11 research outputs found

    Mapping and modelling the spatial variation in strain accumulation along the North Anatolian Fault

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    Since 1900, earthquakes worldwide have been responsible for over 2 million fatalities and caused nearly $2 trillion of economic damage. Accurate assessment of earthquake hazard is therefore critical for nations in seismically active regions. For a complete understanding of seismic hazard, the temporal pattern of strain accumulation, which will eventually be released in earthquakes, needs to be understood. But earthquakes typically occur every few hundred to few thousand years on any individual fault, and our observations of deformation usually only cover time periods of a decade or less. For this reason, our knowledge of the temporal variation in strain accumulation rate is limited to insights gleaned from kinematic models of the earthquake cycle that use measurements of present-day strain to infer the behaviour on long time scales. Previous studies have attempted to address this issue by combining data from multiple faults with geological estimates of long-term strain rates. In this thesis I propose a different approach, which is to observe deformation at multiple stages of the earthquake cycle for a single fault with segments that that have failed at different times. In the last century the North Anatolian Fault (NAF) in Turkey has accommodated 12 large earthquakes (Mw >6.5) with a dominant westward progression in seismicity. If we assume that each of these fault segments are at a different stage of the earthquake cycle then this provides a unique opportunity to study the variation in along-strike surface deformation, which can be equated to variation of deformation in time. In this thesis I use Interferometric Synthetic Aperture Radar (InSAR) and Global Navigation Satellite System (GNSS) observations to examine the spatial distribution of strain along the NAF. InSAR is an attractive technique to study surface displacements at a much higher spatial resolution (providing a measurement every 30 m) compared to established GNSS measurements, with station separations between 10 km to 100 km in Turkey. I specifically address a key technical challenge that limits the wide uptake of InSAR: phase unwrapping, the process of recovering continuous phase values from phase data that are measured modulo 2π radians. I develop a new unwrapping procedure for small baseline InSAR measurements that iteratively unwraps InSAR phase. For each iteration, this method identifies pixels unwrapped correctly in the previous iteration and applies a high cost to changing the phase difference between these pixels in the next iteration. In this way, the iterative unwrapping method uses the error-free pixels as a guide to unwrap the regions that contained unwrapping errors in previous iterations. I combine measurements of InSAR line-of-sight displacements with published GNSS velocities to show that an ∼80 km section of the NAF that ruptured in the 1999 Izmit earthquake (Mw 7.4) is creeping at a steady rate of ∼5 mm/yr with a maximum rate of 11 ± 2 mm/yr near the city of Izmit within the observation period 2002-2010. I show that in terms of the moment budget and seismic hazard the effect of the shallow, aseismic slip in the past decade is small compared to that from plate loading. Projecting the shallow creep displacement rates late into the earthquake cycle does not produce enough slip to account for the 2-3 m shallow coseismic slip deficit observed in the Izmit earthquake. Therefore, distributed inelastic deformation in the uppermost few kilometers of the crust or slip transients during the interseismic period are likely to be important mechanisms for generating the shallow slip deficit. I used similar techniques to confirm that a ∼130 km section of the central NAF near the town of Ismetpasa, is also undergoing aseismic creep at a steady rate of 8±2 mm/yr. Using simple elastic dislocation models to fit fault perpendicular velocities I show that there is an eastward decreasing fault slip rate in this region from ∼32 mm/yr to ∼21 mm/yr over a distance of about 200 km. The cause of this decrease remains unclear, but it could be due to postseismic effects from the 1999 Izmit and Duzce earthquakes and/or long-term influence from the 1943 (Mw 7.4) and 1944 (Mw 7.5) earthquakes. Finally, I combine line-of-sight displacements from 23 InSAR tracks to produce the first high resolution horizontal velocity field for the entire continental expression of the NAF (∼1000 km). I show that the strain rate does not vary significantly along the fault, and since each segment of the NAF is at a different stage of the earthquake cycle, the strain rate is invariant with respect to the time since the last earthquake. This observation is inconsistent with viscoelastic coupling models of the earthquake cycle, which predict a decreasing strain rate with time after an earthquake. My observations imply that strain accumulation reaches a steady-state fairly rapidly after an earthquake (<7-10 years) after which strain is localised on a narrow shear zone centred on the fault and does not vary with time. A time-invariant strain rate is consistent with a strong lower crust in the region away from the fault with a viscosity ≥1020 Pas. My results imply that short term snapshots of the present-day strain accumulation (as long as it is after the postseismic period) are representative of the entire earthquake cycle, and therefore geodetic estimates of the strain rate can be used to estimate the total strain accumulation since the last earthquake on a fault, and be used as a proxy for future seismic hazard assessment. The techniques I developed to explore the spatial and temporal pattern of aseismic fault creep and long-term strain accumulation along the NAF are general and can be ap- plied to all strike-slip faults globally. The archived ERS-1/2 and Envisat satellite data are an extremely valuable resource that can and should be used to extend InSAR time series measurements back to the early 1990s. Together with the new Sentinel-1 data sets, this provides an unprecedented opportunity to explore tectonic deformation over several decades and on continental scales. Despite the availability of numerous correction techniques (in this thesis I use global weather models to calculate the atmospheric contribution), atmospheric delays remain the major challenge to exploiting Sentinel-1 data for global strain mapping, the mitigation of these delays are an important goal for the InSAR community

