwater discharge zones located in the northwestern portion of victoria australia lake tyrrell is a large playa that serves as a major discharge zone of regional groundwater the chemistry of the regional groundwater is unique having salinities of approximately 30 g 1 1 and ph values as low as 2 8 as the regional groundwater is discharged along the western edge of the playa in the spring zone it is evapo concentrated into a dense brine which sinks through the playa sediments macumber s pioneering work on the geohydrology of this region established that this denser reflux brine below the playa surface has evolved from the evaporation of regional groundwaters we have measured the 87sr 86sr ratio of a number of groundwater samples taken at shallow depths along the spring zone at the playa edge and compared them with the regional groundwaters and the reflux brine our data reveal a rather complex pattern of mixing of the various water masses associated with the playa system most probably the result of density driven convective plumes two types of regional groundwater can be identified based on their 87sr 86sr values with groundwater from the south being less radiogenic than that flowing from the north i e 0 7108 vs 0 7142 vertical variations in 87sr 86sr in the regional groundwaters observed in wells adjacent to the spring zone can also be identified along a vertical gradient in the spring zone besides the hydrogeological information provided by these measurements the strontium isotope variations allow us to evaluate the source of strontium to these waters the regional groundwaters south of the lake are very similar in 87sr 86sr ratio to the reflux brines whereas the northern regional groundwaters are comparatively enriched in 87sr this enhanced 87sr signature indicates the addition of sr along the flow path via the weathering of the aquifer material the miocene parilla sand the southern waters have 87sr 86sr values closer to that of present day seawater and or miocene seawater indicating that the major source of sr to the system is from one or both of these sources our results demonstrate that strontium isotopes can be extremely useful in studying the dynamics as well as the geochemistry of surface water groundwater interactions

rupture history of the 1989 loma prieta california earthquake strong motion records of the 1989 loma prieta earthquake are inverted to determine a model of the rupture history uncorrected horizontal and vertical accelerograms are integrated to particle velocity time histories for 38 stations within an epicentral range of 75 km the time histories are bandpassed filtered with corners at 0 05 and 1 0 hz these bandpassed time histories are inverted using a nonlinear method to solve for the distribution of slip amplitudes and rupture times at specified locations on the fault plane the fault plane is specified a priori 38 km long and 17 km wide extending from 3 to 19 km depth at a constant dip of 70 starting models have rupture times based on constant rupture velocities of 2 5 2 8 and 3 0 km sec and uniform slip with rise times of 0 5 1 0 1 5 and 3 0 sec the waveform inversion results show the strike slip displacement is concentrated at the southern end of the rupture rake 156 and the dip slip displacement is concentrated at the northern end of the rupture rake 115 the average total slip is partitioned almost equally between strike slip and dip slip rake 137 the hypocentral area has an unusually small amount of slip with almost no slip in a region just to the north and up dip from the hypocenter the rupture front is complex propagating up dip to the south faster than it propagates to the north the region of maximum strike slip to the southeast radiates simultaneously with the region of maximum dip slip to the northwest the average rupture velocity is 3 0 km sec approximately 0 83 times the local shear wave speed the calculated seismic moment is 3 5 0 5 1026 dyne cm

hydrogeochemistry and weathering on the yilgarn block western australia ferrolysis and heavy metals in continental brines abstract groundwater samples collected from salt lakes over the archean yilgarn block of western australia show the influence of climate on geochemical weathering patterns major element compositions in salt lake groundwaters show little systematic trend but there are marked differences in ph and fe cu and pb concentrations in the alkaline carbonated systems of the arid northeastern part compared to concentrations of the same elements in groundwater systems of the mediterranean type climatic region of the wheatbelt in the southwestern part of the block in this latter region in addition to higher rainfall input the groundwater systems have steeper hydraulic gradients weathering bedrock is closer to the surface and oxidation and hydrolysis of iron ferrolysis is evident in weathering profiles a combination of vertical diffusion and horizontal groundwater flow is thought to be responsible for the development of ferrolysis and acidity in groundwaters in this region anomalously high concentrations of heavy metals particularly lead are observed in some of the acid saline groundwaters the formation of laterite which is a ubiquitous feature of the yilgam block landscape is a process probably closely linked to ferrolysis

coseismic fault slip of the september 16 2015 mw 8 3 illapel chile earthquake estimated from insar data the complete surface deformation of 2015 mw 8 3 illapel chile earthquake is obtained using sar interferograms obtained for descending and ascending sentinel 1 orbits we find that the illapel event is predominantly thrust as expected for an earthquake on the interface between the nazca and south america plates with a slight right lateral strike slip component the maximum thrust slip and right lateral strike slip reach 8 3 and 1 5 m respectively both located at a depth of 8 km northwest to the epicenter the total estimated seismic moment is 3 28 1021 n m corresponding to a moment magnitude mw 8 27 in our model the rupture breaks all the way up to the sea floor at the trench which is consistent with the destructive tsunami following the earthquake we also find the slip distribution correlates closely with previous estimates of interseismic locking distribution we argue that positive coulomb stress changes caused by the illapel earthquake may favor earthquakes on the extensional faults in this area finally based on our inferred coseismic slip model and coulomb stress calculation we envision that the subduction interface that last slipped in the 1922 mw 8 4 vallenar earthquake might be near the upper end of its seismic quiescence and the earthquake potential in this region is urgent

