Gerardo Alguacil

Geodynamic evolution of the lithosphere and upper mantle beneath the Alboran region of the western Mediterranean: Constraints from travel time tomography

An edited version of this paper was published by the American Geophysical Union. Copyright 2000, AGU. See also: http://www.agu.org/pubs/crossref/2000/2000JB900024.shtml; http://atlas.geo.cornell.edu/morocco/publications/calvert2000.htm

Seismovolcanic signals at Deception Island volcano, Antarctica: Wave field analysis and source modeling

The seismovolcanic signals associated with the volcanic activity of Deception Island (Antarctica), recorded during three Antarctic summers (1994–1995, 1995–1996 and 1996–1997), are analyzed using a dense small-aperture (500 m) seismic array. The visual and spectral classification of the seismic events shows the existence of long-period and hybrid isolated seismic events, and of low-frequency, quasi-monochromatic and spasmodic continuous tremors. All spectra have the highest amplitudes in the frequency band between 1 and 4 Hz, while hybrids and spasmodic tremors have also significant amplitudes in the high-frequency band (4–10 Hz). The array analysis indicates that almost all the well-correlated low-frequency signals share similar array parameters (slowness and back azimuth) and have the same source area, close to the array site. The polarization analysis shows that phases at high-frequency are mostly composed of P waves, and those phases dominated by low frequencies can be interpreted as surface waves. No clear shear waves are evidenced. From the energy evaluation, we have found that the reduced displacement values for surface and body waves are confined in a narrow interval. Volcano-tectonic seismicity is located close to the array, at a depth shallower than 1 km. The wave-field properties of the seismovolcanic signals allow us to assume a unique source model, a shallow resonating fluid-filled crack system at a depth of some hundreds of meters. All of the seismic activity is interpreted as the response of a reasonably stable stationary geothermal process. The differences observed in the back azimuth between low and high frequencies are a near-field effect. A few episodes of the degassification process in an open conduit were observed and modeled with a simple organ pipe.

Array tracking of the volcanic tremor source at Deception Island, Antarctica

We have found experimental evidence which shows that the volcanic tremor recorded at Deception Island (South Shetland Islands, Antarctica) is a superposition in time of overlapping hybrid events. We studied data from a small aperture seismic array. Data analysis for tremor and hybrids included: (1) spectral analysis; (2) apparent slowness and back-azimuth determination by using the zero-lag cross-correlation method; and (3) polarization analysis. Both types of events share these common features: (a) two dominant spectral bands at frequencies 1–3 Hz (the most energetic) and 4–8 Hz; (b) several coherent phases with the same back-azimuth to the source and apparent slowness along the whole signal; (c) in the high frequency band, the apparent slowness is very low (around 0.17 s/km), indicating the propagation of body waves; (d) in the low frequency band, the apparent slowness is high (around 1.6 s/km), consistent with the presence of surface waves; and (e) clear P-wave onset followed by a complex pattern of Rayleigh waves. Therefore, both types of events are strongly related because they share the same source region, the same wave-propagation properties, and the same wave composition. Moreover, several arrivals, that resemble a single hybrid event, have been found along the tremor signals. Due to these reasons, we hypothesize that volcanic tremor of Deception Island is a superposition of hybrid type events. The source of both types could be the interaction between thaw water and hot materials in a shallow aquifer.

Basement structure of the Granada basin, Betic Cordilleras, Southern Spain

Abstract The analysis and interpretation of geophysical data (gravity and seismic reflection) has facilitated the definition of the Granada basin structure. The reflector showing the contact between the Betic-Subbetic basement and the Neogene-Quaternary sedimentary filling has been identified. Mapping of the basement in two and three dimensions is presented. The presence of four important depocenters (Genil, Chimeneas, Cubillas and Granada) has been determined. These troughs are limited by ridge areas through important sets of fractures. In some cases the accumulation of Neogene-Quaternary sediments reaches a thickness exceeding 3 km as in the Genii and Cubillas depocenters. The mapping of the most important fractures affecting the basement has been achieved, defining four systems that have influenced and conditioned the genesis and late evolution of the Granada basin. The directions of the most important groups of fractures are: NE-SW, N70W to E-W, N45W and N10-30E.

Spatial‐temporal analysis of a seismic series using the principal components method: The Antequera Series, Spain, 1989

A method for the characterization of a seismically active zone from a distribution of hypocenters is presented. This method is based on principal components analysis, a powerful multivariant statistical technique that is used to find the rupture local ellipsoid (RLE). The ellipsoid is a planar structure with which two variations of the method are developed: the spatial principal components analysis and the spatial-temporal principal components analysis; using these methods, it is possible to find the dominating tendencies in the fracturing of the seismically active volume as well as the temporal evolution of the process. The methodology developed has been applied to a series of earthquakes that occurred near Antequera, Spain, with the result that the main fracture series trends N70°–80°E. Moreover, the temporal evolution of the system from the most relevant RLE has been characterized.

