Perturbation of the Izmit earthquake aftershock decaying activity following the 1999 Mw 7.2 Duzce, Turkey, earthquake

[ 1] Triggering and quiescence patterns of seismicity in western Turkey, following the occurrence of the 12 November 1999 M-w 7.2 Duzce earthquake are investigated. The changes in seismicity rate are analyzed along the North Anatolian Fault segments that had ruptured 3 months earlier during the 17 August 1999 M-w 7.4 Izmit earthquake. Detection of triggering and quiescence is done by comparing the seismicity rate that would be expected if the Duzce earthquake had not happened to the actual observed rate. The expected rates are estimated by extrapolating the preexisting seismicity pattern, using two complementary models: a simple Omori-Utsu's law and the ETAS model. Fault segments located to the east of the Izmit epicenter show a mild case of quiescence following the Duzce earthquake. These segments had previously experienced an anomalous triggering episode in the 5 days preceding the Duzce earthquake, correlated with the occurrence of two strong ( M-w >= 5) Izmit aftershocks. However, it is not clear whether this observed quiescence is real or spurious, as it coincides with a local reconfiguration of the seismological network: a temporary increase of 0.1 to 0.15 in completeness magnitude would be enough to explain this apparent deactivation. Fault segments located to the west of the Izmit epicenter exhibit triggering of seismicity following the Duzce earthquake. This observed triggering is mostly restricted to a cluster of events located in the geothermal area of Yalova. The increase of seismicity rate is delayed by 18 hours after the waves travelling from the Duzce rupture had hit this zone. Our analysis suggests that this 15-day-long reactivation can be explained by a short-lived perturbation, the local seismic activity then sustaining itself for the remaining 15 days. We argue that this particular behavior is likely to correspond to a case of dynamic triggering. By the end of year 1999, the Yalova cluster shows a significant quiescence that lasted for several months; this shutting down does not coincide with the occurrence of any local earthquake that would be large enough to stress shadow this area, suggesting an aseismic cause to this pattern. These results highlight the fact that ( 1) seismicity triggered by a major earthquake is not restricted to areas loaded by static stress and ( 2) that seismic activity associated with geothermal geological settings is highly sensitive to stress perturbations, but also to aseismic processes, that can both delay reactivation and cause this activity to suddenly decrease.