Facts About Earthquakes
Discover why this sudden release of pressure can flatten cities and spawn giant waves.
Even if you’ve never felt an earthquake, you’ll know they can be devastating. Films like 2012 feature ‘mega quakes’, where gaping fissures swallow people and buildings. Real-life earthquakes are less dramatic than those in the movies, but they’re still one of nature’s worst natural hazards. Unstoppable and terrifying, big quakes strike with little or no warning, flattening cities and killing tens of thousands of people.
Most of the world’s earthquakes occur at the boundaries between the Earth’s huge crustal plates. These boundaries are called faults, and the plates – of which there are 15 of varying different sizes here on Earth – jostle on the planet’s surface like the pieces of a giant, floating jigsaw puzzle. In some parts of the world, these crustal plates grate past each other. In other places, they collide or are pulled apart. Faults break open as these rigid plates move and exert forces great enough to crush and tear solid rock.
As the plates move about, the rock slabs at either side of faults are dragged past each other. But rocks are jagged and uneven, meaning there’s lots of friction between them. This friction causes the rocks to become locked together. Pressure builds along the fault as the plates grind along, squeezing and stretching the rocks until, eventually, they break and lurch forward. Huge amounts of pent-up energy are unleashed, and it’s the resulting snap that is an earthquake.
The point at which the Earth’s crust first breaks is called the earthquake focus. This is usually many miles below the Earth’s surface. The epicenter is the point on the surface located directly above the focus.
The released energy speeds through the Earth in the form of shock waves.
There are three main types of Shockwave: primary, secondary and surface waves. Primary waves radiate fastest from the earthquake focus. Secondary waves arrive later and surface waves arrive last. The surface waves travel near the Earth’s surface, rocking the ground and causing the widespread devastation wrought by the largest earthquakes. People barely feel primary or secondary waves.
The size of an earthquake is defined by its magnitude – this is a measure of the energy released. Magnitude isn’t a simple measurement of the relationship between earthquake size and energy. Increasing the magnitude by one increases shock wave size by ten times and total energy released by about 30 times. So for example, a magnitude eight earthquake is a billion times more powerful than a magnitude two. Quite an unimaginable thought.
It’s interesting to note that earthquake damage isn’t directly related to magnitude. Deep, distant earthquakes shake the ground less than close, shallow earthquakes because the energy released at the focus has had a chance to disperse. Big earthquakes often cause longer tremors. For example, an earthquake in 1949, which had a magnitude of 7.1, shook the ground for 30 seconds, while a magnitude 8.3 earthquake in 1964 lasted five minutes.
The majority of the shuddering during an earthquake is caused by Rayleigh waves. These surface waves roll along, convulsing the Earth’s crust. The ground heaves up and down and from side to side much like water waves in the ocean. Earthquakes can shake the ground violently enough to open large fissures but, unlike in the movies, these don’t crunch closed around people’s bodies and legs.
Big, long-lasting surface waves created by large earthquakes can topple buildings, crack roads and buildings and even trigger landslides. Well-built, earthquake-proof buildings on solid bedrock usually suffer substantially less damage than urban areas built on loose debris and sediments. Water-saturated sediments can behave like quicksand when shaken, where loose grains move apart and flow like a liquid. In Niigata, Japan, 1964, earthquake-resistant buildings tumbled onto their sides when the ground underneath liquefied.
The population faces additional hazards once the shaking stops. Fires break out where the ground convulsions sever gas and electricity lines or destroy flammable objects. Nearly 90 per cent of the damage in the 1906 San Francisco earthquake was due to fire. Lives can be endangered and rescue efforts thwarted by collapsed bridges, burst water pipes, broken containers of hazardous chemicals and aftershocks.
Aftershocks are the less powerful earthquakes following the main tremor, when faults shift and readjust after the release of energy and stress. You could think of a tremor as the ground breathing a big sigh of relief. Major earthquakes are usually followed by several noticeable aftershocks within the first hour or so. The number of aftershocks drops over time. However they can happen months, years or decades after the quake.
Undersea earthquakes can be as devastating as those on land, if not more. Fault movements can displace huge volumes of water, which crash to shore as killer waves called tsunamis. An undersea earthquake near northern Indonesia triggered the Indian Ocean tsunami in 2004- the world’s biggest for at least 40 years. At least 120,000 people in Indonesia alone were killed by the giant waves. Rescue teams cleared up bodies for weeks afterwards. The final death toll was over 200,000.
Tsunamis can reach speeds of 97okm/h (6o2mph) in the deep ocean, depending on water depth. As the tsunami races into shallower water, it slows down and can reach a mammoth 30m high when it hits shore. The first sign may be water rushing out to sea, sometimes beyond the horizon, leaving the sea floor bare. The sea pours back onshore as a series of towering waves or a rapidly rising tide. Warning signs such as these can save lives. A ten-year-old British girl saw the sea hurtling away from the beach at a resort in Phuket, Thailand in 2004 and warned her mother and staff that a killer wave was coming. She’d learned about tsunamis in school a fortnight before.
Pacific Ring of Fire
The ‘Ring of Fire’ is a horseshoe of active volcanoes and earthquake zones encircling the Pacific Ocean. About 90% of quakes and more than 50% of active volcanoes above water happen around the ring. It’s violently active because crustal plates carrying the Pacific Ocean I are sliding under the encircling continents into the Earth’s interior. The crust is broken into many rigid plates, which drift across hot rock below. The grinding i and melting of the oceanic crust as it’s forced down near the Pacific coast creates volcanoes and earthquakes.
The big ones – ‘Mega quakes’ above magnitude 10 are impossible -no fracture in the Earth’s surface is long enough to store the vast energies. The biggest ever was 9.5.
Heart shaker – Earthquakes can occur almost anywhere. The 1931 Dogger Bank earthquake was felt throughout the UK – a woman died of a heart attack and a church spire spun around.
Deadly waves – 2004’s Indian Ocean tsunami was among the world’s most devastating natural disasters. It killed at least 200,000 people in 14 countries and had waves up to 30m high.
Living dangerously – Tokyo, Japan, lives with the constant threat of earthquakes. Several quakes happen daily and a catastrophic quake strikes about every 70 years.