Science Friday: Earthquakes and Seismology

A series of destructive earthquakes in Indonesia and the southwest Pacific has attracted a lot of recent media attention. It's unusual for so many destructive earthquakes to occur in such rapid fire succession as they did over three days last week, and especially for one country (Indonesia) to get hit by two powerful quakes on consecutive days. Given the topic's relevance and general badassness (full disclosure: I'm a seismologist and have a particular interest in it), this first edition of what I hope will become a weekly series is devoted to earthquakes and seismology.

Why do earthquakes occur? The Earth is not a calm, still ball of rock—it's churning with activity underneath. Forces powerful enough to drive currents of solid rock in the mantle push pieces of the crust around like tea leaves floating on boiling water. Pieces of the crust (plates) are constantly being rifted apart, crunched against each other, and forced above and below each other in a process called plate tectonics. The vast majority of earthquakes are the result of these processes and consequently occur at the edges of plates.

Plates are constantly being pushed into and against each other. However, most of the time, frictional forces are too strong to let the plates slide to accommodate the stresses. So, stress builds up until it overcomes friction, at which points the plates abruptly slide over a small part of a fault (the focus). This is an earthquake. As a result of the sudden acceleration and deceleration of the rock near the focus, seismic waves are produced, and they radiate out in all directions from the focus. The area on the Earth's surface directly over the focus (which is generally tens of kilometers underground) is called the epicenter, and is typically the site of greatest ground motion and damage.

Nearly all earthquake-related injuries are caused by collapsing man-made structures, not by ground motion itself. Two approaches have been tried to prevent building damage: building very strong, rigid structures, and building flexible ones that sway rather than break. The first approach has been a terrible failure—no matter how strong a rigid building is, a sufficiently strong earthquake will eventually occur and destroy it—so current earthquake-safe architecture has generally rejected it. However, places that suffer infrequent (once per hundred-plus years) but destructive earthquakes often don't require earthquake-safe design. Memphis is probably the best example of this in the US: an early 19th century earthquake in nearby New Madrid, MO was strong enough to ring church bells in Boston, and a similar-sized event will probably happen again.

Tsunamis are among the worst effects of earthquakes. It is never certain when a strong underwater earthquake occurs whether it will produce a tsunami, and tsunamis cannot be detected until they hit land, at which point it's too late to sound an alarm. They often travel far across oceans and strike thousands of miles from their source. Further, a sudden fall in water level precedes their arrival, which often attracts curious bystanders to walk down the beach toward the ocean rather than flee. After the first wave hits, many more can strike, spaced apart by an hour or more, meaning that survivors of the first wave that return to survey the damage can be caught in a later wave. Warning systems have been developed since the Indian Ocean tsunami of 2004, but, as we saw in Samoa, have much room for improvement.

Strong earthquakes generally appear at two types of plate boundaries. The first is where one plate slides underneath another—typically oceanic crust going under either oceanic or continental crust (oceanic crust is chemically different from continental crust, and is thinner and denser) –which is called a convergent plate boundary or a subduction zone. These areas are also the site of widespread volcanism. Examples include the Oregon/Washington coast, Japan, Sumatra, and the Chile/Peru coast. The second type is where plates slide in opposite directions past each other, called a transform boundary. Transform boundaries are found in California (the San Andreas fault), Java, and various other places. Transform boundaries tend to cause more earthquakes, but the ones in subduction zones are often stronger.

You might have noticed that Indonesia has both a convergent and transform boundary. It's not a coincidence that it is one of the most geologically active places in the world. The 2004 tsunami was caused by a Sumatran earthquake. Additionally, the two biggest volcanic eruptions in recorded history, Tambora and Krakatoa, occurred in Indonesia, and were so powerful that temporary cooling effects were felt all over the globe from the ash and gas emissions blocking sunlight. Last week, Indonesia got walloped by two major earthquakes. One might say it's a dangerous place to live.

