Recent tectonic model for the Upper Tagus Basin (central Spain)

Active tectonics within the Upper Tagus Basin is related to the lithospheric flexure affecting the Palaeozoic basement of the basin. This flexure displays NE-SW trending. Besides, this structure is in agreement with the regional active stress field defined by the maximum horizontal stress with NW-SE trending. In this tectonic framework, irregular clusters of instrumental seismicity (Mw< 5.0) fade in the zone bounded by the Tagus River and the Jarama River valleys. These clusters are related to major NW-SE trending faults of suspected strike-slip kinematics. Moreover, reverse faults with NE-SW trending are affected by the strike-slip system as well. Despite the reverse faults are in agreement with the present SHMAX orientation, though, they apparently are blocked as seismogenic sources (scarce instrumental seismicity recorded today). In addition, we have determined the regional and local stress/ strain fields and two different fracture patterns were observed. Hence, we have divided the area in two zones: (1) the lateral bands of the basin, defined by reverse faulting (NE-SW trending) and strike-slip faulting (NW-SE trending) and (2) the central zone of the basin characterized by shallow normal faulting and NE-SW trending strike-slip faults. Furthermore, surface faulting and liquefaction structures are described affecting Middle to Late Pleistocene fluvial deposits, suggesting intrabasinal palaeoseismic activity (5.5 < M < 6.5) during the Late Quaternary. The obtained structural and tectonic information has been used to classify and characterize the Upper Tagus Basin as a semi-stable intraplate seismogenic zone, featured by Pleistocene slip-rates < 0.02 mm/yr. This value is low but it affords the occurrence of Pleistocene paleoearthquakes.


Introduction
The Upper Tagus Basin (UTB) is located at the central part of the Iberian Peninsula, and include the province of Madrid, parts of the provinces of Guadalajara, Toledo, Cuenca and Segovia, and the major mountain range constituted by the Spanish Central System (SCS).The most relevant feature of the UTB to be faced by the seismic risk study is their proximity to large urban and industrial areas (i.e.Madrid, Guadalajara and Alcalá de Henares).The instrumental seismic record of the area displays small earthquakes with magnitudes M < 5.0 (www.ign. es; 1996-2011).Moreover, this instrumental seismicity in the region displays spatial and temporal clusters of smallmoderate earthquakes (3.0 < Mw < 5.0), most of them located within a NE-SW narrow band area defined by the watershed zone of the Tagus and Jarama river valleys, about 30-40 km SE of the Madrid city.The last significant earthquake in the zone occurred in 7 June 2007 (Escopete; Guadalajara) with a magnitude mb 4.2 and 10 km depth (www.ign.es).This earthquake triggered a ground motion with a maximum PGA value of 0.071g (Carreño et al., 2008).This relatively high ground response is related to the relatively thick Cenozoic sedimentary filling of the basin (up to 3 km, i.e.Alonzo-Zarza et al., 2004;Gómez-Ortiz et al., 2005), but also the near-field effect has to be considererd (Carreño et al., 2008).The extensive ancient Late Neogene surface dominating the intrabasinal landscape, as well as the scarce evidence of recent earthquake-related deformations (Silva et al., 1997;De Vicente et al., 2007), make difficult to assign a deformation Quaternary tectonic slip-rate for the Upper Tagus Basin (UTB).However the occurrence of deep Canyonlike valleys at basin center locations, boundend by relevant kilometric lineal scarps between 40-60 m high, related in some cases to moderate historic and instrumental seismicity (Silva et al., 1988;Silva, 2003;De Vicente et al., 2007), support the hypothesis of Late Quaternary tectonic activity in the region.Furthermore, Quaternary fluvial deposits of Middle to Late Pleistocene age, display evidence of strong fracture density, tectonic deformation and a wide variety of liquefaction structures related to synsedimentary faulting and collapse of the underlaying Neogene evaporites (Silva et al., 1988;Giner, 1996;Silva et al., 1997;Silva, 2003).Under these assumptions, the integration of both new and old data as well as the implementation of new techniques are required to perform an updated seismic hazard analysis for the UTB.
