Geoscience Reference
In-Depth Information
specific (shear modulus-shear strain-damping) G-
-D curves have been also proposed
describing thedynamic soilproperties for linear elastic and nonlinear conditions.
γ
The soil-layering from surface to bedrock presents a successive increase of the stiffness
with Vs velocities from 90 to 450m/s (mixtures of recent fills, loose sandy-sands, and
soft clays with gravels at a thickness of some decades of metres) to 500 to 850m/s
(redbedsfromstiffclayswithgravels-stones,conglomerateandsandstonesforathickness
larger than 100m). The velocities and their variation with depth are not uniform in the
whole area of the basin. This knowledge resulted from the synthesis of a great number
of data (drillings, sampling, geophysical-geotechnical, passive (noise) and actual sur-
veys,empiricaltransferfunctions,parametricmodelling,etc.).Finally,the3Dsoilmodel
of the Mygdonian sedimentary basin has a rather asymmetrical semi-cylindrical shape,
whichdeepensattheopenedgeswithdifferentratesofdeepeningandwithoutverystrong
immersions. Table 5.2 presents the classified basic soil formations met in the whole area
of the basin together withthe Vp and Vs velocities.
2.1.3. Instrumentation
Atthebroaderareaofthebasin,alargepermanentseismologicalnetworkwithmorethan
15 broadband stations operates continuously since 1981, providing useful information
fortheseismicityrecordedinthewholeNorthernandCentralGreeceincludingthenorth
Aegean Sea as well as the surrounding northern countries. This array operates under the
responsibilityofAUTh(GL).Additionalnationalpermanentseismologicalandaccelero-
graphicarrayswithmorethan80instrumentseach(fromNationalObservatoryofAthens
and ITSAK) provide fullcatalogues of weak and strongearthquakes in Greece.
Table 5.2. Geophysical parameters and soil description for the basic soil formations. Formations
A,B,C,andDareincludedintheupper(Mygdonian)systemandE,F,G
∗
,andGinthelowerone
(Promygdonian). The characteristic Vs velocities of both main systems and the bedrock basement
are also given as average velocities of the defined sub-layersfrom the top down to the bedrock
Layer
Description
Vs (m/s)
Vp (m/s)
Vpw(m/s)
A
Silty-clayey sand
95-150
220-450
B
Siltysand and sandy
clay
150-250
300-350
900-1650
C
Marly silt and silty
sand
250-400
400-800
900-2200
D
Marly sandy clay and
clayey silt
400-550
1650-1900
E
Clayey silt-sandand
sandy clay
560-650
2200-2750
F
Clayey silt-sandand
sandy clay
600-900
2200-2750
G
∗
Weathered Schist
1000-1700
3000-3500
G
Gneiss
2300-2600
4300-4900
