Dynamics of North Anatolian Fault Zone

1. DYNAMICS OF THE NORTH ANATOLIAN FAULT
N. Canitez
Department of Earth and Planetary Sciences
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
(On leave from the Technical University of Istanbul, Turkey)
Abstract. Recent progress on the genetic and dynamic proper­
ties of the North Anatolian fault has been reviewed. Detailed
geological studies show that the fault started its activity in
Miocene times, and had a 5-6 mm/year average slip-rate for the
last 15 m.y. The slip-rate for the last 1/2 m.y. is about 7 mm/
year. The recent slip-rate associated with earthquakes is between
1.2 and 2.4 cm/year depending on the vertical extension of the
fault. The stress-drop due to earthquakes is not strongly depen­
dent on magnitude for M > 7. Studies of the dependence of the
surface area of faulting on the seismic moment of earthquakes
give different relationships for large and small earthquakes.
Necessary future studies related to the North Anatolian fault
are discussed at the end of the paper.
INTRODUCTION
The North Anatolian fault has attracted wide attention because
of a series of large earthquakes that began with the Erzincan
earthquake in 1939. The fault is quite similar to the famous
active strike-slip faults of the earth such as the San Andreas
fault of California and the Alpine fault of New Zealand. The
rift topography and characteristic features of Quaternary fault­
ing (sag ponds, offset streams, scarps in alluvium) mark the
fault throughout its extent.
The number of earthquakes with magnitude equal to or larger
than 6 in the fault zone is about 40 since 1900, Only 8 of the
major earthquakes have been investigated in detail in the field
(Ketin, 1969; Ambraseys, 1970), The average displacement
associated with these earthquakes varied from 30 to 430 cm.
The fault shows a continuous creep around Gerede (Ismetpasa
station). These are almost the only wellknown characteristics
of the fault. More information is needed for a better under­
standing of static and dynamic characteristics of the fault.
In this paper an attempt has been made to summarize the
recent progress on the genetic and dynamic properties of the
North Anatolian fault.
GEOLOGICAL HISTORY OF THE NORTH ANATOLIAN FAULT
The verifiable length of the North Anatolian fault is about
1300 km. The fault shows an interruption in the region of Marma­
ra in northwestern Turkey. In the eastern part, the fault is
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2. continuous and the earthquake source mechanisms are consistent­
ly strike-slip. The western part, however, is much more compli­
cated both, in structure and fault mechanism.
There are different opinions about the development of the
fault. According to Pavoni (1961), the fault ruptured at the
beginning of Tertiary time and the total slip since that time
is 350-400 km. According to Ketin (1969), however, the fault
started its activity after the Miocene or probably during the
PIio-Quaternary period. Since that time there has been a total
displacement of tens of kilometers across the fault.
The North Anatolian fault is the boundary between the Pontid
and Anatolid tectonic units. There is almost no lithologic cor­
relation between the two sides of the fault. The Mesozoic and
Tertiary development of the eastern Pontids has been investiga­
ted by Tokel (1973). According to him, the Pontid mountains are
assumed as massifs which were tectonically active and upheaved
during Mesozoic and Tertiary times. The granites and the meta-
morphic rocks of the Pontids were affected by the Hercynian
orogeny. The region was above sea-level during Dogger and
Lower Cretaceous times. The development of the Upper Cretaceous
trough in the Pontids, according to Tokel (1973), is attributed
to a subduction zone in the North Anatolian Tethys. After the
Cretaceous period, the North Anatolian Tethys closed as a result
of the northward movements of the African and Arabian plates,
and probably a trench developed near the southern boundary of
the Pontids. The closing of the North Anatolian Tethys was
completed at the Miocene as the two continents moved towards
each other. The boundary between the Anatolian and the Pontid
plates is the North Anatolian fault.
A part of the North Anatolian fault (Resadiye region) has
been investigated by Seymen (1974). According to him, the Pontid
unit does not contain the Alpine ofiolites and epimetamorphites.
