A scenario of major plates (EUR: Eurasia, NAM: North America; PAC: Pacific) and microplates (AMU: Amurian, OKH: Okhotsk) investigated in this work. GPS observations confirmed that Siberia to the east of the Chersky Range belongs to the North American tectonic plate. Existence of the microplates was not confirmed. The pole of relative rotation of EUR and NAM determined from GPS (blue circle) differs from the geologically determined pole (green circle), probably because of reorganization of the plate motion within the last 3 million years. The recent large Hokkaido earthquake (red star) illustrates the fact that the seismic activity is associated with plate boundaries.

About 99 percent of earthquakes occur at the faults between plates such as the San Andreas Fault in California, which marks the boundary between the Pacific and North American Plates. Geophysicists generally use concentrated belts of seismic activity to pinpoint the exact location of boundaries, but some are notoriously diffuse. The junction between Eurasian and North American Plates, which together comprise most of the northern hemisphere, is not only poorly defined seismically, but it passes through a sparsely populated, arctic region of Eastern Siberia where there is very little in the way of seismic detection equipment. As a result, the precise location of the boundary has been unverified since the theory of plate tectonics was widely accepted in the late 1960s.

By using GPS data instead of the geologic record, the researchers also hoped to discern the actual movement of the plates relative to one another. Geophysicists have for years relied on the so-called geologic plate model to calculate the motion of plates. The model uses geologic data such as anomalies in the record of the earth's magnetic field over the past three million years to estimate the present rate and direction of tectonic plates.

From 1996 to 2002, the team of scientists measured the position of more than 50 points throughout the region. By looking for very small changes in relation to one another and to the stable interior of both plates, the team was able to determine that the boundary of the Eurasian and North American Plates passes through the Cherskiy Range in Eastern Siberia. In addition, they calculated the relative rates of rotation of the Eurasian, North American and Pacific plates and found that they differ by five to seven percent from the geologic plate model.

Their research was supported by grants from the National Science Foundation, NASA's Jet Propulsion Laboratory and the Incorporated Research Institutions for Seismology.

The Lamont-Doherty Earth Observatory, a member of The Earth Institute at Columbia University, is one of the world's leading research centers examining the planet from its core to its atmosphere, across every continent and every ocean. From global climate change to earthquakes, volcanoes, environmental hazards and beyond, Observatory scientists provide the basic knowledge of Earth systems needed to inform the future health and habitability of our planet. For more information, visit www.ldeo.columbia.edu.

The Earth Institute at Columbia University is the world's leading academic center for the integrated study of Earth, its environment, and society. The Earth Institute builds upon excellence in the core disciplines --earth sciences, biological sciences, engineering sciences, social sciences and health sciences -- and stresses cross-disciplinary approaches to complex problems. Through its research training and global partnerships, it mobilizes science and technology to advance sustainable development, while placing special emphasis on the needs of the world's poor.