SOURCE PARAMETERS OF MODERATE AND STRONG EARTHQUAKES IN THE BROADER AREA OF ZAKYNTHOS ISLAND (W. GREECE) FROM REGIONAL AND TELESEISMIC DIGITAL RECORDINGS

The source parameters of all the moderate and strong events that occurred in the broader area of Zakynthos Island for the period 1997–2009 are determined using two different techniques depending on epicentral distance. For the strong events we employed teleseismic body-wave modeling between 30° and 90°, while for moderate events a regional moment tensor inversion approach was used. In both cases we used broadband waveform data with a good signal-to-noise ratio. The calculated focal mechanisms are representative of the displacement and strain fields of the broader area. Those associated with the Cephalonia transform fault are consistent with dextral strike-slip motion. The area between the island of Zakynthos and the Western coast of Peloponnese is also characterized by strike–slip faulting, while reverse faulting is mainly observed south of Zakynthos Island. Using both techniques, we also retrieved the source parameters of the biggest events of the seismic sequence of April 2006 near the southern coast of Zakynthos Island. The depths of the located events of the sequence vary mainly between 10 and 25 km. The fault plane solutions revealed thrust type faulting in all cases, something which can be related with upward motions in the southern part of the Island observed by DGPS measurements

The Making of the NEAM Tsunami Hazard Model 2018 (NEAMTHM18)

The NEAM Tsunami Hazard Model 2018 (NEAMTHM18) is a probabilistic hazard model for tsunamis generated by earthquakes. It covers the coastlines of the North-eastern Atlantic, the Mediterranean, and connected seas (NEAM). NEAMTHM18 was designed as a three-phase project. The first two phases were dedicated to the model development and hazard calculations, following a formalized decision-making process based on a multiple-expert protocol. The third phase was dedicated to documentation and dissemination. The hazard assessment workflow was structured in Steps and Levels. There are four Steps: Step-1) probabilistic earthquake model; Step-2) tsunami generation and modeling in deep water; Step-3) shoaling and inundation; Step-4) hazard aggregation and uncertainty quantification. Each Step includes a different number of Levels. Level-0 always describes the input data; the other Levels describe the intermediate results needed to proceed from one Step to another. Alternative datasets and models were considered in the implementation. The epistemic hazard uncertainty was quantified through an ensemble modeling technique accounting for alternative models’ weights and yielding a distribution of hazard curves represented by the mean and various percentiles. Hazard curves were calculated at 2,343 Points of Interest (POI) distributed at an average spacing of ∼20 km. Precalculated probability maps for five maximum inundation heights (MIH) and hazard intensity maps for five average return periods (ARP) were produced from hazard curves. In the entire NEAM Region, MIHs of several meters are rare but not impossible. Considering a 2% probability of exceedance in 50 years (ARP≈2,475 years), the POIs with MIH >5 m are fewer than 1% and are all in the Mediterranean on Libya, Egypt, Cyprus, and Greece coasts. In the North-East Atlantic, POIs with MIH >3 m are on the coasts of Mauritania and Gulf of Cadiz. Overall, 30% of the POIs have MIH >1 m. NEAMTHM18 results and documentation are available through the TSUMAPS-NEAM project website (http://www.tsumaps-neam.eu/), featuring an interactive web mapper. Although the NEAMTHM18 cannot substitute in-depth analyses at local scales, it represents the first action to start local and more detailed hazard and risk assessments and contributes to designing evacuation maps for tsunami early warning.publishedVersio

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. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

The spatially extended 2006 April Zakynthos (Ionian Islands, Greece) seismic sequence and evidence for stress transfer

The seismic sequence at the southern part of Zakynthos Island (Ionian Sea, Western Greece) during April and May of 2006 is investigated. It consists of four moderate earthquakes (5.3 ≤M w≤ 5.7) that were followed by significant seismic activity. Source parameters of the significant events were determined using two different techniques. Teleseismic body-wave modelling was employed for the major events of the sequence (M w > 5.6), although for the moderate ones the frequency-wavenumber integration method was used. The calculated fault plane solutions of the 2006 April seismic sequence revealed compressional regime in all cases. More than 3500 microearthquakes were recorded and located using a local temporary network. Their spatial distribution indicated high concentration of seismic activity within an expanded area, taking into account the magnitudes of the major events. Within this expanded area (approximately 10 × 30 km 2), the focal depths varied between 10 and 25 km. Two main clusters were revealed. The major events occurred in an area of low seismic activity that separates the two clusters. Seismic cross-sections indicated a complex pattern of the hypocentre distribution with the activation of two nearly antithetical faults, consistent with the determined focal mechanisms. The northern cluster can be associated with a fault plane trending NNW-SSE and dipping towards SW, although the southern cluster with a fault plane striking NW-SE and dipping towards ENE. Finally, Coulomb stress analysis was performed to calculate the stress transfer and correlate it with the activated area. Positive lobes with stress more than 0.3 bars were obtained, indicating that these values are large enough to increase the seismicity towards the observed NNW-SSE direction. © 2012 The Authors Geophysical Journal International © 2012 RAS

