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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS005 Oral Presentation 1914
Seismic characterization of the Vilaria Fault in NE Portugal: Comparison
between a segmentation model and paleoseismology
Dr. Thomas Rockwell
Geological Sciences San Diego State University
Chris Madden, Tim Dawson, Lewis Owen, Susan Villanova, Paula Figueiredo, Joo
Fonseca
As part of a seismic hazard assessment for a new dam in NE Portugal, we studied the geomorphology
and conducted paleoseismic trenching along the Vilaria fault in northeastern Portugal to explore the
faults late Quaternary rupture history. The Vilaria fault is a major, 250 km-long sinistral strike-slip
structure orientated NNE to SSW. This fault has no historical seismicity for large earthquakes, although
it may have generated a moderate (M5.8) earthquake in 1858 (Vilanova, 2004). Evidence of continued
left horizontal displacement is shown by the presence of Cenozoic pull-apart basins as well as late
Quaternary stream deflections. We identified three major segments based on structural complexities,
with the dam adjacent to the 75 km-long central segment. Based on an inferred rupture of this length,
we would expect earthquakes of about M7.2 with about 1.6 m of average displacement using the Wells
and Coppersmith (1994) regressions. Combined with the very sparse published inferences on the
Quaternary slip rate, about 0.5 mm/yr (Cabral, 1989; 1995), we estimate a return period for such
events of about 3000 years. To test this segmentation model and more directly determine the seismic
hazard posed by the Vilaria fault, we excavated a number of trenches at three sites along the central
segment. At one site at Vale Meo winery, we determined the occurrence of 2-3 events in the past 14-18
ka, suggesting a return period of 5-9 ka. In the same area, a small offset rill suggests 2.2 m of slip in
the MRE. At another site along the Vilaria River alluvial plain, northeast of the Vale Meo site, we
excavated several trenches in late Pleistocene and Holocene alluvium, and exposed the fault displacing
channel deposits dated to between 18 and 32 ka. In a succession of closely-spaced parallel cuts and
trenches, we traced the channel riser into and across the fault to resolve 9 m of cumulative slip, which
yields a slip rate of 0.3-0.5 mm/yr, consistent with but a bit lower than earlier estimates. This
displacement is also about four times that inferred from the deflected channel at Vale Meo, consistent
with its older age. Combining the information of timing at Vale Meo winery and displacement at Vilaria
argues for earthquakes in the M7.2-M7.4 range, similar to that inferred from the segmentation model,
but suggests that the displacements are slightly larger and the return periods longer. It also
demonstrates that there are potential seismic sources in Portugal that are not associated with the 1755
earthquake or the Tagus Valley and, although rare, large events on the Vilaria fault could be quite
destructive for the region.
Keywords: vilarica, paleoseismology, portugal