Seismotectonics and consequences of the 1930s large earthquakes in eastern Mazandaran, north of Iran

Early in the 1930s, two relatively large earthquakes (Kosout, magnitude 6.8, and Talarrud, magnitude 5.8) shook the eastern Mazandaran, northern Iran. Despite the historical and instrumental seismic activity of the eastern region of Mazandaran, little is known about the status of seismotectonics and consequences of these earthquakes. This paper presents a compilation of available data from early reports of these earthquakes with new structural, geomorphic and local data on the effects of this earthquake, especially co-seismic landslides and liquefaction, to assess the seismotectonics and probable causative faults of the earthquakes. It is proposed that the close times of occurrence of two earthquakes might be due to local loading or triggering effect of the first earthquake on the second one. Like many other instrumental earthquakes in the Central Alborz, it is difficult to find the exact causative faults of important earthquakes, however the Qadikola, Chachkam or North Alborz Fault have the potential of producing Kosout earthquake and the Lalehband fault is more promising for Talarrud earthquake. Additionally, the structural complexity of the area is also discussed in the form of a hybrid tectonic model. In this model, the boundary zone of thick-skinned and active thin-skinned domains has more structural complexity than outer portions. Konim-Badeleh Shahvar pop-up structure is bounded by major faults with thick-skinned deformation style. The role of older inherited fault structures and their interaction with low-slope Neogene thrusts driven from north to south by crustal tectonics and deformation is discussed. Co-seismic landslides and rock falls have great potential to be investigated in the Alborz Range for identification of prehistoric earthquakes.