MSc Thesis
Figure: Fundamental frequency map of Arak, Iran.
Abstract: Following the earthquake swarms that occurred in a short period on January 15, 2012, between the northwest of Arak and the southeast of Ashtian, the city of Arak, initially considered a quiet seismic zone, garnered increased attention. Studies performed within the framework of seismic microzonation revealed that the H/V spectral ratio calculated in some areas showed clear amplification at lower frequencies.
To further investigate the site effect and the potential existence of a thick alluvial deposit, we attempted to extract the velocity structure through inversions of Rayleigh wave ellipticity.
Ellipticity is defined as the ratio between the horizontal and vertical motions of Rayleigh wave particles, which is strongly related to the local soil structure. Since the 1970s, this method has been promoted and popularized both experimentally (Nogoshi and Igarashi 1971, Yamakana et al., 1994) and numerically (Lachet and Bard 1994, Bard 1999), showing its potential for investigating deep sedimentary layers.
Ellipticity depends on the noise wavefield; therefore, two methods were introduced to reduce the effects of body and Love waves from the wavefield.
The HVTFA method is defined based on time-frequency analysis of continuous wavelet transforms. In this method, the maximum values of the vertical signal on time-frequency representations of the records are extracted at each frequency. These maximums in the vertical spectra correspond to the presence of Rayleigh waves. Averaging the calculated spectral ratios of these values indicates the ellipticity of the Rayleigh waves.
The RayDec method is executed by summing and stacking buffered signal windows. The ellipticity ratio is calculated by identifying all times where the vertical components of the filtered signal change their sign and stacking these parts in windows. The ratio is then calculated with a novel approach to horizontal components by introducing a correlation factor between horizontal and vertical components. Finally, the ratio is defined as the square root of the energies in the buffered signals.
This summing and stacking procedure eliminates the effects of other waves. Body waves are also dampened with phase-shifting of stacked signals, and Love waves do not appear in vertical components. Additionally, the incidental correlations of Love and Rayleigh waves arriving simultaneously are dissolved by stacking a large number of time windows.
We used these methods to extract and invert Ellipticity in the Arak region. The obtained profiles, alongside geological and geophysical observations, confirmed the existence of a thick alluvial deposit in the northern and eastern parts of Arak's urban area. For the southern parts, this method indicates higher velocity and shallower bedrock, consistent with the geology of the southern high-elevated areas.
