Palaeoseismology of the Vilariça Segment of the Manteigas-Bragança Fault in northeastern Portugal

Abstract

The Manteigas-Bragança fault is a major, 250-km-long, NNE-striking, sinistral strike-slip structure in northern Portugal. This fault has no historical seismicity for large earthquakes, although it may have generated moderate (M5þ) earthquakes in 1751 and 1858. Evidence of continued left horizontal displacement is shown by the presence of Cenozoic pull-apart basins as well as late Quaternary stream deflections. To investigate its recent slip history, a number of trenches were excavated at three sites along the Vilariça segment, north and south of the Douro River. At one site at Vale Meão winery, the occurrence of at least two and probably three events in the past 14.5 ka was determined, suggesting an average return period of about 5-7 ka. All three events appear to have occurred as a cluster in the interval between 14.5 and 11 ka, or shortly thereafter, suggesting a return period of less than 2 ka between events within the cluster. In the same area, a small offset rill suggests 2 -2.5 m of slip in the most recent event and about 6.1 m after incision below a c. 16 ka alluvial fill event along the Douro River. At another site along the Vilariça River alluvial plain, NE of the Vale Meão site, several trenches were excavated in late Pleistocene and Holocene alluvium, and exposed the fault displacing channel deposits dated to between 18 and 23 ka. In a succession of closely spaced parallel cuts and trenches, the channel riser was traced into and across the fault to resolve c. 6.5 m of displacement after 18 ka and c. 9 m of slip after c. 23 ka. These observations yield a slip rate of 0.3-0.5 mm/a, which is consistent with earlier estimates. Combining the information on timing at Vale Meão winery and displacement at Vilariça argues for earthquakes in the M7þ range, with coseismic displacements of 2 -3 m. This demonstrates that there are potential seismic sources in Portugal that are not associated with the 1755 Lisbon earthquake or the Tagus Valley, and, although rare, large events on the Vilariça fault could be quite destructive for the region. This work provides an analogue for the study of active faulting in intracontinental settings and supports the view that earthquakes within intracontinental settings tend to cluster in time. In addition, this study highlights the usefulness and application of multiple field, remote sensing and geochronological techniques for seismic hazard mitigation.

Contribution of active faults in the intraplate area of Iberia to seismic hazard: The Alentejo-Plasencia Fault

“…For the slip rate values associated with the initiation of the pull-apart basins we use ~ 16 Ma years (pull-apart basin initiation; Villamor et al, 2004). Those values are 0.05 to 0.1 mm/ yr (Table 1), which are also in agreement with slip rate values of 0.3-0.5 mm/yr in similar NE trending strikeslip faults in the Iberian Massif from paleoseismic studies (Rockwell et al, 2009). For slip rates associated with the S-Raña surface, we use a range of ages from 5 to 2 Ma for the Raña sediments (based on different authors, see discussion in Villamor, 2002 and section 4).…”

“…In this latter area, the drainage pattern has also been described as controlled by NE-SW faults (Martín-González, 2009). More recent paleoseismic studies have revealed the Quaternary activity of the Vilariça fault (NE trending fault; fault 23 on Fig 1), with at least two Mw>7 events in the last 14.5 ka with a slip rate of 0.3-0.5mm/yr (Rockwell et al, 2009).…”

Seismicity and potencially active faults in the Northwest and Central-West Iberian Peninsula

“…The main seismic sequences (Lugo and Orense sequences) are related to faults belonging to the Orense corridor. These corridors penetrate in Portugal and have the same orientation as the Bragança-Vilariça fault corridor, which has a slip rate of 0.2-0.5 mm/a (Cabral, 1989 andRockwell et al, 2009;Cabral et al, 2010), the Verin-Vila Real fault corridor, and some segments of the AlentejoPlasencia fault (Villamor, 2002) (Figs. 11 and 2).…”

“…The National Seismic Catalogue, managed by the Instituto Geográfico Nacional (IGN), contains 4,380 historic and instrumental 1989; Cabral and Ribeiro, 1993;Rockwell et al, 2009;Cabral et al, 2010) (Fig. 1) and, although there is plenty of petrologic and structural information about the zone (e.g.…”

Ocean loading effects on stress at near shore plate boundary fault systems

“…This would be an example of a physical process external to the seismic cycle affecting the faulting process. There are not currently many paleoseismic indicators that sufficiently span the time since the LGM in a way that would allow these predictions to be rigorously tested, though a few studies do offer some observations of long-term fault behavior [e.g., Rockwell et al, 2009]. We therefore present a model-based analysis of the stress changes ocean loading is likely to create and the impact such perturbations would have on observable paleoseismic indicators, so as to better understand the influence sea level rise may have on long-term plate boundary behavior.…”