Vassilis Sakkas

Magnetotelluric images of the crustal structure of Chyulu Hills volcanic field, Kenya

Electromagnetic experiments were conducted in 1995 as part of a multidisciplinary research project to investigate the deep structure of the Chyulu Hills volcanic chain on the eastern flank of the Kenya Rift in East Africa. Transient electromagnetic (TEM) and broadband (120–0.0001 Hz) magnetotelluric (MT) soundings were made at eight stations along a seismic survey line and the data were processed using standard techniques. The TEM data provided effective correction for static shifts in MT data. The MT data were inverted for the structure in the upper 20 km of the crust using a 2-D inversion scheme and a variety of starting models. The resulting 2-D models show interesting features but the wide spacing between the MT stations limited model resolution to a large extent. These models suggest that there are significant differences in the physical state of the crust between the northern and southern parts of the Chyulu Hills volcanic field. North of the Chyulu Hills, the resistivity structure consists of a 10–12-km-thick resistive (up to 4000 V m) upper crustal layer, ca. 10-km-thick mid-crustal layer of moderate resistivity (f50 V m), and a conductive substratum. The resistive upper crustal unit is considerably thinner over the main ridge (where it is ca. 2 km thick) and further south (where it may be up to 5 km thick). Below this cover unit, steep zones of low resistivity (0.01–10 V m) occur underneath the main ridge and at its NW and SE margins (near survey positions 100 and 150– 210 km on seismic line F of Novak et al. [Novak, O., Prodehl, C., Jacob, A.W.B., Okoth, W., 1997. Crustal structure of the southern flank of the Kenya Rift deduced from wide-angle P-wave data. In: Fuchs, K., Altherr, R., Muller, B., Prodehl, C. (Eds.), Structure and Dynamic Processes in the Lithosphere of the Afro-Arabian Rift System. Tectonophysics, vol. 278, 171– 186]). These conductors appear to be best developed in upper crustal (1–8 km) and middle crustal (9–18 km) zones in the areas affected by volcanism. The low-resistivity anomalies are interpreted as possible magmatic features and may be related to the low-velocity zones recently detected at greater depth in the same geographic locations. The MT results, thus, provide a necessary upper crustal constraint on the anomalous zone in Chyulu Hills, and we suggest that MT is a logical compliment to seismics for the exploration of the deep crust in this volcanic-covered basement terrain. A detailed 3-D field study is recommended to gain a better understanding of the deep structure of the volcanic field. D 2002 Elsevier Science B.V. All rights reserved.

THE KRISP-94 MAGNETOTELLURIC SURVEY OF EARLY 1995 : FIRST RESULTS

Abstract In early 1995, high-quality, broad-band (10 −2 –10 4 s) magnetotelluric (MT) data were collected at 20 sites in southern Kenya. These sites were located to lie along pre-existing geophysical profiles along which seismic and gravity data had previously been acquired in 1994. Transient electromagnetic soundings (TEM) were also performed in order to facilitate removal of static shift effects from the MT data. Preliminary analyses of the MT transfer functions before and after static shift correction are presented, together with a review of those lithospheric processes which may plausibly exert an influence on the geoelectric signature, particularly in the context of a rift environment. Consideration of both the regional geology and induction arrow transfer functions implies the importance of two principal strike directions, one delineated by the faulting bounding the N-S-trending rift, the other controlled by a continental-scale, Proterozoic, NW-SE fault fabric. The western boundary of the rift appears more sharply imaged than the eastern margin, with the onset to conductivity enhancement enigmatically offset to the west of the Nguruman fault bounded western flank. Significantly enhanced conductivities are implied in the vicinity of the rift below the Chyulu Hills.

GROUND DEFORMATION OF ZAKYNTHOS ISLAND (WESTERN GREECE) OBSERVED BY PSI AND DGPS

Ground deformation studies based on Differential GPS (DGPS) measurements and Permanent Scatterers InSAR (PSI)* analysis using ERS (1992-2000) and ENVISAT (2003-2010) radar data have been conducted on Zakynthos Island (Western Greece) covering the period 1992 to 2012. These results were compared, validated and integrated with geological, geotectonic and seismological data to evaluate possible pre-earthquake deformation process, and the present tectonic regime. The PSI results indicate that a slight subsidence had occurred during the period 1992-2000, while uplift has mainly observed in 2003-2010. DGPS results for 2005-2006 indicated strong opening of the southern part of the island, while in the period 2006-2012 the amplitude of deformation is relatively smaller. The occurrence of the seismic outbreak that took place offshore to the south of Zakynthos during 2005-2006 may have contributed to the different deformational pattern as revealed by the ERS and ENVISAT PSI products, and also elucidate the DGPS results.