Hence, the main goal of this work is to propose a innovative interpretation of the seismic potential of the UTB by the integration of (a) paleoseismic evidence for Pleistocene deposits within the main river valleys; (b) seismotectonic data from the focal mechanism solutions and instrumental seismicity; and (c) the widespread existing structural and geological data on active faulting for the area and rheological models for the lithosphere within the basin as well.
Active faulting and Quaternary tectonics are the eventual responsible for the landscape and shaping of the SCS border, the uplift of the Neogene materials at the basin
Palabras clave: Sismotectónica, Mecanismos focales de terremotos, Paleosismicidad, Pleistoceno, Cuenca alta del Tajo.centre and the paleoseismic evidence within the valleys, with recording estimated event-magnitude of c.a. 5.5 to 6.5, are the way to go a step beyond for the seismic hazard assesment in this zone of scarce intrumental seismicity.

Geodynamic and geologic background
The Upper Tagus Basin (UTB) covers an area of about 12,000 km 2 , and is located at the central part of the Iberian Peninsula, including the provinces of Madrid, Guadalajara, Toledo, Cuenca and Segovia.The basin is a complex zone of Paleogene to Neogene sedimentary infilling (Alonso Zarza et al., 2004), limited by three intracratonic mountain ranges: the Spanish Central System (SCS) to the north, the Altomira Range (AR) to the east,and the Toledo Mountains (TM) to the south (Fig. 1).The SCS is interpreted as a Cenozoic pop-up controlled by E-W and NE-SW structures linked to large-scale lithospheric flexure triggered by the SE-NW far-field stress propagation of the Africa-Eurasia collision (De Vicente et al., 2007).These authors indicated that the aforementioned fault systems reach and affect to shallow crustal levels within the basin.
The topography and the crustal structure of the SCS and adjacent areas can be explained by lithospheric folding according to the analogue experimental models performed by Fernández-Lozano et al. (2011), and based on previous proposals (i.e.Giner et al., 1996 ;Cloetingh et al., 2002 ;De Vicente et al., 2007).The shallow seismicity at the intraplate western area of Iberia is explained by Fernández-Lozano et al. (2011) as a consequence of the stress transfer from the plate boundaries to the interiors.Relationships between the topography, the Bouger anomaly, the temperature at 100 km depth and the temperature at the Moho, suggest a hot-zone between the SCS and the TB, in agreement with the zone in which is presently recorded the instrumental seismicity (time-period between 1980-2010) (Fernández-Lozano et al., 2011).
The basement of the basin is composed by Variscan granitic and metamorphic rocks, with a Mesozoic and Paleogene cover (pre-tectonic Alpine unit), whilst the main infilling materials are constituted by overlaying Neogene sedimentary sequences and "cut and fill" Quaternary deposits (post-tectonic Alpine unit) related to the development of the present drainage network.In this geological framework, the main tectonic structures have a conjugate orientation, with NE-SW and NW-SE trending (Giner et al., 1996).All of these structures are related with the aforementioned lithospheric flexure defined by the basement geometry and differential filling and thickness of Neogene sequences (De Vicente et al., 1996;Giner et al., 1996;Cloetingh et al., 2002;De Vicente et al., 2007, 2009;Martín-Velázquez et al., 2009).
Thermal and rheological models for the SCS and adjacent areas were developed by Suriñach and Vegas (1988) and Tejero and Ruiz (2002), with a surface heat flow ranging between 80 and 60 mWm -2 , and with the Moho located at 31-34 km in depth.Jiménez-Díaz et al. (2012) suggested a value ranging between 81-83 mWm -2 of the surface heat flow for the Tagus Basin, involving mantle Upper Tagus Basin (UTB), composed by the Spanish Central System (SCS), Altomira Range (AR) and the Toledo Mountains (TM).Fig. 1.-Situación geológica de la zona de estudio (recuadro pequeño), con las principales unidades geológicas: Cuenca Alta del Tajo, Sistema Central, Sierra de Altomira y Montes de Toledo. .processed to the continuous uplift of the SCS.Models developed by these authors indicate a maximum thickness for the upper crust of 16 km within the Tagus Basin and of 11 km for the SCS.Moreover, two crustal discontinuities, at 11 and 31 km depth respectively, have been described from the spectral analysis of gravity data (Gómez-Ortiz et al., 2005).In consequence, in this work is assumed the interval between 11-16 km depth as the preferred crustal level for brittle deformation and seismogenic source, in order to evaluate the area for faulting rupture according to the main Quaternary faults traces described in the following sections.
