HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) References Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Navarro, M. Articles by Vidal, F. Search for Related Content GeoRef GeoRef Citation

Inversion of R_g waveforms recorded in southern Spain

Department of Applied Physics Polytechnic School of Almería University of Almería, La Cañada, 04120 Almería, Spain
Department of Theoretical Physics-Geophysics University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain
Civil Engineering School Polytechnic University of Catalunya, Gran Capitán, 08034 Barcelona, Spain
Andalusian Institute of Geophysics University of Granada, P.O. Box 2145, 18080 Granada, Spain

Abstract

Group velocity dispersion measurements of R_g waves generated either by blasts or by local earthquakes are used to investigate the shallow crustal structure of Almería (southern Spain). In principle, the usable frequency range of 250 to 2000 mHz allows determination of structures to depths of about 4 km. For this purpose, the main operations are a detailed dispersion analysis of high-frequency Rayleigh waves propagating along very short paths and the inversion of R_g-wave group velocities. A total of 21 seismic events were studied. These events had small magnitudes (2.0 to 2.5 approximately) and very shallow focal depths (about 100 m) and were taken from a set of 214 events that occurred in 1991 during a Spanish-Italian seismic experiment. The events were recorded at seven single-component stations belonging to the Regional Seismic Network of Andalucía at approximate distances of between 15 and 57 km from the source. These events were grouped into six seismic sources according to specific criteria. We used digital filtering techniques providing a significant improvement in signal-to-noise ratio to determine ray-path group velocities, and we inverted dispersion data via generalized inversion. In order to obtain refined dispersion data, we have carried out a further regionalization of group velocities, and thus six small subregions have been resolved in group velocities. The highest group velocity values, from 1.93 to 2.25 km sec^–1, correspond to the Filabres mountain range, which is an area containing materials of the Nevado-Filábride complex of Paleozoic and Triassic age. On the other hand, low velocity values, between 1.39 and 1.56 km sec^–1, correspond to the Alhamilla mountain range, which belongs to the Alpujárride complex and contains conglomerates of the Cambrian and Tortonian periods. The velocities obtained for the neogene-quaternary basin of the Andarax river, with materials of the Tortonian and Pliocene periods, are also very low, between 1.35 and 1.68 km sec^–1. We inverted the regionalized group velocities in order to obtain the shear velocity structure of the region for depths down to 4 km. According to the regional Earth models that we obtained, we find clear variations in velocity both laterally and vertically for several zones with different composition. The Filabres mountain range shows high shear velocity values: 2.14 to 2.83 km sec^–1. In the opposite end, we have the Andarax basin that presents the lowest shear velocity values, consistent with its sedimentary structure: 1.56 to 2.55 km sec^–1. Intermediate shear-wave velocities characterize the remaining regions: the Tabernas-Sorbas basin, the Gádor mountain range, and the volcanic region of Nijar-Cape of Gata. Although the relationship between lateral changes in R_g dispersion and geologic structure may not be straightforward, in this study, we have observed a correlation between those changes and the sharply contrasting geology between adjacent geological formations.

Footnotes

Present address: Instituto Geográfico Nacional, General Ibáñez de Ibero, 3, 28003 Madrid, Spain.

This article has been cited by other articles:

				Imaging of the Near-Surface Shear-Wave Velocity Structure of the Granada Basin (Southern Spain) Bulletin of the Seismological Society of America, February 1, 2003; 93(1): 430 - 442.