Magazine article Oceanus

Learning to Live with Earthquakes

Magazine article Oceanus

Learning to Live with Earthquakes

Article excerpt

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When I was a boy growing up in China, a 7.8-magnitude earthquake near the city of Tangshan killed more than 242,000 people and severely injured 170,000 more. More than 7,000 families perished entirely. It was the deadliest quake of the 20th century.

The quake left an imprint on my generation that is perhaps as profound and indelible as Sept. 11 is on today's generation. It encouraged me to become a geophysicist and to seek to understand the fundamental physics of earthquakes.

My research has focused on many seismically vulnerable areas of the world, most recently including Haiti and Chile, and we have come to learn some simple lessons:

* Poorly constructed buildings are the primary cause of deaths from earthquakes.

* People who live in areas of high seismic hazard must be educated about earthquake preparedness.

While the earthquake in Chile on Feb. 27, 2010, released 500 times more energy than the previous month's quake near Port-au-Prince, the death toll and devastation in Haiti was 200 times worse because, as a very poor nation, Haiti does not mandate building codes, and its structures are inadequately built. Furthermore, unlike in Chile, which has frequently felt earthquakes, Haiti experienced its last major quake 240 years ago. It affected long-gone ancestors but had dimmed in the memory of their descendants.

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Stress buildup

Our research, and that of other scientists, has shown that Haiti faces a precarious seismic situation. The island is cut by the Enriquillo fault, the border between two of Earth's tectonic plates. The southern part of Haiti is on the Caribbean Plate, moving generally eastward; the northern part is on the Gonave Microplate, moving generally westward.

The plates are moving at a rate of only a few millimeters a year--less than the length that the nail on your pinkie finger grows in a year. But over 240 years, those millimeters add up to meters. That builds up enormous stress along the fault, which is essentially stuck. When the stress builds up sufficiently, parts of the fault rupture suddenly to release the stress.

We know the release of a 240-year buildup of stress caused the catastrophic 7.0 earthquake that killed some 230,000 people in Haiti and destroyed much of the capital city of Port-au-Prince. But the area of stress released by the fault rupture in that quake was restricted only to a 35- to 50-kilometer-long segment of the Enriquillo fault. While the Jan. 12 earthquake alleviated stress along this segment of the fault, we have learned from our studies of this and similar faults elsewhere in the world that this quake also increased the stresses along adjacent segments of the fault. One of those segments, just to the east of the rupture zone, is even closer to Port-au-Prince than the January rupture.

Whether the next major quake on this fault segment abutting Port-au-Prince will happen in the next year or 10 years or 50 years, we cannot say for sure, but the chances of another major quake within our lifetimes is real. Such an occurrence would not be unprecedented. Historical records show us that large earthquakes often come in clusters. Two large and possibly adjacent earthquakes occurred in this part of Haiti only 19 years apart in 1751 and 1770. …

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