Tectonics/Volcanoes 3

The use of L-band InSAR data for observing the surface displacements caused by earthquakes can be very beneficial. The retrieved signal is generally more stable against temporal phase decorrelation with respect to C-band and X-band InSAR data, such that fault movements also in vegetated areas can be observed. Also, due to the longer wavelength, larger displacement gradients occurring particularly close to the ruptures can be measured. A serious draw back of L-band data on the other hand is that it more strongly reacts to heterogeneities in the ionosphere. The spatial variability of the electron content causes spatially long wavelength trends in the interferometric phase, distorts the surface deformation signal therefore impacts on the earthquake source analysis. A well-known example of the long-wavelength distortions are the ALOS-1 InSAR observations of the 2008 Wenchuan earthquake.

To mitigate the effect of ionospheric phase in the modelling of earthquake sources, a common procedure is to remove any obvious linear or quadratic trend in the displacement data that may have been caused by ionospheric phase delays. Additionally, remaining trends may be accounted for by including so-called ambiguity (or nuisance) parameters in the modelling. The introduced ionospheric distortion, however, is only approximated arbitrarily by such simple ramp functions with the true ionospheric phase screen unknown. As a consequence, either a remaining ionospheric signal may be mistaken for surface displacement or, the other way around, long-wavelength surface displacement may be attributed to ionospheric distortion and is removed. The bias introduced to the source modelling results by the assumption of linear or quadratic ionospheric effects is therefore unknown as well.

We present a more informed and physics-based correction of the surface displacement data in earthquake source modelling by using a split-spectrum method to estimate the ionospheric phase screen superimposed to the interferogram. This method is based on the dispersive nature of the ionosphere and separates the ionospheric component of the interferometric phase from the non-dispersive component related to topography, ground motion, and tropospheric path delay.

We study the ionospheric bias on the fault modelling results in a real-data case study of a non-complex earthquake rupture using the ionospheric phase screen estimations and the uninformed ramp removal in comparison. The test earthquake is the March 24 2011 Myanmar earthquake (Mw6.8), which is covered by ALOS-1. Here, the coseismic interferograms of both ascending and descending tracks show significant ionospheric distortions. We use the standard formulations of rectangular dislocations in an elastic half-space and run fully non-linear optimizations of the first-order fault parameters. We complement the optimization with a Bayesian model parameter uncertainties estimation to quantify the modelling bias in source model estimation with arbitrarily trend-corrected L-band displacement data compared to the correction based on the estimated ionospheric phase screen.

The ALOS data for this study are kindly provided by JAXA through the RA4 program under the project ID 1349.

Askja is an active volcano located in North Iceland and lying within a spreading segment of the mid-Atlantic ridge. From historical records, its eruptions are relatively infrequent but can be very powerful such as the 1875 Plinian event that blew ash to Scandinavia. To improve the mitigation of any similar future episode, a better understanding of its complex magmatic system is necessary. 

A continuous subsidence centred on Askja caldera has been recorded since at least 1983. Previous GPS and InSAR studies have shown that this subsidence has been decaying exponentially until at least 2009. It is postulated that rifting extension and some magmatic processes (e.g. outflow, crystallisation) at shallow depth are responsible for this deflation, although they have not been well defined. From the record of crustal deformation, different source geometries have been suggested in the literature but all studies agree that there is at least one shallow magmatic source at 3-3.5 km depth with a volume change rate of around -0.0014 to -0.0021 km³ yr^-1. 

Assuming a Mogi source at 3 km depth below the centre of the caldera, where the maximum elevation changes were recorded, and using time-dependent microgravity data, a mass decrease of ~1.6x10¹¹ kg was computed for the period 1988-2003 and a mass increase of ~0.68x10¹¹kg between 2007 to 2008, which continued until at least 2009. Those mass changes that did not affect the continuous subsidence were interpreted as being due to a magma drainage and a magma intrusion, respectively.

Our aim is to understand better the magmatic system at Askja by determining the potential physical processes (magmatic- or tectonic-related) that could explain the changes in crustal deformation and microgravity records.

Here, we present an update on the crustal deformation at Askja, from an InSAR time-series analysis of new radar data (ENVISAT, COSMO-SkyMed, Sentinel-1), spanning 2002 to 2015. We test several source geometries while constraining the location and magnitude of volume changes and extract the subsidence rate.

We also constrain the magnitude and location of mass changes from a microgravity time-series analysis combining previous data collected since 2002 with new data we collected in August 2015, considering the same source.

The next step of our work will be to jointly-invert InSAR and microgravity data to determine the nature of the body causing unrest by constraining its density. Any temporal changes in density will narrow down what could be the physical processes responsible for both deformation and gravity signals.

