Determination of earthquake inter-event empirical Green’s function using virtual seismometer method, Case study: Kahurak Fault in the Rigan area, southeastern Iran

Analysis of earthquake events provides an efficient tool to extract the empirical Green's function (EGF) between pairs of earthquakes by interferometry approach. Because of sparse distribution of stations or low seismicity, many classical seismic studies (earthquake-receiver systems, ambient seismic noise and etc.), may yield a poor or noisy calculation of the tomographic result maps. However, inter-event EGFs between two earthquake locations can be retrieved by virtual stations, first outlined by Curtis et al. (2009). These EGFs are equivalent to the waveform produced as an impulse at one receiver location and that recorded by another receiver. Several researchers (e.g. Hong and Menke, 2006; Curtis et al., 2009) used a source-receiver reciprocity theorem, to indicate that inter-event EGFs could be retrieved when their waveforms are recorded by a set of receivers surrounding the events. According to this theorem, the stacked cross-correlations of event-pair (between a pair of earthquake event) waveforms recorded by these set of receivers, are equivalent to the estimated EGFs (Curtis et al., 2009). This technique can, therefore, provide new insight and useful tool to study fault planes and Earth's interior where real receivers can not be installed. However, the fault plane where earthquake ruptures occur at depth is often interpreted as being a transitional zone which is characterized by asperities and barriers (Aki, 1984). Thus the aftershock events interferometry approach could be applied to study fault plane by retrieving accurate, stable and reliable inter-event EGFs. After the Rigan earthquake occurred on 20 December 2010 (Mw 6.5) in Kerman province (southeastern Iran), aftershock events extended along the hidden part of the Kahurak Fault. In this paper, the cross-correlation of aftershock events was applied to retrieve the inter-event EGF on the hidden part of the Kahurak Fault plane in the Rigan area. This event-pair example was selected based on some criteria that the most important of these conditions is the similar (approximately) depth of events due to the ease of operation and processing. Aftershock event-pair projection and data processing is similar to that explained in detail by Bensen et al. (2007). The mean and trend were removed and the data were decimated to 10 sps. Time and frequency domain normalizations were then applied to suppress the influence of instrument irregularities and high energy events. After cross-correlation and stacking procedure, event-pair EGF signal was extracted. Then, 1D and 2D synthetic signals were generated using computer program in seismology (Herrmann and Ammon, 2013) and SPECFEM , respectively. Horizontal velocity result at depth of ~4 km, which is calculated by Shirzad et al. (2017), was applied for both 1D and 2D synthetic input modeling. Comparison between inter-event EGF and synthetic signals indicates that the inter-event EGF is in agreement with the synthetic models. Also, inter-event EGF signal propagates on the hidden part of the Kahurak fault plane. The correlation coefficient of 1D and 2D synthetic inter-event 031-163 EGF signals are of the order of ~75% and 80% within the signal window. In conclusion, these inter-event EGFs can be used for investigating the laterally varying the 2D mapping of surface wave group and/or phase velocities.