6. Conclusion
I infer a seismic quiescence in the central segment of the NTF, North of
SND from 2006 until August 2012 AVD. While the two eastern and western
segments of NTF show much higher seismicity with two remarkable peaks of
cumulative scalar seismic moments on both lobes of the central segment
near SND. The existing heat by the SND magma chamber near the fractured
area of central NTF raises the pore-fluid pressure and decreases the
effective normal stress there, consequently unclamp the fault, and
facilitate the right-lateral creep. An Mw6.8 half-creep slip model is
suggested for this segment considering half of 7 mmy-1 constant geodetic
deformation rate on it since the 1721 AD historical earthquake.
The creeping segment is situated in almost the same longitude range to
the 2012 AVD and transferred positive Coulomb stress fields of
>1 bar on them. This segment also transferred
>4 bar of positive Coulomb stress on its neighbor segments,
where the two peaks of cumulative seismic moments were observed. Some of
the right-lateral deformation stresses on central NTF transferred to the
North and released during the 2012 AVD on the Ahar-Varzaghan complex
fault system (Fig. 6).
After the AVD until TKC, two new peaks of the cumulative scalar seismic
moment have observed for earthquakes that occurred on NTF and NMF. One
is exactly in the middle of the previously creeping central segment of
NTF, consistent to the obtained transfer of positive normal and Coulomb
stresses on this segment by AVD (i.e. Momeni et al., 2019). The
transferred stress changed the rheology of the creeping segment from
mostly creeping to temporary stick-slip.
The other peak of the cumulative scalar seismic moment is on the NMF
North of Urmieh Lake, and, may be due to the existing pore-fluid
pressure or a recent dramatic decline in the water level of the lake
over the past two decades (90% decrease in its volume has happened
during 1995 to 2013; Schulz et al., 2020) that both reduce the effective
normal stress on NMF, unclamp it, and facilitate slip on it (e.g. Saar
& Manga, 2003).The two peaks of cumulative scalar seismic moments
observed before AVD on the western and eastern lobes of the creep
segment of NTF near SND were disappeared after AVD until November 2019
TKC, suggesting a change in seismic activity of NTF along its segments
by the transferred stress fields produced by AVD, at least for the first
8 years. This change is probably temporary and NTF will return to
continue its creep behaviour in the central segment near SND.
In terms of rupture dynamics, the two highly stressed neighbor segments
of NTF are prone to nucleate earthquakes (e.g. Vadacca et al., 2020). If
an earthquake nucleates on the stressed lobes of the creeping segment
and its rupture grow toward the West, it will cause a strong directivity
effect for that earthquake toward Tabriz city. However, the creeping
segment may work as a barrier and probably does not allow NTF to rupture
in both Central and Western segments in one larger earthquake.
The change in seismic activity on the different segments of NTF and
having mix behaviour of lock and creep deformation on them raise the
seismic hazard in the region, especially for the Tabriz city that
currently host > 2 million people. I suggest continuous
monitoring of seismicity along NTF will help to understand the
rheological behaviours of segments of this mature fault system, with a
concentration on the central and western segments that did not rupture
in large events since the 1721 AD and 1780 AD historical earthquakes.