José Antonio Peña

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.

Magnitude scales for very local earthquakes. Application for Deception Island Volcano (Antarctica)

Abstract Different magnitude scales are calculated for a set of volcano-tectonic earthquakes recorded in Deception Island Volcano (Antarctica). The data set includes earthquakes recorded during an intense seismic series that occurred in January–February 1999, with hypocentral distances that range between 0.5 and 15 km. This data set is enlarged to include some regional earthquakes with hypocentral distances up to 200 km. The local magnitude scale, M L , fixed at a hypocentral distance of 17 km, is used as the reference for the other magnitude scales studied in the present work. M L is determined on a standard Wood–Anderson simulated trace assuming a gain of 2080. Maximum peak-to-peak amplitudes are measured on the vertical components of a short-period sensor. The M w scale is calculated, in the vertical component, both for P and S waves. The attenuation correction of the ground motion displacement spectra is introduced using data from coda waves studied in the area. The comparison between M L values and M w estimations indicates severe discrepancies between both values. A magnitude–duration scale is calibrated from the comparison between coda durations of the recorded events and their assigned local magnitude scales. In order to investigate the causes of the discrepancy between the M L and M w values we analyze two possible error sources: a wrong coda Q value, or the effects of the near-surface attenuation that initially are not taken into account in the correction of the ground displacement spectra. The analysis reveals that the main cause of this discrepancy is the effect of the near-surface attenuation. The near-surface attenuation is also the cause of the determination of an anomalous spectral decay slope, after the corner frequency, and the determination of this corner frequency value. This near-surface attenuation, represented by κ , is estimated over the data set, obtaining an average value of 0.025. With this κ value, the M w scale is recalculated using an automatic algorithm. The new M w values are more consistent with the M L values, obtaining a relationship of M w =0.78 M L −0.02.

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.

Spatial and temporal analysis of a seismic series using a new version of the three point method: application to the 1989 Antequera (Spain) earthquakes

Abstract The three point method (TPM) has been successfully applied to several seismic series and has provided information about the spatial characteristics (azimuth and dip) of the fault planes activated in the rupture process. A new development of the TPM to determine temporal characteristics, is presented, to obtain the evolution of the fracturing process of an active fault system. For the analysis of the 158 microearthquakes and earthquakes that took place in the seismic series of Antequera in June 1989, the choice of a threshold magnitude (mu = 2.5) has permitted the events related to the most relevant fractures to be distinguished. Secondly, only the events between two concentric spheres (here named Spatial Crown) with respect to a given earthquake, have been used in order to avoid taking into account earthquakes that are too close to each other, together with the very distant events that have little relation to the event analysed. The Spatial Crown has permitted some clear results in the Antequera series, where we have found that the fracturing process began fundamentally with N 80° E planes and evolved to N 65° W planes. Finally, an error analysis enables an estimation of the uncertainty in the results to be obtained from the errors in the data.

Application of the CMP refraction method to an archaeological study (Los Millares, Almerı́a, Spain)

Obtaining information at an archaeological site by means of geophysical methods can reduce the need for intensive excavation. This paper addresses the use of seismic methods to reveal details in a non-destructive manner at the archaeological site of Los Millares (Almeria, Spain). The seismic refraction method provides information on the low frequency component of the model for the shallowest layers. In this way, it is possible to fix the thickness of the surface layer, as well as to determine a velocity model. Use of the refraction method in Los Millares has resulted in the determination of the depth of the calcaric surface upon which the foundations were built. The application of a recently developed method, common-midpoint (CMP) refraction, allows the detection of local heterogeneities in the near subsurface. This method uses the amplitude, phase and frequency information of the first arrivals. The results highlight the location of anomalous zones characterized by early first arrivals. According to a priori geological and archaeological information, these anomalies can be correlated with buried foundations providing the key information for planning future excavations.

The Utility of Geophysical Models in Archaeology: Illustrative Case Studies

This chapter uses case studies to illustrate the way geophysical techniques are integrated into archaeological approaches. The general idea is to show how the different geophysical models obtained using magnetic, electric, seismic and GPR data, are used in the archaeological praxis and the capacity they have to respond to particular questions.