Description: Seismic moment tensors can provide information on the size and orientation of fractures producing acoustic emissions (AEs) and on the stress conditions in the sample. The moment tensor inversion of AEs is, however, a demanding procedure requiring carefully calibrated sensors and accurate knowledge of the velocity model. In field observations, the velocity model is usually isotropic and time independent. In laboratory experiments, the velocity is often anisotropic and time dependent and attenuation might be significant due to opening or closure of microcracks in the sample during loading. In this paper, we study the sensitivity of the moment tensor inversion to anisotropy of P -wave velocities and attenuation. We show that retrieved moment tensors critically depend on anisotropy and attenuation and their neglect can lead to misinterpretations of the source mechanisms. The accuracy of the inversion also depends on the fracturing mode of AEs: tensile events are more sensitive to P -wave anisotropy and attenuation than shear events. We show that geometry of faulting in anisotropic rocks should be studied using the source tensors, since the P - and T -axes of the moment tensors are affected by velocity anisotropy and deviate from the true orientation of faulting. The stronger the anisotropy is, the larger the deviations are. Finally, we prove that the moment tensor inversion applied to a large dataset of AEs can be utilized to provide information on the attenuation parameters of the rock sample. The method is capable of measuring anisotropic attenuation in the sample and allows for detection of dilatant cracking according to the stress regime.

Description: We investigate spatiotemporal variations of the crustal stress field orientation along the rupture zones of the 1999 August Izmit M w 7.4 and November Düzce M w 7.1 earthquakes at the North Anatolian Fault zone (NAFZ) in NW Turkey. Our primary focus is to elaborate on the relation between the state of the crustal stress field and distinct seismotectonic features as well as variations of coseismic slip within the seismogenic layer of the crust. To achieve this, we compile an extensive data base of hypocentres and first-motion polarities including a newly derived local hypocentre catalogue extending from 2 yr prior (1997) to 2 yr after (2001) the Izmit and Düzce main shocks. This combined data set allows studying spatial and temporal variations of stress field orientation along distinct fault segments for the pre- and post-seimic phase of the two large earthquakes in detail. Furthermore, the occurrence of two M  〉 7 earthquakes in rapid succession gives the unique opportunity to analyse the 87-d-long ‘inter-seismic phase’ between them. We use the MOTSI (first MOTion polarity Stress Inversion) procedure directly inverting first-motion polarities to study the stress field evolution of nine distinct segments. In particular, this allows to determine the stress tensor also for the pre- and post-seismic phases when no stable single-event focal mechanisms can be determined. We observe significantly different stress field orientations along the combined 200-km-long rupture in accordance with lateral variations of coseismic slip and seismotectonic setting. Distinct vertical linear segments of the NAFZ show either pure-strike slip behaviour or transtensional and normal faulting if located near pull-apart basins. Pull-apart structures such as the Akyazi and Düzce basins show a predominant normal faulting behaviour along the NAFZ and reflect clearly different characteristic from neighbouring strike-slip segments. Substantial lateral stress field heterogeneity following the two main shocks is observed that declines with time towards the post-seismic period that rather reflects the regional right-lateral strike-slip stress field.