Terceira island, Azores: crustal imaging and correlation of volcano-tectonic structures with seismicity

Terceira is a volcanic Island of the Azores Archipelago, located in a tectonic triple junction expression of the intersection of the Mid-Atlantic Ridge with the complex western sector of the Eurasian-Africa Boundary. The island sits on the so-called Terceira rift, the Azorean segment of the triple junction, oblique to the main spreading direction with ultra-slow spreading rate.

The main active volcanic structure is the Santa Barbara volcano to the west of the island, geologically the younger part. The island is divided by a roughly WNW-ESE fissural volcanic system, that intersects Santa Bárbara volcano to the north, crosses the active Guilherme Moniz volcano, in the mid of the island, and the extinct Serra do Cume volcano to the south. Pico Alto, another active volcano, is located north of the fissural system, whereas to the NE lies the significant tectonic structure of the Lajes Graben.

We will present a 3D tomographic model of Terceira Island, obtained from coupled inversion of local earthquake and ambient noise tomography, using a dense temporary seismic deployment (2019-2020) coupled with data from the permanent IPMA Seismic Network (2000-2019).

The Santa Bárbara volcano presents a distinct tomographic signature, with a high Vp and Vp/Vs anomalies located beneath the caldera and extending ~4-6 km, and significant seismicity up to ~6 km depths. On the other hand, although with the same signal the Vp and Vp/Vs anomalies beneath the Pico Alto volcano are much weaker, with few seismicity associated. The shallow layers beneath Guilherme Moniz present weak perturbations on both Vp and Vp/Vs, but a stronger deep high Vp anomaly is clear around 7 km depths with no significant Vp/Vs variation associated. The fissural system signature is usually associated with shallow anomalies, with high Vp north of Santa Bárbara volcano, but low Vp and very low Vp/Vs ~1.6 beneath the area between Pico Alto and Guilherme Moniz volcanoes; the low Vp/Vs anomaly in the center of the island is very shallow <3 km and is probably due to the presence and migration of geofluids in the vicinity of the geothermal powerplant.

The fissural system in this area presents a strong seismic activity, with the deepest events recorded by the networks ~9 km; a relatively high number of very shallow and low magnitude events were recorded around the area of the geothermal powerplant.

The SW area of the Serra do Cume volcano presents high Vp values and usually a normal Vp/Vs ~1.77, suggesting an already frozen volcanic system. It has some significant seismicity associated probably due to the intersection of the fissural system that splits the island. To the NE, a bit surprisingly, no significant seismicity was recorded along the Lajes Graben; the lack of seismicity also gave a low tomographic coverage.

This is a contribution to RESTLESS (DOI:10.54499/PTDC/CTA-GEF/6674/2020) and GEMMA (DOI: 10.54499/PTDC/CTA-GEO/2083/2021) and funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC): UID/50019/2025 and LA/P/0068/2020 https://doi.org/10.54499/LA/P/0068/2020), and UIDB/04683 and UIDP/04683 – Instituto de Ciências da Terra.