Insight on the Crustal Stress State in Faial and Pico Islands (Azores), from Analysis of Aftershocks of the 1998 Earthqu
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Pure and Applied Geophysics
Insight on the Crustal Stress State in Faial and Pico Islands (Azores), from Analysis of Aftershocks of the 1998 Earthquake NUNO AFONSO DIAS,1,3
JU´LIA TE´LLEZ,2 and LUI´S MATIAS3
Abstract—The 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°–114°, and an associated transition of normal-fault to strike-slip dominant regime consistent with the estimated regional field of 110°–120°, 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°–153° and are consistent with the effect of a NNW–SSE fault system present in the islands, rotating to 50°–77° 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° to 135°, in a radial pattern around the island’s main stratovolcano, overlapping the regional stress field. Keywords: Seismicity and tectonics, anisotropy, waveform cross-correlation, crustal stress, oceanic plateaus.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00024-020-02558-1) contains supplementary material, which is available to authorized users. 1
Instituto Superior de Engenharia de Lisboa, Instituto Polite´cnico de Lisboa, Rua Conselheiro Emı´dio Navarro 1, Lisbon 1959-007, Portugal. E-mail: [email protected]; [email protected] 2 Facultad de Ciencias Fı´sicas, Universidade Complutense de Madrid, Madrid 28040, Spain. E-mail: [email protected] 3 Instituto Dom Luiz (IDL), Faculdade de Cieˆncias, Universidade de Lisboa, Campo Grande, 1749-016 Lisbon, Portugal. E-mail: [email protected]
1. Introduction The Azores archipelago (Portugal) rises from an oceanic plateau located in a tectonic triple junction between the American, Eurasian and African plates. The Azores Plateau (Fig. 1) is defined by the intersection of the Mid-Atlantic Ridge (MAR) with
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