1. Introduction
On 11 August 2012, an earthquake doublet with Mw6.5 and Mw6.3 occurred in NW Iran near the Ahar and Varzaghan cities (AVD), 40 km Northeast of Tabriz city (TBZ), the fourth megacity of Iran with >2 million population. They located in 50 km North of Sahand Volcano (SND), which situated 40 km Southeast of TBZ. AVD caused >300 fatalities. In terms of mechanism, the first mainshock exhibits an almost pure right-lateral strike-slip faulting, and the second one had two slip patches with right-lateral strike-slip and reverse mechanisms (Fig. 1a, Momeni & Tatar, 2018; Momeni et al, 2019). The first mainshock produces almost 12 km of surface rupture. The mainshocks mechanisms and their rupture geometries were almost the same to the North Tabriz Fault (NTF), the main active fault of northwestern Iran, that crosses just North of TBZ. Before AVD, the seismicity on the Ahar-Varzaghan fault system was negligible, so that these events were a surprise in that region.
Tectonic Settings
NW of Iran is a part of Turkish-Iranian plateau, bounded by the Caucasus Mountains to the North, Zagros Fold and thrust belt to the South, and Talesh Mountains to the East. This region is under complex tectonics which resulted in high deformation and seismic activity (Berberian and Arshadi, 1976; Jackson, 1992; Ghods et al., 2015; Momeni & Tatar, 2018) (Fig. 1). The main tectonic regime is the oblique convergence of Arabia-Eurasia with the current deformation rate of about 13–15 mmy−1, at the longitude of 46.1 °E (Vernant et al., 2004; Reilinger et al., 2006). The Talesh Mountains on the eastern termination of the region is either colliding to the relatively thick South Caspian sedimentary cover (∼20 km) (Zamani and Masson, 2014) or, the South Caspian crust is under-thrusting westward beneath it (Jackson et al., 2002). Two different northeastward and northwestward deformation directions are observable in the GPS velocity vectors at the longitude of 44°E, which suggest it as the western margin of the region (Momeni & Tatar, 2018).
Earthquakes mechanisms (GCMT) and surface deformation studies (Reilinger et al., 2006; Djamour et al., 2011) suggest the resulting deformation in NW Iran is mainly the right-lateral strike-slip faulting on the W-NW striking faults within the region. While thrust faulting is partitioned to the North on the Great Caucasus (Jackson, 1992; McClusky et al., 2000; Momeni & Tatar, 2018). The two N-S to NE- SW (N018°) shortening and NW-SE (N287° and N155°) strike-slip deformation stress regimes were obtained by Zamani and Masson, (2014). The first stress system developed the three E-W striking fold and thrust belts, Arasbaran, Ghoshe-dagh and Bozkosh mountain ranges. While the N-S striking structures like Talesh mountains corresponds to the second stress system (Fig. 1a).
NTF with the main right-lateral strike-slip mechanism and deformation rate of 7 ± 1 mmy−1 accommodates most of the deformation of the region (Djamour et al., 2011). Rizza et al., (2013) estimated that the current deformation rate has been mostly constant since late quaternary until now (during the past 45 ka). However, they remarked a decrease in the strike-slip deformation rates along the NTF from about 7 mmy−1 to 5 mmy−1 from the West to the East of 47° longitude, and they relate it to the fault orientation changes from about NW-SE to W-E (Djamour et al., 2011; Rizza et al., 2013) (Fig. 2a). Later, Su et al., (2017) reported subsidence in the closest GPS station to the Northeast SND near BA and interpreted it as magmatic activities of Sahand.
A microseismicity study along the NTF using a local seismic network confirmed the main shear deformation mechanism of NTF in the upper crust (Moradi et al. 2011). On the western and eastern terminations of NTF, the North Mishu Fault (NMF) and the South Bozkosh Fault (SBF) are the two oblique reverse faults with right-lateral strike-slip components that strike ~E-W and dip to the South and North, respectively. Solaymani Azad et al., (2015) called the NTF, NMF, and SBF as Tabriz Fault System (TFS).
