Spatial characterization of Agron, southern Spain, 1988–1989 seismic series

Abstract

During November 1988 to April 1989, a seismic swarm of about 400 earthquakes of magnitude between 0.4 and 4.0 occurred near the village of Agron, Andalusia, southern Spain. The pattern of the energy release versus time clearly shows that the dataset is representative of a seismic swarm, where it is not observable a typical mainshock-aftershock sequence. Basing on the Gutenberg-Richter relationship the catalogue is considered complete down to a magnitude of 1. We revise the dataset by selecting those digital data which were recorded by at least four stations. The revised dataset is relocated using a probabilistic approach based on a Bayesian formalism. The epicenters are barely aligned along two main structures trending NNE-SSW, NW-SE and WNW-ESE, respectively. The focii depth is confined in the 5-20 km interval. Application of a clustering technique based on the principal component analysis (PCA) of the local rupture ellipsoids depicts a spatial clustering of the seismicity according to five main planes, of which only one coincides with the horizontal alignment of epicenters. Cross-correlation among waveforms allows to individuate five different groups of similar events (multiplets). High-resolution estimate of time differences among events belonging to the same multiplet and least-squares adjustment of arrival times allow for precise relative locations, from which we derive the corresponding rupture planes. Focal mechanisms are calculated for three of the five multiplets: for these cases, the planes obtained from the relative locations are coincident with one of the fault plane solutions. The results are consistent with those previously inferred from PCA, and indicate a high variability in the spatial setting of fault planes over a short distance range. These observations are consistent with a fragmentation of the upper crust into small (2-4 km) tectonic blocks, and indicate an heterogenous stress regime, with different stresses acting simultaneously in adjacent areas.

Seismotectonic constraints at the western edge of the Pyrenees: aftershock series monitoring of the 2002 February 21, 4.1 Lg earthquake

“…The high waveform similarity of events of a given cluster may also be used to highly improve the picking of the phase onsets, allowing an accurate relative hypocentre determination that can provide enhanced resolution on the location and extent of the seismotectonic structure (Frémont & Malone 1987;Deichmann & García-Fernández 1992;Got et al 1994;Augliera et al 1995;Shearer 1997;Rubin et al 1999;Saccorotti et al 2002).…”

“…These time differences can be calculated using either the cross-spectral analysis method (Poupinet et al 1984;Frémont & Malone 1987;Got et al 1994;Rubin et al 1999), or the time-domain, cross-correlation approach (Deichmann & García-Fernández 1992;Augliera et al 1995;Saccorotti et al 2002), which is used in this study. These time differences can be calculated using either the cross-spectral analysis method (Poupinet et al 1984;Frémont & Malone 1987;Got et al 1994;Rubin et al 1999), or the time-domain, cross-correlation approach (Deichmann & García-Fernández 1992;Augliera et al 1995;Saccorotti et al 2002), which is used in this study.…”

“…Repeating the correlations with all possible pairs of events within a given set, and applying a leastsquares fitting procedure, it is possible to determine arrival-time differences between master and slaves events, which minimize discrepancies, allowing a more precise determination on the onset times (see García-Fernández 1992 andSaccorotti et al 2002 for the mathematical formulation of the procedure). Repeating the correlations with all possible pairs of events within a given set, and applying a leastsquares fitting procedure, it is possible to determine arrival-time differences between master and slaves events, which minimize discrepancies, allowing a more precise determination on the onset times (see García-Fernández 1992 andSaccorotti et al 2002 for the mathematical formulation of the procedure).…”

Repeated palaeoseismic activity of the Ventas de Zafarraya fault (S Spain) and its relation with the 1884 Andalusian earthquake

“…4). The occurrence of series and seismic swarms is frequently observed, affecting the depression (Vidal 1986;Posadas et al 1993;Saccorotti et al 2001). The depth of this shallow activity ranges between 5 and 15 km to a maximum of 20 km and includes earthquakes with M W %5 or more.…”

Precise determination of the relative wave propagation parameters of similar events using a small‐aperture seismic array

“…The main advantage of the enhanced waveform similarity is that we are able to determine very accurately the time delay between the occurrence of two earthquakes. This precise relative timing of multiplet earthquakes can be used to estimate relative locations of clusters of seismic sources [ Got et al , 1994; Phillips , 2000; Stich et al , 2001; Saccorotti et al , 2002; Brancato and Gresta , 2003]. Other applications of precise relative timing include studies of the origin of the coda [ Antolik et al , 1996; Aster et al , 1996; Got and Coutant , 1997], temporal variations of velocity in tectonic [ Poupinet et al , 1984; Haase et al , 1995] or volcanic settings [ Ratdomopurbo and Poupinet , 1995; Poupinet et al , 1996], and the detailed properties of the inner structure of the Earth [ Song and Richards , 1996; Poupinet et al , 2000].…”