Tuna Eken

An earthquake gap south of Istanbul

Over the last century the North Anatolian Fault Zone in Turkey has produced a remarkable sequence of large earthquakes. These events have now left an earthquake gap south of Istanbul and beneath the Marmara Sea, a gap that has not been filled for 250 years. Here we investigate the nature of the eastern end of this gap using microearthquakes recorded by seismographs primarily on the Princes Islands offshore Istanbul. This segment lies at the western terminus of the 1999 Mw7.4 Izmit earthquake. Starting from there, we identify a 30-km-long fault patch that is entirely aseismic down to a depth of 10 km. Our evidence indicates that this patch is locked and is therefore a potential nucleation point for another Marmara segment earthquake-a potential that has significant natural hazards implications for the roughly 13 million Istanbul residents immediately to its north.

Seismic Anisotropy from SKS Splitting beneath Northeastern Tibet

The northeastern boundary of the Tibetan high plateau is marked by a 2 km topographic drop and a coincident rapid change in crustal thickness. Surface tectonics are dominated by the Kunlun strike?slip fault system and adjacent Kunlun concealed thrust. The main objective of the current study is to map lateral variations of seismic anisotropy parameters in this region along the linear INDEPTH IV array in order to investigate the link between surface and internal deformation in the context of crust and mantle structure. To achieve this aim, we performed Minimum?Transverse?Energy based SKS splitting measurements using 23 stations of the INDEPTH IV array deployed across the northeastern margin of Tibet. Average fast polarization directions and splitting time delays are obtained by averaging stacked misfit surfaces of all analyzed events at each station. The agreement of fast directions with the strikes of major active strike?slip faults and strike?slip focal mechanisms, but not with fossil structures such as the Jinsha suture, implies that the anisotropy records lithospheric petrofabric formed by recent deformation within the lithosphere rather than representing frozen?in anisotropy or shear within the asthenosphere due to absolute plate motion. The distribution of large splitting delays throughout the northern plateau suggests that deformation is distributed rather than focused onto narrow shear zones associated with the Kunlun strike?slip faults. The drop in splitting delays toward the Qaidam is then a natural consequence of the much lower degree of deformation there.

The Use of Direct Shear Waves in Quantifying Seismic Anisotropy: Exploiting Regional Arrays

Abstract To overcome the potential contamination of the direct S waves by source‐side anisotropy in shear‐wave‐splitting analysis, we describe a new approach that we call the reference station technique. The technique utilizes direct shear waves recorded at a station pair and depends on maximizing the correlation between the seismic traces at reference and target stations after correcting the reference station for known receiver‐side anisotropy and the target stations for arbitrary splitting parameters probed via a grid search. The algorithm also provides a delay time between both stations caused, for example, by isotropic heterogeneities. Synthetic tests demonstrate the stability of the estimated parameters, even where variability in near‐surface properties (thickness and velocity of sediment layer) exists. We applied the reference station technique to data from seismic experiments at the northern margin of Tibet. Average splitting parameters obtained from the analysis of direct S ‐wave results are consistent with those obtained from previous SKS splitting measurements. Where differences exist, shear‐wave fast polarization estimates resolved from direct S indicate a higher degree of internal consistency for closely spaced stations than those derived from SKS . This is probably due to the much larger number of direct S waves available for splitting measurements compared to SKS for the same observational period, resulting in higher quality measurements. We also demonstrate the ability of the technique to provide improved splitting measurements for temporary stations by following a bootstrap approach in which only a few stations with well‐constrained SKS splitting parameters are used as seeds to determine the splitting parameters of a large array in an iterative manner. In addition, the S measurements sample the anisotropic layer with different angles of incidence and back azimuths, thus potentially providing additional constraints on more complicated anisotropic structures, and the interstation delay times could be used for tomographic studies to reduce the bias from anisotropic structure. Online Material: Multisplit software package (C++) with instructions.