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Integrated Nonseismic Geophysical Studies to Assess the Site Effect of the EUROSEISTEST Area in Northern Greece

Final Report Summary - IGSEA (Integrated Nonseismic Geophysical Studies to Assess the Site Effect of the EUROSEISTEST Area in Northern Greece)

Marie Curie project: IGSEA – Integrated Nonseismic Geophysical Studies to Assess the Site Effect of the EUROSEISTEST Area in Northern Greece

IGSEA addresses primarily topics out of the main scientific area such as Geophysics, Tectonics, and Seismology in combination with Earthquake Engineering, Engineering Seismology and building-soil & soil-water processes through the interaction between geophysicists and civil engineers. This project aims to assess site effects in the surrounding area of the EUROSEISTEST site by means of low-cost integrated nonseismic geophysical methods that are capable of detecting vertical geotectonic boundaries and fault zones with high accuracy and are able to confine and improve one of the first 3D resistivity inversion models of the area. The project will benefit from the use of state of the art devices and techniques which allow for a comprehensive view inside the inner structure of the basin. Through the implementation of hydrogeophysical and hydrogeological aspects we believe to significantly support the waveguide propagation modelling process of researcher and civil engineers in Greece and Europe who are working on strong motion data.

Subdivision 1:

Multidimensional Interpretation of Near Surface Electromagnetic Data Measured in the Mygdonian Basin, northern Greece

The Mygdonian Basin is located in an alluvial valley 45 km northeast of the city of Thessaloniki in northern Greece. It is a neotectonic graben (6 km wide) structure with increasing seismic activity where the large 1978 Thessaloniki earthquake occurred (A01). The seismic response at the site is strongly influenced by local geological conditions. Therefore, the European test site EUROSEISTEST for studying site effects of seismically active areas was installed in Mygdonian Basin. The ambient noise measurements from the area east of the EUROSEISTEST give strong implication for a complex 3D tectonic setting. Hence, near surface EM (electromagnetic) measurements are carried out to understand the location of the local active fault and the top of the basement structure of the study area.

The RMT (radiomagnetotelluric) and TEM (transient electromagnetic) measurements were carried out along eight profiles, which include 500 RMT and 104 TEM soundings (A02). The correlation between the borehole data and the interpreted TEM and RMT data generally shows a four layer model. The layers are identified as sediment and metamorphic rocks, which are in detail: silty sand (10-30 Ohmm), sandy clay (30 – 50 Ohmm), marly sand with clay (50 – 100 Ohmm) and marly sand with gravel (> > 100 Ohmm) with varying thicknesses. Due to the high resistivity of the top layer, the skin depths of the RMT soundings are approximately 35 m (A03). The TEM data give detailed information about the deeper structure up to the depth of 200 m. The joint and sequential inversion of RMT and TEM data proves to be an effective tool to investigate complex geological structure. The 1D and 2D interpretation of RMT and TEM data, respectively, indicate a local fault structure and helps in explaining the direction of the fault structure. In order to understand the local geological background and the impact of the 3D effects in the area, 3D forward modeling of RMT data was realised. According to the analysis, a normal fault is located next to the EUROSEISTEST site, with a strike direction of N 70 ° E.

Subdivision 2:

Magnetic survey
Magnetic data were collected on a regular grid in the Mygdonian Basin during 2009 and 2010. Whenever possible, we also collected magnetic susceptibility data at the same location to get reliable estimates for the modeling tasks. First results of the magnetic field survey during April 2009 are showing the predominant serbomacedonian strike of the basement structure (A04). Nevertheless, the Mygdonian Basin around the EUROSEISTEST is characterised by a more complex 3D anomaly pattern. The analysis of existing AMT data revealed an increase of the complexity of the conductivity distribution from west to east. Similar results are obtained by the observed magnetic anomaly field (A05). From the west to the east, the influence of a granite structure becomes more dominant and the magnetic field increases. Whereas to the west, a more simple tectonic setting can be assumed. From the decrease of the magnetic field from north to south we can deduce an asymmetric sediment filling of the basin with maximum thickness to the south. This observations implies a detachment fault system in the survey area.

We have to deal with man made electromagnetic noise in the Mygdonian Basin caused by the intensive utilisation of irrigation systems. To study this influence on the collected data we used information from a reference profile conducted through the noise free Philippi Basin (approx. 200 km east of Thessaloniki) that has been investigated with the same instrumentation and methodology as the Mygdonian Basin. This study revealed a tectonic setting that is presumably similar to the one in the Mygdonian Basin (A06).

During 2010 the magnetic survey was extended to cover the western part of the Volvi lake. We deployed a multi sensor platform to capture magnetic, temperature and self-potential data continuously on the hydrothermal active part of the lake (A07). Analysing the magnetic anomaly pattern, we can deduce the location of a granite formation, that is presumably responsible for the complex 3D tectonic setting for the central part of the Mygdonian Basin. The sediments reach in the western part of the survey area a thickness of approx. 600 m. In the eastern part of the survey area we expect less thick sediments.

To the west, the southern limit of the basin is not well defined. The resistivity model based on the AMT 3D inversion implies in the southwest thick layers of sediments. This finding was not very well supported by the AMT site distribution and gave reason for discussion. Hence, this area was in focus for our first magnetic model approach. First models of the total magnetic field through the basin indicate a good accordance with the 3D AMT resistivity model (A08). The magnetic model confirms the existence of thick sediment layers. Please note that the bottom of a conductivity structure is not well resolved (the vertical goes down to-4100 m) in an inversion, so that the magnetic model is helpful to delineate the top-of basement and can be used as a constraint in ongoing resistivity model studies

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