Surface creep on the North Anatolian Fault at Ismetpasa, Turkey, 1944–2016

We reevaluate the 72 year history of surface slip on the North Anatolian Fault at Ismetpasa since the Mw = 7.4 1944 Bolu/Gerede earthquake. A revised analysis of published observations suggests that days after the earthquake the fault had been offset by 3.7 m and 6 years later by an additional 0.74 m. Creep was first recognized on the fault in 1969 as a 0.13 m offset of a wall constructed in 1957 that now (2016) has been offset by 0.52 m. A carbon rod creep meter operated across the fault in the past 2 years confirms results from an invar wire creep meter operated 1982–1991 that surface slip is episodic. Months of fault inactivity are interrupted by slow slip (≤10 µm/d) or multiple creep events with cumulative amplitudes of 2–10 mm, durations of several weeks, and with slip rates briefly exceeding >2.5 mm/h. Creep events accommodate 80% of the surface slip and individually release ≈ 10−6 shear strain on the flanks of the uppermost 3–7 km of the fault. GPS and interferometric synthetic aperture radar methods yield a current fault slip rate of 7.6 ± 1 mm/yr suggesting that creep meters incompletely sample the full width of the surface shear zone. The slip rate has slowed from >10 mm/yr in 1969 to 6.1 mm/yr at present, 4.65 mm/yr of which appears to be due to steady interseismic creep driven by plate boundary stressing rates. We calculate that a further 1 m of aseismic surface slip will precede the next major earthquake on the fault assuming an ≈ 260 year main shock recurrence interval on this segment. Plain Language Summary We describe slow surface slip on the North Anatolian fault in Turkey, similar to that occurring on the San Andreas and Hayward Faults of California. Rapid slip was initiated by a magnitude 7.4 earthquake in 1944 (several inches/year) but by 1969 it had slowed to less than 1/2″ per year and by 2016 had slowed to 1/4″ per year. The slip occurs in brief episodes at roughly 8 month intervals with durations of a few hours to weeks, and appears to be confined to the uppermost 3 miles of the 7 mile deep fault. When it occurs this slip stresses the top of the region at depth that slips in large earthquakes, suggesting that monitoring surface slip close to the anticipated time of the next earthquake may provide an indication of the imminence of its occurrence. Key Points Afterslip decayed rapidly in the first two decades following the 1944 Mw=7.4 Bolu earthquake, and surface slip is now driven almost entirely by plate boundary shear stress Eighty