inorganic isotopic and organic composition of high chloride water from wells in a coastal southern california aquifer abstract chloride concentrations were as high as 230 mg l in water from the surface discharge of long screened production wells in pleasant valley calif about 100 km nw of los angeles wells with the higher cl concentrations were near faults that bound the valley depending on well construction high cl water from different sources may enter a well at different depths for example cl concentration in the upper part of some wells completed in overlying aquifers influenced by irrigation return were as high as 220 mg l and cl concentrations in water sampled within wells at depths greater than 450 m were as high as 500 mg l these high cl waters mix within the well during pumping and produce the water sampled at the surface discharge changes in the major ion minor ion trace element and 34s and 13c isotopic composition of water in wells with depth were consistent with changes resulting from so4 reduction precipitation of calcite and cation exchange the chemical and isotopic composition of high cl water from deep wells trends towards the composition of oil field production water from the study area chloride concentrations in oil field production water present at depths 150 m beneath freshwater aquifers were 2200 mg l and cl concentrations in underlying marine rock were as high as 4400 mg l high cl concentrations in water from deeper parts of wells were associated with dissolved organic c composed primarily of hydrophobic neutral compounds believed to be similar to those associated with petroleum in underlying deposits these compounds would not be apparent using traditional sampling techniques and would not be detected using analytical methods intended to measure contamination

damped regional scale stress inversions methodology and examples for southern california and the coalinga aftershock sequence 1 we present a new focal mechanism stress inversion technique to produce regional scale models of stress orientation containing the minimum complexity necessary to fit the data current practice is to divide a region into small subareas and to independently fit a stress tensor to the focal mechanisms of each subarea this procedure may lead to apparent spatial variability that is actually an artifact of overfitting noisy data or nonuniquely fitting data that does not completely constrain the stress tensor to remove these artifacts while retaining any stress variations that are strongly required by the data we devise a damped inversion method to simultaneously invert for stress in all subareas while minimizing the difference in stress between adjacent subareas this method is conceptually similar to other geophysical inverse techniques that incorporate damping such as seismic tomography in checkerboard tests the damped inversion removes the stress rotation artifacts exhibited by an undamped inversion while resolving sharper true stress rotations than a simple smoothed model or a moving window inversion we show an example of a spatially damped stress field for southern california the methodology can also be used to study temporal stress changes and an example for the coalinga california aftershock sequence is shown we recommend use of the damped inversion technique for any study examining spatial or temporal variations in the stress field

the 2002 denali fault earthquake alaska a large magnitude slip partitioned event the mw moment magnitude 7 9 denali fault earthquake on 3 november 2002 was associated with 340 kilometers of surface rupture and was the largest strike slip earthquake in north america in almost 150 years it illuminates earthquake mechanics and hazards of large strike slip faults it began with thrusting on the previously unrecognized susitna glacier fault continued with right slip on the denali fault then took a right step and continued with right slip on the totschunda fault there is good correlation between geologically observed and geophysically inferred moment release the earthquake produced unusually strong distal effects in the rupture propagation direction including triggered seismicity

earthquake clusters in southern california i identification and stability 1 we use recent results on statistical analysis of seismicity to present a robust method for comprehensive detection and analysis of earthquake clusters the method is based on nearest neighbor distances of events in space time energy domain the method is applied to a 1981 2011 relocated seismicity catalog of southern california having 111 981 events with magnitudes m 2 and corresponding synthetic catalogs produced by the epidemic type aftershock sequence etas model analysis of the etas model demonstrates that the cluster detection results are accurate and stable with respect to 1 three numerical parameters of the method 2 variations of the minimal reported magnitude 3 catalog incompleteness and 4 location errors application of the method to the observed catalog separates the 111 981 examined earthquakes into 41 393 statistically significant clusters comprised of foreshocks mainshocks and aftershocks the results reproduce the essential known statistical properties of earthquake clusters which provide overall support for the proposed technique in addition systematic analysis with our method allows us to detect several new features of seismicity that include 1 existence of a significant population of single event clusters 2 existence of foreshock activity in natural seismicity that exceeds expectation based on the etas model and 3 dependence of all cluster properties except area on the magnitude difference of events from mainshocks but not on their absolute values the classification of detected clusters into several major types generally corresponding to singles burst like and swarm like sequences and correlations between different cluster types and geographic locations is addressed in a companion paper

