Monitoring the fault zone of the Sefid Sang earthquake (5th April 2017, Mw 6.1) via ambient seismic noise analysis utilizing the moving window cross-spectral technique

Passive image interferometry (PII) is an effective tool for studying seismic velocity changes by reconstructing the green function between two stations using cross-correlation. However, monitoring fault zones with PII can be challenging due to various processes that can cause velocity changes in the crust. Recently, ambient seismic noise has become a popular subject of study due to the possibility of recovering the Green's function between two seismometers from the cross-correlation. Seismic noise, which is recorded continuously and not limited to earthquake occurrences, can be used to observe variations in seismic velocities. Different natural phenomena produce measurable velocity changes using ambient noise, including co- and post-seismic changes following tectonic earthquakes, atmospheric-temperature-induced thermoelectricity, tidal modulation, and artificial changes related to variation in the noise source. Differentiating between the various processes inducing crustal velocity changes can be challenging because the amplitudes of velocity changes resulting from these influences can be similar to those resulting from tectonic phenomena.

In this study, we investigated the usefulness of PII on noise recorded in two seismic stations located near the fault zone that caused the ML = 6.1 Sefid-Sang earthquake. Our study covers a period of 15 months, including 12 months before and 3 months after the earthquake. We examined velocity changes in layers of different widths and analyzed the effect of stacking on the results. Our analysis revealed a 0.3% increase in seismic velocity during the two months before the earthquake. These findings provide valuable information about the earth's structural and mechanical properties between the stations.