aftershocks halted by static stress shadows the role of permanent versus transient crustal stress changes in triggering earthquakes is debated analysis of the stress imparted by the joshua tree and nearby landers earthquakes in california in 1992 implies that a permanent drop in stress can halt aftershocks so such static stress changes should be incorporated into seismic hazard assessments

forecasts of the 1989 loma prieta california earthquake abstract the magnitude m w 6 9 loma prieta earthquake struck the san francisco bay area of central california at 5 04 p m local time on 17 october 1989 killing 62 people and generating billions of dollars in property damage scientists were not surprised by the occurrence of a destructive earthquake in this region and had in fact been attempting to forecast the location of the next large earthquake in the san francisco bay area for decades this article summarizes more than 20 scientifically based predictions made before the 1989 loma prieta earthquake for a large earthquake that might occur in the loma prieta region the predictions geographically closest to the actual earthquake primarily specified slip on the san andreas fault northwest of san juan bautista a number of the predictions did encompass the magnitude of the actual earthquake and at least one approximately encompassed the along strike rupture length post loma prieta studies of the 1906 san francisco california earthquake in the loma prieta region of the san andreas fault zone show the loma prieta and 1906 events with different senses of slip and fault plane dip therefore some have argued that the 1989 earthquake was not foreseen even though 1 this earthquake appears to have released much of the horizontal strain accumulated since 1906 and 2 not all of the forecasts were based on 1906 behavior

aftershock triggering by postseismic stresses a study based on coulomb rate and state models the spatiotemporal clustering of earthquakes is a feature of medium and short term seismicity indicating that earthquakes interact however controversy exists about the physical mechanism behind aftershock triggering static stress transfer and reloading by postseismic processes have been proposed as explanations in this work we use a coulomb rate and state model to study the role of coseismic and postseismic stress changes on aftershocks and focus on two processes creep on the main shock fault plane afterslip and secondary aftershock triggering by previous aftershocks we model the seismic response to coulomb stress changes using the dieterich constitutive law and focus on two events the parkfield mw 6 0 and the tohoku mw 9 0 earthquakes we find that modeling secondary triggering systematically improves the maximum log likelihood fit of the sequences the effect of afterslip is more subtle and difficult to assess for near fault events where model errors are largest more robust conclusions can be drawn for off fault aftershocks following the tohoku earthquake afterslip promotes shallow crustal seismicity in the fukushima region simple geometrical considerations indicate that afterslip induced stress changes may have been significant on trench parallel crustal fault systems following several of the largest recorded subduction earthquakes moreover the time dependence of afterslip strongly enhances its triggering potential seismicity triggered by an instantaneous stress change decays more quickly than seismicity triggered by gradual loading and as a result we find afterslip to be particularly important between few weeks and few months after the main shock

geodetic estimate of coseismic slip during the 1989 loma prieta california earthquake offsets in the relative positions of geodetic stations resulting from the loma prieta earthquake can be explained with a dislocation model that includes buried oblique slip on a rupture surface extending 37 km along the strike of the san andreas fault dipping 70 degree to the sw and extending from a depth of about 5 to 17 5 km assuming uniform slip on a rectangular surface the mean values for a range of reasonable fault geometries are 1 6 plus minus 0 3 m right lateral strike slip and 1 2 plus minus 0 3 m reverse slip slip on an adjacent extension of the rupture to the southeast recorded in the aftershock sequence is not well constrained by the geodetic data the geodetic data clearly preclude rupture extending near the surface

faulting geometry and slip from co seismic elevation changes the 18 october 1989 loma prieta california earthquake abstract leveling surveys conducted before and after the loma prieta earthquake provide observations of the co seismic elevation changes these data are used to determine the faulting geometry and distribution of slip considering planar listric and negatively listric fault shapes both the planar and nonplanar models produce elevation changes consistent with the observations most of the observed elevation changes can be modeled with a rupture surface that extends from 4 to 15 km depth if the rupture surface is planar the observations require 2 4 m of right lateral strike slip and 1 7 m of reverse slip on a 34 km long plane that dips 60 sw the best fitting model faults lie above and to the southwest of the aftershock zone a significantly better fit to the observations is obtained when these fault geometries are allowed to have two rake values with a larger thrust component northwest of the epicenter and a larger strike slip component southeast of the epicenter when a low modulus layer over a half space is used for consistency with the seismic p wave velocity structure the fault deepens coming within 3 km of the hypocenter but still locates several kilometers southwest of most aftershocks

