Near-surface structure of the North Anatolian Fault zone from Rayleigh and Love wave tomography using ambient seismic noise

George Taylor,Sebastian Rost,Gregor Hillers,Gregory A Houseman

doi:10.5194/se-10-363-2019

Abstract

Abstract. We use observations of surface waves in the ambient noise field recorded at a dense seismic array to image the North Anatolian Fault zone (NAFZ) in the region of the 1999 magnitude 7.6 Izmit earthquake in western Turkey. The NAFZ is a major strike-slip fault system extending ∼1200 km across northern Turkey that poses a high level of seismic hazard, particularly to the city of Istanbul. We obtain maps of phase velocity variation using surface wave tomography applied to Rayleigh and Love waves and construct high-resolution images of S-wave velocity in the upper 10 km of a 70 × 30 km region around Lake Sapanca. We observe low S-wave velocities (<2.5 km s−1) associated with the Adapazari and Pamukova sedimentary basins, as well as the northern branch of the NAFZ. In the Armutlu Block, between the two major branches of the NAFZ, we image higher velocities (>3.2 km s−1) associated with a shallow crystalline basement. We measure azimuthal anisotropy in our phase velocity observations, with the fast direction seeming to align with the strike of the fault at periods shorter than 4 s. At longer periods up to 10 s, the fast direction aligns with the direction of maximum extension for the region (∼45∘). The signatures of both the northern and southern branches of the NAFZ are clearly associated with strong gradients in seismic velocity that also denote the boundaries of major tectonic units. Our results support the conclusion that the development of the NAFZ has exploited this pre-existing contrast in physical properties.

Highlights

The formation of fault zones appears to be a balance between the accommodation of the tectonic strain field and the exploitation of pre-existing weak zones such as tectonic suture zones or lithological boundaries (e.g. Bercovici and Ricard, 2014; Dayem et al, 2009; Gerbi et al, 2016; Tapponier et al, 1982)
Sensitivity kernels representing the vertical resolution for Rayleigh and Love waves within our period range can be found in the Supplement (Fig. S8), along with synthetic checkerboard recovery tests to illustrate the horizontal resolution of the inversion
The initial and final data misfit of the tomography models for both Rayleigh and Love wave phase velocities are shown in Supplement Figs

Summary

Introduction

The formation of fault zones appears to be a balance between the accommodation of the tectonic strain field and the exploitation of pre-existing weak zones such as tectonic suture zones or lithological boundaries (e.g. Bercovici and Ricard, 2014; Dayem et al, 2009; Gerbi et al, 2016; Tapponier et al, 1982). Studying how structural changes affect strain localization in the upper crust is critical to understanding the earthquake cycle (Bürgmann and Dresen, 2008). Imaging the seismic velocity structure of fault zones provides information essential to understanding the long-term behaviour of faults and the earthquakes that occur on them. We interpret images from ambient noise surface wave tomography of the upper 10 km of the North Anatolian Fault zone (NAFZ), Turkey, in the rupture zone of the 1999 Izmit earthquake. This allows us to study the near-surface structure of a recently ruptured fault. The motion of Anatolia is driven by a gradient of lithospheric gravitational potential energy that extends across the Anatolian Peninsula (England et al, 2016) and is sustained by the collision between the Arabian and Eurasian plate in the east and the roll-back of the Hellenic

Results

Discussion

Conclusion