A GEOLOGIC CONTRIBUTION TO THE EVALUATION OF THE SEISMIC POTENTIAL OFTHE KAHRIZAK FAULT (TEHRAN, IRAN)

In this paper we present the results of preliminary geomorphic and tre nching investigations along the Kahrizak fault. This fault is located south of the highly populated metropolis of Tehran and represents one of the main structures in the area containing important seismic potent ial. The Kahrizak fault has a very clear expression at the surface whe re it forms a prominent 35-km-long, 15-m-high scarp on Holocene alluvi al deposits. The fault strikes N70 degrees-80 degrees W and dips to th e north. Movement is prevalently right-lateral with the northern side of the fault up. Trench excavations exposed a sequence of weathered, m assive, alluvial deposits which are dated, by means of radiometric met hods, to the Holocene. In the trenches the sequence is intensely defor med by north-dipping, high-and low-angle faults within a 30-m-wide zon e. On the basis of stratigraphic and structural relations, some eviden ce for individual Holocene earthquakes is found; however, we were not able to reconstruct the seismic history of the fault nor to evaluate t he size of deformation produced by each event. Because of the possible similar to 10 m offset of ancient linear hydraulic artifacts (qanats) , that cross the fault, we hypothesize that the most recent event may have occurred in historical times (more recent than 5000 yr B.P.) and it may be one of those reported in this area by the current catalogues of seismicity. Based on these preliminary investigations we estimate an elapsed time between 5000 and 800 years, a maximum slip per event d (max) of similar to 10 m, a minimum Holocene vertical slip rate of sim ilar to 1 mm/yr versus a horizontal slip rate of similar to 3.5 mm/yr, a maximum of similar to 3000 years for the average recurrence time, a nd an expected M-w = 7.0 to 7.4. These can be considered as a first-ha nd reference for the activity on this fault. (C) 1998 Elsevier Science B.V. All rights reserved.