geochemistry of the onyx river wright valley antarctica and its role in the chemical evolution of lake vanda the onyx river wright valley antarctica is a dilute meltwater stream originating in the vicinity of the wright lower glacier it acquires a significant fraction of its salt content when glacial meltwaters contact wright valley soils at lake brownworth and the concentrations of all ions increase with distance along the 28 km channel down to lake vanda average millimolar concentrations of major ions at the vanda weir during the 1980 1981 flow season were ca 0 119 mg 0 061 na 0 212 k 0 033 q 0 212 so4 0 045 hco3 0 295 and sio2 0 049 based on the flow measurements of chinn 1982 this amounts to an annual flux in moles to lake vanda of ca 0 238 106 mg 0 122 106 na 0 424 106 k 0 066 106 cl 0 424 106 so4 0 09 106 hco3 0 59 106 sio2 0 098 106 in spite of the large salt input from this source equilibrium evaporation of onyx river water would have resulted in early calcite deposition and in the formation of a na mg cl hco3 brine rather than in the ca na mg cl waters observed in lake vanda the river alone could not have produced a brine having the qualitative geochemical features of the lower saline waters of lake vanda it is proposed that the vanda brine is instead the result of past 1200 yrs bp mixing events between onyx river inflows and calcium chloride rich deep groundwaters derived from the don juan basin the mixing model presented here shows that the onyx river is the major contributor of k hco3 so4 and possibly mg found in the lake and a significant contributor approximately one half of the observed na calcium and cl on the other hand came largely from deep groundwater sources in the don juan basin all concentrations except mg are well predicted by this model the chemical composition of the geologically recent upper lake is explained in terms of ionic diffusion from the pre formed brine coupled with onyx river inflow ionic ratios calculated from this latter model are in very good agreement with those observed in the lake at 35 meters

holocene dolomitization of calcium carbonate sediments from the coastal sabkhas of abu dhabi u a e a stable isotope study secondary dolomitization of holocene calcium carbonate sediments occurs beneath the coastal sabkhas of abu dhabi u a e a stable isotope study of the sabhka sediments and brines indicates that the

a search for evidence of secondary static stress triggering during the 1992 mw7 3 landers california earthquake sequence secondary triggering of aftershocks is widely observed and often ascribed to secondary static stress transfer however small to moderate earthquakes are generally disregarded in estimates of coulomb stress changes cfs either because of source parameter uncertainties or a perceived lack of importance we use recently published high quality focal mechanisms and hypocenters to reassess the role of small to moderate earthquakes for static stress triggering of aftershocks during the 1992 mw7 3 landers california earthquake sequence we compare the cfs imparted by aftershocks 2 m 6 onto subsequent aftershocks with the total cfs induced by the m 6 main shocks we find that incremental stress changes between aftershock pairs are potentially more often positive than expected over intermediate distances cumulative aftershock stress changes are not reliable for receivers with nearby aftershock stress sources because we exclude unrealistic aftershock stress shadows that result from uniform slip models nonetheless 27 of aftershocks receive greater positive stress from aftershocks than from the main shocks overall 85 of aftershocks are encouraged by the main shocks while adding secondary stress encourages only 79 we infer that source parameter uncertainties of small aftershocks remain too large to convincingly demonstrate or rule out that secondary stress transfer induces aftershocks an important exception concerns aftershocks in main shock stress shadows well resolved secondary stress from detected aftershocks rarely compensates negative main shock stress these aftershocks require a different triggering mechanism

calcification of cyanobacterial mats in solar lake sinai pore water samples were obtained from the shallow water part of solar lake sinai where luxurious cyanobacterial mats grow these samples were analyzed for ca sup 2 mg sup 2 sr sup 2 cl sup so sub 4 sup 2 and titration alkalinity ta to determine the role of cyanobacterial growth and degradation on the calcification of the mats the data are modeled thermodynamically to predict mineral pore water equilibria the data support earlier bacterial and sedimentological studies suggesting that the degradation of the cyanobacterial mat via sulfate reduction is of major importance in the calcification process 34 references 1 figure 2 tables

