Near-field Source Model for the 2017 Bodrum-Kos Earthquake (Mw=6.6): Triggered off-fault Events to the Northeast of the Ruptured Fault Plane

Pınar A., Alçık H., Irmak T. S.

7. International Earth Science Colloquium on the Aegean Region, pp.79-83, 2020 (Conference Book)


The Bodrum-Kos earthquake initiated a long-lasting debate on the dip of the fault plane where authors give arguments both on a south-dipping and north dipping planes. Here, we contribute to the subject by using Coulomb stress change estimation technique as a tool to discriminate between the two nodal planes. The stress changes associated with a mainshock primarily dependent on the properties of the source model. A large variety of fault planes solutions have been derived for the Bodrum-Kos mainshock mechanism showing faults striking from NNW-SSE to NNE-SSE, rakes indicating oblique to pure normal faulting and also mechanisms portraying from shallow to steeply dipping planes. As such, here we first try to establish a source model based on near-field data and then use that model to estimate the Coulomb stress changes. The resulting stress maps estimated for the optimally oriented strike-slip and normal faults are compared with the aftershock distributions to see which of the nodal planes provide better correlation with the region of increased stresses. A near-field five-station strong motion network located in Bodrum Peninsula has been used to infer a rupture model for the July 20th, 2017 Bodrum-Kos earthquake. The accelerometric stations of the local network are located on the hanging wall of the plane extending northeast from its surface trace located offshore Kos island. The data used have been augmented including nearby accelerographs operated by AFAD and NOA-EIDA node. Thus acquired mainshock accelerograms have been integrated twice to get displacements which were then inverted to their sources to retrieve source parameters for the mainshock through searching for the best CMT location on a 3-D grid scheme. The results indicate that the best CMT point is located at 6 km depth and shows predominantly normal faulting mechanism with a considerable strike-slip component; where left-lateral strike-slip motion has been derived on the nodal planes striking NW-SE and dipping NE, while the sense of motion is right-lateral-strike-slip on the plane extending NE-SW and dipping SW. A 50x20 km fault plane has been parameterized to have 40 elements each of 5x5 km size; 10 of them are located along the strike and 4 along the dip. Ultimately, the best fit CMT parameters have been utilized to get a slip distribution model where the moment release on each sub-fault is derived; essentially, needed for Coulomb failure stress calculations. The results indicate that the Coulomb stress changes associated with the northeast dipping nodal plane show very well fit between the aftershock distributions taking place to the NE of the ruptured fault plane and the region of increased coulomb failure stress changes estimated for the optimally oriented normal and strike-slip faults. Despite the distance to the ruptured fault plane and the time delay since the origin of the mainshock those off-fault events can be classified as aftershocks taking into account space and time window length for Mw=6.6 event.