The “Avaj Region” earthquake (48º.93 E Longitude and 35º.69 Latitude) occurred in western Iran, about 220 km West of Tehran on June 22, 2002. The moderate moment magnitude of 6.5 (USGS, 2002) [1] was calculated for this earthquake and the depth of focus was about 5-10 Km . The Fault plane Solution (USGS, 2002) indicates that the seismic event was occurred on a reverse fault having trend about N115º(Figure 1).

Fig. (1): The Focal mechanism of Avaj Region earthquake (June 22, 2002). (USGS, 2002)

Extensive field investigations were undertaken by The IIEES seismotectonic group during the summer 2002 (June 30 to August 27). Results of this group’s investigation during the first days after the event is presented as follows. It should be considered that, these results were not presentable before publishing [2] (because of the ِCopyright of authors).
Based on Active tectonic and seismotectonic studies, the causative fault of this earthquake (which has been accompanied by visible Surface faulting) has located at distance more than 8 km north of Avaj Fault. Before presenting of the research team’s results [2,3] most of the other researchers had presented several faults (Such as Avaj Fault and … .) as the causative Fault of Avaj earthquake )for example: Javan doluie et al. 2003, Zare 2003 and … ). However, based on the present study, it becomes clear that Avaj and other known faults (figures 2) were not causative fault of the earthquake. In addition, the coordinations of the earthquake epicenter (which have been reported by numerous seismological sites) do not coincide with the location of these faults.

Fig. (2): The location of previous known faults of the area on the Base map [2].

So, in the next step, emphasising the location of most damaged villages and studing of aerial photos of the area, we found two fault trends that had not been presented in previous Geological and Seismotectonical maps. One of them is located along the strike of Abdareh and Hossein Abad completely damaged villages, and the other one along to the strike of Changureh and Didar, damaged villages. On the aerial photos the azimuth of these two fault trends are N110º -120º (similar to the trends of epicenteral planes in Focal mechanisms) (Figure 3).

“Abdareh-Hossein Abad” and eastern part of “Changureh-Didar” can be seen.

At the beginning of third step we found the surface faulting evidences of Avaj Region earthquake on the “Abdareh-Hossein Abad” fault trend at the eastern side of the Abdareh village (Figures 4 and 5).

Fig. (4): Surface faulting of the Avaj earthquake at the east of Abdareh village (looking south). Date of Photography: July 16, 2002.

of Abdareh village (looking east). Date of Photography: July 17, 2002.

The investigations and measurements on the plane of the earthquake fault (through the east and west of Abdareh village) indicated the following results [2 and 3]:

• The mechanism of faulting was Reverse.
• The dip of causative fault plane was 30º-32º toward SW.
• The Strike of causative fault plane was N105º-115ºE.
• The plane of faulting was parallel to the bedding plane of Neogene unites (which is cut by the fault) and opposite to the topographical slope (Figure 6).

• At the east of Abdareh village, vertical displacement of Surface faulting was 50 cm (figure 7), however, The maximum measured amount was 65 cm about three kilometers west of this village (closing to the Emamzadeh Esmail road) (Figure 8).

(A)

(B)

Fig. (7): A and B: 50 cm vertical displacement of the earthquake Surface faulting at the east of Abdareh village (looking west). The scale is geological hammer. Date of photography and drawing the sketch: July 17, 2002.

Fig. (8): The maximum amount of vertical displacement of the Surface faulting (65 cm) about 3 km west of Abdareh village (east of Emamzadeh Esmail road) (looking east-northeast). The mechanism of faulting is reverse and at this view: southern block (right side) is thrusted on the northern block (left side). Date of photography: July 30, 2002.

As noted above, this fault trend is visible on aerial photos (Figure 3). The variation of vertical surface displacement along this fault which occurred during the earthquake has been presented in figure 9. The amounts of vertical surface displacement have a very good similarity with surface topography of the fault scarp.

More field investigations (on July 30, 2002 to August 19, 2002) indicated that the fault trend is a fault zone with 200 meters width and minimum length about 14 kilometers. 9200 meters of this length (from Charpo valley at the east to north of Balaban village at the west) have been continuously ruptured the surface during the earthquake [2]. The length of noncontinuous Surface faulting along the fault zone was about 13 kilometers (figure 10). The causative fault of this earthquake there fore, has been named “Abdareh fault” by this team [3].

Considering the moderate magnitude and shallow focal depth of the earthquake and high plastic materials of surficial Neogene beds, the amounts of vertical displacement and the length of this surface faulting could be expectable. The continuation of Abdareh fault trend at eastern part of it has a relation and structural connection with another fault trend (Saied Abad-Shurab fault trend with Azimuth: N160º) (Figure 10).
Abdareh fault trend has been cut the large alluvial terrace (which is generally sloped to the north). One of the typical Active tectonic and Morphotectonic features which has been found at the edge of Abdareh fault trend and this alluvial terrace, is a Quaternary “Back Tilted Terrace” (Figure 11).

The second fault trend (Changureh-Didar) having Surface faulting evidences (but too weaker than the first one, Abdareh-Hossein Abad) was found at the east of Changureh village (at the south of Jen-Dareh valley) (figure 12).
The length of this weak surface faulting was 900 meters having azimuth N110º and steeply dip to the south and left lateral Strike slip mechanism with a small reverse component (figures 10 and 12). The total amount of horizontal surface displacement which occurred on this fault plane during the recent earthquake, was 30 cm (Figure 12). The location of this faulting was at distance about 70 meters south of big Changureh Landslide. It seems that, the mechanism of faulting have been occurred due to slip Partitioning and the fault have been triggered by maximum energy on the epicenteral area of this earthquake. It should be considerated that, some of other faults around the causative fault (Abdareh fault) had been reactivated in some parts, specially in connection to Abdareh fault (such as: Saied Abad-Shurab and Kuh-e Aban fault trends) (Figure 10).

Fig. (12): The Surface faulting of Changureh-Didar fault trend with 30 cm left lateral displacement (looking south) [2]. Date of Photography: June 30, 2002.

Based on the results of this study, two “seismotectonic reasons” of damaging in most damaged villages of the area presented as follows:

• The complete damaging of Abdareh and Hossein Abad villages has been occurred because they are located exactly on the earthquake Surface faulting trend at Abdareh fault scarp.
• The complete damaging of Changureh village and considerable damaging of some others (such as: Didar and Kord-e Jin villages) have been occurred due to the location of these villages exactly on the epicenteral area of the earthquake.

References:

1. http://neic.usgs.gov/neis/FM/neic_fsac_q.html, 6/22/2002.
2. Feghhi, Kh. and Solaymani, Sh. (2003), Geometry of Fault Ruptures of the Avaj Region Earthquake. Research Bull. of Seismo. And Earthq. Engin. Vol. 5, No. 4, winter 2003 (in Farsi).
3. Solaymani, Sh. and Feghhi, Kh. (2003), Seismotectonics and Neotectonics of Changureh Earthquake on June 22, 2002. a part of internal report. International Institute of Earthquake Engineering and Seismology (in Farsi).
4. Javan D., Gh., Asatoriance, K. and Mokhtari, M. (2002), Evaluation of Avaj Earthquake and Its Aftershocks. Research Bull. of Seismol. And Earthq. Engin. Vol. 5, No. 3, Autumn 2002 (in Farsi).
5. Zare, M. (2003), A Study of the Strong Ground Motion and Process of the Accelerometric Data of the 22 June 2002 Changureh (Avaj) Earthquake. Research Bull. of Seismo. And Earthq. Engin. Vol.5, No. 4, winter 2003 (in Farsi).