improved methods for observing earth s time variable mass distribution with grace using spherical cap mascons we discuss several classes of improvements to gravity solutions from the gravity recovery and climate experiment grace mission these include both improvements in background geophysical models and orbital parameterization leading to the unconstrained spherical harmonic solution jpl rl05 and an alternate jpl rl05m mass concentration mascon solution benefitting from those same improvements but derived in surface spherical cap mascons the mascon basis functions allow for convenient application of a priori information derived from near global geophysical models to prevent striping in the solutions the resulting mass flux solutions are shown to suffer less from leakage errors than harmonic solutions and do not necessitate empirical filters to remove north south stripes lowering the dependence on using scale factors the global mean scale factor decreases by 0 17 to gain accurate mass estimates ocean bottom pressure obp time series derived from the mascon solutions are shown to have greater correlation with in situ data than do spherical harmonic solutions increase in correlation coefficient of 0 08 globally particularly in low latitude regions with small signal power increase in correlation coefficient of 0 35 regionally in addition to reducing the error rms with respect to the in situ data reduction of 0 37 cm globally and as much as 1 cm regionally greenland and antarctica mass balance estimates derived from the mascon solutions agree within formal uncertainties with previously published results computing basin averages for hydrology applications shows general agreement between harmonic and mascon solutions for large basins however mascon solutions typically have greater resolution for smaller spatial regions in particular when studying secular signals

contemporary deformation and stressing rates in southern alaska summary the subduction of the pacific plate beneath north america induces broad scale stressing of the alaskan crust that has led to the development of the highest mountains in north america the highest slip rates along some of the longest strike slip faults on earth and widespread seismicity that includes the 1964 m9 2 alaska earthquake the second largest ever recorded these features are a consequence of deformation associated with three primary processes interseismic loading due to relative plate motions large earthquakes and postseismic processes how these mechanisms contribute to the evolution of stress in the alaskan crust is not well understood here we use observed contemporary surface velocities to constrain 2 d and 3 d viscoelastic numerical models of relative pacific north american plate motions coseismic slip associated with the 1964 m9 2 megathrust event and strike slip earthquakes on the transform boundary in 1949 m8 1 1958 m7 8 and 1972 m7 6 the four largest events prior to the 2002 m7 9 denali quake viscoelastic relaxation following these events and afterslip to gain insight into how these processes are shaping alaska today results suggest that interseismic deformation and on going post seismic deformation following the 1964 earthquake both contribute significantly to the gps measured contemporary velocity field viscoelastic relaxation associated with a mantle with a viscosity of 10 19 pa s is required to explain southerly directed velocities that are observed in the cook inlet region to well north of the denali fault results also suggest that subduction of the pacific plate leads to a broad zone of deformation with high stressing rates concentrated in a band that lies several hundred kilometres from the plate boundary coeval with the inboard location of the maximum locking depth of the megathrust interseismic deformation and stressing rates remain high further inland across the yakutat microplate where flat subduction extends the width of the locked plate interface calculations show that post seismic relaxation following the large strike slip events serves to reload these rupture surfaces while relieving stress on the eastern denali fault post seismic relaxation following the 1964 earthquake combined with coseismic stress changes promoted the triggering of the 2002 denali quake calculations also suggest that over the past 50 years high stress has accumulated on part of the thrust interface to the west and east of the 1964 rupture surface and along the queen charlotte fault to the south of the 1949 rupture surface

