Combined Ground Deformation Study Of Broader Area Of Patras Gulf (W. Greece) Using PSI-WAP, DGPS And Seismicity Analyses

Long-term ground deformation monitoring using the Persistent Scatterer Interferometry Wide Area Product (PSI-WAP) technique for the period 1992-2003, combined with Differential GPS measurements and seismicity analysis has provided useful information about the tectonic motions of the tectonically complex area of Patras Gulf (Western Greece), and lead to new insights on the geotectonic regime of this region. Descending ERS radar images were used to compile the PSI-WAP product that has been calibrated using the absolute velocity field of available GPS stations in the area. It has been found that the deformation of the southern part of Patras Gulf near the coastline has been characterized by considerable subsidence (>-5mm/yr), where unconsolidated sediments usually prevail, compared to the northern part of the gulf. Significant subsidence has also been identified in areas along the down-throw side of possible faults, as well as areas where extensive ground water pumping has occurred for irrigation. These results correlate well with local GPS and seismicity data

Seismological and Ground Deformation Study of the Ionian Islands (W. Greece) during 2014–2018, a Period of Intense Seismic Activity

Seismicity in the Ionian Sea (W. Greece) is mainly generated along the Cephalonia–Lefkada Transform Fault Zone (CLTFZ) in the central Ionian, and on the northwestern termination of the Hellenic subduction margin in the south. Joint pre-, co- and post-seismic ground deformation and seismological analysis is performed at the broad Ionian area, aiming to homogeneously study the spatiotemporal evolution of the activity prior to and after the occurrence of strong (M > 6) earthquakes during the period of 2014–2018. The 2014 Cephalonia earthquakes (Mw6.1 and Mw5.9) were generated on a faulting system adjacent to CLTFZ, causing local ground deformation. The post-seismic sequence is coupled in space and time with the 2015 Lefkada earthquake (Mw6.4), which occurred on the Lefkada segment of the CLTFZ. Co-seismic displacement was recorded in the broader area. Seismicity was concentrated along the CLTFZ, while its temporal evolution lasted for several months. The 2018 Zakynthos earthquake (Mw6.7) caused regional deformation and alterations on the near-velocity field, with the seismicity rate remaining above background levels until the end of 2021. In the northern Ionian, convergence between the Apulian platform and the Hellenic foreland occurs, exhibiting low seismicity. Seismic hazard assessment revealed high PGA and PGV expected values in the central Ionian

Combined transient electromagnetic and magnetotelluric study of the southern Kenya Rift Valley

The transient electromagnetic (TEM) method and the magnetotelluric (MT) technique have been applied to determine the electrical resistivity structure across the southern Kenya Rift Valley. The main profile extends from the shores of Lake Victoria, west of the Rift Valley, to the north of the Chyulu Hills volcanic chain, 150 km SE of the rift. A second profile runs parallel to the Chyulu Hills volcanic trend. Data from 19 stations along the two profiles have been processed using classical techniques and in the case of MT, analysed with modern tensor decomposition methods.;The TEM data have facilitated the removal of static shift effects from the MT data and recovery of the near-surface (2000 .m) Archaean crust 30 km thick, with a 10-12 km mid-crustal conductive (100 .m) zone, resting on a moderately resistive (100 .m) mantle appears at the west end of the main profile. A conductive fault-like zone extending to mantle depths in the area of the Oloololo Escarpment coincides with the exposed boundary between the Archaean Nyanza Craton and the Proterozoic Mozambique Belt. A poorly constrained highly resistive (>10000 .m) (Proterozoic ?) crust is found at the western flank of the rift. Low resistivities (<50 .m) are found down to the base of the crust in the rift zone and are possibly due to the presence of sedimentary fill deposits at shallow depths, and the presence of magmatism and partial melt at deeper levels. East of the rift a less sharply defined geoelectric margin, offset from the accepted topographic and geologic boundary of the rift, marks the transition to a more resistive (1000 .m.) Proterozoic crust. Significantly enhanced conductivities (<100 .m) are implied in the complex 3-D region of the Chyulu Hills

Ground Deformation in the Broader Area of the Atalanti Fault Zone (Central Greece) Based on GPS & PSI-WAP

The Persistent Scatterer Interferometry Wide Area Product (PSI-WAP) based on ERS1 & ERS2 radar data has been used in the seismic active area of Atalanti Faulting Zone (Central Greece) to spatially and temporally study the ground deformation for the period 1992-2003. The observed LOS velocity field, with values ranging between -0.5 to -5.0 mm/yr, combined with small standard deviation velocity values reveals an almost linear type of ground deformation. The most intense subsidence was associated to alluvia deposits and man made activities (intense water pumping). Differential motions along the main faulting zones have also been clearly identified. GPS results reflected a similar pattern of motions (subsidence) with the identified interferometric image. The recorded seismicity in the area is not significant for the PSI-WAP period. The micro-seismic activity (M<3) is mainly confined peripherally and does not seem to confidently affect the observed ground deformation

