An event of magnitude Mw 6.1(EMSC) occurred on 12/02/2007 at 10:35 UTC off coast of South-Western Portugal. The earthquake had its epicentre in the eastern Horseshoe Abyssal Plain, at 175 km South-West of San Vicente Cape (Figure 1). This earthquake is the largest earthquake since the great instrumental earthquake, Ms=8.0 (USGS), occurred on February 28th, 1969 in the same epicentral area. This earthquake was followed by four small aftershocks with magnitude less or equal to 3.5. There has been no reported damage associated to the event since habitated regions are too far away from the epicentre. This event has been widely felt in Portugal, particularly in the Algarve Region (I=IV – IM information), Southern Spain and Western Morocco and up to 700 km away of the epicentre (Salamanca, Madrid) (EMSC report in http://www.emsc-csem.org)

Bezzeghoud, M.

Borges, J.F.

Caldeira, B.

Grandin, R.

English

Repositório Científico da Universidade de Évora

Tectonic and seismicity of thearea In the last four years there was an increasein the seismic activity in the area betweenGorringe Bank and the Horseshoe Fault(Figure 1 and Table 1).The region at East of 16°W is dominated bya transpressive tectonic regime, with a verylow convergence rate of 4mm/year trendingNW to NNW, consistent with the observedmaximum horizontal stress direction(Borges et al., 2001). In this region we don’tsee a clear limit between plates and thedeformation is distributed over anincreasingly large area that can reach an N-S width of 300 km near the continentalmargin of Iberia. The seismicity isscattered, but most events are concentratedalong a 100 km wide band, trending ESE-WNW from 16°W to 9°W. In the area, aseries of topographic structures trendingWSW-ENE occur (Figure 1). The Horseshoescarp and the Marquês de Pombal scarp,parallel to the San Vicente canyon, haveexperienced deformation since, at least, theMiocene. The latter scenario is supported bythe occurrence of unusually large oceanicearthquakes inside the area of scatteredseismicity such as the 1969 earthquake(Ms=8.0) and the historical 1755 Lisbonearthquake.Seismic data and Peak GroundaccelerationDue to the proximity to the epicentre andthe magnitude of the event, the Portugueseseismic network (IM), the WesternMediterranean Network (WM) and Spanishnetwork (IGN) (Bento et al., this issue),have recorded a high quality andunsaturated broadband seismic data. Theregional broadband records of this eventwill provide, in the future, an excellentopportunity to an accurate analyses of thesource parameters and of the seismicground motion effects of this earthquake.As example a seismic record section of thisearthquake is presented (Figure 2) as wellas the peak ground acceleration (PGA) asfunction of the epicentral distance, for adistance between 200 km and 1200 km.From this plot one can observe that themaximum PGA reached 7.5 cm/s2 and wasmeasured in the PFVI station.The 2007 earthquake in thecontext of the SW C.S. VicenteareaThe 2007/02/12 earthquake was thestrongest event occurring in the area sincethe 1969 earthquake (M8.0). All the fourevents (events 2 to 4, Table 1) fall in thesame region, and could have beenoriginated on the same fault, although theuncertainties on epicentral locations do notallow this hypothesis to be confirmed (Table1 and Figure 1 shown thrust mechanisms,with a strike-slip component, consistentwith a N-S to NNW-SSE direction ofmaximum compression (Borges et al.,2001)). All four solutions agree on a NE-SWstriking nodal plane which has almost thesame configuration as the inferred faultplane of the 1969 earthquake. Both 2004and 2007 events occurred closer to thecoast, at the south-western extremity of theSan Vicente Canyon (CSV), and could beattributed to the seismic activity of theHorseshoe Fault (Figure 1), a NE-SWstriking thrust fault, with a dip towards SE.Discussion and implications forthe 1755 earthquake LisbonearthquakeThe November 1st, 1755 earthquake wasthe strongest earthquake ever reported inEurope, and was extremely destructivethroughout Portugal, and Morocco; the27 Mai 2007IntroductionAn event of magnitude Mw 6.1(EMSC)occurred on 12/02/2007 at 10:35 UTC offcoast of South-Western Portugal. Theearthquake had its epicentre in the easternHorseshoe Abyssal Plain, at 175 kmSouth-West of San Vicente Cape (Figure1). This earthquake is the largestearthquake since the great instrumentalearthquake, Ms=8.0 (USGS), occurred onFebruary 28th, 1969 in the sameepicentral area. This earthquake wasfollowed by four small aftershocks withmagnitude less or equal to 3.5. There hasbeen no reported damage associated to theevent since habitated regions are too faraway from the epicentre.  