mohammadreza abbassi

Investigation of historical and instrumental seismicity and fault kinematics of major faults are used to deduce the stress state in the northeast of Greater Tehran. In the present study, we have identified the Mw 5.1 earthquake and its related aftershocks in northeast Tehran that occurred on May 7, 2020. In this regard, after combining the waveforms of seismograms recorded in the seismic stations of Tehran and neighbouring provinces, the location, magnitude, and exact time of occurrence of the main shock and its six aftershocks have been calculated. Then, using three methods, including waveform modelling, P wave polarity and the ratio of P and S wave amplitudes, the focal mechanism of the fault causing seismic events is estimated. Fault kinematic study and the epicenter of the seismic event and related aftershocks suggest that the Mosha fault could be responsible for the event. Furthermore, the regional tectonic stress field has been calculated by focal mechanism inversion. Comparisons between stress field orientations and stress ratio provide new information on the local stress field. The variation of the stress ratio in the lower and upper crust is considerably high, demonstrating an inhomogeneity of deformation related to the Mosha fault.

The abrupt topographical change in northern Tehran divides the Eocene rock formation from Quaternary and Plio-Quaternary alluvial deposits situated in the piedmont and the plain. This phenomenon has been interpreted differently by two Geologists in the 70's and 80's. The term North Tehran Fault (NTF) is the first interpretation of rock-alluvium boundary, coined by Tchalenco (1975). According to this interpretation consist the NTF of the fault system, arranged in an en-echelon manner, not necessarily forming the rock-alluvium boundary in northern Tehran. A later interpretation, by joining all the fault systems, as a single line, called North Tehran Thrust (NTT). According to this interpretation forms (NTT) a single line, defining the boundary of rock-alluvium in northern Tehran (Berberian et al., 1983), not confusing with North Tehran Fault (NTF). The mentioned two interpretations of faulting in northern Tehran include two different faulting mechanisms opposing each other diagonally. By time are the two interpretations wrongly melted together as (NTF) and were used by several authors without paying attention to its original meaning interpreted by Tchalenko (1975).In later works (Langraf et al., 2009; Ritz et al., 2012) is (NTF) the main structure responsible for the rising of highly elevated rock formation in the hanging wall of North Tehran Thrust (NTT).The present study deals strictly with the boundary of rock and alluvium in northern Tehran, which was called (NTT). Kinematic study along the contact of rock and alluvium revealed two characteristic features: 1- Rock-Alluvium boundary occurs not along with a single faulting trend, it is rather arranged along NW-, N-S, E-W, and NE-striking faults. 2- Obtained stress direction associated with fault plane solution show different directions.In many places is the NTT covered by rock slides, obscuring the trace of the rock-alluvium boundary. Older rockslides are thrusted over alluvium units of different ages. The contact of such boundaries shows striations compatible with present-day stress direction. The slow deformation rate in northern Tehran is not concentrated along a single fault. Therefore, it seems that absorbed deformation in northern Tehran is distributed over a wide range adjacent to the rock-alluvium boundary. These observations suggest an unrecognized fault, which needs more careful geological and seismological study.Considering the three different trends of NTT, namely NW-, E-W- and NE-trending, no fold axis run parallel to those trends. The results obtained in this study suggest the NTT is not a major fault and in addition, it could not be regarded as a single fault responsible for the rising of the rock formation on the hanging wall of NTT in northern Tehran.

