Seismotectonics of the Canary Islands

Marine Geophysical Researches, 2003

A series of clastic dikes and tubular vents were identified in southern Tenerife (Canary Islands). These features are the result of seismic liquefaction of a Holocene sand deposit, as the consequence of a high intensity paleoearthquake. The peak ground acceleration (pga) and magnitude of the paleoearthquake generating these liquefaction features were estimated by back calculation analysis. A representative value of 0.30 ± 0.05 g was obtained for the pga. From this, an earthquake intensity of IX was estimated for the liquefaction site. Magnitude bound methods and energy based approaches were used to determine the magnitude of the paleoearthquake, providing a moment magnitude M = 6.8. The zone in which the liquefaction structures are found has undergone tectonic uplift and is affected by two faults. One of these faults was responsible for displacing Holocene materials. Dating of the uplifted sand formation indicates an age of 10,081 ± 933 years, the liquefaction features ranging from this age to 3490 ± 473 years BP. This paleoearthquake was of much greater magnitude than those known historically. Faults with neotectonic activity are significant features that should be borne in mind when assessing the seismic hazards of the Canary Islands, presently considered as low and mainly of volcanic origin.

Tectonophysics, 2007

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.

Earth and Planetary Science Letters, 1997

Seismic data have been used to determine the crustal and upper mantle structure of Tenerife, Canary Islands, a volcanic island of Tertiary age located on > 140 Ma oceanic crust. Reflection data show that oceanic basement dips gently towards the island, forming a flexural moat which is infilled by 2-3 km of well stratified material. The moat is characterised by a major angular unconformity, which we attribute to volcanic loading of pre-existing oceanic crust and overlying sediments and the subsequent infilling of the flexure by material that was derived, at least in part, from the islands. Refraction data show that the flexed oceanic crust has a mean thickness of 6.41 + 0.42 km and upper and lower crustal velocities of 4.8-5.4 km s-' and 6.7-7.3 km s-' respectively. The flexure, which has been verified by gravity modelling, can be explained by a model in which Tenerife and adjacent islands have loaded a lithosphere with a long-term (> IO6 yr) elastic thickness of approximately 20 km. Seismic and gravity data suggest that up to 1.5 X lo5 km3 of magmatic material has been added to the surface of the flexed oceanic crust which, assuming an age of 6-16 Ma for the shield building stage on Tenerife, implies a magma generation rate of about 0.006 to 0.02 km3 a-'. This rate is similar to estimates from other African oceanic islands (e.g., Reunion and Cape Verdes), but is significantly less than that which has been calculated at Hawaii. There is no evidence in either the seismic or gravity data that any significant amount of magmatic material has "underplated" the flexed oceanic crust. The crustal and upper mantle structure at Tenerife therefore differs from other oceanic islands such as Hawaii and Marquesas where > 4 km of underplated material have been reported.

Tectonophysics, 1976

New seismicity and focal-mechanism data from the area of the Azores Islands, in the Mid-Atlantic Ridge, to the Alboran Sea and the southern part of Spain are presented. As a consequence of the different characters in the focal-mechanism solutions and b-values associated, the area has been divided in four different parts, namely, Mid-Atlantic and Terceira Ridge, Azores-Gibraltar fault, Gulf of Cadiz, Alboran Sea and Betica. The last two form the interaction between the Eurasian and African continental plates. The fracture zone is the locus of very large earthquakes with mechanisms showing a predominant right-lateral horizontal motion. Seismic foci in the continental interaction zone are spread over the whole region with mechanisms changing in character from west to east. It is suggested that this may be consequence of the behaviour of the Spanish Peninsula as a partly independent subplate. In the eastern part of the studied zone, the so-called Alboran plate may be considered as a buffer plate.