1
-9 -8 -7 37º 38º 39º 40º 41º 42º 0 100 OMZ CIZ GTMZ SPZ BAO TBCSZ PTFASZ 8.5º 8º 7.5º 38.5º 39º 0 km 10 km 20 km 0 2009/02/17 - M w = 3.4 1989/09/23 - M L = 4.0 2011/05/22 - M L = 3.0 1987/06/04 - M w = 4.4 2011/12/25 - M w = 2.0 2005/12/29 - M w = 3.5 2005/12/29 - M w = 3.7 2011/11/01 M w = 2.1 2010/03/27 - M L = 4.1 2012/07/13 - M w = 2.5 L 1998/07/31- M L = 4.0 1997/01/19 - M L = 3.6 2012/05/28 M w = 1.7 2011/12/19 M w = 2.0 2011/09/02 M w = 1.9 2012/04/13 - M w = 1.5 2011/09/24 - M w = 1.3 1987/07/04 - M w = 4.4 Custódio et al. (2016) FM computed in this study A seismicity boundary in the low-strain region of Alentejo, south Portugal Catarina Matos 1 , Jiří Zahradník 2 , Pierre Arroucau 3 , Graça Silveira 4 and Susana Custódio 1 Affiliations and contact References 1 - Faculty of Sciences - University of Lisbon, Instituto Dom Luiz, Lisbon, Portugal. Email: [email protected]. 2 - Charles University - Faculty of Mathematics and Physics, Prague, Czech Republic. 3 - Dublin Institute for Advanced Studies, Dublin, Ireland. 4 - Instituto Dom Luiz, Lisbon, Portugal/Instituto Superior de Engenharia de Lisboa, Lisbon, Portugal. Acknowledgments - Temporary seismic stations whose data was used in this study were deployed in the framework of the DOCTAR project (Potsdam - Germany). - FCT PhD grant PD/BD/106019/2014. - FCT SPIDER project PTDC/GEO-FIQ/2590/2014. Uneven spatial distribution of earthquakes in mainland Portugal Instrumental seismicity for the period 1996-2016 (black dots - IPMA catalogue) and significant earthquakes (M > 4) since the 12 th century overlaid on topography (Smith & Sandwell, 1997). Surface traces of potentially active faults are displayed in white (Cabral, 2012) and grey (Basili et al., 2013). Thick dashed line separates regions with distinct levels of seismic activity. -8.5º -8º 39º 0 km 10 km 20 km 30 km -7.5º 0 km 10 km 20 km 30 km 38.5º Meso-Cenozoic sedimentary rocks Upper Paleozoic basins Northeast Alentejano massif Plutonic rocks of the Évora massif Beja igneous complex Medium high grade metamorphic rocks Very low grade metamorphic rocks Cadomian basement High grade metamorphic rocks South Portuguese Zone Key questions Earthquake detection and location using a dense network -11º -10º -9º -8º -7º -6º 37º 38º 39º 40º 41º 42º 0 100 A B 1146-1899 (Stucchi et al., 2013) 1900-1968 (Grünthal et al., 2013) Eurasia Nubia North America 1969-2016 (IPMA catalogue) M5 M6 M4 Basili et al. (2013) EDSF, doi: 10.6092/INGV.IT-SHARE-EDSF. Cabral J (2012) I. Iber. Geol., doi: 10.5209/rev_JIGE.2012.v38.n1.39206. Grünthal et al. (2013) J. Seismol, doi: 10.1007/s10950-013-9379-y. IPMA - Instituto Português do Mar e da Atmosfera earthquake catalogue. Lima et. al. (2012) J. Pet., doi:10.1093/petrology/egs037. Lomax et al. (2000), doi: 10.1007/978-94-015-9536-0 5. Miranda et al. (1989) Earth Planet. Sci. Lett., doi: 10.1016/0012-821X(89)90174-X. Sandwell et al. (2014) Science, doi: 10.1126/science.1258213. What controls the asymmetric distribution of earthquakes? Are the observed earthquake clusters associated with geologic features? 8.5º 8º 7.5º 38.5º 39º 0 km 10 km 20 km 0 5 10 15 20 25 30 Depth km Permanent network (1996-2016) A Viana do Alentejo Arraiolos N=1276 N=1276 N= 6 8.5º 8º 7.5º 38.5º 39º 0 km 10 km 20 km 0 5 10 15 20 25 30 Depth km Arraiolos A Viana do Alentejo Temporary network (2011-2012) N=535 MT inversion of small magnitude earthquakes M1.9 20110902 16:58:50 WOMZ N=1687 9º 8º 7º 37º 38º 39º 40º 41º 42º 0 100 300 150 0 150 300 450 Magnetic Anomaly nT Miranda et al. (1989) Évora massif 9º 8º 7º 38º 40º 42º 0 100 50 0 50 100 150 Gravity Anomaly mGal Sandwell et al. (2014) 9º 8º 7º 38º 40º 42º 0 100 8 4 0 -4 -8 S /V V S % Silveira et. al. (2016) Depth = 25 km V s = 3.91 km/s 8.7º 8.4º 8.1º 7.8º 7.5º 7.2º 38.6º 38.7º 38.8º 38.9º 39º 0 km 10 km 20 km 0 5 10 15 20 25 30 Depth km Shear zone? N=917 We present a case study from Alentejo, Portugal, southwest Europe. Mainland Portugal (MP) lays on a stable continental region characterized by low strain rates (convergence velocities <1 mm/yr). However, regardless of the low strain rates, the historical and instrumental earthquake catalogs show that MP has been the source region of large to moderate magnitude events, which indicates the existence of active structures. The spatial distribution of earthquakes in MP is asymmetric. The northernmost region presents low earthquake activity. Most of the earthquakes occurring in this region correlate with major mapped