The earthquakes that occurred recently in Italy have sparked a debate among professionals involved in the structural design of buildings, mainly about the response of buildings in Malta should a similar event occur locally. Questions asked include, what is the likelihood of a similar event occurring locally? Would it be of the same magnitude? What should we be designing for?

The structural design of buildings for resisting earthquakes is carried out in accordance with international standards and codes of practice. When Malta became a member of the EU, the use of Eurocodes, as design standards, became mandatory. The information in Eurocodes sets the general design parameters to provide a homogenous and harmonised design approach across European states.

Nevertheless, it should be noted that the design parameters for designing structures for earthquakes depends on the location of the building or of the civil engineering structure under consideration. In fact, Eurocodes are supplemented by National Annexes that are used for defining local parameters particular to each state. For instance, Italy has a National Annex for the design of buildings to resist forces generated by earthquakes. The National Annex includes maps of the Italian peninsula and the islands, in which the parameters for designing for earthquakes are given for different zones.

The maps generally show each national territory subdivided into seismic zones. Each zone has a single value which defines the hazard or seismic risk of the zone under consideration. A value of a design parameter referred to as the reference peak ground acceleration on outcropping bedrock is given for each zone. The reference peak ground acceleration is chosen by the national authorities for each seismic zone depending on various factors, like the risk of a seismic occurrence in that particular location.

Italian authorities, in conjunction with seismic and volcanic research institutions, have published an interactive online map for the Italian territory. The map can be used for selecting an appropriate value of the reference peak ground acceleration, referred to as Pga. (reference http://esse1-gis.mi.ingv.it/s1_en.php)

Fig 2. Reference Peak ground acceleration for Pantelleria

Based on the interactive map for Italy, shown in Fig 1, in particular Sicily and the surrounding territory, it can be noted that in the seismic zone close to Malta, the value of the Pga varies between 0.025 and 0.05g. The value of the reference peak ground acceleration for Pantelleria, for instance, shown in Fig 2, is 0.05g. It should be noted that a volcanic eruption below sea level was recorded in 1891 in Pantelleria. Today, volcanic activity consisting of hot springs and plumes can be observed in Pantelleria.

It is imperative that there should be no compromises for achieving a safe design for all buildings in the case of a seismic event, where one of the objectives is to safeguard the lives of the users of the buildings

The European Commission has also addressed the harmonisation process for defining seismic hazards across Europe.

‘Share’ is a collaborative project in the Seventh Framework Programme of the European Commission. Its main objective is to provide a community-based seismic hazard model for the Euro-Mediterranean region with update mechanisms. The project aims to establish new standards in Probabilistic Seismic Hazard Assessment (PSHA) practice by a close cooperation of leading European geologists, seismologists and engineers. (www.share-eu.org).

Share has published a map of Europe in which the seismic hazard for different states and zones is given. This map is reproduced in Fig. 3.

From the Map given in Fig 3, it can be noted that the value of the reference peak ground acceleration for Malta is given between 0.04g and 0.06g. A detailed view of the map showing Malta and Sicily is given in Fig. 4.

Fig 3. European Seismic Hazard Map. Reproduced from www.share-eu.org

Despite the lack of a Maltese National Annex, a number of public buildings and civil engineering structures in Malta have, in the past, been designed to cater for the forces that would be generated during an earthquake. In many cases, it is likely that the professional responsible for the structural design of such buildings has used engineering judgement for selecting a suitable value for the reference Pga value, based on information available at the time of design.

Also, values of reference peak ground acceleration have been given in tender documents for a number public projects. For instance, for the Power Station, the value of the reference peak ground acceleration was stipulated as 0.12g. This value may be considered to be over-cautious, particularly when compared to the data available in seismic hazard maps around the Maltese region, which range between 0.02g and 0.06g.

The design of buildings and civil engineering structures to cater for earthquakes has a significant impact on the cost of construction, and this increases with higher Pga design values used for the design.

It is also imperative that there should be no compromises for achieving a safe design for all buildings in the case of a seismic event, where one of the objectives is to safeguard the lives of the users of the buildings. In addition, public buildings such as hospitals, power stations, transmission towers, major infrastructural works, among others, need to continue operating in full during emergency situations. However, overdesign is inefficient and has a serious impact on the costs of construction. All projects, whether private, State or European funded, need to be cost-effective.

To date, Malta does not have a National Annex for defining the reference peak ground acceleration for different seismic zones in Malta. Based on the Share seismic hazard map, Italian seismic hazard maps for the Mediterranean region and locally recorded earthquakes (found in historical data for Malta), it is evident that buildings and infrastructural projects need to be designed to cater for potential seismic events.

It may also be the case that several buildings require retrofitting, and a strategy for addressing such works is required. Given the upcoming trend of tall buildings, the need for a National Annex for defining seismic hazard parameters is becoming even more compelling. Malta has been a member of the EU since 2004 and publishing a National Annex is now long overdue.

Mark Arrigo is an architect at Mark Arrigo Architects and Structural Engineers.

Fig 4. Detailed view of map given in Fig. 3, showing the Maltese Islands.