GEOLOGY, SEISMICITY AND SEISMIC RISKS ASSESSMENT OF EASTERN SALT RANGE, PUNJAB - PAKISTAN.
ABSTRACT: Lithology, neotectonics and seismic behavior of the Eastern Salt Range was studied. For this purpose, the field analysis of complemented Geological, historical and instrumental earthquake data was carried out. The continuing collision between the Eurasian and Indian plates resulted in compressional forces in the North - West Himalayan Fold and Thrust Belt. The Eastern Salt Range experienced 190 earthquakes of magnitude 1 to 3.9, 13 earthquakes of magnitude 4.00 to 4.9 and only four events of magnitude from 5.00 to 5.9. On the basis of historical and instrumental records, the area has not experienced moderate to large earthquakes. The seismogenic sources and active faults of this zone are Salt Range Thrust, ChoaSayyaidan Shah Fault,KallarKahar Fault and Jhelum Fault.
Keywords:Seismicity, Seismogenic sources, Neotectonics, Antiseismic.
This study examines neotectonics, seismotectonics and seismic risks on the basis of historical and instrumentally recorded earthquakes in order to help in mitigation of earthquake risk as well as post-earthquake disaster management in the EasternSaltRange. This study is also meantto assessantiseismic parameters.
Based upon seismotectonic setting, several countries have established guidelines, standards and code of practice for important civil structures (Khalid et. al.2002). In addition, presence of vertical cliffs in tectonically fragile zone (Khan et. al. 2011) located in the Eastern Salt Range may cause permanent ground displacement duringearthquakes. The international Atomic Energy Agency (IAEA) developed an inclusive safety guide by combining and consulting recommendations from different countries for nuclear, chemical and important civil structures. The methodology adopted for the present study is in accordance with IAEA Safety Guide 50SG.S1-Rev 1. An effort has been made, for the first time, to identify the seismic behavior and the seismogenic sources of the Eastern Salt Range with the help of available historical, instrumental seismic records and neotectonic data. For this purpose, available earthquake data (1904 to 2008) from international seismological networks and seismic observatories have been collected.
Stratigraphy: Seismic wave propagation is considerably influenced by lithological variations (Duroy, 1986). Regional and semi regional stratigraphy play a significant role in the attenuation of the seismic waves because different rocks behave differently when subjected to wave transmission. For this reason lithological units are very carefully described in Table-1. The lithological units of Eastern Salt Range are composed of sandstone, limestone, marl and shale (Gee,1989).Pre-Cambrian and Cambrian rocks are mainly composed of sandstone, marl, shale and dolomite, Permian rocks mainly comprise sandstone and shale while Eocene rocks are limestone and shale. Pliocene to Recent sediments are comprised of alternative layers of sandstone and shale while Miocene to Recent fluvial sandstone, shale and conglomerates (Siwalik Group) dominate the local stratigraphic sequence.
Tectonics: Continuing convergence between India and Asia produced the Himalayas. The Indus suture is the northern boundary of the Indian continent in the Western Himalaya of Pakistan (Figure 1).
The Tethys Ocean was closed about 50 M.a ago when India collided with Eurasia (Burg et. al. 2005). The oblique collision in the west, along the Chaman-Ornach- Nal Transform Fault zone, connects the collision zone with the MakranSubduction Zone, where the Arabian Plate is being subducted beneath the Eurasia (Figure 3). In the offshore region, the Murray Ridge and the Owen Fracture Zone mark the boundary of the Indian and Arabian plates.
The northward movement of Indian Plate with respect to stable Asia is still going on at a rate of 42mm/year (Leathers, 1987).Shortening is manifested by stacking of thrust sheets (Figure 4), which progressively shifted from north to south.
The Indus Suture (Figure 2 and Figure 4) is a north dipping fault that was mostly active before about 11Ma ago (Bilham, 2006). The deformation progressively shifted to the Main Central Thrust and then to Main Boundary Thrust (MBT). The MBT is a zone of north- dipping thrusts (Figure 4).Fault plane solutions of earthquakes reported to the MBT indicate that the Indian plate is under-thrusting the Himalayas. The E-W striking MBT in India extends into Pakistan as a narrow fault zone that takes a sharp bend in Kaghan to coalesce with the NNW striking sinistral Jhelum strike slip fault. The MBT in Hazara area consists of imbricate thrusts (Figure 4) that rejoin in SufaidKoh to form a narrow fault zone again. The Punjal Thrust located to the north of the MBT runs parallel to it.
Table -1. Stratigraphic Succession of Eastern Salt Range.
Quatenry###Recent###Alluvium###Cong and ornerate. gravel sir and sand
###Pleistocene###Lel Conglomerala7soan###Brown and Gray conglomerate with interbedded sandstone.
###and Late###Formation###Light colored sandstone and conglomerate. light red and gray clay.
###Pliocgne and###Siwalik###Dhok Pathan Formaon###Red Brown Clays with gray sandsione, conglomerate
###MiOcene###Nagri Formation###Greenish gray sandstone clay and conglomcerates
###Middle###Chinji Formation###Bright red color clay with alternative beds of sandstone.
