Christmas quake presents geologic gift.

An earthquake that rocked a remote section of northern Canada on Dec. 25, 1989, has broken a new ground in the realm of geology. At a meeting in Baltimore this week, scientist described the event as the first known instances of a quake fault rupturing the land surface in the eastern half of North America.

The shock, centered in the middle of of lake-strewn Ungava peninsula (see star on map) between Hudson Bay and Ungava Bay, registered a magnitude 6 on the Richter scale. Because it occurred during winter, when snow and darkness blankett the region, geologists could not inspect the spicentral area until last summer. After several days of searching, a team from the Geological Survey of Canada in Ottawa found the unusual surface fault.

Despite its location in the uninhabited interior of the peninsula, the Ungava fault is capturing considerable attention from earth scientists. "It is something unique and will provide very valuable insights into the process of earthquakes in stable continental regions," says Robert J. Wetmiller, who led the Canadian team.

Some of the largest earthquakes in North American history have originated in the "stable" heart of the continent. Unlike temblors in active tectonic areas. such as California or the Himalayas, seismic shocks in continental interiors rarely break the surface - a trait that has hampered efforts to the define quake risk for these regions. The Ungava rupture represents one of only 10 exceptions to that rule and is the sole example in North America east of the Rockies, says geologist Thomas F. Bullard of Geomatrix Consultants in San Francisco, Bullard and Arch C. Johnston of Memphis (Tenn.) State University accompanied the Canadian team and discussed the quake this week at a regional conference of the Geological Society of America.

Johnson, a specialist in midplate earthquakes, contacted the Canadian researchers soon after the Ungava tremor to suggest looking for a visible rupture. He suspected surface faulting because the shock was large and shallow - two traits shared by the other ground-breaking quakes in midcontinents.

The investigators located the fault after spotting unusual turquoise water in two of the thousands of slate-blue lakes near the shock's epicenter. The earthquake had lifted the shorelines of these lakes, causing sediments to alter the water's color. The surface fault measured 8 kilometers long, running in a northeast direction. The southeast side of the fault had slipped up over the northwest side during the quake.

The fault direction raises difficult questions for experts in plate tectonics, says Randall Richardson of the University of Arizona in Tucson. Scientists have assumed that tectonic forces squeeze the Ungava peninsula in a northeast-southwest direction and that this stress arises primarily from seafloor spreading in the Atlantic. But the recently discovered fault challenges the idea of northeast-southwest stress, because such forces are oriented in the wrong direction to trigger the observed rupture. The quake, says Richardson, "is making us reevaluate what we think is going on there."

Researchers have virtually no information about subsurface stress in northeastern Canada, and the quake raises the possibility that they have miscalculated the direction of forces throughout the region. Alternatively, the Ungava site may represent a unique place where local features have redirected the stress, Richardson says.

In the past, scientist haven't had to consider seriously the chance of earthquakes breaking the ground surface east of the Rockies. "Seismic-hazard analysts must now allow for the possibility," says Gilbert A. Bollinger of Virginia Polytechnic Institute & State University in Blacksburg. He notes that faulting could seriously damage a nuclear power plant or pierce a hazardous-waste site, allowing dangerous chemicals to enter underground water systems.