Seismic swarms, fault plane solutions, and stress tensors for São Miguel Island central region (Azores)

Abstract

Resumen. La historia del archipiélago de Azores, desde el descubrimiento y colonización en la primera mitad del siglo XV hasta ahora, está marcada por los impactos sociales y económicos producidos por los terremotos, principalmente los de alta intensidad. La información compilada nos lleva a concluir que en este periodo 33 terremotos han afectado las islas de Azores con una intensidad igual o superior a VII, causando unas 6.300 muertes y la destrucción generalizada en algunas Islas del Archipiélago, principalmente en S. Miguel, Terceira, Graciosa, Faial, S. Jorge y Pico. La acomodación de los movimientos diferenciales que se producen debido al límite entre las placas eurasiática (EA), africana (AF) y norteamericana (NA) y también al volcanismo que ocurre en la región, son los principales responsables de la intensa actividad sísmica que ocurre en este archipiélago. Este trabajo revisa los temas científicos de los terremotos conocidos que han interferido severamente con la vida del pueblo azoreano a lo largo de su historia, a esos terremotos llamamos grandes terremotos. Palabras clave: Terremotos de Azores; Sismotectónica de Azores; Mecanismo focal de los terremotos de Azores.[en] Large earthquakes in the Azores Abstract. The history of the Azores archipelago, from its discovery and settlement in the first half of the 15th century through the present, is marked by the social and economic impacts produced by earthquakes, mainly the high-intensity ones. Information that has been compiled leads to the conclusion that in this period, 33 earthquakes with intensity equal to or greater than VII have affected the Azores, which caused approximately 6,300 deaths and widespread destruction on some islands of the archipelago, principally S. Miguel, Terceira, Graciosa, Faial, S. Jorge and Pico Islands. The accommodation of strain resulting from the dynamics of the Azores triple junction (ATJ) plate boundary and volcanism, which also occurs in the region, are the main factors responsible for the intense seismic activity in this archipelago. This work reviews the scientific issues of the known earthquakes that have severely interfered with the lives of the Azorean people throughout their history, which we call large earthquakes. Key words: Azores earthquakes; historical earthquakes; seismotectonics of Azores; focal mechanisms.

CodaQin Different Tectonic Areas, Influence of Processing Parameters

“…This area is also the seismically most active area in the Azores Islands (Silva et al, 2012). Because we use a lapse time of 30 s and a window of 30 s, a quite large area is sampled (radius ∼80 km), much larger than the Fogo volcanic area so the real coda Q in the Fogo area might be much lower.…”

“…The age of the islands is around 1 Ma (Johnson et al, 1998;Calvert et al, 2006). The area is seismically very active (Matias et al, 2007) and there is significant volcanic activity (Silva et al, 2012). Data from the Portuguese digital seismic network on the Azores (25 stations), currently operated by Instituto Português do Mar e da Atmosfera from 1996 to the present day, are used.…”

“…Data from the Portuguese digital seismic network on the Azores (25 stations), currently operated by Instituto Português do Mar e da Atmosfera from 1996 to the present day, are used. A few events from the University of the Azores (used by the first author for another study, Silva et al, 2012) were also used. Magnitudes were in the 0.6-6.1 range and the data covers the period 1998-2014.…”

The 8 October 2011 Earthquake at El Hierro (Mw 4.0): Focal Mechanisms of the Mainshock and Its Foreshocks

“…Seismic sources of crustal tectonic earthquakes are related to local stress patterns and an analysis of their focal mechanisms can help to understand the physical process of magma migration in active volcanic islands (Roman and Cashman, 2006;Umakoshi et al, 2011;Silva et al, 2012). However, scarce focal mechanism data is a common characteristic in the whole Canarian archipelago.…”

Grandes terremotos en Azores

“…In May 2005, a seismic sequence started, and lasted until late 2006, and thousands of earthquakes were recorded. Silva et al (2012) analysed the focal mechanisms of several earthquakes located in different seismic clusters and concluded that on the Fogo Volcano, a normal regional stress field prevails, and at the contiguous volcano Congro, there is a highly heterogeneous stress field. The b-value in the central part of São Miguel Island is low (Fontiela et al, 2017a), which can be explained by the hypothesis that 75% of the displacement between the Eurasian and African plates is located on a narrow strip of 10-15 km in the central part of São Miguel (Jónsson et al, 1999).…”