• SARFRAZ KHAN

Articles written in Journal of Earth System Science

• Mapping sediment thickness of Islamabad city using empirical relationships: Implications for seismic hazard assessment

Soft sediments make an important component of the subsurface lithology, especially in areas underlain by river/stream basins. Occupying a position directly above the bedrock up to the land surface, these soft sediments can range in thickness from few centimeters to hundreds of meters. They carry a specialnuisance in seismic hazards, as they serve as a source of seismic amplification that may enhance the seismic shaking of many folds. Determination of the thickness of the soft sediments is therefore crucial in seismic hazard analysis. A number of studies in recent years have demonstrated that frequency andamplitude spectrum obtained from the noise measurements during the recording of natural seismicitycan be used to obtain thickness of soft sediments covering the bedrock. Nakamura (1989) presented atechnique to determine such spectrum using ratio of horizontal to vertical components of the Rayleighwaves. The present study is based on an extensive set of microtremor measurements carried out in theIslamabad city, Pakistan. Fundamental frequencies were obtained from weak motion sensors and TrominoEngy Plus instruments to show that the correlation is clearly valid for a wide range of sediment thickness.A simple formula was derived for the investigated area to determine directly the thickness of sedimentsfrom the main peaks in the H/V spectrum for seismometer and Tromino data separately. A comparisonis made between sediment thicknesses derived from empirical relations developed in this study withthose given in literature to demonstrate a positive correlation. The correlation of instrumental resonantfrequencies with calculated resonant frequencies (theoretical) suggests that the relation derived from thenoise measurements mostly depends on the velocity depth function of the shear wave. The fundamentalfrequency of the main peak of spectral ratio of H/V using the both instruments correlates well withthe thickness of sediments at the site obtained from the borehole data. It is found out that there is awide variation in soft-sediment thickness in the Islamabad area, but as a general rule, soft sedimentsare thicker adjacent to stream courses compared to the areas intervening the streams. The distributionof sediments in the studied area is illustrated by means of cross sections constructed from results ofthe microtremor analyses and available borehole data, which provides a visual distribution of the softsediments underlying the Islamabad city.

• Probabilistic seismic hazard assessment of Peshawar District, Pakistan

The seismic provisions for the Building Code of Pakistan were revised after the 2005 Kashmir earthquake and these have resulted in the introduction of a macrozonation ground motion hazard map in the seismic provisions. The macrozonation map proposes a peak ground acceleration (PGA) for the return period of 475 yr for Pakistan for flat rock sites. After the macrozonation, the next step is to develop the surface ground motion assessment studies for the cities, districts and tehsils of Pakistan. In this study, the probabilistic seismic hazard analysis (PSHA) approach is used for the Peshawar District. The PSHA, consistent with the classical Cornell approach, is carried out to obtain the seismic hazard curves and uniform hazard spectra of PGA values for the return periods of 150, 475, 975 and 2475 yr at a grid spacing of 0.1$^{\circ }\times$ 0.1$^{\circ }$. The PGA for Peshawar at 150, 475, 975 and 2475 yr return period is estimated as 0.23, 0.34, 0.39 and 0.45g, respectively, for rock flat outcrop site conditions. The surface ground motion maps proposed in this study incorporate the local soil effects using amplification factors based on shear wave velocity obtained as a proxy to the topographic slope. The resultant ground surface hazard assessment proposes the PGA value of 0.63g for the return period of 475 yr and 0.89g for the return period of 2475 yr. The maps developed in the current study are important inputs for the structural designing, risk assessment and land use planning of the Peshawar District.

• Mapping sediment thickness of the Abbottabad basin, Pakistan

The Abbottabad basin is mainly composed of different loose and indurated sediments such as fine to medium grain silt and clay and large to medium sized boulders and cobbles, occupying a low land between the hills. These sediments are primarily stream deposits and variably compacted in the form of rock, suggested name Havelian group after their maximum thickness into Havelian area. Numerous streams converge at the Abbottabad intermontane basin from the north–northeast and join to form a single channel that passes through a narrow gorge on the western side of the Sirban hill. Geomorphically, the Abbottabad city is underlain by a thick sequence of loose Quaternary–Recent alluvial sediments, making it vulnerable to seismic hazards. This research determines the sediment thickness for the Abbottabad basin using a geophysical approach. In this regard, thirteen lithologic profiles were developed in the Abbottabad basin at different locations. These profiles were ultimately combined to develop a Fence diagram showing a generalized stratigraphic pattern of the Quaternary–Recent unconsolidated sediments in the basin. Standard Penetration Test (SPT) and H/V analysis were used to characterize the site and shear wave velocity at a different location of Abbottabad basin and surrounding area. Based on H/V data (using Tromino Engy Plus instrument) Abbottabad basin and immediate surroundings have an average fundamental frequency from 0.5 to 9 Hz, which represents the deposition of alluvial sediments (i.e., stiff and dense soil).

• Landslides assessment using geophysical and passive radon exhalation detection techniques in Murree Hills, northern Pakistan: Implication for environmental hazard assessment

Geophysical investigation of three landslides in Murree Hills was carried out using geophysical techniques (i.e., seismic refraction and electrical resistivity) and geochemical tool (passive radon exhalation detection method). The seismic data was acquired by using reverse shooting scheme employing placement of source after the last active geophone in the spread. The acquired data was analyzed, and layer velocities were estimated by using Hagedoorn’s method. The resistivity data was modeled in terms of true resistivity of subsurface material by curve matching technique. The radon emission was determined as alpha track densities for each detector planted in dosimeter in the sub-surface along survey profiles. The results of all the methods employed were interpreted and correlated in the context of local geology, and also considering seasonal and anthropogenic factors. The study guides the importance of local geological structure and lithologies in the formation of thick weathering layer. The weathered layer wet/moistened through rains in the winter and summer seasons or daily use of water due to urbanization of the area, exerts more downslide force thus resulting landslides. This thickness of weathered layer is determined by using seismic refraction and resistivity methods for the three landslides (MIT, Kuldana and Chitta Mor) which is in agreement. Also, the passive radon exhalation detection technique (geochemical investigation) has delineated the stable and unstable areas within the three landslide zones. These geophysical and geochemical investigations are recommended on the major landslides of the area prior to damage control measures.

• # Journal of Earth System Science

Volume 129, 2020
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019