Eduardo Lauría

The Nazca -South America Euler vector and its rate of change

We present velocities relative to the South American plate for five GPS stations on the Nazca plate and use these measurements to estimate the modern Euler vector. We find a pole at 55.88N, 92.58W with a rotation rate of 0.60 8/Myr. Because the GPS station at Easter Island appears to be moving at approximately 6.6 mm/yr relative to the other Nazca stations, we repeat our analysis with this station excluded from the inversion to obtain a second and preferred result (called CAP10) with a pole at 61.08N, 94.48W and a rate of 0.57 8/Myr. We compare these results with published finite rotation vectors and infer that during the past 10 – 20 Myrs, the Nazca – South America rotation rate has decelerated by 0.048 – 0.06 8/Myr 2 .

Crustal motion in the zone of the 1960 Chile earthquake: Detangling earthquake-cycle deformation and forearc-sliver translation

Temporary deformation in great earthquake cycles and permanent shear deformation associated with oblique plate convergence both provide critical clues for understanding geodynamics and earthquake hazard at subduction zones. In the region affected by the Mw 9.5 great Chile earthquake of 1960, we have obtained GPS observations that provide information on both types of deformation. Our velocity solutions for the first time span the entire latitudinal range of the 1960 earthquake. The new observations revealed a pattern of opposing (roughly arc-normal) motion of coastal and inland sites, consistent with what was reported earlier for the northern part of this region. This finding supports the model of prolonged postseismic deformation as a result of viscoelastic stress relaxation in the mantle. The new observations also provide the first geodetic evidence for the dextral motion of an intravolcanic arc fault system and the consequent northward translation of a forearc sliver. The sliver motion can be modeled using a rate of 6.5 mm/a, accommodating about 75% of the margin-parallel component of Nazca–South America relative plate motion, with the rate diminishing to the north. Furthermore, the new GPS observations show a southward decrease in margin-normal velocities of the coastal area. We prefer explaining the southward decrease in terms of changes in the width or frictional properties of the megathrust seismogenic zone. Because of the much younger age of the subducting plate and warmer thermal regime in the south, the currently locked portion of the plate interface may be narrower. Using a three-dimensional viscoelastic finite element model of postseismic and interseismic deformation following the 1960 earthquake, we demonstrate that this explanation, although not unique, is consistent with the GPS observations to the first order.

The History, State, and Future of the Argentine Continuous Satellite Monitoring Network and Its Contributions to Geodesy in Latin America

Since its creation in 1998, the Argentine Continuous Satellite Monitoring Network (Red Argentina de Monitoreo Satelital Continuo [RAMSAC]) has grown to include more than 100 continuously operating Global Navigation Satellite Systems (GNSS) stations in Argentina. RAMSAC Receiver Independent Exchange Format (RINEX) data and their derived positioning products (e.g., Networked Transport of RTCM via Internet Protocol [NTRIP] streams and time series) have been used in more than 20 peer-reviewed publications studying the inter-, co-, and postseismic geodynamic evolution of the subduction interface between the South America and Nazca plates. Most of this research has focused on the deformation associated with the near-field megathrust earthquake cycle. Nevertheless, many authors have begun to include in their analyses far-field GNSS observations, which in general do not follow the elastic/viscoelastic deformation predicted by current models. We review the contribution of RAMSAC to scientific knowledge of earthquake elastic deformation and associated phenomena. We also describe the future plans for RAMSAC and the societal impact beyond geodetic and geophysical science. INTRODUCTION AND HISTORY OF RAMSAC In 1993, the Argentine Military Geographic Institute (Instituto Geográfico Militar [IGM]), now the Argentine National Geographic Institute (Instituto Geográfico Nacional [IGN]), began a campaign to acquire Global Positioning System (GPS) measurements on ∼120 benchmarks to produce Argentina’s first GPS-based geodetic reference frame (RF), the Geodetic Argentine Positions (Posiciones Geodésicas Argentinas [POSGAR]) RF. This RF, later called POSGAR94, replaced the original national local system (Campo Inchauspe 69) and was based on GPS observations obtained during field campaigns (Lauría et al., 2002) in collaboration with the National Science Foundation-funded Central Andes GPS Project (CAP). In the mid 1990s, the Jet Propulsion Laboratory at National Aeronautics and Space Administration (NASA) and the Deutsches GeoForschungsZentrum (GFZ) deployed the first three continuous GPS (CGPS) stations at the Universidad Nacional de La Plata (Buenos Aires), Universidad Nacional de Salta (Salta), and Estación Astronómica Río Grande (Tierra del Fuego) as part of the early global International GNSS Service (IGS) network. At the same time, CAP deployed stations at the seismic station Coronel Fontana (San Juan),Universidad Nacional de Tucumán (Tucumán), Parque Nacional Lihué Calel (La Pampa), and Aeropuerto de Ushuaia (Tierra del Fuego). Both the NASA/GFZ and CAP groups collaborated with Argentine universities and national laboratories, and CAP also collaborated with the IGN and the National Parks Administration. The IGN proposed creation of an open collaborative GPS network using data provided by these seven sites in Argentina to support governmental, commercial, and scientific geodesy and surveying. This network was named the Argentine Continuous Satellite Monitoring Network (Red Argentina de Monitoreo Satelital Continuo [RAMSAC]), and its main goal was to be the foundation for the development and maintenance of the national geodetic RF. Figure 1 shows a summary of the RAMSAC stations grouped by installation date. Until 2009, RAMSAC depended mainly on other agencies and institutions (both national and international) to provide GPS/Global Navigation Satellite Systems (GNSS) stations to expand the network. During this period, thanks to the effort of multiple national and international collaborators, the network incorporated stations such as Universidad Nacional de Rosario (Santa Fe), and Centro Regional de Investigaciones Científicas y Tecnológicas (Mendoza; MZAC), among others. In late 2009, IGN obtained Argentine government funding to begin to build and operate its own GPS/GNSS stations. This triggered the rapid RAMSAC expansion shown in Figure 1. One of the strengths of RAMSAC is that all installations are performed using monumentation that guarantees the stability of the GPS/GNSS antennas. Also, in an effort to maintain the best possible continuity of the time series (with the least possible time-series jumps), IGN has always tried to keep the antenna changes to a minimum unless a degradation in the solutions quality is noticed. doi: 10.1785/0220170162 Seismological Research Letters Volume XX, Number XX – 2018 1 SRL Early Edition