Global Positioning System Sites Research Articles

Distinguishing shallow from mid-crustal magmatic processes at Soufrière Hills Volcano using Finite Element Modelling and co-analysis of EDM and GPS data

We modelled ground deformation at Soufrière Hills Volcano (SHV), using data collected by the Montserrat Volcano Observatory from 2010–2019. We investigate the combined use of Electronic Distance Measuring (EDM) and Global Positioning System (GPS) to distinguish shallow from mid-crustal magmatic processes and their surface deformation profiles. Our results suggest that the EDM network responds predominantly to changes in the shallow magmatic system, whereas GPS records variation at mid-crustal levels. In addition, we show that the behaviour of the EDM network, and of the GPS site HERM, can be explained by underpressurisation in a shallow dyke conduit orientated NNW–SSE, while the mid-crustal system was still undergoing pressurisation. The modelled dyke may be responding to magma cooling and contraction associated with a previous intrusion. We find that geodetic monitoring coverage of multiple flanks within 1 km of the vent can improve our understanding of shallow magmatic system processes with asymmetric deformation fields.

Estimation of crustal strain in Kashmir Himalayan region of north India using continuous GPS measurements

Global positioning system (GPS) measurements of crustal deformation provide insight into the crustal plate velocity and strain rate variation in the actively deforming Earth's crust. GPS measurements in the Kashmir Himalaya indicate that most of the crustal motion between the Indian Plate and southern China is accommodated along the Himalayan arc, resulting in the accumulation of crustal strain and locking of the Main Himalayan Thrust (MHT). Here, we present geodetically estimated spatial and temporal crustal strain rates in the Kashmir Himalaya from GPS measurements to understand the nature of crustal deformation. We processed and analysed GPS data from 21 sites across Kashmir Himalaya along with several IGS sites. We find that the average deformation rate of the GPS sites in Kashmir Himalaya with reference to the Indian Plate range between 2.8 and 15.5 ± 1 mm/year. The GPS results illustrate that the strain rate field is dominated by a compression component with its rate as 7.2 × 10−8 strain/year and the maximum shear strain, γmax, of 1.9 × 10−7 strain/year. The earthquake focal mechanisms suggest that earthquake activity in the Kashmir Himalaya is predominantly manifested in the form of thrust motion. The maximum principal shear strain value reflects that the Kashmir Himalaya is under NE–SW compressional regime. The spatial distribution of crustal strain indicates that the region north‐east of Kashmir Valley is accumulating maximum strain, resulting in the stress build‐up and could be the region of a future large earthquake.

Estimating Coseismic Deformation of Southwestern Puerto Rico from the 7 January 2020 Mw 6.4 Earthquake: Constraints from Campaign and Continuous GPS

ABSTRACT The Puerto Rico–Virgin Islands (PRVI) block lies within the Northern Caribbean Plate Boundary Zone—a zone accommodating stresses between the larger North America and Caribbean plates. Data from Global Positioning System (GPS) sites throughout the PRVI block have been used to confirm the existence of a distinct microblock in the southwest. It is no coincidence that this portion of the PRVI block is the epicentral region of the 7 January 2020 Mw 6.4 earthquake and the ensuing seismic sequence. Prior to the mainshock, the southwestern Puerto Rico (SWPR) region exhibited most of the onland seismic activity. The 2020–2021 SWPR earthquake seismic sequence has been characterized by having an atypical aftershock decay distribution occurring along multiple faults. As a result, fault parameters of the 7 January 2020 mainshock have been poorly defined by conventional seismic methods. Here, we present results from campaign and continuous GPS sites in SWPR, and compare GPS-derived displacements to those computed from the U.S. Geological Survey National Earthquake Information Center (NEIC) focal mechanism. We conclude that irrespective of which nodal plane is used, the observed coseismic displacements from GPS differ from those predicted using a simple elastic model and the NEIC focal mechanism. We infer based on these observations that the complex mainshock rupture resulted in a suboptimal double-couple solution.