    Seismic hazard assessment and volcanogenic seismicity for the Democratic Republic of Congo and surrounding areas, Western Rift Valley of Africa

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    The Western Rift Valley of Africa has experienced several severe earthquakes with magnitude M≥6 and volcanic eruptions in recent historical times. Most of these earthquakes occurred in the Democratic Republic of Congo (DRC) and neighbouring countries such as Uganda and Tanzania. The largest earthquake on record, which occurred at Kasanga (Tanzania) on 13 December 1910 in the Lake Tanganyika area, had a magnitude of Ms7.3. The most recent significant earthquake occurred approximately 20 km north of Bukavu City (DRC) on 03 February 2008 with magnitude Mw=6.0 in the Basin of Lake Kivu. The Virunga volcano group, located at the northern edge of Lake Kivu, consists of eight major volcanoes (Muhavura, Gahinga, Sabinyo, Visoke, Karisimbi, Mikeno, Nyiragongo and Nyamuragira). The volcanoes Nyiragongo and Nyamuragira have been the most active since 1882. A probabilistic approach was used to map the seismic hazard in Democratic Republic of Congo and surrounding areas, and assess the seismic hazard level for 14 cities in the region. Seismic hazard maps for 2%, 5% and 10% chance of exceedance of the indicated ground accelerations in 50 years were prepared using a 90-year catalogue compiled for homogeneous magnitudes (Mw); the attenuation relations of Mavonga (for the Western Rift Valley of Africa), Atkinson and Boore (for eastern and North America) and Jonathan (for eastern and southern Africa); and the EZ-Frisk software package. A Poisson model of earthquake occurrence that assumes that events are independent was adopted. Therefore, foreshocks, aftershocks and earthquake swarms were removed from the initial catalogue of 2249 events. Furthermore Mw=4 was selected as the lower magnitude bound (Mmin) because smaller earthquakes are considered unlikely to cause damage, even to houses that are poorly designed and built. Thus, any remaining events with Mw<4 were also excluded from the catalogue, leaving a sub-catalogue of 821 events The highest estimated levels of seismic hazard were found in the Lake Tanganyika Rift seismic zone, where peak ground accelerations (PGA) in excess of 0.32g, 0.22g and 0.16g are expected to occur with 2%, 5% and 10% chance of exceedence in 50 years, respectively. The seismic hazard in the Congo basin diminishes with distance away from the Western Rift Valley until, at a distance of about 450 km, the chance of exceeding 0.05 g (the threshold value of engineering interest) is less than 10% in 50 years. Finally, from the probabilistic seismic hazard analysis of the DR Congo and surrounding areas, four seismic zones were identified and rated as follows: Zone A (very high hazard), Zone B (high hazard), Zone C ( moderate hazard), and Zone D (low hazard). The zone A includes the Lake Tanganyika Rift zone where PGA values of 0.32g, 0.22g and 0.16 g are expected to occur with probability 2%, 5% and 10% in 50 years, respectively. Zone B includes the Lake Kivu Basin, Mount Ruwenzori and Lake Edouard region. Zone C includes Rutsuru, Masisi, Upemba area and a part of the Congo basin close to the Western Rift. The remainder of the Congo basin constitutes the zone D. To understand how volcanoes work and reduce the risk due to the Virunga volcanic eruptions in the Western Rift Valley of Africa, the crustal structure beneath the Virunga volcanic area was investigated and studies of volcanogenic seismicity were carried out. From these studies, it was found: High velocity material (6.9 to 7.3 km/s) lies beneath the Kunene (KNN) and Kibumba (KBB) broadband stations at depths of 3-20 km and 3-10 km, respectively, which is indicative of magma cumulates within the volcanic edifice. A low velocity zone was observed below KNN and KBB at depths of 20-30 km and 18-28 km, respectively, and with average velocity 6.1 km/s and 5.9 km/s. This low velocity zone may sample the magma chamber or a melt-rich sill. The depth of the crust-mantle transition zone beneath the area sampled by the KNN and KBB in the Virunga area was determined to be about 39 to 43 km and 30 to 39 km, respectively. Swarm-type seismicity composed mainly of long-period volcanic earthquakes preceded the eruptions of Nyamuragira volcano and was probably enhanced by tectonic seismicity related to rifting. A steady increase in seismicity at a constant rate from a deep magma feeder (located at about 20 to 30 km depth) was observed ten or eleven months before eruption. In the last stage (1 or 2 months) before the eruption, the hypocenters of long-period volcanic earthquakes became shallower. The new model of the local crustal seismic velocity for the Virunga area is useful to map the migration of hypocenters of earthquakes accurately and reveals trends that could be precursors of volcanic eruptions This pattern of seismicity prior to the volcanic eruptions, integrated with other available data (e.g. INSAR, GPS), may be used to characterize the volcanic process and forecast volcanic eruptions in the Virunga area

