plate boundary observatory and related networks gps data analysis methods and geodetic products the geodesy advancing geosciences and earthscope gage facility global positioning system gps data analysis centers produce position times series velocities and other parameters for approximately two thousand continuously operating gps receivers spanning a quadrant of earth s surface encompassing the high arctic north america and caribbean the purpose of this review is to document the methodology for generating station positions and their evolution over time and to describe the requisite tradeoffs involved with combination of results gage gps analysis involves formal merging within a kalman filter of two independent loosely constrained solutions one is based on precise point positioning produced with the gps inferred positioning system orbit analysis simulation gipsy oasis software at central washington university cwu and the other is a network solution based on phase and range double differencing produced with the gps at mit gamit software at new mexico institute of mining and technology nmt the primary products generated are the position time series that show motions relative to a north america reference frame and secular motions of the stations represented in the velocity field the position time series themselves contain a multitude of signals in addition to the secular motions co seismic and post seismic signals seasonal signals from hydrology and transient events some understood and other not yet fully explained are all evident in the time series and ready for further analysis and interpretation we explore the impact of analysis assumptions on the reference frame realization and on the final solutions and we compare within the gage solutions and with others

real time reliable magnitudes for large earthquakes from 1 hz gps precise point positioning the 2011 tohoku oki japan earthquake 1 the early warning issued after the onset of the mw9 0 tohoku oki earthquake significantly underestimated its magnitude saturating 120 seconds after the earthquake began at mw8 1 here we investigate whether real time deformation data from japan s dense network of continuously recording global positioning system gps stations could have been used to provide a more reliable rapid estimate of the earthquake s magnitude and ultimately a more robust tsunami forecast we use precise point positioning in real time mode with broadcast clock and orbital corrections to give station positions every 1 s we then carry out a simple static inversion on a subset of stations to determine the portion of the fault that slipped and the earthquake magnitude unlike most previous methods our method produces estimates of seismic moment before the earthquake rupture has completed we find that the deformation data allow a robust magnitude estimate just 100 s after the earthquake onset we also investigated the density of stations required for a robust moment magnitude estimate fewer than 1 station every 100 km are needed we recommend that gps data be incorporated into earthquake early warning systems for regions at threat from large magnitude earthquakes and tsunamis

non tidal ocean loading effects on geodetic gps heights 1 gps observations used in geophysical studies are not usually corrected for non tidal ocean loading ntol displacement here we investigate ntol effects on 3 4 year gps height time series from 17 sites around the southern north sea and compute the ntol displacement according to two ocean models the global ecco model and a high resolution regional model polssm which covers the northwest european continental shelf to assess the susceptibility of gps height estimates to ntol reprocessed gipsy ppp daily solutions are corrected for the resulting displacement together with atmospheric pressure loading atml we find that the displacements due to ntol are comparable in size to atml and the combined correction reduces the variance by 12 22 mm2 20 30 reduction in rms we find that polssm outperforms ecco with around an 11 greater variance reduction and hence high resolution models are recommended to correct gps heights for ntol

the use of gps horizontals for loading studies with applications to northern california and southeast greenland received 23 august 2012 revised 15 january 2013 accepted 20 january 2013 published 25 april 2013 1 we describe how gps measurements of horizontal crustal motion can be used to augment vertical crustal motion measurements to improve and extend gps studies of surface loading we show that the ratio of the vertical displacement to the horizontal displacement combined with the direction of the horizontal motion can help determine whether nearby loading is concentrated in a small region for example in a single lake or glacier and where that region is we illustrate this method by applying it to two specific cases an analysis of gps data from northern california to monitor the level of lake shasta and the analysis of data from a single gps site in southeast greenland to determine mass variability of two large nearby outletglaciers helheim glacier and midgaard glacier the californiaexampleserves largely as a proof of concept where the results can be assessed by comparing with independent observations lake shasta tide gauge data in this case our greenland results show that both helheim and midgaard have experienced notable interannual variations in mass loss rate over the last decade helheim sm ass loss accelerated rapidly in mid 2003 decelerated in late 2005 and increased again in 2008 2009 before returning to about its pre 2003 rate in late 2010 midgaard s mass loss accelerated in mid 2004 and remained more or less constant before returning to its pre 2003 rate in late 2008