Enhanced all-weather precipitable water vapor retrieval from MODIS near-infrared bands using machine learning

Four novel PWV retrieval approaches based on machine learning methods are for the first time developed to estimate all-weather precipitable water vapor (PWV) from near-infrared (NIR) measurements of the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument. The four retrieval approaches are Back Propagation Neural Network (BPNN), Gradient Boosting Decision Tree (GBDT), Generalized Regression Neural Network (GRNN), and eXtreme Gradient Boosting (XGBoost). The transmittance, latitude, longitude, elevation, cloud, season, and solar zenith angle information, in association with the MODIS NIR PWV’s performance, are utilized. The in-situ one-year PWV data collected in 2017 from 453 Global Positioning System (GPS) sites in Australia and 214 GPS sites in China are utilized as target water vapor estimates for model training. Independent of the 2017 training data, two-year data observed in 2018–2019 in Australia and China are utilized to validate the four models’ performance. The results indicate that the retrieval algorithms can greatly improve the PWV retrieval accuracy from MODIS NIR observations under all-weather conditions, reducing the impact of clouds on NIR PWV retrieval. The new all-weather PWV estimates obtain R2 in the range of 0.83 ∼ 0.86, root-mean-square-error (RMSE) in the range of 4.71 mm ∼ 5.28 mm, and mean bias (MB) in the range of 0.18 mm ∼ 0.51 mm, significantly outperforming the official MODIS NIR PWV product (R2 = 0.31, RMSE = 12.03 mm, and MB = -3.04 mm). The reduction in RMSE is 60.85 % for BPNN, 59.68 % for GBDT, 56.69 % for GRNN, and 57.27 % for XGBoost. The new all-weather PWV results show a superior retrieval accuracy compared to the official MODIS NIR confident-clear PWV product, illustrating the effectiveness of the models. This could be because the retrieval models have considered multiple dependence parameters that affect the performance of MODIS-observed NIR PWV. The retrieval algorithms exhibit little spatial or temporal dependence and they can be applied to other regions and periods. This work provides a more accurate way to retrieve all-weather PWV estimates from satellite NIR measurements considering multiple dependence parameters – location, cloud, season, and solar zenith angle information.

Tracking the source direction of surface mass loads using vertical and horizontal displacements from satellite geodesy: A case study of the inter-annual fluctuations in the water level in the Great Lakes

Climate oscillations with seasonal and longer periods drive surface water cycles and quasi-cycles at regional and global scales. Changes in terrestrial water storage produce responses in the Earth's gravitational field and crustal deformation. Here, we use techniques from the Global Positioning System (GPS) and the Gravity Recovery and Climate Experiment (GRACE) to reveal a normal pattern of inter-annual fluctuations in the water level in the North American Great Lakes (GL). The GRACE-estimated time series was in good agreement with in situ water level measurements from 2002 to 2018, especially in terms of phases. The amplitude of 3–4 inter-annual signals in water thickness was 4–6 cm, which is equivalent to a 50–75 km3 oscillation in surface water volume within the entire GL. The slightly larger annual and inter-annual fluctuation amplitudes estimated using GRACE data indicate that the aquifer system of the GL and its surroundings also contributes to seasonal mass changes. After 2013, water levels in the GL region rose abruptly, and the water mass increased by nearly 270 km3 until the end of 2018. We also used GPS- and GRACE-derived three-component displacements (vertical, northward, and eastward) to identify load patterns. GPS- and GRACE-estimated maximum probability source directions based on multi-channel singular spectrum analysis showed that most of the selected GPS sites point to the GL region, although the direction deviations of GPS results at a few sites are mainly caused by the combined effect of the local load and superposition of the distant GL. Our findings indicate that inter-annual displacement changes at different frequencies (1- to 8-year cycle) are primarily due to water volume fluctuations in the GL. The amplitudes estimated by GPS are greater than the GRACE-based and GL-modeled results at most stations, which reflects sensitivity differences between these geodetic solutions to the surface load, as hydrological processes in the local area around a station are difficult to identify using GRACE data with a resolution of ~300 km.

