Source Process and Slip Model of 2005 Dahuiyeh-Zarand Earthquake (Iran) Using Inversion of Near-Field Strong Motion Data

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 nearfield strong motion waveforms to obtain the complete earthquake rupture history and slip distribution. Accelerograms are bandpass filtered with 0.2\u20131.0 Hz, and a length of 15\u201317 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\u201312 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

The transition between Makran subduction and the Zagros collision: recent advances in its structure and active deformation

South-east Iran is the site of a rare case of young transition between subduction and collision. We synthesized recent results in geodesy, tectonics, seismology and magnetism to understand the structure and kinematics of the Zagros-Makran transition. Surface observations (tectonics, magnetism and geodesy) draw a transpressive discontinuity consisting of several faults striking obliquely to the convergent plate motion, whereas deeper observations (seismology) support a smooth transition across the fault system. No lithospheric transform fault has been created, even though the transition already behaves like a major boundary in terms of tectonic style, seismic structure, lithology and magnetism. The Zendan-Minab-Palami fault system consists of several faults that accommodate a transpressive tectonic regime. It is the surface expression of a southward propagation of the N-trending right-lateral strike-slip fault system of Jiroft-Sabzevaran. Within each system the numerous faults will coalesce into a single, lithospheric, wrench fault

The vertical separation of mainshock rupture and microseismicity at Qeshm island in the Zagros fold-and-thrust belt, Iran

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 (Mw 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, Mw 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 Mw 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. © 2010

The 2013 Mw 6.2 Khaki-Shonbe (Iran) Earthquake: insights into seismic and aseismic shortening of the Zagros sedimentary cover

Determining the relationship between folding and faulting in fold and thrust belts is important for understanding the growth of geological structures, the depth extent of seismic slip, and consequently, the potential earthquake hazard. The 2013 Mw 6.2 Khaki-Shonbe earthquake occurred in the Simply Folded Belt of the Zagros Mountains, Iran. We combine seismological solutions, aftershock relocations, satellite interferometry, and field observations to determine fault geometry and its relationship with the structure, stratigraphy, and tectonics of the central Zagros. We find reverse slip on two along-strike, southwest dipping fault segments. The main shock rupture initiated at the lower northern end of the larger northwest segment. Based upon the hypocenter and rupture duration, slip on the smaller southern segment is likely aseismic. Both faults verge away from the foreland, toward the high-range interior, contrary to the fault geometries depicted in many structural cross sections of the Zagros. The modeled slip occurred over two mutually exclusive depth ranges above 10 km, resulting in long (∼16 km), narrow rupture segments (∼7 km). Aftershocks cluster in the depth range 3–14 km. This indicates reverse slip and coseismic shortening occurred mostly or exclusively in the sedimentary cover, with slip distributions likely to be lithologically controlled in depth by the Hormuz salt at the base of the sedimentary cover (∼10–12 km), and the Kazhdumi Formation mudrocks at upper levels (∼4–5 km). Our findings suggest lithology plays a significant role in the depth extent of slip found in reverse faults in folded belts, providing an important control on the potential size of earthquakes

The vertical separation of mainshock rupture and microseismicity at Qeshm island in the Zagros fold-and-thrust belt, Iran

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 (Mw 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, Mw 5.8 event, which was concentrated within the underlying basement at depths of 10-20 km. 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 Mw 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

Complex pattern of seismic anisotropy beneath the Iranian plateau and Zagros

We performed shear wave splitting analyses on core-refracted teleseismic shear waveforms from 150 broad-bandstations across the Iranian plateau and Zagros to investigate seismic anisotropy in the region. Seismic anisotropyis quantified by shear-wave splitting parameters, i.e. fast polarization direction and split delay time.Our measurements revealed a complex pattern of splitting parameters with variations in the trend and strength ofanisotropy across the tectonic boundaries. This complex pattern implies that a system of simple asthenosphericflow related to the absolute plate motion cannot alone explain our observations and that the lithosphere also hasa significant contribution in many parts. We compare our results to the surface deformation and velocity fieldsinferred from geodetic measurements to assess the role of the mantle in continental deformation. The rotationalpattern of the fast directions around the collision zone in Central Zagros may indicate the presence of a mantleflow around a continental keel beneath the Zagros. The agreement between the crustal and mantle deformationfield in Central Iran implies a vertically coherent deformation in this region, whereas the azimuthal variations insplitting parameters in the collision zone may suggest multi-layered anisotropy with different contributions fromthe crust and mantle

The kinematics of the Zagros Mountains (Iran)

International audienceWe present a synthesis of recently conducted tectonic, global positioning system (GPS), geomorphological and seismic studies to describe the kinematics of the Zagros mountain belt, with a special focus on the transverse right-lateral strike-slip Kazerun Fault System (KFS). Both the seismicity and present-day deformation (as observed from tectonics, geomorphology and GPS) appear to concentrate near the 1000 m elevation contour, suggesting that basement and shallow deformation are related. This observation supports a thick-skinned model of southwestward propagation of deformation, starting from the Main Zagros Reverse Fault. The KFS distributes right-lateral strike-slip motion of the Main Recent Fault onto several segments located in an en echelon system to the east. We observe a marked difference in the kinematics of the Zagros across the Kazerun Fault System. To the NW, in the North Zagros, present-day deformation is partitioned between localized strike-slip motion on the Main Recent Fault and shortening located on the deformation front. To the SE, in the Central Zagros, strike-slip motion is distributed on several branches of the KFS. The decoupling of the Hormuz Salt layer, restricted to the east of the KFS and favouring the spreading of the sedimentary cover, cannot be the only cause of this distributed mechanism because seismicity (and therefore basement deformation) is associated with all active strike-slip faults, including those to the east of the Kazerun Fault System