seasonal hydrological loading in southern alaska observed by gps and grace 1 we compare vertical seasonal loading deformation observed by continuous gps stations in southern alaska and modeled vertical displacements due to seasonal hydrological loading inferred from grace seasonal displacements are significant and gps observed and grace modeled seasonal displacements are highly correlated we define a measure called the wrms reduction ratio to measure the fraction of the position variations at seasonal periods removed by correcting the gps time series using a seasonal model based on grace the median wrms reduction ratio is 0 82 and the mean is 0 73 0 26 with a value of 1 0 indicating perfect agreement of gps and grace the effects of atmosphere and non tidal ocean loading are important we add the aod1b de aliasing model to the grace solutions because the displacements due to these loads are present in the gps data and this improves the correlations between these two geodetic measurements we find weak correlations for some stations located in areas where the magnitude of the load changes over a short distance due to grace s limited spatial resolution grace models can correct seasonal displacements for campaign gps measurements as well

temporal and spatial variations of local magnitudes in alaska and aleutians and comparison with body wave and moment magnitudes abstract we evaluated temporal and spatial variability of local magnitudes m l in the earthquake catalog of alaska earthquake information center regionally recorded hypocenters in mainland alaska are available in the catalog beginning in the early 1970s no comprehensive aleutian wide m l statistics exist prior to the mid 1990s we identify four time intervals with similar magnitude residual trends between m l versus body wave magnitude m b 1971 1976 1977 1989 1990 1999 5 and 1999 5 2008 the three latter intervals are also identified in m l versus moment magnitude m w residual trends these time intervals correspond to the periods with different data processing procedures we find that the latest time interval has the best correspondence between m l and m w values in mainland alaska especially for shallow depth m l versus m b and m l versus m w are present in the catalog for the pre 1990 data the largest discrepancies are observed for the gulf of alaska earthquakes for the latest time period 1999 5 2008 the largest residuals between m l and m b or m w are observed for the earthquakes located within the oceanic segment of the aleutian arc and within 100 km of the trench we calculated a set of magnitude corrections for mainland alaska and the aleutians that need to be applied to homogenize magnitude values in the catalog in time and space

hydrospheric modulation of stress and seismicity on shallow faults in southern alaska abstract shallow 40 km low magnitude m 2 0 seismicity in southern alaska is examined for seasonal variations during the annual hydrological cycle the seismicity is declustered with a spatio temporal epidemic type aftershock sequence model the removal of aftershock sequences allows detailed investigation of seismicity rate changes as water snow and ice loads modulate crustal stresses throughout the year the grace surface loads are obtained from the jpl global land and ocean mass concentration blocks mascon solutions the stress changes at a depth of 10 km are calculated using a 1d spherical layered earth model to evaluate the induced seasonal stresses of 10 kpa we use 30 yr of earthquake focal mechanisms to constrain the tectonic background stress field orientation and assess the seasonal stress change with respect to the principal stress orientations the background stress field is assumed to control the preferred orientation of faulting and stress perturbations are expected to increase or decrease seismic activity on the faults the number of excess earthquakes is calculated with respect to the background seismicity rates for discrete stress intervals the results indicate a 25 increase in regional seismicity rates that correlate with a 3 month time lag following failure encouraging annual mean normal stress differential stress and least principal compressive stress no immediate earthquake rate variations are observed in this region wide analysis the correlation with a 3 month time lag suggests increased mobility of preexisting fluids at seismogenic depths is varying the pore pressure within fault zones to modulate the seismicity rates throughout the seasonal loading cycle

iceberg calving as a primary source of regional scale glacier generated seismicity in the st elias mountains alaska 1 since the installation of the alaska regional seismic network in the 1970s data analysts have noted nontectonic seismic events thought to be related to glacier dynamics while loose associations with the glaciers of the st elias mountains have been made no detailed study of the source locations has been undertaken we performed a two step investigation surrounding these events beginning with manual locations that guided an automated detection and event sifting routine results from the manual investigation highlight characteristics of the seismic waveforms including single peaked narrowband spectra emergent onsets lack of distinct phase arrivals and a predominant cluster of locations near the calving termini of several neighboring tidewater glaciers through these locations comparison with previous work analyses of waveform characteristics frequency magnitude statistics and temporal patterns in seismicity we suggest calving as a source for the seismicity statistical properties and time series analysis of the event catalog suggest a scale invariant process that has no single or simple forcing these results support the idea that calving is often a response to short lived or localized stress perturbations our results demonstrate the utility of passive seismic instrumentation to monitor relative changes in the rate and magnitude of iceberg calving at tidewater glaciers that may be volatile or susceptible to ensuing rapid retreat especially when existing seismic infrastructure can be used

