Short period Rayleigh-wave dispersion applied to the determination of shallow structure.

Abstract

Very short-period surface waves are useful to obtai.n the models of shear velocity structure at shallow crustal depths. In this study, we have investigated the shear velocity shallow crustal structure of the Andarax basin (Almeria province) and Granada basin (Granada province) located in southern Spain, from inversion of fundamental mode Rayleigh wave group velocity curves generated by blasts. The analysis and inversion of Rayleigh waveforms is performed using digital filtering techniques and a generalized inversion approach. A timevariable filter has reduced the influence of noise and removed higher mode interference and multiple filtering is then used to compute group velocities from 1.13 to 1.50 km s-1 for a period range between 0.6 and 2.0 sin Andarax basin. We have obtained group velocities values between 0.83 and 1.22 km s-1 for a period range from 1.4 to 3.4 s in Granada basin. We have performed inversion for velocity structure by stochastic inversion of the dispersion data, so that we have obtained shear wave velocity model down to 2 km depth for Andarax basin which shows velocities between 1.26 and 2.58 km s-1 with a standard desviation band of 0.03-0.08 km s-1 and down to 5 km for Granada basin which shows velocities from 1.01 to 3.14 km s-1 with a standard desviation band of 0.05-0.26 km s-1. The shear velocity model for Andarax basin exhibits a low-velocity channel at depths between 200 and 500 m whereas in Granada basin the shear velocity values increases with depth. In order to guarantee the reliability of our results, we have proceeded to compute the resolution kemels at various reference depths for and observed a good coincidence between the absolute maximun of any kernels and the respective referece depth, which implies a good agreement between the calculated solution and the true solution at these depths.