## 6 Discussions
### The Influence of Vertical Coupling in Rainfall and Barometric Responses  
In many studies, it has been mentioned that BSM data is calibrated by theoretical tides [5; 32; 39; 40; 41; 58; 59; 60]. [PERSON] et al. proposed a calibration method in isotropic condition [41], [PERSON] [40] found some BSMs data set by the Plate Boundary Observatory (PBO) have strong vertical coupling, which will strengthen the response for rainfall or snow the surface [40]; [PERSON] et al. believes that the orientation of gauges often erroneous due to the magnetic properties of rock or malfunction of the instrument compass, the proper adjustment of the initial orientation is suggested [39]. In our study, we can find some strong influence of vertical strain in the barometric response. We can effectively diminish  
Figure 7: Comparison of the cumulative rainfall of each rainfall event with the data of the two groups at the DARB station. The threshold is 250 mm, which shows that landslides are likely to happen when the cumulative rainfall is larger than 250 mm, increasing the sediment volume in the rivers. The threshold is in good agreement with the rainfall triggering index (RTI) (250 mm) of this area published by the Soil and Water Conservation Bureau of Taiwan (SWCB) of Taiwan.  
the vertical strain by using [PERSON]'s method, but the major purpose of this paper is focused on the dealing with those vertical strain caused by the surface loading such as rainfall, air pressure, and river loads. Therefore, we used the isotropic coupling method to correct the observations and highlight the vertical parts of the data. We compare two calibration methods of different considerations to see its difference.  
Comparing the results of two different ways of calibration, the two tend to be consistent in the long-term but there may be some differences in the short-term, these differences may show the magnitude of the vertical strain. Taking the results of real strain in JING as an example, Figure 8 shows the comparison of corrected results and rainfall. The strain drop responding to rainfall events are still spotted on the results calibrated by the method of isotropic coupling. According to the results, the response of barometric change was around 10 nanostrain drops compared to a 1 hPa rise in air pressure. Nevertheless, it was about 7 nanostrain drops in the same situation in the results calibrated by the method of vertical coupling, decreasing the ratio about 30%. In addition, some rainfall response was weakened or even difficult to be observed after vertical coupling correction, and this will be not conductive the purpose of discussion of rainfall or river loads on the strain observation in this paper, so the method of isotropic coupling was preferred. According to the results, this response caused by local and external loads just can be observed in the swallow depth underground, it did not affect the strain state in the deeper strata in ten or even tens of kilometers, such as non-tectonic signals can be effectively reduced or even eliminated by appropriate correction methods, and the identifiability of the tectonic activity will be increased.  
Figure 8: The diagram shows the results of calibrated areal strain in JING with the (**a**) isotropic coupling method and (**b**) vertical coupling method, respectively. The dark colored line means the result of removing tidal response, while lighter color means no deal with tides. In some cases, the rainfall responses clearly spotted in isotropic-coupling data show weaker or even difficult to be identified after vertical coupling calibration.