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

seasonal water storage stress modulation and california seismicity establishing what controls the timing of earthquakes is fundamental to understanding the nature of the earthquake cycle and critical to determining time dependent earthquake hazard seasonal loading provides a natural laboratory to explore the crustal response to a quantifiable transient force in california water storage deforms the crust as snow and water accumulates during the wet winter months we used 9 years of global positioning system gps vertical deformation time series to constrain models of monthly hydrospheric loading and the resulting stress changes on fault planes of small earthquakes the seasonal loading analysis reveals earthquakes occurring more frequently during stress conditions that favor earthquake rupture we infer that california seismicity rates are modestly modulated by natural hydrological loading cycles

relationships between volcano gravitational spreading and magma intrusion volcano spreading with its characteristic sector grabens is caused by outward flow of weak substrata due to gravitational loading this process is now known to affect many present day edifices a volcano intrusive complex can form an important component of an edifice and may induce deformation while it develops such intrusions are clearly observed in ancient eroded volcanoes like the scottish palaeocene centres or in geophysical studies such as in la reunion or inferred from large calderas such as in hawaii the canaries or galapagos volcanoes volcano gravitational spreading and intrusive complex emplacement may act simultaneously within an edifice we explore the coupling and interactions between these two processes we use scaled analogue models where an intrusive complex made of golden syrup is emplaced within a granular model volcano based on a substratum of a ductile silicone layer overlain by a brittle granular layer we model specifically the large intrusive complex growth and do not model small scale and short lived events such as dyke intrusion that develop above the intrusive complex the models show that the intrusive complex develops in continual competition between upward bulging and lateral gravity spreading the brittle substratum strongly controls the deformation style the intrusion shape and also controls the balance between intrusive complex spreading and ductile layer related gravitational spreading in the models intrusive complex emplacement and spreading produce similar structures to those formed during volcano gravitational spreading alone i e grabens folds en echelon fractures therefore simple analysis of fault geometry and fault kinetic indicators is not sufficient to distinguish gravitational from intrusive complex spreading except when the intrusive complex is eccentric from the volcano centre however the displacement fields obtained for 1 a solely gravitational spreading volcano and for 2 a gravitational spreading volcano with a growing and spreading intrusive complex are very different consequently deformation fields like those obtained from geodetic monitoring can give a strong indication of the presence of a spreading intrusive complex we compare the models with field observations and geophysical evidence on active volcanoes such as la reunion island indian ocean ometepe island nicaragua and eroded volcanic remnants such as ardnamurchan scotland and suggest that a combination between gravitational and intrusive complex spreading has been active

analytical prediction of seismicity rate due to tides and other oscillating stresses oscillatory stresses are ubiquitous on earth and other solid surface bodies tides and seasonal signals perpetually stress faults in the crust relating seismicity to these stresses offers fundamental insight into earthquake triggering we present a simple model that describes seismicity rate due to perpetual oscillatory stresses the model applies to large amplitude non harmonic and quasi periodic stressing however it is not valid for periods larger than the characteristic time t a we show that seismicity rate from short period stressing scales with the stress amplitude but for long periods with the stressing rate further that background seismicity rate r is equal to the average seismicity rate during short period stressing we suggest a sigma 0 may be underestimated if stresses are approximated by a single harmonic function we revisit manga et al 2019 which analyzed the tidal triggering of marsquakes and provide a re scaling of their seismicity rate response that offers a self consistent comparison of different hydraulic conditions

