# Monitoring of Possible Activities of Yangsan Fault Zone Using GNSS
## 1 Introduction  
The Korean Peninsula is in the southeastern part of the Eurasian Plate that is hundreds of kilometers away from the plate boundary [1]. In this region, earthquakes have rarely exceeded magnitude 5.0 since the monitoring of instrument seismic earthquakes in 1978. Based on the historic records and instrumental earthquakes it shows that they were concentrated in the southeastern part of the Korean Peninsula [2; 3; 4]. In addition, it has been reported since the 1990s that the existence of the quaternary faults is suspected and might be active on the Korean Peninsula. The quaternary faults were found in the inland areas, such as Pohang, Gyeongiu, and Ulsam, mostly located in the southeastern area [4]. Recently, in this area, the Gyeongiu earthquake with a magnitude of 5.8 (a foreshock of 5.1) occurred on 12 September 2016 [5]. The seismic fault is known as the subsidiary fault in the NNE-SSW direction within the Yangsan Fault Zone [6]. Almost 14 months later (15 November 2017), a 5.4 magnitude earthquake occurred again in Pohang. Therefore, it is assumed that frequent earthquakes in that area are related to the activity of the Yangsan Fault Zone. It is a large-scale fault that develops about 170-200 km (Busan-Yangsan-Gyeongiu-Pohang-Yeongdeok from south; see Figure 1). Most geological disasters, such as earthquakes and volcanoes, are caused by fault motion, and it is important to determine the exact distribution and kinetic characteristics of the fault, and its activity to forecast and identify geological disasters. The geodynamical processes are an important factor in analyzing and understanding the active tectonic deformation. Most of the prior research has steadily been conducted to analyze the movementand deformation of the crust through GPS [7; 8; 9; 10; 11]. Therefore, many countries around the world have established and been operating systems to precisely measure and monitor displacement of crustal deformation, which uses geophysical and geological methods and geodetic technologies. With recent developments in remote sensing technology, for example GPS (Global Positioning System) and InSAR (Interferometric Synthetic Aperture Radar), it has been widely used for linear structural analysis for tracking and investigation of active faults [12; 13]. The GNSS is used as fundamental data for global scale tectonic movement and local deformations that are not detected by seismometers. In the case of the USGS (United States Geological Survey), GPS is used to monitor periodic earthquakes in the San Andreas Fault (in the vicinity of Parkfield). The seismic pattern, formed along the fault near Parkfield and Chollaim in California, showed a distinct change over 50 km, which was investigated to detect displacement by sampling 13 nearby GPS data (1 to 30 s intervals). This allows for the detection of large creep events outside the creeper range (10-30 mm) and/or large seismic displacements, the enhancement of existing measurements due to afterslip [14]. In Italy, GPS systems allow for the estimation of Castrovilari displacement from a series of GPS data obtained in networks across active faults, and studies have been conducted to forecast seismic magnitudes of active faults [15]. It is also possible to measure the displacement that may occur during an earthquake or volcanic eruption from the coordinates accurately calculated from the collected GNSS data. In addition, GNSS can be used to observe seismic deformation processes that have not released seismic energy yet, such as interseismic strain, movement of magma, afterslip, postseismic relaxation, and creep [16]. Immediately after a major earthquake, it is known that the stress around the epicenter is redistributed over the years (or even decades) after an immediate stress change occurs. Earthquakes generally causes a long-term post-seismic displacement after the co-seismic displacement. Thus, the Tohoku earthquake might have been affected by a prior earthquake and the accompanying post-seismic tectonic movements.