Ireland’s earthquake watchers

This article was first published in Science Spin, in March 2010.

Ireland might not experience strong earthquakes, like many other parts of the world, but that doesn’t mean there is no interest here in earthquakes. The Dublin Institute for Advanced Studies (DIAS) is part of a Europe-wide network of earthquake monitoring stations, which means it gets scientific information on significant earthquake events, no matter where they occur around the globe, within minutes of them occuring. DIAS also has established a very popular seismology in schools programme, where students and their teachers do their own earthquake monitoring, whether that involves a small local event, or a massive event like recently in Haiti.

The faultline responsible for the Haitian earthquake can be seen in this image on the right,  starting in the foreground, and cutting through the mountains in a NW to SE orientation. This faultline, called the Enriquillo Plantium Garden Fault, was captured by a NASA satellite image. Geologists had been warning for more than 20 years that a major earthquake could occur along this faultline. [Credit: NASA] 

The earthquake that hit Haiti on the 12th January happened at 21:53. By 10:10 Tom Blake, the man in charge of earthquake monitoring and the seismology in schools programme at the Dublin Institute for Advanced Studies (DIAS) had received an automatic SMS message to his phone, from the European seismic network, informing him that it had occurred and its strength.Though the earthquake had occurred some 5,000 miles away, Tom had received details about the event less than 20 minutes later in Dublin to his mobile phone.

The earthquake was ‘shallow’ meaning it had occurred close to the surface, its epicentre was close to the capital city Port Au Prince, and there was a tsunami warning. The likelihood was that buildings were not earthquake-proof in this very poor country, and there was potential for very large loss of life in densely populated Port Au Prince. DIAS is part a Europe-wide network of seismic, or earthquake monitoring stations organised by Potsdam University.

There is a mobile phone alert system in place so that when an earthquake greater than 6 on the Richter scale occurs anywhere in the world then members of the network are alerted by SMS message. “At that time the advised magnitude was 7.2, so I knew immediately that this was going to be a serious event,” said Tom of the Haitian earthquake. ” I checked our own monitoring station in the Dublin mountains and saw that the ‘P wave’ took just nine minutes to arrive from Haiti, and there was quite a strong surface wave element to it. So, I knew there would be serious casualties.”

WAVES

The nature of the seismic waves associated with an earthquake, the so-called P and S waves, is vital, as along with the overall power of the earthquake, as determined by the Richter scale measurement, these are what will determine the extent of the damage done to buildings and the likely loss of life.

The bulk energy created by an earthquake is contained in the P wave, and this travels out from the epicentre of the earthquake – the location where rocks moved and pressure and energy was released – at a speed of about 10km per second. The S waves, or surface waves, are slower. They travel at about 60 per cent of the speed of the P waves, over the surface of the earth, but they are deadly. It is the S waves that can cause the Earth to ripple and buckle and cause buildings to collapse in a heap.

HAITI

The reason that powerful earthquakes occur in Haiti has to do with the fact that two massive tectonic plates, or pieces of the earth’s crust, come together there. These plates are called the Caribbean Plate and the North Atlantic Plate. They are moving, albeit very slowly past each other, but sometimes at the edges the plates can lock, preventing movement and leading to a build up of pressure. When that happens the only possible outcome is the pressure builds until the plates move rapidly in relation to one another, causing huge pressure to be released – this is an earthquake.

Earthquakes of the power of the 12th January event are rare in Haiti and the last one of this size to occur there was two centuries ago.

The margins of these giant tectonic plates occur in the northern and southern part of Haiti, and there is a big fault running right through the island, which is visible from space, along which the plates can move. It was movement along this fault that caused the recent earthquake. Thankfully, for the beleaguered people of Haiti the fault associated with the earthquake was a strike-slip fault, which means plates glide past each other, rather than a thrust fault, where one plate moves over another.

The importance of the difference, in human terms, is huge, because movement on thrust faults, when offshore, as this earthquake was, can result in the displacement of large amounts of water, which in turn can trigger a tsunami. At least the Haitian people avoided a killer tsunami.

