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.