Microtremor measurements in Yanbu city of Western Saudi Arabia: A tool for seismic microzonation

Abstract

Microtremor measurements are one of the most popular world-wide tool for estimation of site response especially within the urban area. This technique has been applied over 85 sites distributed regularly through Yanbu metropolitan area with an ultimate aim of seismic hazard microzonation for ground-shaking site effects. The horizontal to vertical spectral analysis (H/V) was carried out over all the sites to estimate both the fundamental resonance frequency and its corresponding amplification for the ground vibration. In most sites, H/V curve for amplitude spectra display a clear peak suggesting the presence of a soil-bedrock impedance contrast. Other sites, however, show more than one peak indicating the presence of more than one impedance contrast through sedimentary cover. The estimated values of fundamental frequency range from 0.25 Hz up to 7.9 Hz increases with decreasing depths of basement rock. It has lower values at the central zone extending from north to south compared to the eastern and western parts of Yanbu area. On the other hand, the estimated values of amplification factor ranges from the value of 2 to 5, where the higher values prevailing through the central zone with increasing thickness of sediments. Analyses of the acquired data set have clearly shown that, both of two parameters vary considerably through Yanbu city. This could be due to lateral variations in soil thickness and/or variations in the soil type at Yanbu area. These results show the 2D and 3D effect of basin geometry. The estimated values for the fundamental frequency from microtremor data are compared with that from shear-wave velocity structure within the area of interest and show an excellent agreement.

Application of the microtremor measurements to a site effect study

“…2f); these secondary peaks of the H/V curve reflect the presence of two large impedance contrasts (SESAME 2004). Figure 2d presents some of broad peak curves, and this could be related to presence of an underground sloping of the interface between softer and harder layers (SESAME 2004;Fnais et al 2010). …”

Vulnerability Index Assessment Using Neural Networks (Viann): A Case Study of Nicosia, Cyprus

“…Several studies have been performed for identifying the microtremors (Paudyal et al, 2013;Fnais et al, 2010;Beroya et al, 2009;Jiyang et al, 2016). The ground vibrations as we called microtremors have amplitude values vary between 0.1 and 1 micron and velocity amplitude changing between 0.001 and 0.01 cm s -1 .Microtremors can be classified in two major groups according to their frequency values: frequency values greater than 1 Hz (up to 20 Hz) known as "short period" which refer to shallow soil layers.…”

“…These are inactive ground motions with displacement amplitudes that can be signified by seismograph with high amplification. Many scholars use the Horizontal Vertical Spectral Ratio (HVSR) Technique to observe soil properties that affect the vulnerability of sites to earthquake (Rezaei and Choobbasti, 2014;Nakamura, 1989;Paudyal et al, 2013;Fnais et al, 2010;Beroya et al, 2009;Jiyang et al, 2016;Stanko et al, 2017;Gosar, 2007;Mucciarelli et al, 2001;Pamuk et al, 2017). By utilizing the H/V ratio, the soil fundamental period (T0) and the amplification factor (Ag) of the site were determined (Tarabusi and Caputo, 2017;Stanko et al, 2017;Hardesty et al, 2010;Özdağ et al, 2015).…”

Frequency Analysis and Sismic Vulnerability Index by Using Nakamura Methods For Microzonation at Prambanan Temple and Its Surrounding Area, Yogyakarta Province, Indonesia

“…The H/V spectral ratios were computed according to the following methodology: (1) baseline correction; (2) band-pass filtering to retain the frequencies in the range from 1 to 25 Hz; (3) windows of 10 s length were automatically selected using an anti-STA/LTA trigger algorithm and tapered with a 5% cosine function before the computation of spectra; (4) Fourier spectra were calculated for each noise component and smoothed using Konno-Omachi window having a smoothing constant bvalue of 40; (5) the resulting spectral amplitudes of horizontal components were geometrically averaged and divided by the vertical spectra to calculate the H/V function. Moreover, stability of the peak in the H/V curve have been checked through three tests conducted at every measuring point, where the same frequency peaks were picked for each site at different times [11].…”