Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The ∼55 km-long Firouzkuh fault is located in the Central Alborz Mountains of Iran. It is a left-lateral fault, which dips to the south, and possesses a small dip-slip component of motion that we interpret to result from extension. The ratio of horizontal to vertical displacement across the fault, calculated from the cumulative displacement of landscape features, is 7.6. We provide constraints on the timing of the last earthquake on the Firouzkuh fault from two trenches (T1 and T2) across the fault zone, excavated in 2004, and located east of Firouzkuh city. The trenches expose faulted sedimentary deposits. Two optically-stimulated luminescence (OSL) ages from sediments in the lower part of trench T1 date from the late Pleistocene (15.9 ± 0.9 ka and 27.1 ± 1.7 ka). The younger of the two dated units in T1 is displaced vertically across the fault by 2.2–4.4 m, from which we estimate a strike-slip displacement of 18.2–33.4 m, and hence a average horizontal slip-rate of 1.1–2.2 mm/yr. The sediments exposed in T1 do not yield constraints on the most recent earthquake history. In trench T2, however, human skeletal remains of a middle aged male, which yield a radiocarbon age of 1159 ± 28 BP (corresponding to a mean calendar age of 791 AD), were found within a faulted alluvial layer at a depth of 60–70 cm from the surface. The existence of these medieval human places shows that a surface-rupturing earthquake occurred at some time after 1159 ± 28 BP. The amount of slip in each earthquake on the Firouzkuh fault is difficult to estimate, but assuming the entire ∼55 km fault length ruptures in each event, they will have had a maximum magnitude of 7.1. At our estimated late Quaternary slip-rate of ∼1.1–2.2 mm/yr magnitude 7.1 earthquakes, involving ∼1.2 m average displacement, would be expected to occur every ∼1100–540 years. As the last earthquake on the Firouzkuh fault may be up to ∼700 years in age we suggest that the Firouzkuh fault is a major hazard for earthquakes in the near future.
ISSN: 0743-9547

hal-01122835
https://hal.archives-ouvertes.fr/hal-01122835
DOI : 10.1016/j.jseaes.2013.12.018

Éditeur(s) : HAL CCSD
Résumé : The Firouzkuh Fault in most of its extension is located at the southern side of Firouzkuh Valley in west of the Central Alborz. It is a SW - NE prominent structure with a 70 km long on a sinistral -normal fault. It has been known as a south-dipping reverse fault.Paleoseismic analysis and C14 dating along the Firouzkuh left -lateral strike-slip fault indicate that Central Alborz has been shaken by large earthquakes during the Holocene. Here we present the data carried out of one of the two excavated trenches with 15 m long, 2m wide, ~ 4m deep in the east of Firouzkuh, where we found some evidence for last paleoearthquake associated to seismic re-activity on Firouzkuh Fault in the second trench (F2) that dug across a gauge zone. The last event is evidenced by cutting young superficial deposit where this faulted unit stratigrafically covered alluvium deposits yielded dated human bones fragments. The bones found in 60-70cm-deep of trench which a C14 age of 1159 ± 28 BP. Regarding to estimated slips per event on paleoseismological log of eastern wall of trench F1, event magnitudes for each paleoearthquakes are M≈7.
Geosciences Scientific Quarterly Journal

hal-00800375
https://hal.archives-ouvertes.fr/hal-00800375