The Earthquake Loading Cycle and the Deep Structure of the North Anatolian Fault

Abstract:

Deformation of the Earth’s upper crust is localized onto narrow fault zones, which may slip suddenly and catastrophically in earthquakes. Strain in the upper mantle is more broadly distributed and is thought to occur by continuous ductile creep. The properties of the lower crust are the primary control on the behavior of the coupled system during the earthquake loading cycle. However, the distribution of strain in the lower crust is poorly understood.
Here we show that the North Anatolian Fault (NAF) continues as a narrow structure through most of the crust. We use scattering migration from an 18 month deployment of 73 broadband seismometers in a dense array (~7 km spacing) across the NAF in the location of the 1999 Izmit earthquake rupture. The results reveal clear discontinuities in the lower crust across the northern and southern branches of the NAF, which extend to a depth of at least 25 km. Seismicity, by contrast, is confined to the upper ~15 km.
Deformation on the fault was well recorded both before and after the 1999 earthquakes. Prior to the earthquake, strain was focused in a ~50 km region around the fault. Following the earthquake, a rapid post-seismic transient was observed, which slowly decayed over the subsequent decade. Earthquake cycle models require at least two relaxation time constants to explain these observations – a strong material to allow focused interseismic strain, and a material capable of relaxing quickly, to give rapid postseismic deformation. We present two models capable of reproducing these observations – (i) a visco-elastic model in which the weak material is found in a zone beneath the seismogenic fault, and (ii) a model in which postseismic deformation occurs through afterslip on a deep extension of the seismogenic fault plane. The latter appears more consistent with the seismic images. We also present results from geological analogues of the mid-lower crust beneath the NAF, which are also consistent with the idea that strain is focused in narrow shear zones in the lower crust beneath major faults.