The Geological features of the Torkmanchai NW Iran Earthquake on November 8, 2019 (Mw=5.9)
The Geological features of the Torkmanchai NW Iran Earthquake on November 8, 2019 (Mw=5.9)
Dr. Shahryar SOLAYMANI AZAD, Cyrus ESMAEILI, Dr. Mahasa ROUSTAI, Dr. Sanaz VAJEDIAN, Abdolhamid SARTIPI, Taher KHOSH ZARE, Hamid Reza RAJAB ZADEH
On November 8, 2019, in NW Iran, an earthquake of magnitude 5.9 (Mw) devastated the northwest of Mianeh City (in Torkamanchai Area) with extensive damage, especially in old structures. Based on historical evidence, this area had been experienced another earthquake in 426 years ago (1593 AD, M=6, I0=VIII). The day after the quake, a team of GSI Northwest branch and the Seismotectonics and Seismology Department of Geological Survey of Iran started surveying geological features of the event.
Our preliminary field work, focal mechanism solutions, the distribution of severely damaged villages, InSAR analysis, and the pattern of aftershock locations could be indicated that the causative faulting zone (as the southern sector of a longer active fault zone) extends ~10 km with NNE-striking and left-lateral movement between Helemsi (to the south) and Ardeha (to the North) villages within E-W Bozghush mountain range. Distribution of concentrated-dynamic surface and slope instabilities and ground deformation, regional and local Remote sensing data; satellite imageries, photogrammetry and Drone study are also confirming this suggestion. No evidence of complete surface faulting and also sudden decrease in the number of aftershocks (especially since 4 days after the main shock) for this moderate-shallow earthquake "could be" suggested that; this event is not yet complete. Up to relaxation of the lithosphere during the time, it is therefore an important note to scientists and Rescue responsible and officials.
Within the Alpine-Himalayan orogenic belt, NW Iran is located in the central portion of a convergence zone between Arabian (to the south) and Eurasian (to the north) lithospheric plates (e.g. Jackson and McKenzie., 1984; Solaymani Azad et al., 2019). This active deformation domain manifests as N-S shortening and E-W extension related to faulting, earthquakes and Cenozoic-Quaternary volcanic phenomena (e.g. Karakhanian et al., 2002 and 2004; Solaymani Azad et al., 2009, 2015 and 2019; Faridi et al., 2017) (Figure 1). According to geodetic GPS measurements, the convergence occurs with a rate of about 20-30 mm/yrs (e.g. Vernant et al., 2004; Reilinger et al., 2006). The GPS observations and Paleoseismological features of NW Iran reveal that the amount of slip rate is about 6-8 mm/yr for active dextral faults (e.g. Hessami et al., 2003; Vernant et al., 2004; Reilinger et al., 2006; Solaymani Azad, 2009 and 2015; Djamour et al., 2011). The main concentration of right-lateral strike-slip faulting movements is detected along the North Tabriz fault and its continuation to the west (Gailatu-Siah Cheshmeh-Khoy and Chaldiran faults) (e.g Solaymani Azad et al., 2009 and 2015; Djamour et al., 2011). Structurally, the main NW-striking North Tabriz dextral fault has two E-striking terminations, containing oblique east-striking reverse-dextral faults along with Misho and Bozghush mountain ranges to the west and east, respectively (Solaymani Azad et al., 2015). All these faults have been associated with historical and instrumental earthquakes (e.g. Berberian, 1997; Solaymani Azad et al., 2009). Moreover, the faults such as Maragheh (Solaymani Azad et al., 2009 and 2019 and Taghipour et al., 2018), Nahand (Faridi, 2006; Solaymani Azad et al., 2009) and Khoja (Faridi, 2006; Solaymani Azad et al., 2019), Herzandat and Bilverdi (Faridi, 2017b) have also been recognized in NW Iran that could play an important role in active deformation of Iranian Azerbaijan. Within eastern termination of the North Tabriz fault and in the Bozghush mountain, there are however some NNE-striking sinistral active seismogenic faults (such as; Garmachay or East Bozghush faults) (Solaymani Azad et al., 2009 and 2019; Faridi et al., 2017 a, and b) intersects the E-striking oblique-dextral faults (Figure 2). This seismic event contains an affected ~NE-striking area between northern and southern flanks of the East Bozghush range and then it has no direct relation to the North Tabriz fault. According to the Mezoseismal zone of the quake, interferometry analysis and our field observations and…, in this preliminary report, we suggest a NNE-striking documented fault zone (Solaymani Azad et al., 2015; Faridi et al., 2017) as the causative fault of this moderate seismic event. In the mapping of Faridi et al., 2017, this NNE-striking fault is named Shalghun-Yelimsi (Helemsi) fault.
Figure 1. Active fault map of NNW Iran and its surrounding areas (adopted from Solaymani Azad et al. 2019). The GPS velocity vectors and instrumental earthquakes are from Reilinger et al. 2006 and USGS Earthquake Catalog, respectively. The black rectangle shows the location of the Bozghush area, at the eastern termination of the North Tabriz fault (NTF). For more details refer to Solaymani Azad et al. 2019.