ultrasonic measurements of vp and qp relaxation spectrum of complex modulus on basalt melts abstract ultrasonic compressional wave velocity v p and quality factor q p have been measured in alkali basalt olivine basalt and basic andesite melts in the frequency range of 3 4 22 mhz and in the temperature range of 1100 1400 c velocity and attenuation of the melts depend on frequency and temperature showing that there are relaxation mechanisms in the melts complex moduli are calculated from the ultrasonic data the results fit well a complex modulus of arrhenius temperature dependence with log normal gaussian distribution in relaxation times of attenuation the analysis yields average relaxation time its activation energy relaxed modulus unrelaxed modulus and width of gaussian distribution in relaxation times relaxed modulus is smaller 17 5 gpa for basic andesite melt of high silica and high alumina contents than for the other two basalt melts 18 1 18 4 gpa the most probable relaxation times decrease from 3 10 10 s for basic andesite to 10 11 s for alkali basalt at 1400 c activation energies of attenuation ranging from 270 to 340 kj mol 1 in the three melts are highest in basic andesite longitudinal viscosity values and their temperature dependences are also calculated from v p and q p data the volume viscosity values are estimated from the data using the shear viscosity values longitudinal volume and shear viscosities and their activation energies are highest in the basic andesite melt of the most polymerized structure

ionospheric gps total electron content tec disturbances triggered by the 26 december 2004 indian ocean tsunami 1 tsunami ionospheric disturbances tids of the 26 december 2004 mw 9 3 sumatra earthquake are detected by the total electron content tec of ground based receivers of the global positioning system gps in the indian ocean area it is found that the tsunami waves triggered atmospheric disturbances near the sea surface which then traveled upward with an average velocity of about 730 m s 2700 km hr into the ionosphere and significantly disturbed the electron density within it results further show that the tids which have maximum height of about 8 6 17 2 km periods of 10 20 min and horizontal wavelengths of 120 240 km travel away from the epicenter with an average horizontal speed of about 700 km hr 190 m s in the ionosphere

transient signal detection using gps measurements transient inflation at akutan volcano alaska during early 2008 1 continuous global positioning system gps networks record station position changes with millimeter level accuracy and have revealed transient deformations on various spatial and temporal scales however the transient deformation may not be easily identified from the position time series because of the large number of sites in a network low signal to noise ratios snr and correlated noise in space and time here we apply state estimation and principal component analysis to the daily gps position time series measured in alaska sites of the plate boundary observatory network our algorithm detects a transient signal whose maximum displacement is 9 mm in horizontal and 11 mm in vertical that occurred at akutan volcano during the first half of 2008 a simple mogi source inversion suggests inflation at shallow depth 3 9 km beneath the volcano although the detection was not easy because the signal was aseismic non eruptive and weak not apparent in raw daily time series our detection method improves the snr and therefore provides higher resolution for detecting the transient signal

noise properties of continuous gps data from concrete pillar geodetic monuments in new zealand and comparison with data from u s deep drilled braced monuments 1 reducing noise levels in geodetic data is critical for the interpretation and modeling of geophysically interesting signals in this paper i estimate noise properties of continuous gps time series collected on concrete pillar monuments and compare them with similar data from deep drilled braced monuments that are considered the most stable three dimensional monuments available i show that the time series noise properties are very similar between the monument types at least for time series up to 4 5 years duration differences between the monument types are therefore not a dominant influence on these time series implying that the less expensive concrete pillar monuments are a valid choice for recording many signals of geophysical interest

gps detection of ionospheric perturbations following the january 17 1994 northridge earthquake sources such as atmospheric or buried explosions and shallow earthquakes producing strong vertical ground displacements are known to produce pressure waves that propagate at infrasonic speeds in the atmosphere at ionospheric altitudes low frequency acoustic waves are coupled to ionospheric gravity waves and induce variations in the ionospheric electron density using global positioning system gps data recorded by the permanent gps network operating in southern california we computed ionospheric electron content time series for several days preceding and following the january 17 1994 mw 6 7 northridge earthquake we observe an anomalous signal beginning several minutes after the earthquake with time delays that increase with distance from the epicenter the signal frequency and phase velocity are consistent with results from numerical models of atmospheric ionospheric acoustic gravity waves excited by seismic sources as well as previous electromagnetic sounding results we believe that these perturbations are caused by the ionospheric response to the strong ground displacement associated with the northridge earthquake

