Irregular earthquake cycle along the southern Tianshan front, China (Aksu area)

A long deformation record (􏰩16.5 ka) showing nonsteady earthquake recurrence is established for the Aksu thrust (Tianshan, China) on the basis of cosmogenic 10Be dating of faulted surfaces. Topographic leveling across the Aksu fault scarp indicates up to 􏰩10 m uplift across a moraine abandoned 􏰩16.5 ka and similar uplift of 􏰩6 m across two inset surfaces yielding ages of 􏰩12.5 and 􏰩5 ka. Successively smaller uplifts of terraces younger than 5 ka indicate that three or more major earthquakes occurred during this period. These data show that the Aksu thrust fault was quiet for at least 7500 years and active in the last 􏰩5000 years and probably in the interval 16.5–12.5 ka. The seismic cycle along the Aksu thrust fault over the last 􏰩16,500 years was thus strongly irregular showing long quiescence and clustering, which is a challenge for paleoseismology and hazard assessment. This irregularity also makes it more difficult to estimate long-term shortening rates. Nevertheless, we are able to constrain a minimum total shortening rate (>7 mm/yr) across the southern Tianshan front for the last 12,500 years that is about a third of the total geodetic rate for the western Tianshan (􏰩20 mm/yr).