Before the occurrence of the 2012 earthquake doublet, the Ahar-Varzaghan fault system has been in a long seismic quiescence (Momeni & Tatar, 2018; Fig. 1a). However, field observations revealed the continuation of fault to the both East and West of the ruptured segment during the 2012 AVD (e.g. Copley et al. 2013, Donner et al. 2015, Ghods et al. 2015). The clear segmentation has been observed along this fault system with different kinematics and without clear connections that suggest it as a young fault system (Donner et al., 2015). The occurrence of the AVD suggested that some of the shear strain of the region is not compensating by the NTF, specifically at the longitudes east of 46°E (Donner et al. 2015; Ghods et al. 2015).
Sahand volcano with 3707 m elevation is an isolated, extensively distributed (~3000 km2 area) stratovolcanic complex (Ghalamghash et al., 2019). Together with Sabalan and Saray, they are three Late Miocene-Quaternary volcanoes that formed as results of the collision between the Arabian and Eurasian plates along the Neo-Tethyan suture zone in NW Iran. The formation of Neo-Sahand is estimated to ca. 600 to b173 ka (Ghalamghash et al., 2019). They mostly observed within and outside of the widely eroded caldera margin. Neo-Sahand units include basaltic andesitic to rhyolitic domes in the center of the complex as well as small parasitic cones along with subvolcanic dikes toward the northeast.
Historical earthquakes of NW Iran
Historical seismicity is widely distributed in the region, and mostly on the NTF and NMF. Tabriz city has been the capital of Iran and Azerbaijan in the past. Several large destructive historical earthquakes (magnitudes up to Ms=7.6) have reported for Tabriz (Ambraseys and Melville, 1982; Berberian and Yeats, 1999). Most of them are related to the activity of NTF (Fig. 1a). The last two M>7 destructive historical earthquakes have occurred on 1721 AD and 1780 AD, that ruptured more than 50 km of the southeastern and northwestern segments of the NTF and caused over 40,000 and 200,000 fatalities, respectively (Ambraseys and Melville, 1982; Berberian and Yeats, 1999; Momeni & Tatar, 2018).
Paleoseismological studies revealed at least three major (M∼7.5) historical earthquakes for the southeastern segment over the past 33.5 ka, including the 1721 AD M7.6–7.7 earthquake (Solaymani Azad et al., 2015). On the other side, four historical earthquakes are reported for the northwestern segment of the NTF during the past 3.6 ka, the most recent being the Ms∼7.4 1780 AD earthquake (Hessami Azar et al., 2003). The estimated slip per event and slip rate on this segment are 4 ± 0.5 m and 3.1–6.4 mmy−1, respectively, and the average recurrence interval of large earthquakes on it is estimated in the range of 350 to 1430 years.
NMF and SBF also ruptured during historical earthquakes, the last two being the 1786 AD Marand and 1593 AD Sarab earthquakes that suggest a seismic migration from the southeast toward the northwest of TFS. They also reveal earthquake clustering (e.g. Kagan and Jackson, 1991; Berberian, 1997; Karakhanian et al., 2004) and interaction between fault segments of the TFS (Solaymani Azad et al., 2015).
Instrumental earthquakes of NW Iran
Mechanism of deformation strongly changes from the western Alborz to the Iran-Turkey borders. The 1976 Mw7.0 Chalderan earthquake has occurred near the borders of Iran with Turkey on the right-lateral strike-slip Chalderan-Khoy fault that strikes E-W. While, the 1990 Mw7.4 Rudbar-Tarom earthquake on the western Alborz mountains has occurred on a left-lateral strike-slip fault trending WNW- ESE (Momeni & Tatar, 2018).