A double seismic antenna experiment at teide Volcano: existence of local seismicity and lack of evidences of Volcanic tremor

Data analyzed in the present work correspond to a 40 days field experiment carried out in Teide Volcano (Canary Islands, Spain) with two short-period small-aperture dense seismic antennas in 1994. The objective of this experiment was to detect, analyze and locate the local seismicity. We analyzed also the background seismic noise to investigate the possible presence of volcanic tremor. From a set of 76 events, we selected 21 of them in base of their good signal-to-noise ratio and their possibility to locate their seismic source by using the seismic antennas. A visual classification based on the S‐P time and seismogram shape has permitted to establish three groups of events: local seismicity (S‐P time between 3 and 5 s), very local earthquakes (S‐P time smaller than 3 s) and artificial explosions. These earthquakes have been located by applying the Zero Lag CrossCorrelation technique and the inverse ray-tracing procedure. Those earthquakes that were recorded simultaneously by both seismic antennas were also located by intersecting both back-azimuths. The analysis of the seismicity has revealed that the amount of seismicity in Teide Volcano is moderate. This seismicity could be distributed in three main areas: inside the Caldera Edifice (below the Teide‐Pico Viejo complex), in the eastern border of the Caldera Edifice and offshore of the island. At present, this activity is the only indicator of the volcano dynamics. The analysis of the back-ground seismic noise has revealed that at frequencies lower than 2 Hz, the Oceanic Load signal is predominant over other signals, even over local earthquakes with a magnitude of 2.0. Due to this, although if in the Teide area were present a weak volcanic tremor, or other volcanic signals with predominant peaks below 2 Hz, to observe them would be a very difficult task. q 2000 Elsevier Science B.V. All rights reserved.

Space clustering properties of the Betic-Alboran earthquakes in the period 1962–1989

Abstract In order to obtain a way to quantify the spatial dependence between earthquakes in the Betic-Alboran (south Spain) region, the Single Link Clustering ( slc ) method has been applied to a catalogue of earthquakes, with magnitude greater or equal to 4.0, that occurred in this region from 1962 to 1989. The significant distances of links obtained, which show alignments whose direction is similar to the direction of some surface faults, are 15, 30 and 40 km, and the limit of inconsistency appears at distances greater than or equal to 70 km. Most of the historical destructive earthquakes have taken place within the clusters obtained, and the majority within the dc = 30 km cluster. This implies that the most relevant seismic activity occurs in some source areas where there is a strong nesting of activity.

Spacial and energetic trends of the microearthquakes activity in the Central Betics

AbstractSince early 1983, when the Andalusian Seismic Network began to operate, to late 1987, 4198 microearthquakes have been located in the Central Betics. This activity is quite nonuniform in space and time. The most important fault systems drawn by the geologic cartography and the observed lineaments by Landsat images are N20-40E N60-80E and N120-140E. Those fault systems which have shown activity during the period of study have been identified through the alignment of microearthquake epicentres and the diagrams of consecutive earthquakes relative azimuths. The most active systems in the whole region have been N70-80E and N90-100E. In the south zone appear lineaments N0-30E. There seems to be a gap zone in the area near Arenas del Rey, where the earthquake of December 24, 1984 (I0 = X.M.S.K.) occurred, and during the studied period only one earthquake of m1 = 5.0 has been detected, with neither precursors nor aftershocks. In the Granada Basin, the wellknown fracture system cabia-Santa Fe-Pinos Puente h...

Effect of a sedimentary basin on estimations of Qc and QLg

Abstract The effect which a sedimentary basin (Granada basin, southern Spain) exercises on the estimated Q c and Q Lg attenuation values within a determined frequency range are described. It has been shown that for the values of Q c there is a decrease in the number of fits, with a correlation coefficient > 0.8 in the 1.5 Hz frequency band, in comparison with the other frequency bands. On the other hand, an overestimation of the values of the coda- Q is observed between a station located on the sediment in the interior of the basin and the stations situated on the borders of the basin. The values of the attenuation for Lg waves in the paths which cross the Granada basin show a series of peaks centred in the 1–3 Hz frequency band, which overestimate the values of Q Lg with respect to other frequency bands. All these phenomena indicate that the presence of a low-velocity sedimentary basin modifies the processes of generation of coda waves and the propagation of Lg waves.

Geometrical spreading function for short-period S and coda waves recorded in southern Spain

Two methods to estimate the geometrical spreading factor n are described; the first is applied to S waves recorded at various distances from the source, whereas the second is applied to coda waves. The direct S-wave method is a new technique which provides the geometrical spreading factor independently of the quality factor Q. This method is based on the double spectral ratio. The coda method provides the geometrical spreading factor along with the Q value. To check their reliability, these methods are applied to real data (from the Andalusian Seismic Network) and to synthetic data. The synthetic test of the coda method indicates that this method to obtain n must not be used to calculate the attenuation factor Q. The results obtained with real data show a geometrical spreading coefficient that is greater than unity and slightly frequency dependent. For S waves, the following frequency dependence was obtained: n=(1.19 ± 0.14)+(0.052 ± 0.007)ƒ and for coda waves, n=1.33 ± 0.19 at 1.5 Hz n=1.57 ± 0.19 at 3 Hz n=1.29 ± 0.24 at 6 Hz The results obtained in this paper (n > 1) suggest a velocity increase with depth and/or strong lateral inhomogeneities which modify the wavefront geometry.