Slip history of the 1944 Bolu-Gerede earthquake rupture along the North Anatolian fault system: Implications for recurrence behavior of multisegment earthquakes

Recent research shows that active fault systems produce multisegment earthquakes; however, we have yet to understand the faulting behavior of various spatial patterns of segments. We conducted a three-dimensional trenching survey to reconstruct the detailed slip history of a fault segment that ruptured as one of the multisegment ruptures along the North Anatolian fault system. The trench site, on the Gerede segment, recorded a maximum right-lateral slip of up to 6 m that was associated with the 1944 Bolu-Gerede earthquake (M 7.4). Fault exposures show evidence of four paleoearthquakes. Radiocarbon dates, a refined probability density distribution, and correlation with historical earthquakes place the mean repeat time at similar to 330 years. Four discrete paleoslips yield a slip per event of 5.0 +/- 0.8 m with a coefficient of variation of 0.2. Our research suggests that multisegment earthquakes exhibit various spatial patterns, regardless of recurrence with quasiperiodicity and characteristic slip. Coincidentally, the fault geometry exhibits extremely linear traces, suggesting simple stress accumulation and release through earthquake cycles. Furthermore, the 1944 event did not occur in a single segment, and the Gerede segment probably ruptured within a slip-pulse-like rupture during a multisegment earthquake. A comparable geological slip rate of similar to 17 mm a(-1) based on a GPS-based strain rate supports the persistence of macroscopic asperity through recent geological time. Therefore we conclude that a segment with simple fault geometry along a strike-slip fault system plays an important role in forecasting the timing of future multisegment earthquakes, but the spatial extent of such earthquakes needs to be explored further.