Wednesday, February 22, 2017

The science of earthquakes explained

Earthquakes can cause immense damage to buildings and infrastructure, trigger tsunamis, and reshape the Earth's surface with their force.
Each year, seismology bodies record tens of thousands of earthquakes, with some parts of the world at far greater risk of a damaging quake than others.
But how do earthquakes begin — and why are some much more devastating than others?

How do earthquakes start?

The seeds of an earthquake lie in the tectonic plates that make up the Earth's surface and on which the continents sit.
These plates are ever-so-slowly moving — typically a couple of centimetres a year — as they float on the planet's liquid mantle.
These miniscule movements mean plates are either colliding (converging), moving away from (diverging) or sliding past (transforming) one another.
It's that movement at the plate boundaries that's responsible for most earthquakes.

The most active earthquake zone on the planet is the Pacific rim, which is also known as the "Ring of Fire" and includes New Zealand, Indonesia, Japan, the Alaskan peninsula, and the west coasts of North and South America.
Many of these countries lie along the Pacific plate, which is simultaneously colliding with its neighbouring plates and moving alongside or away from them at other points.
It's for this reason that the largest earthquakes often occur in these countries.
Another high risk area is Italy, which lies on the boundary of the convergent African and Eurasian plates. Nearby, Turkey sits on top of the meeting point of those same two plates and the Arabian plate, another seismically active area.
Nepal and India are also earthquake hotspots because of the collision of the Indian plate and the Eurasian plate, a process which formed the Himalayas.

What are the most powerful types of earthquakes?

The largest earthquakes happen where one plate collides and slips under another (a megathrust earthquake) or collides with and slips past another (slip-strike earthquake)

Megathrust earthquakes

"[Megathrust] quakes that can be some of the largest that we've seen," Geoscience Australia seismologist Dr Trevor Allen said.
These include the magnitude-9.1 Sumatran earthquake in 2004, which triggered a series of tsunamis that hit coastlines across the Indian Ocean and killed at least 230,000 people, and the magnitude-9 Japanese earthquake in 2011 that also generated a large tsunami and led to the meltdown of three reactors at the Fukushima power plant.
They occur when the plates collide and lock together forcing one plate below the other. This causes stress to build up until it's released with often devastating force.
"Once the strain reaches a certain point, that locked section of the subduction interface will break and that's effectively what the earthquake is," Dr Allen said.

Slip-strike earthquakes

A separate type of earthquake is caused by a transform fault, also known as a strike-slip fault, which involves two plates sliding past each other, rather than pulling away or pushing together.
"The most common example of that is the San Andreas fault, where the Pacific plate is moving northwards relative to the North American plate at a rate of about five centimetres a year," Dr Allen said.

He said strike-slip earthquakes can be very large — up to magnitude 8 — and leave plates ten metres away horizontally from where they previously stood.
Other transform faults include the Alpine Fault in New Zealand, which runs along its South Island, the North Anatolian Fault, in Turkey, and the Queen Charlotte Fault in North America.

What happens when the plates move apart?

When plates move apart, molten lava breaches the Earth's crust to create new land, ocean floor, and volcanoes.

It's for that reason Iceland — which sits on the divergent boundary of the European and North American plates — is known for having lots of volcanoes which have bubbled up over thousands of years while the plates move apart.
Divergent boundaries can also generate earthquakes, but they usually aren't as powerful as those that happen on convergent boundaries.
"Historically, we haven't seen very large earthquakes in these environments, but that doesn't mean they can't occur. We've seen roughly up to [magnitude] 7.5 or so," Dr Allen said.
"In the subduction case, the stresses are compressive, whereas in the divergent case the stresses are tensional. They're the same types of stresses but they're acting in opposite directions."

Can you get earthquakes in the middle of a tectonic plate?

Earthquakes occasionally occur away from plate boundaries in areas such as India, the western United States and Australia.
One of the most devastating of these, the 2001 Gujarat earthquake, reached 7.7 on the magnitude scale.
We're not really sure what causes these so-called intraplate earthquakes. One recent study suggested they may they may be triggered by the rise and fall of hot material through the Earth's mantle — a layer that sits between the crust and the upper core.

