Slip Rate and Past Earthquakes Along the Astaneh Fault (iran)
Abstract
The Astaneh fault is classically described as the seismic source that generated the 856 AD Qumis earthquake, the most destructive earthquake to have occurred in the northeastern part of the Alborz range in Iran (estimated magnitude M 7.9; over 200,000 people killed). We present the results of a morphotectonic and paleoseismic study along the eastern part of the Astaneh Valley allowing to characterize the activity of the Astaneh fault. From a morphological analysis, we identify at least three generations of fans in the studied area; all of them have been displaced along the left-lateral strike-slip Astaneh fault. Luminescence analyses (OSL and IRSL) of the coarse matrix material (250-180 μm) from the alluvial fans provide ages of 55 ka, 32 ka and 16 ka, for the 3 successive generations of fans, respectively. These ages allow estimation of a mean horizontal slip rate of ~2mm/yr along the Astaneh fault, which is consistent with published GPS data and slip rates determined for the Holocene period along faults in the continuation of the Astaneh fault to the West. Two paleoseismological studies, within 2 trenches (AT3 and AT5), carried out on a playa area crossed by the fault, allow us to estimate the age of past seismic events, their recurrence intervals and their magnitudes. Within the AT3 trench, our paleoseismic observations reveal a rupture prior to 1300 cal A.D and significantly later than 600 cal B.C. Within the AT5 trench the most recent surface rupture occurred between 700 ± 20 cal BP and 1,370 ± 140 yrs (IRSL age). In both trenches, the most recent surface ruptures could therefore correspond to the Qumis historical earthquake. Morphological analyses along the fault zone yield a co-seismic offset of 3.9 ± 0.3 m for this last surface rupture, suggesting a moment magnitude comprised between 7.3 to 7.5, consistent with the magnitude estimated for the Qumis historical earthquake. Further paleoseismological investigations within the AT5 trench show that six repeated earthquakes, including the 856 AD historical event, were recorded during the past 12 000 years with a mean recurrence intervals of 1800 ± 230 years. Analyses of cumulative offsets and the distribution of past events through time suggest a quasi-periodic seismic model for the Astaneh fault.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T51B2323R
- Keywords:
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- 1105 GEOCHRONOLOGY / Quaternary geochronology;
- 5475 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Tectonics;
- 7221 SEISMOLOGY / Paleoseismology;
- 8175 TECTONOPHYSICS / Tectonics and landscape evolution