new constraints on tectonics of interior alaska earthquake locations source mechanisms and stress regime we used the joint hypocenter determination jhd method to relocate 3611 crustal earthquakes that occurred from 1988 to 1999 in central alaska the new earthquake locations provide more details on the structure of the kantishna cluster and better locations for the aftershock sequence of the 29 november 2000 m l 5 6 earthquake in the minto flats seismic zone the jhd locations for the aftershocks of the 1995 m w 6 0 minto flats earthquake and 22 october 1996 m w 5 8 earthquake near the denali fault are also available a catalog of 196 fault plane solutions consisting of the moment tensor solutions for the earthquakes with magnitude 4 0 or above and p wave first motion solutions for the earthquakes with magnitude 3 4 and above that occurred from 1988 to 2000 was composed moment tensor solutions were calculated using regional broadband data this catalog was used to calculate principal stress orientations in the crust the stress orientations change across central alaska in particular the maximum principal stress orientation rotates clockwise from se nw to ssw nne direction as one moves from west to east these stress orientations are consistent with the stress field transferred from the plate convergence in southern alaska in addition we tested different velocity structures in the moment tensor inversion procedure to identify velocity models for calculating the green s functions the moment tensor inversion study shows that it is possible to obtain a reliable moment tensor solution for moderate sized earthquakes m l 4 using three component records from a single broadband station when the epicentral distances are between 50 and 300 km manuscript received 8 june 2001

seismic detection and analysis of icequakes at columbia glacier alaska 1 contributions to sea level rise from rapidly retreating marine terminating glaciers are large and increasing strong increases in iceberg calving occur during retreat which allows mass transfer to the ocean at a much higher rate than possible through surface melt alone to study this process we deployed an 11 sensor passive seismic network at columbia glacier alaska during 2004 2005 we show that calving events generate narrow band seismic signals allowing frequency domain detections detection parameters were determined using direct observations of calving and validated using three statistical methods and hypocenter locations the 1 3 hz detections provide a good measure of the temporal distribution and size of calving events possible source mechanisms for the unique waveforms are discussed and we analyze potential forcings for the observed seismicity

seasonal patterns of 3d crustal motions across the seismically active southeastern tibetan plateau abstract we study the three dimensional 3d seasonal crustal deformation and its relationship with local seismicity based on continuous gps cgps mass loading models and seismicity data covering the period from 2010 to 2018 in the southeastern tibetan plateau setp of the 3d seasonal motions vertical motion is the most significant followed by the northward and eastward directions with average annual amplitudes from cgps of 9 28 mm 1 62 mm and 0 72 mm respectively the annual motions predicted by the hydrological mass loading hydl model in all directions are about 3 4 times larger than that of the atmospheric mass loading atml model but smaller than the cgps measurements the initial phase difference between the cgps and loading models can be nearly eliminated by combining the atml and hydl models which helps to fit the cgps results better the average weighted root mean square wrms reduction ratios of the atml hydl model and their combination are 0 30 0 77 and 0 88 respectively this indicates that the seasonal vertical crustal motions observed by cgps in the setp are dominated by seasonal loading motions from the atml and hydl in four subregions where no m 4 earthquakes had occurred in the past decade we observe an anti correlation between equivalent water height ewh from the grace gravity recovery and climate experiment and the vertical displacement derived by cgps it shows an anti correlation relationship between the ewh and seismicity in three subregions and positive correlation in one subregion which underscores the importance of hydrological loading on seismicity in the setp

causes and mechanisms of the 2011 2012 el hierro canary islands submarine eruption 1 el hierro eruption started on 10 october 2011 after an unrest episode that initiated on 17 july 2011 this is the first eruption in the canary islands that has been tracked in real time although being submarine and not directly observable the data recorded allowed its reconstruction and to identify its causes and mechanisms seismicity surface deformation and petrological data indicate that a batch of basanitic magma coming from a reservoir located at a depth of about 25 km below the el hierro island was emplaced at shallower depth creating a new reservoir about 10 12 km above where magma evolved until the initiation of the eruption the characteristics of seismicity and surface deformation suggest that the necessary space to accumulate magma at this shallower position which coincides with the crust mantle boundary beneath el hierro was created in about 2 months by elastic deformation and magma driven fracturing of the crust after this first intrusion episode part of the magma started to migrate laterally toward the se for nearly 20 km always keeping the same depth and following a path apparently controlled by stress barriers created by tectonic and rheological contrasts in the upper lithosphere this lateral migration of magma ended with a submarine eruption at about 5 km offshore from the southern corner of el hierro island the total seismic energy released during the unrest episode was of 8 1 1011 j and the total uplift previous to the onset of the eruption was of 40 mm combining geological geophysical and petrological data and numerical modeling we propose a volcanological model of the causes and mechanisms of el hierro eruption that shows how the stress distribution in the crust beneath el hierro which was influenced by rheological contrasts tectonic stresses and gravitational loading controlled the movement and eruption of magma we also discuss the implications of this model in terms of eruption forecast in the canary islands

