1. Analysis of the subsidence of Yilan area in the Republic of China in 2020
The formation compression behavior in the Yilan Zhuang Encircled area continues to occur, but the cumulative compression amount from 2009 to 2020 is only 4.6 cm, and the long-term and short-term average compression rates are less than 1 cm/year, indicating that the formation compression rate in this area is relatively slow at this stage
2.Analysis of the subsidence in the Yilan area from July, 2014 to July, 2018 . The average subsidence rate map from 2014 to 2018 in the Yilan area is shown in Figure 4-54. According to the figure, the significant subsidence area in the Yilan area was 0 Square kilometers, the maximum annual subsidence rate was 2.0 cm/year. The main subsidence location occurred in Yilan City, the second subsidence location occurred in Wujie Township and Suao Town (the annual subsidence rate is 1.4 cm/year), and the remaining subsidence rate is mostly about 1 cm/year, which showed that there is no obvious subsidence in Yilan area in overall. Only a slight subsidence occurred in Yilan City, Suao area, and Wujie area. Table 4-54 is the analysis table of major subsidence detection points in Yilan area in 2018. Figure 4-55 is about the average subsidence rate from 2012 to 2014 and from 2014 to 2018. It shows that the subsidence rate of the Yilan area in the past five years was more obvious in the Suao area (from 2012 to 2014) and the rest of the regions are below 3 cm/year, which indicated the slowing down of the sinking trend in Yilan.
3.Analysis of the subsidence in Yilan region (1992 to 2018) analyzes the cumulative subsidence map from 1992 to 2018 years in the Yilan region. Figure 4-56 shows that the subsidence area of Yilan has mainly concentrated on both sides of estuary of Lanyang creek, including Wujie Township and Zhuangwei Township where the accumulated subsidence exceeds 30 cm. Although there is no large-scale subsidence in the Yilan area in the short term, it was still found in Zhuangwei Township, Wujie Township, Suao Town and Yilan City in the past. Therefore, it is recommended to have a long-term observation for its future trend. Table 4-55 shows the analysis table of the subsidence area from 1999 to 2018 in Yilan.