Three‐Dimensional Magnetic Models of La Gomera (Canary Islands): Insights Into the Early Evolution of an Ocean Island Volcano

An aeromagnetic data set from the island of La Gomera was studied through two inverse modeling approaches that produced complementary views of the inner structure of this volcanic island: (1) a variable magnetization model that identified the main lateral magnetization contrasts and (2) a constant magnetization model that imaged the main magnetic source by assuming that it was a uniformly magnetized body. The modeling reveals intense magnetizations beneath the northern part of La Gomera, which occupy an important portion of the northern submarine edifice, correspond well with outcrops of the submarine volcano (Basal Complex), and confirm that most of the magnetic signal revealed by aeromagnetic mapping in the Canary Islands is due to the intense magnetizations of the intrusive complexes (plutonic bodies and dike complexes) emplaced during the initial stages of growth of the volcanic edifices. The consistency of our models with the results of a previous gravimetric study suggests that these intrusive complexes are denser and more magnetic than the surrounding rocks. The location of the main magnetic source reinforces the interpretation, first suggested by geological evidence, that the submarine and early subaerial growth of La Gomera started to the north of the present island. The elongated shape of these intrusive complexes with a nearly E-W strike agrees with the orientation of analogous structures on Tenerife and Gran Canaria, suggesting that the initial formation of the central islands of the Canary Archipelago was controlled by a set of regional fractures in a strike-slip tectonic framework.Spanish Ministry of Science, Innovation and Universities (Project CGL2015-63799-P) and by the Junta de Castilla y León and the European Regional Development Fund (ERD) (Project BU235P18)

Detección gravimétrica de anomalías del subsuelo en una zona urbana afectada por hundimientos

[ES] En este trabajo se describe un estudio microgravimétrico realizado en Alcalá de Ebro (Zaragoza) destinado a localizar las anomalías del subsuelo que pudieran justificar los hundimientos del terreno acaecidos en la zona. A partir de los datos gravimétricos reducidos se procede a un análisis de la coyarianza y a una predicción mínimos cuadrados para filtrar el ruido observacional y obtener un modelo de anomalías gravimétricas. Mediante un ajuste polinomíal robusto se determina una componente regional y su correspondiente residuo, que es identificado como campo de anomalías locales. Para la inversión de estas anomalías locales se recurre a un ajuste de cuerpos esféricos de densidad anómala.[EN] This paper describes a microgravimetric study carried out in Alcalá de Ebro (Zaragoza) to locate the subsoil anomalies that could justify terrain sinkings which take place in tbis zone. From reduced gravimetrie data, we apply a coyanance analysis and a least squares prediction to filter observational noise and to obtain a model of gravimetnic anomalies. By means a robust polynomial adjustment, a regional component is detennined and its corresponding residue which is identified as local anomalies field. For inversion of those local anomalies an adjustment of spherical bodies with anomalous densitywas done.Peer reviewe

Study of Shadow structures in Sao Miguel Island (Azores Archipelago) through analysis of gravity data

Uno de los principales retos en el estudio de áreas volcánicas activas es determinar un modelo de estructuras corticales que ayude a establecer tanto su origen como la evolución que han tenido a través de las distintas fases de actividad volcánica ocurridas en la zona. El análisis de anomalías gravimétricas permite identificar la geometría de los bordes de estas estructuras, aun cuando su efecto esté enmascarado por otros. En muchos trabajos previos se ha demostrado que este tipo de información, complementada con otros estudios, es de gran utilidad para el conocimiento de la historia volcánica de una determinada zona. En este caso, se presenta un nuevo mapa de anomalías gravimétricas de Bouguer de la isla de San Miguel (Azores) a partir de los datos de gravedad utilizados por Camacho et al. (1997), mejorando tanto el procesado de las observaciones como la corrección topográfica. El análisis de estos nuevos datos de anomalías se ha realizado calculando mapas de derivadas horizontales y gradiente de gravedad. Estos mapas se correlacionan con elementos tectónicos, mostrando estructuras asociadas a las principales calderas, pudiendo inferir sus diferentes orígenes. Los resultados ayudan a clarificar la distribución y posible geometría de estructuras someras ayudando a una mejor comprensión de la actividad volcánica de la isla.El proyecto CGL2015-63799-P del Ministerio de Ciencia e Innovación ha financiado parcialmente esta investigación

