Ahar Varzaghan Earthquakes, August 11, 2012 Seismotectonics investigation using focal mechanism solution and GPS velocity Data
Ahar Varzaghan Earthquakes, August 11, 2012 Seismotectonics investigation using focal mechanism solution and GPS velocity Data
T. Malekzadeh Dilmaghani ˙ Y. Golestan
Abstract Ahar Varzaghan Twin earthquakes, August 11, 2012 (mb 6.2 and 6.0) Caused by right-lateral strike-slip faults, caused great damage in ahar varzeghan region.
These earthquakes with Depth about 10 km are shallowest kind of earthquakes.
In this paper we study the earthquakes by using focal mechanism solution and GPS velocity Data. As well as we compared The Recent earthquakes movement with regional Tectonic motions. The results reflects that movement of recent earthquakes has good Convergence with regional Tectonic motions and velocity Data that recorded by GPS. Recorded after shakes close in the causative faults accord shows strike slip movments.
Motion of This right lateral strike slip faults can be compared with Major Regional fault (North Tabriz Fault) in type of motion.
Twin Earthquakes occur in Ahar Varzaghan area, that not investigation for active fault until recent Earthquakes, this issue caused wrong imagine of low possibility for seismic hazard in this region.
Key Words Ahar Varzaghan earthquakes. Tectonics. right lateral faults. using focal mechanism solution. Northwest Iran. GPS velocity Data
1 Introduction
Earthquake is a one of worst Natural Hazard that Caused enormous damage to Humanity all Over the history. Iran specially Azarbayjan region is one of most active Seismic region on the world. Knowledge of this phenomena and causative ingredient can be valuable to prospecting and risk management.
Ahar Varzeghan Twin Earthquakes occur in Ahar area, that not investigation for active fault until recent Earthquakes, this issue caused wrong imagine of low possibility for seismic hazard in this region. additionally Background of historical earthquakes not recorded for major damage in paste (fig. 2) (Ambraseys, 1982).
In this paper we will study Focal Mechanism of this Earthquakes and relation between regional geodynamics with recent Earthquakes.
2 Metods
NW Iran is a region of intense deformation and seismicity situated between two thrust belts of the Caucasus to the north and the Zagros Mountains to the south (fig. 1). Earthquake focal mechanisms suggest that the convergence between Arabia and Eurasia has been accommodated mainly through WNW-trending right-lat-eral strike-slip faults in this region (fig. 1) (Jackson, 1992). (Westaway1990, 1994)
These strike-slip faults appear to be the southeastern continuation into NW Iran of the North Anatolian Fault and other right-lateral faults in SE Turkey.
However, right-lat-eral faulting in the SE Turkey-NW Iran region is not continuous but consists of several discontinuous fault segments (fig. 1) (Westaway 1990, 1994) ( Jackson, 1992)
According to regional Faults orientation major pusher force has been south-North orientation. (Emami, M, H. 1992)).North Tabriz fault is major fault of Azarbayjan region with high seismic activity, Exist many Historical earthquakes toward Evidence (fig. 2)
Fig. 1 Location map of the NW Iran-Eastern Turkey, adapted from (Jackson 1992), with focal mechanisms of some of the large earthquakes (mb > 5.3) in the Tabriz-Chaldiran Seismogenic zone.
Fig. 2 Map of Areas That destructive by earthquakes in Iran (4 th century B.c. to 1976 A.D) (Berberian. M, 1976)
2 Recent Event
Two relatively large shallow (10 Km) earthquakes struck NW- Iran, about 60 and 50 km northwest of Tabriz city, on August 11, 2012. The first earthquake (mb 6.2) occurred at 16:53 in local time and the second one (mb 6.0) happened with about 11 minutes later. First Earthquake occurred in 20 km of west Ahar and second Earthquake occurred in 28 km of Northwest Ahar.
several victims and huge damage caused by this twin Earthquakes.
Different agencies reported location of these Earthquakes according to (fig3) (Tables 1 and 2) (Hosseini, S, k. 2012).
Table 1 Earthquake reports by different agencies for the first quake. IRSC: Iranian Seismological Center,
IIEES: International Institute of Earthquake Engineering and Seismology, USGS: U.S. Geological Survey.