These mountain ranges display different tectonic frameworks and deformation ages, as a consequence of the evolving stress-field throughout the Cenozoic (De Vicente et al., 1996;2007, 2009;Babín-Vich andGómez-Ortiz, 1997, Martín-Velázquez et al., 2009;Fernández-Lozano et al., 2011).The active stress-field within the SCS is defined by the horizontal maximum stress orientation, SHMAX, NW-trending (De Vicente et al., 1996;Herraiz et al., 2000).This stress-field is still working from the Late Miocene (De Vicente et al., 1996).The stress-field promoting SCS building corresponds with an intraplate response to far-field effect from the active tectonic boundaries.Commonly, the lithospheric folding instead of mantle process is invoked to explain the active tectonic deformation and vertical movements in this zone (Cloetingh et al., 2002;De Vicente et al., 2007, 2009).Polyphase deformation from the Miocene to the present may explain the stress convergence at the SCS (Cloetingh et al., 2002).This convergence comes from the Pyrenean compression (N-to NE-trending), the Betic compression (SSE-trending), the Mid-Atlantic Ocean Ridge (MAOR) push (W-trending) and the Valencia Through extension (WNW-trending).At this point, different authors explain vertical movements at SCS as result of the lithospheric folding (Giner, 1996;De Vicente et al., 1996;Cloetingh et al., 2002;De Vicente et al., 2007, 2009;Martín-Velázquez et al., 2009).Since the lithospheric folding implies large-scale deformation and large wavelength in vertical movements, the active tectonic slip-rate operating in the SCS has to be estimated from Pleistocene, at least.Pérez-López et al. (2005) described the stress-field at the South Border of the SCS as strike-slip to uniaxial extension from Eocene to present.Martín-Velázquez et al. (2009) modeled this stress-field by using finite elements and a punctual rheological model obeying the thermal model of Tejero and Ruiz (2002).The relevance of this model is the simulation of the stress state at the surface of the SCS, where the present topography plays as tectonic loading for faults.
In this work, firstly, major faults with surface trace larger than 30 km (see Wells and Coppersmith, 1994) are recognized.Secondly, striate fault-orientation and stress agreement are studied and finally, Quaternary tectonic markers and paleoseismic evidence are described.However, fault parametrization for hazard purposes is still a controversy.In this sense, the geothermal features (Jiménez-Díaz et al., 2012), the analogue models of the lithosphere (Fernández-Lozano et al., 2011) and numerical models (Martín-Velázquez et al., 2009) underneath the SCS, could shed light on the fault width which could be load for trigger either moderates or destructive earthquakes.

Macro-scale structural analysis
Despite the asymmetric character of the deformation spatial distribution within the basin (De Vicente et al., 2007), the geomorphology features and lineaments interpreted from digital elevation models suggest the existence of two different fracture patterns (Fig. 2): (1) Guadarrama Fracture Pattern (GP1), mainly oriented towards NW-SE, and secondary toward NE-SW.We assume these lineaments orientations (NW-SE trending) with strike-slip faults NW-trending, and the secondary set with reverse faults with NE-trending (De Vicente et al., 1996;Giner, 1996).( 2) Guadalajara Fracture Pattern (GP2): defined by lineament sets oriented towards NE-SW.This fracture pattern could be related with the local stress tensor defined by NE-trending horizontal maximum stress orientation (Giner, 1996;Giner et al., 1996).GP2 re-activates normal faults and secondary strike-slips with NE-trending.This stress field is orthogonal to the Guadarrama stress field and it is interpreted as a switch between the main axes (SHMAX, SHMIN) as a response for the basement flexure (Giner et al., 1996).Assuming these stress fields could be coeval and SHMAX and SHMIN could switch through time and thus generate the roughly orthogonal compression and extension directions.
Hence, we have divided the UTB in two subzones according to both fracture patterns described above.

