Seismic microzoning studies in urban areas of Tenerife and La Palma islands

Over the last 500 years, Tenerife and La Palma (Canary Islands) have suffered several destructive earthquakes, mostly linked to volcanic activity but also generated by regional tectonics. These seismic events can be very shallow and reach moderate magnitudes, as observed in recent volcanic eruptions in the archipelago. The islands' geological complexity can lead to local seismic amplification due to site effects. Therefore, detailed in situ studies of local seismic responses are necessary to assess the seismic hazard correctly.

For these reasons, INVOLCAN has conducted various seismic microzonation surveys in different areas of both islands since 2019. These studies involved principally measuring microtremors in urban areas. The HV method was applied to the large amount of data recorded to determine the predominant frequencies of the ground. The results were compared with existing geological information and geotechnical borehole data.

The first study was conducted in San Cristóbal de La Laguna (Tenerife), whose old town has been declared a universal heritage site by UNESCO. The city is located in a valley filled with lacustrine deposits and lava flow layers, so its local geology makes it susceptible to local seismic amplification effects. In La Laguna, we performed 453 microtremor measurements using broadband stations.

The second study was conducted in La Orotava Valley (Tenerife), where 236 microtremor measurements were taken. This valley originated 500.000 years ago due to a giant gravitational landslide, and nowadays, it is an area hosting significant population centres and key tourist infrastructure.

Finally, the third study was performed in the Aridane Valley (La Palma), where 200 microtremor measurements were obtained. This valley also results from a gravitational landslide of the Cumbre Nueva volcanic edifice. This area was recently affected by the Tajogaite eruption in 2021.

Our main findings are: (1) in the first study, La Laguna Valley is characterised mainly by low frequencies, possibly related to thick lacustrine deposits, but also by secondary high-frequency peaks revealing the existence of thin layers at the surface; and (2) in the other two study areas the frequencies vary between medium-low values that are likely associated with the gravitational landslide deposits.