Seth Stein, students, and coworkers have studied how global plate motions over geologic time (millions of years) compare with those over a few years measured from space geodesy, and explored these data's implications for the evolution of the continents. This was addressed by programs using the Global Positioning System (GPS) satellites to study the New Madrid seismic zone and Andes. These studies are among the many now using the Global Positioning System satellites and other space-based techniques to measure positions on the earth far more precisely than would have been dreamed possible not long ago. Before this, geologists could only study the motion of the great plates of Earth's lithosphere (which move at speeds of a few inches per year, about the speed fingernails grow) over periods of millions of years, long enough for large motions to accumulate. Now, however, very precise positioning makes it possible to study plate motions over a period of years.
Working with former students A. Newman (now at the Los Alamos National Laboratory), J. Weber (now at Grand Valley State University), J. Engeln (now Assistant Director of the Missouri Department of Natural Resources), and T. Dixon from the University of Miami Geodesy Lab, we conducted a multi-institutional GPS program to quantify the rate and distribution of strain accumulation in the New Madrid, Missouri, seismic zone (NMSZ), where great earthquakes occurred in 1811-1812. This area provides a type example of large earthquakes within a relatively stable continental interior. Assessing the scale and causes of deformation in such regions, as well as estimation of the recurrence interval for the large earthquakes within them, has long been recognized as an important but challenging issue, for which the advent of space-based geodesy provides a powerful tool.
Using GPS, the positions of a network of geodetic markers in Missouri, Tennessee, Illinois, Arkansas, and Kentucky have been measured to accuracies of better than an inch since 1991. Data were collected with the help of many NU, Missouri, and GVSU students, and technical support by UNAVCO, a national consortium of universities using the Global Positioning System for geological research. The GPS results show little or no motion across the seismic zone, implying that the seismic hazard of a great New Madrid earthquake may have been greatly overestimated.
This result is consistent with an analysis of GPS platewide data, conducted with T. Dixon and A. Mao (University of Miami Geodesy Lab). These data show that the plate is stable to better than 2 mm/yr, in that it can be described by a single Euler vector, and find insignificant motion across the NMSZ.
Data from the Andes program termed SNAPP (South America - Nazca Plate motion Project), a joint project with graduate student L. Lefffler, and E. Norabuena and T. Dixon (University of Miami Geodesy Lab) , and others were reduced. The data provide a profile of relative plate motion across the Nazca-South America plate boundary zone through the Central Andes derived by a combination of space geodetic techniques. The profile extends from the stable interior of the oceanic Nazca plate, across the Peru-Chile trench to the coastal forearc, across the high Altiplano and foreland thrust belt, and into the stable interior of the South American continent. Space geodetic data directly measure rates and directions of motion in different portions of the plate boundary zone, which could previously only be estimated indirectly. The data indicate that about 30-40 mm/yr of slip, roughly half of the overall convergence rate, is accumulating on the locked plate boundary thrust fault and should be released in future great earthquakes. This estimate avoids some of the difficulties inherent in previous aseismic slip estimates based on the earthquake history. We also estimated that about 10-15 mm/yr of crustal shortening occurs inland at the sub-Andean foreland fold and thrust belt, indicating that the Andes are continuing to build. We observed little (5-10 mm/yr) along-trench motion of coastal forearc slivers, despite the oblique convergence geometry. The results illustrate the value of space geodesy for investigating ocean-continent convergence and continental mountain building. They are shown on the figure above, with topography of the central Andes (mountains shown in brown) and nearby regions, GPS site (triangles), and their velocities relative to stable South America. Arrows show the direction the site moves, and the ellipses show the uncertainty associated with each measurement. The motion of the sites toward the stable interior of the South American continent demonstrate the accumulation of strain that will be released in large earthquakes, and the permanent deformation that builds the Andes.
In addition, a recent analysis with T. Seno (University of Tokyo) of the long-standing question of the plate geometry in Northeast Asia shows that the Sea of Okhotsk and northern Japanese islands are better regarded as part of the North American plate than as a separate Okhotsk plate, a result with interesting implications for understanding earthquakes along Sakhalin Island and the eastern margin of the Japan Sea.Back to top