Crustal deformation measurements by global positioning system (GPS) along NSL, western India

The Narmada Son Lineament (NSL) is a major palaeo-rift system and seismically active intra-plate region. In this paper, we processed and analyzed the GPS data from 2009 to 2016 to study the geodynamic characteristics of NSL. The velocities derived from GPS were used to calculate the associated deformation and crustal strain, while the slip deficit was estimated using the horizontal motion of GPS sites. The investigation reveals that the maximum deformation of western NSL is 1.6 mm per year, and the upper bound of the seismic moment (M0) is 2.0 × 1024 dyn/cm, corresponding to an earthquake of about 6.0 magnitude. The study highlights the significance of Aravalli and Satpura mobile belts. The estimated strain of 0.03 μstrain/yr is low but comparable to the stable continental region, and a close association of factors (the fragility, crustal conductors, and compressive tectonics) is considered responsible for seismogenic activities in western NSL.

Upper Mantle Viscosity Underneath Northern Marguerite Bay, Antarctic Peninsula Constrained by Bedrock Uplift and Ice Mass Variability

AbstractWe constrain viscoelastic Earth rheology and recent ice‐mass change in the northern Marguerite Bay region of the Antarctic Peninsula. Global Positioning System (GPS) time series from Rothera and San Martin stations show bedrock uplift range of ∼−0.8–1.8 mm/year over 1999–2005 and 2016–2020 but ∼3.5–6.0 mm/year over ∼2005–2016. Digital elevation models reveal substantial surface lowering, but at a lower rate since ∼2009. Using these data, we show that an elastic‐only model cannot explain the non‐linear uplift of the GPS sites but that a layered viscoelastic model can. We show close agreement between GPS uplift changes and viscoelastic models with effective elastic lithosphere thickness and upper‐mantle viscosity ∼10–95 km and ∼0.1−9 × 1018 Pa s, respectively. Our viscosity estimate is consistent with a north‐south gradient in viscosity suggested by previous studies focused on specific regions within the Antarctic Peninsula and adds further evidence of the low viscosity upper mantle in the northern Antarctic Peninsula.

Postseismic Relaxation Following the 2019 Ridgecrest, California, Earthquake Sequence

ABSTRACT The 2019 Ridgecrest, California, earthquake sequence involved predominantly right-lateral strike slip on a northwest–southeast-trending subvertical fault in the 6 July M 7.1 mainshock, preceded by left-lateral strike slip on a northeast–southwest-trending subvertical fault in the 4 July M 6.4 foreshock. To characterize the postseismic deformation, we assemble displacements measured by Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar. The geodetic measurements illuminate vigorous postseismic deformation for at least 21 months following the earthquake sequence. The postseismic transient deformation is particularly well constrained from survey-mode GPS (sGPS) in the epicentral region carried out during the weeks after the mainshock. We interpret these observations with mechanical models including afterslip and viscoelastic relaxation of the lower crust and mantle asthenosphere. During the first 21 months, up to several centimeters of horizontal motions are measured at continuous GPS and sGPS sites, with amplitude that diminishes slowly with distance from the mainshock rupture, suggestive of deeper afterslip or viscoelastic relaxation. We find that although afterslip involving right-lateral strike slip along the mainshock fault traces and their deeper extensions reach a few decimeters, most postseismic deformation is attributable to viscoelastic relaxation of the lower crust and mantle. Within the Basin and Range crust and mantle, we infer a transient lower crust viscosity several times that of the mantle asthenosphere. The transient mantle asthenosphere viscosity is ∼1.3×1017 Pa s, and the adjacent Central Valley transient mantle asthenosphere viscosity is ∼7×1017 Pa s, about five times higher and consistent with an asymmetry in postseismic horizontal motions across the mainshock surface rupture.

