## 4 Data and Calculation  
The observed strain change is often influenced by many environmental factors [32; 39; 40; 41]. In additional to dominant trend of borehole relaxation and grout curing, the strain changes are associated with tidal response, barometric pressure, rainfall, groundwater fluctuation, seismic deformation, and interseismic strain accumulation due to tectonic loading. To better understand the tectonic process from the observed strain, the non-tectonic factors such as barometric loading and rainfall response should be removed from the strain data. The borehole relaxation and grout curing can be removed by fitting the model of two exponential attenuation functions. Together with the removal of non-tectonic effects on the strain response, the intensity of tectonic signals will be largely improved. In removing the tidal response, we used a software of SPOTL (Some Programs for Ocean-Tide Loading), see [44] to calculate theoretical tidal constituents with NAO.99B [45] as global, and TWITDE08 [46] as the regional model. A program of BayTap-G [47] was used to analyze and extract the tidal component out of the observed data. Observed strain can be disturbed by severe hydrologic changes, such as typhoon and flood. Therefore, in order to reduce noise and improve the readability of signal, we mainly chose the data in the period of dry season (Oct.-Apr.) to calculate tidal constituents. In addition, averaging the calculation results of several years can be used as a reference basis for removing tidal response. The changes of air pressure also influence the observation in some linear relation. A period of 45 days was set as the sampling window; the barometric pressure and strain were respectively band-pass-filtered to exclude frequencies outside 3-5 days band [40], because in this time period, semi and diurnal signal can be ignored in a better signal/noise ratio and a more accurate response can be obtained. The barometric response strain, nstrain/hPa can be obtained by comparing both. The observed strain is not only affected by the surface water load but also by deep groundwater variations. Currently, the relation between the areal strain and groundwater pressure can be roughly correlated using co-sited water pore pressure transducer. However, the generated strain and the water pressure variation usually make it difficult to identify the tectonic activity-related strain, if some observation stations encounter the problems of water pressure gauge failures. The present work mainly focuses on the rainfall impact in a short period of time; the relation between the groundwater pressure variation and areal strain will be discussed in future publications. The data from ten observation stations are selected for the calculations except SLIN in which data has been affected by borehole adjustment severely so that it is not capable to be taken involved in this study. The topography of the areas where the stations is located is so diverse that we can discuss the impact of rainfall on the strain observed in different environments. For reducing the cross-influences of various factors, seismic effects have been excluded from the priority data. The basic information about the stations is listed in Table 1.  
Rainfall events with a cumulative rainfall over 20 mm were selected because a recognizable rainfall response can hardly be observed from the data of events with little rainfall. Moreover, the comprehensive effect of multiple rainfall events may be observed frequently because a large number of long-lasting continuous rainfall events generally occur in the rainy season from May to November, demonstrating the relatively strong noise during this period. Therefore, independent heavy rainfall events in the dry season (December to April) are the priority to be selected as the study samples. Subsequent to the preliminary selection of rainfall events, relatively strong noise signals with a fairly high standard deviation would be removed before calculating the average value to reduce the impact of noise on the observed data.