Raul Perdomo

Earthquake cycle dominates contemporary crustal deformation in Central and Southern Andes

With the advent of global positioning system (GPS) technology and its applications to high precision geodesy, earth scientists have gained an unprecedented opportunity to study the kinematics and dynamics of present-day deformation processes along plate boundary zones. However, until now our knowledge of the deformation processes associated with the Andean subduction along the western coast of South America has been limited. Here we present first estimates of present-day crustal deformation rates in the central and southern sections of the Andes, between latitudes 22‡S and 42‡S. We find that the deformation in the central section of our study area is dominated by the interseismic phase of an earthquake deformation cycle, caused by 100% coupling of the thrust interface between the subducting Nazca and the overriding South America plates. The estimated depth of coupling is not uniform along strike: north of 30‡S it is V33 km deep, while south of 35‡S it reaches 50 km depth. In addition, we observed postseismic relaxation effects in the northern part of our network in the area of the 1995 Mw8.0 Antofagasta earthquake. South of 38‡S, we detected a similar deformation pattern as in the Antofagasta area which we attribute to postseismic relaxation effects of the 1960 Mw9.5 Valdivia earthquake. fl 2001 Elsevier Science B.V. All rights reserved.

Long-Term Signals in the Present-Day Deformation Field of the Central and Southern Andes and Constraints on the Viscosity of the Earth’s Upper Mantle

As part of the South American Geodynamic Activities project we observed the present day deformation field in the territories of Chile and Argentina using the Global Positioning System. The results clearly show that the earthquake cycle dominates the contemporary surface deformation of the central and southern Andes. Compared to geological timescales, the transient elastic deformation related to subduction earthquakes presents a short-term signal which can be explained by interseismic, coseismic, and postseismic phases of interplate thrust earthquakes. We constructed the Andean Elastic Dislocation Model (AEDM) in order to subtract the interseismic loading from the observed velocities. The estimated parameters of the AEDM, and the amount and depth of coupling between the subducting Nazca and overriding South American Plates, represent long-term features and show that the seismogenic interface between both plates is fully locked and that the depth of coupling increases from north to south.

Regional geoid determination in Tierra del Fuego including GPS levelling

A regional geoid model for the Argentine part of the Isla Grande de Tierra del Fuego, established in previous works on the basis of GPS levelling, suffers a lack of observation data in the remote south-western investigation area. In order to improve the data distribution in this region, the mean lake level of Lago Fagnano has been regarded as a natural indicator for the local geoid. Using a GPS buoy and pressure tide gauges, a method to determine the mean lake surface topography with respect to the ellipsoid has been developed. It is shown that the obtained lake level geometry is essentially controlled by the regional gravity field. The derived information on the mean lake level has been included in the geoid model, which results in a more detailed and plausible representation of the regional geoid.

Anomalous ocean load tide signal observed in lake‐level variations in Tierra del Fuego

[1] We demonstrate the application of a 100 km long lake as a sensor for studying the tidal effects on Tierra del Fuego main island. The lake-level variations observed in Lago Fagnano reflect both the direct response to the tidal potential and the indirect effect of the ocean tidal loading. Modeling both contributions explains the observed tidal signal in the lake to about 70%. Underestimated model load tide amplitudes are found to be probably responsible for the remaining difference. We interpret this discrepancy as a hint for regional elastic lithosphere properties differing substantially from those represented by currently available global models.

Geoid modelling in the area of Fagnano Lake, Tierra del Fuego (Argentina): insights from mean lake-level observations and reduced gravity data

This paper evaluates a combined geoid model in the area of Fagnano Lake located in Tierra del Fuego, Argentina. The model includes GPS/levelling points, gravity data and GPS buoy observations on the lake. The GPS buoy information allowed to determine a mean lake level (MLL) surface which was used to extend the geoid model to an area with restricted access by land (Del Cogliano et al. 2007). An approach to optimize the selection and distribution of the MLL data is developed in order to use them as input in the Equivalent Source Technique, and to combine them with different types of observations.Furthermore, the global geopotential model EGM2008 is validated in the remote lake area. This model does not include observed gravity data in this region. Its behaviour is compared to the results of our observations along the levelling lines. Differences of several decimetres are found when EGM2008 geoid undulations are compared to geoid undulations derived from observations in this region. In the areas where EGM2008 relies on real gravimetric observations, differences between model and observations are only of a few centimetres.The comparison of the EGM2008 model with our observations reveals the effect of missing or not representative gravity information may have on the estimation of geoid undulations in high mountain regions. The data on which EGM2008 is based in the Fagnano Lake area does not result from gravimetric observations on land. Therefore, we consider them as not representative and we show that their effect can be significant in such areas.