This event hasbeen widely felt in Portugal, particularly inthe Algarve Region (I=IV – IMinformation), Southern Spain and WesternMorocco and up to 700 km away of theepicentre (Salamanca, Madrid) (EMSCreport in http://www.emsc-csem.org). CSEM / EMSC  Newsletter26The recent 2007 Portugal earthquake (Mw=6.1) in the seismotectonic context of the SW Atlantic area J.F. Borges (1), M. Bezzeghoud (1), B. Caldeira (1) and R. Grandin (2)(1) CGE and Physics Dept., Universidade de Évora, Évora, Portugal (jborges@uevora.pt)(2) Institut de Physique du Globe de Paris (IPGP), Laboratoire de Tectonique et Mécanique de la Lithosphère, 4 place Jussieu, 75252 Paris, France.Figure 1: Seismicity provided by the Institute of Meteorology (IM- Portugal), focal mechanisms and major active faults of the region close to theepicentral area of 2007/02/12 earthquake; 1a, 1b and 1c are the 1969 earthquake and two major aftershocks; red mechanism represent the recent2007 earthquake. CR= Coral Patch Ridge; HF= Horseshoe Fault; MP=Marquês de Pombal Fault; GB= Gorringe Bank; CSV= San Vicente Cape.Figure 2: Seismic section of the 2007 earthquake recorded by the Instituto de Meteorologia(PM), Western Mediterranean (WM), Geofon  (GE) and IGN Networks. In the top right corner,the Peak Ground Acceleration (PGA), in m/s2, as function of the epicentral distance.Figure 3: Synthetic isoseismic maps for three proposed scenarios of the 1755 Lisbon earthquake. The most realistic hypothesis to fit theobserved isoseismic pattern is in the right panel.Nº Date Lat Lon Depth M Strike Dip Rake REF.(d/m/yr)(°N) (°W) (km) (o) (o) (o)1a 28/02/1969 36.1 10.6 22 8.0Ms 324 24 142 Buforn et al. (1988)1b 28/02/1969 36.2 10.5 37 5.6Ms 46 63 38 Borges (2003)1c 05/05/1969 36.0 10.4 29 5.5Ms 231 47 54 Buforn et al (1988)2 29/07/2003 35.90 10.51 30 5.4Ml 245 70 110 Carrilho (2005)3 13/12/2004 36.29 9.88 29 5.4Ml 260 25 105 Carrilho (2005)4 21/06/2006 36.10 10.47 30 4.6Mw 249 60 90 Grandin et al. (2007)5 12/02/2007 36.08 10.29 44 5.9Mw 125 49 144 CMT(Harvard)Table 1: Epicentral coordinates and focalmechanisms projected in the Figure 1The offshore Mw=6.1, SW Cape St. Vincentearthquake of 12 February 2007 is one of thelargest seismic events in the surroundingareas of Spain in last decades. Thehypocentral coordinates estimated by theInstituto Geográfico Nacional (IGN) werelat=35.96°N, long=10.41°W, depth=65 km.The earthquake was extensively felt in theSW provinces of Spain, such as Huelva,Sevilla or Córdoba, with EMS intensities III-IV, but also in some distant places (more than900 km far away from the epicentre) likeMadrid, Zaragoza or Santiago de Compostelaamong others, with EMS intensity II.The event triggered eight accelerographs ofthe IGN strong motion network, located atepicentral distances between 326 and 623km. These sites and the epicentre of theearthquake are represented in the map of thefigure 1. The maximum horizontal groundaccelerations recorded by these instrumentsrange between 1.65 and 12.95 cm/s2. Table 1shows these peak values together withgeographical coordinates, epicentraldistances and epicentre-station azimuths ofthe recording stations.Acceleration response spectra for a 5%critical damping ratio, SA, were calculatedfrom the acceleration records, after beingcorrected for baseline errors and bandpassfiltered between 0.1 and 25 Hz. Horizontalspectra of six of these accelerograms areshown in figure 2.As a general characteristic, all responsespectra show at least two distinctamplification peaks. The first, thepredominant period in most of the cases, islocated roughly in the range of periods from0.1 to 0.2 seconds, while the second peakranges between 0.4 and 0.6 seconds,approximately. Ordinates of both peaks arerelatively small, as it is expected as a resultof the epicentral distances of the stations andthe magnitude of the earthquake.The largest values of PGA and SA areobtained for the three nearest stations oftable 1, all of them on rather soft soils.Specially, this is the case of the stations CTYand MTC that are situated on marine deepdeposits of sand and gravel. When most of thespectra of the earthquake of February 12 arecompared with those spectra calculated fromlocal earthquakes recorded at the same stations (magnitudes lesser than 4.5), itcan be seen (not shown here), that thequoted first peak appears inside the samerange from 0.1 to 0.2 seconds, but thesecond peak disappears. Also interesting isthe case of CAR, situated on the top of a hillwith steep slopes and in the middle of theGuadalquivir valley, at an epicentraldistance of almost 460 km. Its PGA valuesare greater than other nearer stations forthe same event and the response spectrashow several peaks, possibly as acontribution of topographical effects29 Mai 2007shock was even felt in Northern Germany,the Azores and the Cape Verde Islands. Thelarge tsunami-waves generated by theearthquake also provoked extensivedamage