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The Iranian Plateau has been subjected to destructive earthquakes throughout its history. Reliable assessment of the seismic hazard in this earthquake-prone region is therefore essential. Our study focuses on estimating the maximum earthquake magnitude as one of the main parameters of seismic hazard analysis. We implemented two quantitative approaches, namely, probabilistic and deterministic. The probabilistic method allows combining the historical (i.e. incomplete) and the instrumental parts of a catalogue with different levels of completeness and considers the uncertainties in earthquake magnitude determination. In this study, we used a unified, declustered, and complete catalogue of earthquakes in Iran, covering the period from the fourth century BC to 2019. We calculated the maximum possible magnitudes for hundreds of grid points by using the seismicity data in a 200-km radial region around each grid point. The maximum possible earthquake was observed to vary between 6.0 and 8.2, and the highest values were found in the Alborz-Azarbayejan seismotectonic province, Kopeh-Dagh, central east Iran, Makran, and the southeast Zagros. The lowest mmax values were found in the Persian Gulf, Arabian Platform, Esfahan-Sirjan region, and the Dasht-e-Kavir Desert in central Iran. As a second part to this study, we calculated the maximum credible earthquakes for 1103 identified major faults by using five empirical magnitude-scaling relationships. Our results were consistent with both the observed earthquakes and the seismic potential of the various seismogenic zones of Iran. The study results can be used in future seismic hazard analyses and have fundamental implications for mitigating seismic risk in Iran. [ABSTRACT FROM AUTHOR]

The 2017 Mw 7.3 Iran–Iraq earthquake occurred in the seismically less active northern Zagros Mountains. In this study, DInSAR technique is used to derive the surface displacements of the earthquake using Sentinel-1 and ALOS-2 images. One preseismic, three coseismic and a postseismic interferogram are generated. The coseismic interferograms did not show any signs of a surface rupture and indicate a maximum of 80 cm uplift and 50 cm subsidence. The zenith total delay maps derived from the Generic Atmospheric Correction Online Service for InSAR are used to correct tropospheric delays in the interferograms. After correction, an error of 0–2 cm is observed in the line of sight measurements. The preseismic displacement reveals an indication of ground motion before the earthquake occurrence. The postseismic phase indicates afterslip after the earthquake. Localized displacements corresponding to landslides are observed on the mountainous slopes. The 3D displacements reveal ∼40 cm of surface motion suggesting westward movement and reverse faulting. [ABSTRACT FROM AUTHOR]

This study is a revised inventory of landslides triggered by the Rudbar-Manjil earthquake (Mw = 7.3), 1990, obtained through the study of Komak Panah and Hafezi Moghadas (1993), interpretations of the stereoscopic aerial photos, and field data. We mapped the 223 coherent landslides with a total area of 11.3 km2 and a calculated volume of 3.8 × 108 m3 using a digital elevation model (DEM, 12.5 m) within the geographical information system (GIS) software. The correlation between landslide occurrence (size and distribution) and the controlling factors was analyzed using the bivariate model and the linear automated modeling (LINEAR) procedure in the SPSS software. Both LINEAR models of landslide volume (LV, m3) and landslide area (LA, m2) indicate the distance from the fault surface rupture is the most influencing factor on size prediction of coseismic landslides. The LA is more dependent on seismic factors, whereas the LV is on the lithology and topographic factors. The result of principal component analysis (PCA) model, using the GIS, showed that most landslides occurred in the red band (including 95% of the variance of controlling factors). Thus, the selected factors to investigate the landslide occurrence are valid and the PCA can be used to identify regions prone to coseismic landslides. The curves of landslide number density (LND, landslides/km2), landslide area percentage (LAP, landslide area/total area%), LA, and LV have a multimodal distribution (several patterns of response) by the topographic and seismic factors. It is recommended that other models be applied to better investigate the local effects of factors on the coseismic landslides. [ABSTRACT FROM AUTHOR]

We provide a comprehensive earthquake catalogue for Iran that is suitable for detailed seismological analysis and seismic hazard assessment. Based on the work by (Mousavi-Bafrouei et al. Ann Geophys 57:S0653, 2014) and new sources of data, we extend the uniform Iranian earthquake catalogue to include events from fourth century B.C. to December 31, 2018. Conversion relationships between moment magnitude (Mw) and body wave, surface wave, Nuttli and local magnitudes are presented and used to estimate Mw based on a priority system. The updated earthquake catalogue includes 25,937 earthquakes with Mw between 2.6 and 8.1. Catalogue declustering, using the method of conjugate windows, results in 1772 clusters with 17,123 events with Mw between 2.9 and 8.1. We also evaluate magnitude of completeness and seismicity parameters including the Gutenberg-Richter b-value, mean annual seismic activity rate and maximum expected magnitude in five seismotectonic units. Overall, the Zagros unit has the highest b-value among all units, whereas the Makran unit shows the lowest b-value indicating possibility of the largest range of magnitudes in the region. [ABSTRACT FROM AUTHOR]

