Big Plates, Small Steps: Earthquakes and What We Can Do

Unknown | unknown.ysm@gmail.com September 1, 2010
 
 Death From Below

At 4:53 PM on January 12, 2010, Port-au-Prince, the capital of Haiti, began to shake. Within minutes, much of the city had collapsed under the force of the tremors. The event eventually left over 110,000 people dead, nearly 200,000 more injured, and 1 million homeless. Despite hundreds of millions of dollars in aid from governments and private donors, Haiti will need years, perhaps decades, to recover from the loss of lives and infrastructure. Less than two months later, on the morning of February 27, 2010, a larger magnitude 8.8 earthquake, struck the coast of Chile. The magnitude of destruction that these earthquakes caused reminded observers around the world that despite major advances in seismology in the past century, humans are still wholly vulnerable to earthquakes. Technology has yet to defeat natural disasters.

The earthquake that devastated Port-au-Prince was a magnitude 7.0 earthquake, releasing an amount of energy comparable to that released by a dozen hydrogen bombs. Strikingly, the Haiti earthquake was only moderately powerful compared to the magnitude 7.9 earthquake in 2008 that devastated Sichuan Province, China, and the magnitude 9.1 earthquake in 2004 that triggered a deadly tsunami across the Indian Ocean. These magnitudes are measured on a logarithmic scale; every step of 1.0 in magnitude represents a 31.6-fold increase in the energy released by the earthquake. Therefore, the Sichuan earthquake released more than 20 times the energy released by the Haiti earthquake, and the Indian Ocean earthquake released over 1400 times as much. Given these staggering numbers, we are forced to face the reality of the raw power of earthquakes. But how do they arise?

Causes of Earthquakes

The processes that trigger such enormous releases of energy involve huge masses of the Earth’s lithosphere, which extends down to about 150 km below the Earth’s surface. Dr. Jeffrey Park, professor of geology and geophysics at Yale University, cited several situations that can trigger earthquakes. Most earthquakes are caused when masses of hot material in the lower reaches of the mantle rise by convection, releasing heat into the lithosphere. As these masses cool, they sink back down, carrying some of the lithosphere with them in a process called subduction. Subduction usually occurs with oceanic lithosphere, since the lithosphere under continents is lighter and less prone to sinking. The stresses generated by subduction and other movements of the lithosphere build up at fault lines, boundaries between the tectonic plates that make up the lithosphere. When these plates suddenly move, they trigger an earthquake.

Earthquakes can also be attributed to other processes that stress brittle parts of the crust. For example, after the last ice age, huge ice sheets that had covered much of North America receded. This removal of weight caused the lithosphere to rebound upwards, a process that is still underway and still has the potential to cause earthquakes in unexpected places, such as Canada and the east coast of the United States, which do not lie on fault lines. These types of earthquakes, though, are relatively rare and usually do not approach the magnitudes of quakes that regularly occur on fault lines. Whether on a fault line or elsewhere, earthquakes are caused by processes involving huge volumes of lithosphere and mantle. This does not mean, however, that humans are powerless to prevent or mitigate the dangers that these tremors pose.

Preventing Earthquake Damage

Especially in the past century, engineers have developed methods of construction that can significantly reduce earthquake damage. The United States has invested a significant amount of money in earthquake-proof construction and the effects are evident. Earthquakes in the United States usually cause significantly fewer deaths than earthquakes of similar magnitudes in other regions of the world. For the recent Chilean earthquake, the benefits of earthquake-proof construction were again made clear; the Chile tremor caused far fewer deaths despite being 500 times as powerful.

In order to withstand an earthquake, a structure must be able to handle all types of forces. Much attention is paid to the types of materials used in building this infrastructure. The hardness of the materials used is not as crucial as the flexibility. Indeed, wood is far superior to brick in terms of resilience under earthquake conditions. Brick has low tensional strength and breaks easily when stretched, while wood can deform significantly in all directions before it breaks. Dr. Park describes cases of wooden houses in which “the house will stand but the chimney will fall” during an earthquake. Sadly, in tropical countries such as Haiti, wood construction has its own problems; wood splinters easily under the forces generated by hurricanes. Only more expensive construction methods, such as reinforcing brick buildings with metal wires, can yield good resistance to both earthquakes and hurricanes.

Through similar applications of more flexible materials, roads, power lines, and pipelines can also be made more resistant to earthquakes, though these longer structures can still experience significant damage. The added resistance, however, can sometimes make the difference between needing to repair a water main and being forced to replace it entirely. For example, the Alaska oil pipeline is engineered to earthquake-resistant standards; after a magnitude 8 earthquake a few decades ago, it required only a few repairs.

Looking Ahead for Earthquakes

Besides building with earthquakes in mind, predicting their onset is a useful way to prevent damage and especially loss of life. If seismologists could somehow accurately predict an earthquake, even a few hours before it was due to occur, thousands of lives each year would be saved. Unfortunately, there are currently no methods that can yield such forewarnings. Though impending earthquakes often generate smaller tremors called pre-shocks, these are rarely consistent enough to alert seismologists to the larger quake that they herald. Usually, the best seismologists can do is forecast the probability of an earthquake occurring in a given region within a certain time span. Earthquake prediction methods, which aim to determine the time when the “next big one” will occur in any given area, are far less precise; according to Dr. Park, “We don’t understand enough about how earthquakes start” to determine exactly when and where they will occur.

Seismology is an active science, though. Recently-introduced measurement techniques and new conceptual advances may provide seismologists with more opportunities to reliably forewarn governments of earthquakes. One of the more promising techniques involves the measurement of small movements of the earth with geodetic tools: GPS markers and space-based radar interferometry. Knowledge of these movements can yield predictions of stress transfer around fault lines, which can provide estimates of which parts of a fault are most likely to slip and trigger an earthquake. This information can gain more power if it is combined with the results of new research into the ways that stress can initiate earthquakes. Currently, nobody is completely sure why faults tend to generate intermittent large earthquakes instead of releasing all their stress in a series of small tremors. When this answer is determined, seismologists may actually be able to give people the few hours’ warning that they need to evacuate before an earthquake begins.

Illuminating the Planet

As geologists work towards predicting earthquakes and minimizing the damage they cause, they are also finding valuable applications of the shockwaves that earthquakes can generate. These shockwaves spread through the Earth’s mantle and core, and geologists can analyze how these layers reflect and bend the waves in order to learn more about the Earth’s internal structure. Recently, such an analysis revealed that the Earth’s inner core is rotating slightly faster than other layers, a discovery that would have been impossible without the “illumination” of the inner core by earthquakes. If this is the only silver lining that we can find as we sift through the ruins left by recent earthquakes, then so be it.