A Loading Correction Model for GPS Measurements Derived from Multiple-Data Combined Monthly Gravity

Time-dependent loading deformations of the Earth’s surface, due to nontidal changes in the atmosphere, ocean, land water/ice, etc., contribute significantly to the seasonal and secular Global Positioning System (GPS) site displacements, especially for the up component. While loading deformations derived from general circulation model (GCM) outputs are usually used to correct loading signals in the GPS site displacements, this study aims to provide a loading correction model based on the multiple-data combined monthly gravity products LDCmgm90. We have adopted GPS measurements from 249 IGS reference frame stations and 3 different GCM-based loading models to test the reliability of the LDCmgm90 model. Compared to the GCM-based models, the LDCmgm90 loading correction is more effective in attenuating seasonal (especially annual) loading signals and can bring more significant improvements to most stations for both the data-trend-removed and the data-trend-retained cases. Thus, we have validated the LDCmgm90 model from the loading aspect and proved it to be a reliable loading-correction model for GPS displacements. The relatively better secular loading signals provided by the LDCmgm90 loading model may provide us a chance to study the long-term, nonloading signals in GPS data.

Ionospheric perturbation\xa0during\xa0the South American total solar eclipse on 14th December 2020 revealed with the Chilean GPS eyeball

The influence of the South American total solar eclipse of 14th December 2020 on the ionosphere is studied by using the continuous Chilean Global Positioning System (GPS) sites across the totality path. The totality path with eclipse magnitude 1.012 passed through the Villarrica (Lon. 72.2308°W and Lat. 39.2820°S) in south Chile during 14:41:02.0 UTC to 17:30:58.1 UTC and maximum occurred ~ 16:03:49.5 UTC around the local noon. The vertical total electron content (VTEC) derived by GPS sites across the totality path for two PRN’s 29 and 31 show almost 20–40% of reduction with reference to ambient values. The percentage reduction was maximum close to totality site and decreases smoothly on both sides of totality sites. Interestingly, the atmospheric gravity waves (AGWs) with a period ~ 30–60 min obtained using wavelet analysis of VTEC timeseries show the presence of strong AGWs at the GPS sites located north of the totality line. But the AGWs do not show any significant effect on the VTEC values to these sites. Our analysis suggests, possibly an interplay between variability in the background plasma density and eclipse-generated AGWs induced plasma density perturbation could explain the observations.

Investigation of strain accumulation along Tuzla fault – western Turkey

The Aegean region is one of the most seismically active regions in Turkey and comprises the Hellenic Arc, Greece, and Western Turkey. The Tuzla Fault, which lies between the town of Menderes and Cape Doğanbey, is one of the major seismic threats in western Turkey due to its seismic potential to generate a major earthquake (M 6, near Doğanbey Cape in 1992) and proximity to the city of İzmir which sustained damage due to the earthquake that occurred in the Aegean Sea on October 30th, 2020. In order to estimate strain rates and seismic potential around the Tuzla Fault, five global positioning system (GPS) surveys were carried out between 2009 and 2012. Estimated GPS velocities in the study area exceed 20 mm/year, which is in line with previous studies. We use two different approaches to calculate the strain accumulation on and around the Tuzla Fault. The first method adopts forming triangles using the GPS sites as corners and calculating strain rates within those triangles. The second method adopts a bicubic interpolation approach described by Holt and Haines, 1993. Maximum values of strain accumulation were found to reach up to 200 nanostrain/year.