How is the strength of an earthquake measured?

Earthquakes are measured using a network of seismometers, instruments that record the motion of the Earth as it vibrates or shakes.
Many countries have hundreds or thousands of seismometers, allowing them to accurately predict the size and location of an earthquake using a process of triangulation.
The maximum motion of the earthquake measured by these seismometers is then used in calculating the earthquake's magnitude, which signifies the amount of energy released by the quake.
There's no theoretical upper limit to the magnitude scale, though limitations in the amount of energy that can build up in plate compression mean it's unlikely there'd ever be a quake above magnitude 10 on Earth.
Every time the magnitude increases by one — like from magnitude 5 to magnitude 6 — there's a 30-fold increase in the amount of energy released by the quake.
But magnitude will not always predict how destructive an earthquake will be, because it also depends on where the earthquake occurs and at what depth its hypocentre is.
The hypocentre of an earthquake is the point in the Earth at which a rupture actually starts, which could be kilometres below the surface, while the epicentre is the point on the surface vertically above the hypocentre.
The more shallow the hypocentre of the earthquake, the greater the shaking at the Earth's surface. Seismic waves that are generated deeper below the Earth's surface have further to travel and thus lose their power along the way.

How frequent are the biggest earthquakes?

GIF: Landslides triggered by earthquakes north-east of Christchurch, NZ

The largest earthquake ever recorded was a magnitude 9.4 - 9.6 quake near Valdivia, Chile in 1960.
From 1990 to 2015, an earthquake above magnitude 8 occurred once a year on average.
No single year had more than two earthquakes above magnitude 8 except for 2007, with quakes in the Solomon Islands, Peru, Indonesia, and near the Kuril Islands.
Earthquakes below magnitude 8 can still be highly destructive depending on the region in which they occur, and United States Geological Survey (USGS) data says an average of 17 earthquakes between magnitude 7 and 7.9 occur globally each year.
Earthquakes above magnitude 4 can cause destructive landslides and avalanches, such as those recently seen in New Zealand and Italy. The larger the magnitude of the earthquake the greater the area over which landslides can occur.

Can you predict an earthquake?

Scientists are not currently able to predict when a particular earthquake will happen. What seismology agencies can do is calculate the risk of an earthquake occurring in a particular area based on how frequently it has happened in the past, combined with a knowledge of faults that exist in that area.
Seismologists use these probabilities to calculate hazard maps and models that show regions where an earthquake is likely to occur.
USGS has used its model to forecast a 76 per cent probability of a magnitude 7 earthquake occurring in the next 30 years in northern California, and a 93 per cent probability of one occurring in the greater California region.
These earthquake hazard models are also used to determine building codes.

Do earthquakes have a domino effect?

It will not always be the case that a large earthquake will trigger subsequent earthquakes, but Dr Allen says it is not unusual either.
When a fault ruptures, the stress placed on that part of the fault is relieved, but additional stress can be placed on adjacent parts of the fault that have not moved with it.
"In the 2010/2011 Christchurch earthquake sequence, we saw there was a very large earthquake that occurred in September 2010, and was preceded by a very active aftershock sequence, and we continued to see large earthquakes gradually migrating along a network of faults from the west to the east," Dr Allen said.
"Once the earthquake has ruptured the fault it will put additional loads or stresses on adjacent areas and the aftershocks are the Earth's crust trying to relieve those stresses and come back to an equilibrium state."
These aftershocks have the potential to further damage already weakened structures and can often be as dangerous as the main earthquake.

Where do the worst losses of life happen?

While countries like Chile and Japan are prone to severe earthquakes, the loss of life in those disasters is often far less than in developing countries, even when the earthquake is considered more severe.
Dr Allen said developing countries have lower compliance with building codes that protect structures from earthquakes, leaving homes and other buildings vulnerable.
He said regions of South America and the Middle East also have a greater tendency to be built with adobe bricks (typically made with mud) and other masonry that isn't resilient to an earthquake.
"They're quite brittle structures and whenever they experience ground shaking they will tend to fail," he said.
Building codes in Europe are stronger than those in developing countries, but historical buildings that haven't been retrofitted to protect them from ground shaking remain vulnerable when an earthquake hits.
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