Despite their deadly and destructive effects, seismic waves can be used in positive ways. Seismology began to mature as a science around the end of the 19th century, when instruments used to measure ground motion called seismometers were first developed. For the first time, seismic waves from an earthquake in Japan were measured across the world in Europe. It was discovered about this time that, just as x-ray waves could be used to see beneath skin, seismic waves could be used to see inside the Earth. It didn't take long to find out from this that the outer core of the Earth was liquid, the inner core was solid, and that the Earth's crust was a distinct unit from the mantle. Deployments of seismometers on the Moon have allowed similar investigations into its subsurface.

Before long, people started using controlled seismic events to look inside the Earth, mainly by setting off explosives on the surface. By deploying large arrays of seismometers, they could detect when waves reflected off of rock layers, allowing them to see the structure below them. This proved invaluable in prospecting for oil and other geological resources. (It was actually seismic reflection data collected by oil companies that determined the location of the buried impact crater from the asteroid that killed the dinosaurs.) After the development of nuclear bombs, seismology grew in importance, as seismometers could be used to detect bomb tests across the world.

These days, seismology is, unfortunately, mainly used for oil exploration. However, the days of oil are numbered. It will continue to be used to monitor nuclear testing, prospect for geothermal resources, study the interior of the Earth and the inner workings of volcanoes, and examine ice sheet stability (Greenland and Antarctica have “icequakes.”) Unfortunately, the holy grail of seismology, predicting earthquakes, remains unsolved: despite the efforts of hundreds of mainly American and Japanese seismologists, essentially no progress has been made on this problem.

Comments

Weekly series?

Bring.it.on!

I'll send you an email. I'm happy to help you insert images and links to images that we don't own copyright to. Any government images are typically owned by "we the people" so we can use almost anything found in the public areas of government web sites.

Thank you. This is a great post.



***************************
Vote Democratic! The ass you save may be your own.

Science

is always welcome. My brother lives in Jakarta and he says the USA got major props from our military's handling of relief efforts in the aftermath of the December 2004 quake and resultant devastating tsunami. Lots of heroism just to help people in need. Sometimes we can still be (one of the) good guys.

-b

--

There cannot fail to be more kinds of things, as nature grows further disclosed. - Sir Francis Bacon

I'm glad that the military was helpful there

Providing support related to disasters is one excellent way for us to do good and improve our image abroad, and one we are very good at. For example, the biggest recent volcanic eruption was Mt. Pinatubo (1991) in the Phillipines--because of the US military presence there, American scientists were on site very early on, and successfully predicted when it would blow. Tens of thousands of lives were saved. Imagine how our relations with other countries would change if the face of America were scientists and relief workers rather than planes dropping bombs.

Now, wouldn't THAT be nice?

You are so right, Jake. In actuality, our country has been involved in hundreds of good-faith, humanitarian efforts all around the world. Look at our involvement in Chernoble. But, alas, America is seldom seen in this light almost as if it is expected by the world community that we make these kinds of efforts. We are the "rich and plentiful" nation. God knows what kind of world we would have had if America hadn't gotten involved in eliminating the Nazi scourge. It is disappointing that the real fact is that even if our efforts internationally were only humanitarian, there are those entities that would and most certainly could put an extemely mean, unsafe and controlling face on not only international countries but America as well. This is not to say we haven't made some ridiculous mistakes along the way militarily (Vietnam, Iraq come to mind).

Maybe someday, Jake, maybe someday.

there is also this:

(sorry to toot my own horn, but i had relatives telling me about this after my post)

There is evidence of a world wide destabilization of tectonic fault lines due to the Ice quakes in Greenland and Antarctica.
How big are these quakes?

In Greenland, and to a lesser extent, Antarctica, ice sheets and glaciers are melting and more importantly, sliding in rapid bursts. This is caused by moulins, which are holes that melting water form from the top of a glacier to the bottom. The water then lubricates and melts the underside of the glacier, causing them to detach from the bedrock -- and creating a 'slip-n-slide' for glaciers that weigh in the megatons -- some the size of Manhattan.

The latest scientific discipline to enter the fray over global warming is geology. And the forecasts from some quarters are dramatic - not only will the earth shake, it will spit fire.