the origin of a mississippi valley type deposit in the viburnum trend southeast missouri petrologic and isotopic studies of the base metal sulfide mineralization in the buick mine have established the following paragenesis an early stage of fine grained pyrite and marcasite replaced the dolostone host rock the iron disulfides and dolostone were replaced by sphalerite during the main stage of sphalerite precipitation delta 34 s sp 16 5 to 19 0ppm following the sphalerite two main stages of galena were precipitated a cuboctahedral stage delta 34 s gn 21 3 to 10 0ppm during which galena replaced dolostone and sphalerite and grew in vugs in the dolostone and in the sphalerite and a cubic stage delta 34 s gn 10 5 to 0 9ppm the two stages of galena precipitation were separated by a period during which the main stage of sphalerite and the cuboctahedral galena were partially dissolved and bladed marcasite delta 34 s mc 2 3 to 4 1ppm was precipitated and followed by a period of mild etching of the cubic galena minor amounts of pale sphalerite delta 34 s sp 14 0 to 16 5ppm were then precipitated white and colorless rhombs of dolomite were precipitated at the same time as some of the main stage of sphalerite the cubic galena and the pale sphalerite after the pale sphalerite was precipitated chalcopyrite quartz and siegenite were precipitated followed by calcite and finally kaolinite the paragenetic and textural data for both galena and sphalerite in the buick mine strongly suggest that these minerals were precipitated and dissolved repetitively during ore formation similar textural evidence for galena reported by other workers for different mines in the viburnum trend implies that repetitive precipitation and dissolution of galena was probably an important process during the formation of the entire viburnum trend fluid inclusions in late quartz crystals coprecipitated with chalcopyrite have filling and freezing temperatures within the ranges reported for sphalerite from other parts of the viburnum trend demonstrating that the ore forming solutions in the viburnum trend were probably at temperatures in the range 105 degrees to 141 degrees c assuming a maximum pressure correction of about 11 degrees c and were extremely saline containing about 4 0 to 5 0 molal chloride consideration of typical chemical analyses of fluid inclusions from other mississippi valley type deposits suggests that the maximum likely concentration of dissolved co 2 in the ore forming solutions was about 0 05 molal at these concentrations the ore forming solutions that travelled to the site of deposition could have had ph9s about two units less than neutral and still have been in equilibrium with dolomite or calcite the ranges of sulfur and lead isotopic compositions of galena from the buick mine are very similar to those established by earlier studies of the old lead belt and adjacent subdistricts however the correlation of isotopically lighter sulfur with more radiogenic lead is much stronger for the viburnum trend suggesting that galena was precipitated from many solutions each of which carried lead and sulfur together and had different lead and overall sulfur isotopic compositions the textural paragenetic and isotopic data for the buick mine cannot be easily explained either by a sulfate reduction model in which sulfate was reduced by organic matter or methane or mixing models in which base metals and reduced sulfur were transported separately to the site of deposition instead the data strongly support a reduced sulfur model for ore formation according to which the base metals were transported together with reduced sulfur as h 2 s in the same solutions to the sites of deposition the reduced sulfur model is also supported by a theoretical calculation of the solubilities of galena and sphalerite which demonstrate that low but economically significant concentrations of lead or zinc could have been transported as chloride complexes together with equivalent concentrations of reduced sulfur at temperatures of 100 degrees to 150 degrees c and ph9s of one to two units less than neutral and could have been precipitated by ph increases cooling or dilution at the sites of deposition

sensitivity of static stress calculations to the earthquake slip distribution 1 the apparent strong correlation between coulomb stress changes and the spatial distribution of aftershocks suggests the possibility of making near real time estimations of areas at risk of experiencing off fault aftershocks in order to do this in practice a number of issues must first be addressed including the extent to which the main shock slip must be known in detail before a meaningful stress map can be constructed here we investigate this issue by constructing a time ordered sequence of slip solutions for the landers earthquake computing coulomb stress changes for each solution and quantitatively comparing the stress field with the observed aftershocks by 1 resolving the coulomb stress change onto the aftershock nodal planes and calculating the percentage of events consistent with triggering and 2 constructing a two dimensional map of coulomb stress and computing the correlation coefficient between the positive and negative areas and the locations of the aftershocks we find that slip solutions based on empirical relations and either focal mechanism or moment tensor data produce stress fields inconsistent with the observed spatial distribution of aftershocks whereas slip solutions incorporating the correct rupture geometry but greatly simplified slip produce stress fields consistent with the aftershock distribution when very near fault events are excluded we further find that resolving stress perturbations onto earthquake nodal planes and computing the percentage of events experiencing positive coulomb stress provides a poor measure of success because of the limited range of structures on which events occur and the compatibility of the main shock stress field with these structures our results support the hypothesis that coulomb stress changes affect the spatial distribution of aftershocks and suggest that meaningful calculations of coulomb stress can be made as soon as an earthquake s rupture geometry is well constrained

the october 17 1989 loma prieta california earthquake and its aftershocks geometry of the sequence from high resolution locations hypocenters of the loma prieta sequence form a dipping zone that rises from the mainshock hypocenter and is parallel to the mainshock nodal plane most aftershocks cluster around the perimeter of the zone surrounding a relatively aseismic center which approximates the region of mainshock rupture at its southeastern end the dipping aftershock zone warps into a vertical surface that corresponds to the san andreas fault in the central and northwestern parts of the zone at depths above 10 km the aftershocks define numerous disjoint fault structures the large component of reverse slip observed in this event agrees with a simple model for slip on a dipping plane within a compressional fault bend we do not believe that the loma prieta earthquake occurred on the sargent fault however we are unable to conclude whether it ruptured the principal plate boundary fault or a less frequently active fault