The active tectonic deformation and hazardous earthquakes in the south and west of the Lut block have been investigated for a long time. In this study, we compute the geodetic and seismic strain rates using focal mechanism data from the Harvard CMT catalogue and various other sources including the published GPS velocities. Moreover, we also perform Focal Mechanism Stress Inversion (FMSI) to deduce a stress model for the region. Our study shows an expected correlation between the stress orientations, seismic and geodetic strain rates. Our results show that the south and west of the Lut block is generally exposed as a compressional strike-slip tectonic regime. The tectonic convergence in this area is taken up not only by motions along and across the faults but also by the rotation of those blocks which bounded by these faults. The maximum amount of rotation rate is observed where there are the main right lateral strike slip fault systems such as Sabzevaran, Gowk, Nayband, Bam, Kuhbanan, and Kahurak. The orientation of the mean stress direction, obtained from the FMSI results in the west and south of the Lut block, is approximated ~N19 E. In this area, faults are almost oblique relative to the tectonic motion direction. Moreover, there are right-lateral and left-lateral shears, in addition to the dip movements in different parts of the south and west of the Lut block. Our analyses show three main categories of the stress regimes including strike-slip faulting (43.2%), thrust faulting (38.6%), and unknown or oblique faulting (18.2%). We also calculated seismic and geodetic moment rates for this area, which indicate the seismic moment rate is relatively high between Bam and Shahdad where there are some segments of the Gowk fault. • We analyzed seismological and geodetic data to characterize seismic and a-seismic deformation in the west and south Lut block. • Crustal strain rate, stress and moment rates are presented and linked to the existing faults. • In the southwest of Lut block the ratio of seismic/geodetic moment rate is high where there are segments of the Gowk fault. [ABSTRACT FROM AUTHOR]

On 2012 August 11, an earthquake doublet (M w6.5 and M w6.3), separated in time by 11 min, occur in the northwest of Iran. The hypocentres of these earthquakes are close (∼6 km) and located near the cities of Ahar and Varzaghan. The rupture process of both main shocks is retrieved by inverting the near-field strong motions data and using the elliptical subfault approximation method. Our calculations show that the two earthquakes are occurring on two distinct fault planes: the first main shock (M1) has nucleated at a depth of ∼8.5 km, and is located ∼4 km east of the eastern termination of the E-W trending surface rupture. The slip reaches the ground surface west of the hypocentre on an E-W striking fault (N88°E) that dips almost vertically (80°S). This earthquake exhibits a right-lateral strike-slip mechanism. The entire slip is imaged on a single patch that ruptures with an average speed of 2.4 km s−1. The rupture duration is ∼5.6 s and the earthquake releases a seismic moment of ∼8.41E + 18 N·m. The slip reaches the surface with a right-lateral dislocation value of ∼1 m, which is consistent with the observed surface rupture. About 11 min later, the second main shock (M2) nucleates ∼5 km to the west and 4 km to the north with respect to the hypocentre of the M1, and at a depth of ∼16.5 km. The M2 rupture evolves toward shallower depths and to the west on an ENE-WSW oriented fault plane (strike ∼256°) with a dip of ∼60° northward. The slip is essentially distributed on two distinct patches with strike-slip and reverse mechanisms, respectively. The first patch has a pure right-lateral strike-slip mechanism, and ruptures at a relatively fast speed of over 2.8 km s−1, and last for about 2.6 s until it reaches the second patch. The latter has a reverse mechanism (rake∼112°) and extends the rupture toward shallow depths, and to the west at a speed of ∼2.5 km s−1, and its rupture lasts for ∼2.5 s. The top of the slip distribution of M2 stops at a depth of ∼8 km. We observe that aftershocks surround the M1 and most of the M2 slip models. They are not distributed in the region of high slip (∼3.1 m) of M1. We show that the rupture of M2 is controlled by the static Coulomb stress changes caused by M1, with the maximum slip of M2 located in the positive Coulomb stress caused by M1. The M2 rupture stops where it reaches the area of high negative Coulomb stress change (over −10 bars). The cumulative Coulomb stress fields of both main shocks show a transfer of positive static Coulomb stress change of >0.1 bars on the eastern segment of the North Tabriz Fault. This segment did not rupture since the 1721 M∼7.6–7.7 event that has destroyed the city of Tabriz, and that currently hosts 2 million people. The occurrence of this earthquake doublet with different mechanisms reveals the slip partitioning of the oblique convergence regime of NW Iran on the Ahar–Varzaghan complex fault system. [ABSTRACT FROM AUTHOR]

Geometric and seismic parameters of the Qoshadagh Fault (QDF) were investigated to evaluate seismic hazard along this fault, which consists of three segments. The central E–W striking, dextral-reverse segment is the longest and terminates at both ends into NW–SE striking splay arrays. Both eastern and western splay arrays form locally transtensional bends. Paleoseismic data obtained from three excavated trenches across the fault combined with dated offset geomorphic features revealed that the central segment experienced at least 5 surface rupturing earthquakes during the past 2.5 ka, with maximum moment magnitude of Mw = 6.8 ± 0.2. The mean recurrence interval for the identified paleoearthquakes is 452 ± 143 years (±2σ) and the calculated amount of slip per event is ca. ≈0.85 m. These results imply that the QDF slips at an average rate of 1.9 ± 0.1 mm yr–1 for over the past 2.5 ka. The obtained values define the seismic behavior of the fault and are essential to remediate ensuing seismic risks. [ABSTRACT FROM AUTHOR]

An intraplate earthquake doublet, with 11-min delay between the events, devastated the city of Varzeghan in northwestern Iran on August 11, 2012. The first Mw 6.5 strike-slip earthquake, which occurred after more than 200 years of low seismicity, was followed by an Mw 6.4 oblique thrust event at an epicentral separation of about 6 km. While the first event can be associated with a distinct surface rupture, the absence of a surface fault trace and no clear aftershock signature makes it challenging to identify the fault plane of the second event. We use teleseismic body wave inversion to deduce the slip distribution in the first event. Using both P and SH waves stabilize the inversion and we further constrain the result with the surface rupture extent and the aftershock distribution. The obtained slip pattern shows two distinct slip patches with dissimilar slip directions where aftershocks avoid high-slip areas. Using the estimated slip for the first event, we calculate the induced Coulomb stress change on the nodal planes of the second event and find a preference for higher Coulomb stress on the N-S nodal plane. Assuming a simple slip model for the second event, we estimate the combined Coulomb stress changes from the two events on the focal planes of the largest aftershocks. We find that 90% of the aftershocks show increased Coulomb stress on one of their nodal planes when the N-S plane of the second event is assumed to be the correct fault plane. [ABSTRACT FROM AUTHOR]