A number of geologists say glacial melting due to climate change will unleash pent-up pressures in the Earth's crust, causing extreme geological events such as earthquakes, tsunamis and volcanic eruptions.

A cubic metre of ice weighs nearly a tonne and some glaciers are more than a kilometre thick. When the weight is removed through melting, the suppressed strains and stresses of the underlying rock come to life.

(from Alan Glazner, a volcano specialist at the University of North Carolina)

"When you melt glacial ice, several hundred metres to a kilometre thick . . . you've decreased the load on the crust and so you've decreased the pressure holding the volcanic conduits closed.

"They're cracks, that's how magmas gets to the surface . . . and where they hit the surface, that's where you get a volcano."

this will be something that will result in many more displaced environmental refugees.
And it is not likely to slow down, but may instead speed up:

...quakes ranged from six to 15 per year from 1993 to 2002, then jumped to 20 in 2003, 23 in 2004, and 32 in the first 10 months of 2005 - matching an increase in Greenland temperatures.

thanks
wade norris


"What's the use of a fine house if you haven't got a tolerable planet to put it on?"

Henry David Thoreau

praer.org

That's interesting; I hadn't heard that before

It makes sense though. The ice sheets' weight really depresses the rock beneath them--much of Greenland's sub-ice ground is beneath sea level because of the weight of the ice. If the ice sheet did melt, ground level would rebound significantly, causing tons of earthquakes in the process. And it would be easier for magma to move to the surface with that load gone. Pretty cool, if it wasn't terrifying.

Allen Glazner taught my igneous/metamorphic petrology class a couple years ago, incidentally.

Extremely interesting

This is really interesting stuff and since I have read a great deal about the glacier advancements and how they have carved out just so many areas in our world, it's curious that we are seeing something that seems to be happening exponentially now in that regard.

However, are we sure that this isn't just Mother Nature progressing as she does or do we have science that says that this is something that is occuring as a result of man?

I also read both sides of this "global warming" issue as it relates to "climate change" and the affects on our solar caps and those kinds of zones on mother earth. Both "sides" have convincing arguments, of course, and a person has to decide on the legitimacy of these arguments based on their own beliefs.

Thanks for the information. Very interesting.

There is no disagreement on this in the scientific community

Every credible study done on this issue has found strong, often overwhelming evidence that the climate is changing and that release of greenhouse gases driven by human activity is the cause.

Global warming absolutely is happening, and every argument to the contrary has been debunked thoroughly by scientists.

Lest people suspect that scientists have conflicts of interest due to grant money or something like that, their grants are awarded for research and publication expenses. They live off of their salaries (usually from research labs or universities), which are separate from grants. Occasionally, they can claim summer salaries in grants, but that's a small amount of money. Further, if all they cared about was getting grants, they'd likely get more if their findings were contrary to the consensus on the issue.

Global warming is definitely happening. We can stop it, and we need to stop enabling dirty businesses that are trying to obscure the facts and actually act to stop this disaster.

Whether or not you believe this "global warming"

2 things:

1.) Better to ere on the side of caution, no?

2.) A lot of the cleaning up our act that needs to be done to halt global warming will have other substantial benefits: create jobs, cement cooperation globally, improve the health of every living being on Earth.

Probably not in our lifetimes

Nor in the lifetime of the United States, most likely. It's not so much a giant earthquake we have to look forward to, but a colossal volcanic eruption. Yellowstone is a caldera, a volcanic system that periodically forms a huge magma chamber, empties it via volcanism, and collapses in on itself. The last major caldera eruption, that of Toba 75,000 years ago (also in Indonesia, btw) caused a volcanic winter and drove the human population down to less than 20,000 people--nearly to extinction.

Yellowstone has earthquake swarms every once in a while that bring a lot of media attention (because of Yellowstone's explosive prehistory) but cause essentially no damage and signify little. Given the long waits between eruptions, I think it's likely that civilization will either die off or engineer a way to mitigate its eruptions before Yellowstone erupts again.