Paleoseismic evidence for surface rupturing earthquakes on the West Caspian Fault (Azerbaijan): an active right lateral strike slip fault zone in an over-pressured sedimentary basin

We provide the first geomorphic evidence for active right-lateral strike-slip faulting at the eastern margins of the Greater Caucasus and western margins of the South Caspian Basin. The West Caspian Fault comprises several parallel strands, exhibiting numerous right-lateral stream deflections, following mud volcano alignments. Mud volcanoes occur predominantly at bends and folds along the otherwise strike-slip trace, indicating a structural control on emplacement. Mud volcano distribution and alignments in seismicity allow us to infer a total fault length of ~150 km. We estimate a Holocene slip-rate of 3.9-4.8 mm/yr, based upon a displaced terrace riser of assumed age. Two paleoseismic trenches reveal between five and six surface rupture events over the last 3000 years, and an average recurrence interval of 500-600 years. The events are unevenly spaced in time, with a cluster of 3 events in the last 650 years, and longer intervals earlier. From earthquake scaling relationships we expect rupture of the entire fault length to produce Mw 7.5 events with average slip of 3 m, consistent with the smaller measured lateral offsets. However, a lack of historical earthquakes combined with inferences on fault behaviour within the fluid-rich over-pressured Kura basin sediments lead us to speculate that the ruptures may represent aseismic slip or low-magnitude surface-rupturing earthquakes.