S U M M A R YThe Zagros mountains of SW Iran are one of the most seismically active intra-continental fold-and-thrust belts on Earth, and an important element in the active tectonics of the Middle East. Surface faulting associated with earthquakes is extremely rare, and so most information about the active faulting comes from earthquakes. We use long-period teleseismic P and SH body waves to determine the orientation and depth of faulting in 16 new earthquakes, and then evaluate and synthesize all the available teleseismic data on earthquake source parameters in the Zagros. We use this information to investigate the style and distribution of active faulting in the Zagros, and how it contributes to the N-S shortening of the Arabia-Eurasia collision. When the data are ranked in quality and carefully evaluated, simple patterns are seen that are not apparent when routine catalogue data are taken at face value. An important change in the fault configuration occurs along strike of the belt. In the NW, overall convergence is oblique to the trend of the belt and the surface anticlines, and is achieved by a spatial separation ('partitioning') of the orthogonal strike-slip and shortening components on separate parallel fault systems. By contrast, in the SE, overall convergence is orthogonal to the regional strike and achieved purely by thrusting. In the central Zagros, between these two structural regimes, deformation involves parallel strike-slip faults that rotate about vertical axes, allowing extension along the strike of the belt. The overall configuration is similar to that seen in other curved shortening belts, such as the Himalaya and the Java-Sumatra trench. All the Zagros earthquakes we have been able to check have centroids shallower than ∼20 km and are confined to the upper crust. Many of the larger earthquakes are likely to occur in the basement beneath the sedimentary cover, which is active even beneath areas of known shallow structural decollement such as the Dezful embayment. The dominant style of shortening is high-angle reverse faulting with dips >30 • though some lower-angle thrusting occurs in places. Active thrust and reverse faulting is relatively confined to the lower topography on the SW edge of the belt today, and only strike-slip faulting affects the higher topography. Profound vertical changes in structural and stratigraphic level indicate that a similar style of deformation was once active across the width of the Simple Folded Belt, but has progressively migrated SW over the last 5 Ma. There is no evidence for a seismically active structural decollement, such as a low-angle thrust, beneath the Zagros, nor is there any seismic evidence for active subduction, either beneath the Zagros or beneath central Iran. Instead the Arabian margin seems to have shortened by distributed thickening of the basement. Only in the syntaxis of the Oman Line, at the SE end of the Zagros, is there any evidence for a low-angle thrust of regional extent. Here, earthquakes continue 50 km north of the Zagros Thrust Line (t...

Summary We use drainage patterns, geological markers and geomorphological features to determine a right‐lateral offset of ∼50 km, and possibly as much as ∼70 km, on the Main Recent Fault in NW Iran. This fault trends NW–SE and forms the NE border of the Zagros mountains. It accommodates the strike‐slip component of the N–S convergence between Arabia and Eurasia, with the NE–SW shortening component being accommodated in the Zagros Fold Belt. Its ∼50 km strike‐slip offset implies a shortening of ∼50 km in the fold belt and ∼70 km total N–S convergence accommodated in the NW Zagros. This is a substantial fraction of the 85–140 km overall Arabia–Eurasia convergence expected over the last 3–5 Ma. If the Main Recent Fault initiated at that time, as seems likely from geological arguments, it has a horizontal slip rate of at least 10–17 mm yr−1 and should be the source of frequent earthquakes of Ms 6–7, as has been seen in the 20th century and the earlier historical record. The similarity of the offsets and probable ages of the North Anatolian and Main Recent Faults suggests that they have been active as an almost continuous zone of right‐lateral shear on the north edge of the Arabian and Anatolian plates since the early Pliocene.

An Mw 6.5 earthquake devastated the town of Bam in southeast Iran on 26 December 2003. Surface displacements and decorrelation effects, mapped using Envisat radar data, reveal that over 2 m of slip occurred at depth on a fault that had not previously been identified. It is common for earthquakes to occur on blind faults which, despite their name, usually produce long‐term surface effects by which their existence may be recognised. However, in this case there is a complete absence of morphological features associated with the seismogenic fault that destroyed Bam.