Ground deformation monitoring of the Santorini volcano using satellite radar interferometry

The island complex of Santorini is located at the central part of the Hellenic Volcanic Arc in the Southern Aegean Sea. This volcano complex basically consists of five islands, Thera, Therassia, Palea Kammeni, Nea Kammeni and Aspronisi forming a caldera of 83 km/sup 2/ with 390 m depth. The last significant volcanic activity took place between 1925 and 1950. However, the volcano is at a rest state during the last 50 years. The present work refers to the ground deformation monitoring of the area using ERS1, ERS2 and Envisat radar scenes from 1993 to 2004. A total number of four ERS1&2 SLC radar images covering the period 1993 to 1999, and two ENVISAT ASAR images for the period 2003-2004 were used. The method applied was the two-passes interferometry with a contribution of a high resolution Digital Elevation Model. The interferometric results show that although the volcano is at a rest phase, in the two volcanic centers of Pales and Nea Kammeni ground deformation (subsidence) of 62 mm along the line of sight of the satellite was detected.

Ground deformation study of KOS island (SE Greece) based on Squee-SAR™ interferometric technique

The SE part of the Hellenic Volcanic Arc (HVA), including Kos, Yali and Nisyros islands, is geodynamically very active and of high tectonic unrest, where the largest volumes of volcanic products were emitted during the past 160,000 years. The SqueeSAR™ Interferometric technique based on ENVISAT radar data has been applied in Kos to spatially and temporally study the ground deformation for the period 2003-2010. The observed LOS velocity field, with values ranging between -4 to +4 mm/yr, combined with small standard deviation velocity (<;1.4 mm/yr) and acceleration (-0.5 to 1.5 mm/yr2) values reveal an almost linear type of ground deformation. These small velocity values in conjunction with intermediate seismicity occurring only off-shore, indicate ground deformation of aseismic local character. The strongest subsidence is associated with local geothermal areas, as well as rural areas due to intense water pumping. Motions along the main faulting zones have also been clearly identified.

Crustal stress and seismic activity in the Ionian archipelago as inferred by satellite- and ground-based observations, Kefallinia, Greece

Abstract Different observational techniques are compared in order to investigate possible correlations in seismic activity. The study site is the island of Kefallinà (Greece), where measurements available included (1) DInSAR, DGPS, and DEM data, (2) soil exhalation measured by monitoring Radon (Rn) well content, and (3) acoustic emissions (AE) at high and low frequency (point-like records with high temporal resolution). AE records provide: (1) relative time variation of the applied stress intensity and (2) the state of fatigue of stressed rock volumes, the AE source. Our results indicate that the large spatial scale (poor time resolution) may be considered quite satisfactory, whereas fractal analysis of the AE time series displayed some discrepancies when compared to analogous investigations in the Italian Peninsula. Therefore, some refinement is needed in order to reach more precise interpretations of the relevant information available with this kind of data. However, both sets of observations appear in agreement with each other, although more exhaustive investigations would require a suitable array of point-like AE and Rn (or other) measuring sites, as well as longer data series. The latter are particularly helpful for detailed interpretations of the different occurrences within tectonically complex settings where crustal stress crises are marked by various types of geological phenomena.

Deformation monitoring in Kos Island Hellenic volcanic arc, Eastern Greece) using differential interferometry

In this study SAR interferometry technique has been applied over Kos Island (Hellenic Volcanic Arc) in order to detected pre-, coand post-seismic deformation and to compare with the deformation detected in Nisyros Island for the same period using the same interferometric pairs. The seismic unrest activity observed in Nisyros volcano (1996-1999) could produce crustal deformation in the adjacent island of Kos. The results of the interferometric processing show that coand post-seismic deformation was not possible to be detected due to the obtained low coherence. Contrary the results of the interferometric pair of the pre-seismic unrest period show two fringes of deformation in the southwest part of the island, the same amount of deformation was observed in Nisyros island.

