In this study, we employed network RTK GPS to measure the benchmarks (stations, Figure 2, Appendix 1) established near Yuli downtown, providing a comprehensive understanding of the deformation patterns associated with the 0918 Taitung earthquake. These benchmarks were set up by multiple agencies, including Township and Hualien County, the River Management Office, and the Ministry of Interior, for various reasons, reflecting the importance of monitoring tectonic activity in the region. The benchmarks have been surveyed on several occasions over the past few years, offering a valuable dataset for comparative analysis. The benchmarks that have been installed for several years, for the most part, have not been measured periodically and only provide data from when they were initially installed. In reality, even though there has been a geodetic measurement of a plate shift of 82 mm/year (Takahashi et al. 2019; Yu & Kuo, 2001; Yu et al. 1997), from the time of benchmark installation (between 2010-2015) until the occurrence of the 0917-0918 Taitung earthquake in 2022, various minor earthquakes did not result in significant surface deformation, therefore we consider measurements of benchmarks are capable of representing the coseismic deformation resulted from 0918 earthquakes. Essentially, in the case of this earthquake which resulted in a shift of more than 100 mm, we are confident that our measurements are representative of the coseismic shift.
Between October 17, 2022, and January 16, 2023, we measured a substantial number of benchmarks using network RTK GPS for data collection. This approach allowed for more detailed spatial coverage and enhanced the accuracy of the observed coseismic deformation patterns. Although RTK is less accurate than campaign GPS and continuous GPS, with an accuracy range of 1 - 10 cm (Saghravani et al. 2009), it is still sufficient for determining the location of coseismic deformation in the context of this study. The primary network RTK station in this study is based Continuously Operating Reference Station (CORS) at the Dongli station (DOLI) (figure 2). We used local Taiwan Datum 1997 (TWD97) or EPSG:3826 as the projection for the entire process, from measurement to analysis. Comprehensive records of the benchmarks can be found in the supplementary data.
The measurement results reveal that there are 207 stations with measured longitude, latitude, and altitude, out of a total of 274 stations obtained from the local geodetic agency (see Appendix 1). The remaining 67 stations have no measured altitude and are marked with N/A data in Appendix 1. For analysis to enable the integration of GPS measurements with various dates of measurements, the GE18x station was selected as the local reference due to its proximity to the DOLI station and roughly equal distance to both blocks. The average horizontal error (HRMS) value of 0.009 m with a maximum value of 0.06 m was recorded at station GH55. Similarly, the average vertical error (VRMS) value of 0.018 with a maximum value of 0.09 m was measured at station GL24. The largest Northing displacement (∆N) of 1.882 m was recorded at station L29-1, while the smallest ∆N value of -2.082 m was recorded at station U216. The maximum Easting displacement (∆E) value of 0.056 m was measured at station GN36, and the minimum ∆E value of -1.411 m was recorded at station GL35. The maximum Height displacement (∆H) value of 2.697 m was recorded at station GE21y, and the minimum ∆H value of -0.858 m was measured at station GK02. Lastly, the maximum Length (calculated from Northing and Easting) value of 2.309 m was recorded at station U216, whereas the minimum Length value of 0.000 m was recorded at station GE18x. These measurements provide valuable insights into the deformation patterns associated with the 0918 Taitung earthquake. Please refer to Appendix 1 for a detailed list of the stations and their measurements.
Situated west of Yuli downtown, the Central Range block, represented in purple in Figure 1, reveals various displacement measurements. The northing component (∆N) exhibits an average displacement of approximately -0.860 m which has a standard deviation of about 0.319 m, with the GE16x station recording a minimum displacement of -1.623 m and the GF09 station noting a maximum of -0.341 m, which also denotes the least overall displacement. In the easting component (∆E), an average displacement of roughly -0.618 m is recorded with a standard deviation of about 0.114 m. Here, the GN10 station presents the minimum displacement of -0.808 m, whereas the maximum displacement of -0.408 m is found at the GL63 station, marking the least displacement in this category. Regarding the height component (∆H), there is an average displacement of about 1.33 m which has a standard deviation of about 0.599 m. The GE6x station reports a maximum displacement of 2.603 m, while the GF09 station observes a minimum displacement of 0.504 m, signifying the least displacement in this category. The length component, combining the northing and easting components, shows an average displacement of 1.072 m with a standard deviation of about 0.294 m toward a southwest direction, the GE16x station records a maximum displacement length of 1.767 m, while a minimum displacement of 0.589 m is observed at the U011 station.
The Longitudinal Valley block, located beneath Yuli downtown and depicted as the Alluvial area in Figure 1, necessitates the consideration of the rupture line for accurate interpretation of results. Our rupture line investigation along the Longitudinal Valley yielded various measurements that were integral to our analysis. To further enhance our understanding, we have segmented the area into two sections, demarcated by the rupture line: the western part, designated as LV1 in Figure 5, and the eastern part, marked as LV2 in Figure 5.
