Major Asperities Along The Izmit Rupture Of The North Anatolian Fault Zone Obtained From Analysis Of Aftershocks Of The August, 17th 1999 Izmit Earthquake

We investigated spatial and temporal clustering of 10066 aftershocks along the Izmit Mw=7.4 August, 17th 1999 earthquake (EQ) rupture zone with a total length of 140 km. In the time window from August, 24th 1999 to the day the Duzce Mw=7.2 EQ occurred (November, 12th 1999), we identified three spatial clusters, namely Izmit (IC), Sakarya (SC) and Karadere-Duzce (KDC). The cumulative number of EQ versus time (N(t)) significantly decreases in two of the clusters (SC and KDC) at a critical time (tc= September 29th, 1999), 45 days before the Duzce EQ occurred. We interpret this to represent seismic quiescence in aftershock activity and are able to quantify it by the change in slope of the cumulative number (dN (t)/dt) from 63 EQ/day before to 42 EQ/day after tc. Restricting our aftershock catalogue to highly precise located events with horizontal and vertical errors < 5 km, we calculated b-values (Gutenberg-Richter law) and p-values (Omori law) for 7348 events. Mean values of the aftershock sequence are determined to be b=0.75±0.01 and p=0.71±0.02, respectively. The lowest b-value (0.6) was calculated in the Izmit hypocentral region; the highest b-value (1.0) was observed east of Sapanca Lake, and at the eastern and western rim of KDC. High b-values at Sapanca lake correlate with maximum surface displacement of 5.2 m. Highest p-value (1.2) was identified again in the Izmit hypocentral area, and in the SC. Low p-values (0.6) were calculated from aftershock sequence east of Sapanca Lake and in the KDC. A pronounced (50 per cent) drop in b-value (from 0.9 to 0.45) is observed in the KDC. At SC the drop occurs 11 days earlier compared to KDC and is slightly less pronounced (0.8 to 0.6). We discriminated two zones in the depth variation of b-value. At shallower depth (1-10 km), b-values are below 0.6 (brittle crust), below 10 km the b-values monotonically increase to a value of 1.2 at a depth of 14 km, i.e. in the depth range of brittle-ductile transition where also the hypocenters of the Izmit and Duzce EQ were located. Key words: Fault asperities, Gutenberg-Richter law, Izmit earthquake, North Anatolian Fault Zone, Omori law.