Abstract
The central region of Sao Miguel Island is one of the most seismically active areas of the Azores archipelago. A revised analysis of the seismicity distribution at this region has, for the first time, shown that the seismicity is clustered in two distinct areas: the area around Fogo Volcano (Fogo) and the area around Congro maar (Congro), with each area having a highly localized swarm activity. From a total of about 15,000 events in the period from 2002 to 2010, 78 best located events were selected to make fault plane solutions using P-wave polarities and amplitude ratios. This set of fault plane solutions, and another six subsets derived from it, were inverted for the best fitting stress tensor. The stress tensor using all the 78 fault plane solutions is characterized by a subhorizontal σ1 striking WNW-ESE and a σ3 striking NNE-SSW, consistent with the regional stress field for this region. A similar result, using only the fault plane solutions located in the Fogo area, was obtained. On the other hand, for the Congro area, a local stress field seems to be superimposed on the regional field: subhorizontal σ3, striking NNE-SSW, and a near-vertical σ1. The same stress regime persists in the first 5 km depth, probably related to the upwelling of thermal fluids. The rising fluids generate horizontal extension at shallow depths, which favour the opening of cracks and the circulation and ascension of hydrothermal fluids. The stress regime deeper than 5 km is more uncertain; however, it is indicative of a compressional regime. Thus, it can be conclude that the smaller Fogo area appears to be dominated by the normal regional stress field while the high active Congro area seems to have a different, highly heterogeneous stress field dominated by local conditions.
Highlights
The historical information available, which describes the destructive effects caused by earthquakes, shows spatial and temporal gaps that are due in large part to the geographical distribution of the population
The epicentre map of instrumental seismicity (Fig. 1) shows the branches of the triple junction of the Azores region well defined to 24° W, along a sector that includes the Azores Plateau (AP) and extends from the Mid-Atlantic Ridge (MAR) to 24o W
The high level of seismicity along the MAR and the Terceira Rift (TR) (Fig. 1) is strongly associated with seafloor spreading and the northeastward motion of the Eurasian plate with respect to the African plate (Luis et al, 1994).This argument was supported by Fernandes et al (2002), who proposed an elastic model with two possible locations of the Azores triple junction: along the extension of the TR at the same latitude as Graciosa Island or on the Faial fracture zone at the same latitude as Faial Island
Summary
The historical information available, which describes the destructive effects caused by earthquakes, shows spatial and temporal gaps that are due in large part to the geographical distribution of the population. (Oliveira, 1986), in the Azores region, such information is not available before the beginning of the sixteenth century, when a sizable population began to occupy the islands. Miguel Island, and buried that capital, not being saved of her more than a small suburb of the West side, and two houses on the beach Of these places there survived little more than seventy people.... In addition to the destruction of a large part of the housing stock in Faial and the eight deaths, the earthquake damaged approximately 20% of the houses on the neighbouring island of Pico These are three descriptions of the many possible ones regarding the consequences of the earthquakes, sometimes with relatively low magnitudes, that throughout history have perturbed the stability of the Azoreans. Considering that the focal mechanisms of recent significant earthquakes are the information basis to understanding geodynamic and seismotectonic models of the Azores, this issue will be addressed in the current study
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