D18: Results of in-situ microtremors surveys and array measurements at selected sites

Rough classification of the soils in the Medieval City of Rhodes, according to EC8 (CEN, 2004).

The report describes the work performed by the Aristotle University of Thessaloniki for the Tasks
7.2 & 7.3 of the WP7 according to Perpetuate contract. It contains a description of the field noise
surveys performed in selected sites within the Medieval city of Rhodes. The surveys include array
and single station microtremor measurements close to selected monuments of the city, which will
be used for the validation of the methodology which will be developed in PERPETUATE. The aim
is to define the dynamic properties of the local soils and to contribute in the experimental seismic
response of the soils and the monuments. We applied the well known SPAC method (Aki, 1957) in
10 sites. Concentric circles of different radii were deployed in each of these sites and the noise
wavefield was recorded using four broad-band triaxial seismometers. The dispersion curve of the
Rayleigh waves was determined from the correlation coefficients of the different station pairs
existing for each site and a detailed Vs profile was finally obtained by inverting the corresponding
dispersion curve in 9 out of the 10 sites. According to the Vs profiles the subsoil condition in the
medieval City of Rhodes are characterized by stiff geological formations to the depth of 250-300m.
The crystalline rock may be even deeper. Examining the Vs velocities between the investigated
sites, we were able to identify an interface at 80-100m depth, which is characterized be Vs>600-
650m/s. This interface can be considered as the seismic bedrock of the area for theoretical site
response analysis. Ambient noise was also recorded at 14 sites within the city and processed in
order to calculate their soil resonant and amplification characteristics. The data from the SPAC
measurements were also processed for this scope. In total, the experimental site response in the
medieval City of Rhodes is based on 24 field measurements. The HVSR method (Nakamura,
1989) was applied to calculate the resonant frequency and the amplification factor. A dominant
frequency ranging between 0.35 and 0.45Hz with amplification factor ranging between 3.0 and 8.0
was determined; a second peak with lower amplitude (around 3.0) appears in frequencies around
1.35 and 1.45Hz in almost all investigated sites. The field campaign including the SPAC and
HVSR measurements close to important monumental buildings of the Medieval city of Rhodes,
offer a quite good knowledge of the dynamic properties and the seismic response of the soils
located in their vicinity. The results show the existence of heterogeneous and stratified soil
conditions in almost all investigated sites. The estimated Vs profiles and the amplification
characteristics of the local soils will be considered in the pilot studies foreseen for the medieval
City of Rhodes.