Extension at divergent plate boundaries is episodic and occurs during rifting events. As most divergent plate boundaries are located on the sea floor, rifting events are challenging to study and near-field data of past events are limited. Here we summarize our findings of studying the near-field deformation using SAR image offsets for several recent rifting events in the Red Sea region and in Iceland. Three volcanic eruptions occurred in the southern Red Sea during the past several years, on Jebel at Tair Island (2007-8) and within the Zubair archipelago (2011-12 and 2013).  By studying SAR and optical images of the activity we constrained the geometry of the dikes feeding the eruptions and obtained information about the associated stress directions. On Jebel at Tair we find evidence for temporarily varying stress orientations within the island’s volcanic edifice that appear isolated from the regional Red Sea stress field. The two eruptions in the Zubair archipelago resulted in the formation of two new islands and were fed by dikes much larger than the small size of the new islands might suggest. The dike geometry and the related near field deformation were constrained from SAR image-offset tracking on neighboring islands, which unlike InSAR provides unambiguous relative displacements between the different islands in the archipelago. Together these three volcanic eruptions, along with several seismic swarms, indicate that the southern Red Sea has been experiencing a rifting episode with multiple diking events and meter-scale extension. The reawakened activity in the southern Red Sea comes after more a century of quiescence and shows that this plate boundary is more active than previously thought. The 2014-15 Bárðarbunga rifting event in central Iceland was monitored with real-time earthquake and geodetic observations, showing that it originated from the Bárðarbunga central volcano located under the Vatnajokull ice cap. Magma propagated over 40 km to the northeast from the central volcano and well beyond the periphery of the glacier where the intrusion eventually made it to the surface and produced a large lava field. We used high-resolution TerraSAR-X and COSMO-SkyMed image-offset tracking to map the near-field deformation within and around a reactivated graben between the glacier’s edge and the eruption site. We find that the meter-scale opening across the graben was accompanied with a significant amount of left-lateral shear, which is also seen in some of the earthquake focal mechanisms in the area. The left-lateral shear is in accordance with the mis-alignment of the rift segment to the overall plate motion in the region and implies that pre-existing fracture zones play a key role in controlling dike emplacements in rifts.

Santorini Volcanic Complex (SVC) is the most active part of the South Aegean (Hellenic) Volcanic Arc, which marks the subduction of the African underneath the Eurasian plate, and is considered a prime site for conducting geophysical research. A series of ground based observational networks have been installed in SVC, including cGPS and seismographs, and data from several instrument campaigns have become available. Consequently, the exploitation of the dense geodetic measurements offered by InSAR and PSI in combination with in-situ measurements presents an exceptional opportunity for an innovative trans-disciplinary approach for monitoring an Santorini volcano.

The SVC islands showed signs of unrest for the first time in over half a century back in January 20112, when a series of small earthquakes began beneath the islands. Ground deformation was also detected by use of GPS receivers and an island-wide network of triangulation stations. Envisat data confirm that the islands have risen as much as 14 cm since January 2011 until March 2012, in the LOS. The whole island group has been inflating – slowly rising and moving outward – almost systematically around a point just north of the Kameni islands. Published results point to molten rock that has accumulated in a magma chamber at a depth of about 4 km.

The satellite data showed that the amount of molten rock that has arrived beneath Santorini over the 12+ month period (2011-2012) is the equivalent of 10–20 years’ growth of the volcano. The 2011-2012 magma movements didn’t indicate that an imminent eruption is about to happen. In fact, the rate of earthquake activity dropped off in early 2012. However, deformation has not fully recovered. The ultimate goal of this work is to enhance our understanding for the physical processes that govern Santorini volcano, covering the period from April 2012 until now.

Here, we present ground velocities for the SVC as obtained from the multi-temporal analysis of Synthetic Aperture Radar (SAR) imagery after the uplift event in order to monitor the process. We use high resolution SAR data from COSMO-SkyMed and TerraSAR-X satellites. The basic processing of the SAR imagery is based on Persistent Scatterer Interferometry, using a long series of co-registered, multi-temporal data and employing multi-interferogram techniques. StaMPS implementation is used, which adopts a spatial correlation approach to identify coherent pixels that retain their scattering characteristics through time, has been developed to suit volcanic areas, and implements a robust and innovative three-dimensional phase unwrapping algorithm. The methodology incorporates both PSInSAR and SBAS.Part of the processing is being performed on top of the Geophysical Exploitation Platform that is supported by ESA.

In addition to the satellite based measurements, the final ground motion rates are calibrated and validate via the exploitation of the dense GPS network installed in Santorini, consisting of eleven permanent stations.