triggering of the 1999 mw 7 1 hector mine earthquake by aftershocks of the 1992 mw 7 3 landers earthquake 1 there is strong observational evidence that the 1999 mw 7 1 hector mine earthquake in the mojave desert california was triggered by the nearby 1992 mw 7 3 landers earthquake many authors have proposed that the landers earthquake directly stressed the hector mine fault our model of the landers aftershock sequence however suggests there is an 85 chance that the hector mine hypocenter was actually triggered by a chain of smaller earthquakes that was initiated by the landers main shock we perform our model simulations using the monte carlo method based on the gutenberg richter relationship omori s law bath s law and assumptions that all earthquakes including aftershocks are capable of producing aftershocks and that aftershocks produce their own aftershocks at the same rate that other earthquakes do in general our simulations show that if it has been more than several days since an m 7 main shock most new aftershocks will be the result of secondary triggering these secondary aftershocks are not physically constrained to occur where the original main shock increased stress this may explain the significant fraction of aftershocks that have been found to occur in main shock stress shadows in static coulomb stress triggering studies

stress fields associated with the growth of a large shield volcano la palma canary islands 1 measurements of dyke orientations and fault slip data have been taken in 39 stations located in the northern part of the island of la palma canary archipelago these structures affect the materials belonging to the submarine growth stage and to the lower units of the subaerial shield volcano stage four sets of dykes have been distinguished groups i ii iii 1 and iii 2 the two younger dyke groups iii 1 and iii 2 with highly variable strikes are clearly related to the subaerial growth stage most of the measured faults are normal field relationships and the use of stress inversion methods allowed us to separate three different stress tensors tensor 2 is associated with the dykes of group iii 1 given the spatial distribution of the stations with faults of tensor 2 and dykes of its coeval group iii 1 it has been possible to obtain a map of trajectories of the maximum sh and minimum sh compressive stress axes in the horizontal comparison between this map and the results of theoretical models suggests that the stress field active during at least the early stages of growth of the shield volcano in la palma from near 1 7 myr to at least 1 2 myr ago includes a compressive radial volcanic component plus a regional tensile stress of a tectonic origin this second component might be the imprint in the atlantic oceanic lithosphere of the cenozoic convergence between the african and eurasian plates

three armed rifts or masked radial pattern of eruptive fissures the intriguing case of el hierro volcano canary islands abstract using new surface structural data as well as subsurface structural data obtained from seventeen water galleries we provide a comprehensive model of the volcano tectonic evolution of el hierro canary islands we have identified measured and analysed more than 1700 volcano structural elements including vents eruptive fissures dykes and faults the new data provide important information on the main structural patterns of the island and on its stress and strain fields all of which are crucial for reliable hazard assessments we conducted temporal and spatial analyses of the main structural elements focusing on their relative age and association with the three main cycles in the construction of the island the tinor edifice the el golfo las playas edifice and the rift volcanism a radial strike distribution which can be related to constructive episodes is observed in the on land structures a similar strike distribution is seen in the submarine eruptive fissures which are radial with respect to the centre of the island however the volcano structural elements identified onshore and reflecting the entire volcano tectonic evolution of the island also show a predominant ne sw strike which coincides with the main regional trend of the canary archipelago as a whole two other dominant directions of structural elements n s and wnw ese are evident from the establishment of the el golfo las playas edifice during the second constructive cycle we suggest that the radial striking structures reflect comparatively uniform stress fields during the constructive episodes mainly conditioned by the combination of overburden pressure gravitational spreading and magma induced stresses in each of the volcanic edifices by contrast in the shallower parts of the edifice the ne sw n s and wnw ese striking structures reflect local stress fields related to the formation of mega landslides and masking the general and regional radial patterns