On the contrary, the Anatolids are composed of shelf and deep
sea deposits accumulated on the pre-Alpine metamorphic basement,
and of an allochthonous ofiolite belt and o1istostroms. The
emplacement of these ofiolitic rocks bearing the Alpine epi-
metamorphites occurred in the subduction zone which was de­
veloped in the northern branch of the Tethys at the beginning
of Upper Cretaceous time. According to Seymen (1974) the paroxism
of the Alpine mountain building (Pontian-phase) within the Niksar-
Resadiye region was caused by the continent/is1and arc collision
during the passage from Upper Paleocene to Eocene. This phase
of the Alpine movements created the border folds trending in
an E-W direction. Seymen is of the opinion that the second
episode of the Alpine orogeny together with the calc-alkaline
volcanism and plutonism of the eastern Pontids took place since
the trench zone in the northern Tethys rejuvenated during Late
Lutetian-Early Priabonian time. The last Alpine movements,
according to him, occurred due to the continent/continent
(i.e. Pont id/Anato1 id) collision in Lower-Middle Miocene times.
In this period, the North Anatolian fault zone was also developed,
as a result of the Anatolian plate being gradually pushed westwards
by the Arabian block, so that this fault cut the regional trends
and the boundary between the Pontids and Anatolids with an acute
angle. Thus, the right-lateral movements along the fault dis­
placed the boundary of the Anato1id-Pontid in the vicinity of
Susehri (Sivas) and Refahiye (Erzincan) by 90 + 5 km. Seymen
(.1974) states that the average slip-rate along the North-Anato­
lian fault is about 0.5-0.6 cm/year since Middle Miocene times.
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3. SLIP-RATE AND STRESS DROP ALONG THE NORTH ANATOLIAN FAULT
The slip-rate of the right-handed motion along the North
Anatolian fault has been investigated by several authors (Brune,
1968; Ambraseys, 1970; McKenzie, 1972; Canitez and Ezen, 1973;
Seymen, 1974). Brune (1968) found a value of 11 cm/year for the
period 1939-1967. As it was also pointed out by Brune himself,
this value might be overestimated because this time interval
has an episode of large dislocations. The average slip for the
whole length of the fault since 1939 was reported to be about
90 cm by Ambraseys (1970). This value corresponds to an average
slip-rate of about 3 cm/year. Even though the evidence for a
90 cm average displacement is not reported, a slip-rate of
3 cm/year seems more reasonable.
According to McKenzie (1970, 1972), the sense of the motion
of the Anatolian and the Aegean plates is almost the same. Thus,
in order for a tensional zone to develop in western Turkey, the
slip-rate of the Aegean plate has to be higher than that of the
Anatolian plate. The deepest point of the sinking slab in the
Aegean area is only 200 km deep (Caputo et al., 1970; Galanopou-
los, 1972; Papzachos, 1973). One can think about two possibilities:
1) The Aegean lithosphere is assimilated by the mantle, and 2)
the structure is quite young. If the first one is the case, and
10 m.y. is enough for the assimilation (Isaacs et al., 1968),
the convergence rate in the Aegean area is 2 cm/year. If the
second possibility is the case, and furthermore if the Aegean
plate started moving as early as the Anatolian fault started,
say 15 m.y. ago in the middle of the Miocene (Seymen, 1974),
the convergence rate is 1.3 cm/year. McKenzie (1972) suggests
a 4 cm/year slip-rate for the Turkish plate.
Some recent tectonic events in Turkey have been investigated
by Arpat and Saroglu (1975) using aerial photographs and field
observations. The East Anatolian fault crossing the North
Anatolian fault zone has been mapped. This left-handed fault
is connected with the Dead Sea fault system. They also mapped
some parts of the North Anatolian fault, and they showed some
evidence indicating an average slip-rate greater than 7 mm/
year for the last 1/2 m.y.
Another study of the static and dynamic characteristics of
the North Anatolian fault has been conducted by Canitez and
Ezen (1973). Using the static parameters (fault lengths and
average dislocations) observed in the field, they found the
relation:
log M = 1.64 M + 14.75 for 6 < M < 8 (1)
O S s
between seismic moment and magnitude from which the total seismic
moment for the period 1900 - 1971 has been obtained as
1.77 x l o z o
dyne cm. Using this value, and following Brune (1968),
they calculated the average slip-rate for the period of interest
with different fault-depth assumptions. For instance, they found
2.4 cm/year for W = 20 km, 1.6 cm/year for W = 30 km, and
1.2 cm/year for W = 40 km.