    Impact of Etna’s volcanic emission on major ions and trace elements composition of the atmospheric deposition

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    Mt. Etna, on the eastern coast of Sicily (Italy), is one of the most active volcanoes on the planet and it is widely recognized as a big source of volcanic gases (e.g., CO2 and SO2), halogens, and a lot of trace elements, to the atmosphere in the Mediterranean region. Especially during eruptive periods, Etna’s emissions can be dispersed over long distances and cover wide areas. A group of trace elements has been recently brought to attention for their possible environmental and human health impacts, the Technology-critical elements. The current knowledge about their geochemical cycles is still scarce, nevertheless, recent studies (Brugnone et al., 2020) evidenced a contribution from the volcanic activity for some of them (Te, Tl, and REE). In 2021, in the framework of the research project “Pianeta Dinamico”, by INGV, a network of 10 bulk collectors was implemented to collect, monthly, atmospheric deposition samples. Four of these collectors are located on the flanks of Mt. Etna, other two are in the urban area of Catania and three are in the industrial area of Priolo, all most of the time downwind of the main craters. The last one, close to Cesarò (Nebrodi Regional Park), represents the regional background. The research aims to produce a database on major ions and trace element compositions of the bulk deposition and here we report the values of the main physical-chemical parameters and the deposition fluxes of major ions and trace elements from the first year of research. The pH ranged from 3.1 to 7.7, with a mean value of 5.6, in samples from the Etna area, while it ranged between 5.2 and 7.6, with a mean value of 6.4, in samples from the other study areas. The EC showed values ranging from 5 to 1032 μS cm-1, with a mean value of 65 μS cm-1. The most abundant ions were Cl- and SO42- for anions, Na+ and Ca+ for cations, whose mean deposition fluxes, considering all sampling sites, were 16.6, 6.8, 8.4, and 6.0 mg m-2 d, respectively. The highest deposition fluxes of volcanic refractory elements, such as Al, Fe, and Ti, were measured in the Etna’s sites, with mean values of 948, 464, and 34.3 μg m-2 d-1, respectively, higher than those detected in the other sampling sites, further away from the volcanic source (26.2, 12.4, 0.5 μg m-2 d-1, respectively). The same trend was also observed for volatile elements of prevailing volcanic origin, such as Tl (0.49 μg m-2 d-1), Te (0.07 μg m-2 d-1), As (0.95 μg m-2 d-1), Se (1.92 μg m-2 d-1), and Cd (0.39 μg m-2 d-1). Our preliminary results show that, close to a volcanic area, volcanic emissions must be considered among the major contributors of ions and trace elements to the atmosphere. Their deposition may significantly impact the pedosphere, hydrosphere, and biosphere and directly or indirectly human health