There were some clues that Haiti was due for a big earthquake, had anyone been looking. Haiti, in seismic terms had been relatively quiet for some time compared to its neighbour on the same island the Dominican Republic. The movement along the Haitian fault had stopped. This led to a big build up of strain, as the two huge plates struggled to continue their movement past each other. The build up inevitably led to a massive rupture as the two big plates rapidly moved to relieve the pressure.

Unlike Haiti, San Francisco has a sophisticated earthquake monitoring system, and when there is a lack of movement across the San Andreas fault, scientists take note, and people start to worry. The reason is that if there is movement along the fault, pressure is being dissipated. If there is no movement, there is a build up of stress, and no-one has much idea of how, when or where that stress is going to be released, or whether the release will trigger a major earthquake event or not.

The lack of monitoring was a problem for Haiti, as was their poor building standards, and also, the bad luck that the earthquake occurred at a time of the evening when people were mostly inside relaxing in their homes, or apartments. But, these are small mercies, as earthquake proof buildings, such as are seen in San Francisco or Tokyo are too expensive for Haitian people to construct. Such building are designed to withstand an earthquake of 7 on the Richter Scale – about the size of the Haitian earthquake. So, in those cities, a similar earthquake would not cause such huge damage.

In the aftermath of the earthquake, many survivors of the Haitian earthquake refused to go inside buildings again, even when they were still standing, and preferred to sleep outside. This, in fact, was an entirely sensible thing to do, as the threat from strong aftershocks was significant. Geologists understand that strong aftershocks can occur up to 15 days after major earthquakes and that is what happened in Haiti with one aftershock recording 6.1 on the Richter Scale.

DONEGAL

Ireland does experience earthquakes, albeit very small ones compared to Haiti, and the two regions that are most prone to the shakes, are Donegal and Wexford. On the 7th January 2010 Donegal had an earthquake of 1.5, and shortly after that, another one, this time measuring 1.7. The reason why Donegal gets more earthquake activity than most of the rest of the country is that it is part of the major structural fault system that extends right down from Scotland. There is movement of rocks along the fault, which can result in release of pressure and energy and cause minor tremors.

“A lot of people felt it, and the schools have a recording of it,” said Tom Blake of DIAS. “St Egneys a primary school had the recordings. People in Donegal are very clued in to earthquakes. They are alerted by the sound of rumbling and the erratic behaviour of animals, dogs or cats, that go berserk almost. The erratic behaviour of animals has been reported around the world as the precursor to earthquakes.”

If people are interested in finding out more about the Irish monitoring network they should clickhere.

To see a record of all the earthquakes that have been recorded in Ireland throughout historical time. The DIAS would also encourage people that felt the Donegal tremors to fill out an earthquake questionnaire on the site.

1984

The year of ‘Big Brother’ is also the year when the largest earthquake ever reported in Ireland occurred off the coast of Anglesea, measuring 5.4 on the Richter Scale. “That was felt by a lot of people along the east coast and it caused structural damage along the east coast,” recalled Tom. “I remember feeling the earthquake at home, and I said my God there has been an earthquake. My wife thought I was mad. Then when I got into DIAS the phones were hopping. It happened in the morning. There was a blue sky, absolutely no wind, sunny morning in Dublin, deadly calm. So, when I heard the rumbling I felt what else could it be? And our doors fell off.”

MÉDECINS SANS FRONTIÉRES

One of the first organisations to go into action following the quake was Médecins Sans Frontiéres, an organisation delivering medical care to areas devastated by war, poverty or natural disasters, such as this one. Some of the MSF staff who were already in Haiti since 1991 are among the dead and missing, and according to doctors working in improvised conditions under canvas, they have never had to deal with so many serious injuries. MSF has managed to send in additional medical staff, and one of their tasks is to set up mobile clinics to replace those that have been destroyed.
To find out more about MSF in Ireland click here

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