The major asperities of the 1999 M w = 7.4 Izmit earthquake defined by the microseismicity of the two decades before it

Summary We compare the rupture location of the M w  7.4 Izmit earthquake to the local seismic hazard estimated by the technique of mapping local recurrence time, T L , based on the microseismicity. After correcting for a magnitude shift in 1990, the declustered earthquake catalogue, produced by the University of Istanbul for the Marmara Sea region, is homogeneous for M d  ≥ 2.9 during 1983–1999. We mapped T L in the area bounded by 40°–41° latitude and 27.6°–30.5° longitude. T L is the probabilistic estimate of recurrence time, calculated from the a ‐ and b ‐values of the frequency–magnitude relation of the seismicity within a radius of 20 km from every point on a grid with 5 km spacing. T L varies strongly as a function of space, since a ‐ and b ‐values also vary strongly. In our interpretation, the 5–20 per cent of locations with the shortest recurrence times map major asperities. In the Marmara region, we mapped four anomalies of short T L , together covering about 12 per cent of the total area. They are centred near 40.25°/29.4°, 40.8°/28.3°, 40.75°/28.8° and 40.7°/29.8°. The last two of these coincide with the western end of the rupture and the epicentre location of the Izmit earthquake, respectively. Thus, we suggest that the major asperity of this rupture and a point past which it could not propagate were mapped out by the background seismicity during the years before the event as locations that produced more large microearthquakes than average, and hence showed anomalously short T L . The T L method does not contain information about when earthquakes are expected, and the absolute values of the recurrence time could be inaccurate. The method only specifies the most likely locations of main shocks. Since the method is new, it will have to be tested for many cases and in many areas before its reliability can be assessed.