The 26 November 2019 Mw 6.4 Albania destructive earthquake

With the strong 26 November 2019 earthquake that struck western Albania, several buildings collapsed, causing 51 casualties, mainly in the areas of Durrës and Thumanë. The destruction is attributed to several factors, including strong ground motion (maximum peak ground acceleration = 192 cm= s2 in Durrës), soil liquefaction, site amplification, poor building workmanship and construction quality, aging of building materials, impact on buildings of the strong 21 September 2019 Mw 5.6 foreshock, and pre-existing stress on buildings sustaining differential displacements because of soft soil conditions in their foundations. In both areas, we estimated maximum seismic intensity of VIII-IX (modified Mercalli intensity and European Macroseismic Scale 1998 scales). Fault-plane solutions indicated reverse faulting striking northwest-southeast. From regional tectonics, we assumed that the causal fault dips to east-northeast, implying that the affected area is situated at the hanging wall domain of the causative fault. Using the Non-Linear Location program algorithm and ak135 velocity model and 71 P and S phases, we manually located the mainshock hypocenter offshore, at distance of ∼ 17 km from Durrës and at depth of ∼ 22 km. Adopting this solution, a finite-fault model of space-time seismic slip was developed from the inversion of teleseismic P waveforms. Strike 345°, dip 22°, rupture velocity 2:6 km= s, and total rupture duration ∼ 16 s fit the data. The rupture was complex, showing one main patch at the south and a second at the north with maximum slips of ∼ 1:5 and ∼ 1 m, respectively. The rake vector at the main slip area was 99°, indicating that the thrust-type component played the most important role in the rupture process. The total seismic moment released was estimated at Mo = 5:0 × 1018 N·m corresponding to Mw 6.4. © Seismological Society of Americ

Tsunami Potential of Moderate Earthquakes: The July 1, 2009 Earthquake (M w 6.45) and its Associated Local Tsunami in the Hellenic Arc

On July 1, 2009, a Mw 6.45 earthquake ruptured south of Crete Island (Greece) along the Hellenic Arc triggering a local tsunami. Eyewitness reported the tsunami from Myrtos and Arvi (south-eastern Crete) and from the north of Chrisi Islet, located to the southeast of Crete. The earthquake occurred offshore, about 80 km south of Crete, where routine earthquake locations are poor. The hypocentre is relocated using a 2-D velocity model and several local 1-D velocity models. Epicentral locations obtained by using the different velocity models show very minor variations. Instead, relocated hypocentres can be grouped into two sets of solutions: (1) those with a shallower depth (depth < 12 km) obtained with the 2-D velocity model and the 1-D velocity models having a shallower Moho at less than 30 km, and (2) those with a larger depth (depth of 28 and 40 km) obtained with the velocity models having a Moho at about 40 km. Shallower hypocentres are more consistent with the tsunamigenic nature of the earthquake as also supported by tsunami numerical simulations. In fact, shallow sources (depths < 12 km) are capable of generating tsunami waves, while it is not the case for deeper sources (depth > 25 km) either in the upper-plate or along the plate interface. Models accounting for either homogeneous or heterogeneous slip on the causative fault are tested, with the heterogeneous one better reproducing the observations in terms of number of tsunami waves reaching the shoreline and duration of the sea disturbance. The short travel time, about 10 min, of the first tsunami arrival at the southern coast of Crete represents a big challenge for tsunami early warning systems operating in the area. © 2019, Springer Nature Switzerland AG

Seismic and Geodetic Imaging (DInSAR) Investigation of the March 2021 Strong Earthquake Sequence in Thessaly, Central Greece

Three strong earthquakes ruptured the northwest Thessaly area, Central Greece, on the 3, 4 and 12 March 2021. Since the area did not rupture by strong earthquakes in the instrumental period of seismicity, it is of great interest to understand the seismotectonics and source properties of these earthquakes. We combined relocated hypocenters, inversions of teleseismic P-waveforms and of InSAR data, and moment tensor solutions to produce three fault models. The first shock (M-w = 6.3) occurred in a fault segment of strike 314 degrees and dip NE41 degrees. It caused surface subsidence -40 cm and seismic slip 1.2-y1.5 m at depth similar to 10 km. The second earthquake (M-w = 6.2) occurred to the NW on an antithetic subparallel fault segment (strike 123 degrees, dip SW44 degrees). Seismic slip of 1.2 m occurred at depth of similar to 7 km, while surface subsidence -10 cm was determined. Possibly the same fault was ruptured further to the NW on 12 March (M-w = 5.7, strike 112 degrees, dip SSW42 degrees) that caused ground subsidence -5 cm and seismic slip of 1.0 m at depth similar to 10 km. We concluded that three blind, unknown and unmapped so far normal fault segments were activated, the entire system of which forms a graben-like structure in the area of northwest Thessaly

Fault models for the Bodrum–Kos tsunamigenic earthquake (Mw6.6) of 20 July 2017 in the east Aegean Sea

We investigate a fault model for the shallow, strong (Mw6.6) tsunamigenic earthquake of 20 July 2017 with source area in the east Aegean Sea between Bodrum peninsula (Turkey) and Kos Isl. (Greece). Fault plane solutions are consistent with the regional active tectonics indicating normal faulting striking about E–W. A relocated hypocenter of the main shock was obtained and the rupture time-space history for two alternative fault solutions dipping either to the south or to north was approached by inverting P-waves recorded at teleseismic (30°–90°) distances. In both fault solutions the seismic moment calculated corresponds to magnitude Mw = 6.6 while the earthquake had total rupture duration of less than 10 s. The maximum co-seismic slip was close to the hypocenter ∼1.6 m and ∼2.2 m for the south and north dipping faults, respectively. Close to the surface co-seismic slip of ∼0.3 m was calculated. Our analysis of Sentinel satellite InSAR images showed ground deformation fringes indicating ∼13 cm vertical displacement change between the North and South coasts of the Karaada islet. This result is in line with our preference of the south dipping fault, supported primarily by GPS displacements, regional tectonics, aftershock activity and tsunami observations, although results of InSAR inversion fit equally well both solutions. © 2019 Elsevier Lt