Seismo Tectonic Investigations of Gogai – Ziarat Earthquake in Balochistan
Seismo Tectonic Investigations of Gogai – Ziarat Earthquake, Balochistan of October 29, 2008
The earth scientists have been warning about recurrence of a major earthquake in Quetta region after every 70-80 years and a moderate one in 10-15 years and should be taken seriously by all the stake holders. The GSP Museum of Earth Sciences Quetta has been running a campaign for earthquake preparedness from the past 3 years to pre-empt the clear and present danger. We believe that hazard is natural but disaster is not. Disaster management and seismic trainings have to be imparted to all the stake holders at every levels, including senior administrators such as the governor, chief minister, chief justice and members of legislative assemblies, and farmers, engineers, builders, architects, doctors, sociologists, psychologists, the common people, as well as in educational institutions, etc. The Finishing School students of Women’s University Brewery Quetta are actively collaborating with GSP Museum in delivering lectures, presentations and evacuation drills in educational institutions of the provincial metropolis.
A team of earth scientists from the Geological Survey of Pakistan started post earthquake investigation shortly after 6.4 earthquake which struck most parts of Balochistan in the wee hours of October 29th, 2008. The team comprising Mr. Asif Nazeer Rana, Deputy Director / Museum Curator, GSP Quetta, Sardar Saeed Akhtar, and Ghulam Qadir, Assistant Directors GSP Islamabad and Quetta collected post seismic data and made field observations in Khanozai, Gogi epicentral areas and Ziarat areas. Seismotectonic investigations were carried out in the worst affected areas; Khanozai, Gogi, Ahmedoon, Kach, Wam, Varchum, Kan Depot, Kawas and Zandra. The field investigation by GSP team of experts have now been completed and the post earthquake data has been collected about causative fault, fault plane, surface rupture, vertical and horizontal displacement, rock fall, landslide, slope failures, and related geological phenomenon like liquefaction, subsidence and mass wasting.
The field investigation were carried out starting from the USGS reported epicenter near Khanozai ( 30. 653 N, 67.323 E ), 60 km NNE of Quetta. No major structural damage, no surface rupture except some hairline ground cracks were observed at Khanozai. No reportable ground rupture was recorded at other places near Kach and Kawas. Major structural damage and ground failure was observed in a narrow belt 3 by 10 km in the E-W direction in Ziarat valley (N/7 & N/11) from Lat 30 26.241, 67 14.202 just after Kadi Kach. The worst affected area was between Wam, Verchoom, Kan Banglow and Kawas. The main event of 6.4 M at 5.09 was preceded by foreshocks of 5 M striking at 4.33 AM followed by second 6.4 M event at 5.15 PM. The epicenter of this event and most of the M5 aftershocks was located near Gogi. Arcuate type of cracks and fractures were observed on the Gogi-Kach-Wam road segment built on the dump filled strata along the causeways.
Although the tremors were felt over an area of 300 sq.km. but the intensity of IX to X was felt more at Wam, Verchoom, Killi Pio Khan, Killi Sahibzad, Kan Depot & Kawas. Destruction was more severe on the left bank of the Ziarat Manda stream on the terraces composed of unconsolidated Ghazij shale of Eocene age as compared to the right bank where the settlements are built on solid bed works of Paleocene Dungan limestone. It was also observed that certain houses were damaged due to ground failure and slumping. Mercifully, Ziarat town itself was unscathed but Wam and Kan villages are the worst effected areas in the scenic Ziarat valley. The houses are totally collapsed as poorly constructed mud houses were built atop escarpments and terraces of loose sediments along dangerous slopes of 30. Most of the veranda pillars, boundary walls and electric polls were found to be tilted to the south indicating that shock waves traveled from north-east to the south west along a hybrid fault resulting in complete destruction. A previously mapped active fault Kawas Tangi near Zandra, 30 km east of Ziarat town saved the Switzerland of Balochistan but due to uplifting between this fault and an offshoot of Gogai Fault brought havoc in the settlements in 10X3 km area.
Some major thrusts namely Zhob valley thrust, Chinjan, Bibai and Gogai thrust running parallel to each other are located in this region but no evidence was recorded about any displacement along these faults. Ghazaband-Zhob Fault and Gwal-Bagh Fault system connected with the active Chaman Fault system are also in close vicinity of the area. It seems that only localized faults were activated which destroyed Wam and Kan villages. After recording these preliminary field investigations, the team is now focusing its attention to record observations to ascertain the location of epicenters of 874 aftershocks and delineating the causative fault which might be running beneath the surface from Khanozai to Gogai in N-S direction. The alignment of aftershocks epicenters supports this idea.
More than a thousand multiple aftershocks have been reported to continue even after a lapse of one month after the two major earthquakes of 6.4 M to rock the Ziarat, Sanjawi, Quetta, Harnai and Pishin valleys on 29th October, creating fresh fears that badly damaged buildings which already have developed major cracks might be collapsed in the aftershocks. People are braving sub zero temperatures under open sky and tents in quite inhospitable terrains after the major tremors brought down their poorly constructed dwellings built on loose sediments. Majority of the population has been migrating to warm areas before the onset of harsh wet winter which brings more than 4-7 feet thick snow and mercury plunging down to -11 C.
Due to below freezing temperatures in Ziarat valley, the third phase of rehabilitation after the initial rescue and relief operations has to put on halt for the next three months. For reconstruction of the affected villages, proper town planning is the need of the hour based on geo hazard zonation mapping which should include the local geology, geomorphology, geohydrogeology, and geotechnical investigation as GSP has done after October 8, 2005 earthquake in Bagh and Muzaffarabad areas. If we fail to plan, we plan to fail.
The phenomenon reported by the locals in the electronic and print media about rising of dust clouds from the fissures in the rocks and earth cracks being developed in the Dungan Limestone was also checked. It was observed that due to rock falls, mass wasting at certain places dust clouds arose and remained suspended in the air due to cold climate for some time. In dry and cold climatic regions like Quetta, Ziarat and Pishin valleys, on steep rock slopes, if the rain soaks deeply in the rocks, such type of weathering may proceed to a considerable depth and dislodge the rock layers which are of inter bedded hard and soft earth materials. The weathered material forms a mantle of rock waste and slides downwards due to gravity. It is possible that the dust/smoke coming out of the mountain top and emission of light rays by the locals may be dust cloud when the upper most layer of the rock moved down to the bottom and hit ground with a bang triggering cracks in the soft layers and rolling of boulders. This is another positive aspect of present event that no rain is forecast in the region and there is no eminent danger of landslides in this region as was common after Oct 8 earthquake in AJK. Another positive point is that the infrastructure is intact and road network is accessible in most parts of Ziarat and Pishin valleys, hence aid relief and rescue operation is going on unhindered. Another observation made by the team is that the there is less likelihood that death toll will increase considerably as almost all the injured and dead have been taken out of the rubble of the destroyed mud houses with tin roof compared to concrete structure in the Muzaffarabad area. The results of GSP’s research will be presented in the 35th All Pakistan Science Conference being held at the University of Karachi from December 20-23, 2008. The complete report with geologic maps on the seismo tectonic investigations on the Ziarat Earthquake is downloadable from GSP’s website at www.gsp.gov.pk
Asif Nazeer Rana, Curator, Museum of Historical Geology, Geological Survey of Pakistan, Quetta, Pakistan. Ph#081-9211381, Fax 9211361.
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