Crust-Mantle Kinematics in and around the Hellenic Arc Elucidated by Local Shear Wave Splitting and Receiver Function Analyses

<p>The fundamental knowledge on seismic anisotropy inferred from various data sets can enhance our understanding of its vertical resolution that is critical for a better interpretation of past and current dynamics and resultant crustal and mantle kinematics in the Hellenic Trench and its hinterland. To investigate the nature of deformation zones, we perform both local S-wave splitting (SWS) measurements and receiver functions (RFs) analysis. Our preliminary findings from the harmonic decomposition technique performed on radial and tangential RFs suggest relatively more substantial anisotropic signals in the lower crust and uppermost mantle with respect to upper and middle crustal structure in the region. Apparent anisotropic orientations obtained from RFs harmonic decomposition process show several consistencies with those discovered from local SWS measurements at selected stations. The actual anisotropic orientation for the structures, however, requires further modelling of the receiver functions obtained.</p>

Short-Term Foreshocks as Key Information for Mainshock Timing and Rupture: The Mw6.8 25 October 2018 Zakynthos Earthquake, Hellenic Subduction Zone

Significant seismicity anomalies preceded the 25 October 2018 mainshock (Mw = 6.8), NW Hellenic Arc: a transient intermediate-term (~2 yrs) swarm and a short-term (last 6 months) cluster with typical time-size-space foreshock patterns: activity increase, b-value drop, foreshocks move towards mainshock epicenter. The anomalies were identified with both a standard earthquake catalogue and a catalogue relocated with the Non-Linear Location (NLLoc) algorithm. Teleseismic P-waveforms inversion showed oblique-slip rupture with strike 10°, dip 24°, length ~70 km, faulting depth ~24 km, velocity 3.2 km/s, duration 18 s, slip 1.8 m within the asperity, seismic moment 2.0 × 1026 dyne*cm. The two largest imminent foreshocks (Mw = 4.1, Mw = 4.8) occurred very close to the mainshock hypocenter. The asperity bounded up-dip by the foreshocks area and at the north by the foreshocks/swarm area. The accelerated foreshocks very likely promoted slip accumulation contributing to unlocking the asperity and breaking with the mainshock. The rupture initially propagated northwards, but after 6 s stopped at the north bound and turned southwards. Most early aftershocks concentrated in the foreshocks/swarm area. This distribution was controlled not only by stress transfer from the mainshock but also by pre-existing stress. In the frame of a program for regular monitoring and near real-time identification of seismicity anomalies, foreshock patterns would be detectable at least three months prior the mainshock, thus demonstrating the significant predictive value of foreshocks.

InSAR Time-Series Analysis for Monitoring Ground Displacement Trends in the Western Hellenic Arc: The Kythira Island, Greece

Kythira Island is situated at the western Hellenic Arc, which is a region of very high seismicity and tectonic activity. On 8 January 2006, a large seismic event of Mw = 6.7 occurred close to Kythira, in association with the Hellenic subduction zone. We present an extensive multi-temporal interferometry study of ground displacements in Kythira Island exploiting SAR data of a decade, from 2003 to 2009, and from 2015 to 2019. The line-of-sight displacement field for the 2003–2009 time period presents small-scale displacements that do not exceed −3 mm/y, identified to the East of Kythira’s airport, and 3 mm/y in the central part of the island. We exploit then Sentinel-1 data from 2015 to 2019, of both descending and ascending Sentinel-1 SAR imaging modes to decompose the line-of-sight measured deformation to vertical and east–west motion components. Higher vertical displacement rates characterize the central part of the island, with a maximum value of 5 mm/y. This short-term uplift rate exceeds by an order of magnitude the long-term geologic uplift rate of ~0.13 mm/y found in the island during the Quaternary. We discuss possible regional geophysical explanations for this discrepancy. Strike-slip components are detected in the N–E coast of the Potamos village, where horizontal displacements occur, with an east-ward trend and a maximum value of −3 mm/y. These insights are valuable input for the systematic monitoring of this high seismic risk island and the dynamic assessment of its hazard potential.

Tsunami Propagation and Flooding in Sicilian Coastal Areas by Means of a Weakly Dispersive Boussinesq Model

This paper addresses the tsunami propagation and subsequent coastal areas flooding by means of a depth-integrated numerical model. Such an approach is fundamental in order to assess the inundation hazard in coastal areas generated by seismogenic tsunami. In this study we adopted, an interdisciplinary approach, in order to consider the tsunami propagation, relates both to geomorphological characteristics of the coast and the bathymetry. In order to validate the numerical model, comparisons with results of other studies were performed. This manuscript presents first applicative results achieved using the weakly dispersive Boussinesq model in the field of tsunami propagation and coastal inundation. Ionic coast of Sicily (Italy) was chosen as a case study due to its high level of exposure to tsunamis. Indeed, the tsunami could be generated by an earthquake in the external Calabrian arc or in the Hellenic arc, both active seismic zones. Finally, in order to demonstrate the possibility to give indications to local authorities, an inundation map, over a small area, was produced by means of the numerical model.

