Domino-style earthquakes along blind normal faults in Northern Thessaly (Greece): kinematic evidence from field observations, seismology, SAR interferometry and GNSS

Here we present a joint analysis of the geodetic, seismological and geological data of the March 2021 Northern Thessaly seismic sequence, that were gathered and processed as of April 30, 2021. First, we relocated seismicity data from regional and local networks and inferred the dip-direction (NE) and dip-angle (38°) of the March 3, 2021 rupture plane. Furthermore, we used ascending and descending SAR images acquired by the Sentinel-1 satellites to map the co-seismic displacement field. Our results indicate that the March 3, 2021 Mw=6.3 rupture occurred on a NE-dipping, 39° normal fault located between the villages Zarko (Trikala) and Damasi (Larissa). The event of March 4, 2021 occurred northwest of Damasi, along a fault oriented WNW-ESE and produced less deformation than the event of the previous day. The third event occurred on March 12, 2021 along a south-dipping normal fault. We computed 22 focal mechanisms of aftershocks with M≥4.0 using P-wave first motion polarities. Nearly all focal mechanisms exhibit normal kinematics or have a dominant normal dip-slip component. The use of InSAR was crucial to differentiate the ground deformation between the ruptures. The majority of deformation occurs in the vertical component, with a maximum of 0.39 m of subsidence over the Mw=6.3 rupture plane, south and west of Damasi. A total amount of 0.3 m horizontal displacement (E-W) was measured. We also used GNSS data (at 30-s sampling interval) from twelve permanent stations near the epicentres to obtain 3D seismic offsets of station positions. Only the first event produces significant displacement at the GNSS stations (as predicted by the fault models, themselves very well constrained by InSAR). We calculated several post-seismic interferograms, yet we have observed that there is almost no post-seismic deformation, except in the footwall area (Zarkos mountain). This post-seismic deformation is below the 7 mm level (quarter of a fringe) in the near field and below the 1 mm level at the GNSS sites. The cascading activation of the three events in a SE to NW direction points to a pattern of domino-style earthquakes, along neighbouring fault segments. The kinematics of the ruptures point to a counter-clockwise change in the extension direction of the upper crust (from NE-SW near Damasi to N-S towards northwest, near Verdikoussa)

Contemporary crustal stress of the Greek region deduced from earthquake focal mechanisms

A massive dataset of over 1900 focal mechanisms of crustal earthquakes with M ≥ 3.5 in the Greek region was employed to resolve the contemporary stress-field using a damped least-squares inversion. The results are in good agreement with the strain-rate field of the Global Strain Rate Model, which was used as reference, both in terms of their principal axes orientations and expected faulting styles. Dual stress-states were identified using the Multiple Inverse Method in regions delineated by joining neighboring Area Sources of the European Seismic Hazard Model 2013 (ESHM13). North-western Greece is mostly affected by transpressional tectonics characterized by NE-SW contraction. Northern/central Greece and the Corinth Rift are dominated by E-W normal faulting, with secondary oblique-normal to strike-slip faulting at the western margin of the latter. North and central Aegean are mainly governed by transtensional regime, characterized by stable N-S extension. The stress-tensor was found to be compatible with the Fault Sources (FS) of ESHM13, in terms of orientation and expected faulting type. Differences were observed in regions of low strain-rate, such as the Southern Aegean, where left-lateral, E-W strike-slip instead of normal faulting was inferred. Discrepancies in areas with strong local heterogeneities were highlighted by anomalies in the stress-ratio, Φ, indicating transtensional regime in the pull-apart basins of Western Greece and transpressional tectonics in north-western Greece and south of Crete. The latter is characterized by stable N-S contraction, SW-NE sinistral strike-slip and E-W reverse faulting in the vicinity of the subduction zone. A low Φ, E-W oriented zone was identified along the active volcanic arc, where a remarkable 90° rotation occurs in the stress field. This rotation is related to the transition from E-W (in the north) to N-S (in the south) normal faulting in Peloponnese and Dodecanese Islands, as well as rearrangement from dextral to sinistral SW-NE strike-slip faulting in North and South Aegean, respectively. © 2018 Elsevier Lt

On the spatial distribution of seismicity and the 3D tectonic stress field in western Greece

We analyzed a large number of focal mechanisms and relocated earthquake hypocenters to investigate the geodynamics of western Greece, the most seismically active part of the Aegean plate-boundary zone. This region was seismically activated multiple times during the last decade, providing a large amount of enhanced quality new information that was obtained by the Hellenic Unified Seismological Network (HUSN). Relocated seismicity using a double-difference method appears to be concentrated above ∼35 km depth, exhibiting spatial continuity along the convergence boundary and being clustered elsewhere. Earthquakes are confined within the accreted sediments escarpment of the down-going African plate against the un-deformed Eurasian hinterland. The data arrangement shows that Pindos constitutes a seismic boundary along which large stress heterogeneities occur. In Cephalonia no seismicity is found to be related with the offshore Cephalonia Transform Fault (CTF). Onshore, N[sbnd]S crustal extension dominates, while in central and south Peloponnesus the stress field appears rotated by 90°. Shearing-stress obliquity by 30° is indicated along the major strike-slip faults, consistent with clockwise crustal rotation. Within the lower crust, the stress field appears affected by plate kinematics and distributed deformation of the lower crust and upper mantle, which guide the regional geodynamics. © 2016 Elsevier Lt