Gravimetric study of the Cueva de Maltravieso (Cáceres)

Comunicación presntada a la 3ª Asamblea Hispano-Portuguesa de Geodesia y Geofísica = 3ª Assembleia Luso-Espanhola de Geodesia e Geofisica, celebrada en Valencia entre el 4 y el 8 de febrero de 2002.A micro-gravimetric study carried out upon a karstic cavity of archaeological interest is presented. A whole of 505 stations (with step of about 4171) was observed upon the terminal portion of the cavity and the surrounding areas using two gravimeters LaCoste-Romberg mod G. The aim of the study is 10 locate a possible continuation of the known cavity. The drifts and jumps adjustment of the gravity data gave a residuals with standard deviation 7 μGal, and the correlation analysis of the gravity anomalies gave a value 6 μGal for the noise values. A further non-linear gravity inversion produced a model of the subsoil anomalous structures confirming the actual cavity and showing other interesting low density bodies. The presentation gives some details of the results and of some methodological aspects.Peer reviewe

Strain Pattern and Kinematics of the Canary Islands from GNSS Time Series Analysis

Following the 2004 seismic unrest at Tenerife and the 2011-2012 submarine eruption at El Hierro, the number of GNSS observation sites in the Canary Islands (Spain) has increased, offering scientists a useful tool with which to infer the kinematics and present-day surface deformation of the Canary sector of the Atlantic Ocean. We take advantage of the common mode component filtering technique to improve the signal-to-noise ratio of the velocities retrieved from the daily solutions of 18 permanent GNSS stations distributed in the Canaries. The analysis of GNSS time series spanning the period 2011-2017 enabled us to characterize major regions of deformation along the archipelago through the mapping of the 2D infinitesimal strain field. By applying the triangular segmentation approach to GNSS velocities, we unveil a variable kinematic behaviour within the islands. The retrieved extension pattern shows areas of maximum deformation west of Tenerife, Gran Canaria and Fuerteventura. For the submarine main seismogenic fault between Tenerife and Gran Canaria, we simulated the horizontal deformation and strain due to one of the strongest (mbLg 5.2) earthquakes of the region. The seismic areas between islands, mainly offshore Tenerife and Gran Canaria, seem mainly influenced by the regional tectonic stress, not the local volcanic activity. In addition, the analysis of the maximum shear strain confirms that the regional stress field influences the E-W and NE-SW tectonic lineaments, which in accordance with the extensional and compressional tectonic regimes identified, might favour episodes of volcanism in the Canary Islands.Project CGL2015-63799-PPeer reviewe

Features of the deep structure of some volcanic islands of the Canary Archipelago from the passive seismo-prospecting method

Trabajo presentado en el 26º UGG General Assembly, celebrado en Praga (República Checa) del 22 de junio al 2 de julio.The Microseismic Sounding Method (MSM) [Gorbatikov, etc., 2008,2011,2013] belongs to group of passive methods of seismic prospecting and can be applied at the solution of geologic-geophysical and structural problems to various classes of geological objects, and considering various geographical and climatic conditions. The model of formation of a microseismic field in MSM doesn't proceed from the obligatory horizontal lamination of the medium. In this regard, it is considered that the main contribution to a microseismic field is provided by fundamental Rayleigh modes, and the existence of the highest modes is minimum. Informative parameter (a useful signal) in MSM is extent of distortion of initial amplitude field of microseisms due to interaction with velocity inclusions. The phase information is not used. The form and depth of the heterogeneity is estimated through the distribution of the distortions of the initial field at the surface, and the frequency at which these distortions are manifested as well. The MSM method was applied to estimate the models of the deep structure of several islands of the Canary Archipelago: El Hierro, Lanzarote and Gran Canaria. The respective velocity models obtained were then compared in independent studies: 1) with distribution of micro-seismicity and inversion of the gravity field at El Hierro Island, 2) with inversions of gravity and magnetic fields, and with morphological and structural maps at Lanzarote and Gran Canaria islands. A good correlation with the results of various methods has been found.Peer Reviewe