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

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.

Detailed analysis of spatiotemporal variations of the stress field orientation along the Izmit-Düzce rupture in NW Turkey from inversion of first-motion polarity data

“…This pattern was also observed by Seeber et al (2000) who found highly diverse focal mechanisms reflecting normal faulting as well as strike slip mechanisms. Moment tensor solutions from Stierle et al (2014b) also indicate a normal faulting regime at the southern part of the Karadere fault stressing the influence of the nearby east-west extensional Akyazi pull-apart basin. Koulakov et al (2010) investigated the distribution of Vp, Vs and attenuation in the crust beneath the fault based on local earthquake tomography.…”

Microscopic Characterization of Tensile and Shear Fracturing in Progressive Failure in Marble

“…First of all, microcrack density in Table 1 suggests two possible Journal of Geophysical Research: Solid Earth 10.1002/2017JB014581 distinctions of AE events in shear fracturing compared to tensile fracturing. It is probably why some nearly pure shear aftershocks (with insignificant non-double-couple components) exhibit a larger magnitude than some other tensional aftershocks (with substantial positive non-double-couple components) in the 1999 Izmit earthquake (Stierle et al, 2014). Logically, the former will be accompanied by more seismic events.…”

“…Shear cracks often propagate along the crystal boundaries leading to a damage zone 5 to 10 times wider than tensile cracks, which mainly propagate along and across crystal boundaries. Additionally, many seismic studies have explored the correlation between stress regimes (tensile or strike slip) and the non-double-couple components of the seismic events (Julian et al, 1998;Kwiatek et al, 2014;Stierle et al, 2014;Vavryčuk, 2002). Multiple microcracking zones oriented nearly parallel with the loading direction were also observed in the region between the preexisting flaw tips where a macroscale shear crack forms finally.…”

M ≥ 7 earthquake rupture forecast and time‐dependent probability for the sea of Marmara region, Turkey

“…These segments are bounded by geometric fault complexities and discontinuities (e.g., jogs and fault bends) that act as barriers to rupture propagation [ Segall and Pollard , ; Barka and Kadinsky‐Cade , ; Wesnousky , ; Lettis et al , ; An , ]. Where possible, the reconstruction of the fault traces and boundaries at the surface is informed by seismotectonic studies of recent earthquakes (e.g., 1999 Izmit and Düzce seismic sequence) [ Gülen et al , ; Barka et al , ; Bohnhoff et al , ; Bulut et al , ; Pucci et al , ; Stierle et al , ] and historical surface ruptures [ Ambraseys and Jackson , ].…”