a global search for stress shadows 1 debate continues regarding the relative proportion of earthquakes triggered by passing seismic waves versus static stress changes from a main shock static stress changes are expected to have long term effects on earthquake probabilities whereas dynamic stress changes due to the passing of seismic waves should not both mechanisms are expected to raise seismicity rates in some areas but only static stress change calculations predict rate decrease shadows thus identification of post main shock earthquake suppression is diagnostic of a static stress change process we note that in principle static stress change theory predicts suppression of particular earthquake mechanisms in a shadow zone rather than an overall rate reduction a stress shadow can therefore be characterized by a change in the average earthquake focal mechanism before and after a main shock that results from suppression of a given mechanism type we examined average mechanisms from 2 radii and 5 year periods before and after 119 ms 7 main shock earthquakes drawn from the harvard centroid moment tensor cmt catalog significant average mechanism changes caused by earthquake suppression were found in only two cases however by stacking the data we were able to resolve statistically significant suppression of particular post main shock focal mechanisms this indicates that while static stress shadows are subtle they are indeed present in the global catalog

mechanism diversity of the loma prieta aftershocks and the mechanics of mainshock aftershock interaction the diverse aftershock sequence of the 1989 loma prieta earthquake is inconsistent with conventional models of mainshock aftershock interaction because the aftershocks do not accommodate mainshock induced stress changes instead the sense of slip of the aftershocks is consistent with failure in response to a nearly uniaxial stress field in which the maximum principal stress acts almost normal to the mainshock fault plane this orientation implies that i stress drop in the mainshock was nearly complete ii mainshock induced decreases of fault strength helped were important in controlling the occurrence of after shocks and iii mainshock rupture was limited to those sections of the fault with preexisting shear stress available to drive fault slip

organic geochemistry of the pine point lead zinc ore field and region northwest territories canada middle devonian barrier complex within the pine point lead zinc ore field located within the immature to marginally mature zone with respect to petroleum generation certain of the host rocks of the ore field were deposited in highly reducing conditions such that organic matter of algal or microbial origin has been preserved to a remarkable degree yields of extractable soluble material relative to organic carbon are extremely high which has resulted in the local migration of immature bitumens and heavy oil within the shallow regime polymerization of nitrogen sulfur and oxygen compounds and asphaltenes biodegradation of bitumens is a relatively late process the question of a genetic relationship between lead zinc sulfide and organic matter remains open modified journal abstract

forecasting the evolution of seismicity in southern california animations built on earthquake stress transfer 1 we develop a forecast model to reproduce the distribution of main shocks aftershocks and surrounding seismicity observed during 1986 2003 in a 300 310 km area centered on the 1992 m 7 3 landers earthquake to parse the catalog into frames with equal numbers of aftershocks we animate seismicity in log time increments that lengthen after each main shock this reveals aftershock zone migration expansion and densification we implement a rate state algorithm that incorporates the static stress transferred by each m 6 shock and then evolves coulomb stress changes amplify the background seismicity so small stress changes produce large changes in seismicity rate in areas of high background seismicity similarly seismicity rate declines in the stress shadows are evident only in areas with previously high seismicity rates thus a key constituent of the model is the background seismicity rate which we smooth from 1981 to 1986 seismicity the mean correlation coefficient between observed and predicted m 1 4 shocks the minimum magnitude of completeness is 0 52 for 1986 2003 and 0 63 for 1992 2003 a control standard aftershock model yields 0 54 and 0 52 for the same periods four m 6 0 shocks struck during the test period three are located at sites where the expected seismicity rate falls above the 92 percentile and one is located above the 75 percentile the model thus reproduces much but certainly not all of the observed spatial and temporal seismicity from which we infer that the decaying effect of stress transferred by successive main shocks influences seismicity for decades finally we offer a m 5 earthquake forecast for 2005 2015 assigning probabilities to 324 10 10 km cells