(For local time of the events you may add +04:30:00 to UTC time) (Hosseini, S, k. 2012).
|
Ref. |
UTC time |
Lat. |
Long. |
Mag. type |
Mag. |
Depth |
|
IRSC |
12:23:15.3 |
38.495 |
46.865 |
Mn |
6.2 |
10 |
|
IIEES |
12:23:16.2 |
38.55 |
46.87 |
mb |
6.1 |
15 |
|
USGS |
12:23:17.0 |
38.322 |
46.888 |
Mw |
6.4 |
9.9 |
Table 2 Earthquake reports by different agencies for the second quake. IRSC: Iranian Seismological Center,
IIEES: International Institute of Earthquake Engineering and Seismology, USGS: U.S. Geological Survey.
(For local time of the events you may add +04:30:00 to UTC time) (Hosseini, S, k. 2012).
|
Ref. |
UTC time |
Lat. |
Long. |
Mag. type |
Mag. |
Depth |
|
IRSC |
12:34:34.8 |
38.449 |
46.731 |
Mn |
6.0 |
10 |
|
IIEES |
12:34:35.0 |
38.58 |
46.78 |
mb |
6.1 |
16 |
|
USGS |
12:34:35 |
38.324 |
46.759 |
Mw |
6.3 |
9.8 |
Fig. 3 Location of epicenters and focal mechanisms of twin Earthquakes, reports by different agencies (Davodi, M. 2012)
3 Focal Mechanism
Focal Mechanism of twin Earthquakes presents major pressure Force in orientation of Southwest to Northeast direction that have good coincidence with regional geodynamic (figs 3and 5). Causative faults of this Earthquakes shows a right lateral strike slip that can be compared with famous north Tabriz fault sake displacement and tectonics regime (fig 4).
Fig. 4 surface rupture of right lateral strike slip fault in khaje to varzeghan road iiees (Davodi, M. 2012)
More than 300 after shakes accorded close the causative faults shows strike slip movments (fig 5)
Fig. 5 Distribution of 300 aftershocks of Ahar - Varzaghan double earthquakes. Numbers 1 and 2 refers
to the causative earthquake faults and their mechanisms (Hosseini , S, k. 2012)
4 GPS Data
National Cartographic Center of Iran established a GPS network called Iranian Permanent GPS Network (IPGN) due to Permanent record of crust motion exist 25 stations on Azarbayjan region.
exist 2 stations in our study area Ahar and khajeh stations (fig 6).
According to record data of this network; in Azarbayjan region in tow side of Tabriz fault , khoy makuo fault and Nakhjavan fault can see 1.8 mm per year right lateral strike slip displacement in eastern region and this movement increased about 4.5 mm per year in western region. reason of this increase in right lateral strike cane be approaching to Anatolian plate and north Anatolian faults. Recent study by (Vernant et al., 2004) emphasized to right lateral displacement in this region that have good coincidence with recent twin Earthquakes displacements. (NCC, 2012)
After Ahar Varzghan Earthquakes, Ahar GPS Station report crust displacement About3-4 cm (figs 7and 8) (NCC, 2012)
Fig. 6 Permanent GPS Network (IPGN) motion rate and direction of crest movement in Azarbayjan (NCC, 2012)
Fig. 7 Ahar GPS Station report crust displacement during first Earthquake (NCC, 2012)
Fig. 8 Ahar GPS Station report crust displacement during second Earthquake (NCC, 2012)
5 Conclusion
Focal Mechanism of twin Earthquakes presents major pressure Force in orientation of Southwest to Northeast direction that has good coincidence with regional geodynamic. Causative faults of this Earthquakes shows a right lateral strike slip that can be compared with famous north Tabriz fault sake displacement and tectonics regime.
More than 300 after shakes accorded close the causative faults shows strike slip movments.
Movement of crust that monitored by GPS network have good coincidence with regional geodynamic, also have accordance between GPS network result and recent Earthquakes Movement.
Twin Earthquakes occur in Ahar area, that not investigation for active fault until recent Earthquakes, this issue caused wrong imagine of low possibility for seismic hazard in this region.
It is necessary to investigation for active faults in Azarbayjan region.
References
Ambraseys, N., Melville, C. (1982). A history of Persian earthquakes, Cambridge University Press
Berberian. M, (1976): Map Areas of destructive earhquakes in Iran (4 th century B.c. to 1976 A.D), Geological Survey of Iran, Tehran
Davodi, M. (2012): preliminary Report of Ahar Earthquakes, August 11, 2012 International Institute of Earthquake Engineering and Seismology, Tehran (in Farsi)
Emami, M, H. (1992): Geological map of Varzeghan 1:100000. Geological Survey of Iran.Tehran
Hosseini, S, k. (2012):Two source faults of Ahar-Varzaghan Earthquakes, August 11, 2012, NW- Iran. Earthquake Research Center, Ferdowsi University of Mashhad, Iran
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