Within the NW Iran before the AVD, the 1997 Mw = 6.1 Ardebil earthquake occurred on an N-S striking fault situated ~150 km East of the AVD, showing a pure left-lateral strike-slip faulting (Aziz Zanjani et al., 2013). On 11 August 2012, the AVD occurred that were very well recorded by different instruments. The first mainshock with Mw6.5 had a right-lateral strike-slip mechanism. While the second one with Mw6.3 was more complex and contained two slip patches: the first patch had a right-lateral strike-slip mechanism, and the second one had a reverse mechanism (Momeni et al., 2019). The last large earthquake in the region is the 7 November 2019 Mw5.9 Torkamanchay earthquake (TKC) that occurred on an NE striking left-lateral strike-slip fault between North and South Bozkosh Faults (Valerio et al., 2020).
The EHB catalog (Engdahl et al., 2006) shows most of the seismicity on the Talesh mountains and also on the NMF (Fig. 1a). However, no large earthquake (M>5) reported on the segments of NTF in the GCMT catalog (Fig. 1b).
In 1995 a local permanent seismic network consisting of eight stations, known as Tabriz sub-network of the Iranian Seismological Center (IRSC), was installed in the region (Fig. 1b). However, this sparse network (station spacing ~50 km) limited the number of precisely located earthquakes. There are only 70 earthquakes in IRSC catalog from 1995 until the end of 2005, and none of them has located close (distance <5km) to NTF. From 2006, after the installation of the Broadband Iranian Network (BIN) maintained by the International Institute of Earthquake Engineering and Seismology (IIEES) in Tehran, Iran, and improvement of IRSC network data, the IRSC locations improved in terms of accuracy and magnitude completeness.
Figure 1b shows the precisely located seismicity of the region recorded in the IRSC network from 1995 until the occurrence of AVD in August 2012. They have located by at least five stations, have a location error of <5 km, RMS of <0.5 s, and azimuthal gap of <270°. They are all smaller than Mw5.0 unless the 1997 Ardabil earthquake. They mostly have shallow focal depths (< 20 km) and are distributed mainly along the NTF and NMF. However, the central segment of NTF near SND shows much lower seismic activity compared to the western and eastern segments (Fig. 1b). A seismic cluster on the North of AVD is mainly related to mining activities in that area. The rest of the seismicity is related to the Astaneh (ASF), West Alborz (WAF), South Qoshadagh (SQF), Goshachay (GCF), South Bozkosh (SBF), North Mishu (NMF), Maragheh (MGF), Khajeh (KHF), and Nehram (NHF) faults. After AVD, the only large event of the region is the 7 November 2019 Mw5.9 Torkamanchay earthquake that occurred on an NE striking vertical dip fault between NBF and SBF near the eastern termination of NTF.
Detailed microseismic monitoring on the NTF by a local dense seismic network data confirmed its right-lateral strike-slip mechanism with an East-Southeastward oriented fault plane (Moradi et al., 2011; Fig. 4). Microearthquakes located by Moradi et al., (2011) on NTF situated in the upper crystalline crust at depths shallower than 20 km. Moradi et al. proposed a vertical dip fault plane for the NTF.
The goal
In this study, the seismic activity of the NTF from historical earthquakes until 2020 is investigated. A cumulative slip model has proposed for the creeping segment of NTF near SND, and, the transferred Coulomb stress by this aseismic deformation on the AVD ruptures has computed. Ultimately, the relation between NTF seismic-aseismic deformation activity to SND and AVD, and changes in the rheology of the central segment of NTF near SND after AVD investigated.
Seismicity along NTF and NMF
As mentioned before, earthquake locations by IRSC seismic network in NW Iran started in 1995. However, the IRSC catalog is poor until the end of 2005 due to data quality and sparsely distanced stations (>50 km). There are no earthquakes in the distance of 5 km to the NTF from 1995 to the end of 2005. From 2006, earthquake monitoring has improved in this region in terms of location accuracy and magnitude completeness.
