No 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.

S U M M A R Y Some results of a study of the propagation of monochromatic Rayleigh waves normally incident on the western ocean-continent boundary of the Iberian Peninsula are described. This seismological problem is modelled in two dimensions using the finite element method for periods in the range of 1.5 to 60.0s. The following results for ocean-continent direction of propagation are presented: (i) the estimate of the reflected and transmitted wavefields; (ii) the phase velocity curves for the fundamental mode in the transition zone; (iii) the transmission and reflection coefficients; and (iv) the seismic energy distribution for transmitted and reflected modes. Experimental data from P O R T 0 (PTO), a continental station, and PONTA DELGADA (PDA), an oceanic station, were compared to ascertain the effect on Rayleigh wave propagation across the continental margin. Comparison of the experimental amplitude ratios and transmission factor suggests a thickness of about 4 km for the sediment layer in the western oceanic zone of the Iberian Peninsula.