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In this study, we analyze 25 RADARSAT-2 images from ascending and descending geometries to study the creep rate on the Hayward fault and landslide motions near Berkeley, CA. We applied a coherence optimization technique from polarimetric synthetic aperture radar interferometry (PolInSAR) to increase the accuracy of the measurements. We resolve 3–5 mm/year of motion along the Hayward fault, in agreement with earlier creep estimates. We identify a potential motion on secondary fault, northeast and parallel to the Hayward fault, which is creeping at a lower rate of ~1.5 mm/year. In addition, we identify a number of landslides along the hills east of the fault that agree with earlier results from advanced interferometric synthetic aperture radar (SAR) analysis and field investigations. We investigate four particular slope instabilities, one of which was marked as moderately active, and three as highly active, by earlier field investigations. The resolved along-hill slope displacement is estimated at ~23 mm/year. Our results demonstrate that PolInSAR is an effective method to increase the interferometric coherence and provide improved resolution of deformation features associated with natural hazards.

Ground deformation in volcanic regions can be a precursor to resumption of activity. Volcanic eruptions are typically brief periods of activity punctuating very long inter-eruptive periods. This makes hazard evaluation a difficult task for volcanoes with low-recurrence eruptive activity, which often are poorly monitored. As a result, analysis of inter-eruptive periods by use of remote sensing techniques can provide important information on precursory activity and improve volcano hazard assessment. In August–September 2010 Mt Sinabung, Indonesia, reawakened after at least 400 years of dormancy. The ground deformation before this eruption was investigated by use of differential interferometric synthetic aperture radar data obtained from Japanese ALOS-PALSAR radar imagery between 05 January 2007 and 31 August 2010. Results from InSAR time series processing detected significant ground deformation (subsidence) at several locations on the Karo plateau, and uplift in the summit area of Mt Sinabung. The persistent scatterers density obtained by use of ALOS data is sufficient to enable extraction of temporal and spatial patterns of the deformation. The surface deformation at the summit can be modeled by using a spherical point-source model. Source data are consistent with a very shallow (hydrothermal) reservoir, with a linear increase in overpressure before the 2010 Mt Sinabung eruption. Hydrothermal origin is consistent with seismicity, tiltmeters, and analysis of ash products collected during and after the 2010 eruption. These results support the potential of L-band interferometry for hazard assessment in poorly monitored and highly vegetated volcanic areas and also indicate that hazard assessment for Indonesian volcanoes could potentially be improved by identification of precursory (inter-eruptive) uplift periods.

Rapidly accelerating ground subsidence in the south-western part of British Columbia, the third largest metropolitan area in Canada with over 2.3 million of inhabitants, is estimated using the Multidimensional Small Baseline Subset (MSBAS) advanced Differential Interferometric Synthetic Aperture Radar (DInSAR), an effective processing strategy for multi-mission, multi-temporal SAR data. The Synthetic Aperture Radar (SAR) data used in this study consists of seven independent data sets: one ascending and one descending ERS-1/2 and ENVISAT frames, together spanning July 1995–September 2010, and three RADARSAT-2 frames spanning February 2009–October 2012. During the July 1995–October 2012 period we observe fast ground subsidence with a maximum rate of about 2 cm/year in the Greater Vancouver region that includes the Fraser River Delta and the cities of Burnaby, Richmond, Surrey, New Westminster and Vancouver. The rapidly accelerating subsidence is observed beneath the Vancouver International Airport, SkyTrain terminal as well as several agricultural and industrial locales. These time series suggest that the subsidence rate at the studied regions does not decrease with time, as suggested in previous studies, but remains steady or increases. These results also demonstrate the importance of acquiring and appropriately estimating longer time series, as previous studies on the same Greater Vancouver area may have misinterpreted the long term ground deformation rate and direction and underestimated the potential hazard. The long term impact of this subsidence on urban infrastructure can be significant and needs to be investigated further.

