Using Seismic Ambient Noise Cross-Correlation and Probabilistic Power Spectral Density (PPSD) to Monitor Changes in the Seismic Ambient Noise Field in Central Iraq

Abstract

     Seismic ambient noise is the continuous background vibration of the Earth, and it varies temporally and spatially, remaining poorly understood. The objective of this study is to monitor these variations and evaluate data from the Mesopotamian Seismological Network (MPSN) stations (ANB1, KAR2, and SAM2) during 2024-2025. Probabilistic Power Spectral Density (PPSD) was employed to assess data quality, and monthly stacked Cross-Correlation (MSCC) was used to extract Green’s functions for each month to identify noise-source variations. The PPSD analysis identified periods of poor-quality data and instrumental artifacts at stations ANB1 from March to June and KAR2 in March and August during 2025. The resulting MSCC functions displayed asymmetrical causal and acausal parts, indicating an uneven distribution of noise sources. A distinct seasonal pattern emerged, with peak correlations for all station pairs occurring in July. This aligns with maximum wind speeds and temperatures, supported by strong statistical relationships: wind speed (r=0.877, R²=0.77) and temperature (r=0.797, R²=0.64). Additionally, the ANB1-KAR2 pair exhibited a secondary peak (October–February) that correlates strongly with rainfall (r=0.872, R²=0.76). This secondary signal suggests a dominant noise source near the centre of the interstation path. The findings suggest that seasonal environmental factors could be a source to drive temporal changes in the ambient seismic noise source field in the area.