Paleoseismological and morphological evidence of slip rate variations along the North Tabriz fault (NW Iran)

International audienceNorthwest Iran is characterized by a high level of historical and instrumental seismicity related to the ongoing convergence between the Arabian and Eurasian plates. In this region, the main right-lateral strike-slip fault known as the North Tabriz fault (NTF) forms the central portion of a large crustal fault system called the Tabriz fault system (TFS). The NTF is a major seismic source along which at least three strong and destructive earthquakes have occurred since 858 AD. The two most recent destructive seismic events occurred in 1721 AD and 1780 AD, rupturing the SE and NW fault segments, respectively. This paper reports paleoseismological and quantitative geomorphologic investigations on the SE segment of the NTF, between the cities of Bostanabad and Tabriz. These observations help to improve our understanding of the seismic hazard for Tabriz city and its surrounding areas. Our field investigations revealed evidence of successive faulting events since the Late Quaternary. Paleoseismic investigations indicate that since 33.5 kyr, the SE segment of the NTF has experienced at least three major (M>7.5) seismic events, including the 1721 AD earthquake (M=7.6–7.7). Along the NW segment of the fault, however, our results suggest that the amount of strong (M~7.5) seismic events during the same period is significantly greater than along the SE segment. One possible explanation of such a difference in seismic activity is that the Late Quaternary-Holocene coseismic slip rate is decreasing along the NTF from the northwest to the southeast. This explanation contradicts the former hypothesis of a constant slip rate along the whole length of the NTF. In addition, more distributed deformation along several parallel fault branches, in a wider fault zone of the SE segment of the NTF may be considered as additional evidence for the estimation of lower rate of deformation along the fault segment. Such a slip distribution pattern can explain the existence of smaller (~300 m) Pliocene-Quaternary cumulative dextral offsets along the SE fault segment than the measured cumulative offsets along the NW segment (~800 m) of the NTF

Paleoearthquakes and slip rates of the North Tabriz Fault, NW Iran: preliminary results

The North Tabriz Fault is a major seismogenic fault in NW Iran. The last damaging earthquakes on this fault occurred in 1721, rupturing the southeastern fault segment, and in 1780, rupturing the northwestern one. The understanding of the seismic behavior of this fault is critical for assessing the hazard in Tabriz, one of the major cities of Iran; the city suffered major damage in both the 1721 and 1780 events. Our study area is located on the northwestern fault segment, west of the city of Tabriz. We performed geomorphic and trenching investigations, which allowed us to recognize evidence for repeated faulting events since the Late Pleistocene. From the trenches, we found evidence for at least four events during the past 3.6 ka, the most recent one being the 1780 earthquake. On the basis of different approaches, horizontal slip per event and slip rates are found in the ranges of 4 ± 0.5 m and 3.1-6.4 mm/yr, respectively. We also attempted an estimate of the average recurrence intervals which appears to be in the range 350-1430 years, with a mean recurrence interval of 821 ± 176 years. On the basis of these results, the northwestern segment of the North Tabriz Fault does not appear to present a major seismic potential for the near future, however, not enough is known about the southeastern segment of the fault to make a comparable conclusion

Neotectonic stresses in the Sangchal earthquake (1957) area – Mazandaran

The Sangchal earthquake occurred on July 2, 1957, in the central part of the North Alborz fault zone. The North Alborz fault is an arched shape fault that continued from Lahijan to Gonbad-e Kavos. This fault, along with Khazar, Lalehband, Kandovan and Mosha faults affected by the shortening and rising of the Alborz Mountains. This shortening and uplift accompanied by large earthquake events that continue today. With knowledge of the geometry of faults and how faults are displaced and the use of computational methods for fault analysis and also, the Focal mechanism, direction of maximum tension governing the part of north Alborz fault has been determined. The two methods show acceptable results. In addition, the comparison of the findings with the results of geodetic studies of the scope, accuracy of the findings confirmed. The results of the study show that during the North Alborz Fault, the dominant vector is compressive, and the left-lateral slip is observed. From the central part of the Alborz to the east, the left lateral displacements increases and cause shear mechanism in the eastern parts of the Alborz. This movement has caused compressive earthquakes in the central Alborz and compressive-shear earthquakes in the eastern parts. Therefore, the magnitude of future earthquakes in the eastern part is slightly smaller than the central Alborz earthquakes

Young vertical movements in Qara-Chai basin (Markazi province)

Identification of active geological structures can make a lot of help to identify seismic zones. There are several ways to study and identify structures. Accordingly, the horizontal and vertical movements can be detected in the area. In order to identify the intensity of vertical movement on the part of the Markazi province, morphometry indices in Qara-chai watershed have been studied. Qara-Chai basin, following of Zagros and Central Iran Tectonics. In this study, topographic data, geological maps and geological information were scrutinized qualitative and quantitative. These data along with the streams data in the basin of choice, utilizes various specialized software’s are analyzed. There are many faults, some of which quaternary, and are young, it is a sign of high seismic potential range. Young Quaternary sediments, fractures and faults compliance and various geomorphometry indices surveys to indicate Indes and Koshk-e-Nosrat fault zone in west, south Saleh abad and Yal abad faults, Tafresh fault in north, North West and west of Tafresh city and Talkhab fault in North West of Arak from the perspective of uplift, are active and relatively high seismic potential. According to the relative tectonic activity indices of East and North Markazi province around Ashtian, Tafresh, Farahan and Saveh cities have high tectonic activity. These cities require special planning in the Markazi province. Undoubtedly, geomorphometry is not everything in mobility of Qara-Chai basin. Some fault zone has a strike-slip motion, which requires the use of methods other than geomorphometry indices