M.J. Borque

Active faulting in the internal zones of the central Betic Cordilleras (SE, Spain)

The internal zones of the Betic Cordilleras show a present-day relief that is mainly controlled by kilo- metre-size, symmetrical or north-vergent folds which developed mostly since Middle Miocene times. The Sierra Nevada, Sierra Alhamilla, Sierra de Los Filabres, Sierra Tejeda and Sierra de Gador, among others, are roughly E-W trending high mountain ranges, corresponding to antiforms where metamorphic rocks crop out. The surrounding depressions are located in synforms, where Neogene rocks are preserved from erosion. Field evidence shows that the growth of the folds is coeval with fault development, and that at least three of them, i.e. the Padul Fault, the Zafarraya Fault, and the Balanegra Fault, may be considered to be active seismogenetic structures. The Zafarraya Fault, in particular, is thought to be responsible for the 1884 Andalucia Earthquake. The fault is located at the northern limb of the Sierra Tejeda antiform, and could be interpreted as a collapse structure developed along the external arch of the uplifted fold. The Padul and Balanegra faults are located at the southeastern border of the Granada Basin and south of the Sierra de Gador, respectively. They belong to a set of NW-SE oriented faults that are mainly normal in character and indicate NE-SW extension. The set up, since 1999, of a GPS network within and around the Granada Basin and the planed installation of a new network in the Sierra Tejeda, will give us new insights on the present-day deformation behaviour of both folds and faults in the area.

Levelling Profiles and a GPS Network to Monitor the Active Folding and Faulting Deformation in the Campo de Dalias (Betic Cordillera, Southeastern Spain)

The Campo de Dalias is an area with relevant seismicity associated to the active tectonic deformations of the southern boundary of the Betic Cordillera. A non-permanent GPS network was installed to monitor, for the first time, the fault- and fold-related activity. In addition, two high precision levelling profiles were measured twice over a one-year period across the Balanegra Fault, one of the most active faults recognized in the area. The absence of significant movement of the main fault surface suggests seismogenic behaviour. The possible recurrence interval may be between 100 and 300 y. The repetitive GPS and high precision levelling monitoring of the fault surface during a long time period may help us to determine future fault behaviour with regard to the existence (or not) of a creep component, the accumulation of elastic deformation before faulting, and implications of the fold-fault relationship.

A GRAVIMETRIC GEOID COMPUTATION AND COMPARISON WITH GPS RESULTS IN NORTHERN ANDALUSIA (SPAIN)

Two new GPS surveys have been carried out to check the accuracy of an existing gravimetric geoid in a test area located in northern Andalusia (Spain). The fast collocation method and the remove-restore procedure have been used for the computation of the quasigeoid model. The Spanish height system is based on orthometric heights, so the gravimetrically determined quasigeoid has been transformed to a geoid model and then compared to geoid undulations provided by GPS and levelling at benchmarks belonging to the Spanish first-order levelling network. The discrepancies between the gravimetric solution and GPS/levelling undulations amount to ±2 cm for one survey and ±5 cm for another after fitting a plane to the geoid model.

The Use of GNSS/Levelling and Gravity Data for the Spanish Height System Unification

From 2001 to 2008, the National Geographic Institute of Spain (IGN) carried out the REDNAP project to the establishment of a National High Precision Levelling Network in the whole Spanish territory. Within REDNAP, spirit levelling and gravity observations were complemented by GNSS data. The levelling network of the continental area and those of the main islands were referred to different tide gauges thus producing different height systems. In the paper, the GOCO-05S model from the GRACE and GOCE gravity missions is used to unify such systems. More precisely, it is used to estimate the normal heights of the mean sea level at the reference tide gauges, i.e. the height system biases. In the proposed solution such biases are determined through the least squares adjustment of the differences between height anomalies derived from GNSS/levelling and height anomalies derived from a proper combination of global gravity models. An accurate modelling of the observation error covariances is taken into account as well. The estimated accuracies of the resulting biases are in the order of few centimetres apart from that of continental Spain which is an order of magnitude better.