Ahar-Varzeghan Twin Earthquakes of 11 August 2012, Mw.6.4 and Mw.6.3

Two 
earthquakes 
occurred
 in 
11 
August 
2012
 NW
 Iran, 
Ahar 
region 
(60 km
 NE 
Tabriz), 
first 
one 
in 
16:53 
local 
time 
(Mw 6.4; 
23 km 
west 
of 
Ahar) 
and 
the 
second
 one 
in 
17:04 
local 
time 
(Mw 6.3;
 30 km 
west 
of 
Ahar)
 [Iran 
local 
time; 
GMT +4:30; 
which 
is 
GMT+3:30,
 and
 there 
is 
+1:00 
for
 summer
 time]. 
These 
earthquakes 
caused 
306 
killed, 
3037 
injured
 and 
30000 
homeless 
people
 (based
 on 
the 
declaration
 of 
the 
Iranian 
health 
minister, 
13 
August 
2012). 
These 
events 
caused
 panic
 in 
the 
cities 
of 
the 
epicentral 
region 
(specially 
in 
the 
cities 
of 
Ahar, 
Varzaghan,
 Kalibar,
 Haris,
 and
 in
 Tabriz 
the 
greatest 
city
 in
 NW
 Iran, 
having 
a 
population 
of 
about
 1,5 
million).
 After
 the 
mainshocks 
the 
people 
settled
 in
 outside 
of 
the 
buildings 
and 
slept 
in
 the 
2 
nights
 after 
the 
events.
 Most
 of
 the 
damages 
are 
reported 
from
 the
 villages 
of 
Gourdeh,
 and
 Dino 
(of
 Ahar
city).
This 
earthquake 
felt 
in 
the 
cities 
of 
Tabriz,
 Marand, 
Shabastar,
 Mamaghan
and
 Bostanabad 
in 
East 
Azarbayjan 
Province, 
Ardabil 
and 
Meshkin shahr 
in 
Ardebil 
Province,
 Urumieh, 
Khoy 
and 
Salmas 
in
 West 
Azarbayjan
 Province, 
and
 Astara, 
Rasht 
and 
Somehsara 
in
 Gilan 
Province. 
There 
are 
110 
aftershock
s with
a 
M>3.0 
recorded
 in 
IIEES
 broadband
 seismic
 network
 in 
the 
1st
 40 
hours 
after
 the 
twin 
events. In general, based on the aftershocks, the fault that caused this earthquake is a strike-slip with left-lateral motion in the Northeastern-Southwestern direction, with about 20 kilometers length, that is situated 15 kilometers west of Ahar. Considering the two big earthquakes, with many aftershocks, there is a low probability of a bigger earthquake from this fault. Based on the information from the national seismology center, there has been no sign of increased seismicity in Tabriz fault; therefore, this earthquake has not triggered the tension on the Tabriz fault. The city of Varzaghan is located on mild slope bedrock. Fine sediments could be seen near the river, and the ground water level is near the ground surface in these parts. Based on the structural damages seen in this city, damages decrease as one walks away from the riverside. However, structures constructed on the thick layers of saturated alluvium have been damaged more (surprisingly, with more damages on the lower stories). There was no sign of liquefaction. Also in villages with partial destruction, damages were focused on the steep lands, which could be related to topography effects. Due to lithology and morphology characteristics of Pliocene sediments (loose and discontinuous Conglomerates and layers of silt – clay) and their disperse distribution, there has been some great landslides during the past years in this area. By considering this fact, and the high probability of another landslide in the area, the survey group of IIEES focused on identifying this hazard. Beside some old landslides, the survey group found some new landslides caused by the recent earthquake. Most of these cases were seen in the slopes of Ahar-Khaje road (10 to 20 kilometers from Ahar, between Zanjirblagh and Yangje villages) and Varzaghan-Khaje (between Zaghanabad and Sarad villages), caused by the high loose embankments on the side road gables, and were not a big threat. However, in some cases, signs of big landslides in the mentioned areas could be seen, which threatens the main roads. One of these cases could be seen in the picture below on the eastern hillside of Khaje-Varzaghan road (after Chaykandi village). Two big landslides in the southern and northern slopes of the mountain have been activated. Also signs of the old and newly activated landslides could be easily seen in the eastern side of the river. There were many signs of small and big rock falls; however, they were more focused on the asphalted roads of Ahar-Kahje (10 to 20 kilometers from Ahar) and Varzaghan-Kahje (in Sarant to Zaghanabad villages). Most of the residential buildings in the mentioned villages were adobe, mud or un-reinforced masonry without any technical or engineering provisions. Lack of construction regulations and heavy materials has caused the destruction of more than 200 villages among the total 500 villages in this area (claimed by the local authorities). Inefficient connection of non-structural elements to the structure or huge structural deformations has caused big damages to the buildings expected to be constructed based on the codes and regulations, which has made the residents to leave the building and stay on tents beside their house. Based on the received information, there are hospitals in two of the three Cities of Ahar, Vazaghan and Haris. Haris Hospital was highly damaged and out of service. The Bagher’ol-oloum Hospital in Ahar is a two-storey concrete building, which has received high damages in the non-structural parts and the facilities. Separation of dividing walls, big diagonal cracks in the walls, damaged ceilings, displacement of facilities like the fallen oxygen capsules and damages in the extension joints, were some of the defects seen by the group. Damages were more extensive in the second floor compared to the first floor. There was no structural damage in the Hospital’s structure. However, the extent of non-structural damages has lowered the performance of the hospital, which is crucial after each disaster.