Broad-band strain amplification in an asymmetric fault zone observed from borehole optical fiber and core

Abstract

To unravel fault zone properties, high-resolution imaging and probing are pivotal. Here we present a comprehensive study utilizing Distributed Dynamic Strain Sensing of a 3D array of optical fiber with a surface-to-depth installation, traversing an active fault zone at depth, combined with a geological analysis of drilled cores. We identify an asymmetric fault zone with a narrow fault core, exhibiting a notable amplification in strain rate across a broad-band frequency range, which is attributed to the presence of weak gouge material within a much stronger host rock. This feature heightens responsiveness of the weak fault zone with low rigidity to ground motion, and intensifies strain from both nearby and remote seismic events. We demonstrate that optical fiber facilitates long-term monitoring of ground motion amplification with high spatial resolution, thus indicating where strain and deformation accumulate through repeating ruptures. This progress is pivotal to the understanding of the role of low rigidity and changes thereof within fault gouge material.