diverse pliocene quaternary tectonics in a transform environment san francisco bay region california the san francisco bay region occupies part of the diffuse transform boundary zone between the pacific and north american plates although dextral strike slip faulting is dominant the plate motion is expressed in a variety of ways some strike slip faults are parallel with the plate boundary but some are slightly oblique the major strike slip faults are zones in which an interplay occurs between strands some of which are en echelon this interplay may be responsible for some apparent pull apart basins and presumed normal faults along the calaveras fault zone in extensive areas between strike slip fault zones there are many compressional structures folds and reverse faults that are pliocene quaternary in age hence largely coeval with the strike slip faults some of the compressional structures are oblique to and others are parallel with the major strike slip faults we have divided the san francisco bay region into domains on the basis of the types orientations and relationships of structures or in one case lack of strong deformation each domain has responded in its own way to the regional northwest southeast dextral shear imposed by the relative plate motion in an attempt to understand the origin of various structures and the interrelationships among them we compared established models with our observations some observed geometric relationships agree quite well with classical coulomb anderson and simple shear models but others require further explanation models based on fault interaction seem to apply to certain cases for example the east bay hills domain which is between the left stepping calaveras and hayward rodgers creek fault zones is under compression resulting from interaction between the two strike slip zones

the trace metal geochemistry of pore water brines from two hypersaline lakes abstract pore water brines from modern hypersaline lake sediments have been analyzed to determine the nature of brine trace metal interaction in these environments brine samples were collected from the surface waters and sedimentary pore waters of two hypersaline lakes solar lake eastern sinai and goto meer bonaire netherlands antilles metal concentrations in the oxic brines appear to be related to the strength of their respective chloride complexes with pb and zn more enriched than cu pore water cu zn and fe concentrations increased across the sediment water interface in both lakes all three metal profiles follow a similar pattern with depth in the pore water higher concentrations were observed in the goto meer brines strontium isotopes were utilized to qualify better the mixing of the brines in solar lake although these metals were observed at higher concentrations than sea water in these sulfidic brines their concentrations are not close to what is needed for the formation of ore deposits

the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a abstract land l s and prezbindowski d r 1981 the origin and evolution of saline formation water lower cretaceous carbonates south central texas u s a in w back and r letolle guest editors symposium on geochemistry of groundwater 26th international geological congress j hydrol 54 51ndash 74 systematic chemical variation exists in formation water collected from a dip section through lower cretaceous rocks of south central texas chemical variation can be explained by an interactive water rock diagenetic model the cyclic lower cretaceous shelf carbonates of the edwards group dip into the gulf of mexico coast geosyncline and can be considered to a first approximation as part of a complex aquifer contained by paleozoic basement beneath and by relatively impermeable upper cretaceous clay and chalk above the hydrodynamic character of this carbonate system is strongly controlled by major fault systems major fault systems serve as pathways for vertical movement of basinal brines into the lower cretaceous section formation water movement in this sytem has strong upfault and updip components the parent na ca cl brine originates deep in the gulf of mexico basin at temperatures between 200 and 250 c by the reaction halite detrital plagioclase quartz water albite brine other dissolved components originate by reaction of the fluid with the sedimentary phases k feldspar calcite dolomite anhydrite celestite barite and fluorite significant quantities of pb zn and fe have been mobilized as well as the brine moves updip out of the overpressured deep gulf of mexico basin and encounters limestones of the stuart city reef trend the buried platform margin small amounts of galena precipitate in late fractures continuing to rise upfault and updip the brine becomes progressively diluted on encountering significant quantities of dolomite in the backreef facies the ca rich brine causes dedolomitization although thermochemical consideration suggests that small amounts of several authigenic phases should precipitate most have yet to be found minor amounts of several kinds of calcite spar are present however as the brine evolves by dilution and by cooling no systematic changes in any cation cl ratio occur except for regular updip gain in mg as a result of progressive dedolomitization the formation water highly diluted by meteoric water eventually discharges along faults as hot mineral water

decay of aftershock density with distance does not indicate triggering by dynamic stress the most predictable earthquakes are aftershocks which invariably follow mainshocks so what triggers aftershocks it was recently argued controversially that the decay of aftershocks with distance from the main earthquake could only be explained by dynamic triggering keith richards dinger and colleagues have tested this hypothesis and conclude that the observed decay can be better explained by static triggering resolving whether static or dynamic stress triggers most aftershocks and subsequent mainshocks is essential to understand earthquake interaction and to forecast seismic hazard it has recently been argued that the decay of aftershocks with distance from the main earthquake could be explained only by dynamic triggering this hypothesis has now been tested the conclusion being that the observed decay can be better explained by static triggering resolving whether static1 2 3 or dynamic4 5 6 7 8 stress triggers most aftershocks and subsequent mainshocks is essential to understand earthquake interaction and to forecast seismic hazard9 felzer and brodsky10 examined the distance distribution of earthquakes occurring in the first five minutes after 2 m 3 and 3 m 4 mainshocks and found that their magnitude m 2 aftershocks showed a uniform power law decay with slope 1 35 out to 50 km from the mainshocks from this they argued that the distance decay could be explained only by dynamic triggering here we propose an alternative explanation for the decay and subject their hypothesis to a series of tests none of which it passes at distances more than 300 m from the 2 m 3 mainshocks the seismicity decay 5 min before the mainshocks is indistinguishable from the decay five minutes afterwards indicating that the mainshocks have no effect at distances outside their static triggering range omori temporal decay the fundamental signature of aftershocks is absent at distances exceeding 10 km from the mainshocks finally the distance decay is found among aftershocks that occur before the arrival of the seismic wave front from the mainshock which violates causality we argue that felzer and brodsky10 implicitly assume that the first of two independent aftershocks along a fault rupture triggers the second and that the first of two shocks in a creep or intrusion driven swarm triggers the second when this need not be the case

