(Nadine Pedoe)
On 28th May 2009, Central America was rocked by a strong earthquake measuring 7.1 on the Richter scale, which has luckily caused far less damage and fewer fatalities than it may have done. Earthquakes affecting Belize are extremely rare, so it has caused a lot of panic. The Pacific Tsunami Warning Center issued a tsunami warning in the immediate aftermath of the quake, causing further panic amongst island and coastal dwellers. Hopefully understanding some of the science behind it can relieve some of those fears.
Why did it happen and is it likely to happen again?
Earthquakes and volcanoes are caused by movements of massive plates that make up the crust of the earth. All the land and ocean rests on one or other of these plates, which form a kind of jigsaw pattern around the earth. The plates move very slowly in relation to each other, just a few inches a year, caused by heat cells in the liquid mantle (made of magma) below them. Where these plates meet is known as a plate boundary, of which there are four different kinds.
Three of the kinds of plate boundary (from Wikipedia)
Constructive plate boundary (or divergent)
The plates move apart from each other, on land this is known as a rift valley like the one in East Africa. Scientists believe that eventually this will fill with water meaning that there will be two Africas. In the sea, as the plates move apart and magma comes up inbetween them forming new crust. This is called an Oceanic ridge. This is happening in the mid-Atlantic which is slowly spreading and will get larger. The country of Iceland sits on top of the mid Atlantic ridge, which is why they have so much volcanic activity – that’s how the whole country was formed.
Destructive plate boundary (or convergent)
Two continental plates push against each other and both rise up in a destructive plate boundary. This is how the Himalayas have been forming over millions of years and will get even higher. This is the one not in the above diagram as some scientists classify only three types of boundary. There is usually some element of subduction in this boundary too, but that varies.
Subduction zone (or convergent)
The less dense, lighter continental plate pushes against a heavier more dense oceanic plate and pushes it underneath causing a lot of heat and friction, and thereby melting it back into the mantle. This type of plate boundary causes the most earthquakes and volcanoes. The edges of the Pacific Ocean are mostly subduction zones, so that in many thousands of millions of years, the Pacific will cease to exist, and the continents on each side will come together.
Subduction zone (from Wikipedia)
Tranform boundary
The two plates grind against each other in a transform boundary. The San Andreas fault in California is a very active transform fault, but it is also affected by a subduction zone. This is also the kind of fault that caused our earthquake, but it is less active than San Andreas.
Most earthquakes and volcanoes occur at these different kinds of plate boundaries as the plates slip and grind against each other, causing friction. It is not a smooth process so energy builds up and then they suddenly slip, causing the movement on the land above. This friction builds up at some boundaries far more than others. Subduction zones cause the most movement (perhaps around 80%), followed by transform boundaries. Ancient landforms can eventually be left behind when the plates move away, so that new oceans can close up, volcanoes can become extinct, or rift valleys or mountains stop growing, but this happens over a huge time scale.
There are also hot spots underneath some areas, which is how volcanic islands like Hawaii are formed. They are sometimes but not always caused by the heat rising from the melting plate in a subduction zone. The plates slowly move over the hot spots so that islands on top form in an arc, known as a ‘Volcanic Island Arc’. The older ones are the furthest away from the hot spot and may no longer have active volcanoes. Eventually these hot spots just stop or the plate moves too far away, so that the arc stops growing, and volcanic activity stops.
It is also possible to experience earthquakes in the middle of a plate, but with much less frequency and they are rarely strong ones. These are caused by more minor faults, which criss cross the earth’s surface. These are still caused by the plate movements, but most of the stress is further away.
Plate boundaries in Central America (from Wikipedia)
Central America is surrounded by plate boundaries as the countries sit on five of these plates: the Caribbean, North American, Cocos, Nazca and Panama plates. Belize and Mexico sit well onto the North American plate, Just to the south of us on land is a constructive boundary between this plate and the Caribbean plate, running on land approximately at the border between Guatemala and Honduras, meaning that eventually Guatemala will have the Caribbean coast it so desperately wants, but it will take a few million years.
Under the sea, this boundary is a transform fault, where the Caribbean and North American plates grind against each other. The faults are different as the plates move in different directions, so that a kind of rotation occurs on the smaller ones, such as the Caribbean plate. Subduction zones exist to the Pacific side of Central America which is where most earthquakes and volcanoes originate from in this region. Belize is the only country in the region that has no volcanoes, and has very rarely experienced earth tremors. The reason for that is Belize has less active faults that the other Central American countries. In fact the only major fault runs in the south, which is the one that caused the earthquake. The fact that this earthquake occurred at sea is why it was less devastating that it could have been, without the risk of a tsunami. It was a fairly shallow earthquake which can be more destructive on land.
To put it in context, there are hundreds of earthquakes worldwide every year. Strong earthquakes measuring 7 or over on the Richter scale happen on average 17 times a year, with one major one. Even areas with very low seismic activity report some earthquakes, such as the UK, although these are very gentle ones.
Some doomsayers claim that there are more earthquakes happening, so the end of the world is coming. Even some scientists claim that global warming is increasing the frequency of earthquakes. However, most scientists agree that it is because of much improved equipment, global monitoring, and sharing of data and news reports between countries that there seems to be a slight increase. It’s just that we are more aware of them nowadays.
PART TWO COMING SOON
Basic advice in the event of an strong earthquake:
Try to get outside of your building, with all of your family. Watch for falling objects. If you can’t get outside, try to stand in a doorway, or get under the heaviest furniture you can find, usually a table. Once the tremors stop, be aware that aftershocks can occasionally be worse than the original quake. Stay away from tall trees that may fall.
When living in, or visiting an earthquake prone area, plan out your evacuation route in advance so that you can remember it when you are half asleep and find the way in the dark.
It is worthwhile having an emergency kit. Of course we should all have those in the form of our hurricane preparedness kits already- flashlights, lots of spare batteries, one litre of water per person for at least three days, canned food that can be eaten cold, tin opener, medicines, basic first aid, few toys for the kids, toilet roll etc. A battery radio can be useful but they run down very quickly so just listen in occasionally. Try to keep your cell phone charged up, but remember lines may be down, or just jammed from too many people using them. You should also have some available cash. After a large quake, banks and ATMs will be closed, and credit card machines won’t work.
After the quake, check that the gas is turned off. Often fires and explosions will start as gas is ignited when lines are displaced.
Be careful re-entering your building. It may be completely unsafe, or displaced objects may fall on you, or it could be fine.
After a devastating quake, make a list of your family members and who is safe, who still missing, where they were last seen etc and tape it to the outside of your building, along with details of any way to contact you, what you have done to make it safe (turning off gas etc), and where you are going. This will help the emergency services enormously, as well as anyone that is searching for you.
Find your important ID and travel documents, and valuables and keep them with you. You may need them to claim any aid, or get out of the area.
Try not to panic. Afterwards you may need counseling to get over the trauma, but during the event and immediately after, you need to hold it together for your own safety and to reassure your children.
Tags: caribbean, earthquake, education, faults, global, ocean, plate tectonics, research, threat, tsunami, volcanic, warning