The 2006 March 25 Fin earthquakes (Iran)-insights into the vertical extents of faulting in the Zagros Simply Folded Belt

We investigate the depth of faulting and its connection with surface folding in the Zagros Simply Folded Belt of Iran. Our focus is a sequence of earthquakes (Mw 5.7, 5.5, 5.2, 5.0, 4.9) that struck the Fin region, in the south-eastern Simply Folded Belt, on 2006 March 25. Modelling ground displacements measured with radar interferometry, we find that either N- or S-dipping model reverse faults can reproduce the observed fringe patterns. Despite the uncertainty in fault orientation, we can constrain the vertical extents of rupture to between a top depth of ~5-6 km and a bottom depth of ∼9-10 km, consistent with the ∼8 km centroid depth of the largest earthquake. We suggest that the faulting ruptured the thick 'Competent Group' of Paleozoic and Mesozoic conglomerates and platform carbonates, which makes up the lower part of the sedimentary cover. The rupture probably terminated within the Precambrian Hormuz salt at its base, and the Cretaceous Gurpi marls at its top. These mechanically weak layers act as barriers to rupture, separating faulting within the Competent Group from deformation in the layers above and below. The pattern of coseismic surface uplift is centred on the common limb of the Fin syncline and Guniz anticline, but is oblique (by 20°) to the trend of these open, symmetric, 'whaleback' folds, and also overlaps a section of the Fin syncline axis. These observations suggest that locally, surface folding is decoupled from the underlying reverse faulting. Although the Fin syncline and Guniz anticline are symmetric structures, some other nearby folds show a strong asymmetry, with steep or overturned southern limbs, consistent with growth above N-dipping reverse faults. This suggests that the Simply Folded Belt contains a combination of forced folds and detachment folds. We also investigate the distribution of locally recorded aftershocks in the weeks following the main earthquakes. Most of these occurred at depths of ∼10-30 km, with a particularly high concentration of events at ∼20-25 km. These aftershocks therefore lie within the crystalline basement rather than the sedimentary cover, and are vertically separated from the main rupture. This study confirms earlier suggestions that earthquakes of Mw 5-6 are capable of being generated within the thick 'Competent Group' of Paleozoic and Mesozoic sediments, as well as in the basement below the Hormuz Salt Formation. © 2010 The Author Journal compilation © 2010 RAS

The 2006 March 25 Fin earthquakes (Iran) - insights into the vertical extents of faulting in the Zagros Simply Folded Belt

We investigate the depth of faulting and its connection with surface folding in the Zagros Simply Folded Belt of Iran. Our focus is a sequence of earthquakes (Mw 5.7, 5.5, 5.2, 5.0, 4.9) that struck the Fin region, in the south-eastern Simply Folded Belt, on 2006 March 25. Modelling ground displacements measured with radar interferometry, we find that either N- or S-dipping model reverse faults can reproduce the observed fringe patterns. Despite the uncertainty in fault orientation, we can constrain the vertical extents of rupture to between a top depth of 223C520136 km and a bottom depth of 223C9201310 km, consistent with the 223C8 km centroid depth of the largest earthquake. We suggest that the faulting ruptured the thick 'Competent Group' of Paleozoic and Mesozoic conglomerates and platform carbonates, which makes up the lower part of the sedimentary cover. The rupture probably terminated within the Precambrian Hormuz salt at its base, and the Cretaceous Gurpi marls at its top. These mechanically weak layers act as barriers to rupture, separating faulting within the Competent Group from deformation in the layers above and below. The pattern of coseismic surface uplift is centred on the common limb of the Fin syncline and Guniz anticline, but is oblique (by 20°) to the trend of these open, symmetric, 'whaleback' folds, and also overlaps a section of the Fin syncline axis. These observations suggest that locally, surface folding is decoupled from the underlying reverse faulting. Although the Fin syncline and Guniz anticline are symmetric structures, some other nearby folds show a strong asymmetry, with steep or overturned southern limbs, consistent with growth above N-dipping reverse faults. This suggests that the Simply Folded Belt contains a combination of forced folds and detachment folds. We also investigate the distribution of locally recorded aftershocks in the weeks following the main earthquakes. Most of these occurred at depths of 223C10201330 km, with a particularly high concentration of events at 223C20201325 km. These aftershocks therefore lie within the crystalline basement rather than the sedimentary cover, and are vertically separated from the main rupture. This study confirms earlier suggestions that earthquakes of Mw 520136 are capable of being generated within the thick 'Competent Group' of Paleozoic and Mesozoic sediments, as well as in the basement below the Hormuz Salt Formation