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We perform a detailed synthetic study on the resolution of non-double-couple (non-DC) components in the seismic moment tensors from short-period data observed at regional networks designed typically for monitoring aftershock sequences of large earthquakes. In addition, we test two different inversion approaches-a linear full moment tensor inversion and a nonlinear moment tensor inversion constrained to a shear-tensile source model. The inversions are applied to synthetic first-motion P-and S-wave amplitudes, which mimic seismic observations of aftershocks of the 1999 M w = 7.4 Izmit earthquake in northwestern Turkey adopting a shear-tensile source model. To analyse the resolution capability for the obtained non-DC components inverted, we contaminate synthetic amplitudes with random noise and incorporate realistic uncertainties in the velocity model as well as in the hypocentre locations. We find that the constrained moment tensor inversion yields significantly smaller errors in the non-DC components than the full moment tensor inversion. In particular, the errors in the compensated linear vector dipole (CLVD) component are reduced if the constrained inversion is applied. Furthermore, we show that including the S-wave amplitudes in addition to P-wave amplitudes into the inversion helps to obtain reliable non-DC components. For the studied station configurations, the resolution remains limited due to the lack of stations with epicentral distances less than 15 km. Assuming realistic noise in waveform data and uncertainties in the velocity model, the errors in the non-DC components are as high as ±15 per cent for the isotropic and CLVD components, respectively, thus being non-negligible in most applications. However, the orientation of P-and Taxes is well determined even when errors in the modelling procedure are high.

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 M w = 7.1 Düzce 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 analysed earthquake catalogue contains waveforms of more than 4000 accurately located events observed at 35 three-component short-period seismic stations. To ensure high-quality data with 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-Düzce and the Akyazi segments. Events located in the Izmit-Sapanca and Karadere-Düzce 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 per cent, 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.

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