Figure 2. Active faults of Bozghush Mountain at the eastern termination of the North Tabriz fault (Solaymani Azad et al., 2015). Note to the NE-striking faults which crosscut the E-striking oblique reverse-dextral faults. The black rectangle shows the location of our study area. For more details refer to Solaymani Azad et al., 2015.
Figure 3. NNE-striking valleys (bracketed by red arrows) within eastern Bozghush Mountain, which have been formed by active faulting. The main one is located between Ardeha and NE of Helemsi villages along meizoseismal zone. Compare with figure 2. For more details see the text.
Field study and observations
The major destruction of the quake occurred in Varankesh, Dastjerd, Helemsi, Khalifehlu, Cheran and Bloukan villages, which are located too close to the causative fault zone. Most of the damages are confined to heavy and old mud-stone structures (reveals; I0=VII+ to VIII in modified Mercalli; similar to the historical earthquake of 1593 “e.g. Ambraseys and Melville, 1982; Berberian, 1997”). According to our comprehensive field investigations in three NNE-striking faulted valleys of Helemsi, Varankesh and Khalifehlu as well as the radar interferometry analysis (e.g Roustaie, 2019; Vajedian, 2018 and 2019; Valkaniotis, 2019: Figure 4), there is no evidence of complete surface faulting within the Mezoseismal zone of this moderate and shallow earthquake.
Figure 4. The radar interferometry analysis of the earthquake (Mw=5.9, D~10-17 km); Geomatic Dept. of GSI: Roustaie, 2019; Valkaniotis, 2019; Vajedian, 2019. Note to the NNE-striking smooth discontinuity and aftershocks distribution along Ardeha-Helemsi valley. In this research, we have been studied the faulted valleys by using field and Drone studies. Based on our study, there is no clear evidence of complete surface faulting within the Mezoseismal zone of this moderate and shallow earthquake.
In the study area, the secondary failures correspond mostly to some dynamic surface and slope instabilities (e.g. centimetric cracks, rockfalls, landslides, stone jumping features, and change in spring water-colors), which have been concentrated within the NNE-striking meizoseismal zone, especially on the hanging-wall of the causative fault zone (Figure 5). According to our field study, all these surface deformation features concentrated in north of Helemsi and Varankesh villages, to the south and Ardeha and Shalgoun villages, to the north (coherent with the NNE-striking lineaments in the radar interferometry analysis, destroyed villages and aftershocks distribution pattern and the sinistral plane in the focal mechanism solution) which correspond to the NNE-striking faults in Eastern Bozghush (Solaymani Azad et al., 2015) and the previously mapped, sinstral Shalgun-Yelimsi Fault (Faridi et al., 2017). In this short technical note, we have presented some examples of the mentioned features (Figures 6 and 7).
Figure 5. Within the study area, the secondary co-seismic surface failures (along the yellowish dashed traces) have been concentrated along the NNE-striking faulted valleys, especially on the hanging-wall of the causative fault zone (Shalgun-Helemsi or Yelemsi Fault zone; the red dashed line and arrows).The figure adopted from Faridi et al., 2017a).
Figure 6. Some examples of dynamic surface and slope instabilities such as stone jumping, rockfalls concentrated along the yellow traces in the figure 5.
Figure 7. Since the quake and along the NNE-striking valley of Helemsi, the color of some springs has changed.
Helemsi (Yelimsi) faulted valley
The fault structures of the NNE-striking valleys within the Bozghush Mountain have previously mapped by Solaymani Azad et al., 2015 and Faridi et al., 2017 (see figures 2 and 5). These faults contain mostly sinistral active faults, which crosscut the ~E-striking oblique-dextral faults of eastern termination of the North Tabriz fault. Based on Faridi et al. (2017), the Shalgoun-Yelmsi or Helemsi fault (as the southern portion of Aghmiun and East Sarab NNE-SSW faults) act as sinistral active fault and cut the Bozghush range. The total length of the fault zone reaches to ~90 km. Morphologically, the left-lateral offset feature of cumulative faulting (hundreds of meters) is visible in both northern and southern mountain fronts of the E-striking range (see figures 2 and 5). Along the faulted valley, the eastern Bozghush range has been rotated in an anti-clock wise direction to its central portion and then, the faulted valley is generally separated the rotated East Bozghush from its western part (see figures 2 and 5). Along its southern part and close to Helemsi village, an outcrop of the NNE-striking fault is visible (Figure 8).
Figure 8. The NNE-striking sinistral Shalgun-Yelimisi Fault, which is located at the base of Helemsi valley.
In addition, no evidence of complete surface faulting and also sudden decrease in the number of aftershocks (especially since 4 days after the main shock) for this moderate-shallow earthquake "could be" suggested that; this event is not yet complete. Up to relaxation of the lithosphere during the time, it is therefore an important note to scientists and Rescue responsible and officials.
We are grateful to Aram Fathian Baneh and Dr. Mohammad Faridi for their insightful comments and discussions.
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