Relative moment tensor inversion method using synthetic data and its application to the aftershocks of the Faryab Earthquake on February 28, 2006 (Mw 6.0)

Relative Moment Tensor Inversion, RMTI, method is a suitable method to determine the focal mechanism of some close-lying earthquakes (a cluster of earthquakes) recorded by common seismic stations. With enough data on the propagation path, a Green's function is needed in classical source studies. However, the effect of the propagation path is minimized in RMTI applications using relative data. The observed data in an RMTI method consists of the relative amplitudes of the direct P phase and direct S phase on vertical and rotated horizontal components of some close-lying earthquakes recorded by common seismic stations. The effect of the propagation paths from the source-region to a given seismic station is minimized using the relative amplitudes of the corresponding phases (e.g. P-phase) at a given station. The focal mechanisms of the earthquakes are then determined using a linear weighted least-squares approach for the six components of the seismic moment tensor without knowing the complete Green's functions. The calculation is done using only a simple velocity model at the source region. In this research, we have implemented the RMTI method for four groups of synthetic data. Each group consists of 14 events that their corresponding locations and focal-mechanisms are taken according to the events of March 25, 2006 occurred in the Fin region, South of Iran. These events were recorded by Iranian National Broad-Band Seismic Network (INSN) in local distances and their magnitudes (M_L) vary between 3.5 and 5.6. In this synthetic data-set, we tried to analyze the effect of a velocity-model and also non-double couple components (CLVD) in the moment tensor inversion using different scenarios. The synthetic data were firstly inverted using a classical moment tensor inversion in a time domain and then were inverted using the RMTI method in both time and frequency domains. The effects of the velocity model and CLVD components in the data were then analyzed using the synthetic data at different frequency bands used in the inverse algorithm. Our results indicate that the RMTI method can be easily implemented to retrieve the focal mechanism of close-lying earthquakes in local distances. Our results also indicate that an unrealistic increase of non-double-couple components causes wrong results, but the crustal model does not cause any significant effect and the effect of the propagation path in a relative inversion is minimized. The aftershock focal mechanisms of the February 28, 2006 earthquake were determined using an RMTI method. The focal mechanisms of the aftershocks were consistent with main shock.