along the coasts of Portugal,southern Spain and Morocco.The problem of epicentral location has beenaddressed by various early studies, and,since the beginning of the instrumentalperiod, a consensus attributed the origin ofthe earthquake to a structure locatedbetween the Gorringe Bank and the CoralPatch Ridge, area where the recent 2007earthquake occurred. Several scenarioswere tested and we concluded that a NE-SW trending fault, possibly outcropping atthe base of the NW flank of Gorringe Bank,could be responsible for the 1755earthquake, as shown in Figure 3.The epicentral region of the February 12th,2007 and the events presented in the Figure1 and listed in Table 1 (events 1 to 5), mustbe considered as a possibility also for aseismogenic area of 1755 earthquake. Thisnew possibility will be tested using thesame methodology already employed for thescenarios built for the Gorringe area(Figure 3).   AcknowledgementsThis work has been partially funded by thePortuguese Science and TechnologyFoundation (FCT) of the Ministry ofScience and Superiors Education (MCES)through the projects: MODSISNAC-FCT-2005, SEISMOLITOS-FCT-2004,FCT/POCTI/CTE-GIN/59750/2004,FCT/POCTI/CTE-GIN/55994/2004.ReferencesBorges J. F., A. J. S. Fitas, M. Bezzeghoud, and P.Teves-Costa, 2001. Seismotectonics of Portugaland its adjacent Atlantic area, Tectonophysics,337, 373-387.Buforn, E., Udías, A. and Colombás, M.A., 1988.Seismicity, source mechanisms andseismotectonics of the Azores-Gibraltar plateboundary. Tectonophysics, 152, 89-118Caldeira B., F. Carrilho,, M. Miranda,, M.Bezzeghoud, P.M. Alves, G. Silveira, F. Villalonga,J.A. Pena, L. Matias, J.F. Borges, D. Vales, C.Corela, and G. Madureira. Recent improvementsin the Broadband seismic networks in Portugal,this issue.Carrilho J., 2005. Estudo da sismicidade da ZonaSudoeste de Portugal Continental, M. S. thesis,172 pp., Univ. of Lisboa.Grandin R., J.F. Borges, B. Caldeira, M.Bezzeghoud and F. Carrilho, 2007. Simulations ofstrong ground motion in SW Iberia for theFebruary 28th, 1969 (MS=8.0) and the November1st, 1755 (M~8.5) earthquakes. Submitted to JGI.CSEM / EMSC  Newsletter28On the 12 February 2007, at 10:35:31GMT, a strong earthquake ofmagnitude (Mw=6.1 EMSC; Mb=6.2USGS) took place off-shore of Portugal,Spain and Morocco. This seismic eventcaused panic in several moroccan cities.The epicentre was located by themoroccan network in the AtlanticOcean south of Cape St. Vincent atLatitude 35.45° N and Longitude 9.94°W, Md=6.0, with an estimated focaldepth of 45 km. The 12 February 2007 earthquake wasfelt throughout northern cities ofMorocco. This earthquake had amaximum macroseismic intensity of IV(all intensities are given in MMS scale)in costal cities of Morocco, Tangier,Rabat, Casablanca, El Jadida and Safi.The event was felt more strongly inCasablanca where the highestbuildings in Morocco are grouped.The occurrence of the 12 February 2007Cape St. Vincent earthquake that wasdiffusely felt in the most cities ofMorocco, allowed us to arrange themacroseismic survey in the regionwithin the Western Rif and Atlasmountains zone.The earthquake was slightly felt(intensity II), in Agadir city, Ouarzazet,Errachidia, Taza which are locatedsome 650 km from the epicenter. In thisregard, the large extent of themacroseismic area (see figure 1), maybe explained by the depth of thehypocenter.The macroseismic map is somewhatsimilar to those obtained for the 1969and 1755 earthquakes and shows ageneral pattern of macroseismicphenomena of Atlantic seismic events,(Levret, 1991).ReferencesLevret A. (1991): The effects of November 1,1755 “Lisbon” earthquake in Morocco.Tectonophysics, 193, 83-94.The Cape St. Vincent Earthquake of February 12, 2007Macroseismic effects Jabour N., Hahou Y., Benchekroun S., Timoulali Y., Menzhi M., Hni L., Badrane S., Kasmi M. and Birouk A.Institut National de Géophysique, CNRST-Rabat; Morocco, jabour@cnrst.ma.Recorded ground accelerations in the 2007/02/12 SW CAPE ST. Vincent EarthquakeCabañas L. (1), Martín A.J (1). and Alcalde J.M. (1)(1) Instituto Geográfico Nacional (IGN). Madrid. Spain, lcabanas@fomento.esFigure 1: Morocco macroseismicity map of the February 12th 2007, Cape St Vincentearthquake.Figure 1: Strong motion stations (green triangles) that were triggered by the earthquake of 12February 2007Figure 2: Acceleration response spectra (5% damping) for components EW and NS from six ofthe recorded motionsTable 1: Stations of the IGN strong motion network that recorded the earthquake

The recent 2007 Portugal earthquake (Mw=6.1) in the seismotectonic context of the SW Atlantic area

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The recent 2007 Portugal earthquake (Mw=6.1) in the seismotectonic context of the SW Atlantic area

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