effects of lithospheric viscoelastic relaxation on the contemporary deformation following the 1959 mw 7 3 hebgen lake montana earthquake and other areas of the intermountain seismic belt 1 the 1959 mw 7 3 hebgen lake mt normal faulting earthquake occurred in an extensional stress regime near the yellowstone volcanic field time dependent crustal deformation data following this major earthquake were acquired by precise trilateration and gps surveys from 1973 to 2000 around the hebgen lake fault zone modeling the changes of baseline lengths across and near the fault reveals a lateral variation of transient rheology in which the lithosphere is stronger near the hebgen lake fault zone than in the vicinity of the yellowstone volcano system the models also imply that the lower crust is stronger than the upper mantle in agreement with results from studies of postseismic and post lake filling relaxations 100 years in addition evaluations of the postseismic motion produced by the hebgen lake and the 1983 mw 6 9 borah peak id earthquakes indicate that horizontal transient motion of up to 1 mm yr contribute significantly to the contemporary regional crustal deformation near the epicentral areas for the eastern basin and range 500 km south of the hebgen lake fault similar rheologic models were derived from the observed uplift associated with the lake bonneville rebound and were used to evaluate the postseismic deformation associated with six most recent paleoearthquakes of the wasatch fault zone and three m 5 6 historic earthquakes of northern utah the results show 0 1 mm yr of horizontal postseismic motion at present time that are within the horizontal uncertainties of continuous gps velocity from the basin and range and significantly smaller than the contemporary extension of 1 3 mm yr in the wasatch front

space geodesy constrains ice age terminal deglaciation the global ice 6g_c vm5a model a new model of the last deglaciation event of the late quaternary ice age is here described and denoted as ice 6g c vm5a it differs from previously published models in this sequence in that it has been explicitly refined by applying all of the available global positioning system gps measurements of vertical motion of the crust that may be brought to bear to constrain the thickness of local ice cover as well as the timing of its removal additional space geodetic constraints have also been applied to specify the reference frame within which the gps data are described the focus of the paper is upon the three main regions of last glacial maximum ice cover namely north america northwestern europe eurasia and antarctica although greenland and the british isles will also be included if peripherally in the discussion in each of the three major regions the model predictions of the time rate of change of the gravitational field are also compared to that being measured by the gravity recovery and climate experiment satellites as an independent means of verifying the improvement of the model achieved by applying the gps constraints several aspects of the global characteristics of this new model are also discussed including the nature of relative sea level history predictions at far field locations in particular the caribbean island of barbados from which especially high quality records of postglacial sea level change are available but which records were not employed in the development of the model although ice 6g c vm5a is a significant improvement insofar as the most recently available gps observations are concerned comparison of model predictions with such far field relative sea level histories enables us to identify a series of additional improvements that should follow from a further stage of model iteration

an extraordinary episode of yellowstone caldera uplift 2004 2010 from gps and insar observations 1 geodetic measurements of yellowstone ground deformation from 2006 to june 2010 reveal deceleration of the recent uplift of the yellowstone caldera following an unprecedented period of uplift that began in 2004 in 2006 2008 uplift rates decreased from 7 to 5 cm yr and 4 to 2 cm yr in the northern and southwest caldera respectively and in 2009 rates further reduced to 2 cm yr and 0 5 cm yr in the same areas elastic dislocation modeling of the deformation data robustly indicates an expanding sill at 7 10 km depth near the top of a seismically imaged crystallizing magma reservoir with a 60 decrease in the volumetric expansion rate between 2006 and 2009 reduction of hydrothermal volcanic recharge from beneath the northeast caldera and seismic moment release of the 2008 and 2010 large earthquake swarms are plausible mechanisms for decelerating the caldera uplift and may have influenced the change in recent caldera motion from uplift to subsidence

improved stability of a deeply anchored geodetic monument for deformation monitoring a test of two different monument designs used in geodetic networks shows that monuments installed to depths of 5 to 10 meters and laterally braced exhibit less environmentally caused displacement than those installed to 2 meters depth at parkfield california we have been monitoring the lengths of 17 baselines over the past decade with a 2 to 3 day interval between measurements using a two color geodimeter with a nominal precision of 0 5 mm over 5 km long baselines significant variations are observed on many of these baselines which appear to be related to the seasonally occurring rainfall with the larger variations approaching 10 mm over the past decade to test whether we could improve upon the measurements on some of the more susceptible lines at two sites we installed new monuments within about 30 meters of the original monuments after 1 5 years of measurements it is evident that the new monuments significantly attenuate the seasonal displacements to less than 1 mm the use of deeply anchored monuments should improve the ability of fault scale geodetic monitoring arrays to detect small tectonic displacements

