Non-linear Focal Mechanism Solutions: a Parametric Study of the Earthquake near Plovdiv (Bulgaria) on February 21, 2018

Abstract

The focal mechanism of the M_W 4.4 earthquake that struck on February 21, 2018, near the city of Plovdiv (Bulgaria) was investigated by using the INPAR method. INPAR is a non-linear inversion approach that estimates a moment tensor by comparing observed and synthetic seismograms. Waveform data from multiple seismic data centres were collected and processed to ensure consistency in the sampling rate and time window. Synthetic seismograms were computed using various parameters, such as velocity structure, origin time, location, depth, and rupture duration. The impact of the above parameters on the reliability of the obtained moment tensor was evaluated by conducting numerous tests. The final focal mechanism, computed with INPAR, was determined in two steps: a linear search and a non-linear search in the parameter space. The computed rupture mechanism was then  compared with the parameters of the known seismogenic faults in the region. The study establishes a set of parameters suitable for exploring the rupture characteristics of the moderate earthquake near Plovdiv and contributes to a better understanding of the faulting processes in the Maritsa fault zone.