Magnitude of 9.5 = around 60 million tonnes of TNT

1. There are about 500 000 earthquakes a year. Only about 100 000 of these can be felt and about 100 cause significant damage each year, the rest are recorded by sensitive instruments. There might even be one happening right now and you wouldn’t know it!

2. The largest ever earthquake recorded was in Chile, May 22 in 1960. This had a magnitude of 9.5 – which would be about 60 million tonnes of TNT exploding.

3. The deadliest earthquake was in Shansi, China on January 23^rd, 1556 – an estimated 830 000 people died.

4. Earthquakes that occur on one side of the Earth can shake the other side, sometimes for days in something called ‘oscillation’‘. The massive earthquake which caused the 2004 tsunamis around the Indian Ocean, is believed to have caused a weakening of the San Andreas fault in California.

Each red triangle denotes volcanic activity

5. The Pacific ‘Ring of Fire’ (shark bait ooh ha ha) which is actually more a horseshoe shape than a ring…but ‘horse shoe of fire’ just sounds wrong. It’s the most geologically active region of the Earth as there’s a whole lot of tectonic plates bumping and grinding against each other here.  In fact, around 80% of major earthquakes occur here. The -cough- ‘ring’ circles the Pacific Ocean and passes through North and South America, China and Russia. It contains 452 volcanoes which is over 75% of the Earth’s active and dormant volcanoes.

6. The fastest seismic wave has been recorded at 360km/hour

7. The 2011 earthquake in Japan created a massive rift 24 km underneath the ocean, measuring about 300 km long and 150km wide.

8. The city, Concepción, in Chile moved three meters when a huge 8.8 magnitude earthquake struck in February 2010. This caused a rift in the Earth’s crust that shifted the city towards the west. This earthquake is thought to have had bigger implications – by changing the planet’s rotation slightly and shortening the Earth’s day!

9. Earthquakes have been a big part of mythology in civilisations through time. In Japanese mythology, a giant catfish called Namazu is responsible. In Hindu mythology, the Earth is balanced on the backs of eight elephants which rest on the back of a turtle which lies on a snake – if any of those animals move then an earthquake occurs. In ancient Greece, it was believed that earthquakes were the wrath of sea-god Poseidon, who would angrily strike his trident on the ground and cause an earthquake…which seems fitting as many earthquakes are also associated with tsunamis!

10. The largest tsunami caused by an earthquake was over 85 meters high, which occurred in 1771 on the coast of Ishigaki Island, Japan.

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Old Faithful, Yellowstone NP

Some hot spots heat up the groundwater that lies beneath the Earth’s surface. This causes water and steam to erupt from the ground with great force. These eruptions are called geysers. One of the biggest geyser’s is Old Faithful in Yellowstone National Park in the U.S. At the point of eruption, the water is 95.6 C (204 degrees F) and reaches 55 meters (180 feet) high. Impressive!

In other areas such as New Zealand, not only are there volcanoes, but thermal areas of heated pools of water or boiling mud…like at Rotarua, which stinks like crazy with the smell of sulphur.

There are around 50 hot spots across the globe. Hot spots generally are gathered in regions like Iceland, Indonesia or Hawaii.

Nobody quite knows for sure how they came about, but there’s a couple of theories.

Theory 1. A mantle plume – which is where a thermal abnormality causes an upwelling of magma (basically, for some reason, an area of magma gets VERY hot). This super-heated magma then wells upwards, melting rock as it rises through the lithosphere (upper, rocky layer of the earth). This produces volcanoes at the surface, and can be formed in either oceanic or continental conditions.

Theory 2. Movement in the lithosphere causes a weak point or crack which allows the magma to leak to the surface. Like the first theory, the magma then rises to the surface forming volcanoes.

Volcanoes are initially formed over hot spots, but because tectonic plates are moving, the volcano slowly drifts away from the hot spot and will eventually become extinct (not capable of errupting). A new volcano will then be formed above the hot spot, and so on. If the volcanoes are formed in the oceans, as they cool they produce islands, or atolls (ring shaped reefs) and seamounts (mountains rising from the seafloor).

The volcanic islands of Hawaii are an example of these oceanic volcanic masses which have moved away from the hot spot. These are within the Pacific Plate, meaning they have anomalous volcanism (not at a plate boundary) and originate from the Hawaiian hot spot, which has been active for at least 70 million years. They have moved 6000km away from the hot spot and have produced a volcanic chain.

Volcanoes originating from continental hot spots move away from the hot spots where they cool and sometimes become extinct, such as the Yellowstone hot spot which has been active for about 15 million years.

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Elephants cause earthquakes said the ancient Hindi faith

Earthquakes are the vibrations caused by rocks breaking under stress, usually when tectonic plates rub up against each other. (tectonic plates). But basically, these tectonic plates are always on the move….but veeeerry slooowly.

Tectonic plates don’t have smooth boundary lines that can gently slide over each other; they’re rough, so when each end moves against each other, they get ‘stuck’ for a while against the jagged edges, the pressure builds up until the rocks break under tremedous stress, causing an earthquake.

The place where two tectonic plates slip against each other is called the fault. The place where the rock breaks underground is called the focus or hypercentre. The area directly above at the surface is the epicentre.

Sometimes foreshocks occur before an earthquake; these are smaller earthquakes that happen before the mainshock and can be followed by aftershocks. Aftershocks can continue for days to years after the mainshock, but aren’t as strong.

How are Earthquakes measured?

If you think the answer is ‘using the Richter scale’, then you’re wrong. Just like the Mercali scale, the Richter has had its day. It just wasn’t accurate.

Earthquakes are now measured using the Moment Magnitude Scale (MMS).

The MMS measures the seismic moment which is essentially the amount of energy released (magnitude). The exact MMS is measured by

(1) The distance the rock face slips at the fault, this is called the, wait for it….,  fault slip. (2) the area of the fault surface which is broken by the earthquake and (3) the type of rock at the fault. Rocks like granite are rigid and need more energy to break, whereas sandstone is soft and easily broken.

Three types of fault slip

Therefore: Fault Slip + Fault Area + Rigidity + a whole lot of complicated maths = MMS

The level of magnitude is calculated using a scientific instrument called a seisomograph which detects the amplitude (level and frequency) of the seismic waves. Seismic waves are produced when the energy is released at the fault, they radiate out just like the ripples you see when you throw a stone in a pond.

There are two types of seismic waves:
1. Body waves – these move underground and are divided into ‘P’ (fast waves) and ‘S’ (slower waves). The move through the earth and to the surface dislodging rock and stones.
2. Surface waves – also known as ‘L’ waves, have the slowest frequency. They roll along the surface causing most of the destruction.

Seismographs are extremely sensitive, they can detect earthquakes from around the world, and as these waves all have a different frequency, the results from several seismographs can be put together, using a process called trilateration to accurately calculate the location of the epicentre, fault slip etc to provide an MMS.

Images:

pbs learning media

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