Finite-Element Updating of the University of Exeter Forum Walkway

This paper presents the finite element (FE) updating of a walkway, part of the University of Exeter’s Forum building. The walkway is a relatively slender structure with natural frequencies in the range that can be excited by human induced vibrations. It is often difficult for designers to model and predict as-built modal properties of such structures with an adequate degree of precision. Thus updating of their FE models is highly instructive for situations like this one. The aim of this paper is to assess the ability to predict accurately the modal properties of this particular structure and to correlate and update the initial FE model with experimental results. The initial model is made based on technical drawings and on best engineering judgement. However, comparing the natural frequencies and mode shapes of the FE model with experimental results, it was seen that the model needs to be updated to match the real structural properties (natural frequencies and mode shapes) as accurately as possible. After the updating, an FE model was obtained where the first five vibration modes were updated. The obtained properties are by no means a unique solution. However, they lead to a reasonable model of the walkway which more accurately represents its modal properties.UK Engineering and Physical Sciences Research Counci

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)

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

Tomographic imaging of the Andravida blind strike-slip fault (Western Greece)

On 8 June 2008 at 12:25 GMT, a large (Mw6.4) earthquake occurred NE of the town of Andravida in Western Peloponnese, Greece – an area characterized by high seismicity during the last decade. In this study, the local velocity structure of the Andravida Fault Zone (AFZ) is investigated primarily using data recorded during the period 2012-2017 by the Hellenic Unified Seismological Network (HUSN). We selected about 1,500 seismic events recorded by the local HUSN stations as well as the Hellenic Strong-Motion Network (HSMN). By applying tomographic inversion, we produced and interpreted 3D models of VP, VS, and VP/VS ratio in the study area. The spatial distribution of the aftershocks, as well as the 3D model derived by Local Earthquake Tomography (LET), provided evidence for the rupture plane. Surface breaks and minor faults are found to be oblique to the main direction of the AFZ, as a result of a restraining bend in Mtn. Movri and the formation of a positive flower-structure in the shallow layers of the upper crust.         ARK: https://n2t.net/ark:/88439/x014750 Permalink: https://geophysicsjournal.com/article/29 Reprints & Permissions &nbsp

Deep structure of the Hellenic lithosphere from teleseismic Rayleigh-wave tomography

This research provides new constraints on the intermediate depth upper-mantle structure of the Hellenic lithosphere using a three-step Rayleigh-wave tomography. Broadband waveforms of about 1000 teleseismic events, recorded by∼200 permanent broad-band stations between 2010 and 2018 were acquired and processed. Through a multichannel cross-correlation technique, the fundamental mode Rayleigh-wave phase-velocity dispersion curves in the period range 30-90 s were derived. The phase-velocities were inverted and a 3-D shear velocity model was obtained down to the depth of 140 km. The applied method has provided 3-D constraints on large-scale characteristics of the lithosphere and the upper mantle of the Hellenic region. Highlighted resolved features include the continental and oceanic subducting slabs in the region, the result of convergence between Adria and Africa plates with the Aegean. The boundary between the oceanic and continental subduction is suggested to exist along a trenchperpendicular line that connects NW Peloponnese with N. Euboea, bridging the Hellenic Trench with the North Aegean Trough. No clear evidence for trench-perpendicular vertical slab tearing was resolved along the western part of Hellenic Subduction Zone; however, subcrustal seismicity observed along the inferred continental-oceanic subduction boundary indicates that such an implication should not be excluded. The 3-D shear velocity model supports an N-S vertical slab tear beneath SW Anatolia that justifies deepening, increase of dip and change of dip direction of the Wadati-Benioff Zone. Low velocities found at depths <50 km beneath the island and the backarc, interrelated with recent/remnant volcanism in the Aegean and W. Anatolia, are explained by convection from a shallow asthenosphere. © The Author(s) 2020