We constrain epicentroid locations, magnitudes and depths of moderate-magnitude earthquakes in the 2013-2014 Minab sequence using surface-wave cross-correlations, surface-wave spectra and teleseismic body-wave modelling. We estimate precise relative locations of 54 Mw ≥ 3.8 earthquakes using 48 409 teleseismic, intermediate-period Rayleigh and Love-wave cross-correlation measurements. To reduce significant regional biases in our relative locations, we shift the relative locations to align the Mw 6.2 main-shock centroid to a location derived from an independent InSAR fault model. Our relocations suggest that the events lie along a roughly east-west trend that is consistent with the faulting geometry in the GCMT catalogue. The results support previous studies that suggest the sequence consists of left-lateral strain release, but better defines the main-shock fault length and shows that most of the Mw ≥ 5.0 aftershocks occurred on one or two similarly oriented structures. We also show that aftershock activity migrated westwards along strike, away from the main shock, suggesting that Coulomb stress transfer played a role in the fault failure. We estimate the magnitudes of the relocated events using surface-wave cross-correlation amplitudes and find good agreement with the GCMT moment magnitudes for the larger events and underestimation of small-event size by catalogue MS. In addition to clarifying details of the Minab sequence, the results demonstrate that even in tectonically complex regions, relative relocation using teleseismic surface waves greatly improves the precision of relative earthquake epicentroid locations and can facilitate detailed tectonic analyses of remote earthquake sequences. [ABSTRACT FROM AUTHOR]

The Ardekul (Zirkuh) earthquake (May 10, 1997) is the largest recent earthquake that occurred in the Ardekul-Ghaen region of Eastern Iran. The greatest destruction was concentrated around Ardekul, Haji-Abad, Esfargh, Pishbar, Bashiran, Abiz-Qadim, and Fakhr-Abad (completely destroyed). The total surface fault rupture was about 125 km with the longest un-interrupted segment in the south of the region. The maximum horizontal and vertical displacements were reported in Korizan and Bohn-Abad with about 210 and 70 cm, respectively; moreover, other building damages and environmental effects were also reported for this earthquake. In this study, the intensity value XI on the European Macroseismic Scale (EMS) and Environmental Seismic Intensity (ESI) scale was selected for this earthquake according to the maximum effects on macroseismic data points affected by this earthquake. Then, according to its macroseismic data points of this earthquake and Boxer code, some macroseismic parameters including magnitude, location, source dimension, and orientation of this earthquake were also estimated at 7.3, 33.52° N-59.99° E, ~ 75 km long and ~ 21 km wide, and ~ 152°, respectively. As the estimated macroseismic parameters are consistent with the instrumental ones (Global Centroid Moment Tensor (GCMT) location and magnitude equal 33.58° N-60.02° E, and 7.2, respectively), this method and dataset are suggested not only for other instrumental earthquakes, but also for historical events. [ABSTRACT FROM AUTHOR]

This work documents for the first time the prehistoric, but morphologically pristine, Emad Deh rock avalanche of the Zagros Mountains. The ca. 420 Mm3 rock avalanche was initiated as a rockslide on a dip slope affecting a weak unit of marls with interbedded limestones overlain by a thick and competent limestone formation. The slope is located on the northern limb of the growing Gavbast Anticline and at the edge of the inflating Gavbast Dome, related to a buried salt diapir of Hormuz salt. The unconfined rock avalanche deposits, covering 32 km2, were accumulated on coalescing alluvial fans and a floodplain environment. The depositional lobe shows sectors with distinctive morphological features attributable to the variable flow-depositional behavior of the debris stream, controlled by the nature of the substrate: (1) proximal continuous breccia flanked by levees; (2) intermediate depression; and (3) flat-topped polygonal hills and conical hills. A morphometric and spatial distribution analysis has been performed with the 550 conical hills (hummocks) mapped in the distal sector. The rock avalanche, with 914 m of maximal height drop (H) and 9280 m of runout (L), displays an extraordinarily high mobility (H/L index of 0.09), that can be attributed to the combined effect of dynamic rock fragmentation and basal lubrication by the soft substrate in the floodplain. Relief rejuvenation and slope over-steepening related to the growth of the anticline and the inflation of the Gavbast Dome are considered the main long-term preparatory factors that shifted the slope to a state of marginal stability. The slope failure was likely triggered by a large M ≥ 6.5–7 earthquake at ca. 5.4 ka, as indicated by OSL ages obtained from folded alluvium situated just beneath the rock avalanche deposit. The source of the earthquake was probably a blind reverse fault situated beneath the asymmetric Gavbast Anticline, as support the structural and geomorphic features of the fold. The identification and dating of large coseismic landslides in the Zagros Mountains, where large earthquakes are rarely accompanied by primary surface ruptures, could help to improve both landslide and seismic hazard assessments. • A 420 Mm3 rock avalanche potentially triggered by a large earthquake at ca. 5.4 ka • Slope failure associated with a growing anticline and an inflating diapir • Changes in flow behavior and morphology controlled by variable substrate • Morphometric and spatial distribution analysis of 550 hummocks • Tectonic geomorphology allows inferring the potential seismogenic source. [ABSTRACT FROM AUTHOR]

