2.Moment Tensor of the 7 May 2020 M5.1 mainshock
The low-frequency full waveforms of the mainshock recorded by eight
Iranian broadband seismic network (BIN) stations situated at distances
ranging from 84 km to 433 km (Fig. 1a) were inverted to obtain the
moment tensor, assuming a point source. The BIN seismic network is
maintained by the International Institute of Earthquake Engineering and
Seismology (IIEES), Tehran, Iran. More distant stations were excluded
from the inversion to keep the maximum possible frequencies of the
displacement waves while avoiding the undesirable effects of crustal
heterogeneity on the centroid location and moment tensor calculations.
The moment tensor inversion was carried out using the latest version of
Isola code (Sokos and Zahradnik, 2008). The mean and trend of data were
removed and bandpass filtered by fourth-order Butterworth filter between
0.03- 0.08 Hz and cut from origin time for a length of 250 s. The
Green’s functions for the trial point source were computed using the
discrete wavenumber (DWN) method (Bouchon, 2003) and the similarity
between the observed and calculated waveforms measured by the Variance
Reduction (VR) parameter of the Isola code. Details of the inversion
procedure are explained in a paper by Momeni and Tatar (2018). We used a
velocity model of the area obtained in a detailed microseismic study by
Tatar et al., (2012) (Table S2). The trial point sources were grid
searched in the activate area (Fig. 2a). The centroid time was searched
as well, from 2 s before to 4 s after the origin time with steps of 0.08
s (Fig. 2b). The best wave-form fit was obtained for a point source
situated at ~4 km WNW of the hypocenter reported by
IRSC, at a depth of 12 km, with a space-time correlation of 85%,
proposing that the rupture grew mostly to the West (toward Tehran city).
The NW-SE striking nodal plane has a strike/dip of 291°/60° (Figs. 2,
4a), which is steeper than the one obtained by IRSC (52°). However, that
is less steep compared to the GCMT result (68°). The centroid depth
could change between 11 km to 16 km, with only a slight change in the
waveform-fit (<2%). A total scalar seismic moment of 4.8 E+16
Nm equivalent to Mw 5.1 was computed, which is larger than the GCMT and
IRSC results (4.1*E+16 Nm and 2.8*E+16 Nm, respectively.