In the LV1 area, the northing component (∆N) demonstrates an average displacement of -0.555 m. The U216 station records a minimum displacement of -2.082 m which has a standard deviation of about 0.363 m, and the GE62 station notes a maximum displacement of 0.778 m, the least displacement for this component is 0.088 m at the GN39 station. The easting component (∆E) has an average displacement of -0.496 m with a standard deviation of about 0.171, with the GE62 station recording a maximum displacement of -1.172 m, and the GE12y station observing a minimum displacement of -0.111 m, which also signifies the least displacement in this category. The height component (∆H) presents an average displacement of 0.977 m which has a standard deviation of about 0.535 m, with the GE21y station marking the maximum displacement of 2.697 m, and the GF21 station indicating the minimum displacement of 0.297 m, which also represents the least displacement. The length component shows an average displacement of 0.791 m with a standard deviation of about 0.364 m towards the southwest, the maximum length displacement of 2.309 m is noted at the U216 station, while the GB07 station records a minimum length of 0.197 m.
In the LV2 area, the northing component (∆N) shows an average displacement of 0.896 m with a standard deviation of about 0.448 m, with the L29-1 station recording a maximum displacement of 1.882 m, and the L50 station noting a minimum displacement of -0.643 m, the least displacement in this component is observed to be 0 m at the GE18x station. The easting component (∆E) has an average displacement of -0.363 m which has a standard deviation of about 0.292 m, with the U243 station recording a minimum displacement of -1.110 m and the GN36 station observing a maximum displacement of 0.056 m, the least displacement for this component is also 0 m at the GE18x station. For the height component (∆H), the average displacement is 0.145 m which has a standard deviation of about 0.323 m, with the GI04 station marking the maximum displacement of 1.535 m, and the GF03 station recording the minimum displacement of -0.755 m, the least displacement is 0 m at the GE18x station. The length component reveals an average displacement of 1.033 m with a standard deviation of about 0.458 m towards the northwest, the U207 station records the maximum length displacement of 2.067 m, while the minimum length displacement of 0 m is observed at the GE18x station.
The Coastal Range block, represented in light red in Figure 5, is situated to the east of Yuli downtown and demonstrates a variety of displacement measurements. The northing component (∆N) has an average displacement of approximately 1.288 m which has a standard deviation of about 0.262 m, with the U031 station noting a maximum displacement of 1.742 m and the GI03 station recording a minimum displacement of 0.181 m, which also marks the least displacement. The easting component (∆E) shows an average displacement of about -0.674 m with a standard deviation of about 0.175 m. The maximum displacement of -0.057 m is recorded at the GJ32 station, and the minimum displacement of -1.411 m is observed at the GL35 station. The least displacement in this category is associated with the GJ32 station. In the height component (∆H), the average displacement is approximately -0.197 m with a standard deviation of about 0.169 m. The GL27 station reports a maximum displacement of 0.472 m, while the GK02 station notes a minimum displacement of -0.858 m, the least displacement in this category, a minuscule 0.001 m, is recorded at the GJ18 station. The length component indicates an average displacement of 1.47 m with a standard deviation of about 0.231 m towards the northwest, the maximum length displacement of 1.974 m is observed at the U031 station, while the 9152 station records a minimum length of 0.580 m.
Figure 2 illustrates the pattern of horizontal coseismic displacement. The black arrow signifies the direction of these displacements, as calculated from the ∆E and ∆N values (refer to Appendix 1). The length of each arrow corresponds to the magnitude of displacement, as determined using the Length value (also found in Appendix 1). For clarity and ease of interpretation, we have incorporated a scale for the arrows in the lower right corner of the figure. The red dashed and solid line delineates the rupture line, which is based on point ruptures recorded during our post-seismic field survey.
Figure 3 presents the distribution of vertical coseismic displacement. The upward-facing orange arrows indicate the vertical uplift displacement, corresponding to a ∆H value greater than 0. Conversely, the downward-facing blue arrows represent subsidence displacement, associated with a ∆H value less than 0. The length of these arrows is determined by the ∆H value (please see Appendix 1 for details). For enhanced readability, we have included a scale for the arrows in the lower right corner of the figure. The red dashed and solid line marks the fracture line, derived from fracture points identified in our post-seismic field survey. The "T" shape with rectangular yellow transparent symbolizes the start-end points with 1 km width of the profile sections x-x' and y-y', which display the vertical displacement.
Figures 2 and 3 provide a clear visual representation of the rupture lines generated in the aftermath of the 0918 Taitung earthquake, based on reported points from the affected area (as shown in Figure 1). The horizontal view (Figure 2) and vertical view (Figure 3) exhibit distinctive patterns that are separated by the main rupture line. Notably, the western region of Yuli experienced a southwestward displacement and vertical uplift, while the eastern region experienced a northwestward displacement and downward vertical movement. By defining the lines connected from the point of ruptures as the structure of a fault plane and considering the east and west displacement of Yuli, the fault structure type of our investigation suggests a reverse left-slip fault. Moreover, by fusing RTK GPS measurement results with the local geological formations and current ruptures, while considering the displacement patterns, we have effectively demarcated the study area into four distinct moving blocks, a subject thoroughly investigated in the discussion section. Overall, these figures provide valuable insights into the impact of the earthquake and its fault structure classification, which can aid in future disaster preparedness and mitigation efforts.