Post on 13-Jun-2020
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2009/02/17 - Mw = 3.4
1989/09/23 - ML = 4.0
2011/05/22 - ML = 3.0
1987/06/04 - Mw = 4.42011/12/25 - Mw = 2.0
2005/12/29 - Mw = 3.5
2005/12/29 - Mw = 3.7
2011/11/01 Mw = 2.1
2010/03/27 - ML = 4.1
2012/07/13 - Mw = 2.5
L
1998/07/31- ML = 4.0
1997/01/19 - ML = 3.6
2012/05/28 Mw = 1.7
2011/12/19 Mw = 2.0
2011/09/02 Mw = 1.9
2012/04/13 - Mw = 1.5
2011/09/24 - Mw = 1.3
1987/07/04 - Mw = 4.4
Custódio et al. (2016)
FM computed in this study
A seismicity boundary in the low-strain region of Alentejo, south PortugalCatarina Matos1, Jiří Zahradník2, Pierre Arroucau3, Graça Silveira4 and Susana Custódio1
Affiliations and contact References1 - Faculty of Sciences - University of Lisbon, Instituto Dom Luiz, Lisbon, Portugal. Email: cpomatos@fc.ul.pt. 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 12th 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.
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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
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A
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1146-1899 (Stucchi et al., 2013)
1900-1968 (Grünthal et al., 2013)
Eurasia
Nubia
NorthAmerica
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?
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Permanent network (1996-2016)
AViana do Alentejo
Arraiolos
N=1276N=1276N=12766
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Temporary network (2011-2012)
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MT inversion of small magnitude earthquakes
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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.
Mes
seja
na fa
ult
Évora
mas
sif
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 ML> 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.
Nex
t ste
ps
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 ML > 1.0.
Fit between observed (black) and synthetic (red) seismograms for the Mw
1.9 2011/09/02 earthquake. We checked our results with first motion polarities.
We calculated focal mechanisms using ISOLA (Sokos and Zahradnik, 2008).
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