A database of Iranian earthquakes during the instrumental period (1900–2014) is used to analyze geometric and kinematic aspects of the surface ruptures in different seismotectonic regions of the country. Majority of the ruptures have occurred along or associated with known geological or active faults. Thrust and strike-slip faults show a maximum surface rupture length of 85 and 125 km respectively. Excluding the distributed displacements on bedding thrusts during the 1979 Tabas earthquake, which was summed up differently to ~ 1.5 and ~ 3.0 m, maximum displacement on individual faults of thrust mechanism is 1.5 m, while strike-slip faults show a maximum horizontal displacement of 4.5 m. Surface rupture zone width of ~ 3 km is observed in an strike-slip event, and a ~ 4 km distance between the known active fault and the surface rupture of an event suggests that care should be taken in evaluation of setback zone for rupture hazards of some faults with complex geometry/kinematics. Partitioning in space and time of ruptures in earthquakes requires a better understanding of exact kinematics of active fault zones. While rerupturing of a fault segment in alternating events complicates the rupture history of the segment, simultaneous rupture of several fault segments and sympathetic faulting should be taken into account for evaluation of earthquake hazard in specific regions with such possible seismotectonic setting. Secondary ruptures and fractures, either in the presence or absence of primary surface ruptures, develop via different mechanisms, and complicate the surface deformations. While surface ruptures are frequently associated with earthquakes in eastern Iran, these features are very rare in Zagros and Makran regions. Empirical relations are suggested among surface rupture length, maximum displacement and moment magnitude for different earthquake fault mechanisms in Iran, which may be more compatible to the events within Iran. Using these relations the maximum surface rupture length, maximum surface displacement, and the associated maximum credible earthquakes are suggested for different regions in Iran. The suggested values may also be used for estimation of earthquake magnitudes in paleoseismological studies. [ABSTRACT FROM AUTHOR]

We use coseismic offsets, derived from continuous GPS measurements at six nearby sites, due to the twin Varzaghan-Ahar earthquakes (northwest Iran) of 11 August 2012 (Mw 6.4 and 6.3) to constrain slip distribution on the ruptures of two earthquakes. We assume that slip during the two earthquakes occurred on conjugate faults. Majority of the slip occurred during the first earthquake which involved dextral slip on the east west trending vertical plane. The earthquake involved slip at shallow depth which reached up to ∼0.9 m at the surface, consistent with the evidence of surface rupture and the observed offsets. During the second earthquake oblique slip occurred on a north–south rupture having steep dip towards east and extending towards north from the eastern edge of the first earthquake. Maximum slip of ∼0.4 m occurred at depth of ∼6–8 km. Derived moment magnitude of the first earthquake (6.45) from our analysis is consistent with the reported moment magnitude, while that of the second earthquake (6.1) is slightly underestimated from our analysis. [ABSTRACT FROM AUTHOR]

The crustal structure around the Bam fault located southeast of Iran was investigated by applying surface and P-wave tomography to locally recorded aftershocks of the destructive Bam earthquake (M w 6.6) on December 26, 2003. The aftershock dataset was recorded for 1 month after the main shock by 23 temporary seismic stations operated by the International Institute of Earthquake Engineering and Seismology (IIEES). After applying the selection criteria, 331 well-located events, including more than 1700 waveforms, remained. By calculating the surface wave dispersion measurements in the period range of 0.6–5 s, a tomographic procedure was then performed, and the local dispersion curves were extracted for each geographic grid point (1.8 km × 1.8 km). We applied an iterative damped least-squares inversion procedure to obtain the V SV and V SH models at depths of up to 6.5 km. The discrepancy between these V SV and V SH models also resulted in the radial anisotropy model. Moreover, the P-wave first-arrival times were inverted by the computer program SIMULPS to calculate the V P model for a depth range of 6.5–14 km, while the calculated V P model does not have sufficient resolution at depths of up to 6 km. The tomographic results indicate that a ~ 2 km thick upper sedimentary layer with a shear wave velocity of less than 2.4 km/s and relatively negative radial anisotropy covers the study area (except Bam City, with a thin 500 m positive radial anisotropy layer), which overlay on relatively positive radial anisotropy. Our results indicate that the main shock occurred on the Bam fault escarpment; then the rupturing was shifted to the co-seismic surface rupture (approximately 5 km west of the escarpment) at a depth of ~8–10 km. This dividing of energy on the fault plane (an oblique Y shape) leads to damping most of the energy as a deformation in the lower sedimentary layer. [Display omitted] • Aftershocks of the Bam earthquake can picture a 3D model of the fault; • The mainshock shifts from Bam fault to coseismic rupture at depths 8–10 km; • The main part of deformation occurred on coseismic rupture part. [ABSTRACT FROM AUTHOR]

SUMMARY On 2005 February 22, the Dahuiyeh-Zarand earthquake, Mw 6.5, struck one of the most seismically active regions in south-central Iran, east of Zarand City in Kerman province, causing more than 500 fatalities. The causative fault of the 2005 Dahuiyeh-Zarand earthquake, a reverse fault, striking nearly EW and dipping to the north, was located within a mountainous region and therefore more difficult to identify compared to the range-bounded faults. Its identification, after the 2005 event, is very important for both the estimation of seismic hazard as well as for the damage and fatality functions. We have inverted six three-component near-field strong motion waveforms to obtain the complete earthquake rupture history and slip distribution. Accelerograms are bandpass filtered with 0.2-1.0 Hz, and a length of 15-17 s of the waveforms is inverted. The lack of absolute timing has been successfully overcome by estimating, from the velocity model of the region, the propagation of P and S waves from the epicentre to the stations. The final fault slip model and the estimated source parameters are able to explain the observed waveforms. The rupture is found to be bilateral with a maximum slip of 2.4 m concentrated on two asperities in the west and east sides of the nucleation point at depths of 6-12 km. The western asperity is located to the east of Zarand City and beneath the Dahuiyeh village, which might explain why the Dahuiyeh village was totally destroyed by this earthquake. [ABSTRACT FROM AUTHOR]

