Much of what we know about the nucleation of earthquakes comes from the temporal and spatial relationship of foreshocks to the initiation point (hypocentre) of the mainshock. The 1999 Mw 7.6 Izmit, Turkey, earthquake was preceded by a 44-minute-long foreshock sequence, which built in intensity as the time of the mainshock approached. Here we apply a series of high-resolution methods to the sparse seismological observations to determine the spatial and temporal relation of the foreshocks to the mainshock hypocentre. We find that the foreshocks form a contiguous series of ruptures that progressed systematically from west to east towards the mainshock hypocentre, located at the extreme eastern edge of the foreshocks. The Izmit foreshock sequence occurred as a triggered cascade in which one foreshock loads the adjacent fault patch causing it to fail, with the mainshock initiation no different than another foreshock. We find no evidence to support a hypothesized precursory aseismic driving process. If we are to resolve the role of aseismic deformation and possible role of fluid overpressure in the earthquake nucleation process it will likely require measurements made in the near field of the hypocentre.
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We thank the seismic network operators of Kandilli Observatory and Earthquake Research Institute, the Marmara Research Center, GFZ Potsdam and Turkish Disaster Affairs for access to their seismic recordings of the Izmit earthquake foreshock sequence. R. Abercrombie, G. Beroza, M. Brehme, E. Dunham and F. Tilmann provided valuable input.
The authors declare no competing interests.
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Ellsworth, W.L., Bulut, F. Nucleation of the 1999 Izmit earthquake by a triggered cascade of foreshocks. Nature Geosci 11, 531–535 (2018). https://doi.org/10.1038/s41561-018-0145-1
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