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The Global Seismic Hazard Map

The UN has put the world's earthquake risk on one chart

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In a triumph of science in the political setting, the whole world has been mapped consistently in terms of its earthquake hazard. The resulting map is reproduced here on this site.

Challenges of a World Hazard Map

The map was made as part of GSHAP (Global Seismic Hazard Assessment Project) by some 500 scientists over seven years, and their challenges were great.

First, the state of science differs around the world. Earthquake monitoring is advanced in prosperous countries, while the Russian system is in a state of decay. Much of Africa had never been properly assessed at all. And scientists in different countries use different standards. So when this project, part of the United Nations International Decade of Natural Disaster Reduction, arrived at a single standard of hazard and a clean worldwide database, it was quite an achievement.

Second, seismic hazard can be sensitive knowledge. Prosperous nations make builders meet building codes that are based on the degree of earthquake shaking a site can expect. Poor nations that want international loans for their important projects, like dams, pipelines or airports, need sound data on seismic risks. The safer you can make a place appear, the cheaper it is to build there. Many countries therefore consider this subject part of their national interest. But that obstacle, too, was overcome.

Third, simply discussing a region's geology can be difficult when the wrong border crosses it. Geologists don't care about borders, but their governments do. India and China, for example, are longtime enemies that took three years to meet and adjust their hazard map—as a result, India raised its hazard levels by as much as 40 percent in some places. Greece and Turkey surmounted similar hurdles for hazard zoning in the Adriatic Sea.

What the Map Shows

So what's on the map? Seismic hazard is basically the degree of earthquake shaking that you can expect in a given place during a given time span. To simplify things, the mappers assume that your building is on solid rock and that you're interested in relatively fast shaking (0.2 second period), a frequency that strongly affects ordinary houses. Different ground and different periods are important in determining the exact seismic risk for a particular structure.

The map shows the peak ground acceleration (PGA) that a site can expect during the next 50 years with 10 percent probability. The color scale goes from very low (white) to about half the acceleration of gravity (red-brown)—remember, this is acceleration sideways. A structure of modern design and sound construction can survive that degree of shaking without killing the people in it, although it may well need to be torn down afterward.

How the Map Is Useful

This map makes a difference for everyone. For instance, Africa was not well mapped in the geologically active Rift Valley region on its east side. Making this map required new research there. Now many East African nations have their very first official seismic hazard map, and they can begin useful planning for preparedness, for mitigation, and for public education. They can begin saving lives.

Australia has a low to moderate seismic hazard level, but that is still higher than previously thought. But now Australian planners can compare their building standards to other areas of similar geology and hazard level, such as the eastern Canada, or western Europe, or parts of China.

India has higher but more accurate hazard levels along the Himalaya, thanks to its meeting with Chinese scientists. Now the nation is taking a fresh look at its infrastructure in light of this information, seeing what must be reevaluated before the public can be assured of its safety.

The seismic hazard map for the United States does not change—in fact, American knowledge of seismic hazard far exceeds what is on the new global map—but this country still benefits. America has examples of every kind of tectonic terrain, from hard continental rocks to volcanic areas to soft sedimentary basins, and these can be compared to similar areas in other countries. Many of those places have earthquake records that go back thousands of years, and American scientists can learn from that data.

The new map was released December 16, 1999 and is available from the Global Seismic Hazard Assessment Program website in Switzerland along with much supporting material and reports. The compressed file expands to a large (77MB) Adobe Illustrator 8.0 vector document. But I've digested it into many smaller GIF maps that show all the important parts of the globe. Make use of this historic achievement!

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