Tertiary###Early###Kamlial Formation###Massive sandstone with alernative layers of red clay.
###Miocene###Rawalpindi###Muree Formation###Massive sandstone beds dark red and purplish clay/shale with basal conglomerates.
###Chore Gal' rormat3on###Olive gseen shale with bedded limestoile.
###Earty###Cheral###Sakessar Limestone###Massive and nodular limestone. with marl and thert in upper part.
###Eocene###Nammal Formation###Light gray calcareous shale and limestone.
###Patala Formation###Green shale with coal seanis thin limestone
###Paleacene###Makarwa###Lockhart Formation###Grey, nodular and Marly limestone.
###Hangu Formation###Impure limestone, sandstone, and shale. often carbonaceous.
###Sardhai Formation###Dark piirpe and lavender clay with subordinate sandstone
###Warcha Sandstone###Red and light sandstone and girt, in pads arkosic, clay intetbreects.
###Dandot Formation###Olive green and gray sandstone and shale occasionally carbnaceuous.
Palaezonic###Early Permain###NIIawanhan###Tobra formation###Conglomeratic sandstone and shale, boulders are mainly ignecos or metamorphic.
###Baghanwala Formation###Red Blood sftale and fiaggy sLlndstone ~Iti salt pseudomotphs.
###Middle###Jhelum###Jutana Formation###Massive light colored dolomite and dolomite sandstone, wish
###and Early###subordinate shale.
###Cambrian###Kussak Formation###Gray and purplish shale and glauconitic sandstone. pebble bed at base.
###Khewra Sandtone###Massive maroon fine textured Sandstone, maroon shale
###Precambrian###Salt Range Formation###Red gypsiferous marl with rock salt, gypsum dolomite above
###occasional oil shale (Modified from Ibrahim Shah. 2009)
The main deformation front shifted southwards for the last 2Ma (Yeats, et. al. 1984), resulting in the emergence of the Salt Range Thrust.
Neotectonics: The Salt Range is a still active fold and thrust belt (Molnar et. al. 2009). Continuing deformation is manifested by neotectonic features related to the Recent and Sub-recent sediments at different places. These sediments include conglomerates, gravel, sand and silt lying over the Siwalik Group and older rocks. The dominant, so-called "Potwar Silt deposits" in Eastern Potwar were dated, using the thermo-luminescence method up to 0.17Ma (Yeats, et. al. 1984).
The Major active tectonic structure of the area is the Salt Range Thrust (SRT) along which, Palaeozoic rocks are tectonically emplaced over syn- orogenicfranglomerates. The Salt Range makes an abrupt escarpment against the Recent Punjab alluvial deposits (Figure 5). In EasternSaltRange, near Jalalpur, the Siwaliks have faulted contact with Recent alluvial deposits. (Figure 5). The silt and gravel beds are tilted and offset, indicating that deformation is continuing, even after the deposition of Recent and Sub-recent sediments.
Recent gravel and gritty sand are tilted near Khewra village. The progradation of fans and denudation deduced from the gravel dispersal suggests that the Recent and Sub-recent deposits are continually uplifted, due to deformation along the Salt Range Thrust.
The association of neo-tectonic features with Salt Range Thrust clearly indicates that Salt Range Thrust is active. Average slip rate along the Salt Range Thrust is estimated to be 14 mm/yr (Leathers, 1987). Localization of brittle neo-tectonic features with the fault traces, tilt of sandstone and conglomerate without brittle deformation are the evidences of aseismic creep movement along the faults.
The sinisteral Jhelum strike slip fault marks the eastern limb of the Salt Range and Potwar Plateau (Figure 5). Jhelum River more or less follows the fault zone. Recent terraces in the northwest of Dina and River terraces near Mangla are tilted as a consequence of active movement along the Jehlum fault.
Seismogenic Sources: The Eastern Salt Range is 48 km long from Jalalpur to ChoaSayyaiden Shah Fault. The Salt Range Thrust and decollement zone under the Potwar Plateau are the main tectonic features of the Eastern Salt Range (Figure 5).
Movement along the Salt Range Thrust and decollement is usually aseismic (Burbank et. al. 1986) due to low strength rocks (i.e. Shale, Gypseferiousetc)that lubricate the decollement and Salt Range Thrust. At places,microseismic events are associated with the decollement zone,which is not unusual forweak detachments with small asperities. Higher average slip rate (14mm/year) of Salt Range Thrust with contrasting low frequency of micro-seismicity indicates that mainly aseismic creep is ongoing along Salt Range Thrust (Leathers, 1987).
The association of neotectonics with the ChoaSayyaidan Shah Fault, the existence of micro- seismic epicenter along the fault and its structural association with Salt Range Thrustclassify theChoaSayyaidan Shah Fault as active and capable. It terminates to the southwest against the Salt Range Thrust. A number of neotectonic features exist along this structure; however, only one instrumentally recorded micro-seismic event is attributed to theChoaSayyaidan Shah Fault. It can be assumed that generally the movement along the fault is aseismic due to the occurrence of Salt Range Formation within fault zone and at the decollement zone (Coward, et. al.1986).
