Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

this method, the magnitude of the earthquakes (M s) for a
100‐year return period within the radiuses of 50, 100, 200
and 300 kilometers are 4, 5.1, 6.4 and 6.8, respectively.
ASSESSMENT OF FAULTS ACTIVITY BASED ON THE
REGIONAL TECTONICS
It is evident for the estimation of maximum parameters of
earth movement with the deterministic method and even
the introduction of parameters for various seismic design
levels with the probabilistic method based on the seismic
line sources, identification of active faults at the periphery
of the site are highly important. Although the
seismotectonic maps at the periphery of the site within
200 to 300 kilometers are usually prepared, the
performance of the active faults within the 30 to 50‐
kilometer radius is considered as the most important
seismic scenario. In other words, the faults beyond the
50‐kilometer radius of the dam site impose little
acceleration (less than 0.1g) to the dam site, in case they
become reactivated. Normally, this range of acceleration
inflicts no damage to the dam and power plant structures.
The conducted studies point out that with the 60‐
kilometer radius of the Moshampa dam and power plant
site, the prominent faults in terms of seismic potential
and location are the Takht‐e‐Solayman and NE1 faults.
The Takht‐e‐Solayman fault with the thrust mechanism
and northwest‐southeast strike is related to the two
epicenters, therefore it could be considered as an active
fault in the seismic analysis. The distribution of the
epicenters for earthquakes occurred within the 300
kilometer radius is shown in (Fig. 3).
Fig. 3: The seismotectonic map of the Moshampa dam & power
plant site within the 300‐kilometer radius
It is evident that the site is located in a place where the
frequency of events is inconsistent. According to Figure 3,
within the 300‐kilometer radius, the closest earthquake to
the site was 40 kilometers to its southwest and within the
50‐kilometer radius only one historical earthquake has
taken place. The two mentioned earthquakes are related
to the Takht‐e‐Solayman Fault. At the 60‐kilometer
distance, only one more historical earthquake has been
recorded and within the 90‐kilometer radius, no other
earthquake is recorded. The other earthquakes are
concentrated beyond the radius of 90 kilometer from the
site mostly around the North Tabriz fault, western end of
Alborz and northwestern part of Zagros seismotectonic
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology)
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provinces. From the distribution pattern of the
earthquake epicenters and their lack of presence at the
vicinity of the site, it is not possible to comment on the
nearby fault activities by the customary method. At least,
it could be concluded that they have had no activity
during the 20 th century. On the other hand, the lack of
recent alluvium has prevented the study of shear effects
by the faults activities in the recent period. In this regard,
only the tectonic and paleotectonic studies are to be
conducted.
These studies indicate that the NE1 Fault is the middle
segment of a major basement fault comprised on North
Tabriz fault in the northwest extended to the Soltaniyeh
fault in the southwest with the length of 450km. This is
the separating border between the western Alborz and
Soltaniyeh‐Mishu structural zones. It is mentioning that
the geological maps show this basement fault after a
distance gap is connected to Qom‐Zefreh fault towards
the southwest. Thus, its total length from the North Tabriz
fault to the Qom‐Zefreh fault is over 900km.
The seismotectonic studies of this major fault, which is
comprised of several segments, indicate that most of its
segments have had seismic activities during the 20 th
century. In other words, they could be considered as
active fault segments. However, there are segments like
the NE1 fault segment that did not reactivate in historical
or 20 th century time. Albeit, due to their tectonic
characteristics and structural links with the other fault
segments, their inactivity in the 20 th century does not rule
out the potential for future activities. NE2, NE3 and NE4
faults have a completely parallel trend with the NE1 fault
segment. They are located at the southwest of the NE1
fault segment. The field studies and the tectonic and
seismotectonic assessments indicate that these faults are
inactive and probably will not impose any danger to the
dam during its normal life time. Moreover, due to their
considerable distance to the NE1 fault, the activity of the
NE1 fault will not affect them. During the tectonic
evolution of the Moshampa dam and power plant site, at
least from the Cenozoic (Miocene) up to now, no
significant fault at the upper crust has occurred. From
what has been mentioned above, no essential change in
the dam during its normal life time will be expected. It
could be concluded that the faulting hazard at the dam
and power plant foundation from the general tectonics
point of view is not relevant.
SITE CHARACTERISTICS
The basement rock of the Moshampa dam and power
plant site is generated by the Qom formation (Oligo‐
Miocene). This formation in the studied region is
comprised of limestone, schistose limestone, tuff and
schistose tuff. However, the intercalations of marl are
present between the above‐mentioned units. The
geophysical studies for determination of shear wave
velocity in these rock bodies at the dam axis are
conducted via the seismic (refraction) method. The results
indicate that the average velocity of the primary wave at
the site is 3500m/s. Since the average density of the site's
rocks is 2.5t/m 3 , the velocity of the shear wave is
1400m/s. Thus, the rocks contained in the basement of
the Moshampa dam and power plant site are classified as
stiff rocks.