Microseismicity and seismotectonics of the North Tabriz fault (Iran)
Research highlights
► The North Tabriz fault is seismically active. ► Focal mechanisms show right-lateral strike-slip motion. ► The North Tabriz fault is rather simple. ► The North Tahgbriz fault is probably mature.
Introduction
The city of Tabriz is the fourth largest city of Iran and comprises ~ 1.4 million inhabitants. During the past 20 years it has grown by about 50%, and it is now one of the largest industrial cities in Iran. Tabriz is located on a large and active tectonic fault which belongs to the complex system that connects the North Anatolian fault system, located in Turkey, to the Alborz in Iran. It accommodates both the northward motion of Arabia and the westward motion of Anatolia relative to Eurasia, both of which occur at 20–30 mm/year. Tabriz was founded in the 3rd century AD by the Sassanids, and it was the capital of Azerbaijan for a long time and the capital of Iran several times between the 14th and the 16th century, before the capital was moved to Ghazvin. Because of its old and continuous record of history, we know that Tabriz experienced several destructive earthquakes in the past.
Instrumentally recorded seismicity is rather imprecisely located in this area. Teleseismic locations, based on the comparison to “ground truth” locations in Iran, is accurate to about 15 km for a magnitude completeness of Mw ≥ 5.5 (Engdahl et al., 2006), which is not sufficient to study seismicity at the scale of an active fault. In 1995, the Institute of Geophysics of the University of Tehran (IGTU) installed a seismological network of 8 stations around Tabriz to monitor the seismic activity. However, the spacing between stations, which is about 50 km, limits the number of precisely located earthquakes and those with reliable focal mechanisms.
In 2004 we installed and operated for approximately 3 months a dense network of 30 seismological stations around the city of Tabriz, with a spacing of 10 km, to complement the permanent IGTU network. We recorded 80 local events and were able to compute 22 focal mechanisms (Table 2). This paper presents the results from this temporary network and updates results of the permanent network.
Section snippets
Tectonic setting and historical seismicity
The present-day tectonics of Iran (Fig. 1) is mainly the result of the motion between the Arabian and Eurasia plates, which at the location of Tabriz converge at a rate of ~ 20 mm/year (Vernant et al., 2004). This convergence is distributed between the shortening across the Zagros mountains, the internal deformation accommodated by the large strike-slip faults in Central Iran, and the shortening of the Alborz mountains. To the west, Anatolia moves westward relative to Eurasia, at a rate of about
Data and procedure
From April 27 to July 24, 2004, we installed a temporary network of 30 seismological stations in the area of Tabriz (Fig. 2). These stations were complemented with 10 stations located within the city of Tabriz to study the variability of the ground motion and site effects within the city. The sensors were Mark-Product 2 Hz L22 3-component velocimeters. The recorders were Agecodagis Mini-Titan 3XT recording continuously at a rate of 125 Hz. The time was calibrated with a GPS signal every 3 h.
Earthquakes locations and focal mechanisms
The 1171 events (selec1), which we expect to be located with uncertainties of 20 km are spread out of the North Tabriz fault sensu stricto (Fig. 4). It is difficult to associate them with any specific fault or to infer possible blind faults. They attest to deformation throughout this area and the release of seismic energy over a broad region and especially north of the North Tabriz fault. We observe, however, that most seismicity is located near the NTF. Moreover, dense clusters are located on
Cross-sections
We plot sections across the NTF to examine its geometry with depth (Fig. 7). Sections are drawn to strike locally perpendicular to the fault, but because some segments of the fault are not linear, it is difficult to choose an azimuth with a robust logic. After many tests, we decided to plot 3 families, of 4 sections each, striking locally perpendicular to the fault system (Fig. 4). In the western part, they strike perpendicular to the Mishu and Sufian faults, in the center part perpendicular to
Discussion
The North Tabriz Fault constitutes only one part of the large complex fault system which transfers part of the motion from the North Anatolian and East Anatolian faults to the east, toward the Alborz and the Zagros. Our maps and cross-sections show a relative simple pattern for the selected seismicity, consistent with a system of faults starting with the GSK fault to the NW, the NTF in the center, and the N. Bozghush fault to the SE. All these fault segments seem to dip almost vertically, which
Conclusion
The task of this study was to improve our knowledge on the North Tabriz Fault that crosses one of the largest city of Iran which suffered of several destructive earthquakes of magnitude larger than 7 in the historical time.
Indeed, we confirm that the NTF is associated with microearthquake seismicity. Because the pattern is rather simple, the seismicity seems to be associated with the fault itself, and not a response to stress in the region surrounding the fault.
Hessami et al. (2003) compute a
Acknowledgments
We thank A. Bitarafan, M. Massoudi, A. Paul, and M. Zolfaghari who participated in the field work. We thank F. Tavakoli for providing the NCC lodging and P. Molnar for suggestions. Constructive reviews by B. Engdahl and an anonymous reviewer contributed to improve the manuscript. This work has been supported by Insu-CNRS program Dyeti, by the French Embassy in Tehran, and by IIEES. The seismological stations belong to the French Lithoscope pool. A. M. benefited from a fellowship of the French
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