A. J. Gil

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.

Geodetic measurements of crustal deformation on NW–SE faults of the Betic Cordillera, southern Spain, 1999–2001

Abstract The Granada basin, located in the central sector of the Betic Cordillera, is one of the most seismically active zones of the Iberian Peninsula. For the first time, a geodetic network along the Padul fault and two levelling profiles (Genil and Viznar) crossing the Granada fault were operated to detect crust microdeformations in this area. Three years of terrestrial geodetic measurements are analysed to characterize the behaviour of these faults in a low to moderate strain rate environment. After the comparison of these three campaigns, we can conclude that there is no significant short-term movement of the Padul fault. Granada fault measurements show significant differences between 1999 and 2001 campaigns in Viznar profile. However, more data are needed to correlate this displacement to the tectonic activity.

Establishment of a Non-Permanent GPS Network to Monitor the Recent NE-SW Deformation in the Granada Basin (Betic Cordillera, Southern Spain)

The Granada Basin (Central Betic Cordillera), one of the most seismically active areas of the Iberian Peninsula, is currently subjected to NW-SE compression and NE-SW extension. The present day extension is accommodated by normal faults with various orientations but particularly with a NW-SE strike. At the surface, these active NW-SE normal faults are mainly concentrated on the NE part of the Basin. In this part we have selected a 15-km long segment where several active normal faults crop out. Using the marine Tortonian rocks as a reference, we have calculated a minimum extensional rate of 0.15-0.30 mm/year. The observed block rotation, the listric geometry of faults at depth and the distribution of seismicity over the whole Basin, indicate that this rate is a minimum value. In the framework of an interdisciplinary research project a non-permanent GPS-network has been established in the central sector of Betic Cordillera to monitor the crustal deformations. The first two observation campaigns were done in 1999 and 2000.

Comparing RTK positioning from updated REGAM and MERISTEMUM CORS networks in Southeast Spain

Abstract For many countries, regional networks of Continuously Operating Reference Station (CORS) are the basic infrastructure to support the needs of users requiring positioning with centimeter-scale accuracy. However, nationwide NRTK (RTK Network) coverage is often not economically viable for countries with large rural regions. In contrast, for administrative reasons and social proximity sometimes several networks may cover the same territory. Reference station placement affects the quality of the positioning services and the cost of the network. Spain has a high density of regional NRTK networks. Indeed, the Region of Murcia has two such networks: REGAM and MERISTEMUM. REGAM uses the MAC (Master Auxiliary Concept) and MERISTEMUM uses the VRS (Virtual Reference Station), which are currently the most commonly used approaches to generate network RTK corrections. Control tests based on simultaneous determination of position have been performed in the Region of Murcia to analyze the performance of the REGAM and MERISTEMUM networks. The results confirm that both the VRS and MAC network solutions provide centimeter-scale accuracy: the positioning error for the test points inside the NRTK cells is lesser than 1.5 cm in planimetry and around 3.5 cm in height. The ambiguity resolution, the coverage and the connection to the networks services were highly satisfactory.

Testing precise positioning using RTK and NRTK corrections provided by MAC and VRS approaches in SE Spain

RTK networks can provide an active service through the computation and transmission of error correction data to support the application of location-based services that require high accuracy positioning, e.g. land surveying, precision agriculture, machinery control etc. At present, the most used approaches to generate the NRTK (Network RTK) corrections are VRS (Virtual Reference Station) and MAC (Master Auxiliary Concept). In the Region of Murcia (SE Spain), two GNSS active networks (i.e. REGAM and MERISTEMUM) are available and they use MAC and VRS respectively. In order to analyse these RTK networks, four tests were designed and applied at five representative locations distributed throughout the Region of Murcia. The results indicate that both VRS and MAC solutions provide an accuracy within 2.5 cm in horizontal positioning and 5.0 cm in vertical positioning.

Geoid Determination in Central Spain from Gravity and Height data

SummaryThe least-squares collocation method has been used for the computation of a geoid solution in central Spain, combining a geopotential model complete to degree and order 360, gravity anomalies and topographic information. The area has been divided in two 1°× 1° blocks and predictions have been done in each block with gravity data spacing about 5′ × 5′ within each block, extended 1/2°. Topographic effects have been calculated from 6″ × 9″ heights using an RTM reduction with a reference terrain model of 30′ × 30′ mean heights.

The Gravimetric Geoid in Spain: First Results

A method for validation of gravity anomalies, and the prediction of geoid undulation in a small zone in the center of Spain, using the least squares collocation method are presented.

Surveying at the limits of local RTK networks: Test results from the perspective of high accuracy users

Precise GNSS-based differential positioning in real time is usually known as the real-time kinematics (RTK) technique. RTK reduces the effects of orbit errors and ionospheric and tropospheric refraction by forming differences between the observables (e.g. double-differences). These effects, however, grow with increasing baseline length, although the use of corrections generated in real-time from an active GNSS network allows these distance-dependent errors to be reduced. This technology increases the reliability of the system and thereby the accuracy of real-time positioning. In this study, the test results of RTK positioning at different test points located in the border area between the Autonomous Communities of the Region of Murcia and the Community of Valencia, in SE Spain, are presented. The analysis is based on three GNSS local active networks present in this border area, namely MERISTEMUM, REGAM and ERVA networks. Test measurements with RTK rover were performed in this region in order to analyze the real-time services offered by these three networks. Moreover, precise coordinates for each test point were determined using Bernese 5.0. The results confirm that it is possible to achieve centimetre-scale accuracy with RTK positioning based on the networks studied, even in border regions.

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.

Multivariate outlier detection based on robust computation of Mahalanobis distances. Application to positioning assisted by RTK GNSS Networks

Abstract RTK GNSS Networks for real time and post-processing positioning services are becoming more and more numerous throughout the world. In order to study the quality of the real time positioning services offered by these facilities with particular respect to outlier identification and rejection and accuracy assessment, classical statistical methods do not appear adequate. In fact, they are mainly based on Least Squares adjustment results and on the hypothesis of normally distributed samples; in addition, most outlier detection tests are set up for univariate samples. This paper presents a method based on the robust computation of Mahalanobis distances able to detect outliers in multivariate samples, and its evaluation by comparing the results obtained from randomly generated data with those stemming from other classical methods. The application of this method in the processing of RTK positions, recorded in real time with a GNSS receiver assisted by a RTK Network for positioning services is shown. In the various tests performed using both simulated samples and those from real GPS observations, the proposed method has been found effective for the outlier detection.