SUMMARY The sequence of seismicity in the Dasht-e-Bayaz and Zirkuh region of northeastern Iran, which includes 11 destructive earthquakes within a period of only 30 years, forms one of the most outstanding examples of clustered large and intermediate-magnitude seismic activity in the world. We perform a multiple-event relocation analysis, with procedures to remove systematic location bias, of 169 earthquakes, most of which occurred in the period 1968-2008, to better image the distribution of seismicity within this highly active part of Iran. The geographic locations of the clustered earthquakes were calibrated by the inclusion of phase arrivals from seismic stations at short epicentral distances, and also by matching the relative locations of the three largest events in the study to their mapped surface ruptures. The two independent calibration methods provide similar results that increase our confidence in the accuracy of the distribution of relocated epicentres. These calibrated epicentres, combined with the mapping of faults from high-resolution satellite imagery, and from an InSAR-derived constraint on fault location in one case, allow us to associate individual events with specific faults, and even with specific segments of faults, to better understand the nature of the active tectonics in this region during the past four decades. Several previous assumptions about the seismicity in this region are confirmed: (1) that the 1968 August 30 Mw 7.1 Dasht-e-Bayaz earthquake nucleated at a prominent segment boundary and left-step in the fault trace, (2) that the 1968 September 11 Mw 5.6 aftershock occurred on the Dasht-e-Bayaz fault at the eastern end of the 1968 rupture and (3) that the 1976 November 7 Mw 6.0 Qayen earthquake probably occurred on the E-W left-lateral Avash Fault. We show, in addition, that several significant events, including the 1968 September 1 and 4 ( Mw 6.3 and 5.5) Ferdows earthquakes, the 1979 January 16 ( Mw 6.5) and 1997 June 25 ( Mw 5.9) Boznabad events and the 1979 December 7 ( Mw 5.9) Kalat-e-Shur earthquake are likely to have ruptured previously unknown faults. Our improved description of the faulting involved in the 1968-1997 earthquake sequence highlights the importance of rupturing of conjugate left- and right-lateral faults in closely spaced events, or potentially even within a single earthquake, as was likely the case at the eastern end of the 1979 November 27 ( Mw 7.1) Khuli-Buniabad main shock. The high level of clustered seismic activity probably results from the simultaneous activity on left- and right-lateral faults, an inherently unstable arrangement that must evolve rapidly. The combination of high-resolution satellite imagery and calibrated earthquake locations is a useful tool for investigating active tectonics, even in the absence of detailed field observations. [ABSTRACT FROM AUTHOR]

SUMMARY The Zagros fold-and-thrust belt is amongst the world's most seismically active mountain ranges, and is influential in our understanding of continental collisions. The presence of Hormuz salt at the base of the ∼10 km-thick folded sedimentary cover, together with the general absence of coseismic surface rupturing, have led to an assumption that earthquakes are concentrated within the basement. Here, we reinterpret the seismicity of the Zagros in light of InSAR studies that show some recent earthquakes ruptured the 'Competent Group' of mechanically strong strata in the lower sedimentary cover. Local network data demonstrate that microseismicity occurs within the basement, reaching depths of ∼20 km and in places ∼30 km. Centroid depths of larger ( Mw > 5) earthquakes are mostly ∼4-14 km, with rare events up to ∼20 km (and ∼28 km along the Oman Line). Within the Simply Folded Belt, most of these events are contained within the Competent Group and are limited to Mw < 6.1. However, the Mw 6.7 Ghir and Khurgu earthquakes were too large to have been contained within the Competent Group and probably ruptured both basement and cover. Both are associated with asymmetric anticlines containing rare exposures of Palaeozoic strata, features that may be symptomatic of places where larger ( Mw > 6.5) earthquakes occur. Earthquakes within the Competent Group can account for much of the overall shortening at this level, but the seismic strain rate in the basement is much lower. It is unclear whether the basement deforms by aseismic creep or folding beneath the SFB or by ductile shortening beneath the High Zagros or Iranian Plateau. [ABSTRACT FROM AUTHOR]

We present the first detailed source model of the 1997 M7.2 Zirkuh earthquake that ruptured the entire Abiz fault in East Iran producing a 125 km long, bended and segmented fault trace. Using SAR data from the ERS and JERS-1 satellites we first determined a multisegment fault model for this predominately strike-slip earthquake by estimating fault-segment dip, slip, and rake values using an evolutionary optimization algorithm. We then inverted the InSAR data for variable slip and rake in more detail along the multisegment fault plane. We complement our optimization with importance sampling of the model parameter space to ensure that the derived optimum model has a high likelihood, to detect correlations or trade-offs between model parameters, and to image the model resolution. Our results are in an agreement with field observations showing that this predominantly strike-slip earthquake had a clear change in style of faulting along its rupture. In the north we find that thrust faulting on a westerly dipping fault is accompanied with the strike-slip that changes to thrust faulting on an eastward dipping fault plane in the south. The centre part of the fault is vertical and has almost pure dextral strike-slip. The heterogeneous fault slip distribution shows two regions of low slip near significant fault step-overs of the Abiz fault and therefore these fault complexities appear to reduce the fault slip. Furthermore, shallow fault slip is generally reduced with respect to slip at depth. This shallow slip deficit varies along the Zirkuh fault from a small deficit in the North to a much larger deficit along the central part of the fault, a variation that is possibly related to different interseismic repose times. [ABSTRACT FROM AUTHOR]

