Microzonation in the Mġarr harbour area (Gozo) : case study at the Fort Chambray area

Abstract

Formed by a series of strike-slip faults south of the African-Eurasian tectonic plate, the central Mediterranean is subject to a high amount of seismic activity. As a result, seismic events such as earthquakes and volcanic eruptions could trigger secondary effects such as landslides and infrastructural damage. The Nakamura method of seismic site recording (namely, the Horizontal-to-Vertical Spectral Ratio [HVSR] method) was identified by the European SESAME project in order to better predict and identify particularly vulnerable areas on the continent. In the Maltese Islands, the sedimentary geological formations have recently been studied by geophysical HVSR measurements, with particular focus on the North-Western horst and graben structures. These were done in order to study the vibrational effects of the unconsolidated soft Blue Clay rock on the overlying hard Upper Coralline Limestone as well as identify the possible types of landslides that could locally occur. This microzonation study took place in order to fill the gap in seismic site response knowledge in the economically vital Mġarr Harbour area, on the South of the Gozitan island. A total of 41 data points were recorded around the built-up Fort Chambray plateau using the Tromino® seismometer, both on Upper Coralline Limestone and Blue Clay. The results were then processed and HVSR graphs extracted, showing a correlation between the geology and its recorded fundamental frequency. The average frequency values of Upper Coralline Limestone and Blue Clay were 1.5Hz and 3.5Hz respectively. Although more studies around the area are necessary in order to compile a stronger basis of seismic site response information, the awareness of possible rock-falls as a result of earthquake triggers is lacking to date and this also requires addressing.