Analysis of the Discrepancies Between the Vertical Reference Frames of Argentina and Brazil

The vertical reference frames for Argentina and Brazil present discrepancies due to their different datums and realizations. Thus, since 2008, we have started a series of activities with the aim of unifying the Argentine and Brazilian national vertical networks (NVNs). To achieve this goal, we have connected the two NVNs at three border points by using the geodetic levelling approach. Additionally, the gravity field approach was also applied, based on a suitable representation of the geoid by considering the Earth Gravitational Model (EGM2008) in its full resolution. In this regard, 1266 co-located Global Positioning System (GPS) and levelling benchmarks regularly distributed over Argentina (612) and Brazil (654) were considered. The geodetic levelling approach shows an offset value of 54 cm, which implies that the Argentine vertical reference frame is above that of the Brazilian vertical reference frame. However, the result of the gravimetric approach shows an offset of 57 cm, which implies a difference of approximately 3 cm between both methods. Hence, since Brazil and Argentina represent a significant part of South America, the solution to the datum problem between both countries could point towards a common vertical reference frame for the Atlantic side.

Geoid Study in Tierra del Fuego

In 1993 a precise GPS network was established in the region of Tierra del Fuego. In 1998, a levelling line was measured with origin at Ushuaia tide gauge. Gravimetric measurements were also made to apply the orthometric correction.

EVALUATION OF TERRAIN CORRECTIONS THROUGH FFT AND CLASSICAL INTEGRATION IN TWO SELECTED AREAS OF THE ANDES AND THEIR IMPACT ON GEOIDAL HEIGHTS

As part of the regional geoid modeling, terrain corrections were computed in Tierra del Fuego island and in the west side of the province of Mendoza. The first place is located in the southernmost region of Argentina and Mendoza is in the center-west of this country. Considering both study areas, elevations range from 0 m to 6500 m. The classical integration of prism contribution and the 2-D FFT technique were used to estimate terrain corrections. This study aims at discussing the results obtained by both approaches and their applicability considering their advantages and disadvantages according to the regions under investigation. The analysis allowed us to conclude that classical integration has a better performance than FFT methods, especially in the highest regions where terrain corrections can be overestimated in more than 20 mGals by FFT. Both techniques described show similar results in flat areas. Finally, the effect that the error of terrain corrections computation has on geoidal heights is also discussed and numerically tested. It is proved that an error in gravity anomalies of 20 mGals may cause up to 2 m geoid height error.

Advances in the Calculation of a Height Transformation Model in Buenos Aires Province

In order to refer local frames to SIRGAS, the Buenos Aires Province GPS network was transformed to POSGAR98, the best Argentine geocentric frame related to SIRGAS. The comparison with previous results is satisfactory, the differences are few centimetres.

Towards a tidal loading model for the Argentine-German Geodetic Observatory (La Plata)

Abstract We present a regionalized model of ocean tidal loading effects for the Argentine-German Geodetic Observatory in La Plata. It provides the amplitudes and phases of gravity variations and vertical deformation for nine tidal constituents to be applied as corrections to the observatory’s future geodetic observation data. This model combines a global ocean tide model with a model of the tides in the Río de la Plata estuary. A comparison with conventional predictions based only on the global ocean tide model reveals the importance of the incorporation of the regional tide model. Tidal loading at the observatory is dominated by the tides in the Atlantic Ocean. An additional contribution of local tidal loading in channels and groundwater is examined. The magnitude of the tidal loading is also reviewed in the context of the effects of solid earth tides, atmospheric loading and non-tidal loads.