Resolution of non-double-couple components in the seismic moment tensor using regional networks—II: application to aftershocks of the 1999 Mw 7.4 Izmit earthquake

Abstract

SUMMARY We study potential non-double-couple (non-DC) components in aftershocks of the 1999 Izmit earthquake. The Izmit earthquake ruptured a ∼140-km-long segment of the North Anatolian Fault Zone in northwestern Turkey and was followed by the Mw = 7.1 D¨ uzce earthquake that extended the rupture further to the east. Focal mechanisms of Izmit aftershocks clearly indicate a segmentation of the rupture into several segments, one of which is the Akyazi Plain, a pull-apart structure, where significant non-DC components might be observed. The analysedearthquakecataloguecontainswaveformsofmorethan4000accuratelylocatedevents observedat35three-componentshort-periodseismicstations.Toensurehigh-qualitydatawith good focal coverage, we apply strict quality criteria to the aftershock catalogue reducing the number of events to only 33 aftershocks for which stable moment tensors were calculated using P- and S-wave amplitudes. The moment tensors of the 33 analysed aftershocks display significant differences in the percentage of the non-DC components for the three distinct fault segments: the Izmit-Sapanca, Karadere-D¨ uzce and the Akyazi segments. Events located in the Izmit-Sapanca and Karadere-D¨ uzce segments exhibit only small percentages of the non-DC components and if existent they are mainly positive. This correlates well with the predominant strike-slip stress regime along these segment and also with the main shock rupture being rightlateral strike-slip. In contrary, we found a substantial percentage of non-DC components for events below the Akyazi Plain where the Sapanca Fault splits into the Mudurnu and Karadere faults. There, the observed non-DC components are entirely positive indicating a tensional regime and ranging from 20 to 48 percent, clearly exceeding the defined error bounds found in a synthetic study. This observation is in accordance with the post-seismic setting following the Izmit main shock that left a remarkable slip deficit of 3.5 m below the Akyazi bend.