Focal mechanisms for sub‐crustal earthquakes in the Gulf of Cadiz from a dense OBS deployment

An eleven‐month deployment of 25 ocean bottom
seismometers provides an unprecedented opportunity to
study low‐magnitude local earthquakes in the complex
transpressive plate boundary setting of the Gulf of Cadiz,
known for the 1755 Lisbon earthquake and tsunami. 36
relocated earthquakes (ML 2.2 to 4.8) concentrate at 40–
60 km depth, near the base of the seismogenic layer in
∼140 Ma old oceanic mantle lithosphere, and roughly
align along two perpendicular, NNE‐SSW and WNWESE
striking structures. First motion focal mechanisms
indicate compressive stress for the cluster close to the
northern Horseshoe fault termination which trends
perpendicular to plate convergence. Focal mechanisms for
the second cluster near the southern termination of the
Horseshoe fault indicate a strike‐slip regime, providing
evidence for present‐day activity of a dextral shear zone
proposed to represent the Eurasia‐Africa plate contact. We
hypothesize that regional tectonics is characterized by slip
partitioning.

Baptista, M. A., and J. M. Miranda (2009), Revision of the Portuguese catalog
of tsunamis, Nat. Hazards Earth Syst. Sci., 9, 25–42, doi:10.5194/
nhess‐9‐25‐2009.
Brillinger, D. R., A. Udias, and B. A. Bolt (1980), Probability model for
regional focal mechanism solutions, Bull. Seismol. Soc. Am., 70,
149–170.
Buforn, E., A. Udías, and A. Colombas (1988), Seismicity, source mechanisms
and tectonics of the Azores‐Gibraltar plate boundary, Tectonophysics,
152, 89–118.
Buforn, E., M. Bezzeghoud, A. Udias, and C. Pro (2004), Seismic sources
on the Iberia‐African plate boundary and their tectonic implications,
Pure Appl. Geophys., 161, 623–646, doi:10.1007/s00024-003-2466-1.
Carrara, G., L.Matias, W.Geissler, and F.D’Oriano (Eds.) (2008),NEAREST
2008 cruise report R/V Urania, 1st Aug 2008–4th Sept 2008, GOCE Contract
037110, 79 pp., Ist. di Sci. Mar., Bologna, Italy. (Available at http://
nearest.bo.ismar.cnr.it/documentation)
Carrilho, F., and D. Vales (2009), Calibration of magnitude ML for Portugal
and adjacent areas, in 6° Symposio de Meteorologia e Geofisica da APMG,
pp. 48–53, Assoc. Port. de Meteorol. e Geofis., Lisbon.
Carrilho, F., J. C. Nunes, J. A. Pena, and M. L. Senos (2004), Catalogo
Sismico de Portugal Continental e Regiao Adjacente Para o Periodo
1970–2000, 227 pp., Inst. Meteorol. Lisboa, Lisbon.
Engdahl, R., R. van der Hilst, and R. Buland (1998), Global teleseismic
earthquake relocation with improved traveltimes and procedures for
depth determination, Bull. Seismol. Soc. Am., 88, 722–743.
Fukao, Y. (1973), Thrust faulting at a lithospheric plate boundary—The
Portugal earthquake of 1969, Earth Planet. Sci. Lett., 18, 205–216,
doi:10.1016/0012‐821X(73)90058‐7.
Gephart, J. W. (1990), FMSI: A Fortran program for inverting fault/
slickenside and earthquake focal mechanism data to obtain the regional
stress tensor, Comput. Geosci., 16, 953–989, doi:10.1016/0098‐3004
(90)90105‐3.
González, A., M. Torné, D. Córdoba, N. Vidal, L. M. Matias, and J. Díaz
(1996), Crustal thinning in the southwestern Iberia margin, Geophys.
Res. Lett., 23, 2477–2480, doi:10.1029/96GL02299.