    EVOLUTION OF THE SUBCONTINENTAL LITHOSPHERE DURING MESOZOIC TETHYAN RIFTING: CONSTRAINTS FROM THE EXTERNAL LIGURIAN MANTLE SECTION (NORTHERN APENNINE, ITALY)

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    Our study is focussed on mantle bodies from the External Ligurian ophiolites, within the Monte Gavi and Monte Sant'Agostino areas. Here, two distinct pyroxenite-bearing mantle sections were recognized, mainly based on their plagioclase-facies evolution. The Monte Gavi mantle section is nearly undeformed and records reactive melt infiltration under plagioclase-facies conditions. This process involved both peridotites (clinopyroxene-poor lherzolites) and enclosed spinel pyroxenite layers, and occurred at 0.7–0.8 GPa. In the Monte Gavi peridotites and pyroxenites, the spinel-facies clinopyroxene was replaced by Ca-rich plagioclase and new orthopyroxene, typically associated with secondary clinopyroxene. The reactive melt migration caused increase of TiO2 contents in relict clinopyroxene and spinel, with the latter also recording a Cr2O3 increase. In the Monte Gavi peridotites and pyroxenites, geothermometers based on slowly diffusing elements (REE and Y) record high temperature conditions (1200-1250 °C) related to the melt infiltration event, followed by subsolidus cooling until ca. 900°C. The Monte Sant'Agostino mantle section is characterized by widespread ductile shearing with no evidence of melt infiltration. The deformation recorded by the Monte Sant'Agostino peridotites (clinopyroxene-rich lherzolites) occurred at 750–800 °C and 0.3–0.6 GPa, leading to protomylonitic to ultramylonitic textures with extreme grain size reduction (10–50 μm). Compared to the peridotites, the enclosed pyroxenite layers gave higher temperature-pressure estimates for the plagioclase-facies re-equilibration (870–930 °C and 0.8–0.9 GPa). We propose that the earlier plagioclase crystallization in the pyroxenites enhanced strain localization and formation of mylonite shear zones in the entire mantle section. We subdivide the subcontinental mantle section from the External Ligurian ophiolites into three distinct domains, developed in response to the rifting evolution that ultimately formed a Middle Jurassic ocean-continent transition: (1) a spinel tectonite domain, characterized by subsolidus static formation of plagioclase, i.e. the Suvero mantle section (Hidas et al., 2020), (2) a plagioclase mylonite domain experiencing melt-absent deformation and (3) a nearly undeformed domain that underwent reactive melt infiltration under plagioclase-facies conditions, exemplified by the the Monte Sant'Agostino and the Monte Gavi mantle sections, respectively. We relate mantle domains (1) and (2) to a rifting-driven uplift in the late Triassic accommodated by large-scale shear zones consisting of anhydrous plagioclase mylonites. Hidas K., Borghini G., Tommasi A., Zanetti A. &amp; Rampone E. 2021. Interplay between melt infiltration and deformation in the deep lithospheric mantle (External Liguride ophiolite, North Italy). Lithos 380-381, 105855

    Impact of geogenic degassing on C-isotopic composition of dissolved carbon in karst systems of Greece

