干旱区生态环境知识资源中心

Profiles of the relative geodetic velocity variation across the western North Anatolian Fault (NAF) near the location of the 1999 Izmit earthquake show two important characteristics: (i) before the earthquake, strain is localized in a region <100 km wide and constrained by the long-term relative displacement rate of Anatolia with respect to Eurasia (similar to 22 mm/yr), and (ii) after the earthquake, relative velocities of up to similar to 150 mm/yr are observed across this near-fault zone. Using a model of the crust in which a faulted elastic layer above a visco-elastic mid and lower crust slips episodically in response to continuous boundary loading, we previously showed that these observations can be explained quantitatively if a localized weak zone (LWZ) in the mid-crust directly beneath the NAF northern strand is embedded in a relatively high viscosity background crustal layer. Such a spatial viscosity variation however seems inconsistent with electrical resistivity profiles that show a low resistivity zone confined between the NAF northern and southern strands, and furthermore show resistivity continually decreasing with depth in this zone. By further numerical experiments, we here confirm that the LWZ in the mid-crust must be centred on the NAF northern strand; a LWZ between the NAF northern and southern strands is inconsistent with geodetic velocity distribution. We conclude that there is no simple relation between viscosities and resistivities in the crust beneath the western NAF. We also investigate possible depth-variation of the background viscosity structure and show that the background viscosities must be greater than similar to 2 x 10(20) Pa sat depths shallower than similar to 30-35 km in the 40 km thickness of the crust in order to retain the high strain-rate zone in the pre-seismic velocity profiles; a low-viscosity layer in the lower similar to 5-10 km of the crust has no significant impact on the predicted profiles and therefore is neither excluded nor required by these data. (C) 2015 Elsevier Ltd. All rights reserved.