GPS Imaging of Vertical Bedrock Displacements: Quantification of Two‐Dimensional Vertical Crustal Deformation in China

AbstractThe Global Positioning System (GPS) derived bedrock displacements respond to multiple geophysical effects, ranging from surface elastic loads to tectonic sources or viscoelastic uplifts stemming from Earth’s viscous mantle. In this study, the GPS‐inferred vertical crustal velocities are rigorously estimated in mainland China. We integrate the GPS vertical velocity field with Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow‐On (GFO) data, adopting an empirical Spatial Structure Function (SSF), to image tectonic deformation in mainland China with respect to the International Terrestrial Reference Frame (ITRF) 2014. We present four profiles across China, which indicate that our new robust results are superior to kriging. Furthermore, we use the GRACE/GFO products to account for elastic deformation due to surface mass changes to isolate tectonic deformation signals at GPS sites within mainland China from 2002 to 2019. By integrating GPS and GRACE/GFO measurements, our results reveal the long‐term spatial patterns of vertical tectonic motion in different blocks in mainland China. We conclude that significant steep velocity gradients occur at tectonic block boundaries that are attributable to locking and elastic strain accumulation on active block boundary faults.

The Assessment of Hydrologic- and Flood-Induced Land Deformation in Data-Sparse Regions Using GRACE/GRACE-FO Data Assimilation

The vertical motion of the Earth’s surface is dominated by the hydrologic cycle on a seasonal scale. Accurate land deformation measurements can provide constructive insight into the regional geophysical process. Although the Global Positioning System (GPS) delivers relatively accurate measurements, GPS networks are not uniformly distributed across the globe, posing a challenge to obtaining accurate deformation information in data-sparse regions, e.g., Central South-East Asia (CSEA). Model simulations and gravity data (from the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO)) have been successfully used to improve the spatial coverage. While combining model estimates and GRACE/GRACE-FO data via the GRACE/GRACE-FO data assimilation (DA) framework can potentially improve the accuracy and resolution of deformation estimates, the approach has rarely been considered or investigated thus far. This study assesses the performance of vertical displacement estimates from GRACE/GRACE-FO, the PCRaster Global Water Balance (PCR-GLOBWB) hydrology model, and the GRACE/GRACE-FO DA approach (assimilating GRACE/GRACE-FO into PCR-GLOBWB) in CSEA, where measurements from six GPS sites are available for validation. The results show that GRACE/GRACE-FO, PCR-GLOBWB, and GRACE/GRACE-FO DA accurately capture regional-scale hydrologic- and flood-induced vertical displacements, with the correlation value and RMS reduction relative to GPS measurements up to 0.89 and 53%, respectively. The analyses also confirm the GRACE/GRACE-FO DA’s effectiveness in providing vertical displacement estimates consistent with GRACE/GRACE-FO data while maintaining high-spatial details of the PCR-GLOBWB model, highlighting the benefits of GRACE/GRACE-FO DA in data-sparse regions.

An integrated structural and GPS study of the Jalpatagua fault, southeastern Guatemala

Abstract The Jalpatagua fault in Guatemala accommodates dextral movement of the Central America forearc. We present new global positioning system (GPS) data, minor fault analysis, geochronological analyses, and analysis of lineaments to characterize deformation along the fault and near its terminations. Our data indicate that the Jalpatagua fault terminates at both ends into extensional regions. The western termination occurs near the Amatitlan caldera and the southern extension of the Guatemala City graben, as no through-going structures were observed to continue west into the active volcanic arc. Along the Jalpatagua fault, new and updated GPS site velocities are consistent with a slip rate of 7.1 ± 1.8 mm yr−1. Minor faulting along the central section of the fault includes: (1) N-S–striking normal faults accommodating E-W elongation; and (2) four sets of strike-slip faults (oriented 330°, 020°, 055°, and 295°, parallel to the Jalpatagua fault trace). Minor fault arrays support dextral movement along a major fault in the orientation of the Jalpatagua fault. GPS and fault data indicate that the Jalpatagua fault terminates to the east near the Guatemala–El Salvador border. Data delineate a pull-apart basin southeast of the fault termination, which is undergoing transtension as the Jalpatagua fault transitions into the El Salvador fault system to the east. Within the basin, minor faulting and lineations trend to the NW and accommodate NE-directed elongation. This faulting differs from E-W elongation observed along the Jalpatagua fault and is more similar to minor faults within the El Salvador fault system.

