Amplification phenomena observed in downhole array records generated on a subductive environment

The damage caused by earthquakes is strongly dependent on the local site conditions and, as a result, site characterization and amplification phenomenon have become a subject of considerable interest to engineers and seismologists. A vertical array in a soil deposit in Llolleo City, near the coast of Chile, has been installed, being somewhat unique in that it is located in a subduction zone environment. Because the coast of Chile is one of the most seismically active areas of the world, this array is expected to be particularly valuable because it may produce recordings of very strong and very long duration motions. The characteristics of this borehole array are presented first, which include pore water pressure measurements in clayey soils. Also some interesting aspects of the earthquakes recorded to date are described, which open a question about the suitability of one-dimensional analysis. In addition, the H/V spectral ratio proposed by Nakamura to estimate natural periods is analyzed. Microtremor measurements at the surface and the seismic events recorded by the seismic borehole array of Llolleo are used, looking both at the H/V spectral ratio of the motion recorded on the free surface and at the ratio of the response spectra of the motions at the surface and at bedrock. On the other hand, it is observed that the recorded dynamic pore pressures are controlled by the vertical accelerations, or compressional waves.