S U M M A RYThe destructive Zirkuh-e-Qa'enat earthquake of 1997 May 10 (M w 7.2, M s 7.3, m b 6.3) produced 125 km of NNW^SSE right-lateral strike-slip surface faulting on the Abiz fault in the Sistan suture zone of eastern Iran: the longest known surface rupture associated with an Iranian earthquake. Analysis of the body-wave seismograms from the main shock shows that rupture occurred in four main subevents, propagating in a sequence from north to south. Although predominantly strike-slip, the orientation of the faulting in each subevent varies, with appreciable reverse components in the northcentral part and at the southern end of the Abiz fault. This change in fault style along the Abiz fault inferred from the seismograms is also seen in the coseismic surface ruptures and the geomorphology. Average coseismic surface displacements were approximately 2 m, implying a static stress drop of only 5 bar (0.5 MPa). The 1997 surface ruptures followed clear traces of late Quaternary slip on the Abiz fault, and for its northern 50 km re-ruptured fault segments that had slipped in previous earthquakes of M s 6.0^6.6 in 1936 and 1979. The 1997 earthquake ruptured the northern end of the N^S right-lateral strike-slip system of the Sistan suture zone, ending where it abuts a system of E^W left-lateral strike-slip faults which have also slipped in large earthquakes during the last 30 years. The earthquakes on this conjugate system of strike-slip faults form a sequence that may have been triggered by the enhancement of stress on one fault as a result of slip on a neighbouring fault. Together, these faults achieve N^S rightlateral shear of deforming Iran against stable western Afghanistan by N^S slip on the right-lateral faults and clockwise rotation of the E^W left-lateral faults.

SUMMARY The 1998 March 14 Fandoqa earthquake (Ms 6.6) was the penultimate in a series of five substantial earthquakes on the Gowk fault system of southeast Iran since 1981, all of which were associated with co‐seismic surface ruptures. We use observations of surface faulting, analysis of P and SH body waves, SAR interferometry and geomorphology to investigate the ruptures in these earthquakes and how they are related both to each other and to the regional active tectonics. The 1998 Fandoqa earthquake produced 23 km of surface faulting with up to 3 m right‐lateral strike‐slip and 1 m vertical offsets. SAR interferometry and seismic waveforms show that the main rupture plane dipped west at ∼50° and had a normal component, although the surface ruptures were more complicated, being downthrown to both the east and the west on steep faults in near‐surface sediments. In addition, SAR interferometry shows that a nearby thrust with a similar strike but dipping at ∼6°W moved about 8 cm in a time interval and in a position that makes it likely that its slip was triggered by the Fandoqa earthquake. The 1998 surface ruptures in the Gowk valley followed part of a much longer (∼80 km) set of co‐seismic ruptures with smaller offsets that were observed after larger earthquakes in 1981 (Mw 6.6 and 7.1). The main ruptures in these 1981 earthquakes probably occurred on different, deeper parts of the same fault system, producing only minor reactivation of the shallower faults at the surface. Although the 1981–1998 earthquake sequence apparently ruptured parts of the same fault system repeatedly, these earthquakes had very different rupture characteristics: an important lesson for the interpretation of both palaeoseismological trenching investigations and historical accounts of earthquakes. The regional kinematics, which involve oblique right‐lateral and convergent motion, are evidently achieved by a complex configuration of faults with normal, reverse and strike‐slip components. Some of the complexity at the surface may be related to a ramp‐and‐flat fault geometry at depth, but could also be related to the large topographic contrast of ∼2000 m across the fault system, which separates the high Kerman plateau from the low Dasht‐e‐Lut desert. Details of the fault geometry at depth remain speculative, but it must be unstable and evolve with time. It may be this requirement that causes the principal features of geological ‘flower structures’ to develop, such as series of subparallel faults which accommodate dip‐slip components of motion.