Smart Geospatial System Design for Real-Time Risk Assessment at the Highly Active Area of the Ionian Islands, Greece

<p>The Ionian Islands are located in the northwestern part of the Hellenic Arc and constitute one of the most seismically active areas in the Mediterranean. Building a geospatial database including all the available geo-information layers was the initial step for identifying and delineating the earthquake-related environmental effects by using various mapping algebra techniques and algorithms. Landslide, liquefaction and tsunami related inventories were created. Real time recording network of sensors such as meteorological instruments, seismographs, accelerometers etc was designed to trans pond data telemetrically and feed a dynamically interactive geodatabase, which in turn act as a smart tool for declaring an area as vulnerable to a specific hazard. The abovementioned approach can contribute to the reduction of the consequences after a disastrous event, as it will provide useful information to the civil protection authorities for increased alertness during an ongoing threat.</p><p>The identification of the risk areas by using various methods has become significant in recent years due to the fact that among others it serves as a valuable tool for revealing and highlighting sites of significant hazards. In this study we present a smart tool, specially developed for recording and taking under consideration any changing parameters that affect the susceptibility of an area to any of the studied geo-hazards and highlight it on a digital real time updateable map.</p>

The ML = 6.8 25 October 2018 Earthquake Zakynthos Island (Ionian Sea) and the evolution of the aftershock sequence one year later

<p>The area of Zakynthos (Ionian Island) is located at a complex plate boundary region where two tectonic plates (Africa-Nubia and Eurasia) converge, thus forming the western Hellenic Arc. On the midnight of 26<sup>th</sup> October (M<sub>L</sub> = 6.6, 22:54:49 UTC) a very strong earthquake has struck at the eastern part of Zakynthos Island (Ionian Sea, Western Greece). Epicentral coordinates of the earthquake was determined as 37.3410° N, 20.5123° E and a focal depth at 10 km, according to the manual solution of National Observatory of Athens</p><p>(http://bbnet.gein.noa.gr/alerts_manual/2018/10/evman181025225449_info.html).</p><p>This earthquake was strongly felt at the biggest shock was felt as far afield as Naples in western Italy, and in Albania, Libya, and the capital Athens. Nobody was injured by these events but there was significant damage to the local port and a 13th Century island monastery south of Zakynthos.</p><p>A few minutes later (23:09:20, UTC) a second intermediate earthquake with magnitude M<sub>L</sub>=5.1 was followed the first event. The M5+ events of 25 October 2019, as well as the rich aftershock sequence of 10.000+ events with magnitudes 1.0<ML<4.9 of the 12 following months have been relocated using the double – difference algorithm HYPODD.</p><p>For the aftershocks with 3.7<M<sub>L</sub><6.6 we applied the moment tensor inversion to determine the activation of the faulting type, the Seismic Moment (M<sub>0</sub>) and the Moment Magnitude (M<sub>w</sub>). For this purpose, 3–component broadband seismological data from the Hellenic Unified Seismological Network (HUSN) at epicentral distances less than 3˚ were selected and analyzed. The preparation of the data, includes the deconvolution of instrument response, following the velocity was integrated to displacement and finally the horizontal components rotated to radial and transverse. All the focal mechanisms were compared with those from other institutes and they are in agreement. The second part of this study refers to the calculation of the stress tensor using the STRESSINVERSE package by Václav Vavryčuk. The final part of this study includes an extensive kinematic analysis of geodetic data from local GNSS permanent station to further examine the dynamic displacement.</p><p>References:</p><ol><li>Athanassios Ganas, Pierre Briole, George Bozionelos, Panagiotis Elias, Sotiris Valkaniotis, Varvara Tsironi, Alexandra Moshou and Nikoletta Andritsou, 2019. The October 25, 2018 M6.7 Zakynthos earthquake sequence (Ionian Sea, Greece): fault modeling from seismic and GNSS data and implications for seismic strain release along the western Hellenic Arc, 15th, Sp. Pub. 7, Ext. Abs. GSG2019 – 324</li> <li>Konstantaras A.J. Classification of distinct seismic regions and regional temporal modelling of seismicity in the vicinity of the Hellenic seismic arc. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. 6 (4), 1857-1863, 2012.</li> <li>Gerassimos A. Papadopoulos, Vassilios K. Karastathis, Ioannis Koukouvelas, Maria Sachpazi, Ioannis Baskoutas, Gerassimos Chouliaras, Apostolos Agalos, Eleni Daskalaki, George Minadakis, Alexandra Moshou, Aggelos Mouzakiotis, Katerina Orfanogiannaki, Antonia Papageorgiou, Dimitrios Spanos, Ioanna Triantafyllou. The Cephalonia, Ionian Sea (Greece), sequence of strong earthquakes of January – February 2014: A first report, Research in Geophysics 2014; 4:5441</li> </ol>

Lower Ionospheric turbulence variations during the tectonic activity of the last quarter of 2019 in the Hellenic Arc (Greece)

<p>In this paper we investigate the ionospheric turbulence from TEC observations before and during the tectonic activity of the last quarter of 2019 in the Hellenic Arc, Greece (main shock at l=23.26<sup>o</sup>E, j=35.69<sup>o</sup>N, M<sub>w</sub>=6.1). The Total Electron Content (TEC) data of 6 Global Positioning System (GPS) stations of the EUREF network, which are being provided by IONOLAB (Turkey), were analysed using Discrete Fourier Analysis in order to investigate the TEC variations. The results of this investigation indicate that the High- Frequency limit f<sub>o</sub>, of the ionospheric turbulence content, increases by aproaching the site and the time of the earthquake occurrence, pointing to the earthquake location (epicenter). We conclude that the LAIC mechanism through acoustic or gravity wave could explain this phenomenology.</p>