Focal mechanism and surface slip data are used to investigate the relationship between kinematic studies conducted on young geological units, especially in trenches of paleoseismolical sites, Late Pleistocene and Holocene in age. It has shown that there is direct relationship between data obtained on surface and seismic data in depth. Kinematic measurements in three trenches for paleoseismological sites on North Tehran Thrust, Mosha fault and Firuzkuh fault are used to calculate the stress tensor. Stress tensors and their associated fault planes and focal mechanism data are compared. Fault planes obtained from calculated stress tensors whose fault planes lie parallel or close to the general trends of Mosha and Firuzkuh faults are in good agreement with focal mechanism. The third trench on North Tehran Thrust displayed insufficient data to obtain an appropriate stress tensor, however the average of strikes, dips and rakes allow to propose a paleofocal mechanism for an area of seismic quiescence. Based on fault plane inversion and focal mechanisms, overall, maximum (σ1~N045±5) and minimum (σ3) principal stresses are subhorizontal and the intermediate principle stress (σ2) is vertically oriented. This is consistent with a dominant strike-slip regime. The Mosha fault (trending N100E, dipping to North) is a left-lateral strike-slip fault with a minor extensional component and joins Firuzkuh fault (trending N60E, dipping to South) as strike-slip fault with a compressional component.

The studied area lies in the Central Iranian plate. Two reasons for the lack of knowledge concerning the recognition of active faulting in this region could be summarized as following: 1- faults with pure vertical motion are difficult to recognize on aerial photos, 2- especially those cutting through rock units. The mapped active fault, Kushk-e Nosrat is an example of uncertain knowledge of its activity. The existing evidence to prove its activity such as seismicity and geologic data are yet unclear, and concerning morphological aspects it contradicts the present-day stress direction. The present-day stress direction dictates a left lateral deflection of drainages, however the drainage pattern is in accordance with NW-directed paleostress. Therefore, it was necessary to examine the activity of Kushk-e Nosrat fault. The current investigation focuses on two specific issues: 1- The characterization of changes in the Plio–Quaternary state of stress using inversions of slip vectors measured on fault planes with a broad range of displacement values; 2- The identification of present- day stress field by inversion of seismically determined by fault slip vectors (focal mechanism of earthquakes); and 3- The verification of the coherence and mechanical compatibility of the inversion results with the geological and structural characteristics of the region illustrating the kinematic significance of each fault system. The central part of Kushk-e Nosrat trending (N100E) is a dormant fault. The evidence for the inactivity are reflected in prevailing stress direction, drainage pattern and Quaternary deposits. The stress direction measured directly on several station on this fault show a NW-directed stress (σ1), which explains the right lateral deflection of drainage pattern crossing this fault. The last evidence of inactivity of these faults is delivered by mapping of geomorphic surfaces. The young alluvial deposits such as Holocene and late Pleistocene in age are not affected by Kushk-e Nosrat Fault. The deflection of drainage pattern crossing the fault is compatible with a NW-directed stress direction. The inversions of slip vectors measured on fault planes has shown the states of paleostress existing on the faults and the inversion of focal mechanisms revealed the present day stress direction as following: The present day state of stress deduced form focal mechanisms is characterized by a transpressional tectonic regime consistent with the ongoing stress state (N20°E trending σ1); The paleostress field was characterized by a regional mean of N140±10°E trending horizontal compression (σ1), and a transtensional tectonic regime. The change from paleostress to modern stress states has occurred through an intermediate stress state. Characterized by a mean regional N-S trending σ1. According to the obtained results, the assessment of earthquake hazard based on Kushk-e Nosrat fault must be revised in term of considering them as active faults; therefore, caution must be paid by taking them into any calculation. Under the influence of NW-directed paleostress, the faults trending nearly E-W acted as dextral faults, thus the present landscape must be interpreted as the result of a relatively recent reversal of stress direction.

