Movement Occurred Throughout the Continent
The massive magnitude 8.8 earthquake that struck the west coast of
Chile last month moved the entire city of Concepcion at least 10 feet to the west,
and shifted other parts of South America as far apart as the Falkland Islands and
Fortaleza, Brazil.
These preliminary measurements, produced from data gathered by researchers from four
universities and several agencies, including geophysicists on the ground in Chile,
paint a much clearer picture of the power behind this temblor, believed to be the
fifth-most-powerful since instruments have been available to measure seismic shifts.
Buenos Aires, the capital of Argentina and across the continent from the quake's
epicenter, moved about 1 inch to the west. And Chile's capital, Santiago, moved
about 11 inches to the west-southwest. The cities of Valparaiso and Mendoza, Argentina,
northeast of Concepcion, also moved significantly.
Plate Tectonics of the 8.8 Earthquake
The quake's epicenter was in a region of South America that's part of the so-called
"ring of fire," an area of major seismic stresses which encircles the Pacific Ocean.
All along this line, the tectonic plates on which the continents move press against
each other at fault zones.
The February Chilean quake occurred where the Nazca tectonic plate was squeezed under,
or "subducted," below the adjacent South American plate. Quakes routinely relieve
pent-up geologic pressures in these convergence zones.
Movement Calculated from GPS Data
The research team deduced the cities' movement by comparing precise GPS (global positioning
satellite) locations known prior to the major quake to those almost 10 days later. The US
Geological Survey reported that there have been dozens of aftershocks, many exceeding
magnitude 6.0 or greater, since the initial event February 27.
Mike Bevis, professor of earth sciences at Ohio State University, has led a project since
1993 that has been measuring crustal motion and deformation in the Central and Southern Andes.
The effort, called the Central and Southern Andes GPS Project, or CAP, hopes to perhaps triple
its current network of 25 GPS stations spread across the region.
Plate Motion Occurs in "Jumps" and Adjustments
"By reoccupying the existing GPS stations, CAP can determine the displacements, or 'jumps',
that occurred during the earthquake," Bevis said.
"By building new stations, the project can
monitor the postseismic deformations that are expected to occur for many years, giving us new
insights into the physics of the earthquake process."
Ben Brooks, an associate researcher with the School of Ocean and Earth Science and Technology
at the University of Hawaii and co-principal investigator on the project, said that the event,
tragic as it was, offers a unique opportunity to better understand the seismic processes that
control earthquakes.
Importance of the Maule Earthquake
"The Maule earthquake will arguably become one of the, if not the most important great
earthquake yet studied. We now have modern, precise instruments to evaluate this event, and
because the site abuts a continent, we will be able to obtain dense spatial sampling of the
changes it caused."
"As such the event represents an unprecedented opportunity for the earth science community if
certain observations are made with quickly and comprehensively," Brooks said.
Research Team and Contributors
Working with Bevis and Brooks on the project are Bob Smalley, the University of Memphis, who
is leading field operations in Argentina; Dana Caccamise at Ohio State, who is lead engineer,
and Eric Kendrick, also from Ohio State, who is with Bevis now in Chile making measurements
in the field.
Along with Ohio State University and the University of Hawaii, scientists from the University
of Memphis and the California Institute of Technology are participating in the project.
Additionally the Instituto Geografica Militar, the Universidad de Concepcion and the Centro de
Estudios Cientificos, all in Chile, also were partners.
In Argentina. the Instituto Geografica Militar, the Universidad Nacional de Cuyo in Mendoza and
the Unversidad Nacional de Buenos Aires are collaborating in the work. UNAVCO, a consortium of
more than 50 institutions and agencies involved in research in the geosciences, is providing
equipment for the project.
The researchers have constructed a map showing the relative movement of locations after the
Maule, Chile earthquake. The images at right are derived from their map.
Contacts: Mike Bevis (614) 247-5071 and Ben Brooks (808) 228-8356
Written by Earle Holland, (614) 292-8384; holland.8@osu.edu
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Preliminary Coseismic Displacement Field for the M 8.8 Maule Earthquake, Chile, Feb 27 2010
This is the preliminary solution obtained by Project CAP (Central and Southern Andes GPS Project)
for the coseismic displacement field associated with the recent M 8.8 Maule earthquake in
south-central Chile. Peak measured displacement is 3.04 m near the city of Concepcion, Chile.
Significant displacements are evident as far east as Buenos Aires, Argentina (2-4 cm) and as
far north as the Chilean border with Peru.
The areas with the largest expected displacements are not yet re-surveyed, but surveyors are
already occupying several marks in that and surrounding areas, so this initial result will soon
be updated to show the movements at additional locations. Continuous GPS stations are also being
set up as equipment becomes available.
The CAP team led by Mike Bevis (Ohio State University, University of Mempis, University of Hawaii,
the Instituto Geografico Militar (IGM) de Chile, Universidad de Concepcion (Ch.), the Centro de
Estudios Cientificos (CECS, Ch.), IGM de Argentina, Universidad Nacional de Cuyo, Argentina and
Universidad Nacional de Buenos Aires, Argentina) are actively collaborating with Caltech geodesists
(Jeff Genrich and Mark Simons), and are coordinating with the Sergio Barrientos (Universidad de
Chile) and their French partners (ENS/IRD), so as to maximize the amount of geodetic data being
collected. The American team is being supported by UNAVCO, who are sending GPS equipment to supplement
that which CAP and Caltech already had in Chile and Argentina. All these data will be placed in open
archives at Universidad de Chile and at UNAVCO, as soon as the data are recovered.
IGM Chile and Mike Bevis (OSU) are leading field activities in Chile including Geodesists Dana
Caccamise and Eric Kendrick, Bob Smalley (UM) is leading the work in Argentina, and Ben Brooks
(U Hawaii) and Mark Simons (Cal Tech) are coordinating processing and logistical support stateside.
This preliminary geodetic solution was computed by James Foster and Ben Brooks at the University of Hawaii. Enlarge Image |
Processing Description:
All available data for continuous GPS in South America from Jan 30 through Mar 5 were processed
using GAMIT (King et al., 2009) with additional IGS sites included to provide reference frame stability.
Data through Feb 28 were processed using the MIT precise orbits. More recent data used the IGS rapid
orbit solutions. Orbits were held tightly constrained and standard EOP and earth and ocean tides were
applied. Due to the number of stations, two separate subnets were formed with common fiducial sites.
The subnets were merged, and, through Feb 28, combined with MIT's global solution using GLOBK.
Displacements for the Maule earthquake were calculated by performing daily Helmert transformations for
the network solutions prior to the earthquake to define the best pre-quake position for each site, in
an ITRF2005 reference frame. This process was repeated for the post-earthquake solutions (data from the
day of the earthquake had the first 8 hours (earthquake was at 06:34 UTC). The displacements were
calculated by taking the difference between the pre- and post- earthquake positions and performing a
Helmert transformation that minimized the displacements for a subset of sites distant from the earthquake.
Errors for the positions were calculated based on the scatter of the solutions about the mean position,
while the displacement errors were calculated by assuming that the errors in the pre- and post-earthquake
positions were independent.
This preliminary geodetic solution was computed by James Foster and Ben Brooks at the University of Hawaii. Enlarge Image |
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