Micro structural analysis
Figure 3 shows the location of the 43 structural field stations for fault planes affecting Neogene and Quaternary deposits.There, we have measured more than 700 (743) kinematic data on fault planes to obtain the active stress tensor (Fig. 4).We have applied the structural analysis technique proposed by Reches (1987) to obtain the stress tensor (s 1 , s 2 , s 3 and SHMAX) from slip vectors measured on the fault planes.The results suggest the coeval existence of two stress fields: Regional stress tensor (Fig. 5a): featured by SHMAX with NW-trending, activates strike-slips (Fig. 6) and reverse faults (Fig. 7a and 7b).
The spatial distribution of the structural stations covers all the zones defined from the macro-scale structural analysis.Accordingly, we can observe that the structural stations associated with the local stress tensor are located within the central zone, Guadalajara fracture pattern (GP2) (Fig. 2 and 4) whereas the regional stress tensor is related with stations located within the Guadarrama fracture pattern (GP1).Furthermore, the spatial relationship between both deformational patterns, observed in the field stations in the south boundary of the basin, also suggests that faults activated by the local stress tensor are younger in age to those faults activated by the regional stress field.
Finally, it is necessary to note that the sedimentary infilling of the basin may enlarge the deformation recorded at surface by halokinesis of the Neogene evaporates located at basin centre locations (see Alonso Zarza et al., 2004 for spatial location).However, the coherence and arrangement of both fault patterns, in agreement with the tectonic context of the basin, suggest a main tectonic control underneath karstic-collapses assisting large canyonvalley development in central basin locations as previously suggested by several authors (Silva et al., 1988;Giner et al., 1996;Silva, 2003;Alonso Zarza et al., 2004).

The Pleistocene paleoseismic record
Several liquefaction structures have been described within the basin (Giner et al., 1996;Silva et al., 1997;Silva 2003;Silva et al., 2010;Silva et al., 2011), related to recent faulting and with Paleolithic settlements (younger than 780 Ka).However, more detailed studies are required to establish a complete table of paleoseismic parameters (slip-vector, total offset, recurrence intervals, etc.).Furthermore, the most of the structures described here correspond with liquefactions, and consequently it is very difficult to assign seismic sources and recurrence intervals.
All of these paleoseismic structures are in agreement with the local stress tensor (SHMAX NE-trending) and

Instrumental seismicity
Anomalous clusters of seismicity are located in a narrow band between the Tagus River and the Jarama River (Fig. 10).These clusters are classified as anomalous since no Quaternary active faults have been described in this area and no seismic sources have been recognized except those in the works of Silva et al. (1988;1997) and Giner (1996).This zone is elongated with NE-trending in coincidence with the axis of the basement flexure (Figs. 2,  and 3).We have analyzed the focal mechanism solutions recorded within the area (Giner, 1996;De Vicente et al., 1996;Andeweg et al., 1999;Carreño et al., 2008), according to the methodology proposed by De Vicente et al. (1996).The regional stress-field for the Iberian Peninsula is described in Herraiz et al. (2000) and Stich et al. (2006) and(2010).In these works the UTB stress-field is defined by SHMAX with NW-trending, in agreement to the works focused at the UTB by Giner (1996) 10).Normal faults are coherent with the local stress tensor defined here (SHMAX NE-trending) and located at the extensional zone defined by the basement flexure.However, the focal mechanisms of reverse faulting obey the regional stress tensor (SHMAX NW-trending) and affecting mainly fluvial Middle to Late Pleistocene deposits (Fig. 8).Most of the liquefaction structures observed are sand-dikes-like mainly associated to normal and lateral faults activated by the local stress tensor (Fig. 7b and 7c).The nature and dimensions of these structures suggest the occurrence of paleoearthquakes with magnitudes greater than M5-M5.5, according to the proposals of Obermeier (1996) and Rodríguez-Pascua et al. (2000).In addition, there are recorded liquefaction affecting gravel deposits (Rodríguez-Pascua et al., 2000) as well as thick (0.8-1.2 m) individual liquefaction horizons in medium-coarse sands (Silva et al., 2010 and2011), suggesting paleoearthquake magnitudes greater than M6.
