  We analyze the ground motion time histories due to the local seismicity near
the Itoiz reservoir to estimate the near-source, surface 3D
displacement-gradients and dynamic deformations. The seismic data were obtained
by a semi-permanent broadband and accelerometric network located on surface and
at underground sites. The dynamic deformation field was calculated by two
different methodologies. First, by the Seismo-Geodetic method using the data
from a three-station micro-array located close to the dam; second, by
Single-Station estimates of the displacement gradients. The dynamic
deformations obtained from both methods were compared and analyzed in the
context of the local free-field effects. The shallow 1D velocity structure was
estimated from the seismic data by modeling the body wave travel times. Time
histories obtained from both methods results quite similar in the time-window
of body-wave arrivals. The strain misfits between methods vary from 1.4% to
35.0% and rotational misfits vary from 2.5% to 36.0%. Amplitudes of
displacement gradients vary in the range of 10-8 to 10-7 strains. From these
results, a new scaling analysis by numerical modeling is proposed in order to
estimate the peak dynamic deformations for different magnitudes, up to the
expected maximum MW in the region (M5.5). Peak dynamic deformations due to
local MW5.5 earthquakes would reach amplitudes of 10-5 strain and 10-3 radians
at the Itoiz dam. The Single-Station method shows to be an adequate option for
the analysis of local seismicity, where few three-component stations are
available. The results obtained here could help to extend the applicability of
these methodologies to other sites of engineering interest.

Geophysics