Combined Squee-SAR TM and GPS ground deformation study of Nisyros-Yali volcanic field (Greece) for period 2002–2012

This study presents the spatial and temporal ground-deformation pattern of the Nisyros-Yali volcanic field after the period of the seismic crisis (1996-1998) based on the local GPS network remeasurement and the analysis of Interferometric data (2003-2010) applying the SqueeSARTM technique. The GPS results indicated intense subsidence in the northern and central parts of Nisyros that caused the western and eastern flanks of the island to “collapse” towards its center. The observed LOS velocity field of the SqueeSARTM analysis revealed a nearly linear type of ground deformation, exhibiting values ranging between -3 to +3mm/yr with small standard deviations. The interferometric results have also shown subsidence along the northern and central parts of the island, and an inherent eastward horizontal component at its southeastern part. The overall pattern of the observed subsidence in the area after 2000 is consistent with decrease of pressure in the associated magma chambers and hydrothermal system.

Ground deformation studies in Cephallonia island (Western Greece) based on DGPS & PS interferometry

Ground deformation studies based on Differential GPS (DGPS) measurements and Permanent Scatterers (PS) Interferometric analysis have been conducted in the seismically active area of the Cephallonia and Ithaca islands. DGPS measurements for the period 2001 to 2010 revealed horizontal component of deformation generally ranging from 3–8 mm/yr with the largest values at the western and southern parts of the island. Considering the vertical deformation, two periods are distinguished on the basis of DGPS and PS Interferometry: The first one (1992 to 2003) is consistent with anticipated motions associated with the main geological and tectonic features of the island. The second one (2003 to 2010) has been tentatively attributed to dilatancy in which relatively small uplift (2–4 mm/yr) occurred along the southern and southeastern parts of the island, while larger magnitudes (>4 mm/yr) took place at the western part of Cephallonia. These large magnitudes of uplift over an extended area (>50 km) are consistent with the hypothesis of dilatancy. On the basis of the analysis of 53 differential ASAR interferograms and the PS product, it has been derived that dilatancy effect should have commenced some time in mid-2005. If this interpretation is correct, it may foreshadow the occurrence of very strong earthquake(s) sometime in the near future.

Reviewing and Updating (1996–2012) Ground Deformation in Nisyros Volcano (Greece) Determined by GPS and SAR Interferometric Techniques (1996–2012)

The southeastern part of the South Aegean Volcanic Arc (SAVA), including Kos, Yali and Nisyros islands, is geodynamically very active. Intense seismic activity occurred on Nisyros Island during 1996–1998, accompanied with strong ground deformation and temperature increase of the fumaroles. Ground-deformation studies based on GPS and differential Interferometry for the period 1996–2000 revealed an “opening” of the island along the main faulting zones together with intense uplift of up to 90 mm that gradually reduced the following two years (2000–2002). Two Mogi point sources related to magmatic chambers fitted the observed ground deformation that was deduced from differential GPS and conventional Interferometry. One source is located onshore at about the centre of Nisyros, and the other is offshore at about 5 km to the north of Nisyros and 2 km to the southeast of Yali. Additional geophysical and geotectonic data support the existence and the location of these two magma chambers. Remeasurement of the Nisyros-Yali GPS network in 2012, combined with the analysis of SqueeSARTM Interferometric data from ENVISAT ascending and descending imageries (2003–2010), provided the spatial and temporal ground-deformation pattern after the period of the seismic crisis. The GPS results indicated intense subsidence (5–10 mm/year) in the northern and central parts of Nisyros that caused the western and eastern flanks of the island to “collapse” towards its centre, as is evident from the horizontal component of the GPS vectors. The observed line of sight (LOS) velocity field of the SqueeSARTM analysis revealed a nearly linear type of ground deformation for that period, exhibiting values ranging between −3 and +3 mm/year with small standard deviations (<0.8 mm/year). The SqueeSARTM results have generally shown a similar deformational pattern as the GPS one; subsidence is observed along the northern and central parts of Nisyros, while when comparing ascending and descending LOS components of ground velocities, an eastward horizontal component is inferred for the southeastern part of Nisyros. Motions along the main faulting zones that crosscut the island are also clearly identified. Both analyses have shown that the ground deformation on Yali may be associated with a more regional subsidence that takes place to the south and offshore of the islet, as is clearly indicated from the horizontal component of the GPS vectors. The overall pattern of subsidence in the broader area of Nisyros and Yali after 2000 is consistent with the two modelled Mogi sources, and indicates that the pressure inside the two associated magma chambers and the hydrothermal system has likely decreased.