We have recognized liquefaction related to reverse faulting (NE-trending) as well, located at the south zone of Villarubia de Santiago town (Toledo) (Fig. 9).This fault was activated by the regional stress-tensor (SHMAX NW-trending).The liquefaction affects Middle-Late Pleistocene fluvial sediments of the Tagus River, but also all these features are common within the main tributaries such as the Jarama, Tajuña and Manzanares (Silva, 2003;Alonso Zarza et al., 2004;De Vicente et al., 2007).Sand-dikes are injected into the fault plane and the vertical throw was 0.5 m.We assume that this liquefaction corresponds with one single event and consequently, the observed vertical offset corresponds with a coseismic vertical throw.Accordingly, empirical relationships (Wells and Coppersmith, 1994) suggest a maximum magnitude ranging between M6.4 and M6.6.River.These faults are activated by the regional stress tensor with SH-MAX oriented to NW-SE.Fig. 6.-Ejemplo de una de las estructuras de desgarre (SHMAX según NO-SE, tensor regional) en el norte de la cuenca.Las fallas afectan a materiales de terrazas del río Tajo de edad Pleistocena.
spatially located southward of the flexure zone.
The orthogonal relationship between the SHMAX orientation of the regional and local stress tensor points the same genetic relationship for both types of faulting, normal and reverse (Giner et al., 2003).This same genetic origin for both fault sets we assume that is due to the lithospheric folding and basement flexure, provoking a stress axes switch from the regional tensor (SHMAX NW-trending) to the local one (SHMAX NE-trending).Hence, normal faulting in this area works as the seismogenic source related to the flexure of the basement, restricted to the extensional zone of shallow and surface folding (Fig. 11) recognized by several authors on basis to the deformation of Late Neogene intrabasinal surfaces (Fernández-Casals, 1979;Silva et al., 1988;Alonso Zarza et al., 2004;De Vicente et al., 2007).
Reverse faulting with NW-trending are related with the regional stress-field and related with the basement flexure as well.Therefore, shallow normal faulting can be considered as earthquake sources at basin centre locations, whereas earthquakes related to reverse faulting can be produced at deeper crustal levels (Fig. 11).Addi- tionally the deformation is accommodated by strike-slips NE-trending at the NE part of the basin.These faults are transfer faults.The Escopete earthquake (7 th of June, 2007) (Carreño et al., 2008) was triggered by this type of faulting with a focal mechanism of sinistral lateral fault.
The second cluster of instrumental seismicity is located at the Southern border of the SCS (Fig. 10), though the major reverse faults (length > 100 km) have no evidence of paleoseismicity and scarce record of instrumental seismicity.The earthquake cluster of the SCS is more related with strike-slip NW-trending.This fault set is similar to the strike-slips located at the northwestward of the basement flexure.Therefore, the major reverse fault of the South boundary of the SCS would not totally broke as a unique large segment (length > 100 km) since it is segmented by crustal-scale transverse NW-strike-slip faults (Fig. 11).The scarce record and low magnitude of instrumental seismicity in this zone seems to support this seismotectonic scenario.The hyponcentre errors assumed in this work corresponds with focal mechanism solutions described in Andeweg et al. (1999).Since this work, only the Escopete earthquake ( 2007) has occurred within the slip-rate estimated should be related with a strength balance of all of these tectonic driving forces.The strain field observed into the UTB is defined by two different patterns (Fig. 2): (1) active normal and strikeslips faults, SHMAX NE-trending, parallel to the Tajuña River and morpho-lineaments with NE-trending, and (2) morpho-lineaments with NW-trending.Seismicity clusters are related with the second strain pattern.Both strain patterns are in agreement with the strain regime obtained from the structural analysis: (A) the regional strain tensor with SHMAX NW-trending, activating NE-reverse faults and NW-strike-slips faults, and (B) the local strain tensor defined by SHMAX NE-trending.This local tensor we assume a switch off the regional tensor due to the basement flexion, acting surface faulting with 10 km long and 11 km depth, normal NE-faults and lateral NW-faults.
