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

Abstract

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 TDPZ) 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 shearwaves 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 microcracks 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. 266 R . Evans et al.

A review of a quarter century of International Workshops on Seismic Anisotropy in the crust (0IWSA–12IWSA)

“…Shearwave splitting was indeed confirmed in the crust by Crampin et al (1980), and five papers, following Crampin et al (1985), reporting TDP1 and TDP2, set many of the parameters for observations of shear-wave splitting above small earthquakes. Evans et al (1987) reporting TDP3 confirmed and enhanced previous results of TDP1 and TDP2. TDP3 also deployed magneto-telluric (MT) stations (Evans et al 1987).…”

“…Evans et al (1987) reporting TDP3 confirmed and enhanced previous results of TDP1 and TDP2. TDP3 also deployed magneto-telluric (MT) stations (Evans et al 1987). These MT sites showed regional contributions to the geoelectric-strike azimuths approximately parallel to the shearwave polarisations.…”

“…(90 • flips are where the faster and slower split shear waves abruptly exchange polarisations, Angerer et al 2002). modelled observations of VSPs in the Mohave Desert, CA, USA, with synthetic seismograms and found shear-wave splitting consistent with interpretations in terms of sub-parallel sub-vertical cracks in other studies (for example, Evans et al 1987). observed and modelled fault-zone-trapped waves in a thin anisotropic wave guide.…”

“…Note that other papers at 5IWSA also searched for temporal changes in shear-wave splitting above small earthquakes. Graham and Crampin (1993) examined regional earthquakes recorded by the TDP experiments in Turkey (Evans et al 1987) and showed that shear-wave splitting in Sn-waves, refracted along the Moho, indicate substantial crack-induced anisotropy in the lower crust with similar orientation as the upper crust but with larger time delays of up to 1 s. Slater et al (1993) successfully modelled synthetic seismograms using ANISEIS software (Taylor 1990) in walkaway VSPs in an oil field in the Caucasus Basin, matching confirmed field observations of anisotropic cusps.…”

Microearthquakes in the TDP swarm, Turkey: clustering in space and time

“…A swarm of small earthquakes near the North Anatolian Fault (NAF) in Turkey was monitored during the Turkish Dilatancy Projects, TDPl in 1979, TDP2 in 1980 and TDP3 in 1984 (Evans et al 1987). The persistent swarm of microearthquakes is associated with the southern limb of a graben structure beneath the hills south-east of Izmit, at the eastern end of the Marmara Sea (Fig.…”