Abstract: The relationship between the present-day stress field and natural fractures can have significant implications for subsurface fluid flow. In particular, fractures that are aligned in orientations favourable for reactivation by either shear or tensile failure in the in-situ stress field often exhibit higher hydraulic conductivities. The Ilam Formation of southwestern Iran is an important hydrocarbon reservoir containing numerous natural fractures. However, little is known about the state of stress in this region, or any of Iran''s petroleum provinces. We conducted analysis of the present-day maximum horizontal stress orientation and the density, orientation and hydraulic conductivity of natural fractures in the Ilam carbonates using high resolution Formation Micro Imager resistivity logs in two wells. A total of 51 breakouts with an overall length of 215m were observed in the two wells, indicating a maximum horizontal stress orientation of 68°N (±7.6°) in well A and 58°N (±6.3°) in well B. Furthermore, the wellbore-derived stress orientations determined herein are consistent with those inferred from nearby earthquake focal mechanism solutions, indicating that stresses in the sedimentary cover are linked to the resistance forces generated by Arabia–Eurasia collision. Furthermore, the correlation between stress orientations estimated from earthquake focal mechanism solutions and breakouts indicates that focal mechanism solution data, which is often considered to be unreliable for stress field analysis near transform margins, may provide reliable information on the stress orientation near continental collision zones. The image log data also reveals three sets of open, and presumably hydraulically conductive, fractures with strikes of (i) 160–170°N, (ii) 110–140°N and (iii) 070–080°N. Fracture set (iii) is consistent with being formed and open in the present-day stress field. However, fracture sets (i) and (ii) strike at a high angle to the present-day maximum horizontal stress, and are interpreted herein to be the result of either pre- or syn-folding related forces. The observation that different sets of open fractures in the field can be either sensitive or insensitive to the present-day stress is critical for improving hydrocarbon recovery. [Copyright &y& Elsevier]

The Rudbār earthquake of 1990 June 20, the first large-magnitude earthquake with 80 km left-lateral strike-slip motion in the western ‘High-Alborz’ fold-thrust mountain belt, was one of the largest, and most destructive, earthquakes to have occurred in Iran during the instrumental period. We bring together new and existing data on macroseismic effects, the rupture characteristics of the mainshock, field data, and the distribution of aftershocks, to provide a better description of the earthquake source, its surface ruptures, and active tectonic characteristics of the western ‘High-Alborz’. The Rudbār earthquake is one of three large magnitude events to have occurred in this part of the Alborz during recorded history. The damage distribution of the 1485 August 15 Upper Polrud earthquake suggests the east–west Kelishom left-lateral fault, which is situated east of the Rudbār earthquake fault, as a possible source. The 1608 April 20 Alamutrud earthquake may have occurred on the Alamutrud fault farther east. Analysis of satellite imagery suggests that total left-lateral displacements on the Rudbār fault are a maximum of ∼1 km. Apparent left-lateral river displacements of ∼200 m on the Kashachāl fault and up to ∼1.5 km of the Kelishom fault, which are situated at the eastern end of the Rudbār earthquake fault, also appear to indicate rather small cumulative displacements. Given the relatively small displacements, the presently active left-lateral strike-slip faults of the western High-Alborz fold-thrust belt, may be younger than onset of deformation within the Alborz Mountains as a whole. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the depth and geometry of faulting within a cluster of buried, reverse faulting earthquakes that struck Qeshm island, in the Zagros fold-and-thrust belt, over a four year period between November 2005 and July 2009. Of particular interest is our observation that there was a vertical separation between the largest two earthquakes (M w 5.8 and 5.9), which ruptured the lower parts of a ∼10-km thick sedimentary cover, and microseismicity recorded by a local network after the first, M w 5.8 event, which was concentrated within the underlying basement at depths of 10–20km. Although measured in different ways — the largest three earthquakes using radar interferometry, moderate-sized events with teleseismically-recorded, long-period waveforms, and the microseismicity using data from a local seismic network — we used consistent velocity and elastic parameters in all our modelling, and the observed vertical separation is robust and resolvable. We suggest that it reflects the influence of the Proterozoic Hormuz salt, a weak layer at the base of the sedimentary cover across which rupture failed to propagate. Because the full thickness of the seismogenic layer failed to rupture during the largest earthquakes in the sequence, the lower, unruptured part may constitute a continued seismic hazard to the region. Considering the rarity of earthquakes larger than M w 6.2 in the Zagros Simply Folded Belt, we suggest that the Hormuz salt forms an important, regional barrier to rupture, not just a local one. Finally, we note that buried faulting involved in the largest earthquakes is almost perpendicular to the trend of an anticline exposed at the surface immediately above them. This suggests that locally, faulting and folding are decoupled, probably along a weak layer of marls or evaporites in the middle part of the sedimentary cover. [Copyright &y& Elsevier]

