Slip rate and earthquake recurrence of the Düzce fault (North Anatolian fault zone): integrating geomorphological and paleoseismological analyses

Abstract

To learn about recurrence of large earthquakes and strain model on the Düzce segment of the North Anatolian Fault Zone, that ruptured on November, 12, 1999 (Mw 7.1), systematic geomorphological and paleoseismological analyses were integrated. In order to provide new estimates on Quaternary slip rate of the fault., geo-morphological mapping along the fault trace permitted to analyze fault-related cumulative landforms and drainage pattern settings. Remnant of an old alluvial fan modeled by fluvial terraces and 41 right-hand stream deflections were reconstructed, described and used as offset geomorphic markers. Two correlated Late Pleistocene, terrace risers, offset of about 300 and 900 m, respectively, were dated by means of OSL method about 21 kyr BP and 60 kyr BP. Moreover, the onset of the offset of the streams deflected for a total of 100 m was radiocarbon dated about 7000 yr BP. These data translate to a constant rate of deformation of 15.0±3.2 mm/yr for the last 60 kyr. On the basis of “characteristic-earthquake model” and under constant slip rate assumptions, it is possible to estimate that stream deflections across the Düzce Fault may be explained by repetition of 20-30 1999-like earthquakes during the last 7000 years, thus the recurrence time for surface rupturing events of the Düzce Fault is 290±60 years. With the aim to reconstruct the record of last large earthquakes, 10 trenches at five sites were excavated. By merging information obtained from all trenches, evidence for three surface faulting earthquakes prior to 1999 were found. These paleoearthquakes are dated on the basis of radiocarbon, 210Pb and archaeological information and can be summarized at: 1) AD1685-1900, possibly end of 19th century; 2) AD1685-1900, possibly close to AD1700; 3) AD800-1000. Some of them can be correlated to historical earthquakes occurred on AD967, 1719, 1878 or 1894. These paleoseismological results, merged with those from previous papers, are suggestive of bimodal recurrence distribution yielding overall average recurrence time of ~350 yrs for the past two millennia, compatible with that calculated from the geomorphic markers analysis. Under the assumption of “characteristic earthquake”, bimodal paleoearthquakes distribution indicate slip rate fluctuations during the past two millennia, with clustered high strain release. These results suggest Wallace-type strain release model for the Düzce fault, and average strain accumulation of 11.4±1.2 mm/yr, in agreement with slip rate results for the past 60 kyr obtained by geomorphic marker analysis.