Basement faults within the basin, as the South Border of the SCS, are well oriented according to the presentday NW-trending tectonic compression (De Vicente et al., 1996, 2007, 2009).What is the role of this basement faults as seismogenic-sources? Reactivation under the presentday stress field is possible within the most accepted tectonic framework, although the question emerges from the potential total energy accumulation of these faults, fed by the far-field stress transferred from the SE plate boundary.Taking into account that the bulk portion of the stress is released in the Betic Cordillera and another relevant fraction is derived for lithospheric folding and topographical loading, a minimum stress value can be stored at the Southern Border of the SCS.Following this assumption, 125 km of fault length (assuming a complete rupture of the fault of the South border of the SCS) and 16 km width (upper crust) should involve a lot of energy accumulation (6.31 10 16 J) that could be released in a M8 earthquake (according to Wells and Coppersmith, 1994).No such sizedearthquakes have been evidenced along the paleoseismic record since the Middle Pleistocene (~780 kyr B.P.), suggesting a minimum recurrence interval in the same timerange for expected earthquakes of M8, and independent of the fault segmentation for the SCS.
The basement flexure determines the spatial distribution of active faults in the Tagus Basin.The NE boundary of the flexure displays strike-slips faults with NE-trending and 20 km long (Fig. 11).The Escopete earthquake (7 th , June of 2007) with magnitude mb 4.2 and 10 km depth occurred within this area.Maximum peak ground acceleration (PGA) was 0.07g, though the estimated maximum value for 400 years was 0.04g (Carreño et al., 2008).
The analysis of the focal mechanism solutions for the scarce instrumental seismicity within the area (www.ign.es), namely in the surrounding of the flexure axis, reveals two fault geometries with NE-trending: reverse Madrid Basin with a minimum size to obtain the focal mechanism.Despite the scarce instrumental record to perform a statistical analysis to support the spatial distribution of earthquakes in depth, the flexure model fits enough to explain the present instrumental distribution of earthquakes within this basin.

Discussion and conceptual model
Summarizing, three different tectonic sources determine the intraplate tectonic field within the UTB and they may activate faults: (1) the tectonic far-field from the convergence between Africa, Iberia and Eurasia plates, plus the pushing of the MAOR (2) the lithosphere -mantle coupling by the lithospheric flexion evidenced by large basement wave-length folding and (3) the tectonic loading by the topography of the SCS.Therefore, the tectonic   and normal faults.Despite the error of the hypocentral location for these earthquakes, we suggest the presence of a non-finite surface between the extensional (SHMAX NE-trending) and compresional (SHMAX NW-trending) zones.This implies that reverse earthquakes would be deeper than normal ones.The reorganization of the fluvial network within the basin and the generation of large lineal canyon-shaped valleys at basin centre location date from Middle-Plesitocene (Silva et al., 1988(Silva et al., , 1997)), but paleoseismic evidence are characteristic until the last interglacial period (i.e.125-90 kyr B.P., Silva, 2003).Vertical throws of 0.5 m recorded for the Late Pleistocene (125 kyrs), and empirical relationships used for paleoearthquakes and geometrical parameters of Quaternary faults (surface trace and width), suggest a tectonic slip-rate of 0.004 mm/yr, probably related to single isolated events.This value is too small and probably related with secondary faulting as to be considered as representative for the whole basin.

Conclusions
The Upper Tagus Basin (UTB) is featured by two major tectonic structures: The reverse fault of the South border of the Spanish Central System (SCS), with NE-trending, almost 100 km long and with no evidence of Quaternary and recent tectonic activity, according to the geomorphology within the area.Moreover, this fault displays a scarce and lowmagnitude instrumental seismicity for the historical plus the instrumental period , and mainly related with strike-slip faults (NW-trending), which divided and cross NE-reverse-faulting. Strike-slips have a maximum surface length of 8 km.
