High-resolution seismic amplification map of San Cristóbal de La Laguna (Tenerife, Canary Islands) through micro tremor HVSR method

The island of Tenerife (Canary Islands, Spain) suffered in the past centuries various historical eruptions accompanied by intense volcanic seismicity. In particular the 1704-1705 Arafo-Fasnia-Siete Fuentes, caused about 13 victims because of the associated seismicity. Under these premises, Instituto Volcanológico de Canarias (INVOLCAN) started a research program aimed at mapping the seismic amplification in the urban areas of the densely populated island of Tenerife. We selected the historical capital of Tenerife: the city of San Cristóbal de La Laguna as a starting point for this research program. The town of San Cristóbal de La Laguna is located over an old lacustrine area filled with sedimentary deposits, making this the ideal starting point. Previous studies, with 3 measurement points, already evidenced local seismic amplification effects. Furthermore, its urban area, declared World Heritage Site by UNESCO in 1999, is constituted mostly by highly vulnerable historical buildings.

We have conducted an extensive survey in the urban area of San Cristóbal de La Laguna, performing more than three hundred of broadband seismic noise measurement over an area of about 5 km ² . At each measurement point, two hour long recordings of seismic ambient noise were taken. Measurements were obtained by using simultaneous recordings on mini-arrays of 3-4 elements, spacing them a few hundred meters apart.

The use of the microtremor Horizontal to Vertical Spectral Ratio (HVSR) method provided a rapid preliminary assessment of the data quality and the amplification frequencies. Subsequently, a cross correlation between the stations pairs to retrieve empirical Green's functions and Rayleigh waves dispersion curves through Frequency Time Analysis (FTAN), were performed. For each mini-array a joint inversion of HVSR and dispersion curves using trans-dimensional inversion were done, to determine the vertical velocity and density profiles at each site.

This survey presents the first extensive mapping of seismic site amplification effect on the city and its subsurface geological structures. We evidenced a clear relationship between the thickness of the lacustrine deposits under the town and the frequency of the strongest peak in HVSR.