• The Khazar active fault mainly corresponds to a hidden thrust, often associated with forebergs structures. • In its central part, a minimum Late Quaternary slip rate of 3.6 mm.yr -1 is estimated. • The Khazar fault is likely capable to produce large (Mw7) to very large (Mw8) events. • Paleoseismological investigations suggest that at least 5 events occurred in the past 5300 years. The Alborz Mountain accommodates some of the convergence between Central Iran and Eurasia, is characterized by active range-parallel fold and thrust structures. At present, the kinematics of the range involves a strain partitioning mechanism and clockwise rotation of the South Caspian Basin. Range-parallel, left-lateral strikeslip faulting dominates the central part of the mountain range, while reverse faulting affects its northern and southern borders. Several slip-rate studies have been carried out along active faults in the internal and southern parts of the range. However, the characteristics of the main northern bounding fault (the Khazar [Persian: Caspian] Fault) remain poorly known. Our analysis provides new constraints on the activity of this fault. We first show that the fault generally is a hidden thrust fault, often associated with fault-bend and fault-propagation folds (forebergs). In the central part of the fault, the radiocarbon dating of an uplifted terrace allows estimating minimum vertical and average horizontal slip rates of 2.0 ± 0.5 mm/yr and 3 mm/yr respectively. hence, minimum slip rate along the fault reach to 3.6 mm/yr. About 150 km further east, near the city of Behshahr, within the archeological site of Gohar-Tappe, a paleoseismological trench study on a young detachment fold suggests that at least 5 events occurred in the past 5,300 years, 3 of them with surface-rupturing between 5300- and 3900-years cal BP, our results confirm that the Khazar Fault is a major active structure in northern Iran, and represents a significant seismic hazard for the entire Central Alborz region. [ABSTRACT FROM AUTHOR]

The Kuh-e Asmari anticline is an asymmetrical structure located in the central part of the Zagros fold-thrust belt, SW Iran. Fracture analysis of the Kuh-e Asmari is providing key constraints on any modeling of nearby fractured reservoirs. Whereas, field surveys are not sufficient, Remote Sensing (RS) techniques were used for regional faulting / fracturing characterization of the anticline. The fault / fracture pattern of the Kuh-e Asmari anticline has been extracted based on ASTER and IRS-PAN images, and Digital Elevation Model (DEM). The synthesized lineament map of the anticline indicates many fault / fracture sets with varied lengths and orientations. Statistical analysis of the data shows the most common trend of the lineaments is NE-SW The other main directions are NW-SE, N-S, and some deal E-W. Structural analysis of the data implied that the NE-SW and NW-SE systems are comparable to fold--related faults / fractures. As well as, the N-S and E-W systems are independent from folding. These systems are attributed to early origin basement trends, which have been reactivated in the later stages of the Zagros orogeny. This research emphasized that ASTER and IRS-PAN images are worthwhile for structural characterization of fault / fracture systems in folded areas. Furthermore, the new structural findings have improved tectonic understanding of the Asmari fractured reservoirs, SW Iran. [ABSTRACT FROM AUTHOR]

The Bojnurd region of NE Iran experienced a Mw 6.4 earthquake on February 4, 1997. By combining results from teleseismic body-waveform analysis, field observations of structural damage, coseismic deformation, geomorphology, and analysis of the resulting strong ground-motions, we build a coherent picture of the faulting associated with this earthquake. The earthquake resulted from almost pure right-lateral strike-slip motion (0.5-1.0 m), which ruptured a ∼15 km long section of fault, striking ∼340° at its northern end, which changes to ∼320° at its southern end. This fault can be seen clearly on the geological map and satellite imagery. The village of Sheikh lies ∼10-15 km SE of the fault rupture, yet was severely damaged during the earthquake. Analysis of strong-motion records, recorded by the Building and Hazard Research Center of Iran, particularly their significant duration, the polarization of the fault-normal component, and the velocity pulse, indicates a probable directivity effect, in which the rupture propagated from north to south, towards the village of Sheikh. [ABSTRACT FROM AUTHOR]

Abstract: One simple way of accounting for site conditions in calculating seismic hazards is to use the shear-wave velocity (V s) in the shallow subsurface to classify materials. We used the average shear-wave velocity to a depth of 30m to develop site categories for Tehran. This parameter can further be used for modifying a calculated ground motion to account for site conditions. The National Earthquake Hazard Reduction Program (NEHRP) classification was used to group sites into different classes. Our classification was based on values measured in 188 seismic profiles and on the assumption of similar values for similar geological materials. For each geological unit, characteristics of shear-wave velocity distribution including histograms of shear-wave velocity and variation of V s versus depth were calculated and shown. This study reveals that the of the mountainous zones fall into categories A and B of NEHRP site classification. But, in the Tehran Plain, the northern alluvium tends to have values which fall into range of category C of the NEHRP site classification, whereas, those in the southern alluvium would be categorized as NEHRP D. [Copyright &y& Elsevier]

We used new precise leveling data acquired 40 days after the Bam earthquake in combination with radar interferometry observations from both ascending and descending orbits to investigate static deformation associated with the 2003 Bam earthquake. We invert this geodetic data set to gain insight into the fault geometry and slip distribution of the rupture. The best-fitting dislocation model is a steeply east-dipping right-lateral strike-slip fault that has a size of 11 by 8 km and strikes N2°W. We find that such smooth geometry fits available geodetic data better than previously proposed models for this earthquake. Our distributed slip model indicates a maximum strike slip of 3 m occurring about 3 to 5 km deep. The slip magnitude and depth of faulting taper to the north, where the fault approaches the Bam city. Inclusion of crustal layering increases the amount of maximum slip inferred at depth by about 4%. [ABSTRACT FROM AUTHOR]