High-precision triangulat ion and trilateration measurements
were started in 1972 in the western part (Gerede-Cerkes region)
of the North Anatolian fault zone. The comparison of the 1946
(after the 1944 earthquake) and 1972 measurements showed a
75 cm horizontal displacement in the eastern end of the 1944
Gerede-Bolu earthquake fault. The total relative displacement
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4. in the western portion, however, is 20 cm for the same period
(Ugur, 1 974) .
Stress drops for some earthquakes in the North Anatolian
fault zone have been investigated by Chinnery (1969), Hanks
and Wyss (1972) and Canitez and Ezen (1973). The general con­
clusion is that the stress drop due to earthquakes in the
area is less than 50 bars. Canitez and Ezen (1973) concluded
that the stress drop in the North Anatolian fault zone is
between 10 and 15 bars for M > 7 and does not depend on
magnitude as Aki (1972) expressed. For M < 7, however, they
found the stress drop less than 10 bars.
Using the observed fault lengths and relative displacements,
and assuming that the vertical extension of the fault is 20 km,
Canitez and Ezen (1973) compared the seismic moments with the
fault areas. They found different relations for large and small
earthquakes:
log M = 26.76 + 1.66 x lo~4
S for M > 7 (2)
o s
and
log M n = 20.5 + 7.5 x 1 0 ~ 3
S for M < 7 (3)
0 s
2
where, M is in dyne cm and S is in km . The accuracy in these
relations is debatable because of the uncertainties in the
vertical extension of the fault.
Geological and seismological investigations undertaken so far
show that the North Anatolian fault is very similar in character
to the San Andreas fault. In summing up, we can say that right-
handed motion of the fault started 15 m.y. ago in the middle
of Miocene times with a long term slip-rate of 0.5 - 0.7 cm/
year. The average rate of the recent movements associated with
earthquakes, however, is about 1-2 cm/year. The stress drop
due to earthquakes is not dependent on magnitude for M > 7.
NECESSARY FUTURE STUDIES
The significance of the North Anatolian fault comes from its
position as a very clear plate boundary in the eastern Mediterra­
nean area. Not only studied for recent tectonics, the fault
zone is also a very interesting place to investigate the paleo-
tectonics of the eastern Mediterranean region. However, the
properties given in the previous sections are almost the only
ones known presently. The following investigations are still
pending:
a) Only some small parts of the fault have been investigated
so far in some detail. More field work is needed for detailed
mapping of th.e fault zone.
b) The paleogeography of Permian and Triassic times should
be constructed using sedimentological and biostratigraphical
methods. The result of this study will shed new light on the
early development of the Tethys ocean in Anatolia.
c) Geochemical, petrological, geochronological and bio-
stratigraphical correlations must be made between the two
sides of the fault. The questions of "What was the total
amount of displacement during geological times?" and "Was
the motion reversed anytime in its geological history?" might
be answered using these data.
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5. d) The one thing we know about the crustal structure is
that the 36 km thickness of the crust in Central Anatolia is
diminshing toward the Black Sea in the western part of Turkey
(Canitez, 1962). There is no information about the velocity
structure and lateral inhomogeneities within the crust and
upper mantle. An explosion study program has to be started
as soon as possible.
e) The earthquake epicenters along the fault zone are quite
scattered. Because of the unsatisfactory azimuthal distribu­
tions of seismological stations, it is very difficult to make
high-precision relocation. A dense, local, short-period seismic
network is necessary for detailed seismicity and seismo-tectonic
study in the area. A 6-year program for micro-earthquake study
was begun in 1975.
f) The area is quite convenient for earthquake prediction
studies. Although high-precision geodetic measurements have
been started, strain, creep, tilt and magneto-telluric measure­
ments would be very helpful for this purpose. A dense seismic
network also might be very useful to investigate the velocity
changes and abnormal velocity zones in the area.
Acknowledgement. The author wishes to express his sincere
thanks to Professor M. N. Toksbz who read the manuscript. The
study is supported by NATO Science Grant 4=f 568 .
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Special Publications Geodynamics: Progress and Prospects Vol. 5
Copyright American Geophysical Union