dynamics and rapid migration of the energetic 2008 2009 yellowstone lake earthquake swarm 1 yellowstone national park experienced an unusual earthquake swarm in december january 2008 2009 that included rapid northward migration of the activity at 1 km per day and shallowing of the maximum focal depths from 12 to 2 km beneath northern yellowstone lake the swarm consisted of 811 earthquakes 0 5 mw 4 1 aligned on a n s 12 km long vertical plane of hypocenters the largest earthquake of the swarm had a 50 tensile crack opening source determined by a full waveform inversion that we interpret as a magmatic expansion component in addition gps data revealed e w crustal extension coincident with the swarm modeling of gps and seismic data is consistent with e w opening of 10 cm on a n s striking vertical dike our interpretation is that the swarm was induced by magmatic fluid migration or propagation of a poroelastic stress pulse along a pre existing fracture zone

global mapping functions for the atmosphere delay at radio wavelengths i have developed expressions for calculating the ratios mapping functions of the line of sight hydrostatic and wet atmospheric path delays to their corresponding zenith delays at radio wavelengths for elevation angles down to 3 the coefficients of the continued fraction representation of the hydrostatic mapping function depend on the latitude and height above sea level of the observing site and on the day of the year the dependence of the wet mapping function is only on the site latitude by comparing with mapping functions calculated from radiosonde profiles for sites at latitudes between 43 s and 75 n the hydrostatic mapping function is seen to be more accurate than and of comparable precision to mapping functions currently in use which are parameterized in terms of local surface meteorology when the new mapping functions are used in the analysis of geodetic very long baseline interferometry vlbi data the estimated lengths of baselines up to 10 400 km long change by less than 5 mm as the minimum elevation of included data is reduced from 12 to 3 the independence of the new mapping functions from surface meteorology while having comparable accuracy and precision to those that require such input makes them particularly valuable for those situations where surface meteorology data are not available

ongoing drought induced uplift in the western united states the western united states has been experiencing severe drought since 2013 the solid earth response to the accompanying loss of surface and near surface water mass should be a broad region of uplift we use seasonally adjusted time series from continuously operating gps stations to measure this uplift which we invert to estimate mass loss the median uplift is 4 mm with values up to 15 mm in california s mountains the associated pattern of mass loss which ranges up to 50 cm of water equivalent is consistent with observed decreases in precipitation and streamflow we estimate the total deficit to be about 240 gt equivalent to a 10 cm layer of water over the entire region or the annual mass loss from the greenland ice sheet

dating open systems by the 238 u 234 u 230 th method application to quaternary reef terraces abstract the sedimentation conditions and diagenetic processes of biogenic and inorganic carbonates are such that the assumptions of a closed system evolution and the absence of initial th invoked in classical 238 u 234 u 230 th dating methods are very often not fulfilled for such material this problem is of interest for all studies requiring accurate dating of corals or aragonite rich sediments as in palaeoclimatology or neotectonics extending the approach of henderson and slowey nature 404 2000 61 66 we propose a consistent model that takes into account possible initial 230 th excess and where continuous selective redistribution gain or loss of 234 u 234 th and 230 th is controlled by recoil processes this model applied to quaternary marine terraces in barbados marie galante lesser antilles arc and pacific islands is able to explain the large scatter in isotopic compositions observed in the series of cogenetic corals or sediments we show that recoil processes generally tend to increase the 234 u 238 u and 230 th 234 u ratios with time leading to an overestimation of the apparent ages calculated using classical methods on the basis of simple assumptions identical marine initial 234 u 238 u 0 and 230 th 232 th 0 ratios we propose a method for calculating a best estimate of the age of cogenetic series our model offers an alternative way to date old reef terraces where corals are almost systematically altered and where classical dating methods do not apply it may also explain the apparent variations of the 234 u 238 u seawater initial ratio estimated from corals formed in the last 300 ka and the apparent uncoupling between sea level highstands and high latitude northern hemisphere summer insolation our set of equations can be applied to all weathering processes where the mobility of u series isotopes is controlled by recoil

