Salinas, V.; Luzon, F.; Garcia, A.; Sánchez-Sesma, F.; Kawase, H.; Matsushima, S.; Suarez, M.; Cuellar, A.; Campillo, M. Bulletin of the Seismological Society of America Vol. 104, num. 2, p. 995-1001 DOI: 10.1785/0120130202 Data de publicació: 2014-03 Article en revista
It has been recently demonstrated that averaging the autocorrelations of fields produced by various almost-vertical incoming elastic body plane waves upon a layered system approximately leads to the imaginary part of the corresponding 1D Green's functions for deep sources located underneath the receiver (Kawase et al., 2011). Thus, the ensemble of these waves from deep earthquakes recorded in a station located in the epicentral zone is interpreted as a diffuse field. In this short note, we extend the study to consider earthquakes recorded in a station located at epicentral distances of up to hundreds of kilometers. We consider the horizontal-to-vertical spectral ratio (HVSR) of the averaged P, S, and coda waves and full earthquake records at the Cibeles station (Mexico City Accelerometric Network) and compare these with the results obtained with the corresponding HVSR for the 1D (Kawase et al., 2011) and the 3D (Sanchez-Sesma, Rodriguez, et al., 2011) diffuse fields models. Using the signals of 90 earthquakes recorded at Cibeles, we find that the experimental results have distinctive features compatible with the 3D signature of a diffuse field. We interpret this result as a consequence of the multiple paths that seismic waves undergo from the subducting slab to the Mexico City valley and to the multiple scattering in a complex tectonic environment. Our study strongly suggests that we can use strong-motion records from earthquakes and apply similar techniques to the ones used to analyze the ambient seismic field.
Navarro Bernal, Manuel; Corchete, V.; Badal, J.; Canas, J.; Pujades, L.G.; Vidal, F. Bulletin of the Seismological Society of America Vol. 87, num. 4, p. 847-865 Data de publicació: 1997-08 Article en revista
Group velocity dispersion measurements of Rg 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 Rg-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 Rg 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.
Up to now, dispersion analysis of surface waves across the Iberian Peninsula and adjacent zones has been based on analog data recorded at the long-period Iberian stations. Also, the northern region of the peninsula has never been investigated due to the lack of seismological stations. With the ILIHA data set now available, it is possible to investigate the northern part of Iberia from quality digital records. To efficiently remove higher-mode interference and to improve isolation of the fundamental-mode Rayleigh wave from the seismograms, time-variable filtering is employed. Once the signal is filtered, multiple filtering is then used to compute group velocities at each station. The interstation Rayleigh-wave group velocity can thus be easily calculated. Frequency-domain Wiener deconvolution is used to determine the interstation phase velocity. We carried out inversion of velocity dispersion data containing both Rayleigh-wave phase velocities and group velocities according to the generalized inversion theory by means of the stochastic inverse operator. The theoretical 2-D Earth models determined by joint inversion allow us to obtain for the first time the distribution of the shear-wave velocity both laterally and with depth for the northern Iberian region, and to emphasize the main features of the crust-mantle structure of this area.
The coda Q method has been applied to a set of earthquakes recorded by the short-period vertical component of the seismograph station OXM in southern Mexico. A frequency independent Q0 value of 489 for the Lg phase has been found for southern Mexico. This value is similar to the values found for central Rocky Mountains, and higher than those obtained for the Western United States. Assuming that Pg, Pn, Sg, and Sn phases behave as wave trains, attenuation coefficients have been found for periods close to 1 sec. The attenuation coefficient values for the Pg, Sn, and Sg phases are almost equal to that of the Lg phase. The Pn wave shows a slightly higher ¿ value than the corresponding Lg wave. This could be due to scattered Pn data. This study indicates that the 1 Hz Lg attenuation in southern Mexico is about three times lower than in California.