Grimison, N. L., and W.‐P. Chen (1986), The Azores‐Gibraltar plate
boundary: Focal mechanisms, depths of earthquakes, and their tectonic
implications, J. Geophys. Res., 91, 2029–2047, doi:10.1029/
JB091iB02p02029.
Havskov, J., and L. Ottemöller (Eds.) (2005), SEISAN: The earthquake
analysis software for Windows, Solaris, Linux and MacOSX, version
8.1, 259 pp, Cent. Geocienc., Univ. Nac. Auton. de Mex., Querétaro.
McKenzie, D., J. Jackson, and K. Priestley (2005), Thermal structure of
oceanic and continental lithosphere, Earth Planet. Sci. Lett., 233, 337–
349, doi:10.1016/j.epsl.2005.02.005.
Neves, M. C., and R. G. M. Neves (2009), Flexure and seismicity across
the ocean‐continent transition in the Gulf of Cadiz, J. Geodyn., 47,
119–129, doi:10.1016/j.jog.2008.07.002.
Rebull, O. G., J. D. Cusi, M. R. Fernandez, and J. G. Muset (2006), Tracking
fin whale calls offshore the Galicia Margin, north east Atlantic
Ocean, J. Acoust. Soc. Am., 120, 2077–2085, doi:10.1121/1.2336751.
Rovere, M., C. R. Ranero, R. Sartori, L. Torelli, and N. Zitellini (2004),
Seismic images and magnetic signature of the Late Jurassic to Early Cretaceous
Africa‐Eurasia plate boundary off SW Iberia, Geophys. J. Int.,
158(2), 554–568, doi:10.1111/j.1365‐246X.2004.02339.x.
Sartori, R., L. Torelli, N. Zitellini, D. Peis, and E. Lodolo (1994), Eastern
segment of the Azores‐Gibraltar line (central‐eastern Atlantic): An oceanic
plate boundary with diffuse compressional deformation, Geology,
22, 555–558, doi:10.1130/0091‐7613(1994)022<0555:ESOTAG>2.3.
CO;2.
Serpelloni, E., G. Vannucci, S. Pondrelli, A. Argnani, G. Casula, M. Anzidei,
P. Baldi, and P. Gasperini (2007), Kinematics of the western Africa‐
Eurasia plate boundary from focal mechanisms and GPS data, Geophys.
J. Int., 169, 1180–1200, doi:10.1111/j.1365‐246X.2007.03367.x.
Snoke, J. A., J. W. Munsey, A. G. Teague, and G. A. Bollinger (1984), A
program for focal mechanism determination by combined use of polarity
and SV‐P amplitude ratio data, Earthquake Notes, 55, 15.
Stich, D., F. Mancilla, and J. Morales (2005), Crust‐mantle coupling in the
Gulf of Cadiz (SW‐Iberia), Geophys. Res. Lett., 32, L13306, doi:10.1029/
2005GL023098.
Stich, D., F. Mancilla, S. Pondrelli, and J. Morales (2007), Source analysis
of the February 12th 2007, Mw 6.0 Horseshoe earthquake: Implications
for the 1755 Lisbon earthquake, Geophys. Res. Lett., 34, L12308,
doi:10.1029/2007GL030012.
Stich, D., R. Martin, and J. Morales (2010), Moment tensor inversion for
Iberia‐Maghreb earthquakes 2005–2008, Tectonophysics, 483, 390–398,
doi:10.1016/j.tecto.2009.11.006.
Zitellini, N., et al. (2001), Source of the 1755 Lisbon earthquake and tsunami
investigated, Eos Trans. AGU, 82(26), 290–291.
Zitellini, N., M. Rovere, P. Terrinha, F. Chierici, and L. Matias, and the
BIGSETS Team (2004), Neogene through Quaternary tectonic reactivation
of SW Iberian passive margin, Pure Appl. Geophys., 161(3), 565–
587, doi:10.1007/s00024‐003‐2463‐4.
Zitellini, N., et al. (2009), The quest for the Africa‐Eurasia plate boundary
west of the Strait of Gibraltar, Earth Planet. Sci. Lett., 280, 13–50,
doi:10.1016/j.epsl.2008.12.005.

An edited version of this paper was published by AGU. Copyright (2010) American Geophysical Union.