A possible precursor prior to the Lushan earthquake from GPS observations in the southern Longmenshan

Global Positioning System (GPS) stations installed in and around the epicenter of the Lushan earthquake (Mw 6.7), which occurred almost 5 years after the 2008 Wenchuan earthquake, recorded preseismic deformation corresponding to the Lushan earthquake within the southern Longmenshan thrust belt. A half-space dislocation model is used to simulate the theoretical values of the postseismic displacements caused by the 2008 Wenchuan earthquake, and after transforming the reference frame and filtering the GPS displacement time series, the theoretical and observed GPS values are compared to identify the geodetic anomaly preceding the Lushan earthquake. The abnormal extent of this geodetic anomaly decreases with increasing epicentral distance for each GPS site. This geodetic signal reflects preslip along a locked section of the 2013 seismogenic fault, which caused the accumulation of elastic strain energy until the faulting strength was overcome, thereby generating the Lushan earthquake. Hence, this anomaly might be used as an observable and identifiable precursor to forecast an impending earthquake within a period of less than two and half years before its occurrence.

Slip distribution of the 2020 Elazığ Earthquake (Mw 6.75) and its influence on earthquake hazard in the Eastern Anatolia

SUMMARYWe analysed coseismic surface displacements of the 2020 January 24 Elazığ (M 6.75) Earthquake using GPS (Global Positioning System) measurements to investigate the associated fault slip distribution and static stress change. Our geodetic analysis of 13 GPS sites surrounding the epicentre included data from four additional sites. We accurately located the nucleation point of the main shock at 38.310° N, 39.081° E (±1.4 km) and at a depth of 5.2 ± 1.2 km. Both seismograph and GPS-derived fault plane solutions confirmed that it has a nearly pure sinistral mechanism with a negligible obliquity. The main shock generated 29.2 cm average sinistral slip along an approximately 70 km long and 20 km wide section of the East Anatolian Fault. Based on its rupture size and average slip, its magnitude was found to be Mw 6.75. An average of 6.6 bars of stress drop occurred on the rupture plane. The rupture occurred bilaterally failing two separate segments both to the northeast and to the southwest of the nucleation point. Average sinistral slips were 14.6 and 47.4 cm along the southwestern and northeastern segments, respectively. Sinistral slip reached up to 53.1 cm along the southwestern segment and 110.5 cm along the northeastern segment. During the generation process of the 2020 earthquake, 78 per cent of the slip deficit had been released aseismically since 1875. Increasing Coulomb stress by an average of 2.5 bars, it substantially increased earthquake hazard on the 1874 (M 7.1) rupture zone, which might have already accumulated 1.51 m slip deficit on its fully locked patches. Furthermore, increasing Coulomb stress by an average of 0.5 bars, it raised earthquake hazard on the 1893 (M 7.1) rupture zone, which might have already stored 1.01 m slip deficit along the fully locked patches.

Slow Slip Events in New Zealand

Continuously operating global positioning system sites in the North Island of New Zealand have revealed a diverse range of slow motion earthquakes on the Hikurangi subduction zone. These slow slip events (SSEs) exhibit diverse characteristics, from shallow (<15 km), short (<1 month), frequent (every 1–2 years) events in the northern part of the subduction zone to deep (>30 km), long (>1 year), less frequent (approximately every 5 years) SSEs in the southern part of the subduction zone. Hikurangi SSEs show intriguing relationships to interseismic coupling, seismicity, and tectonic tremor, and they exhibit a diversity of interactions with large, regional earthquakes. Due to the marked along-strike variations in Hikurangi SSE characteristics, which coincide with changes in physical characteristics of the subduction margin, the Hikurangi subduction zone presents a globally unique natural laboratory to resolve outstanding questions regarding the origin of episodic, slow fault slip behavior. ▪ New Zealand's Hikurangi subduction zone hosts slow slip events with a diverse range of depth, size, duration, and recurrence characteristics. ▪ Hikurangi slow slip events show intriguing relationships with seismicity ranging from small earthquakes and tremor to larger earthquakes. ▪ Slow slip events play a major role in the accommodation of plate motion at the Hikurangi subduction zone. ▪ Many aspects of the Hikurangi subduction zone make it an ideal natural laboratory to resolve the physical processes controlling slow slip.

