We report estimation of site response in the form of fundamental frequency. Towards this objective, we deploy widely established receiver function technique. Taking locally recorded events as inputs, we implement this technique to estimate resonance frequency in three receiver sites, characterized by varying lithology underneath. It is observed that resonance frequencies varies from 3 to 7 Hz, which is also confirmed by our previous studies of estimates from ambient noise recordings with reference to identical sites. Variation of frequency implies existence of heterogeneity in the study area.
Deep Dive into Delineating site response from microtremors: A case study.
We report estimation of site response in the form of fundamental frequency. Towards this objective, we deploy widely established receiver function technique. Taking locally recorded events as inputs, we implement this technique to estimate resonance frequency in three receiver sites, characterized by varying lithology underneath. It is observed that resonance frequencies varies from 3 to 7 Hz, which is also confirmed by our previous studies of estimates from ambient noise recordings with reference to identical sites. Variation of frequency implies existence of heterogeneity in the study area.
Estimation of spatial variation of site response is one of the prime objectives of microzonation studies. Several literatures emphasized the paramount significance of site dependent factor which largely impact the concentration of damage in some pocket areas pertaining to local geology (King and Tucker, 1984;Aki, 1988;Kawase, 1998;Field and Jackob, 1993;Nath et al., 2000Nath et al., , 2002a, b), b). Even the shaking of man-made structures also get influenced by the spatial variation of site effects. The site effects can be parameterized by factors like resonance frequency, amplification factor which have direct impact on semi-resonance or resonance of certain building types, and thereby causing distortions or failures of constructions. In order to compute site response, there are adoptions of various methodologies (Castro et al., 1990;Field and Jacob, 1995;Bonilla et al., 1997;Riepl et al., 1998). HVSR or receiver function technique has been widely regarded as one of the most powerful methods available to seismologist for acquiring a reliable estimate of site response.
In this work, we endeavor to estimate site response through horizontal to vertical ratio of microtremors. We give a comprehensive analysis of the results attained through horizontal to vertical ratio of locally recorded waveforms.
In order to generate local waveforms exclusively for this study, a temporary network of three stations namely IIG, NEHU and SETUK were installed in Shillong City. This network was operational for two months. The stations were equipped with three Trillium 120P seismometers from Nanometrics having frequency bandwidth of 0.003 to 50 Hz; with 24 bit Guralp Digitizer in synchronization with Guralp GPS. It was a continuous mode of recording in all the three stations. The data were digitized at a sampling frequency of 100 samples /second. The seismic stations are shown in Figure 1. A total of 135 tremors were recorded during the period of deployment of this temporary network. Out of this, a total of 40 tremors recorded by the three stations were precisely located adopting the velocity model of Bhattacharya et al., (2005), compatible for Shillong region with a view to determine the hypocentral parameters. Out of these 40 events, only fourteen events have been selected in order to study the site response from HVSR in this study region within an epicentral distance of less than 50 km. Table 1 provides the hypo-central parameters of the located events. The depths of the events vary from 4 to 25 km whereas the epicentral distance ranges from a mere 1.9 km to 48 km. The root mean square of the located events is below 0.2.
As per reports of Aki, 1988;Kawase, 1998, microearthquake study entailing events at shot epicentral distances facilitates the understanding of physics of source processes as well as local site conditions. HVSR is based on the assumption that vertical component is least affected by near-surface influence. Consequently, when we divide the horizontal component by vertical component, the site effect can be deciphered.
As an input for HVSR, we incorporate S-wave packets. Suppose, ) , ( Assuming k events being recorded by j stations, the amplitude spectrum A (rlm, fn) in frequency domain of fn can be expressed as (Lermo et.al., 1993, Nath et al., 2002a;Mandal et al., 2005) A(rlm, fn) =SIk(fn).P (rlm, fn).SOk(fn)
(2)
The corresponding HVSR can be estimated as
represent the Fourier spectra of the North-South component, East-West and Vertical component, respectively. By taking into account the contribution of all the seismic events, the average receiver function HVSRj ave (fn) is estimated.
HVSR yields a peak with existence of an impedance contrast. The epicentral plot of the events used for implementing this receiver function is illustrated in Figure 1. The horizontal to vertical ratio technique adopted for the locally recorded earthquakes in the present study is described below: During the estimation approach of HVSR, we ensure that the corrected spectra is endowed with noise to signal ratio of less than a factor of 3 to eliminate all sorts of plausible transients, as followed by Nath et al., 2002a.
The receiver function was determined at the three temporary stations viz, SETUK, NEHU and IIG incorporating the waveforms recorded by this temporary network. All these stations are characterized by different type of site geology. The HVSR yields different type of amplification levels and peak frequency corresponding to highest amplification. All these are elaborate station wise.
The average HVSR result for IIG station including all the local events utilized are outlined between 4 and 6.8 Hz, as evident from Figure 2a. In between 3 and 5 Hz, an appreciable level of amplification is found which later on decays. But, with increment of frequency the amplification rises again and reaches the peak at 1.285. The frequency corresponding to the highest amplification, generally referred to as the fundamental frequency, is
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