On the Patterns and Scaling Properties of the 2021–2022 Arkalochori Earthquake Sequence (Central Crete, Greece) Based on Seismological, Geophysical and Satellite Observations

The 27 September 2021 damaging mainshock (Mw6.0) close to Arkalochori village is the strongest earthquake that was recorded during the instrumental period of seismicity in Central Crete (Greece). The mainshock was preceded by a significant number of foreshocks that lasted nearly four months. Maximum ground subsidence of about 18 cm was estimated from InSAR processing. The aftershock sequence is located in an almost NE-SW direction and divided into two main clusters, the southern and the northern ones. The foreshock activity, the deformation area, and the strongest aftershocks are located within the southern cluster. Based on body-wave travel times, a 3-D velocity model was developed, while using combined space and ground-based geodetic techniques, the co-seismic ground deformation is presented. Moreover, we examined the co-seismic static stress changes with respect to the aftershocks’ spatial distribution during the major events of the foreshocks, the Mw = 6.0 main event as well as the largest aftershock. Both the foreshock and the aftershock sequences obey the scaling law for the frequency-magnitude distribution as derived from the framework of non-extensive statistical physics (NESP). The aftershock production rate decays according to the modified Omori scaling law, exhibiting various Omori regimes due to the generation of secondary aftershock sequences. The analysis of the inter-event time distribution, based on NESP, further indicates asymptotic power-law scaling and long-range correlations among the events. The spatiotemporal evolution of the aftershock sequence indicates triggering by co-seismic stress transfer, while its slow migration towards the outer edges of the area of the aftershocks, related to the logarithm of time, further indicates a possible afterslip

On the Patterns and Scaling Properties of the 2021&ndash;2022 Arkalochori Earthquake Sequence (Central Crete, Greece) Based on Seismological, Geophysical and Satellite Observations

The 27 September 2021 damaging mainshock (Mw6.0) close to Arkalochori village is the strongest earthquake that was recorded during the instrumental period of seismicity in Central Crete (Greece). The mainshock was preceded by a significant number of foreshocks that lasted nearly four months. Maximum ground subsidence of about 18 cm was estimated from InSAR processing. The aftershock sequence is located in an almost NE-SW direction and divided into two main clusters, the southern and the northern ones. The foreshock activity, the deformation area, and the strongest aftershocks are located within the southern cluster. Based on body-wave travel times, a 3-D velocity model was developed, while using combined space and ground-based geodetic techniques, the co-seismic ground deformation is presented. Moreover, we examined the co-seismic static stress changes with respect to the aftershocks&rsquo; spatial distribution during the major events of the foreshocks, the Mw = 6.0 main event as well as the largest aftershock. Both the foreshock and the aftershock sequences obey the scaling law for the frequency-magnitude distribution as derived from the framework of non-extensive statistical physics (NESP). The aftershock production rate decays according to the modified Omori scaling law, exhibiting various Omori regimes due to the generation of secondary aftershock sequences. The analysis of the inter-event time distribution, based on NESP, further indicates asymptotic power-law scaling and long-range correlations among the events. The spatiotemporal evolution of the aftershock sequence indicates triggering by co-seismic stress transfer, while its slow migration towards the outer edges of the area of the aftershocks, related to the logarithm of time, further indicates a possible afterslip

GROUND DEFORMATION STUDIES IN CEPHALLONIA ISLAND (WESTERN GREECE) BASED ON DGPS &amp; 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 (&gt;4 mm/yr) took place at the western part of Cephallonia. These large magnitudes of uplift over an extended area (&gt;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

Heterogeneous crust and upper mantle across southern Kenya and the relationship to surface deformation as inferred from magnetoteluric imaging.

We have used magnetotelluric data imaging to determine the resistivity structure across southern Kenya and our results suggest the presence of a buckled blocky or segmented lithosphere across the region. Prominent steep conductive zones at the Oloololo (OLO) escarpment and eastern rift margin allow us to subdivide the region into three crustal domains. West of OLO, a bow-shaped conductor underlies a 10 km thick resistive upper crustal unit spatially correlating with an exposed Archaean greenstone belt. Between OLO and the eastern rift margin are found steeply dipping alternating conductive and resistive zones that appear buckled. East of this belt are found prominent, 5 to 20 km deep, subhorizontal conductors atop steep resistive blocks with flanking conductors. The main steep features in the crust appear to extend below the seismic Moho and thus suggest the presence of anomalously thick crust across the region. A 50 km-wide and 4–8 km deep w-shaped (double half-graben) structure is suggested at the position of the Kenyan rift. We show that our inferred lateral zoning is consistent with collocated gravity and seismic measurements. We propose a link between the deep resistivity heterogeneity and surface deformation pattern in the area

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