Until recently, most researchers trying to explain these enigmatic disasters pointed to wars, fires or flash floods -- or simply shrugged their shoulders and kept digging.

But new research by geophysicists at Stanford and elsewhere is painting a picture of an ancient world in which earthquakes destroyed fortified buildings, changed the course of rivers and made elite rulers vulnerable to attack.

One of the proponents of that picture is Amos Nur, the Wayne Loel Professor of Earth Sciences at Stanford, who moderated a session on the impact of natural disasters on ancient civilizations at the fall meeting of the American Geophysical Union on Dec. 14. Robert Kovach, a professor of geophysics at Stanford, co-moderated the session.

Ancient quakes
The idea that ancient civilizations were shaped by earthquakes is still controversial, but a growing number of archaeologists and geophysicists believe that earthquakes might have intervened at crucial moments in history.

Nur's research on the ancient city of Megiddo, also known as Armageddon, provides one example. By studying ancient texts and archaeological evidence, Nur demonstrated that earthquakes, and not repeated conquests, could have been responsible for the city's sandwich-like layers of ruined buildings.

Other research suggests that earthquakes -- caused when fault lines release built-up tension -- could have done more than just level cities; they may have brought down civilizations as well. According to Nur, storms of earthquakes raging over periods of 50 to 100 years might have helped bring the Bronze Age to an end.

In hierarchical societies where wealth was concentrated in the hands of a few well-fortified elites, says Nur, earthquakes that toppled columns and cracked walls could have instantly changed the balance of power. That would have been especially true during times of war or revolt.

The Harappan enigma
Manika Prasad, a research associate in the Rock Physics Laboratory at Stanford, has helped expand the study of ancient earthquakes beyond the Eastern Mediterranean. Together with Nur, Prasad is studying the contribution of earthquakes to the collapse of the Harappan civilization in South Asia.

The Harappan civilization mysteriously disappeared in 1900 BC, after almost 2,000 years of continuous existence. Some researchers have argued that the civilization slowly declined because of changing trade patterns; others, now mostly discredited, blamed Aryan invaders from the north.

Prasad and Nur blame earthquakes. Last January, a catastrophic earthquake struck the southern edge of the former Harappan territory, a coastal area near the border between India and Pakistan. In 1819, a similar earthquake raised an 80 to 100-kilometer (50 to 62 mile) ridge of earth about 20 feet (6 meters), creating an artificial dam known as the "Allah Bund" (God's Dam). Both earthquakes are evidence that the Harappan region, though not near a traditional fault zone, is seismically active.

The evidence of seismic activity in the region, combined with the recent discovery of an ancient riverbed at the center of the former Harappan region, offers a possible explanation for the civilization's decline.

Four millennia ago, one or more quakes could have blocked or diverted the water that flowed through the riverbed, say Prasad and Nur. That would have helped turn the part of the Harappan region into the desert it is today -- and destroyed the Harappan civilization in the process.

The collapse of the Mayan Classic Period
While Nur has focused on the Mediterranean and, more recently, Harappan civilizations, Stanford Professor of Geophysics Robert Kovach has been studying the role of earthquakes in the Mayan civilization of Central America.

When the Mayan Classic Period ended in the late 9th century A.D., the cities of Quirigua and Benque Viejo (Xunantunich), now located in Guatemala and Belize, were suddenly abandoned. According to Kovach, the cities could have been destroyed by a single earthquake centered on the Chixoy-Polochic and Motagua fault zones.

But evidence for earthquake damage in Mesoamerica is harder to come by than in the Mediterranean. Documents that can be mined for hints about ancient catastrophes are rare, and tropical vegetation quickly overwhelms any archaeological evidence that might point to a major temblor.

With the help of Bernabe Garcia, a former graduate student at the Stanford Center for Latin American Studies, Kovach was able to piece together evidence of past quakes by looking at the effects of recent quakes on structures at archaeological digs. The evidence suggested that an earthquake struck the Mesoamerican region in the late 9th century, just as the Mayan Classic Period was collapsing. Like Nur, Kovach thinks the Mayans' rigidly hierarchical society could have made the quake especially damaging.

Other research
Despite growing evidence of the effects of ancient earthquakes, some researchers remain skeptical. Iain Stewart, a geologist at Brunel University in England, will argue at Friday's AGU session that earthquake damage is hard to distinguish from the effects of poor construction, ground instability or human intervention.

Other researchers, however, have warmed to the idea that earthquakes may be a missing piece of the archaeological puzzle. At the AGU session, Italian geologist Luigi Piccardi will present research linking earthquakes to ancient Mediterranean sanctuaries such as the temple at Delphi. Erhan Altunel of Osmangazi University in Turkey will propose that massive earthquakes may have helped destroy several ancient Turkish cities. And Italian researcher Emanuela Guidoboni will present a case study of two ancient earthquakes that led to the collapse of temples in Sicily.

According to Prasad, the spread of geographical information systems technology, which allows different types of data to be integrated into a single map, should make the study of ancient earthquakes increasingly easy.

"A lot of information is out there," she says. "It's just a matter of piecing it together."

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