Validation of Velocity the Lower Tagus Valley (Portugal) velocity model using H/V technique

Abstract

Along his history the Lower Tagus Valley LTV area was shaken by several earthquakes. The largest reported had their origin in the southwestern part of Iberia. These earth- quakes were destructive, and some of them were produced in large ruptures of offshore structures located southwest of the Portu- guese coastline; other moderates earthqua- kes were produced by local sources such as the 1344, 1531 and the 1909 (Benavente). In the last years, due to 3D structural model improvement and development in numeri- cal methods and computational capacities, several studies have successful obtained strong-ground motion synthesis for the Lower Tagus region using finite difference method [1]. To confirm the velocity model of the LTV sedimentary basin obtained by geophy- sical and geological data, we use broad-band microtremor measurements and application of the horizontal to vertical (H/V) spectral ratio method [2]. Some seismic data collec- ted in a profile with azimuth perpendicu- lar to the basin axis reveals a dependence between the thickness of sediments, the frequency and the amplitude of the low fre- quency peaks (0.15 -1 Hz) of the H/V curve. [1] Grandin R., Borges J.F., Bezzeghoud M., Caldeira B., Carrilho F., 2007. Simulations of strong ground motion in SW Iberia for the 1969 February 28 (MS = 8.0) and the 1755 No- vember 1 (M ~ 8.5) earthquakes – II. Strong ground motion simulations, Geophys. J. Int., Vol. 171, 2, 807-822. [2] Nakamura, Y., 1989, A method for dynamic characteristics estimations of subsurface using microtre- mors on the ground surface, Quarterly Re- port, RTRI, Japan, v. 30, p. 25-33.