S U M M A R YIn 1994, three shallow earthquakes of M w ∼ 6 occurred close together on blind thrusts near Sefidabeh in eastern Iran. In an earlier study of the teleseismic waveforms, the geomorphology and the faulting in the epicentral region, it was suggested that these earthquakes were associated with the growth of a ridge above a blind thrust fault system, whose activity could be detected by its effect on the surface drainage. In this study we present a SAR interferogram that precisely determines the location and amount of coseismic surface displacements, showing that the earthquakes in the Sefidabeh sequence probably occurred on en-echelon fault segments associated with three stepping ridges. We also present U/Th dates of ∼100 ka for lake deposits uplifted by the growing ridge. From the cumulative, dated uplift and knowledge of the surface displacements due to an earthquake sequence, we estimate that ∼120 such events have occurred in the past 100 ka, with an average recurrence interval of 830 yr, and an average convergence rate of 1.5 mm yr −1 on the Sefidabeh thrust; each estimate has an uncertainty of a factor of two, either way. We argue that the Sefidabeh fault originally formed by coalescence of many small fault segments, and has grown in length at about 2 cm yr −1 in the past 100 ka. Though the coseismic surface deformation observed in the SAR interferogram closely resembles folding, the overall topography does not, because of inherited topography associated with earlier geological deformation. In spite of this, the activity of the buried thrust fault can easily be detected by its effect on the surface drainage: a significant lesson when interpreting landscapes that are not entirely due to the present-day deformation.

[1] This paper shows how the Turkish-Iranian Plateau grows laterally by incrementally incorporating adjacent parts of the Zagros fold-and-thrust belt. The limit of significant, seismogenic, thrusting in the Zagros (M w > 5) occurs close to the regional 1250 m elevation contour. The seismicity cutoff is not a significant bedrock geology boundary. Elevations increase northward, toward regional plateau elevations of~2 km, implying that another process produced the extra elevation. Between the seismogenic limit of thrusting and the suture, this process is a plausibly ductile thickening of the basement, suggesting depth-dependent strain during compression. Similar depth-dependant crustal strain may explain why the Tibetan plateau has regional elevations~1500 m greater than the elevation limit of seismogenic thrusting at its margins. We estimate~68 km shortening across the Zagros Simply Folded Belt in the Fars region, and~120 km total shortening of the Arabian plate. The Dezful Embayment is a low strain zone in the western Zagros. Deformation is more intense to its northeast, in the Bakhtyari Culmination. The orogenic taper (across strike topographic gradient) across the Dezful Embayment is 0.0004, and across the Bakhtyari Culmination, 0.022. Lateral plateau growth is more pronounced farther east (Fars), where a more uniform structure has a taper of~0.010 up to elevations of~1750 m. A >100 km wide region of the Zagros further northeast has a taper of 0.002 and is effectively part of the Turkish-Iranian Plateau. Internal drainage enhances plateau development but is not a pre-requisite. Aspects of the seismicity, structure, and geomorphology of the Zagros do not support critical taper models for fold-and-thrust belts.

Summary In 1994 a sequence of five earthquakes with Mw 5.5–6.2 occurred in the Sistan belt of eastern Iran, all of them involving motion on blind thrusts with centroid depths of 5–10 km. Coseismic ruptures at the surface involved bedding‐plane slip on a growing hanging‐wall anticline displaying geomorphological evidence of uplift and lateral propagation. The 1994 earthquakes were associated with a NW‐trending thrust system that splays off the northern termination of a major N–S right‐lateral strike‐slip fault. Elevation changes along the anticline ridge suggest that displacement on the underlying thrust dies out to the NW, away from its intersection with the strike‐slip fault. This is a common fault configuration in eastern Iran and accommodates oblique NE–SW shortening across the N–S deforming zone, probably by anticlockwise rotations about a vertical axis. This style of fault kinematics may be transitional to a more evolved state that involves partitioning of the strike‐slip and convergent motion onto separate subparallel faults.