Shaken to the core: mid-continental earthquakes can be even more damaging than the ones at the boundaries of tectonic plates. The great Indian earthquake of 2001 is a benchmark for geologists seeking to understand how they happen

In the westernmost corner of India, south of a huge salt marsh known as the Rann of Kachchh, lies the old walled city of Bhuj, administrative headquarters of the district of Kachchh. To a Westerner, even a traveler equipped with a Lonely Planet guidebook, the area seems remote. A single train arrives in Bhuj each day; rarely does a tourist disembark. Yet, like most of India, the city and surrounding region are densely populated and rich in architectural heritage.

Two years ago these human and architectural riches became, in an instant, the preconditions for tragedy. On January 26, 2001, a magnitude 7.6 earthquake struck India about forty miles northwest of Bhuj. The quake was the first major temblor to take place on land, and away from a tectonic plate boundary, since the invention of the modern seismometer in the 1880s. It killed more than 18,000 people and wrecked hundreds of thousands of buildings across the Indian state of Gujarat.

Less than a year and a half later a much smaller, magnitude 5.1 mid-continental earthquake hit New York State just south of the Canadian border. This one, the Au Sable Forks quake of April 2002, did little damage, but its unfamiliar rumblings were felt all over the northeastern United States. A temblor that size in Los Angeles would barely be noticed in earthquake-jaded San Diego, a hundred miles away.

Gujarat and New York are on virtually opposite sides of the globe, but in geological terms they have much in common. Unlike California or Alaska, neither Gujarat nor New York is situated atop a seismically active boundary between the Earth's great tectonic plates--rigid blocks of the planet's brittle outer layer that can measure thousands of miles across but are often less than a hundred miles thick. Given such a substantial platform, one would not expect either state to undergo many earthquakes--certainly not as many as take place in the regions' situated above active plate boundaries.

But the earthquakes that do strike places like Gujarat and New York have a particularly long reach. In regions along plate boundaries--coastal California and Alaska, but also the Himalaya and the Andes--the crust is a jumble of once-distinct terrains. Over geologic time, as tectonic plates have ground past each other, rocks have been cracked, ruptured, and folded, thereby mixing and mangling ancient terrains. Earthquake waves cannot propagate efficiently in such a complex, fractured setting. In regions that lie within plates, however, the underlying crust is older, less fractured, and less complicated, and the waves reverberate over much greater distances from an earthquake's epicenter.

The Bhuj temblor strongly shook the ground as far as 300 miles from its epicenter in the district of Kachchh. High-rises toppled in Ahmadabad, a large industrial city almost 200 miles away. In the hardest-hit towns and villages, people and buildings alike were thrown down violently. Writing to BBC News Online, a resident of one devastated town described the scene as she and her father stood on the balcony of her apartment block, watching everything around them shaking and crumbling. "We were in the jaws of death waiting for it to gulp us," she wrote. "Any small jerk could have caused the building to collapse."

Bhuj was truly a shock heard round the world, a wake-up call both for its horrific immediate effects and for its even more frightening implications. Earthquake damage reflects not only the magnitude of a main shock but also a region's population density and the vulnerability of its buildings, roads, and other structures. As recently as October 2002 a magnitude 5.9 earthquake in the southern Italian village of San Giuliano di Puglia took twenty-nine lives; three days later a magnitude 7.9 quake in Alaska claimed none.

It is sobering to contemplate what might take place when the next major earthquake hits the Himalayan region. The combination of potentially great magnitude, efficient earthquake wave propagation, crowded cities, and fragile buildings Could threaten the lives of tens of millions of people. For comparison, consider the effects of the magnitude 7.8 shock that took place in the industrial city of Tangshan, China, in 1976. Tangshan has the unhappy distinction of being the only large city to have suffered a direct hit from a major earthquake in the past hundred years; the death toll there may have been as high as 750,000, though the official tally is closer to 250,000. A future large Himalayan earthquake could cause an immediate six-figure death toll in India and Pakistan--not to speak of the likelihood of widespread epidemics, the obliteration of costly and essential infrastructure, and the possible release of radiation from nuclear power plants.

The Bhuj earthquake offers lessons for other regions as well. Modern seismic monitoring and analysis have established it as the standard against which to compare anecdotal descriptions of other large mid-continental quakes in the historical record. Among the most important such earthquakes, at least for North Americans, were three powerful shocks that struck the southeastern corner of Missouri, near the town of New Madrid, in 1811 and 1812. The New Madrid earthquakes were strong enough to temporarily reverse the course of the Mississippi River and cause damage as far away as coastal South Carolina [see "The Aftershocks That Weren't," by Susan Elizabeth Hough, March 2001].