Areas: Space Geodesy
1. Aqueous Geochemistry
2. Environmental and Theoretical
3. Mineral Physics and Petrology
5. Paleoclimatology and Paleoceanography
6. Planetary Science
7. Sedimentology and Stratigraphy
9. Space Geodesy
10. Tectonics and Structural Geology
and coworkers are engaged in efforts to understand how
global plate motions over geologic time (millions of
years) compare with those over a few years measured
from space geodesy, and exploring these data's implications
for the evolution of the continents. This is being 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 a few
years 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 are conducting 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 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 an important
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 (shown below)
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 are now being
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 estimate 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 observe 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 below, 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.