Geodetic slip rate and locking depth of east Semangko Fault derived from GPS measurement

65 km Semangko Fault is part of southern segments of Sumatran Fault Zone (SFZ) which is complex corresponds to the transition from the strike-slip regime of the SFZ to the normal faulting tectonics of the Sunda Strait. The recent publication showed branches of Semangko Fault: West Semangko Fault (WSF) and East Semangko Fault (ESF). This study estimated geodetic slip rate and locking depth of ESF using Global Positioning System (GPS) time series data from 2007 to 2019 from all available GPS sites. GPS velocities refer to Sundaland Plate were used to estimate the fault parameters of the WSF and ESF simultaneously. Non-uniformity of velocity direction shows the complexities of Semangko Fault possibly caused by the contribution of all faults around ESF. An ESF geodetic slip rate, which is 12.5 ± 2 mm/yr was lower than WSF, which is 16.5 ± 2 mm/yr. It is consistent with the rigid block nature of the SFZ system as northern segment slip rates have similar value. Small slip rates are possibly leading to lower generated seismic moment than the major segment of SFZ.

Present‐Day Vertical Land Motion of Australia From GPS Observations and Geophysical Models

AbstractThe secular rate of Australia's vertical surface deformation due to past ice‐ocean loading changes is not consistent with present vertical velocities observed by a previously sparse network of Global Positioning System (GPS) sites. Current understanding of the Earth's rheology suggests that the expected vertical motion of the crust should be close to zero given that Australia is located in the far field of past ice sheet loading. Recent GPS measurements suggest that the vertical motion of the Australian continent at permanent sites is between 0 and −2 mm/year. Here we investigate if vertical deformation due to previous ice sheet loading can be recovered in the time series of Australian GPS sites through enlarging the number of sites compared to previous studies from ~20 to more than 100 and through the application of improved data filtering. We apply forward geophysical models of elastic surface displacement induced by atmospheric, hydrologic, nontidal ocean, and ice loading and use independent component analysis as a spatiotemporal filter that includes multivariate regression to consider temporally correlated noise in GPS. Using this approach, the common mode error is identified, and subsequent multivariate regression leads to an average reduction in trend uncertainty of ~35%. The average vertical subsidence of the Australian continent is substantially different to vertical motion predicted by glacial isostatic adjustment and surface mass transport models.

Rapid ionospheric variations at high latitudes: Focusing on Greenland

The Helheim glacier, located in southeast Greenland, has more than ten campaign-type Global Positioning System (GPS) sites; data processing led to the observation of a very rapid change in the ionospheric delay. To identify the cause of these sporadic disturbances, we analyzed the slant total electron content (STEC), single-differenced STEC (SD-STEC) and scintillation proxy index called the delta phase rate (DPR). From this analysis, the abrupt change of those ionospheric indicators was attributed to the line-of-sight direction to the satellite and the temporal sequence of the event was found to be highly correlated with the geometry of the GPS sites. In addition, the disturbance based on the result of SD-STEC occurred mostly during the night, from 17 UTC through 7 UTC, and across a band spanning the east-west direction. Based on the DPR indices obtained from GPS stations distributed across all of Greenland, Iceland, and northeastern Canada, the rapid ionospheric variation was found to be correlated with the time of the day and the geomagnetic latitude of the station. The disturbance was larger at the relatively low geomagnetic latitudes at night but was more significant at higher latitudes in the daytime. These rapid ionospheric variations tended to appear in band shapes parallel to the geomagnetic field. These results allow us to attribute such disturbance observed at the Helheim glacier to aurora-related phenomena.