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    The Earth C-cycle is complex, where endogenic and exogenic sources are interconnected, operating in a multiple spatial and temporal scale (Lee et al., 2019). Non-volcanic CO2 degassing from active tectonic structures is one of the less defined components of this cycle (Frondini et al., 2019). Carbon mass-balance (Chiodini et al., 2000) is a useful tool to quantify the geogenic carbon output from regional karst hydrosystems. This approach has been demonstrated for central Italy and may be valid also for Greece, due to the similar geodynamic settings. Deep degassing in Greece has been ascertained mainly at hydrothermal and volcanic areas, but the impact of geogenic CO2 released by active tectonic areas has not yet been quantified. The main aim of this research is to investigate the possible deep degassing through the big karst aquifers of Greece. Since 2016, 156 karst springs were sampled along most of the Greek territory. To discriminate the sources of carbon, the analysis of the isotopic composition of carbon was carried out. δ13CTDIC values vary from -16.61 to -0.91‰ and can be subdivided into two groups characterized by (a) low δ13CTDIC, and (b) intermediate to high δ13CTDIC with a threshold value of -6.55‰. The composition of the first group can be related to the mixing of organic-derived CO2 and the dissolution of marine carbonates. Springs of the second group, mostly located close to Quaternary volcanic areas, are linked to possible carbon input from deep sources

    Georisks in the Mediterranean and their mitigation

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    An international scientific conference organised by the Seismic Monitoring and Research Unit, Department of Geoscience, Faculty of Science, Department of Civil and Structural Engineering and Department of Construction and Property Management, Faculty of the Built Environment, University of Malta.Part of the SIMIT project: Integrated civil protection system for the Italo-Maltese cross-border area. Italia-Malta Programme – Cohesion Policy 2007-2013This conference is one of the activities organised within the SIMIT strategic project (Integrated Cross-Border Italo-Maltese System of Civil Protection), Italia-Malta Operational Programme 2007 – 2013. SIMIT aims to establish a system of collaboration in Civil Protection procedures and data management between Sicilian and Maltese partners, so as to guarantee the safety and protection of the citizens and infrastructure of the cross-border area. It is led by the Department of Civil Protection of the Sicilian region, and has as other partners the Department of Civil Protection of Malta and the Universities of Palermo, Catania and Malta. SIMIT was launched in March 2013, and will come to a close in October 2015. Ever since the initial formulation of the project, it has been recognised that a state of national preparedness and correct strategies in the face of natural hazards cannot be truly effective without a sound scientific knowledge of the hazards and related risks. The University of Malta, together with colleagues from other Universities in the project, has been contributing mostly to the gathering and application of scientific knowledge, both in earthquake hazard as well as in building vulnerability. The issue of seismic hazard in the cross-border region has been identified as deserving foremost importance. South-East Sicily in particular has suffered on more than one occasion the effects of large devastating earthquakes. Malta, although fortunately more removed from the sources of such large earthquakes, has not been completely spared of their damaging effects. The drastic increase in the building density over recent decades has raised the level of awareness and concern of citizens and authorities about our vulnerability. These considerations have spurred scientists from the cross-border region to work together towards a deeper understanding of the underlying causes and nature of seismic and associated hazards, such as landslide and tsunami. The SIMIT project has provided us with the means of improving earthquake surveillance and analysis in the Sicily Channel and further afield in the Mediterranean, as well as with facilities to study the behaviour of our rocks and buildings during earthquake shaking. The role of the civil engineering community in this endeavour cannot be overstated, and this is reflected in the incorporation, from the beginning, of the civil engineering component in the SIMIT project. Constructing safer buildings is now accepted to be the major option towards human loss mitigation during strong earthquakes, and this project has provided us with a welcome opportunity for interaction between the two disciplines. Finally the role of the Civil Protection authorities must occupy a central position, as we recognize the importance of their prevention, coordination and intervention efforts, aided by the input of the scientific community. This conference brings together a diversity of geoscientists and engineers whose collaboration is the only way forward to tackling issues and strategies for risk mitigation. Moreover we welcome the contribution of participants from farther afield than the Central Mediterranean, so that their varied experience may enhance our efforts. We are proud to host the conference in the historic city of Valletta, in the heart of the Mediterranean, which also serves as a constant reminder of the responsibility of all regions to protect and conserve our collective heritage.peer-reviewe
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