Applying the Multidimensional Small Baseline Subset interferometric synthetic aperture radar algorithm to about 1500 Envisat and RADARSAT-2 interferograms spanning 2003–2013, we computed time series of ground deformation over Naples Bay Area in Italy. Two active volcanoes, Vesuvius and Campi Flegrei, are located in this area in close proximity to the densely populated city of Naples. For the first time, and with remarkable clarity, we observed decade-long elevation-dependent seasonal oscillations of the vertical displacement component with a peak-to-peak amplitude of up to 3.0 cm, substantially larger than the long-term deformation rate (<0.6 cm/yr). Analysis, utilizing surface weather and radiosonde data, linked observed oscillations with seasonal fluctuations of water vapor, air pressure, and temperature in the lower troposphere. The modeled correction is in a good agreement with observed results. The mean, absolute, and RMS differences are 0.014 cm, 0.073 cm, and 0.087 cm, respectively. Atmospherically corrected time series confirmed continuing subsidence at Vesuvius previously observed by geodetic techniques.

Campi Flegrei is one of the most hazardous volcanic areas in the world because of its close proximity to the city of Naples. Here we apply the multidimensional small baseline subset (MSBAS) differential interferometric synthetic aperture radar (DInSAR) technique to obtain vertical and horizontal components of ground deformation for Campi Flegrei at high spatial and temporal resolutions that span, for the first time, 20 years. The area underwent continuous subsidence from 1993 through 1999. Moderate uplift began in 2010 and substantially increased through 2012, reaching approximately 13 cm by 2013. We model the observed deformation to determine source parameters for subsidence and uplift epochs. Both the inflation and deflation mechanisms involve large, extended sources in a layered hydrothermal system whose location is controlled by the caldera structure and stratigraphy. The temporal resolution of MSBAS approaches that of GPS daily time series, with superior precision and spatial resolution, making it an excellent alternative for volcano monitoring.

We apply, for the first time, an equal-scattering-mechanism multibaseline optimization technique on quad-pol satellite images of RADARSAT-2 in order to increase the interferometric coherence. The optimized polarimetric channel results in the selection of new pixel candidates for multibaseline differential interferometric synthetic aperture radar applications, which, otherwise, are not recognized in any single-pol channel. In addition, we analyze the individual performance of this algorithm for rural and urban land covers along with the corresponding optimized scattering mechanisms. The optimized coherence improvement with respect to a single HH channel is shown to be correlated with the interferometric spatial and temporal baselines. Finally, we investigate the performance of this technique on the interferometric phase. The optimized interferograms increase the quality of the phase patterns with respect to those phases recovered in a single copolar channel, e.g., the residual orbital ramps of the RADARSAT-2 images.

We present new results for shallow aseismic creep on the Hayward fault in California using a new DInSAR technique. This method not only provides, for the first time, the ability to map the displacement field on both sides of the fault, it does so over a much shorter time period than earlier results. The results provide a good match in the near-field to both the regional continuous GPS velocities and data from an alinement network that measures long-term creep along the fault. The average slip rate for the northern segment of the Hayward fault is ∼4.4 mm/yr between 2008 and 2011, slightly less than that estimated for longer time periods, suggesting that the slip rate may not be constant. If the slip rate along the fault is variable on the decadal or longer scale, current estimates of its earthquake potential and the associated hazard associated with the slip rate deficit may need to be revised from previous estimates. We demonstrate the potential impact of this method to better define the spatial and temporal complexity of aseismic slip and estimate the accumulated elastic strain along one of the most significant sources of seismic hazard in the San Francisco Bay area.

We present ground deformation maps for the southern Saskatchewan region between Rice Lake and the City of Saskatoon measured by Radarsat-2 Differential Interferometric Synthetic Aperture Radar (DInSAR) during 2008–2011. We acquired and processed Radarsat-2 data from two different tracks (Multi-Look Fine beam and one Standard beam) and performed advanced interferometric analysis that revealed ground deformation with a maximum line-of-sight rate close to 10 cm per year. Decomposition of ascending and descending, line-of-sight images into horizontal (mainly east–west) and vertical components revealed the presence of localized horizontal motion with a maximum absolute rate of 4 cm per year and subsidence with a maximum rate of −10 cm per year. These results suggested that simple line-of-sight time series analysis of interferometric data acquired in single geometry is insufficient for capturing a complete pattern of ground deformation and can be misleading in the presence of both horizontal and vertical components of deformation.