One of the most seismically active parts of Iran is Zagros area. The basement-involved active fold-thrust belt of the Zagros in southwest Iran is underlain by numerous seismogenic blind basements thrust covered by the folded Phanerozoic sedimentary rocks. The present morphology of the Zagros active fold-thrust belt is the result of its structural evolution and depositional history: a platform phase during the Paleozoic; rifting in the Permian Triassic; passive continental margin (with sea-floor spreading to the northeast) in the Jurassic-Early Cretaceous; subduction to the north­east and ophiolite-radiolarite emplacement in the Late Cretaceous; and collision-shortening during the Neogene.Besides, there are a lot of different faults in Zagros, for example, the Main Zagros Reverse Fault (MZRF), the Main Recent Fault (MRF), the High Zagros thrust belt, the High Zagros Fault (HZF), and Mountain Front Fault (MFF). This study is focused on the last-mentioned one. The MFF flexure is introduced for the first time by Falcon (1961) and then is presented as the mountain front fault by Berberian and Tchalenko (1976)], which delimits the Zagros simple fold belt and the Eocene-Oligocene Asmari limestone outcrops to the south and southwest. The Mountain front fault (MFF), is a segmented master blind thrust fault with important structural topographic, geomorphic and seismotectonic characteristics. Therefore, the study and recognition of seismic parts of Iran are important. The aim of this study is to determine the focal mechanisms of Mountain Front Fault (MFF) at a latitude of 46 to 48.5 degree in Zagros. Due to the salt layers, large earthquakes rarely reach the surface. In such cases, the seismic method is an appropriate tool to understand the faulting mechanisms. By means of focal mechanisms, it is possible to gain information about the fault geometry and its related mechanism. The data used in this study are from International Institute of Earthquake Engineering, and Seismology (IIEES) and Institute of Geophysics of the University of Tehran (IGUT). Because of some wrong relocation, during this study relocated them to reach a well-relocated data base and better results. Getting the focal mechanism of an earthquake can occur in various ways. In this study, first, the waveform modeling by Isola software was used to find the focal mechanisms. To determine the accuracy of focal mechanism solutions obtained by waveform modeling, the polarity method was used to solve focal mechanisms. Besides, some of these earthquakes have also been reported by CMT. After determining focal mechanism solutions with the stated method, they were compared with CMT, and all the focal mechanism were mapped in the area so that the trend of this part of MFF can be recognized better. Because there are many earthquakes in this area, a reliable decision can be made. By looking at the maps, it is easily understandable that the trend in this part is obviously EW. Finally, the prevailing trend that obtained in the study area is found. Most of these earthquakes are trending EW. The study of 31 focal mechanisms in the area has permitted to constrain the faulting mechanism of MFF.

The study area is a part of the Alpine-Himalayan orogen. It is formed by the Greater Caucasus Mts., the Lesser Caucasus Mts., the Talesh Mts., the Kura Basin, and the South Caspian Basin. Present-day structures of the study area are controlled by the ongoing collision of the Arabian and African plates with Eurasia. The study area extends over 1342 km2 located in Northwest of Iran, which can be regarded generally as the continuation of structural grain of Lesser Caucasus. However, the existence of rigid block of South Caspian Basin and the different lithology of Azerbaijan including Quaternary volcanic masses such as Sahand and Sabalan has resulted in a complex distribution of deformation in the studied area.
Northwest Iran is a region of vigorous inflection, deformation and seismicity situated between two thrust belts namely Lesser Caucasus to the north and the Zagros thrust belt to the south.
What is known about Seismotectonics of the area located in north of Tabriz fault is limited to the recent works done by Masson et al. (2006). Using dense GPS, they determined that the deformation in NW Iran is characterized by ~ 8 mm/yr of right-lateral movement on the North Tabriz fault, and ~ 8 mm/yr of extension within Talesh Mnt. The duplicate Ahar-Varzeghan earthquake focal mechanisms contradicted the GPS results. Although both main shocks have probable fault planes that strike roughly east–west, it is likely that the mapped surface faulting should be associated with the first main shock because field observations record nearly pure strike-slip motion that would be inconsistent with the transpressional mechanism of the second main shock (Ghods et al., 2015). The fault study of Coopley et al. (2013)
introduced three Segments with a length of 400 m. till 8 km. In 2012 August 11 (12:23 UTC), a moderate earthquake with MW=6.4 (USGS) occurred between Ahar and Varzeghan towns in Azerbaijan Province at northwest of Iran, in a region where there was no major mapped fault or any well-documented historical seismicity. In order to solve this problem, Ghods et al. (2015) have introduced a model to resolve this earthquake focal mechanisms and many active fault earthquake rupture in the range of Ahar - Varzeghan.