The KallarKahar Fault is an active and capable structure. It is constituted by parallel, overlapping faults and subsidiary transverse faults. Tilted silt, abrupt truncations and active salt diapirism have been recorded in close proximity to the fault. Two very shallow epicenters of micro-seismic events are located along this fault, which terminates southwards against the active ChoaSayyaidan Shah Fault.
Jhelum Fault and lineaments parallel to it are marked by shallow seismic events and neotectonic features are also associated with the Jhelum Fault.
All these observations suggest that, Salt Range Thrust, ChoaSayyaidan Shah Fault,KallarKahar Fault and Jhelum Fault are active and capable faults.
Seismicity: Pakistan has been classified into various seismic zones. The trend to construct high rise buildings and important civil structures in Pakistan also demands that the local ground motion data and ground motion characteristics should be studied and incorporated in the structural design (Qazi et. al. 2012).
The magnitude terminology used in this work is as follows.
The magnitude terminology used in this work is as follows.
Great (Greater than) 7.80 magnitude
Large###7.00 to 7.80
Moderate###6.00 to 6.90
Small###5.00 to 5.90
Macro###4.00 to 4.90
Micro###(Less than) 4.00
(Based on U.S Geological Survey Documents)
Historical Seismicity: The record of historical seismicity in the study area covers only the past 200 to 900 years. It indicates that the Eastern Salt Range has experienced rather small earthquakes over thisperiod. Some moderate events have occurred in the north and east of the investigated area where intensity IX was suggested in epicentral area for the biggest known event.
Instrumental Seismicity: The catalogue of seismic events that occurred in the EasternSaltRange and its surroundings from 1904 to 2008 was obtained from various sources such as National Earthquake Information Centre (NEIC) of United States Geological Survey (USGS) and Meteorological Department of Pakistan.
The catalogue includes the events of magnitude (Greater than) 3. The distribution of epicenters in adjoining area is given in (Figure 6). 190 earthquakes of magnitude 1.0 to 3.9 have been recorded,13earthquakes have magnitude 4.0 to 4.9. Four events are of magnitude ranging from 5.0 to 5.9. An event of magnitude (Less than) 3.9 with focal depth of 10 km occurred in the BunhaKas area of Eastern Salt Range. The event was likely associated with basement structure as Salt Range Thrust is shallower.
An earthquake of magnitude (Less than) 4.9 with focal depth of 32.3 km originated from deep seated structure of basement. Two events very shallow of magnitude (Less than) 3.99 (focal depths of 1.6 and 1.3 km) are recorded.
A very shallow earthquake is associated with a lineament along the axis of Pabbi Anticline. The Pabbi Anticline is a growing blind thrust cored fold. It cannot cause surface rupture in the Eastern Salt Range as it is located on a different tectonic structure.
An epicenter of magnitude (Less than) 4.9 with focal depth of 5 km is located just to the south of MandiBaha- ud-din.The event probably originated from some structure within the cover sediments of the shield. It is located on footwall of the Salt Range Thrust.The slips along the Salt Range Thrust are generally aseismic, however, concealed structures of basement can generate small events.
Results and Conclusion: The lithology of the study area is mainly composed of limestone, sandstone, marl, shale and conglomerate / gravel of Eo-Cambrian to Recent age, which plays a significant role in the attenuation of the seismic waves.
SaltRange is an active Frontal Fold and Thrust Belt. Neotectonics are associated with Salt Range Thrust, KalarKahar Fault, ChoaSayyaidan Shah Fault and Jhelum Fault. Tilted silt / Gravel beds, abrupt truncations, active salt diapirism and offset, indicating that deformation is continuing, even after the deposition of Recent and Sub- recent sediments. The association of neo-tectonic features with Salt Range Thrust clearly indicates that Salt Range Thrust is active. The slips along the Salt Range Thrust are generally aseismic, however, concealed structures of basement can generate small events.
The records indicate that the Eastern Salt Range has experienced small earthquakes with shallow epicenters. No moderate or large event has been documented. The area is also sparsely populated. So in case of earthquake, there is limited chance of big damage to human life and property. Permanent ground displacement may occur during earthquake i.e. rock fall, rock blocks detachments etc. It is suggested that, a regulatory body should be constituted in the country, which should define uniform criteria and minimum requirement for determination of Peak Ground Velocities and Peak Ground Accelerations for civil structures. The regulatory body should have legislative powers to enforce its recommendations.
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College of Earth and Environmental Sciences University of the Punjab, Lahore, Pakistan. Department ofEarth Sciences, ETH-Zurich, Switzerland. Corresponding Author E-mail: email@example.com
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|Publication:||Pakistan Journal of Science|
|Date:||Mar 31, 2013|
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