Finally, several geophysical models are tested, accounting for point pressure (Mogi model for magma chambers), elastic dislocations (Okada model for sills), penny-shaped cracks and other models, along with combination of these to create more complex structures. Different inversion approaches are employed, by inverting InSAR data and GPS measurements independently and jointly.

The Gulf of Corinth (GoC) is one of the world's most rapidly extending continental regions. It is bounded on each side by active normal faults, on- and off-shore [Moretti et al., 2003; Palyvos et al., 2007] with a cumulated offset of ~3 km and a series of tilted blocks along the south coast [Doutsos and Poulimenos, 1992; Koukouvelas et al., 1999]. It has one of the highest seismicity rates in the Euro-Mediterranean region. A number of earthquakes with magnitude greater than 5.8 has occurred: Alkyonides (1981, M=6.7), Aigio (1995, M_w=6.1), and Galaxidi (1992, M_w=5.8), Efpalio (2010, M_w=5.3). In average, one M_w>6 earthquake is occurring every six years in the Gulf of Corinth.

The western part of the Gulf of Corinth presents the highest level of micro-seismicity and extension. This area includes the city of Patras, the Rion-Patras fault zone, the Rion-Antirrion Bridge, and many on- and off-shore fault systems [Palyvos et al., 2007]. The Psathopyrgos fault zone, which is considered to be a presently active structure, acts as a transfer zone between the Corinth and Patras rift [Flotte et al., 2005]. According to [Palyvos et al., [2007] no large earthquake has occurred on this fault in the last 400 years. A triple junction-like structure is present within the active fault network of the western Gulf of Corinth at the junction between the Karpenissi, Akarnania and Northwest Peloponnesus fragments. In the Western GoC there are numerous subaerial and subaqueous delta [Piper et al., 1990] and fault-controlled Gilbert-type fun deltas [Ford et al., 2013]. Several faults lay near the delta areas and, in some places, the prodelta slopes are modified by tidal current erosion [Piper et al., 1990]. Land subsidence is a common phenomenon in modern delta plains because of the compaction of sediments by consolidation of the dewatered material, compression of the load of the subsequent overlain deposits and groundwater or oil pumping [Nageswara Rao et al., 2010;  Meckel et al., 2007; Shi et al., 2007).

The presence of a plurality and the intense of geophysical phenomena and morphological features render this area and the Gulf of Corinth generally, as natural laboratory.

The Corinth Rift Laboratory (CRL) project (http://crlab.eu) is based on the joint efforts of various European institutions to study fault mechanics and related hazards in the study area. It is included in Geohazards Natural Laboratories of the GEO Supersites and will be one of the Near Fault Observatories (NFO) of European Plate Observing System (EPOS).

The CRL project focuses in the western part of the rift, including the city of Patras, the Rion-Antirion bridge and many other urban areas exposed to seismic risk. Driven by the multidisciplinary of the studies being carried on, a large number of surface networks has been deployed and are routinely operating , i.e. seismological,  strong motion, permanent and repeated GPS, strain, tilt and tide gage networks, many of them for more than a decade. Simultaneously, in-situ campaign measurements are being conducted for over two decades.

Previous geodetic studies conducted, which were based on GPS observations and Interferometric Synthetic Aperture Radar (DInSAR) observations, revealed North – South extension rates across the gulf of up to about 1.5 cm/yr [Clarke et al., 1997; Briole et al., 2000; Avallone et al., 2004] during the last 20 years.

In the present study we investigate the ground deformations in the area of the western end of the GoC by exploiting satellite geodesy. Multitemporal Interferometry technics [Hooper 2006; Hooper et al., 2007, 2008; Ferretti et al., 2001] – i.e. PSI, SBAS – of Synthetic Aperture Radar data acquisitions ASAR/ENVISAT sensor, as well as GPS observations [Briole et al., in preparation] have been carried out, acting in synergy for the detection, reconnaissance and measurement of the deformation due to aseismic tectonic procedures (deformation discontinuities characterized as candidate creeping faults). Motivated by the lack of such mapping and by the intrinsic GPS limitations (sparse samplings and weak vertical estimations) we exploited the benefits rising from both types of observations in synergy to overcome each ones weaknesses. The SAR interferometric added value data are very powerful for measuring vertical motions, for mapping localized deformations across faults or other features and for mapping and modeling the co-seismic deformation produced by earthquakes

Using multi-temporal interferometry and GPS observations, vertical movements, attributed mainly to tectonic movement, have been measured in the city of Patras, Rio, Antirrio and Aigio, but also to other geophysical processes such as Sellianitika, etc (see attached figure 1).   The geodetic along with seismic and geological observations were used to model a number of tectonic (eartquakes and aseismic) processes exposing their underlying geophysical context.