slip distribution of the february 27 2010 mw 8 8 maule earthquake central chile from static and high rate gps insar and broadband teleseismic data 1 the shallow depth underthrust earthquake of february 27 2010 mw 8 8 ruptured the subduction plate interface in central chile between 34 s and 38 s we retrieve the spatial and temporal distribution of slip during this mega earthquake through a joint inversion of teleseismic records insar and high rate gps hrgps data additionally our model is shown to agree with broadband surface waves rupture initiated at about 32 km depth and propagated bilaterally resulting in two main slip zones located ssw and nne of the hypocenter nucleation did not take place within or at the edge of one of these main asperities but in between during the first 30s slip propagated predominantly southwards later on the rupture evolved more slowly and more symmetrically eventually the northern asperity became predominant with maximum slip reaching about 20 m most of the seismic moment was released within 110s a relatively short time explained by the bilateral propagation the overall average rupture velocity is 2 6 km s but propagation occurred initially faster towards the south 3 2 km s large slip did not reach the trench a result consistent with the moderate size of the tsunami down dip rupture stopped at about 50 km depth in agreement with the lower limit of the locked zone inferred by ruegg et al 2009 from pre seismic gps data

inverse modeling of gps multipath for snow depth estimation part i formulation and simulations snowpacks provide reservoirs of freshwater the amount stored and how fast it is released by melting are vital information for both scientists and water supply managers gps multipath reflectometry gps mr is a new technique that can be used to measure snow depth signal to noise ratio data collected by gps instruments exhibit peaks and troughs as coherent direct and reflected signals go in and out of phase these interference fringes are used to retrieve the unknown land surface characteristics in this two part contribution a forward inverse approach is offered for gps mr of snow depth part i starts with the physically based forward model utilized to simulate the coupling of the surface and antenna responses a statistically rigorous inverse model is presented and employed to retrieve parameter corrections responsible for observation residuals the unknown snow characteristics are parameterized the observation parameter sensitivity is illustrated the inversion performance is assessed in terms of its precision and its accuracy and the dependence of model results on the satellite direction is quantified the latter serves to indicate the sensing footprint of the reflection

real time inversion of gps data for finite fault modeling and rapid hazard assessment 1 responses to recent great earthquakes and ensuing tsunamis in sumatra chile and japan with the resulting loss of life and damage to infrastructure demonstrate that our ability to ascertain the full extent of slip of catastrophic earthquakes and their tsunamigenic potential in the first minutes after the initiation of rupture is problematic regional gps networks such as those in western north america and japan are complementary to seismic networks by being able to directly measure displacements close to the source during large earthquakes in real time we report on rapid modeling of two large earthquakes the 2003 mw 8 3 tokachi oki earthquake 100 km offshore hokkaido island using 356 geonet stations and the 2010 mw 7 2 el mayor cucapah earthquake in northern baja california using 95 crtn stations in southern california about 75 km northwest of the epicenter working in a simulated real time mode we invert for finite fault slip in a homogeneous elastic half space using green s functions obtained from okada s formulation we compare two approaches the first starts with a catalog of pre defined faults while the second uses a rapid centroid moment tensor solution to provide an initial estimate of the ruptured fault plane in either case we are able to characterize both earthquakes in less than two minutes reducing the time necessary to obtain finite fault slip and moment magnitude for medium and greater earthquakes compared to traditional methods by an order of magnitude

determination of slow slip episodes and strain accumulation along the cascadia margin 1 continuous gps stations in the pacific northwest geodetic array network clearly record subduction related strain accumulation and slow slip episodes along the cascadia convergent margin many of the slow slip episodes have been correlated in time and space with seismic evidence for nonvolcanic tremor leading to the previous discovery of episodic tremor and slip ets in this study we use a hyperbolic tangent curve fitting technique for the identification of slow slip times and displacement magnitudes within the gps time series independent of seismic tremor data we then apply this technique to study the patterns of strain accumulation and release associated with ets events and characterize patterns of coupling associated with the locked and transition zones of the plate interface we demonstrate the effectiveness of this automated technique for both identification of slow slip observations and calculation of slow slip displacements recurrence patterns in the distribution of gps observations demonstrate coherence among neighboring stations over time and apparent along strike segmentation of the subduction interface when slow slip events are removed from the time series we can estimate the total site velocities between slow slip events these velocities decay as depth to the subduction interface increases but they diverge from the long term trends expected from the interseismic cycle at 30 60 km above the interface consistent with the location where slow slip displacements occur forward modeling of coupling on the plate interface reveals that in between slow slip events there is a patch of at least 30 coupling from 20 to 35 km depth which is needed to produce the observed back slip displacements intriguingly our best fitting models have a decrease in coupling down to 30 at 20 km depth followed by a peak of greater than 80 coupling at 30 35 km depth suggesting the source zone for ets events acts as a distinct locking zone that releases strain more frequently than the updip seismogenic locked zone although a zone of constant 30 coupling cannot be ruled out with this data set such a scenario indicates that frictional behavior with depth follows a more complex model than a simple temperature controlled transition we propose that coupling initially decreases with depth due to a decrease in strength of the overriding lower crust but then coupling increases again when the subducting plate comes in contact with the stronger overriding mantle

