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- Insight on the crustal stress state in Faial and Pico Islands (Azores), from analysis of aftershocks of the 1998 earthquakePublication . Dias, Nuno; Tellez, Julia; Matias, LuisThe seismicity in the Azores plateau can be of volcanic or tectonic origin, the latter usually more destructive, especially in the central group of islands. In this study we aimed to obtain crustal stress indicators from seismic data collected in July 1998, following the July 9th M5.8 earthquake that struck near Faial and Pico islands. The maximum horizontal stress (SHmax) azimuth was obtained from two methods. For the seismogenic volume, the SHmax was obtained from single and composite focal mechanisms, whereas the state of stress beneath the seismic network was derived from shear-wave splitting analysis of local earthquakes. The composite focal mechanisms were obtained from a waveform clustering method applied to identify "identical" earthquakes. The seismic anisotropy analysis could be applied to only six stations, the polarization of the first S arrival being stable, and with a secondary perpendicular pulse arriving 0.6-0.24 s later. The focal mechanisms provided a NW-SE SHmax with azimuth range of 96 degrees-114 degrees, and an associated transition of normal-fault to strike-slip dominant regime consistent with the estimated regional field of 110 degrees-120 degrees, and coherent with movement along three fault systems. Inland the SHmax azimuths present greater variability. In Faial, the stations located north of the Caldeira present a stable azimuth of 151 degrees-153 degrees and are consistent with the effect of a NNW-SSE fault system present in the islands, rotating to 50 degrees-77 degrees in the eastern sector of the Pedro Miguel Graben, probably due to a complex interaction of the WSW-ENE dominant fault system with old eroded volcanic complexes that segmented the island into several blocks. In Pico island, the SHmax direction azimuth ranges from 93 degrees to 135 degrees, in a radial pattern around the island's main stratovolcano, overlapping the regional stress field.
- The Portuguese national seismic network: products and servicesPublication . Carrilho, Fernando; Custodio, Susana; Bezzeghoud, Mourad; Oliveira, Carlos; Marreiros, Célia; Vales, Dina; Alves, Paulo; Pena, Areosa; Madureira, Guilherme; Escuer, Maria; Silveira, Graça; Corela, Carlos; Matias, Luis; Silva, Matilde; Veludo, Idalina; Dias, Nuno; Loureiro, Afonso; Borges, J. F.; Caldeira, Bento; Wachilala, Piedade; Fontiela, JoaoPortugal, located in the southwest region of the Eurasian plate, has been affected by several destructive earthquakes throughout its history, the most well-known being the 1755 Great Lisbon earthquake. The seismicity of the territory, both in the mainland and in the Azores and Madeira islands, has prompted the continuous development of seismic monitoring, from the first known macroseismic inquiry, following the 1755 Great Lisbon earthquake, to the current state-of-the-art seismic network. Once scattered in separate efforts, at present, most seismic stations in Portugal relay its data to a common data center, at Instituto Português do Mar e da Atmosfera, where data are automatically processed for the downstream generation of both manually revised and automatically generated products and services. In this article, we summarize the evolution of the permanent seismic network, its current status, the products and services that are publicly available, a recent effort of rapid deployment of a dense network following a mainshock, and state-of-the-art ocean-bottom seismometer developments.