A combined study of active tectonic parameters such as geomorphic indices and stress tensor measurement, allowed to recognize a new fault in the northern part of North Tabriz Fault. In addition to the seismicity of North Tabriz fault, seismicity map of Azerbaijan shows the same distribution ratio in the NW part of the North Tabriz fault. In the absence of active faults, the relation of this seismicity is not known. This study was an attempt to introduce one of the active faults at the North of the North Tabriz fault in order to improve the understanding of seismotectonic characteristic of this area.
In order to inquire the relevance between rock rigidity and SL index based on a simplified geological map of the area and field observation, rocks were categorized by their resistance as below: Very low (young alluvial deposits), Low (old alluvial deposits, poorly consolidated conglomerates, marl), moderate (gypsum, gypseous marl), High(limestone, sandstone, dolomite, shale, conglomerate, tuff, schist, flysch sediments), Very high (andesites, trakiandesite, gabbro, dacite) (El Hamdouni et al., 2007). Based on the mentioned category, the distribution map of lithological resistance was obtained using GIS. To assess tectonic activities in the area, geomorphic indices such as: the stream length- gradient index (SL) and hypsometric integral (Hi) have been used to reveal vertical active movements along a particular fault, namely Nahand fault. This fault lying in north of Tabriz fault strikes parallel to it with a length of 168 Km. The combination of morphometric studies and kinematic study show clear vertical movement along Nahand fault. Determination of stress tensor and geomorphic indices has shown that the Nahand fault is a right-lateral strike-slip with a minor vertical component. Field observations and the inversions of stress tensor (geologic and seismologic fault kinematics) revealed that the Nahand fault is an active fault, acting in a transpressional tectonic regime, with the Sh-max oriented NW-SE.

Tehran lies on the southern flank of the Central Alborz, an active mountain belt characterized by many historical earthquakes, some of which have affected Tehran itself. It is an arcuate fold and thrust belt, consisting of two major distinct structural trends: a NW-SE trend characterizing the west-central Alborz, and a NE-SW trend marking the east-central Alborz. The steep slopes of the Alborz on its southern flank adjoin the piedmont, which is covered by various units of post-orogenic alluvium. The border between the Alborz Mountain and the Tehran's piedmont (northern part of Tehran city) is marked by the North Tehran Fault (NTF) [1], dividing the Eocene rock formation from the alluvial units of different ages (Early Pleistocene to the recent alluvium). The oldest alluvial formation, named Hezardarreh is a folded and faulted conglomerate forming hills parallel to the mountain front. The age of this formation is difficult to determine accurately, due to the lack of datable features such as pollen, fossils or lithic industry [2]; however, it is mostly assumed to be Plio-Quaternary (the bottom of formation) to Early Quaternary (the upper part of formation). The formation overlying the eroded Hezardarreh Formation is very heterogeneous in composition, in particular to the north of Tehran and has been considered to be Mid-Pleistocene. The next following formation is widespread deposits named the “Tehran alluvium” have been assigned to Late Pleistocene [3]. The youngest mapped unit in Tehran City overlaying the Late Pleistocene alluvium is Holocene deposit, which is divided in younger part (4000 to 5000 years BP) and an older unit in age of 12000 years (BP) [4]. The assessment of seismic hazard depends upon the understanding of activity, mechanism and trends of faults affecting an area. The rapid urbanization of Tehran with remarkable concentration of people needs to be studied by new studies considering all possible active fault trends. The E-W-trending active faulting in Tehran is delineated by their morphological features recognized on aerial photos of 1955. This study examines the NW-trending faults as a possible seismic source. The measurement of 56 fault planes in 13 outcrops have shown that the morphological features associated with NW-directed faults are preserved in middle and late Pleistocene deposit of Tehran's piedmont. The length of these faults varies between 2.6 to 6.5 Km arranged in an en-echelon manner. The compressive faulting mechanisms of these trends are in accordance with the NE-directed present day stress direction. Morphological features of the NW-directed faults are controversial, because they appear as normal and compressive on aerial photos and in the outcrops. It means that the faulting mechanism in Tehran's piedmont cannot be explained by a single stress direction. In fact, two different stress directions have affected the alluvial plain, namely an older NW-directed prior to the NE-directed one, which explains the normal faulting mechanism along the NW-striking faults and was mapped as old normal fault, e.g. [5]. Thus the NW-trending faults are inherited fault affecting the Early and Mid-Pleistocene deposit as normal faults. According to the present day stress prevailing in the South Central Alborz obtained by P-axis of focal mechanisms and fault slip data in relative young alluvial deposits [6-7] a NE-directed stress explains the faulting mechanisms of active faults in this area. Although there is now field evidences proving the fault activity of NW-striking faults but the present day stress (NE-directed), it is reasonable to assume seismic activity along these faults.

