Wavelet Transform Analysis of the 1963 Al-Marj Earthquake: Signal Processing and Site Effect

Abstract

The 1963 Al-Marj earthquake (Mw 5.6) caused disproportionate structural damage relative to its moderate magnitude, suggesting significant local site amplification effects. This study applies wavelet transform techniques to analyze the seismic signal characteristics and investigate the role of sedimentary basin resonance in damage distribution. A comprehensive signal processing framework was developed using Continuous Wavelet Transform (CWT), Discrete Wavelet Transform (DWT) multi-resolution decomposition, and wavelet-based denoising to process synthetic seismic signals representative of the 1963 event. The analysis demonstrates that wavelet methods outperform traditional Fourier analysis for non-stationary seismic data, providing simultaneous time-frequency localization of seismic phases. Wavelet denoising improved signal-to-noise ratio by 5–10 dB, enabling recovery of usable signals from noise-corrupted historical analog recordings. Multi-resolution decomposition successfully isolated P-waves (6.25-12.5 Hz), S-waves (3.125-6.25 Hz), and surface waves (0.78-3.125 Hz), revealing distinct time-frequency signatures for each phase. Site response analysis identified a fundamental frequency of 1.0 Hz for the 50-m thick Quaternary sedimentary basin, with theoretical amplification factors reaching 10×. The surface wave dominant frequency (1.5 Hz) lies within the amplification bandwidth, explaining the severe damage to masonry structures with natural frequencies of 0.78–1.56 Hz. These findings support the hypothesis that local geological conditions specifically basin resonance in unconsolidated sediments were the primary cause of destruction, rather than earthquake magnitude alone. The methodology provides a robust framework for analyzing historical seismic data and assessing site-specific seismic hazards in sedimentary basins.