time dependent triggered afterslip following the 1989 loma prieta earthquake we employ a modified version of the network inversion filter to investigate time dependent slip following the 1989 loma prieta earthquake previous analysis of global positioning system gps and leveling data suggests afterslip on the loma prieta rupture as well as aseismic slip on a thrust fault northeast of the san andreas fault which we identify with the foothills thrust belt we analyzed 173 daily gps solution files at 62 stations collected from 1989 8 to 1998 3 a total of 1 134 three dimensional relative baseline determinations the observed position changes are assumed to result from secular deformation random benchmark motions and temporally varying fault slip the data reveal temporal variations in slip rate but poorly resolve spatial variations in fault slip the amount of temporal smoothing is estimated by maximum likelihood conditional on this estimate reverse slip on the foothills thrust decays from 45 12 mm yr immediately after the earthquake to zero by 1992 reverse slip on the loma prieta rupture surface decays from 57 11 mm yr to zero by 1994 right lateral slip on the loma prieta rupture surface decays monotonically from 30 10 mm yr to zero by 1994 these results suggest that 1 triggered afterslip can occur off the main rupture zone on adjacent faults 2 shallow afterslip dominated the postseismic deformation for the 8 years following the earthquake and 3 postseismic slip on the foothills thrust may account for a significant portion of its total slip budget

large scale relocation of two decades of northern california seismicity using cross correlation and double difference methods 1 we simultaneously reanalyzed two decades 1984 2003 of the digital seismic archive of northern california using waveform cross correlation cc and double difference dd methods to improve the resolution in hypocenter locations in the existing earthquake catalog generated at the northern california seismic network ncsn by up to three orders of magnitude we used a combination of 3 billion cc differential times measured from all correlated pairs of events that are separated by less than 5 km and 7 million p wave arrival time picks listed in the ncsn bulletin data were inverted for precise relative locations of 311 273 events using the dd method the relocated catalog is able to image the fine scale structure of seismicity associated with active faults and revealed characteristic spatiotemporal structures such as streaks and repeating earthquakes we found that 90 of the earthquakes have correlated p wave and s wave trains at common stations and that 12 are colocated repeating events an analysis of the repeating events indicates that uncertainties at the 95 confidence level in the existing network locations are on average 0 7 km laterally and 2 km vertically correlation characteristics and relative location improvement are remarkably similar across most of northern california implying the general applicability of these techniques to image high resolution seismicity caused by a variety of plate tectonic and anthropogenic processes we show that consistent long term seismic monitoring and data archiving practices are key to increase resolution in existing hypocenter catalogs and to estimate the precise location of future events on a routine basis

the trace metal geochemistry of the lake tyrrell system brines victoria australia abstract we have analyzed the waters of the lake tyrrell system for the trace metals fe mn pb cu and zn in a number of samples both dissolved fe t and fe 2 were also determined great care was exercized to minimize any contamination of the samples not surprisingly the highest metal concentrations occur in the low ph high eh regional groundwaters with much lower values observed in the reflux brines and the neutral wahpool timboram waters it is thought that the source of these metals to the groundwaters is the dissolution of a suite of heavy minerals within the aquifer the pliocene parilla sand as the acid groundwaters flow toward the lake some metals are removed near lake edge at depth the metal concentrations in the spring zone region indicate an extremely dynamic system where at different times of the year due to changes in the hydrologic regime metals can be either removed or solubilized

pyrite and oxidized iron mineral phases formed from pyrite oxidation in salt marsh and estuarine sediments abstract we used scanning electron microscopy and energy dispersive x ray analysis to examine sediments from vegetated portions of three salt marshes the great sippewissett marsh cape cod ma sapelo island georgia and the hackensack meadowlands n j and from the sediments of an estuary newark bay n j pyrite particles were abundant in sediments from all sites both fine grained pyrite crystals and framboids were found single fine grained crystals diameter 0 2 to 2 0 micrometers predominated in all samples strong evidence for rapid formation of pyrite we also found both microcrystalline and framboidal iron oxyhydroxide phases in many of the sediment samples this is evidence of pyrite oxidation within the sediments and suggests that iron is conserved in salt marshes even as pyrite is oxidized the thermodynamic stability of iron phases in marsh sediments and recent pyrite oxidation studies in coal suggest goethite as the crystalline iron oxyhydroxide phase present in addition we sometimes found a red amorphous coating on grass roots from the great sippewissett and sapelo island marshes this coating is likely a form of hydrated iron iii oxide