Deep structure of the Hellenic lithosphere from teleseismic Rayleigh-wave tomography

International audienceThis research provides new constraints on the intermediate depth upper-mantle structure of the Hellenic lithosphere using a three-step Rayleigh-wave tomography. Broadband waveforms of about 1000 teleseismic events, recorded by ~200 permanent broadband stations between 2010 and 2018 were acquired and processed. Through a multichannel cross-correlation technique, the fundamental mode Rayleigh-wave phase-velocity dispersion curves in the period range 30 to 90 s were derived. The phase-velocities were inverted and a 3-D shear velocity model was obtained down to the depth of 140 km. The applied method has provided 3-D constraints on large-scale characteristics of the lithosphere and the upper mantle of the Hellenic region. Highlighted resolved features include the continental and oceanic subducting slabs in the region, the result of convergence between Adria and Africa plates with the Aegean. The boundary between the oceanic and continental subduction is suggested to exist along a trench-perpendicular line that connects NW Peloponnese with N. Euboea, bridging the Hellenic Trench with the North Aegean Trough. No clear evidence for trench-perpendicular vertical slab tearing was resolved along the western part of Hellenic Subduction Zone; however, subcrustal seismicity observed along the inferred continental-oceanic subduction boundary indicates that such an implication should not be excluded. The 3-D shear velocity model supports an N-S vertical slab tear beneath SW Anatolia that justifies deepening, increase of dip and change of dip direction of the Wadati-Benioff Zone. Low velocities found at depths <50 km beneath the island and the back-arc, interrelated with recent/remnant volcanism in the Aegean and W. Anatolia, are explained by convection from a shallow asthenosphere

Seismicity and tomographic imaging of the broader nisyros region (Greece)

Nisyros Island is a Quaternary composite volcano located close to the eastern termination of the South Aegean Volcanic Arc. Large destructive earthquakes have been reported in the study area. Nevertheless, seismic activity during the last decades is moderate to low, consisting of both shallow and intermediate depth earthquakes. The main regions of the broader area with observed spatiotemporally clustered seismicity are between Nisyros and Karpathos, east of Kos and in the gulf of Symi. Major events of intermediate depth have occurred near Karpathos and Rhodes Islands while the most significant zone of deep earthquakes is identified in the western Nisyros basin. Evidence for a non-systematic temporal co-incidence of deep events at different regions as well as increase in shallow seismicity after the occurrence of a strong deep event have been observed. Moment tensor inversion, using recordings in local and regional distances, was applied to determine the focal mechanisms of recent moderate events. The solutions, obtained by minimizing the difference between observed and synthetic waveforms, revealed that shallow events are mainly related to normal faulting, whereas intermediate depth events to reverse faulting with important strike-slip component. A tomography study was performed, using manually located events, and identified two areas of high V p/V s ratio and low velocity perturbations. The first is located SW of Nisyros and can be attributed to magma intrusion of deeper composition containing fluids and melts. The second, reaching 15 km depth, is possibly related to the magmatic chambers that feed the Yali and Strongyli volcanic centers. © Springer International Publishing AG 2018

The Santorini Volcanic Complex: A detailed multi-parameter seismological approach with emphasis on the 2011-2012 unrest period