Mosha is one of the most important and threatening faults in TehranMegacity (Capital of Iran). Instrumental seismology in the region was limited to insufficient data along with location errors especially in depths as well as the small number of available focal mechanisms in bound with the trends in Hedayati et al. (1976) and Ashtari et al. (2005). In the study ahead, by installing 48 local and temporary seismological stations during June to November 2006, micro earthquakes around the eastern part of the southern flank of central Alborz particularly the Mosha fault zone were recorded and processed. The local temporary network consisted of 24 one-vertical-component TAD-2 Hz, 11 3-components MiniTitan- 5 S and 13 3-components Guralp 6TD 0.02-10 S sensors. Sampling rates were 100 samples/sec the for Guralp sensors and 125 samples/sec for the others in continuous and triggering threshold modes. 115 well recorded micro earthquakes with an appropriate azimuthal gap (Gap ≤ 180°), a trivial residual timing and location errors (RMS ≤ 0.3 sec, Erh ≤ 2 km and Erz ≤ 3 km) were selected and applied for the wave velocity ratio (Vp/Vs) calculation based on Wadati (1933) and Chatelain (1978) approaches (1549 P-wave and 1495 S-wave arrival times). A 1-D model of the upper crustal velocity structure was determined as well. SEISAN software (Havskov and Ottemöller, 2005) for phase readings, Hypo71 (Lee and Lahr, 1975) Hypocenter (Barry, 1994.) for seismic event locations, VELEST (Kissling, 1988) for a crustal velocity layers model and FOCMEC program (Snoke, 2003) for focal mechanism solutions were used. Four layers at the depths 3, 7, 16 and 24 km of the crust were determined with P-wave velocities of 5.4, 5.8, 6.1 and 6.25 km/sec, respectively. Accurate locations of 553 micro earthquakes and 15 A and 31 B (excellent for A and good for B groups in red and blue colors in related figures respectively) classes of focal mechanism solutions of the reliable micro earthquakes with a high quality of P-wave first arrival polarities (more than 8 Pg onset’s signs), were provided for the possible analyses of seismicity, the fault geometries-movements and seismotectonic interpretations. We have found that the Eastern part of Mosha fault, longitudinally located from 51.7° to 52.5°, has a northward high dip angle and complex focal mechanisms. The fault mechanisms varied from thrust, strike slip with a small reverse component to reverse with a small normal component from the West to the East. From grouping analysis of the focal mechanism P (or T) axes, the strikes, N 40 (or N 130) were derived for the compression (or tension) stress direction approximately. The focal mechanisms accompanying with the geodynamic analyses from GPS measurements in the studied area reveal a slip partitioning in the local and regional scale compatible with some conclusions from the previous studies (Ritz et al., 2006 and Tatar et al., 2007). Although micro and large earthquakes nonlinearity relation in stress axes orients as true or not proved is important (Mercier et al., 1991 and Hatzfeld et al., 1999), seismotectonics strain analyses of the micro earthquakes in the studied area show the same results of the large earthquakes stress analyses in stress inversion method by Gillard and Wyss (1995). In addition, this study has demonstrated a seismic active trend as mentioned by Jajrood-Pardis-Absard in the South of Mosha fault. Concentrated seismic activities around Mosha fault in the time of data recording have shown that it is a potential hazard for the studied area.