faults. Further to the south there is a sharp transition. This transition is delimited by a narrow 100-km long earthquake belt. Earthquakes occurring bellow this boundary occur in clear clusters without orientation. Of particular significance is the fact that this seismicity has not been associated with tectonic features. Here, we study the westernmost part of the Ossa Morena Zone (OMZ). The evolution of the OMZ is associated with the geological processes that formed Pangea. After the collision between Gondwana and Laurussia, sedimentary basins were developed within the OMZ. They formed in an extensional regime and were accompanied by metamorphism and emplacement of voluminous magmatism. The study area is crossed by a major tectonic feature known as Messejana fault. Geotectonic units of the Iberian massif outcropping in mainland Portugal and geology (Lima et. al, 2012) of the OMZ. Most outcropping rocks in the study area belong to the Évora Massif, a metamorphic dome mainly composed of gneisses, migmatites, schists and amphibolites. This metamorphic complex is contemporaneous with the emplacement granitoid plutons. Messejana fault Évora massif We used a dataset recorded by a temporary BB array to study small-magnitude seismicity. We detected 535 events using an automatic routine (Heimann et al., in prep.). We then located the events using NonLinLoc (Lomax, 2000). Focal mechanisms for earthquakes with M L > 3.5 are routinely computed using the permanent network. Therefore little is known about the small- scale faulting in MP. The temporary deployment allowed us to model waveforms at relatively high frequencies (1 - 2 Hz). Hypothesis for the observed seismicity There is tendency for strike-slip and reverse faulting along the Arraiolos alignment. Next steps The rigidity contrasts between different lithologies in the Évora massif could promote accumulation of stress in the periphery between blocks. The Arraiolos cluster could be associated with a strike-slip shear zone. The correlation with a sharp velocity contrast suggests the existence of a paleoplate junction. The OMZ is well delineated by a complex pattern of positive and negative magnetic anomalies. Earthquakes in OMZ are concentrated in the negative magnetic and gravity anomalies. Outcropping rocks in the Évora massif have different compositions. Thus, earthquakes in the Viana do Alentejo cluster could mark rheological boundaries. Depth slices of S-wave tomography reveal a sharp velocity contrast in OMZ. At intermediate depths (25-30 km) the concentrations of earthquakes follow this contrast. The similar focal mechanisms along the Arraiolos alignment and the favorable orientation of one of the nodal planes with the seismicity suggests that this cluster could be associated with a strike-slip shear zone. In this sense, the reverse focal mechanisms, which are located in a zone of high concentration of earthquakes could be related to a fault stepover. This together with the marked heterogeneous structure of OMZ suggests that the Arraiolos cluster delimits a Precambrian plate boundary which is favorably oriented to be reactivated by the present-day stress field. Silveira et al. (2016) EGU General Assembly Conference Abstracts. Smith W H F and D T Sandwell (1997) Science, doi: 10.1126/science.277.5334.1956. Sokos and Zahradnik (2008) Comput. Geosci, doi: 10.1016/j.cageo.2007.07.005. Stucchi et al. (2013) J. Seismol, doi: 10.1007/s10950-012-9335-2 . Tectonic and Geological setting New detections lie on the previously identified clusters; In the Arraiolos alignment earthquakes predominate at mid to lower crust depths; The seismicity boundary is not an artifact due to sparse network. Obtain a detailed 3D velocity structure for the study region using local earthquakes and active source data. Relocation of earthquakes. We will used the obtained 3D velocity model to relocate the instrumental seismicity. Comparison of earthquake locations using permanent and temporary networks. Both locations where obtained using the 1D layered velocity model used by Instituto Português do Mar e da Atmosfera (IPMA) in the routine earthquake location. We selected for inversion earthquakes with M L > 1.0. Fit between observed (black) and synthetic (red) seismograms for the M w 1.9 2011/09/02 earthquake. We checked our results with first motion polarities. We calculated focal mechanisms using ISOLA (Sokos and Zahradnik, 2008). 1 2 3 4 6 5 7
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