The present study is focused on the examination of the state of the recently activated Santorini Volcanic Complex (SVC) area, located in the Southern Aegean Sea. The seismic activity was investigated in detail and different methodologies have been applied to examine whether the SVC area approached an eruptive phase during the 2011-2012 seismic crisis period. The detailed spatiotemporal analysis for the broader study area revealed two different seismic patterns: low seismic activity until 2010, mainly concentrated within the Anydros basin and close to the submarine volcano, Columbo, and activation during 2011 and 2012 of two previously quiescent regions. The first is the Santorini Caldera, which had been active for more than one year, and the second is the area south of Christiana Islands, which was activated in January 2012 with the occurrence of two major events of magnitude 5.1 and 5.2, respectively, followed by a large number of aftershocks. In this study, manual analysis and relocation of the 2011-2012 seismic crisis in the SVC was performed, in order to obtain a high-resolution image of the activated structures. The seismicity within the Santorini Caldera, which is oriented approximately NE-SW, was rapidly diminished after the activation of the Christiana area. A large number of focal mechanisms were determined which mainly indicated strike-slip faulting inside the Caldera. Furthermore, the fault plane solutions of the major events in the area south of Christiana, derived by waveform modeling, also suggested similar type of faulting. This type differs from the normal faults observed in the Anydros basin. However, the stress field in all cases is consistently oriented in a NW-SE direction. Since the Santorini Volcano was seismically activated for the first time after the 1950 eruption, changes of the physical properties of the medium were examined using different approaches to assess the state of the volcano. The shear-wave splitting analysis revealed the existence of an anisotropic upper crust, while no significant temporal variations of the anisotropy direction were observed. The majority of the recorded events were classified as volcano-tectonic micro-earthquakes using spectral analysis. Nevertheless, certain cases of spasmodic bursts and LF events, which can be related to magma activity, have also been identified. A region of low Vs and high Vp/Vs ratio has been detected north of Nea Kammeni, at depths between 3 and 5km, that is directly associated with the magmatic chamber inside the Caldera. The applied multi-method approach revealed that the Santorini Volcano was activated, while the obtained results indicate that the Volcano did not reach a critical state during the 2011-2012 unrest period. © 2015 Elsevier Ltd

The Santorini Volcanic Complex: A detailed multi-parameter seismological approach with emphasis on the 2011–2012 unrest period

The present study is focused on the examination of the state of the recently activated Santorini VolcanicComplex (SVC) area, located in the Southern Aegean Sea. The seismic activity was investigated in detailand different methodologies have been applied to examine whether the SVC area approached an eruptivephase during the 2011–2012 seismic crisis period. The detailed spatiotemporal analysis for the broaderstudy area revealed two different seismic patterns: low seismic activity until 2010, mainly concentratedwithin the Anydros basin and close to the submarine volcano, Columbo, and activation during 2011 and2012 of two previously quiescent regions. The first is the Santorini Caldera, which had been active formore than one year, and the second is the area south of Christiana Islands, which was activated in January2012 with the occurrence of two major events of magnitude 5.1 and 5.2, respectively, followed by a largenumber of aftershocks.In this study, manual analysis and relocation of the 2011–2012 seismic crisis in the SVC was performed,in order to obtain a high-resolution image of the activated structures. The seismicity within the SantoriniCaldera, which is oriented approximately NE-SW, was rapidly diminished after the activation of theChristiana area. A large number of focal mechanisms were determined which mainly indicated strike-slip faulting inside the Caldera. Furthermore, the fault plane solutions of the major events in the areasouth of Christiana, derived by waveform modeling, also suggested similar type of faulting. This typediffers from the normal faults observed in the Anydros basin. However, the stress field in all cases isconsistently oriented in a NW-SE direction. Since the Santorini Volcano was seismically activated forthe first time after the 1950 eruption, changes of the physical properties of the medium were examinedusing different approaches to assess the state of the volcano. The shear-wave splitting analysis revealedthe existence of an anisotropic upper crust, while no significant temporal variations of the anisotropydirection were observed. The majority of the recorded events were classified as volcano-tectonic micro-earthquakes using spectral analysis. Nevertheless, certain cases of spasmodic bursts and LF events, whichcan be related to magma activity, have also been identified. A region of low Vsand high Vp/Vsratio hasbeen detected north of Nea Kammeni, at depths between 3 and 5 km, that is directly associated with themagmatic chamber inside the Caldera. The applied multi-method approach revealed that the SantoriniVolcano was activated, while the obtained results indicate that the Volcano did not reach a critical stateduring the 2011–2012 unrest period