The ongoing seismic activity in the south of Malta is not uncommon and the island does not have a history of major quakes.
More tremors of similar size are expected, according to seismologist Matthew Agius, from the Seismic Monitoring and Research Group within the University of Malta’s Department of Geosciences. He explained that there are various factors why some people feel a tremor and others do not.
Time of day
The time of day when the tremor occurs makes a difference. In fact, when tremors happen during the evening, as the recent ones did – when most people are at home – they are more likely to be noticed.
In the calmness and quiet of the evening, people are more likely to notice their windows rattle to a tremor. Daily sounds, such as traffic and construction, would conceal it.
Motion
Similarly, what you are doing when the tremor occurs will impact whether or not you feel it.
A person who is in motion, such as driving or jogging, is less likely to feel a tremor as opposed to someone sitting still.
Geology and location
People who live in the south, closer to the epicentre, are more likely to feel the tremor although other factors also come into play.
The geology of the earth can also make a difference since, for example, the geological clay layer which is found in the north of Malta and in Gozo can lead to the earth responding differently.
Type of building
Another factor that determines whether a person feels a tremor is the type of building they are in at the time: the way the building is constructed and its height.
People in higher buildings, such as apartment blocks, tend to feel it more.
Marc Bonello, who heads the university’s Department of Civil and Structural Engineering in the Faculty for the Built Environment, said that the higher up in a building a person is located, the larger the horizontal oscillations experienced in the building relative to the ground.
The horizontal oscillations in a building are also a function of the structural system used for construction as well as the structural and material ductility. Hence, structural steelwork framed buildings tend to sway more than reinforced concrete framed buildings or unreinforced masonry (URM) buildings. However, the magnitude of oscillations is not necessarily a reflection of the seismic vulnerability of a building, but it depends upon the structural and material ductility of the building.
Structural steelwork framed buildings will sway more than URM buildings during an earthquake, but due to their inherent high structural and material ductility, structural steelwork framed buildings have a much lower seismic vulnerability than URM buildings,” he said.
Are buildings in the country ’quake proof?
Bonello notes that the seismic vulnerability depends on both the intensity of the earthquake as well as on the epicentral distance from the earthquake epicentre to the building site.
“Buildings with different structural systems and methods of construction may collapse at different magnitudes of earthquake events.
“Thus, buildings which are well designed could withstand strong earthquakes, whereas buildings of poor design may suffer serious damage and/or collapse due to lower grade earthquakes,” he said.
Which are the stronger buildings?
Bonello noted that the seismic vulnerability of Maltese buildings depends largely upon their structural systems and methods of construction. In this respect, reinforced concrete and structural steelwork-framed structures of good design should fare reasonably well in earthquake events even higher than 5.2.
The most seismically vulnerable buildings are unreinforced masonry (URM) buildings constructed without frames and using load-bearing limestone or hollow concrete blockwork walls supporting horizontal floor slabs (made of reinforced or precast/prestressed concrete or even stone slabs – xorok – supported on timber or steel beams), especially if the basement level is devoid of internal walls as is the case with modern URM apartments or office blocks with a ‘soft storey’ open basement car parking.
On the other hand, Bonello added, low-rise URM buildings of traditional or vernacular construction using thick masonry walls without ‘soft storey’ open plan basements should also fare reasonably well during an earthquake event.