By attempt of the GSI experts of Shiraz center, the preliminary report of the seismic event on October 19, 2011 in Pir City, Zarindasht Township, Fars Province was announced.
According to Public Relations of GSI, an earthquake measuring 5.3 on the Richter struck Izadkhast area, including Pir City and neighbor towns, on Wednesday, October 19, 2011, at 06:22:39 (local time).
Based on this report, the epicenter of this earthquake was detected by the Iranian Seismological Center of the Institute of Geophysics, University of Tehran at 28.09 northern latitude, 54.30 eastern longitude, 25 kilometers from Sharpir, 31 kilometers from zarindasht, and 877 kilometers from Tehran, with 16 kilometers of focal depth.
In addition, the United States Geological Survey reported the earthquake center with almost 6 kilometers deviation at the north, while the International Institute of Earthquake Engineering and Seismology detected the east of epicenter for it. Within 24 hours after this earthquake, four aftershocks measuring 2.7 to 4 Richter have been reported by the Iranian Seismological Center.
A few hours later the event, a group of GSI experts of the Shiraz Center were sent to the region. According to their observation, due to local reports, the intensity of the earthquake rised from Darab to Zarindasht and it reached the highest in Izadkhast, whose center is Pir City. In Shar Pir, the block walls of the houses were deeply fractured, but those ones which had been built based on safety standards were not damaged. In that area people were frightened by earthquake tremendous sound and vibration.
GSI experts went to Panj Chah village, 17 kilometers from Pir City, the nearest village of the center which the USGS Geophysics Institute had detected. The adobe and mud housed in Panj Chah were damaged, and the buildings were fractured. The buildings which had been built based on safety standards were not damaged, while the adobe houses were. There were seen evidences of fractures and cracks on the ground surface. Alluvium sediments in Panj Chah are granule sand, silt and clay, and in the new area there were fine-grains of sand and gravel.
In Golkooyeh Village, 18 kilometers from south of Pir City, the muddy houses were fractured, while a newly built building were shallowly cracked. This village is located at the northeast margin to Gehdis Kooh, on alluvial-cone sediments.
Finally they visited Mazaijan Village, where the plaster-stone and muddy-adobe houses had been damaged and their walls had collapsed. However the houses which had been built based on safety standards were not damaged.
According to the epicenter detected by USGS, Chah Reza Fault (drawn in 1:100000 map of Zarindasht) can be the main cause of this earthquake. Chah Reza is a s thrust fault with northeast slope, and has affected the southwest hillside of Kooh Gach Anticline, causing underthrusting of some part of southwest of this anticline. A part of this fault passes through SE to Kooh Gach Anticline, and then is located at the NW of anticline near the anticline axis, exposing two Domes. One of the domes is at the core of Kooh Gach, and the other is in the NW plunge of this anticline. Length of this fault according to the geological maps is near 70 kilometers, which is dividable in 4 blocks of 13 to 24 kilometers.
Based on filed surveys, the earthquake measure in Panj Chah, Golkooyeh, and Mazaijan villages seems to be similar, fitting 5.3 Richter. Photo frame fall, fractures in muddy and adobe houses, collapse of muddy-adobe and stone-plaster walls, and even small cracks in cement buildings are all evidences of the intensity of this earthquake. As Mazaijan is 12 km far from the epicenter (International Institute of Earthquake Engineering and Seismology) and has had damaged plaster-stone and muddy-adobe walls, it is clear that the epicenter of this earthquake id at the anticline of Kooh Gach. The earthquake effects are more near the plunges of the anticline, while Mazaijan is located near the SE plunge of the anticline, and Panj Chah is near the NW plunge of it.
The anticline of Kooh Gach is asymmetric, with two salt domes exposed. Some of the formations such as Gachsaran and Mishan in SW limb are underthrusted and eliminated. The oldest exposed formation of it is Sachoon, which is related to the latest earthquake (5.3).
The following pictures demonstrate the abundance of seismic events near Pir City, therefore it is required to pay more attention to the structural optimization and enforcement of safety and engineering principals in constructions in this area require more attention.