<p><span lang="EN-US">Shear wave splitting (SWS) is a widely used technique to study the anisotropic properties of the Earth&#8217;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&#215;</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>
<p>The southeastern part of the Korean Peninsula is known to have high seismic activity and many Quaternary faults. Nonetheless, there have been uncertainties in estimating seismic hazards due to insufficient information on potential seismic sources. We investigated the geometrical characteristics of causative faults related to clustered earthquakes in the southeastern Korean Peninsula by detecting microearthquakes and determining their source parameters. We used the seismic data recorded at the Gyeongju hi-density broadband seismic network, the temporary seismic networks operated to monitor the aftershocks of two moderate earthquakes (the 2016 <em>M</em><sub>L</sub> 5.8 Gyeongju and 2017 <em>M</em><sub>L</sub> 5.4 Pohang earthquakes), and the national seismic network of South Korea. An earthquake catalog for the southeastern Korean Peninsula was built using automatic earthquake detection methods based on measurements of energy ratio. We identified the five clustered earthquake regions via the microearthquake distribution: the 2016 Gyeongju earthquake region (GJ), the 2017 Pohang earthquake region (PH), the eastern part of the Ulsan Fault (UF), eastern offshore Gyeongju (EG), and the western part along the Miryang Fault (MF). We determined the relative location and focal mechanisms of the earthquakes occurring in those regions using the double-difference location method and the <em>P</em>-wave first motion polarity method, respectively. Finally, the geometry of the earthquake causative faults was inferred using the spatial distribution of the relative locations and the focal mechanisms. It was found that there are at least two NNE-SSW trending fault segments and multiple NE-SW trending fault segments in the GJ and PH, respectively. In the case of MF, UF, and EG, it is difficult to relate directly to the surface faults, but the strikes of the causative faults, which are confirmed by the spatial distribution of earthquakes, are similar to those of the surface faults.</p>
<p>The Heunghae area of the Cenozoic Pohang Basin, located in the southeastern part of the Korean Peninsula, is a small-scale sub-basin covered with alluvium. The Jurassic granite is overlaid by the Cretaceous sedimentary and volcanic rocks, which form the basement of the basin composed of the Miocene non-marine and marine sediments. Therefore, the vertical distribution of strata in the Heunghae Basin can be summarized as a sequence of Quaternary alluvium, Tertiary and Cretaceous sedimentary layers, and Jurassic granite. Depending on each layer's formation time, a distinct difference in the physical properties of each layer may occur, which mechanically results in the contrast of acoustic impedance of elastic wave energy. The resonant frequency measured from the horizontal-to-vertical spectral ratio (HVSR) curve of microtremor records at a seismograph station is known to be an effective value for determining the depth to the basement with strong contrast in acoustic impedance. Based on the assumption that the boundaries formed by each layer in the Heunghae Basin have a distinct difference in acoustic impedance, we tried to estimate the resonant frequencies corresponding to each boundary from the HVSR. A total of 114 three-component geophones with a natural frequency of 5 Hz were evenly installed to obtain microtremor records over the Heunghae Basin. The distance between geophones is approximately 500 meters. The installation period is from September 24 to November 24, 2021, and the recording time varies from a minimum of 2 hours to a maximum of about 26 hours, depending on the measurement site. The recording was made at a sampling rate of 500 samples per second. The HVSR analysis used two-hour long recordings for all sites. One or more peaks can be identified in the HVSR curve of most sites. Since the resonant frequency that can be confirmed through the HVSR curve is related to the depth of the boundary between the layers where strong impedance contrast occurs under each geophone, the boundary at various depths can be determined from these frequencies of peaks. The range of resonant frequencies was found to be approximately 0.3 &#8211; 26 Hz. To compare the resonant frequency with the known geological information, the HVSR curve near the borehole site was compared with the geological logging information. In the case of some measurement sites, it was difficult to specify other peaks because one resonance frequency peak was dominant over the HVSR curve. Multiple resonant frequencies can be assumed to correspond to major layer interfaces. Due to the uncertainty of the velocity structure model, it was difficult to accurately determine the depth to the interface from these resonant frequencies. Nevertheless, the results show that the multiple resonant frequencies of the HVSR curve indicates the layer boundaries with a strong impedance contrast, and thus it can contribute to reveal the sequence stratigraphy of a basin with multiple episodes of deposits.</p>
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