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2001 -2002

Student: Joanne Ghirxi

Title: Lithospheric Structure of the Ionian Basin from Rayleigh Eave Dispersion

Abstract: In this study, the inversion of Rayleigh wave dispersion data has been applied to investigate the crustal structure of the Ionian Sea region. Digital seismograms from large, shallow earthquakes, generating surface waves and propagating along a 'pure-path' through the Ionian Sea were analysed. For each seismogram, time series analyses were applied to extract dispersion data. A group velocity dispersion curve, characterising the crustal structure of the propagating path was obtained using the computer program TSAP. Inversion of the average group velocity dispersion data for the Ionian Sea region was done using a Hedgehog inversion, in order to obtain a velocity-depth profile. Inversion of the data has established that the crust beneath the Ionian Sea region has a semicontinental character.




2002 -2003

Student: Steve Zammit

Title: A Study of Seismicity and Earthquake Swarms in the Central Mediterranean

Abstract: A study has been made of the seismicity taking place in the Central Mediterranean region during the last 7 years (1996 - 2002). 118 earthquakes recorded at Wied Dalam Broadband seismic station, Malta, have been studied and the magnitude-frequency statistics were calculated for the whole region.

Occurrence of earthquake swarms has been observed in the data. This swarm activity has been observed between the beginning of 2000 and the middle of 2001. Earthquake swarms are characterised by earthquakes taking place in short temporal sequence in a narrowly restricted area. There is no dominant earthquake in the sequence. Swarms tend to build clusters. They were identified by space- time clustering and by the waveform/ frequency characteristics. Although all epicentres could not be located because of the inadequate amount of data accessible, some major events were located by making use of extended phase data from Sicily, Tunisia and Malta.

This study ends with the correlation of swarm activity with active faults in the Central /Mediterranean region. It has been observed that earthquake swarm activity is taking place at the southwest end of the Pantelleria Rift. This indicated that this region has been tectonically active for the last 2 years approximately. The relocated epicentres are more reliable than those located by ING since the azimuthal distribution of seismic recording stations has a major influence on locations of earthquakes.

 


2005 -2006

Student: Graziella Bartolo

Title: Crustal Structure Beneath the Maltese Islands from Receiver Functions at WDD Seismic Station

Abstract: The primary objective of this study was to shed light on the crustal structure underlying the Maltese Islands and using the receiver function analysis technique. The data analysed in this work consisted of teleseismic events recorded by the MEDNET station WDD. Events with an epicentral distance ranging between 25° and 90° and a magnitude higher or equal to 6.0 were considered. Signals with a high signal- to - noise ratio were kept whilst the rest was discarded. A stacked receiver function was obtained after Langston's deconvolution technique was applied to each of the 20 three-component data sets. The forward modelling procedure was then carried out on the stack and the best-fitting velocity model for the region was selected. The major discontinuities were predicted to be at 10km, 22km and 35km. The intermediate layer between the 22- and 35- km boundaries conformed to existing velocity models. Such a feature is particularly associated with continental rift systems. The inversion technique was carried out on the stack of the receiver functions and three distinct velocity models were attained. Even though the two methods did not give drastically different results, the model obtained by forward modelling appeared to be more consistent with known geological data than those obtained from inverse modelling. 

 

2006 -2007

Student: Matthew Richard Agius

Title: Automatic Earthquake Detection and Localisation from a Three-Component Single-Station

Abstract: As more and more seismic stations are being deployed, seismic networks are enhancing their distribution and analysing earthquake location better. Usually networks detect an earthquake when a number of stations trigger and then analyse the different times-of-arrival to determine the source. With the installation of broadband seismometers, instruments are more sensitive to local seismic activity, but due to the large volume of data that load the network, and the threshold set to detect an event, data from single stations is being left unanalysed.