Earthquake initiation, propagation and arrest are influenced by fault frictional properties1, 2 and preseismic stress. Studies of triggered and induced seismicity can provide unique insights into this influence. However, measurements of near-field, surface ground deformation and pre-earthquake stress conditions necessary for such studies are rare. Here, we use geodetic data to determine surface deformation associated with the Mw 5.1 earthquake that occurred in Lorca, southeast Spain, on 11 May 2011. We use an elastic dislocation model to show that earthquake nucleation and the area of main fault slip occurred at very shallow depths of 2–4 km, on a rupture plane along the Alhama de Murcia Fault. Slip extended towards the surface, across fault segments with frictional properties that changed from unstable to stable. The area of fault slip correlates well with the pattern of positive Coulomb stress change that we calculate to result from the extraction of groundwater in a nearby basin aquifer. We therefore suggest that the distribution of shallow slip during the Lorca earthquake could be controlled by crustal unloading stresses at the upper frictional transition of the seismogenic layer, induced by groundwater extraction. Our results imply that anthropogenic activities could influence how and when earthquakes occur.

We present ground deformation measurements in the Taupo Volcanic Zone (TVZ) using differential interferomeric synthetic aperture radar (DInSAR) observations collected by ALOS PALSAR during 2006–2010, and compare them with displacement observations from continuous GPS. We acquired and processed DInSAR images from two ascending paths (324 and 325) and one descending path (628) covering the TVZ, and produced linear deformation rates and time series of deformation. The DInSAR results were improved by using a modified version of the small baseline subset (SBAS) algorithm that simultaneously solves for deformation rates and residual topographic noise. The accuracy of the DInSAR displacement rates along line-of-sight to the satellite is 0.5–2 cm yr−1 depending on the number of SAR images and their coherence. We found good agreement between the DInSAR-derived displacement rates and those measured by continuous GPS for the two ascending paths (correlation 0.94 ± 0.01 and 0.89 ± 0.02); the DInSAR uncertainties were too large to make a useful comparison for the descending path (correlation 0.66 ± 0.03). We identified ground deformation due to groundwater and steam extraction for geothermal power. To demonstrate the geophysical application, we modelled the deformation results using simplified sources for some of the geothermal signals using ellipsoidal and tabular approximations.

In weather forecasting, current and past observational data are routinely assimilated into numerical simulations to produce ensemble forecasts of future events in a process termed “model steering”. Here we describe a similar approach that is motivated by analyses of previous forecasts of the Working Group on California Earthquake Probabilities (WGCEP). Our approach is adapted to the problem of earthquake forecasting using topologically realistic numerical simulations for the strike-slip fault system in California. By systematically comparing simulation data to observed paleoseismic data, a series of spatial probability density functions (PDFs) can be computed that describe the probable locations of future large earthquakes. We develop this approach and show examples of PDFs associated with magnitude M > 6.5 and M > 7.0 earthquakes in California.

In this study we present modeling results derived from ground deformation observed in the Auckland Volcanic Field (AVF) by the C-band Envisat Synthetic Aperture Radar. Auckland, the largest city in New Zealand with a current population of over one million, coincides with the AVF, which comprises about 50 individual, largely monogenetic, basaltic volcanoes distributed across a total area of 360 km2. The most recent and largest eruption there occurred 600 years ago. While it is anticipated that the chance of any existing volcano reawakening is very low, a new volcano could be created at any time in a new location within the field. The aim of this work is to evaluate the feasibility of interferometric synthetic aperture radar (InSAR) for mapping ground deformation associated with magma ascent, which would be a likely precursor to the onset of volcanic activity. For this study we acquired and processed 23 single look complex (SLC) images from the Envisat satellite (Track 151, Frame 6442, IS2, VV) spanning from July 2003 until November 2007. All possible combinations of differential interferograms were created. Stacking, Small Baseline Subset (SBAS) and Permanent Scatterers (PS) processing algorithms were used to determine spatial and temporal patterns of surface deformation as well as their average rates. A number of localized deformation regions were consistently observed by all three techniques. Due to a lack of evidence pointing toward a relationship with volcanic or tectonic sources it was assumed that they are produced by groundwater withdrawal and recharge. Three largest regions of subsidence (S1–S3) and also three largest regions of uplift (U1–U3) were modeled with the derivative-free simplex algorithms for location, depth and source volume change using a Mogi point source approximation. The results show that InSAR is a viable technique capable of detecting the scale, rate and spatial distribution of precursory deformation that would likely be associated with res

We review the results of the geodetic and structural studies carried out on La Palma Island using geodetic and geophysical data during the period 1992–2007. InSAR and GPS observation techniques were applied to study the existence of deformation on the island and gravity observations were carried out for structural studies. Gravity data were inverted using a nonlinear three-dimensional gravity inversion approach to obtain the geometry of the anomalous bodies constructed in a random growth process with respect to an exponentially stratified background. The main structural feature is a large central body (under the Caldera de Taburiente) with high density, which was interpreted as the Pliocene-age uplifted seamount and a relatively dense intrusive plutonic complex/magma body. The Cumbre Vieja series is characterized by elongated minima distributed according to the rift structure. InSAR results show a clear subsidence located on the Teneguía volcano, where the last eruption took place in 1971. A thermal source is the most probable origin for this deformation. A GPS network composed of 26 stations covering the total island surface was set up. Vertical displacements determined comparing the GPS coordinates obtained in 2007 with coordinates determined in 1994 are consistent with the InSAR results obtained in the southern part of the island. This is not the case for the northern part. From the comparison of 2006 and 2007 coordinates it is clear that more time is needed to obtain significant displacements, but observed trends are also consistent with InSAR results. All the observed significant displacements are in stations located outside of the large high-density central body.

Mass redistribution and deformation cause spatiotemporal gravity changes in active volcano areas. The ability of the gravity measurements to detect subsurface magma movement as a precursor to volcanic eruptions is greatly enhanced if gravity changes are analyzed and modeled jointly with ground deformation data. One effective tool for this analysis is provided by the gravity and elevation change ratio. The proposed numerical formulation allows to simulate a wide range of pressures and mass loads. However, the assessment of gravity changes is especially important when they occur without measurable ground deformation. In such a case it is very difficult to relate the gravity/uplift ratio to changes in subsurface mass movements by using classical models. In this work, we study the role played by pressure and mass sources in the interpretation of observed gravity/uplift ratio when deformation is negligible in volcanic areas associated with rough topographic relief.

Gravity gradients are an effective method for delineating the extent of subsurface density anomalies. The change in subsurface density contrasts due to the seismic deformation gives rise to detectable gravity changes via the dilatational gravity signal or Bouguer anomaly. Solutions for the corresponding gravity gradients of these signals are developed for a vertical strike-slip fault. Gravity gradient solutions exhibit similar spatial distributions as those calculated for Coulomb stress changes, reflecting their physical relationship to the stress changes. The signals' magnitudes, of the order of 10−4 E, are beyond the resolution of typical exploration instruments. Improvements to Superconducting Gravity Gradiometers are necessary for gravity gradients to be used as a viable method for the observation of the stress field changes over large spatial scales.

A general method for calculating the total, dilatational, and free-air gravity for fault systems with arbitrary geometry, slip motion, and number of fault segments is presented. The technique uses a Green's function approach for a fault buried within an elastic half-space with an underlying driver plate forcing the system. The method is easily adapted to investigate a wide range of geophysical applications and is provided as a suite of Fortran90 routines. The Joshua Tree-Landers-Hector Mine sequence is examined and shows remarkable correlation of epicentres with dilatational gravity highs. We also demonstrate the use of the method for large thrusting events, using the Alaska Good Friday earthquake of 1964 as an example.

This work presents results for the three-dimensional displacement field at Tenerife Island calculated from campaign GPS and ascending and descending ENVISAT DInSAR interferograms. The goal of this work is to provide an example of the flexibility of the technique by fusing together new varieties of geodetic data, and to observe surface deformations and study precursors of potential activity in volcanic regions. Interferometric processing of ENVISAT data was performed with GAMMA software. All possible combinations were used to create interferograms and then stacking was used to increase signal-to-noise ratio. Decorrelated areas were widely observed, particularly for interferograms with large perpendicular baseline and large time span. Tropospheric signal was also observed which significantly complicated the interpretation. Subsidence signal was observed in the NW part of the island and around Mount Teide and agreed in some regions with campaign GPS data. It is expected that the technique will provide better results when more high quality DInSAR and GPS data is available.

Here we present a new mathematical tool, the localized Hartley (HL) transform (Hartley, 1942; Bracewell, 1990), that allows for the filtering of 1-D time series through the identification of the power at various spatial and temporal wavelengths. Its application to and the associated results are presented from its application to continuous Global Positioning System (GPS) data from southern California for the time period 1994 through 2006. The HL transform filter removes the high-frequency components of the data and effectively isolates the longer period signal. This long-period signal is modeled as time-dependent postseismic deformation using the viscoelastic-gravitational model of Fernández and Rundle (2004) for six stations selected for their proximity to the Northridge earthquake. The x-, y-, and z-components of the postseismic deformation are compared to the filtered data. Results suggest that this long-period deformation is a result of postseismic relaxation and that the HL transform filter provides an important new technique for the filtering of geophysical data consisting of the superposition of the effects of numerous complex sources at a variety of spatial and temporal scales.

We review several results from geodetic observations carried out in La Palma Island during the period 1992-2007. A gravity survey was done for structural studies, and InSAR and GPS observations techniques were applied to study the existence of deformation areas in the Island. Gravity anomalies have been inverted using a non-linear three-dimensional gravity inversion approach to obtain the geometry of the anomalous bodies. The main structural feature is a large high density body interpreted as a dense intrusive plutonic body and the pliocene-age uplifted seamount. An elongated minimum is detected below the Cumbre Vieja according to the rift structure. InSAR results show two areas of subsidence, a mild long wavelength signal on the western part of Cumbre Vieja and clear subsidence located on the Teneguia volcano where the last eruption took place in 1971. A GPS network composed by 26 stations covering the island has been defined. Vertical displacements determined by comparing the GPS coordinates obtained in 2007 and in 1994 are consistent with the InSAR results. From the comparison of 2006 and 2007 coordinates we conclude that more time span is needed to obtain clearly significant displacements, but observed trends are also consistent with InSAR results. All the observed significant displacements are at stations located outside of the large high density central body.

Three-dimensional displacements and gravity changes were measured at 20 points between 2000 and 2002 at Merapi volcano, Java, Indonesia. Former models focused on the modeling of a single magmatic source located in the summit region of the volcano. Such models do not fit to our measurements between 2000 and 2002. A new hybrid model approach is developed consisting of an elastic-gravitational source described by a mass and energy term as well as a combined dip-slip/strike-slip fault zone in the summit region. Both nonlinear optimization problems, given by the common inversion of three-dimensional displacements and gravity changes, are solved by applying a genetic algorithm. The hybrid model fits the measurements accurately, tested by Fisher test statistics. Furthermore, our model for Merapi volcano confirms previous structural models for this region so that the new model is statistically proven as well as physically reliable.

In this paper, we analyze the stress change associated with the inverted volcanic source models at Mayon volcano, Philippines, where there are gravity changes without significant deformation after the 1993 eruption. We detail the applicable data and the associated inversion techniques and models prior to calculating the appropriate stress changes. It is determined that the stress change associated with the central magmatic source produces compressional stress changes at a secondary source to the northwest, prompting a change in the local water storage in the underlying fractured rock.

Changes in gravity due to volcanic loading of the crust are influenced by topography. We investigate the relative importance of topography and self-gravitation in the interpretation of gravity changes. It is shown that modeling of gravity changes can be more precise with the introduction of topographic relief, although it is neglected self-gravitation of the medium. This paper exploits this result by suggesting a mathematical simplification that could be useful in the future development of a numerical technique to accurately include topographic effects in the modeling of deformation and gravity changes. Finally, we perform an inversion of the gravity changes observed at Mayon volcano (Philippines) between December 1992 and December 1996 including topographic effects by varying the depth of the source. Failure to account for topographic influences can bias estimates of source parameters particularly when the lateral extension of the relief is of the same order of magnitude as the source depth.

Most deformation models of volcanoes assume that the Earth is a linear, elastic, isotropic and homogeneous half-space, although some volcanic areas are associated with significant relief. We investigate the effects of topography on surface deformation and gravity changes caused by a magma intrusion in the Earth's crust. A three-dimensional (3-D) indirect boundary element method (IBEM) that incorporates realistic topographic features is developed in order to perform this analysis. Our results show that the topography alters both the magnitude and pattern of the deformation and gravity signal. As an example of realistic topography, we consider a spherical source of dilatation located at 4 km depth below Teide volcano summit (Tenerife, Canary Islands) in order to simulate the deformation and gravity changes that could be observed at Tenerife if a hypothetical intrusion occurred in the volcanic system. This approach gives a picture of the 3-D topographic effect at Teide that can provide insight in order to improve the geodetic monitoring of the volcano.

We present an evaluation of residual vertical gravity–height change gradients obtained from gravimetric and elevation data between 1982 and 2000 at the Campi Flegrei caldera (CFc) in Italy. Spatial and temporal variations in the gradients are indicative of multiple causative sources during unrest, in particular for ground subsidence from 1988 onwards. Supported by results obtained from time-series inversion for the period 1988–2000 using a random search approach of a purely elastic earth model and a genetic algorithm accounting for elastic-gravitational effects, we propose a centre of dilatation undergoing predominantly pressure changes yet negligible mass changes as the dominant cause for caldera deflation. Mass fluctuations in randomly active secondary sources along the periphery of the CFc can be best explained by dynamic changes along the caldera boundary (ring) faults.

Tenerife is one of the most well monitored islands of the Canaries, but the surveillance generally is centred on Las Cañadas Caldera, where the Teide volcano is located. In the last 180 000 yr, the eruptions on Tenerife Island have never occurred in the same volcanic structure, except for the Teide and Pico Viejo central volcanic system, so that a complete monitoring network would have to cover the whole island. As a result, Synthetic Aperture Radar Interferometry (InSAR) is being used on Tenerife, because this space technique can provide a displacement map of the surface of the earth with centimetre precision. This paper presents the results obtained on Tenerife Island using 18 SAR images acquired by the ERS-1 and ERS-2 satellites during the period 1992-2000. Two important results have been obtained: no deformation on Las Cañadas Caldera, coinciding with results obtained using terrestrial techniques, and two subsidence episodes outside monitoring areas in the NW of the island, in the region of the last historic eruptions. These results show that InSAR is a useful technique for monitoring the entire island, thus allowing us to discover deformations in areas that are not routinely or easily monitored. This technique has been used in combination with Global Positioning System (GPS) observation of a global network on the island to define a new geodetic monitoring system. The possible causes of the deformations observed have been studied in an endeavour to discern if they might be of natural origin, in particular linked to a reactivation of prior volcanic activity. Examination of the geophysical observations on the island, human activities underway and the results of the modelling seem to indicate that at least part of the deformations may be caused by changes in the groundwater level and therefore are not linked to a volcanic reactivation. This result is important because it implies that, if geodetic volcano monitoring is to be performed on the island, the system used

A quantitative global sensitivity analysis (SA) is applied to the non-linear inversion of gravity changes and displacement data which measured in an active volcanic area. The common inversion of this data is based on the solution of the generalized Navier equations which couples both types of observation, gravity and displacement, in a homogeneous half space. The sensitivity analysis has been carried out using Sobol’s variance-based approach which produces the total sensitivity indices (TSI), so that all interactions between the unknown input parameters are taken into account. Results of the SA show quite different sensitivities for the measured changes as they relate to the unknown parameters for the east, north and height component, as well as the pressure, radial and mass component of an elastic-gravitational source. The TSIs are implemented into the inversion in order to stabilize the computation of the unknown parameters, which showed wide dispersion ranges in earlier optimization approaches. Samples which were computed using a genetic algorithm (GA) optimization are compared to samples in which the results of the global sensitivity analysis are integrated by a reweighting of the cofactor matrix in the objective function. The comparison shows that the implementation of the TSI’s can decrease the dispersion rate of unknown input parameters, producing a great improvement the reliable determination of the unknown parameters.