June 2016 Event


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MHA JUNE EVENT

Earthquakes and Tsunamis in Malta

by Joseph Borg

Summary: Charles Gatt

 

Joseph Borg introduced his talk with an introduction to Plate Tectonic Theory.  The solid earth we know is just a thin crust, floating on molten rock, called magma.  The crust is cracked like a broken egg shell and the pieces, called plates, move around in different directions, driven by convection currents, warm magma rising and cool magma sinking.  These slow but powerful forces cause volcanoes and earthquakes, especially at the edges of the plates.

 

 

earthquakes

  

Present earthquakes and faulting in the Mediterranean area are a result of the African and European plates slowly swinging together, pushing up mountains.  Although no igneous rocks are found in Malta it is surrounded by many active volcanoes, including Mt Etna, Stromboli and Lipari, Mt Vesuvius, Pantelleria, and Santorini. 

 

Until 12 million years ago The Maltese islands were underneath the Tethys Sea, being formed by sedimentation of sand, stone, clays, and limestone.  The islands dip 4 degrees to the N.E. probably due to heaving of the Pantelleria rift system.  The Mediterranean had dried up completely on two occasions, forming deep layers of salt in the deeper basins.  Until about 20,000 years ago, Malta was connected to Sicily. 

 

Malta is crossed by two main fault systems.  The older of the two, the Great Fault, trends SW to NE, while the Maghlaq Fault system trends approximately NW to SE along the southern coast of the island and was responsible for the downthrow of Filfla to sea level. A system of horst and graben structures of East Northeast trend characterises Malta north of the Great Fault.  

 Horst Graben

Cross-section of Malta from Marfa on the left to Mtarfa on the right, showing the ‘horsts’ (raised areas) and ‘grabens’ (sunken areas) due to faulting.

 

Localised earthquakes can also occur with the collapse of cliffs and caves or explosions.  Tremors were caused by the cave-in of subterranean hollows (sink holes) such as that at Maqluba on 23 November 1343, the cave-in at Bahrija which killed a boy and several sheep in 1923 and landslips, like that which occurred at Ghajn Tuffieha in 1980.  Explosions can also shake local areas. 

 

Many earthquakes have shaken Malta but the seven that caused most damage were:

 

1542: An earthquake from Eastern Sicily was felt strongly in Malta and a number of single storey buildings collapsed.

 

1693:  A magnitude 7.4 earthquake from Eastern Sicily damaged most buildings in Valletta, some seriously; several church domes collapsed; Mdina and the Cathedral were seriously damaged. Parts of cliff faces reportedly fell into the sea.

 

1743:  An earthquake from the Ionian Sea damaged Mdina cathedral and numerous churches in Malta and Gozo.  Many hillsides collapsed.

 

1856:  A magnitude 7.7 earthquake from Crete seriously cracked many houses in Malta and Gozo, especially the upper floors, and damaged church domes and walls, especially St George in Gozo.

 

1886:  A week long earthquake from the Aegean Sea damaged the Court of Justice in Valletta, some churches, many houses, and the Mdina Cathedral.

 

1911:  An earthquake from the Northwest of Gozo was strongly felt in Gozo.  It caused significant damage to domes and houses and damaged Fort Chambray.  It was much less damaging in Malta.

 

 1923:  An earthquake from the east of Malta was mostly felt around the Harbour area.  It caused non-structural damage to churches and cracks in walls.

 

Mr Borg then drew on his engineering background to describe the desirable features of buildings in earthquake zones: their composition, foundations, dimensions and mass. Elastic bodies are better suited to carry vibrations than solid/brittle ones.  Foundations help to stabilise buildings and absorb the shaking.  Buildings should have shear connections.  Steel structures are better than concrete or stone.  He expressed concern about high rise buildings made from ferro-concrete; with concrete of poor quality and steel reinforcing subject to concrete cancer. Structures over 3 storeys made from unreinforced masonry, with heavy concrete flooring/roofing, and large open basements, are very susceptible to collapse in the event of the ground shaking.

 

Joseph went on to describe Tsunamis.  These large waves are caused by submarine earthquakes, landslides, cliff-falls into the sea, volcanic explosions or asteroid impact into the ocean.  Many reports and geological evidence show that Malta has been hit by tsunamis in the past.  For example, the action of these high waves has moved large boulders to higher places.

 

In the open sea, the waves travel at speeds up to 700 km/h and are not very high.  As they approach the shallower water of the shore, the wave height increases and the velocity slows, so that wave crests are closer together.  The first indication of a tsunami is when the sea recedes for some distance.  The water then rushes in at speed, a high wave followed by others in succession.  It smashes ashore, sweeping everything in its path.  It can travel inland for quite some distance depending on the terrain.  The run-up height is higher when funnelled into a V-shaped inlet (such as Xlendi Bay) or a U-shaped one (such as Marsaxlokk Bay).

 

20% of recorded Mediterranean tsunamis have been damaging. In AD 365, a Mag 7.7 earthquake in Crete created a tsunami which caused extensive damage in Libya, Egypt, Calabria and Spain. This tsunami is unique in the historical record, as it propagated across the entire Mediterranean without major attenuation. Malta must have been affected.

 

Tsunami heights exceeding 20m have been reported in the Eastern Mediterranean.  Thera’s explosion supposedly wiped out the Minoan Civilisation in Crete, and the Messina Earthquake of 1908 (Mag 7.5) caused waves 8.5 m high on the Sicilian coast and more than 10m high on the Calabrian coast. This area averages about 10 tsunamis per 100 years and the last tsunami was recorded in 1954

 

The strongest tsunamis are excited in the Aegean Sea and the Hellenic and Calabrian areas.  Overall, large tsunamis are less frequent than in the Pacific Ocean.  Expected heights of tsunamis in the more exposed parts of the Mediterranean are: 100 years – 1.5m high, 500 years – 4 m high, 1000 years – 7 m high.

 

De Soldanis reports that in the 1693 Earthquake the sea at Xlendi receded for a whole mile then rushed back in as a tremendous wave with thunderous sound. Miraculously, no lives were lost. If it happened now, there would be great loss of property and life, due to building developments and many tourists on the shoreline.

 

The 1908 Earthquake of Messina did not cause any major structural damage but reports suggest that a 5ft high wave hit the island, with the water receding and returning several times at Marsamxett Harbour.  Houses were inundated in Gzira, Sliema and Msida and serious flooding was reported at Marsaxlokk.

 

In 1973, fishermen and residents in Salina Bay reported a marine disturbance, which old fishermen called ‘il-milghuba’, and which was alleged to have also taken place a few years previously. At 3 a.m. the sea level dropped by a couple of feet. A short while later a massive wave caused the sea to rise a couple of feet above the normal level. A rumbling noise was heard.

 

Due to climate change, water levels could rise up to 0.6 m above present level by the end of the century.  Low lying areas, such as Pwales, Marsaxlokk and Birżebbuġa will be inundated.  Loss of land, loss of water purity, especially in water tables close to sea, dying off of plant life in coastal areas, and greater impacts of storms and tidal surges will ensue. Over time as the sea warms, it will expand. Combined with the ice caps melting, the sea level could rise up to 4 m over the next 500 years. 

 

 Wave

 

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