Abstract
Seismic history of the North Tabriz fault (NTF), the main active fault
of Northwestern Iran near Tabriz city, and its relation to the Sahand
active Volcano (SND), the second high mountain of the NW Iran, and to
the 11 August 2012 Ahar-Varzaghan earthquake doublet (Mw6.5&6.3) (AVD),
is investigated. I infer that before AVD seismicity of the central
segment of NTF close to SND was very low compared to its neighbor
segments. Magmatic activities and thermal springs near central NTF close
to Bostan-Abad city and low-velocity anomalies reported beneath SND
toward NTF in tomography studies suggest that the existing heat due to
SND magma chamber has increased the pore-fluid pressure that overcomes
the effective normal stress on the central NTF, resulting in its creep
behaviour. Two peaks of cumulative scalar seismic moments of earthquakes
observed on both lobes of the creeping segment, confirming the strong
difference in the deformation rate between these segments. On 2012, AVD
struck in the 50 km North of NTF, in the same longitude range to SND and
with the same right-lateral strike-slip mechanism to NTF, as a result of
partial transfer of the right-lateral deformation of NW Iran toward the
North of NTF on the Ahar-Varzaghan fault system. A cumulative aseismic
slip equal to an Mw6.8 event is estimated for the creeping segment of
NTF, posing half of the 7mmy-1 geodetic deformation has happened in the
creep mode. This event has transferred a positive Coulomb stress field
of >1 bar on the AVD and triggered them. Also, the western
and eastern NTF segments received >4 bar of positive
Coulomb stresses from the creeping segment and are probable nucleation
locations for future earthquakes on NTF. The observed creep may be the
reason for the NTF segmentation during the 1721AD M7.6 and 1780 AD M7.4
historical earthquakes.
Keywords: Induced seismicity; aseismic deformation; pore-fluid
pressure; effective stress; volcano; triggering