The well-located earthquakes in the distance of 5 km to NTF and NMF are selected from 2006 until before the AVD and from AVD until November 2019 TKC in the IRSC network. These sets are recorded in >=5 stations, with location errors of <5 km, RMS of <0.5 s, and azimuthal gap of < 270°. The first set contains 512 earthquakes in which 45 of them have magnitudes 2.5 and higher. I infer that the central segment of NTF near SND has a long quiescence from 2006 until the occurrence of AVD. However, the two nearby segments show high seismicity (Fig. 2a).
After AVD, 69 M>=2.5 earthquakes are in 5 km distance to NTF and NMF (IRSC catalog) with a distributed seismicity and their most concentration on the NMF (Fig. 2b; Table S3). However, the two relatively large events (3.7<M<4.0) are located in the central NTF near SND (Fig. 2b).
The cumulative scalar seismic moments of the earthquakes that occurred after 2006 until the 2012 AVD, and after that until TKC, is computed to investigate the seismic energy release behaviour along the NTF and NMF. The cumulative scalar seismic moment plot of the first set shows two peaks in the Eastern and Western lobes of central NTF that matches to SND, and one on the NMF (Fig. 2b). However, the central NTF itself was almost silent.
For earthquakes that occurred on NTF After AVD, the cumulative scalar seismic moment plot shows a peak in the middle of the central NTF. However, the previously observed peaks of the scalar seismic moment on its lobes are disappeared (Fig. 2b). There is also a relatively wide peak on the NMF, North of Urmieh Lake, where the high seismicity was also observed before AVD.
Creeping segment of NTF near SND
Creeping behaviour is mainly related to frictional strength of the fault zone material, depending on lithology, temperature, and pore-fluid pressure (i.e. Avouac, 2015, Khoshmanesh & Shirzaei, 2018). The magmatic activities reported in a GPS study by Su et al., (2017) near BA, known thermal springs there, and absence of seismic activity from 2006 until 2012 AVD on the central NTF near BA, propose that the existing heat due to the SND magma chamber decrease the effective normal stress on this segment of the fault by increasing the pore-fluid pressure on the fractured fault area and consequently cause it to creep. The resulting different deformation rates along NTF segments is probably the reason for the observed two peaks of high cumulative scalar seismic moments on both lobes of central segment of NTF near SND.
To prove this idea, a cumulative aseismic deformation on the central segment of NTF is estimated. This segment didn’t rupture since the 1721 M7.6 historical earthquake and the GPS study by Djamour et al., (2011) and Rizza et al., (2013) suggest right-lateral deformation of 7mmy-1 for this segment. A maximum slip of 0.007*(2012-1721) =2.04 meters is expected for this segment until the AVD. This segment has a strike/dip of 300°/90° and covers the silent central segment of NTF with a length of ~30 km and locking depth of ~20 km that is suggested by Djamour et al., (2011). I stress that Rizza et al., (2013) suggested a relatively unchanged deformation rate on NTF since 65 ka. If I fairly pose that only half of this deformation has happened in creep mode and the other half is locked (and may rupture in a future earthquake), the slip model will have a maximum value equal to 1.02 m. Later in section 4, I will explain that even considering the smaller contribution of creep in the whole deformation (i.e. 25%), the transferred stress field by the creeping segment will be high enough to trigger the AVD. An elliptical slip patch (i.e. Ruiz & Madariaga, 2013; Momeni et al., 2019) is proposed with a max slip of 1.02 m at the center and with a Gaussian distribution of slip and half of the max slip equal to ~51 cm on the borders, where high seismic activity is observed (Fig. 2a). The resulting source has a scalar seismic moment of ~2.0*e+19 Nm equal to an Mw6.8 earthquake. For a fully creeping slip model, the source will have a scalar seismic moment of ~4.0*e+19 Nm equal to an Mw7.0 earthquake.
The resulting deformation may transfer positive Coulomb failure stresses on the nearby faults. I note that AVD has occurred in the same longitude to this creeping segment, suggesting that the right-lateral deformation does not fully release on NTF and probably some of it transfer toward North, as also suggested by Donner et al., (2015) by geological investigations.