southern california permanent gps geodetic array error analysis of daily position estimates and site velocities we analyze time series of daily positions estimated from data collected by 10 continuously monitoring global positioning system gps sites in southern california during the 19 month period between the june 28 1992 mw 7 3 landers and january 17 1994 mw 6 7 northridge earthquakes each time series exhibits a linear tectonic signal and significant colored noise spectral power at frequencies in the range 5 yr 1 to 0 5 d 1 is dominated by white noise or possibly fractal white noise and is several orders of magnitude higher than what would be expected from random walk noise in this short period range attributed by others to geodetic monument motions estimating a single slope for the time series power spectra suggests fractal white noise processes with spectral indices of about 0 4 site velocity uncertainties assuming this fractal white noise model are 2 4 times larger than uncertainties obtained assuming a purely white noise model a combination white noise plus flicker noise spectral index of 1 model also fits the data and suggests that the velocity uncertainties should be 3 6 times larger than for the white noise model we cannot adequately distinguish between these two noise models nor can we rule out the possibility of a random walk signal at the lowest frequencies these questions await the analysis of longer time series in any case reducing the magnitude of low frequency colored noise is critical and appears to be best accomplished by building sites with deeply anchored and braced monuments otherwise rate uncertainties estimated from continuous gps measurements may not be improved significantly compared to those estimated from infrequent campaign mode measurements

rapid determination of earthquake magnitude using gps for tsunami warning systems 1 the 26 december 2004 sumatra earthquake mw 9 2 9 3 generated the most deadly tsunami in history yet within the first hour the true danger of a major oceanwide tsunami was not indicated by seismic magnitude estimates which were far too low mw 8 0 8 5 this problem relates to the inherent saturation of early seismic wave methods here we show that the earthquake s true size and tsunami potential can be determined using global positioning system gps data up to only 15 min after earthquake initiation by tracking the mean displacement of the earth s surface associated with the arrival of seismic waves within minutes displacements of 10 mm are detectable as far away as india consistent with results using weeks of data after the event these displacements imply mw 9 0 0 1 indicating a high tsunami potential this suggests existing gps infrastructure could be developed into an effective component of tsunami warning systems

detection of arbitrarily large dynamic ground motions with a dense high rate gps network 1 we describe the detection of teleseismic surface waves from the 3 november 2002 mw 7 9 denali fault earthquake in alaska with a dense network of 1 hz gps stations in southern california about 3900 km from the event relative horizontal displacements with amplitudes in excess of 15 mm and duration of 700 seconds agree with integrated velocities recorded by nearby broadband seismometers with an rms difference of 2 3 mm the displacements are derived from independent 1 hz instantaneous positions demonstrating that a gps network can provide direct measurements of arbitrarily large dynamic and static ground horizontal displacements at periods longer than 1 s and amplitudes above 2 mm with an inherent precision signal to noise that improves indefinitely with amplitude without clipping and in real time high rate real time gps networks can enhance earthquake detection and seismic risk mitigation and support other applications such as intelligent transportation and civil infrastructure monitoring

crustal loading near great salt lake utah two sites of the bargen gps network are located 30 km south of great salt lake gsl lake level records since mid 1996 indicate seasonal water elevation variations of 0 3 m amplitude superimposed on a roughly decadal feature of amplitude 0 6 m using an elastic green s function and a simplified load geometry for gsl we calculate that these variations translate into radial crustal loading signals of 0 5 mm seasonal and 1 mm decadal the horizontal loading signals are a factor of 2 smaller despite the small size of the expected loading signals we conclude that we can observe them using gps time series for the coordinates of these two sites the observed amplitudes of the variations agree with the predicted decadal variations to 0 5 mm the observed annual variations however disagree this difference may be caused by some combination of local precipitation induced site motion unmodeled loading from other nearby sources errors in the gsl model and atmospheric errors

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses