Andy Hooper

This paper presents preliminary results of the subsidence monitoring of a coal-mining field in Zonguldak Province using Interferometric Synthethic Aparture Radar (InSAR) time series. It is a coastal urban area in northwestern Turkey that is prone to human-induced environmental problems such as collapse due to the coal-mining. Severe deformations have been known since the 1900s. Many people have lost their lives, and many buildings have been damaged in the region. The aim of this study is to investigate the subsidence that causes damage in dwelling units and mining areas. For this purpose, ENVISAT ASAR images were acquired in two different geometries (one ascending and one descending track), which cover the same areas. Persistent Scatterer Interferometry (PSI) and Small Baseline Interferometry (SB) time series have been analyzed and compared. Time series were constructed using the Stanford Method for Persistent Scatterers (StaMPS) package with both PSI and SB approaches. Between 2003...

Climate warming at the end of the last glaciation caused ice caps on Icelandic volcanoes to retreat. Removal of surface ice load is thought to have decreased pressures in the underlying mantle, triggering decompression melting, enhanced magma generation and increased volcanic activity. Present-day climate change could have the same effect, although there may be a time lag of hundreds of years between magma generation and eruption. However, in addition to increased magma generation, pressure changes associated with ice retreat should also alter the capacity for storing magma within the crust. Here we use a numerical model to evaluate the effect of the current decrease in ice load on magma storage in the crust at the Kverkfjöll volcanic system, located partially beneath Iceland's largest ice cap. We compare the model results with radar and global positioning system measurements of surface displacement and changes in crustal stress between 2007 and 2008, during the intrusion of a d...

The response of the Earth to crustal loading can provide vital information on crustal strength. We have studied the response of the Earth to the largest man-made water reservoir in Iceland, Hálslón, which was formed in 2006. We address how the Hálslón water reservoir induced crustal displacement and stress changes, by comparing the predictions from a finite element model of the loading process to observed ground deformation mapped by GPS, satellite radar interferometry and extensometers. The impoundment of the Hálslón water reservoir began in September 2006. When the reservoir is full it spans an area of 57 km2 and holds 2.1 km3 of water. The maximum yearly fluctuation of water level is 50 m. The Hálslón reservoir is located on the Eurasian Plate just east of divergent North American-Eurasian plate boundary where it crosses northern Iceland. The location coincides with the edge of a fissure swarm and overlies several lineaments and faults, some of which have been active during the H...

Formation of the oceanic lower crust and migration of magma within it remains enigmatic. These can be studied in subaerial exposure of the mid-oceanic rift system in Iceland. Although the crust in Iceland is thicker (exiting 30 km) than of its oceanic analogue, its internal architecture has been inferred to be comparable with the oceanic crust. A unique opportunity to study magma migration in the lower crust was presented in February 2007 - April 2008 when swarms of deep earthquakes were detected in north-east Iceland. We present detailed GPS and InSAR observation from the area covering the intrusion. The focal depths where concentrated in the lower ductile region of the crust at 14-22 km. The swarms were aligned on a dipping plane, decreasing in depth with time. The main activity continued until April, 2008 but since then a limited shallow activity has occured. Deformation, associated with these events has been observed by GPS and InSAR. A mechanical boundary-element model inverted...

The analysis of radar time series with persistent scatterer techniques usually relies on temporal unwrapping, because phase behavior can be often described by simple models. However, one of the major limitations of temporal algorithms is that they do not take advantage of spatially correlated information. Here, we focus on two types of information that can be spatially estimated, namely, observation precision and the probability density function of the model parameters. We introduce them in phase unwrapping using Bayesian theory. We test the proposed method using simulated data. We also apply them to a small area in the southern Netherlands and compare with conventional temporal unwrapping methods.

Land subsidence poses a serious risk to the low-lying coastal city of Bangkok, Thailand; major flooding occurred there in 1983 and again in 2011. Extreme water pumping in the past led to subsidence rates of up to 120 mm year−1. Although water extraction is now controlled, maximum rates measured by levelling today are still up to 20 mm year−1. In this study, we apply interferometric synthetic aperture radar (InSAR) time-series analysis to study subsidence in Bangkok between October 2005 and March 2010. We validate the InSAR results, by comparing levelling rates and find good agreement between the two techniques. We detect approximately 300,000 coherent pixels overall, with an average density of 120 observations per km2. This is two orders of magnitude greater than the density of levelling benchmarks and reveals subsiding areas that are missed by the levelling network.

Synthetic aperture radar interferometry (InSAR) is a remote sensing technique that is extensively used to measure deformation at the Earth's surface. Although it has proven very successful to estimate small-scale (up to tens of km) deformation phenomena, InSAR still has limitations when applied to study wide areas (hundreds of km). First, radar observations are affected by atmospheric delay and errors in sensor position (also referred to as orbital errors). Both of these translate into large scale signals in InSAR estimations that can be interpreted as a non-existing deformation phenomenon. Second, since InSAR produces deformation maps relative to an area (or pixel) that is part of the observed track, combination of adjacent tracks requires the connection of these relative measurements to a common area or reference point. This operation is, however, non-trivial and is preferably performed in combination with in situ data. In this contribution, we explore different case studies w...

The Tungurahua volcano, in Ecuador, has been experiencing a substantial activity period since 1999, with several eruptions, including those of 2006 and 2008. We use a persistent scatterers approach to analyze a time series of ENVISAT SAR data over the period 2003-2009, to investigate surface deformation in the region of the volcano. We measure a continuous large scale uplift with a maximum line-of-sight displacement rate of about 8 mm/yr, which is the fi�rst evidence of a sustained inflation in the Andes for an active volcano encompassing several eruptions. We model this signal as magma emplacement in a permanent storage zone at 11.5 km below sea level, with a net inflow rate of 7 million m3=yr. The paroxysmal eruptions in 2006 and 2008, did not seem to disrupt this long-term signal. However, we observe signifi�cant deformation during the 2006 eruption consistent with an additional intrusion of 4.5 million m3 of magma.

Excessive groundwater extraction has caused land subsidence in a large rural area located southwest of Tehran, Iran. We used radar images to estimate the temporal and spatial variation in the magnitude of the subsidence. Due to the large perpendicular baselines and rapid temporal decorrelation of the available data, the application of conventional synthetic aperture radar interferometry (InSAR) to monitor the deformation was not possible. Instead, we applied a recently developed Persistent Scatterer InSAR (PSI) method but found that displacements were underestimated in some areas due to high deformation rates that cause temporal aliasing of the signal. We therefore developed a method that combines conventional InSAR and PSI to estimate the high deformation rates in the southwestern Tehran Basin. We used rates estimated from conventional small temporal baseline interferograms to reduce the likelihood of aliasing and then applied PSI to the residual phase. The method was applied to de...

The coal reserves of Limburg, The Netherlands, have been exploited until the mid-1970's, leading to significant land subsidence, a large part of which was due to ground water pumping associated with the mining activities. In 1994, when also the hydrologically-connected neighboring German mining activities ceased, all pumps were finally dismantled. This resulted in rising groundwater levels in the mining areas, continuing until today. Here we report the detection and analysis of heterogeneous surface displacements in the area using satellite radar interferometry. The lack of adequate terrestrial geodetic measurements emphasizes the value of such satellite observations, especially in terms of the temporal and spatial characterization of the signal. Since the lack of direct mine water level measurements hampers predictions on future consequences at the surface, we study the relationship between surface deformation and sub-surface water levels in an attempt to provide rough correlat...

Crust at many divergent plate boundaries forms primarily by the injection of vertical sheet-like dykes, some tens of kilometres long. Previous models of rifting events indicate either lateral dyke growth away from a feeding source, with propagation rates decreasing as the dyke lengthens, or magma flowing vertically into dykes from an underlying source, with the role of topography on the evolution of lateral dykes not clear. Here we show how a recent segmented dyke intrusion in the Bárðarbunga volcanic system grew laterally for more than 45 kilometres at a variable rate, with topography influencing the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred primarily over 14 days, was revealed by propagating seismicity, ground deformation mapped by Global Positioning System (GPS), interferometric analysis of satellite radar images (InSAR), and graben formation. The strike of the dyke segments varies from an initially radial direction away from the Bárðarbunga caldera, towards alignment with that expected from regional stress at the distal end. A model minimizing the combined strain and gravitational potential energy explains the propagation path. Dyke opening and seismicity focused at the most distal segment at any given time, and were simultaneous with magma source deflation and slow collapse at the Bárðarbunga caldera, accompanied by a series of magnitude M > 5 earthquakes. Dyke growth was slowed down by an effusive fissure eruption near the end of the dyke. Lateral dyke growth with segment barrier breaking by pressure build-up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries.

Gradual inflation of magma chambers often precedes eruptions at highly active volcanoes. During eruptions, rapid deflation occurs as magma flows out and pressure is reduced. Less is known about the deformation style at moderately active volcanoes, such as Eyjafjallajökull, Iceland, where an explosive summit eruption of trachyandesite beginning on 14 April 2010 caused exceptional disruption to air traffic. This eruption

ABSTRACT Ground-based monitoring of active volcanoes in Africa can be problematic due to political instabilities, safety issues and poor accessibility. Remote-sensing techniques such as Differential Interferometric Synthetic Aperture Radar (DInSAR, more commonly InSAR), are therefore very useful and provide robust observational tools for natural hazard assessment, regardless of local conditions. Nyiragongo and Nyamulagira volcanoes (which experienced nine eruptions from December 1996 to January 2010) are located in the western branch of the East African Rift (Virunga Volcanic Province, North Kivu, Dem. Rep. of Congo). InSAR has recorded ground displacements related to most of the tectonic and volcanic events that have occurred since 1996 using SAR images from the JERS, ERS-1/2, ENVISAT, RADARSAT-1, RADARSAT-2 and ALOS satellites. This database provides excellent spatial and temporal resolution of deformation, leading to insights into tectonic and volcanic processes. Loss of coherence within the SAR signal due to rapid-changing equatorial vegetation hampers the use of InSAR as a volcano-tectonic monitoring tool. We partially overcome this limitation using 1) a large number of SAR images, including about 150 ENVISAT and more than 100 RADARSAT-1 images, 2) short repeat times of 24 and 35 days for RADARSAT-1 and ENVISAT, respectively, and 3) satellites with longer wavelengths, such as JERS and ALOS. Using a large dataset combining short revisit time SAR images significantly increases the chances of producing interferograms with good coherence. A longer wavelength radar signal better penetrates vegetation cover, also increasing coherence. Furthermore, useful data were retrieved in low-coherence areas by applying the ``StaMPS'' (Stanford Method for Persistent Scatterers) method, which combines a small baseline and persistent scatterers approach, to our largest SAR datasets. Using several look angles from both ascending and descending orbital tracks, we were able to characterize the 3D ground displacement field. A 3D Mixed Boundary Element Method is used to infer geometry and physical characteristics of the sources controlling volcanic unrest and tectonic activity. In addition, we combined our InSAR database with other available data, such as seismicity from a network maintained since 2002 and results from geological and geophysical surveys from the volcanoes, to better constrain source processes. The improved understanding and constraints on volcanic processes in the North Kivu area made possible by this multidisciplinary approach will be highlighted with a few case studies of recent volcanic activity.

Some volcanoes may be so closely spaced that they are coupled; activity in one system has a profound influence on the other. One such pair includes the Katla and Eyjafjallajökull volcanoes, situated under ice caps in the propagating rift zone in southern Iceland. Katla hosts a caldera and is one of Iceland's most active volcanoes, with 20 eruptions in the

Normal faulting earthquakes play an important role in the deformation of continents, and pose significant seismic hazard, yet important questions remain about their mechanics. We use InSAR and bodywave seismology to compute dislocation models and centroid moment solutions for four normal-faulting earthquakes (Mw 5.7–6.2) that occurred in the Pumqu-Xainza Rift (PXR), southern Tibet, a region where lowangle normal faulting has previously been inferred. We also use the fault locations and slip to investigate the correlation between earthquakes and surface topography, and to calculate stress interactions between the earthquakes. The InSAR and body-wave models give consistent focal mechanisms except for the magnitude of the 1996 event, which may be overestimated due to postseismic deformation in the long-interval interferograms. We calculate the static stress changes due to coseismic slip and find that the 1993 event was too distant to cause triggering of the later events, but that the 1998 event pair occurred in regions of increased Coulomb stress resulting from the 1996 event. All the fault planes found here dip at 40–60, reinforcing the absence in observations for low-angle normal faulting earthquakes (dip < 30) whose focal planes can be determined unambiguously. The fault planes of the 1993 and 1996 events are not associated with any obvious surface geomorphology, suggesting that sometimes it is unreliable to resolve the focal plane ambiguity by geomorphology, even for Mw 6.2 events. Furthermore, these events occurred outside the center of the rift, indicating that the active faulting is more distributed and over a length-scale at least 25–50 km east-west in extent, rather than confined to the 20 km width seen in the current mapped faulting and topography. These results suggest that seismic hazard in other extensional zones worldwide might also be more broadly distributed than suggested by geomorphology.