Upper crustal anisotropy in the southeastern Korean Peninsula from shear wave splitting of local earthquakes

Abstract

<p><span lang="EN-US">Shear wave splitting (SWS) is a widely used technique to study the anisotropic properties of the Earth’s interior. The geological structure of the southeastern Korean Peninsula </span><span lang="EN-US">is represented as the </span><span lang="EN-US">Yangsan fault and Ulsan fault, which is controlled by the present-day compressional stress regime in the ENE-WSW direction. We analyzed shear wave splitting to understand the anisotropic features of the upper crust above the hypocentral depth in the southeastern Korean Peninsula using the local earthquake data from the Gyeongju Hi-density Broadband Seismic Network (GHBSN). The GHBSN is a dense array composed of 200 broadband stations, which covers an area of about 60×</span><span lang="EN-US">60 km<sup>2</sup> in the southeastern Korean Peninsula. We used the MFAST program (Savage <em>et al.,</em> 2010) to measure the SWS parameters of fast polarization</span> <span lang="EN-US">and delay time </span><span lang="EN-US">from shear waves of local earthquakes from January 2019 to December 2020. In addition,</span><span lang="EN-US"> the TESSA program (Johnson <span class="notion-enable-hover"><em>et al.,</em></span> 2011) was employed to inspect the</span><span lang="EN-US"> spatial variation in </span><span lang="EN-US">the anisotropy of the study region. </span><span lang="EN-US">To obtain reliable measurements of SWS parameters, rigorous constraints including quality control of the original waveforms were applied, and then, cycle-skipped measurements were manually removed. In final, we obtained the SWS measurements of 4260 records. </span><span lang="EN-US">Because the seismicity in the region is concentrated at </span><span lang="EN-US">the epicentral region of the </span><span lang="EN-US">2016 Gyeongju earthquake sequence and the hanging wall of the Ulsan fault, raypaths are limited to </span><span lang="EN-US">a narrow</span><span lang="EN-US"> azimuthal range. Both the raw and spatially averaged fast-polarization directions are dominant to be parallel either to major faults (structural anisotropy) or to the ENE-WSW (stress-induced anisotropy). Also, some stations and regions show bi- or multi-modal rose diagram of the SWS, representing that there is more than one factor of anisotropy to induce the SWS. The delay time of the SWS showed the right-skewed distribution. Tomographic result of the SWS delay time shows that, relatively high anisotropy is observed at the epicentral region of the 2016 Gyeongju earthquake sequence and the hanging wall of the Ulsan fault. It implies that microcracks at these regions are better developed compared to the remaining regions. </span></p>