بررسی گسلش فعال و خطر گسترش شهرها در پیرامون گسل‌های زمین لرزه‌ای (مطالعه موردی: حوضه‌های شمال تبریز)

           Extended
abstract 1- Introduction The
city of Tabriz is one of the four large city of Iran and the largest
inhabitation center in northwest of Iran. This city is located on a large and
active tectonic fault which enlongate from north Anatolian fault system-located
in Turkey- to the Alborz mountains in Iran. The regions of northwest Iran,
eastern Turkey and Caucasus are one of the most complex regions of the Arabia –
Eurasia collision. The seismicity of this region is due Arabia-Eurasia plates
convergence. The results of convergence indicated by moderate to large magnitude
earthquakes. So that, the cities of this region (as Tabriz) destroyed
repeatedly by large magnitude earthquakes. Right lateral strike-slip motions
along the North Tabriz fault have given rise to strong earthquakes, which
threaten the city of Tabriz with its population of more than 1.5 millon. This
research try while review of seismicity
and seimotectonic situation, evaluate active tectonic by geomorphic indexs in
Varkesh-chai, Gomanab-chai and Nahand-chai and remember city extension in
limites of earthquake fault. Area of the study region is 1258 Km2
and is located northeast of Ormieh lake and north of Tabriz city. North Tabriz
Fault (NTF) pass from north of Tabriz city.   2- Methodology In
this research, seismotectonic characteristics the study area analyzed by
historical and seismography data from Institute of Geophysics Tehran
University. We analyzed six geomorphic indices for evaluating of active
tectonic in the study area. Indices used include: the stream- gradient index
(SL), drainage basin asymmetry (Af), Ratio of valley – floor with to vally height (Vf),
drainage basin shape ratio (Bs), topographic Symmetry
Factor (T) and stream sinusity(S). We the computed a single index (Iat) from the six indices
to characterize relative active tectonics. Iat is cbtained by the average of
the different classes of geomorphic indices (S/n) and divided into four classes
from relatively low to highest tectonic activity.   3- Discussion Studies
of region seismicity indicated that
North Tabriz Fault (NTF) is a NW-SE trending strike – slip fault. Rate of right
lateral slip determined 7±1 mm/yr for the North Tabriz Fault.This value is
consistent with earthquakes of magnitude 7-7.3 and recurrence times of 250-300
yr. This rates conform to historical seismicity of the North Tabriz Fault in
858, 1024, 1273,1304,1641,1721,1780 and 1786. This conditions indicating the
potential for a significant earthquake on the NTF in the next 50 yr. Map
of earthquake focal distribution present relation earthquakes locations with
active faults. This map shows that dense clusters are located on both ends of
the North Tabriz Fault. There is another cluster near the city of Tabriz. The
depth of earthquakes from the surface is almost 20 Km in the seismotectonics. The results of geomorphic indices show active tectonic faulting in
the study drainage basins (Varkesh-chai, Gomanab-chai and Nahand-chai). One of
the indices is Asymmetic factor (AF). This indexcan be used to evaluate
tectonic tilting at the scale of drainage basin. Asymmetic factor shows that
Nahand-chai is classified in class 1(active tectonic), Varkesh-chai and
Gomanab-chai are located in class 2 (moderate). Topographic
Symmetry Factor (T) indicates asymmetry of
Nahand-chai is more than another rivers.Because index values are near 1, Therefore
drainage basins of the study area are active, based up on values topographic
Symmetry Factor.  Another
index sensitive to tectonic uplift is the valley floor width to
vally height ratio. Calculation of ratio of valley floor width to vally height (Vf) is
done at a distance 1 km upstream from mountain front. This index shows valleys
with a wide fioor relative to the height of valley walls (U shape) in the study
area. Therefore the study drainage basins are classified in class 3(low
active). An
other index for studing active tectonic is basin shape index. Relatively, young
drainage badins in tectonically active area tend to be elongated in shape.Based
upon index of drainage basin shape (Bs), Nahand-chai has longer shape than
north Tabriz drainage basins. Varkesh-chai and Gomanab-chai are located in
class3. The results of stream sinusity shows that the study drainage basins
don’t reach to equilibrium. They are active based upon stream sinusity. We
calculated SL index along the study rivers using a digital elevation model and
GIS, and computed its average value for each basin. Values of stream length-
gradient index (SL) differentiantes along the study rivers. So that, average
Value SL index is high in the nahand-chai, but in Varkesh-chai and Gomanab-chai
are moderate.  Iat
indicates high tectonic activity (Class 2) in the study drainage basins. Index
of relative active tectonic (Iat) obtained in the study area, based values of
stream- gradient index (SL), drainage basin asymmetry (Af), Ratio
of valley – floor with to vally height (Vf),
drainage basin shape ratio (Bs), topographic Symmetry
Factor (T) and stream sinusity (S).   4-Conclutions Seismcity
and seismotectonic conditions and analyses of geomorphic indices in the north
Tabriz drainage basins are indicating that Tabriz city is located in hazardous
region. Clusring of strong earthquakes along active faults is an additional
factor of seismic hazard characteristic of the considered area. Historical
evidence indicate that there were several earthquake on the North Tabriz Fault.
The results of geomorphic indices show that the study area is classified in
class 2 (high active tectonic).This problem must to pay attent in urban
planning and development.Now that, Tabriz city sprawl is enlongating toward
slopes of Onebneali heights (location of fault). In recently decades increase
buildings and build towers is disturbed stability of this region.This
conditions increased instability and earthquake hazards. Besides there are
buildings at city margins where located along of Onebneali heights (location of
fault)and build by undesirable material.These houses can´t resist against
moderate magnitude earthquakes.This conditions increase earthquake hazards
intensely in this region.