Morteza Talebian

The 2003 Bam (Iran) earthquake: Rupture of a blind strike-slip fault

A magnitude 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.

Structural variation along the Zagros and the nature of the Dezful Embayment

Structure varies along strike in the Zagros fold-and-thrust belt of Iran, which is a principal element in the Arabia–Eurasia continental collision. Pre-collision, Late Cretaceous ophiolite nappes (Kermanshah, Neyriz) and related nappes of deep marine sediments (Radiolarite Series) were emplaced next to two regions (Pusht-e Kuh arc, Fars) which later developed a consistent structural style across the range from the High Zagros Fault to the foreland limit of deformation. The intervening area has a zone of highly imbricated Arabian plate strata (the Bakhtyari Culmination) thrust southwest towards and over a low relief, low elevation region (the Dezful Embayment). There are no ophiolite nappes northeast of the Bakhtyari Culmination. Isopachs reflect these different structural patterns from Late Cretaceous time but not earlier. In Late Cretaceous time the Dezful Embayment recorded less deposition than adjacent areas to the northwest and southeast. In the Palaeogene there was little net difference between the Dezful Embayment and its margins. The Dezful Embayment has been a depocentre since roughly 35 Ma, which is the likely time of initial collision between Arabia and Eurasia. We propose that the syn-collision structure and stratigraphy of the Zagros is therefore strongly influenced by the variation in Late Cretaceous ophiolite emplacement, but the original cause of this variation is not clear.

Active tectonics of the east Alborz mountains, NE Iran: Rupture of the left-lateral Astaneh fault system during the great 856 A.D. Qumis earthquake

The 856 A.D. Qumis earthquake (M7.9) is the most destructive earthquake to have occurred in Iran, killing more than 200,000 people and destroying the cities of Damghan and the old Parthian capital of Shahr-i Qumis (Hecatompylos). This study combines evidence of historical seismicity with observations of the geomorphology and paleoseismology to provide the first detailed description of active faulting in the Damghan region of the east Alborz mountains, Iran. Regional left-lateral shear is accommodated on the Astaneh, Damghan, and North Damghan faults. Quaternary alluvial fans have been displaced along the Astaneh fault, with 15–20 m stream offsets recording the cumulative displacement over the last two to five earthquakes. A paleoseismology study from a single trench along a 5–10 km segment of the Astaneh fault reveals a rupture prior to 1300 A.D. and significantly later than 600 B.C. Despite the limitations of a single trench in documenting the spatial and temporal evolution of the fault over the late Quaternary, we are nevertheless able to bracket the last event to a time period consistent with the 856 A.D. earthquake. Two older earthquakes were also identified during the Holocene occurring between 600 B.C. and 4600 B.C. and between 4600 B.C. and 9600 B.C. The location of our trench within a bend on the Astaneh fault, which could act as a barrier to rupture propagation, means the three earthquakes recovered from our trench over the Holocene may represent a minimum. Further trenching will reveal how the Astaneh fault ruptures over repeated earthquakes and, consequently, the magnitude and extent of slip during the 856 A.D. earthquake.

The structure and late Quaternary slip rate of the Rafsanjan strike-slip fault, SE Iran

The Rafsanjan right-lateral strike-slip fault in SE Iran has a clear expression in the geomorphology, is sited close to several large population centers, and yet its role in the regional tectonics, its rate of activity, and its potential to generate destructive earthquakes are unknown. We use high-satellite imagery and field investigation to identify the active strands of the fault system and show that the overall north-south right-lateral shearing across the region is spatially separated into almost pure strike slip on the NW-SE–trending Rafsanjan fault and an orthogonal component of shortening on parallel thrust faults in the lowlands. Possible remnants of ruptures, involving right-lateral slip of ∼3 m, from an earthquake of ∼Mw >7 are identified along the eastern part of the Rafsanjan fault. We speculate that these ruptures result from the destructive 1923 Laleh event. An alluvial fan displaced right laterally by 48 ± 4 m and tentatively dated at ∼120 ka yields an average slip-rate estimate of ∼0.4 mm/yr. Our slip-rate estimate is consistent with known estimates of late Quaternary slip rate on other faults within eastern Iran and with global positioning system (GPS) measurements of present-day deformation in this part of the country. Our results therefore suggest that the slip rates of faults in eastern Iran do not vary substantially through the late Quaternary: a result that is important for the interpretation of geodetic and late Quaternary measurements of slip rate in regions of distributed strike-slip faulting.

Determination of Slip-Rate by Optical Dating of Lake Bed Sediments from the Dasht-E-Bayaz Fault, Ne Iran

Abstract The Dasht-e-Bayaz left-lateral strike-slip fault in northeastern Iran ruptured in two destructive earthquakes in 1968 and 1979. The western half of the Dasht-e-Bayaz fault cuts across the dry lake-bed in the Nimbluk valley and has no measurable relief except for at a few localised jogs in the fault trace. We provide the first quantitative constraint on the slip-rate of the Dasht-e-Bayaz fault averaged over the Holocene. The western part of the fault cuts across the Nimbluk valley; the flat surface of which is composed of lake-bed sediments. Small streams cut into the surface of the lake-beds are displaced across the fault by 26 ± 2 m. Two OSL samples of the lake-bed sediments are success-fully dated at 8.6 ± 0.6 and 8.5 ± 1.0 ka, from which we calculate a minimum slip-rate of 2.6 mm/yr. This minimum slip rate remains constant with the previously proposed Holocene slip rate of 2.5 mm/yr and within the range of the Holocene slip rate of 2.4 ± 0.3 mm/yr estimated before on the central section of the Doruneh fault.