The submarine Istanbul-Silivri fault segment, within 15 km of Istanbul, is the only portion of the North Anatolian Fault that has not ruptured in the last 250 years. We report first results of a seafloor acoustic ranging experiment to quantify current horizontal deformation along this segment and assess whether the segment is creeping aseismically or accumulating stress to be released in a future event. Ten transponders were installed to monitor length variations along 15 baselines. A joint least squares inversion for across-fault baseline changes, accounting for sound speed drift at each transponder, precludes fault displacement rates larger than a fewmillimeters per year during the 6month observation period. Forward modeling shows that the data better fit a locked state or a very moderate surface creep—less than 6mm/yr compared to a far-field slip rate of over 20mm/yr—suggesting that the fault segment is currently accumulating stress.