The Turkish Dilatancy Project (TDP3): multidisciplinary studies of a potential earthquake source region

Summary. The section of the North Anatolian Fault lying near the city of Izmit, at the east of the Marmara Sea, has been identified as a seismic gap and the possible site of a future major earthquake. Previously published studies of records from an earthquake swarm within the gap (TDP1 and TDP2) provided the first evidence that shear‐wave splitting occurs in earthquake source regions, a conclusion since verified by many studies at other locations. A third field study (TDP3) was mounted in the Izmit region during the summer of 1984. Observations were made over an eight‐month period and included geomagnetic and geoelectric measurements in addition to a series of observations utilising dense arrays of three‐component seismometers. Earthquake activity in the principal study area was monitored over a period of eight months. Records showed features similar to those observed in the earlier studies. In particular: (1) almost all shear waves emerging within the shear‐wave window displayed shear‐wave splitting; and (2) the polarizations of the first arriving (faster) split shear‐waves showed sub‐parallel alignments, characteristic of propagation through a distribution of parallel vertical cracks striking perpendicular to the minimum compressional stress. These and other observations support the conclusion of earlier studies – that the upper crust is pervaded by distributions of micro‐cracks aligned by stress, known as extensive‐dilatancy anisotropy. A search for time dependence in shear‐wave phenomena has revealed temporal variations in the delays between the split shear‐waves throughout the course of the TDP3 study, but as yet this has not been correlated with specific earthquake activity.