17 August (Mw 7.4) and 12 November 1999 (Mw 7.2) earthquakes have caused major concern about future earthquake occurrences in Istanbul and in the Marmara Region. Stress transfer studies and renewal model type probabilistic investigations indicate about 2% annual probability for a M _w = 7+ earthquake in the Marmara Sea. As part of the preparations for the expected earthquake in Istanbul, an early warning system has been established in 2002. A simple and robust algorithm, based on the exceedance of specified threshold time domain amplitude and the cumulative absolute velocity (CAV) levels, is implemented for this system. Rational threshold levels related to new bracketed CAV window approach (BCAV-W) are determined, based on dataset of strong ground motion records with fault distances of less than 100 km, as 0.2 m/s, 0.4 m/s and 0.7 m/s related to three alarm levels which will be incorporated in the Istanbul earthquake early warning system.