Paleoearthquakes of the past 30,000 years along the North Tehran Fault (Iran)

Abstract

[1] The North Tehran Fault (NTF) is located at the southernmost piedmont of Central Alborz and crosses the northern suburbs of the Tehran metropolis and adjacent cities, where $15 million people live. Extending over a length of about 110 km, the NTF stands out as a major active fault and represents an important seismic hazard for the Iranian capital after historical seismicity. In order to characterize the activity of the NTF in terms of kinematics, magnitude and recurrence intervals of earthquakes, we carried out a first paleoseismological study of the fault within its central part between Tehran and Karaj cities. We opened a trench across a 3 m-high fault scarp affecting Quaternary deposits. Our study shows that the scarp is the result of repeated events along a main N115 E trending shallow dipping thrust fault, associated with secondary ruptures. From the trench analysis and Infrared Stimulated Luminescence (IRSL) dating of fault-related sediments, we interpreted between 6 and 7 surface-rupturing events that occurred during the past 30 kyrs. Their magnitudes (estimated from the displacements along the faults) are comprised between 6.1 and 7.2. The two last events -the largest -occurred during the past 7.9 AE 1.2 ka, which yields a Holocene slip rate of $0.3 mm/yr. The 7 earthquakes scenario suggests a regular periodicity with a mean recurrence interval of $3.8 kyrs. However, the two most recent events could correspond to the two largest historical earthquakes recorded in the area (in 312-280 B.C. and 1177 A.D.), and therefore suggest that the NTF activity is not regular.

Active faulting within a megacity: the geometry and slip rate of the Pardisan thrust in central Tehran, Iran

“…4a), consistent with it being underlain by a north-dipping thrust fault (e.g. Abbassi & Farbod 2009;Landgraf 2010;Ritz et al 2012). However, in the eastern part of the fold sediments in the northern limb are also locally steepened by back thrusting on the Davudieh and 'east-west' faults, which are both visible in the topography as abrupt breaks in the level of incision (Fig.…”

“…Djamour et al 2010). The prominent NTF, which runs along the range-front of the Alborz Mts., has a late Pleistocene slip-rate of only ∼0.3 mm yr −1 (Ritz et al 2012). Our results indicate that the Pardisan fold is related to an active north-dipping thrust fault with a slip-rate of at least 1 mm yr −1 .…”

“…The existence of the Pardisan fault (which has also been called the Milad Tower thrust or foreberg in some studies) has previously been acknowledged (e.g. Jackson 2008;Abbassi & Farbod 2009;Landgraf 2010;Nazari et al 2010;Ritz et al 2012), but our present study presents the first detailed geomorphic analysis and estimate of its slip-rate. The OSL samples analysed in our study were collected in 2008, shortly before the exposures in the vicinity were completely masked or destroyed by urban activities.…”

“…Solaymani Azad et al 2011;Ghassemi et al 2014). Ritz et al (2012) uncovered six or seven surface-rupturing events on the western NTF within the last ∼30 ka, with magnitudes that they estimate at 6.1 to 7.2, and an average slip-rate of ∼0.3 mm yr −1 . They state that the two most recent events in their trench may correspond to the 312-280 B.C.…”

“…Fig. 1; Ambraseys & Melville 1982;Berberian & Yeats 1999;Nazari et al 2009;Ritz et al 2012). However, earthquakes in 312-280 B.C., 855 A.D., 1177 A.D. were located further south, within the Tehran plain, and their sources are not known with certainty, although palaeoseismic events on the North Tehran Fault (NTF; Ritz et al 2012; Fig.…”