The 1980, 1997 and 1998 Azores earthquakes and some seismo-tectonic implications

Abstract

We have studied the focal mechanisms of the 1980, 1997 and 1998 earthquakes in the Azores region from body-wave inversion of digital GDSN (Global Digital Seismograph Network) and broadband data. For the 1980 and 1998 shocks, we have obtained strikeslip faulting, with the rupture process made up of two sub-events in both shocks, with total scalar seismic moments of 1.9 × 10 19 Nm (M w = 6.8) and 1.4 × 10 18 Nm (M w = 6.0), respectively. For the 1997 shock, we have obtained a normal faulting mechanism, with the rupture process made up of three sub-events, with a total scalar seismic moment of 7.7 × 10 17 Nm (M w = 5.9). A common characteristic of these three earthquakes was the shallow focal depth, less than 10 km, in agreement with the oceanic-type crust. From the directivity function of Rayleigh (LR) waves, we have identified the NW-SE plane as the rupture plane for the 1980 and 1998 earthquakes with the rupture propagating to the SE. Slow rupture velocity, about of 1.5 km/s, has been estimated from directivity function for the 1980 and 1998 earthquakes. From spectral analysis and body-wave inversion, fault dimensions, stress drop and average slip have been estimated. Focal mechanisms of the three earthquakes we have studied, together with focal mechanisms obtained by other authors, have been used in order to obtain a seismotectonic model for the Azores region. We have found different types of behaviour present along the region. It can be divided into two zones: Zone I, from 30°W to 27°W; Zone II, from 27°W to 23°W, with a change in the seismicity and stress direction from Zone I. In Zone I, the total seismic moment tensor obtained corresponded to left-lateral strike-slip faulting with horizontal pressure and tension axes in the E-W and N-S directions, respectively. In Zone II, the total seismic moment tensor corresponded to normal faulting, with a horizontal tension axis trending NE-SW, normal to the Terceira Ridge. The stress pattern for the whole region corresponds to horizontal extension with an average seismic slip rate of 4.4 mm/yr.

Revision of the Portuguese catalog of tsunamis

“…The hypocentral coordinates for this shock were (NEIC) 38.81 • N, −27.78 • E, H =10 km, the magnitude is M s =6.7, M w =6.8 (Hirn et al, 1980;Borges et al, 2007). This strike slip event generated a small amplitude tsunami recorded at Angra (Terceira Island) and the maximum amplitude observed was 0.3 m peak to peak.…”

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

“…During the recording period, there were two events of moderate magnitude (magnitudes 3.2 and 3.4) with epicentres located near of the epicentre of the main shock. The focal mechanism solution obtained describes strike-slip motion with nodal planes similar to those of the main event (Borges et al 2007;Buforn et al, 1988). Buforn et al (1988) analysed, among others, eight focal mechanisms for events located on the AP (where they found a diversity of mechanisms without any identifiable patterns) and eleven earthquakes on the MAR (where normal and strike-slip fault mechanisms are typically associated with spreading ridge).…”

“…The first study of Azorean earthquakes using extensive source models was performed by Borges et al (2007). This work was made possible by the existence of digital records of the long period of the 1980 earthquake (Mw = 6.8), obtained by the GDSN network, and the broadband data of the July 27, 1997 (Mw = 5.9), and July 9, 1998 (Mw = 6.0), earthquakes, obtained by worldwide networks (IRIS-Incorporated Research Institutions for Seismology).…”

Seismo-electromagnetic phenomena in the western part of the Eurasia-Nubia plate boundary

“…In the oceanic part, from the Azores archipelago to the Azores-Gibraltar fault approximately 12 • W, the plate boundary is very well delimited. But, from 12 • W to 3.5 • E, the boundary is more disperse and embraces a larger area of deformation, this region comprises the Iberia-African area and prolongs to the western part of Algeria, Buforn et al (2004), Borges et al (2007Borges et al ( , 2008 and Bezzeghoud et al ( , 2010. The WENP region can be separated into six fundamental zones (from the Mid-Atlantic Ridge in the west to Algeria in the east) with different seismtectonic behaviors: zone I -The Azores region; zone II -The central region; zone III -The IberoMaghrebian region.…”