SAN FRANCISCO (Reuters) - Killer earthquakes may have caused the collapse of numerous civilizations through human history, scientists said on Thursday, offering fresh theories for the sudden disappearance of urban cultures ranging from ancient Troy to the Maya of Central America.

``The key question is: why were these places physically destroyed?'' said Prof. Amos Nur, a professor of geophysics at Stanford University. ``We believe that natural disasters, and especially earthquakes, played a major role.''

Nur was one of a number of scientists briefing this week's meeting of the American Geophysical Union on
the growing links between earth science and archeology, contrasting seismic and human historical records in an effort to shed new light on some of the great mysteries of the past.

Nur developed his theory looking at the end of the Bronze Age in the eastern Mediterranean, where cities
including Troy, Mycenae and Knossos were literally wiped from the map around 1200 B.C.

While earlier theories had suggested that a mysterious group of pirates known as the ``Sea People'' caused the destruction, pillaging the cities in a ravenous sweep through the area, Nur has suggested that an ``earthquake storm'' may actually be responsible for the disappearance of so many large urban centers
between 1225 and 1175 B.C.

Other scientists briefing the AGU have taken the theory further, suggesting that historical earthquakes may have caused the collapse of other civilizations ranging from the Harappan of India's Indus River valley in 1900 B.C. to the Mayan Classic Period in central America in the 9th Century A.D.

``A lot of information is out there, it's just a matter of piecing it together,'' said Manika Prasad, a research associate at Stanford's Rock Physics Laboratory who is helping Nur to study the disappearance of the Harappan.

The Harappan, after thriving in the Indus River area for some 2,000 years, disappeared around 1900 B.C. -- a vanishing act some scholars have blamed on everything from changing trade patterns to Aryan invaders from the North.

Nur and Prasad, however, looked at the region's seismic history -- and noted that numerous catastrophic earthquakes have periodically struck the coastal region near the border between India and Pakistan.

According to their theory, one or more such seismic events could have moved enough earth to effectively
dam one of the area's major rivers, disrupting agriculture and leading to floods which eventually covered some of their cities in silt.

Stanford geophysicist Robert Kovach is applying the same theory to the Mayan Civilization of Central America, one phase of which ended abruptly when the cities of Quirigua and Benque Viejo were suddenly abandoned in the late 9th Century.

Kovach's research postulates that both cities -- cultural centers of the Mayan Classic Period -- could have been leveled by a single earthquake on the Chixoy-Polochic and Motagua fault zones.

GODZILLA ATTACKS BABYLON

Nur conceded that the earthquake hypothesis has its critics, at least one of whom has dubbed it the ``Godzilla Attacks Babylon'' theory of human cultural decline.

Major problems in identifying specific faults responsible for past earthquakes, as well as more theoretical challenges to the notion that a brief disaster could wipe out a thriving civilization, have all raised as reasons to doubt the theory.

But Nur said that growing seismic evidence, when coupled with the human historical record, may eventually prove that earthquakes have been major factors in numerous incidents of cultural collapse.

``What really gets destroyed by these earthquakes are the monumental structures, it's not the farmers in the field,'' Nur said -- suggesting that small ruling elites in strictly hierarchical civilizations may find themselves defenseless in the chaos following a major quake. ``The most damage is done to major power
centers,'' he said.

``The evidence is all patchy. It's the ultimate detective work,'' Nur said. ``But when you have a really big earthquake, and weakness in the center of society, then the quake can be the trigger -- it can be the thing that pushes civilization over the edge.''