Scientifiques files
 

30-05-2013 : Earthquake of magnitude 5.7 east of Azores.


The CENALT (Tsunami Warning Centre) detected an earthquake of magnitude 5.7 in the Azores, which occurred on 30 April 2013 at 6h 25 UT (8h25 French local time). The epicentre is located at:
• Onset time: 06.25 UT
• Latitude: 37.46° N
• Longitude: 24.94° W
• Depth: 10 km
• Magnitude: 5.7
As envisaged in the procedures established for earthquakes of this magnitude in the Atlantic, according to the international standards of the Tsunami Early Warning and Mitigation System in the North-eastern Atlantic and Mediterranean (NEAMTWS), the CENALT issued a message of information at 8h37 (local time = UT + 2), for the attention of the ministerial authorities.

Seismo-tectonic context

The seismo-tectonic context of the Azores region is relatively complex because it is located at the triple junction between the North American, Eurasian and African tectonic plates (labelled as NA, EU and AF, respectively, on Figure 1). The Azores-Gibraltar transform zone connects the Mid Atlantic ridge (undergoing extension) to the convergent plate boundary between Africa and Eurasia (Figure 1).


 Figure 1: Context of the earthquake of 30 April 2013 in the region of the Azores (modified after Borges et al., 2007). The epicentre for the earthquake of 30 April 2013 is shown as a red circle.

The seismo-tectonic context of the Azores thus gives rise to domains that are mainly characterized by dextral transform faults, with extensional relay zones. For the earthquake of 30 April 2013, the focal mechanism obtained by inversion of the seismic W-phase (for a description of the method, see Duputel et al., 2012) shows that this event is rather in agreement with the extensional component of the region (see Figure 2).



 Figure 2: Example of inversion of focal mechanism indicating a local tectonic context mainly in extension.

This zone has experienced the strongest strike-slip earthquakes recorded in this area of the Atlantic during the 20th century, including the earthquakes of 1941 (magnitude 8.2 to 8.4) and 1975 (magnitude Ms = 7.9). At such levels of magnitude, even though the focal mechanisms were strike-slip, these earthquakes generated small tsunamis that reached the Portuguese and Moroccan coasts (45 cm maximum in 1941 and 30-35 cm in 1975), (Baptista et al., 2009).

No generation of a tsunami

For the level of magnitude of the earthquake on 30 April 2013 (Mw=5.7), the triggering of a tsunami is highly unlikely. As a result, and considering that this earthquake could be felt at distances of several hundred km, a message of information was transmitted to the authorities in accordance with the decision matrix used by the CENALT. This message of information served to warn the authorities that the event had been processed by the CENALT, but that it did not represent any threat of a tsunami along distant coasts.
In the minutes and hours following this seismic event, the CENALT checked the available real-time data from tide-gauges. No wave was observed, neither at tide-gauge stations in the Azores and Canaries, nor on the Spanish and French coasts.

REFERENCES
  • Borges, J.F., M. Bezzeghoud, E. Buforn, C. Pro et A. Fitas, The 1980, 1997 and 1998 Azores earthquakes and some seismo-tectonic implications, Tectonophysics, 435, 1-4, 37-54, 2007.
  • Baptista, M.A., et J.M. Miranda, Revision of the Portuguese catalog of tsunamis, NHESS, 9, 25-42, 2009.
  • Duputel, Z., L. Rivera, H. Kanamori, G. Hayes, 2012. W-phase fast source inversion for moderate to large earhquakes (1990 - 2010), Geophysical Journal International, v. 189, iss. 2, p. 1125-1147.