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When The Earth Shakes
Earthquakes Can Really Shake Your World
For some reason, earthquakes fascinate me. I've never experienced a quake larger than 5.3 on the Richter scale, and that was quite scary enough. Mostly, where I live, in Melbourne, Australia, we get tremors rather than earthquakes, so it's pretty safe.
One of my favourite countries, New Zealand, isn't so lucky, and suffers from them frequently, being on the "Pacific Ring of Fire", a very seismically active area.
Here, I'll explain a little about earthquakes, and what happens during one, mixed with my personal experiences.
The photo shows a seismological observatory in Japan.
Tectonic plates sit on the aesthenosphere, which is the highest layer of Earth's mantle. All the plates move constantly, and where they interact, earthquakes and volcanoes often occur.
Tectonic plates are approximately 80 km thick, and move at anything up to 130 milimetres (13 cm) a year. This doesn't sound like much, but considering their size, there is tremendous pressure involved. When this pressure builds to a critical point, an earthquake or volcanic eruption takes place.
A book in the science series from the Smithsonian Institute- this is very factual.
A Few Facts
When two plates have been moving and a critical amount of stress has been reached, an earthquake occurs. Sometimes, there will be foreshocks, and it is impossible to tell if these are the main shock until the big quake happens. Often, a series of aftershocks occur after the big quake, and these can go on for months, or even years.
When a temblor (another word for earthquake) occurs, the site underground is called a hypocenter, and the area above ground is the epicenter. The strength of the temblor is measured by the Richter scale, which is open-ended. A quake of 6.4 is ten times as powerful in amplitude as one of 5.4 magnitude. The most powerful earthquake currently known was in 1960, in Chile, and was magnitude 9.4.
The image shows damage in Chile after the 2010 quake, an 8.8 on the Richter scale.
Little Quakes Can Be Scary
In Melbourne, where I live, we are pretty much in the centre of the Australian tectonic plate, so we don't have major temblors. We have had a couple of small ones in the past few years, which have made the house tremble.
When I lived in Darwin, in the Northern Territory, there were a couple of minor quakes a couple of hundred kilometres away. in one, my sister and I were sitting reading, and the objects on the shelves started to vibrate, and the aquarium water sloshed a bit. We dashed out on to the balcony in case it got worse, but it only lasted a few seconds.
The strongest temblor I've felt in Melbourne happened in 2012, and was 5.3 on the Richter scale. It was the scariest, as I was alone in the house. I found out later that it was the largest in our state, Victoria, for 109 years.
I was sitting on the couch, reading, when all of a sudden, there was a huge bang, as if something had hit the house, and it started to shake. This went on for about 40 seconds, then slowly died away. Luckily, there was no damage, but it made me realise how terrifying a quake can be, and how fortunate we are to live in a relatively safe place.
The image is a seismograph of the earthquake.
Have you been there when the earth shook?
Liquefaction - Another effect causing damage
Liquefaction sometimes happens during an earthquake, where the ground is not solid. If a building stands on sandy, or reclaimed land, and there is water in the sub-strata. during a quake the water rises, causing the ground to temporarily become liquid. This causes severe damage, as the foundations of building can move causing them to lean or collapse. Cars can sink into roads. Sometimes sand boils up a through cracks in the ground.
In some earthquake prone areas, after severe damage to suburbs, some places have been declared unsafe for buildings, and homes will be demolished, and the land returned to nature. This has happened in Christchurch, New Zealand, after the 2011 quake, where several suburbs were deemed to be too unsafe to rebuild.
New Zealand Suffers
I've been to New Zealand a couple of times, and although it is known as the "Shaky Isles" I haven't felt any earth movement when I've been there.
On our first trip, we were in Auckland, and an earthquake was reported to have occurred that afternoon, not long after we landed at the airport. We didn't feel a thing. We worked out later that it could have been during the taxi ride, when we went over a bridge, and the car shuddered a bit - we thought it was just rough road, and the driver said nothing. I guess they are used to things like that in NZ!
When we were on the South Island, during our second trip, there was a medium sized earthquake during the night, while we were staying near the Fox Glacier. We both slept through that one, so it couldn't have been too bad where we were, or we would have been woken.
While I've been writing this, New Zealand has had another temblor, in Wellington, the capital. They had one there a few months ago too. Fortunately, this quake, at 6.5 magnitude, didn't cause any deaths, and only minor damage.
Many of the buildings in New Zealand are earthquake-proofed - they have strict building laws. When in Wellington, we visited the Museum of Te Papa (Te Papa is Maori for "Our Place"), and they have a section of their shock absorbers under the buildings open to the public. They are huge!
The image shows a seismic isolator, similar to those used in some buildings in New Zealand.
Disasters can come from many causes, but nature is the most powerful.