The 9 July 1956 Amorgos earthquake (Mw 7.6) was the largest event that hit Greece during
the last century while it was also followed by a tsunami that inundated the coastal areas of the
southern Aegean. This study investigates the rheological properties of the 1956 rupture zone
between Amorgos and Santorini islands, in an effort to place some constraints on
the nucleation depth and rupture extent of this large event. The seismic velocities
inferred from tomographic and surface wave dispersion studies of the area are first
correlated with laboratory determined velocities of known rock types. It is found
that the lithosphere in the southern Aegean can be approximated by three layers
representing the upper/lower crust and upper mantle consisting of quartzite, diabase and
peridotite respectively. Geotherms are calculated by using an analytical solution to
the one-dimensional heat conduction equation, while Yield Strength Envelopes
(YSEs) are produced after assuming laboratory estimated parameters of brittle and
ductile deformation for each rock type. The depth frequency of earthquakes in the
area, as well as other independent observations favour the YSE calculated for a
wet upper crust/upper mantle, a dry lower crust and a geotherm corresponding to
a low surface heat flow of 62 mW m-2. In this YSE, the upper mantle exhibits
maximum strength at 33 km becoming more ductile at greater depths. The lower
crust retains significant strength and therefore cannot flow as it did during the early
stages of extension, but it is relatively weaker than the upper mantle confirming
the ‘jelly sandwich’ model previously proposed for the continental lithosphere.
The downdip rupture width of the Amorgos event can be estimated from empirical
relationships to be 26 km which means that its rupture may have extended from the depth
of peak strength in the upper mantle (33 km) to 7 km upwards. Such a scenario
agrees well with recent modelling results indicating that the Amorgos tsunami was
probably caused by submarine landslides rather than coseismic rupture of the seafloor. |