Measuring slip rate variability on the Eşen Fault, SW Türkiye, with cosmogenic chlorine-36 nuclide analysis

Seismic hazard models often assume near-constant earthquake recurrence intervals on faults since the Last Glacial Maximum, approximately 15,000 years ago. However, it is tricky to show that real fault systems exhibit this behaviour, particularly for distributed networks of normal faults in extensional regimes. Instead, data is limited to historical seismology records, which is likely over a much shorter time than earthquake recurrence intervals, or a single time-averaged Holocene slip rate from paleoseismology methods. Neither method measures slip rate variability over multiple earthquake cycles.

Cosmogenic nuclide analysis on limestone bedrock fault scarps, combined with Bayesian modelling, is an established method to interpret exhumation histories of normal faults since the Last Glacial Maximum. Production of chlorine-36 (³⁶Cl) is primarily by interaction of calcium-40 in the limestone scarp with cosmic rays. Concentration profiles of ³⁶Cl on a fault scarp therefore correlate with fault slip in earthquakes. Previous ³⁶Cl studies demonstrate slip rate variability of normal faults in Italy and Türkiye.

We apply this technique to interpret the slip history of the Eşen Fault, a major normal fault in southwest Türkiye with no known historical seismicity. Bayesian models suggest the last major earthquake was 1000 years ago, but prior to that, there was a period of fast slip of 2-3 mm/yr, which exposed at least 5 m of scarp in 2-3 kyr. Before that, the slip rate was much lower, at about 1 mm/yr. These results demonstrate slip rate variability, which informs our understanding of fault dynamics over millennia, and may help to improve seismic hazard models.