Basement flexure determined by NE-trending axis and located underneath the Tagus Basin (TB).This zone is related with instrumental seismicity of magnitude maximum as M4.2 (Escopete earthquake, 7 th June of 2007), shallow normal earthquakes (depth < 10 km) and located between the area bounded by the Tagus River and the Jarama River.This basement flexure is in agreement with the regional stress/strain tensor defined in this area by others authors (Giner et al., 1996;De Vicente et al., 1996;Herraiz et al., 2000;Tejero and Ruiz, 2002;De Vi-cente et al., 2007, 2009;Fernández-Lozano et al., 2011).Reverse earthquakes are deeper than normal earthquakes (16 km > depth > 10 km) in agreement with the geometry of the flexure and the strain field defined upper and down of the non deformation surface (strain deformation distribution in flexural folding with extrados/stretching and intrados/shortening).The interval of depth is according to the lower limit for the upper crust defined by Tejero and Ruiz (2002).However, new thermal models are required for describe the compressive and extensional distribution of the lithosphere within the Tagus Basin.Lateral NWtrending faults show low-magnitude instrumental seismicity (Carreño et al., 2008).Liquefaction structures for normal faulting indicate a paleoearthquake magnitudes between 5.5<M<6.4for shallow normal faulting.
Analysis for the instrumental seismicity plus the major tectonic structures, suggest a tectonic slip-rate of 0.004 mm/yr in this area.However, taking into account the geodynamic framework of the UTB, liquefaction structures and the empirical relationships between slip-rates and recurrence interval delivered by Villamor and Berryman (1999), we define the UTB as a stable intraplate area featured by Quaternary tectonic slip-rates < 0.02 mm/yr for a time period of 125 Ka.
Taking into account that the studied area (UTB) includes large cities with a huge number of citizens, industrial and critical facilities (Madrid, Toledo, Guadalajara, Alcalá de Henares, etc.), it is necessary a full reevaluation of the potential seismic sources in the zone in order to be included in future seismic hazard analyses.In this sense, we suggest that such seismic sources are oriented bands of tectonic deformation at depth instead particular single large-faults as occur in plate margin locations.These bands of tectonic deformation account for most of the geomorphic anomalies in the studied area (i.e.Silva, 2003;De Vicente et al., 2007), Quaternary lineation and lineal Canyon-shaped valleys into the basin centre, as well as for the spatial clustering of instrumental seismicity.Potential damaging earthquakes (M>5.5)can be produced in the zone like events generated during the time-window of c.a. 780 kyr and 125 kyr.
Figure 2 shows both areas elongated with NE-SW trending mainly located at: (a) the central subzone are in coincidence with the Tajuña River and lineaments interpreted (NE-trending).This subzone overlies the axis of the flexure of the basement, and (b) lateral subzones located at the NW and SE boundary of the basin and lineaments with NW-trending.The South boundary of the subzone

Fig. 5 .
Fig. 5.-Rose diagrams for SHMAX of the stress tensors determined within the Upper Tagus Basin.(a) Regional stress tensor and (b) local stress tensor.Fig. 5.-Resultados del análisis de esfuerzos de las estaciones de análisis estructural con datos de planos de falla estriados.Se representan la rosa de orientaciones medias de SHMAX obtenidas en cada una de las estaciones de análisis: (a) tensor regional y (b) tensor local.
and De Vicente et al. (1996) and, therefore, we have made a kinematic interpretation of the focal mechanisms.The focal mechanisms for earthquakes with M> 3.4 are (Andeweg et al., 1999): (a) reverse and normal faults oriented with NE-trending and (b) strike-slip faults with NW-trending (Fig.

Fig. 6 .
Fig. 6.-Strike-slip faulting affecting Pleistocene fluvial terraces of the TagusRiver.These faults are activated by the regional stress tensor with SH-MAX oriented to NW-SE.Fig.6.-Ejemplo de una de las estructuras de desgarre (SHMAX según NO-SE, tensor regional) en el norte de la cuenca.Las fallas afectan a materiales de terrazas del río Tajo de edad Pleistocena.

Fig. 7 .
Fig. 7.-Photographs of faulting affecting fluvial sediments at the Jarama River and the Tagus River, located at the South of the Tagus Basin.(a) y (b) are reverse faulting activated by the regional stress tensor (SHMAX NW-trending), and (c) and (d) lateral and normal faulting activated by the local stress tensor (SHMAX NEtrending).Fig.7.-Ejemplos de diferentes tipos de fallas afectando a materiales fluviales del río Jarama y del río Tajo en el sur de la cuenca: a) y b) fallas inversas decimétricas (SH-MAX NO-SE), c) fallas direccionales con componente normal y d) falla normal (ambas según SH-MAX NE-SE, tensor local).