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

on the origin of diverse aftershock mechanisms following the 1989 loma prieta earthquake summary we test the hypothesis that the origin at the diverse suite of aftershock mechanisms following the 1989 m 7 1 loma prieta california earthquake is related to the post main shock static stress field we use a 3 d boundary element algorithm to calculate static stresses combined with a coulomb failure criterion to calculate conjugate failure planes at aftershock locations the post main shock static stress field is taken as the of a sum of a pre existing stress field and changes in stress due to the heterogeneous slip across the loma prieta rupture plane the background stress field is assumed to be either a simple shear parallel to the regional trend of the san andreas fault or approximately fault normal compression a suite of synthetic aftershock mechanisms from the conjugate failure planes is generated and quantitatively compared allowing for uncertainties in both mechanism parameters and earthquake locations to well constrained mechanisms reported in the us geological survey northern california seismic network catalogue we also compare calculated rakes with those observed by resolving the calculated stress tensor onto observed focal mechanism nodal planes assuming either plane to be a likely rupture plane various permutations of the assumed background stress field fractional coefficients of aftershock fault planer methods of comparisons etc explain between 52 and 92 per cent of the aftershock mechanisms we can explain a similar proportion of mechanisms however by comparing a randomly reordered catalogue with the various suites of synthetic aftershocks the inability to duplicate aftershock mechanisms reliably on a one to one basis is probably a function of the combined uncertainties in models of main shock slip distribution the background stress field and aftershock locations in particular we show theoretically that any specific main shock slip distribution and a reasonable background stress field are able lo generate a highly variable suite of failure planes such that quite different aftershock mechanisms may be expected to occur within a kilometre of less of each other this scale of variability is less than the probable location error of aftershock earthquakes in the loma prieta region we successfully duplicate a measure of the variability in the mechanisms of the entire suite of aftershocks if static stress changes are responsible for the generation of aftershock mechanisms we are able to place quantitative constraints on the level of stress that must have existed in the upper crust prior to the loma prieta rupture this stress level appears to be too low to generate the average dip across the main shock rupture plane possible reasons for this result range from incorrect initial assumptions of homogeneity in the background stress field friction and fault geometry to driving stresses that arise from deeper in the crust or upper mantle alternatively aftershock focal mechanisms may be determined by processes other than or in addition to static stress changes such as pore pressure changes or dynamic stresses

aftershock patterns and main shock faulting we have compared aftershock patterns following several moderate to large earthquakes with the corresponding distributions of coseismic slip obtained from previous analyses of the recorded strong ground motion and teleseismic waveforms well located aftershock hypocenters are projected onto the main shock fault plane and their positions are examined relative to the zones of coseismic displacement indicated by the estimated distributions of main shock slip we also examine the aftershock focal mechanisms when these data are available in an attempt to identify possible patterns of secondary faulting within the aftershock zone our results are consistent with a hypothesis of aftershock occurrence that requires a secondary redistribution of stress following primary failure on the earthquake fault aftershocks following the earthquakes examined in this study occur mostly outside of or near the edges of the source areas indicated by the patterns of main shock slip the spatial distribution of aftershocks reflects either a continuation of slip in the outer regions of the areas of maximum coseismic displacement or the activation of subsidiary faults within the volume surrounding the boundaries of main shock rupture

post 1906 stress recovery of the san andreas fault system calculated from three dimensional finite element analysis 1 the m 7 8 1906 san francisco earthquake cast a stress shadow across the san andreas fault system inhibiting other large earthquakes for at least 75 years the duration of the stress shadow is a key question in san francisco bay area seismic hazard assessment this study presents a three dimensional 3 d finite element simulation of post 1906 stress recovery the model reproduces observed geologic slip rates on major strike slip faults and produces surface velocity vectors comparable to geodetic measurements fault stressing rates calculated with the finite element model are evaluated against numbers calculated using deep dislocation slip in the finite element model tectonic stressing is distributed throughout the crust and upper mantle whereas tectonic stressing calculated with dislocations is focused mostly on faults in addition the finite element model incorporates postseismic effects such as deep afterslip and viscoelastic relaxation in the upper mantle more distributed stressing and postseismic effects in the finite element model lead to lower calculated tectonic stressing rates and longer stress shadow durations 17 74 years compared with 7 54 years all models considered indicate that the 1906 stress shadow was completely erased by tectonic loading no later than 1980 however the stress shadow still affects present day earthquake probability use of stressing rate parameters calculated with the finite element model yields a 7 12 reduction in 30 year probability caused by the 1906 stress shadow as compared with calculations not incorporating interactions the aggregate interaction based probability on selected segments not including the ruptured san andreas fault is 53 70 versus the noninteraction range of 65 77