Agricultural Research Management Information System

Home
Research Summary
Search
About Us
- ARMIS
- Brochure
Contact Us
Report
User Request
Data Input
Help
- Operation Manual
  - PDF
  - Video
- Program Area & Commodity
We have reached 37600 number of research entries at this moment.

- Logout

Research Detail

Home
Research
Detail

Predominant Period and Amplification Factor Estimation with Respect to Geomorphology - a Case Study of Sylhet City Corporation Area, Bangladesh

Md. Shakhawat Hossain
Department of Disaster Science and Management, University of Dhaka, Dhaka-1000, Bangladesh

Md. Zillur Rahman
Department of Disaster Science and Management, University of Dhaka, Dhaka-1000, Bangladesh

Md. Mostafizur Rahman
Department of Disaster Science and Management, University of Dhaka, Dhaka-1000, Bangladesh

Kamrun Nahar
Department of Geology, University of Dhaka, Dhaka-1000, Bangladesh

A. S. M Woobaidullah
Department of Disaster Science and Management, University of Dhaka, Dhaka-1000, Bangladesh

Abstract/ Executive Summary:

The predominant period and amplification factor of the subsurface soils of Sylhet city corporation area have been estimated using microtremor. Their relation to the geomorphological units of the city has been determined. The estimated predominant periods of alluvial fan, back swamp, flood plain, isolated hills, level hills, piedmont plain, point bar and ridge of the city are 0.60, 0.75, 0.73, 0.23, 0.62, 0.45, 0.73 and 0.66 sec., respectively. With maximum (± 0.88) and minimum (± 0.38) standard deviation amplification factor of alluvial fan, back swamp, flood plain, isolated hills, level hills, piedmont plain, point bar and ridge are 3.1, 4.1, 4.6, 3.75, 2.22, 4.12, 4.18 and 4.42, respectively. According to amplification seismic hazard of different geomorphological units of the city has been ranked from very low to relatively high hazard zone whereas level hill falls under very low, alluvial fan under low, flood plain under a relatively high and back swamp, isolated hills, piedmont plain, point bar and ridge falls under moderate hazard zone.

Keywords: Predominant period, Amplification factor, Seismic hazard, Geomorphological units, Microtremor

Location: the subsurface soils of Sylhet city corporation area, Bangladesh

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Soil Physical Properties

Objectives:

To investigate the seismic response of different geomorphological units of Sylhet city corporation area located in the north-eastern part of Bangladesh.

Methodology:

Spectral ratio between the horizontal and vertical components was first introduced by Nogoshi and Igarashi (1971). To use it as an indicator of the underground structure they showed its relation to the ellipticity curve of the Rayleigh wave and took advantage of the coincidence between the lowest frequency as a maximum of this H/V curve with the fundamental resonance frequency. This technique was later revised by Nakamura (1989) who claimed that the spectral ratio is a reliable site transfer function for S-wave which led him to use the name “Quasi-Transfer Spectrum or QTS”. Although his semiqualitative theoretical explanation appeared questionable to many scientists, this technique is very simple and cheap. That's why, it immediately been circulated all over the world, for further checks or even, and indeed often used, for direct applications. To derive his quasi-transfer spectrum (QTS) model, Nakamura (1989) assumed that the energy of microtremors comprises both body and the surface waves, and the surface sources generated Rayleigh waves equally affect the horizontal and vertical components of motion. On this assumption, he proceeds to estimate the effects of the sources (O_m (f)) as the ratio of the vertical motions registered on the surface (V_s (f)) and at the base layer (V_b (f)): O_m (f) = (V_s (f)) / (V_b (f)) However, the transfer function of interest for a site i(G_i (f)) comprises a ratio involving the horizontal motion of microtremor registered at the surface ( H_s (f)) and at the base layer (H_b (f)): i(G_i(f))=(H_s(f))/(H_b(f)). According to Nakamura (1989), it is effective to identify the fundamental resonant frequency of a sedimentary layer with implied amplification factors that are more realistic than those obtained from sediment to rock site ratios. It has been shown by many researchers (Lermo and Chavez 1993, Field and Jacob 1993, Konno and Ohmachi 1998) that how such H/V ratio of noise can be used to identify the fundamental resonant frequency and amplification factor of sediments. Spectral ratio techniques are most often applied methods for estimating effects and these techniques stem from the basic idea that the severity of damages associated to earthquakes depends heavily on the frequency contents of the corresponding ground motions. Moreover, these techniques take into consideration that most of the operations needed to analyze the energy contents of ground motion turn much easier in the frequency domain than in the time domain. So, in order to determine in which frequency contents of the spectrum, the sedimentary packages caused the larger amplifications. We employed the horizontal to vertical spectral ratio technique on microtremors observed at a site with only a three-component (NS, EW, UD ) sensor. For spectral analysis, we took three noise-free portions of 20.48s from each record as the instrumental sampling frequency was 100 Hz. The steps that we followed for the estimation of site response from spectral ratio curves are as given below (Kamal and Midorikawa 2006): (1) Fourier transformation: We have calculated the Fourier spectra of the two horizontal (north-south and east-west) and the vertical (up and down) components. As the Fourier spectra of the two horizontal components looked alike, their horizontally combined spectra were calculated to obtain the maximum Fourier amplitude spectrum as a complex vector in the horizontal plane. Whereas that of the UD component provides the vertical motion spectra. (2) Smoothing of the spectra: After Fourier transformation, we digitally filtered the combined horizontal and vertical spectra applying a logarithmic window (Konno and Ohmachi 1998, Rodriguez and Midorikawa 2002) with a bandwidth coefficient equal to 15. This filtering technique was applied to reduce the distortion of peak amplitudes. (3) Calculation of the soil response functions: The smoothened combined horizontal spectrum was divided with the vertical counterpart (H/V) which provided the desired predominant period and corresponding amplification factor of the investigated portions (20.48s) of records. (4) Normalizing the data set: After calculating three sets of 20.48s H/V ratios for each record, they are normalized by averaging to obtain a relatively non-biased site-specific H/V ratio. Using above mentioned steps 59 microtremor data have been analyzed for different geomorphic units of Sylhet city corporation area. Each geomorphic unit contains one or more than one microtremor data. Finally, the predominant period and amplification factor have been calculated by averaging the best quality data of those geomorphic units and then the peak amplitude curve with the standard deviation of each geomorphic unit is prepared.

Source of Information: Bangladesh J. Sci. Res. 27(1): 1-10, 2014 (June)

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Amplification factor and predominant period are essential for seismic hazard assessment. Based on amplification factor back swamp, isolated hills, piedmont plain, point bar and ridge fall under moderate hazard zone where amplification value varies from 3.5 to 4.5. level hills fall under a very low hazard zone where value varies from 1 to 2.5. alluvial fan and flood plain fall under low hazard zone and relatively high hazard zone where amplification value varies from 2.5 to 3.5 and 4.5 to 7, respectively. The Predominant periods of different geomorphic units of the city range from 0.23 to 0.75 sec. If the predominant period is known at the site of construction, engineers can design the structure in such that the natural period of the structure does not coincide with the predominant period.

Knowledge Management: Journal