The Physics Department of the University of Malta operates a single, 3-component broadband digital seismograph at Wied Dalam, l/o Birzebbugia in the south of Malta (WDD). Earthquakes in the Sicily Channel have always been difficult to locate, either because they fall outside the southern Italian network, leading to inaccurate location, or because they are poorly recorded on any station other than WDD on Malta. This work has addressed the problem of single-station recording of earthquakes in the Sicily Channel by designing and implementing an automated system for detecting, identifying and locating such events, using a 3-component single-station polarisation analysis. The system, nicknamed LESSLA (Local Earthquake Single-Station Location Analyser) utilises the three components from each of the three sampling streams HH, BH and LH from the Quanterra data acquisition systems, to identify, and pick, major arrivals such as P and S. The standard STA/LTA arrival detection algorithm has been used, and improved for better accuracy. For local and regional earthquakes, event distance is calculated from measured S-P times against a calibrated regional travel time graph, while event azimuth is measured using all three components, after the algorithm of Roberts, Christoffersson and Cassidy (1989). LESSLA uses a weighted scheme on the three different sampling streams to classify an event as a True event of local/regional origin. The system analyses one whole day of data and issues a detailed daily bulletin, containing pick times, event distance, azimuth and location, magnitude, epicentre map, PDF files of event seismograms, links to relevant international bulletins and other analysis information. The report is also sent by email, allowing the user to visually analyse the event seismograms and make rapid judgement on the location reliability.

The method has been implemented and succeeded to locate regional and local events while also identifying quarry blasts. 70% of the earthquakes under test had both P- and S- pick correct and 70% of earthquakes had a 30° or less azimuth variation with respect to INGV / EMSC bulletins. Moreover, more than 17 events were located along a documented fault system to the south and south-east of Malta. which were not reported by any other network. The seismicity of this feature had not been previously identified.

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2008 -2009

Student: Alexander Vella

Title: A Seismic Site Response Survey of the Maltese Islands

Abstract: The Maltese Islands in the central Mediterranean are composed of a simple 4 - layer sedimentary sequence of Oligocene - Miocene age. The sequence is highly disturbed by syn-sedimentary to recent faulting, and the islands are characterised by a variety of outcropping rock types and underlying lithology. The seismic history of the Maltese islands includes a number of earthquakes in which serious building damage was experienced, the maximum intensity being of EMS98 VII - VIII in 1693. A correspondence is observed between building damage and areas underlain by soft clay layers, for example, but a seismic risk assessment of the islands that takes into account such site effects has never been attempted. Since the last damaging earthquake, the building density has increased dramatically, and the building footprint extended to regions of diverse lithologies, while general building practices have not undergone any upgrade in standards. In this study, a seismic site response survey was undertaken to identify the characteristic frequencies and corresponding amplification factors all over the islands. This was done by recording few minutes of ambient noise using a portable seismometer at various sites covering all the outcrop types and major morphological features. The noise was analysed using the Nakamura Technique by finding the Horizontal to Vertical Spectral Ratio (HVSR). The results show a clear correlation between the peak frequencies and the geology. Using statistical methods characteristic frequencies and corresponding amplification factors were identified for the different geological outcropping strata. The lowest and most rigid layer (Lower Coralline Limestone) exhibits a peak frequency at 0.45 ( ± 0.23) Hz and amplification of 1.92 (±0.42). The fine but compact Globigerina Limestone has a frequency of 0.31 (±0.14) Hz and amplification of 3.09 (±1.86). The soft Blue Clay layer exhibits a characteristic frequency of 2.89 (±.064) Hz at an amplification of 5.94 (±2.68) and the overlying hard Upper Coralline Limestone has a frequency of 1.71(± 0.34) Hz and amplification 4.50 (±2.34). Microzonation maps of both peak frequency and corresponding amplifications were plotted. The frequency map confirmed the correlation between frequency and geology showing the determining effect of the Blue Clay layer where layers above it show frequencies larger than 1Hz. The amplification map showed higher amplifications for outcrops over the Blue Clay layer and highlighted areas characterised by topographic features like valley mouths with deposits or alluvium, fault scarps, hill ridges, wide-bed valleys and surficial layers along the coasts. This confirmed that different parts of the islands may amplify ground motion differently due to geological and topographic effects. These results have significant implications on the vulnerability of certain areas of the islands and on the importance of undertaking a holistic assessment of seismic risk. 