Seismicity and geodynamics of western Peloponnese and central Ionian Islands: Insights from a local seismic deployment

The tectonic setting of western Peloponnese and central Ionian Islands, Greece, is characterized by the subduction of the oceanic African plate beneath the Aegean micro-plate. The transition from subduction to continental collision in northwestern Greece is accommodated by the right-lateral Cephalonia transform fault. In this work, we exploit the recordings of a temporary seismic network composed of 15 stations operating from July 2016 until May 2017 to investigate the complex deformation of this region. Our local network fills in a major observational gap in one of the most tectonically active regions of the Hellenic arc. We detected and located more than 1200 local earthquakes and constrained five 1D optimum local velocity models. The relocated seismicity (including the aftershock sequence that followed the October 2018 Mw 6.7 earthquake offshore Zakynthos) and associated focal mechanisms constrained for the major earthquakes point out a complex crustal deformation. We propose a clockwise rotation of the Ionian Akarnania Block accommodated by major marginal strike-slip fault zones that appear segmented along their strike. Additionally, left-lateral motion is observed on the Kyllini-Cephalonia fault along a north-west direction. Finally, the seismicity recorded in north Cephalonia (offshore Myrtos and Fiskardo) suggests that the Cephalonia transform fault is a large deformation zone where secondary WNW-striking sinistral strike-slip faults occur. © 2020 Elsevier B.V

Seismic site characterization at the western Cephalonia Island in the aftermath of the 2014 earthquake series

Background: The site response during a strong earthquake event may be proven crucial for earthquake hazard assessment and risk mitigation. Two moderate magnitude earthquakes that occurred in early 2014 in Cephalonia produced the largest ground motion values ever recorded in Greece, highly exceeding the provisions of the effective seismic code implying for local effects. This motivated the investigation of site response in the epicentral area presented herein. Methods: We applied the HVSR method on free-field ambient noise measurements obtained during an in situ survey. 68 measurements were adopted for site characterization after their validation using earthquakes and geotechnical data. The site response was approximated by the peak frequency and the amplification ratio of the HVSR curves. Results: The majority of measurements exhibit smooth lateral variations in the frequency range 0.7–17 Hz, at a factor up to 7 and they are clearly classified in two bands, a low (0.7–4 Hz) and a high one (5–17 Hz). Some discrepancies that are observed between microtremor measurements and earthquake recordings for peak frequencies <2 Hz and overall underestimated ambient noise HVSR amplification are likely explained by near-source, radiation pattern and/or nonlinear soil effects. Conclusions: High frequencies combined with low amplification correlate with damage in the hardest hit areas. Low frequencies are aligned in a NNE-SSW direction in the epicentral area, similar to the strike of the activated fault, indicating that the properties of rocks along the fault zone have possibly been affected by slippage and/or dynamic effects. © 2017, The Author(s)

Displacements recorded on continuous GPS stations following the 2014 M6 Cephalonia (Greece) earthquakes: Dynamic characteristics and kinematic implications

We report cm-size dynamic displacements of continuous GPS stations onshore the island of Cephalonia, Ionian Sea, Greece, following the passage of seismic waves from two (2) shallow earthquakes on Jan 26, 2014 and Feb 3, 2014, respectively. First, we estimated the displacements from the high-rate GPS data collected at NOA station VLSM, near to the epicenters, by using state-of-art data processing strategies. The time series of displacements were analyzed both in time and frequency domains. From the dynamic analysis of 1Hz data it is shown that the second event was recorded at station VLSM with higher amplitudes on both horizontal components, despite its smaller (22 %) moment magnitude, possibly due to its shallower depth. The static field of deformation is characterized by cm-size permanent motion in opposing directions between stations KIPO (western Cephalonia) and VLSM (eastern Cephalonia), in agreement with the right-lateral kinematics of both ruptures. The 7.4 cm northward motion of station KIPO implies that the western peninsula of Cephalonia island (Paliki) belongs to a separate crustal block with respect to the rest of the island. The northward motion of KIPO also implies that the Cephalonia Transform Fault (CTF) did not rupture during the 2014 events, because KIPO is located at the hanging wall of CTF. It is possible that the amount of accumulated strain along CTF since 1983 (M=6.8) can be released by a seismic event of M6.5-6.7, at any time.Published5-271T. Geodinamica e interno della TerraJCR Journalrestricte