Historical aerial photographs are a valuable source of data for investigating past earthquakes, volcanic activities, ice mass balance and land changes in the pre-modern era. Scan resolution for digitising the historical archives is an important factor in photogrammetric processes, but the effects have not been quantified yet. In this paper, we use the 1978 M s 7.4 Tabas-e-Golshan earthquake in eastern Iran as a case study to test how scan resolution affects digitisation and hence the measurements of deformation. We scanned 6 aerial photographs that were acquired in 1956 with 150, 300, 600 and 1200 dots per inch (dpi), and generated four sets of pre-earthquake digital elevation model (DEM) covering the Tabas fold. By differencing the pre-earthquake DEMs and the post-earthquake DEM derived from SPOT-6 data acquired in 2013, we made four measurements of slip on the Tabas fold. We compared our measurements to the published model which was derived from matching the pre-earthquake Hexagon Keyhole-9 US image and the post-earthquake SPOT-2 and SPOT-6 images, and found that the 600 dpi scans yield the most consistent result among the four different resolutions, with a misfit of 0.7 m. A different resolution might well be better for a different set of aerial photographs, depending on the quality and photographic scale of the images. We scanned 20 aerial photographs that cover the Kurit fold to the south of the Tabas fold with 600 dpi and used them to investigate the slip on that segment. Similarly, DEMs were extracted first to produce the DEM of difference (DoD). Five swath profiles taken from the DoD show clear discontinuities across two fault strands, i.e. the main Kurit fault (the western strand) and a back thrust (the eastern strand). Based on the profile measurements, we implemented Monte Carlo modelling of elastic dislocation and obtained a slip of 1.4 m and 2.2 m on the western and eastern strands respectively. By combining all the information about the earthquake and postseismic slip, we inferred a coseismic slip of 3.5 m on the detachment fault by which most of the seismic moment was released. This slip enhanced the applied load on part of the Kurit fault, which is partly responsible for the long-lasting postseismic deformation in the region. • Deformation of pre-modern earthquakes can be measured using historical archives. • We quantified the effects of different scan resolutions on the accuracy of results. • 600 dpi is recommended for digitising historical aerial photographs. • We extended the analysis of the 1978 Iran earthquake using the new slip model. [ABSTRACT FROM AUTHOR]

On 7 November 2019, an Mw 5.9 earthquake with a left-lateral strike-slip mechanism struck the Bozgush mountain in NW Iran, marking the largest event in the mountain area in the past 44 years. Because no surface rupture associated with the earthquake has been reported, the causative fault of the earthquake is unknown. This event provides an opportunity to gain insight into the active tectonics of the region. Here, we acquire the coseismic deformation associated with this earthquake through Sentinel-1A interferometric synthetic aperture radar (InSAR) data and further invert the fault geometry and the detailed coseismic slip of this event based on the planar fault model. By a joint analysis of the inversion results, aftershocks sequence and local tectonics, we propose that the buried branch fault splaying from the Shalgun-Yelimsi fault (SYF) at depth may be responsible for this earthquake. Both the uniform and distributed slip models infer a smaller fault rupture size than predicted by classical fault scaling relation. We suggest that the subsurface fault geometry of the SYF may stop the rupture propagation further down to southeast at depth, thus resulting in high slip and stress drop at such a small fault plane. Considering the proximity of the epicenter to the SYF, we also invert for the distributed slip model on an anti-listric fault with the purpose to explore the possible complexity of rupture fault. However, due to the lack of obvious improvement to fit the observed data and the limited slip in the deep fault segment, the planar fault model is still regarded as the optimal one. According to the calculated coseismic Coulomb stress changes, we find that 55 % of aftershocks are triggered by the statics stress and that the northern segment of SYF and the eastern segment of SBF have a potential rupture risk. The occurrence of this earthquake, together with the prominent offset feature and slip style of surrounding active fault, indicates an active conjugate fault system, which may accommodate the small-scale northeastward lateral extrusion. [ABSTRACT FROM AUTHOR]

Well-exposed normal fault networks in the Plio-Quaternary sedimentary series of the Zanjan Depression, Iran, were studied in order to analyse the spatial relationships and evolution of fault and damage zone structures in low-burial (~200 m) conditions. Despite the poorly-consolidated, geologically young nature of sediment, fault networks show several characteristics more typical of faulting in lithified strata. Key controls on the evolution of fault networks are found to be fault interactions, segment linkage, and the mechanical stratigraphy resulting in dip refraction, fault segmentation, the development of fault bends and steps, vertical fault overlap zone breaching, and bed-parallel slip. The data show a decrease of fracture frequency away from the fault cores. The obtained data show that damage zones width increases with fault displacement in a non-linear manner. Furthermore, damage zones appear more deformed and wider in the hanging-wall than in the footwall. A plot of damage zone width versus total fault throw reveals a positive but non-linear correlation, and the best-fit function corresponds better to a power-law relationship. The relationship between displacement and length shows a sub-vertical growth path with large displacement gradients typical of faulting in partially-lithified strata, due to shale ductility accommodating fault tip-related strains. A model for the evolution of the normal fault system is proposed based on spatial distribution of deformation, particularly fault segmentation, segment linkage, and damage zone development, as influenced by initial fault spacing, mechanical stratigraphy and the low burial conditions. • Key controls on the evolution of fault networks are found to be fault interactions, segment linkage, and the mechanical stratigraphy. • Damage zones appear more deformed and wider in the hanging wall blocks than in the footwall blocks. • A plot of damage zone width versus total fault throw reveals a positive but non-linear correlation. • A model for the evolution of the normal fault system is proposed based on spatial distribution of deformation, fault segmentation and segment linkage. [ABSTRACT FROM AUTHOR]