Determining focal mechanism of earthquakes with magnitude 2≤ Mn≤5 in Alborz region by using the polarity of P wave first motion and S/P amplitude ratios

Calculating the focal mechanism of earthquakes, helps us to investigate the direction of rupture propagation, the style of faulting, stress field and the seismicity potential of an area. Focal mechanisms classically are determined by using the polarity of P- wave first motion. This method depends on the number of P wave polarities per event and the data set dispersion distribution Significant improvement of long period instruments caused advantages in using waveform inversion method for determining focal mechanism of earthquakes. In this method outspread outlier data can be used.
Extensive availability of small earthquakes makes them useful in determining focal mechanisms. The current study accomplished to determine focal mechanisms of 4204 events with magnitude 2≤ Mn ≤5. The earthquakes were recorded in 41 stations of Iranian Seismological Center (IRSC) in the Alborz region. The focal mechanisms were prepared by using the polarity of P wave first motion and S/P amplitude ratios.
In this study, waveforms of three-component seismograms were filtered with bandpass filter in a 0/1–10 Hz frequency range. Seismograms with velocity components were integrated to the corresponding displacement components. P-wave first motion polarities are revised by reviewing the waveforms carefully. For calculating the S/P ratios, P wave, S wave and background noise amplitudes were measured by selecting 2 second time windows from waveforms. Maximum amount of S/P ratio is near the nodal planes and the minimum amount is far from the nodal planes. Using P wave polarities and ratios, the final focal mechanisms were obtained. With the progressive improvement of IRSC networks since 2012, more P wave first motion polarities can be recorded and the number of the calculated S/P ratios for each event has increased.
In this study, data set of 1997 to May 2015 was investigated. Because of weak qualities of small earthquakes waveforms, small events were omitted and a focal mechanism catalogue containd strike, dip and rake angels for earthquakes with Mn greater than 3.5 was produced. The method also was tested with 12 synthetic earthquakes in the Alborz rigion. Most of the focal mechanisms at north of the region have thrust-faulting components and at south they are strike-slip. Along the left side of the Talaghan fault, most of the focal mechanisms are left-lateral strike-slip which shows the left-lateral motion along the Mosha fault. The Indes fault has north west-south east strike. Along the south east side of this fault, the focal mechanisms of 18/6/2007 event and its aftershocks show the inverse faulting. The results of this study are compatible with most of the previous studies in this region