SUMMARY We investigate the depth distribution of earthquakes within the continental lithosphere of southern Iran, the Tien Shan and northern India by using synthetic seismograms to analyse P and SH body waveforms. In the Zagros mountains of southern Iran, earthquakes are apparently restricted to the upper crust (depths of <20 km), whereas in the Tien Shan and northern India they occur throughout the thickness of the continental crust, to depths of ∼40–45 km. We find no convincing evidence for earthquakes in the continental mantle of these regions, in spite of previous suggestions to the contrary, and question whether seismicity in the continental mantle is important in any part of the world. In some regions, such as Iran, the Aegean, Tibet and California, seismicity is virtually restricted to the upper continental crust, whereas in others, including parts of East Africa, the Tien Shan and northern India, the lower crust is also seismically active, although usually less so than the upper crust. Such variations cannot reliably be demonstrated from published catalogue or bulletin locations, even from ones in which depth resolution is generally improved. In contrast to the oceanic mantle lithosphere, in which earthquakes certainly occur, the continental mantle lithosphere is, we suggest, virtually aseismic and may not be significantly stronger than the lower continental crust. These variations in continental seismogenic thickness are broadly correlated with variations in effective elastic thickness, suggesting that the strength of the continental lithosphere resides in the crust, and require some modification to prevalent views of lithosphere rheology.

SUMMARY The south Caspian Basin is a relatively aseismic block within the Alpine‐Himalayan Belt, but is surrounded by zones of high seismicity. We used the focal mechanisms of 16 earthquakes whose source parameters we determined from inversion of body waves and the mechanisms of 15 other earthquakes to determine the style of faulting in the seismic belts surrounding the south Caspian Basin. Earthquakes beneath the Talesh Mountains of north‐west Iran and immediately off‐shore in the south‐west Caspian Sea have shallow thrust mechanisms, showing that the continental crust of north‐west Iran is overthrusting the ‘oceanic‐like’ crust of the south Caspian Basin. Earthquakes south of the Caspian Sea in northern Iran show a mixture of focal mechanisms. Both high‐angle reverse faulting and left‐lateral strike‐slip faulting mechanisms are observed in the high Alborz Mountains. Farther south, on the edge of the central Iran plateau, oblique left‐lateral reverse mechanisms are observed. It appears that the NE direction of shortening between central Iran and the south Caspian Basin is partitioned into pure left‐lateral strike‐slip and thrusting in the WNW‐trending high Alborz, but is accommodated by oblique faulting in lower elevations. Earthquakes in the Kopet Dag Mountains east of the Caspian Sea also show a mixture of high‐angle reverse and strike‐slip faulting mechanisms and may be another example of the partitioning of oblique slip into strike slip and thrust motion. Normal faulting mechanisms at centroid depths of 35–50 km dominate in the belt of seismicity that extends across the central Caspian Sea. The significance of the normal faulting earthquakes is enigmatic. It is improbable that these events represent the motion between the southern Caspian Basin and Eurasia for they imply a sense of motion that is incompatible with the observed topography and folding in the sediments. Two shallow earthquakes at about 12 km depth in this belt, one a small event and the other a large second subevent of a multiple earthquake, have thrusting mechanisms suggesting that shortening occurs as the continental crust of the northern Caspian Sea is thrust over the ‘oceanic‐like’ crust of the southern Caspian Basin. Shortening is also suggested by the orientations of folds in the sedimentary cover south of the central Caspian Sea seismic belt. We suggest that this shortening does indeed represent a NNE motion of the Caspian Sea relative to Eurasia, but that the motion is slow and has not produced many earthquakes. The deeper, normal‐faulting events may be related to bending or down‐dip extension of the incipient subducted slab. If the motion of the Caspian Sea relative to Eurasia is indeed slow, then the motion in the Alborz between central Iran and the south Caspian Basin will be almost the same as that between Iran and Eurasia, as has previously been assumed. The combined effect of the overthrusting of the south Caspian Basin by the Talesh‐Alborz Mountains in the south, and by the continental crust of the northern Caspian Sea in the nor...

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 YThe Kazerun Line is a transverse valley of about 200 km long that obliquely crosses the regular anticlines of the Zagros fold belt in SW Iran. At its northern end it is a clear fault which can be mapped on the surface. Anticline axes die out or bend towards this valley but do not cross it. Six moderate-sized earthquakes that occurred close the the Kazerun Line, and within a 25 km area involved right-lateral strike-slip motion parallel to the strike of the valley. They indicate that the Kazerun Line is the surface expression of a buried strike-slip fault. Slip vectors in these strike-slip earthquakes are different from those of neighbouring reverse-fault earthquakes, suggesting that the Kazerun Line accommodates some of the shortening between Arabia and central Iran by an elongation of the Zagros mountains parallel to strike. The centroid depths and the source dimensions of these earthquakes, combined with the lack of seismogenic surface faulting in the Zagros, suggest to us that all these earthquakes involve faulting in the metamorphic basement beneath the sedimentary cover. The sedimentary cover is almost certainly decoupled from the basement by several thick evaporite horizons. The seismicity of the Kazerun Line thus demonstrates how lateral interruptions to the regularity of a fold belt can arise from faulting in the basement, and not just from lateral ramps between the thrust sheets that deform the sedimentary cover.

SUMMARY Previously unrecognised thrust faults in eastern Iran were responsible for a destructive earthquake at Tabas (1978, September 16), which produced over 80 km of distributed and discontinuous surface ruptures above a series of low anticlinal hills to the west of a major range‐front. Analysis of long‐period body‐wave seismograms shows a simple rupture on a gently dipping (∼16°) thrust, with a slight right‐lateral component. This is compatible with the locally recorded aftershock distribution. Body wave analysis of two later, smaller events show similar source orientations. Several indicators of long‐term active folding at Tabas can be recognised in the geomorphology, and surface ruptures from 1978 are consistent with co‐seismic fold growth. Drainage incision also indicates uplift at depth on thrust faults dipping eastwards beneath the folds. Body‐wave seismograms for two earthquakes near Ferdows, 150 km east of Tabas, in September 1968 also show thrust faulting at depths of ∼10 km. Again, the surface geomorphology indicates a region of folding above an eastward‐dipping thrust fault, which is ∼10 km west of a fault bounded range‐front. In both Tabas and Ferdows, the active faulting appears to show Quaternary migration away from the range‐front, possibly in response to stresses produced by the elevated topography. The identification of zones of active fault‐related folding is important for earthquake hazard assessment and also for an understanding of the local tectonics. We conclude that the structures which gave rise to the 1968 and 1978 earthquakes could have been recognised beforehand from the clear signals in the geomorphology, had we known what to look for at that time.