 


2011 -2012 

Student: Sharon Pace

Title: Shallow Shear Wave Velocity Structure in the Maltese Islands

Abstract: Even though seismic activity around the Maltese islands is generally of low magnitude, large, damaging events have affected the country in the past (Galea, 2007). It is therefore necessary to undertake studies of seismic risk , which include microzonation studies and site classification, especially since the majority of buildings in Malta are of load-bearing unreinforced masonry, vulnerable to moderate ground shaking. The component of seismic risk arising from local site amplification, resulting from local sedimentary geology has been investigated by means of ambient noise measurements, used to infer the shallow shear wave velocity structure, for which no systematic data exists on the islands.

Xemxija was chosen as a test investigation site due to its varied geology and topography over a small spatial area, as well as for its urban risk. However, measurements at other areas of the Maltese islands were also utilised in order to provide further constraints. A dense microtremor measurement survey at Xemxija was carried out, with over 100 microtremor recordings performed. The time series obtained were processed in order to compute the HVSR (horizontal to vertical spectral ratio) curves (Nakamura, 1989). The resultant curves identify the resonant peak frequencies of the points investigated. HVSR curves obtained from measurements performed in the valley give a large spread of resonant fundamental frequencies ranging between 2 Hz to over 11 Hz. This spread, as well as the equally variable peak amplitudes, is due to the variation in thickness of the soil that overlies the Upper Coralline stratum. On the neighbouring hilltop, where no soil is present, the frequency range is smaller and more stable, spanning 1.19 Hz - 1.56 Hz. On the hill sides and top, where the hard Upper Coralline is outcropping, one might expect no amplification to occur, however a clear sizeable resonance peak of amplitude greater than 2 was consistently evident. These peaks can be generally associated with the Blue Clay layer underlying the UCL. The presence of the Blue Clay layer gives rise to a velocity inversion, a feature that causes the HVSR values to drop below 1 over a wide frequency range (Castellaro and Mulargia, 2009).

Modelling of the sub-surface layers, in order to explain the HVSR curves, was performed at several points using the ModelHVSR program (Herak, 2008). When modelling resonance curves, a trade-off exists between the shear wave velocity and the thickness of the low velocity layer, therefore initial constraints are required. These were provided by existing geotechnical data and other measurements. Modelling of HVSR data using ambient noise has proven to be a useful tool in microzonation studies and for the calculation VS30 (the shear wave velocity in the upper 30 metres of sub-soil) parameter in order to carry out site classification.

Acknowledgements

The research work disclosed in this publication is partially funded by the Strategic Educational Pathways Scholarship Scheme (Malta). The scholarship is part-financed by the European Union- European Social Fund.

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2013-2014 

Student: Kimberly Abela

Title: Seismic attenuation relationships in the Central Mediterranean

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2013 -2015 

Student: Mariya Grech Muscat

Title: Crustal structure of the Central Mediterranean using receiver function analysis

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Calendar
Notices
Outstanding Academic Title
The book "Earthquakes and their Impact on Society" has been selected by Choice magazine as"Outstanding Academic title" for 2016 
Participation at AGU
Dr Sebastiano D’Amico and Ms Daniela Farrugia (Department of Geosciences) have participated in the American Geophysical Union (AGU) Fall Meeting held in San Francisco (USA) from 10th to 15th December 2016.
ESC 2018
Valletta to host 36th General Assembly of the European Seismological Commission in 2018
 
 
Last Updated: 14 October 2015

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