IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Poster presentation 2170
Complex anisotropy structure beneath the Po-Plain and Northern
Apennines (Italy): measurements and modelling
Dr. Simone Salimbeni
INGV Bologna IASPEI
Margheriti Lucia, Levin Vadim, Park Jeffrey
The multidisciplinary RETREAT project (REtreating-Trench, Extension and Accretion Tectonics) is
focusedon the development of a 3D self-consistent dynamic model of the syn-convergent extension in
the NorthernApennines (Italy). A temporary seismic network equipped with 35 seismic stations recorded
betweenOctober 2003 to September 2006. We used the data recorded by these stations, added to those
frompermanent stations (MedNet and INGV network), to analyse seismic anisotropy. Previous
SKSsplitting analysis done for this region allowed to define some anisotropic domains. The Tuscany
domain(Tyrrhenian side) shows a homogeneous NW-SE fast axes directions; the Po-Plain domain
(Eastern side ofthe Apennines) shows a N-S to NE-SW directions. These two domains are separeted by
an abrupt transitionzone (30-40 Km) located in correspondance of the Apenninic crest. In the Po-Plain
domain, the splitting parameters are strongly dependent on backazimuth of earthquakes analysed;
consequently, the presence ofa complex structure (probably multilayers or dipping anisotropy) has been
supposed beneath this region. To evaluate if at depth a double layer structure can justify our dataset
distribution, we use theapproach of Menke and Levin (2003), applying the cross convolution method
they propose to discriminatewhether a two-layer anisotropic model fits better than a simple one-layer
model. Two time-seriescontaining the event data and theorethical earth model are constructed. Varying
the earth model it ispossible to identify the one that minimizes the misfit with observed data and that is
then consideredthe best. The method can work either on the single event recordings or also on group of
events datarecorded at the same station. The analysis confirm the more simple structure beneath the
Tuscany side ofstudy region, where a single anisotropic layer with a NW-SE fast polarization direction is
in agreementwith SKS splitting analysis. The anisotropic structure beneath the Po Plain is the main goal
of the study. Here our results confirm the presence of a multi-layered structure, where a mainly NW-SE
directions and delay time greater than 2.0 sec and NE-SW fast polarizations and smaller delay times are
found respectively for the bottom and upper layers.
Keywords: seismic anisotropy, northern apennines, subduction