## 1 Introduction  
Strainmeters feature high precision and wide frequency ranges and are able to record the crustal deformation caused by tectonic activities. Many of them have been installed in seismically active tectonic settings around the world such as America, Japan and Taiwan. Apart from the application of observations and research of seismic precursors, strainmeters are also frequently utilized to observe and study volcanic activities, slow earthquakes, and tremors [1, 2, 3]. Moreover, strainmeters play an important role in detecting transient deformation associated with earthquakes and exploring whether atmospheric pressure variations in a typhoon are able to trigger fault slip [4, 5, 6, 7]. Taiwan Island is located in a subtropical area with frequent typhoons characterized by significant air pressure drops of several tens of heterospacal (hPa) and heavy rainfalls resulting in gebhazards [8, 9]. Thus, typhoons represent one of the major hydrological factors triggering significant transient deformation in terms of atmospheric events in Taiwan [5, 6]. Taiwan is located at the active orogenic belt between the Eurasian Plate and Philippine Sea Plate, with a convergence rate of 80 mm/yr [10, 11, 12, 13, 14]. The Philippine Sea Plate subducts northward beneath the Eurasian Plate in the northeast of Taiwan, forming the Ryukyu-arc trench system. The continental crust of the Eurasian Plate subducts eastward beneath the oceanic crust of the Philippine Sea Plate in Southeastern Taiwan, forming the LuzonArc System. A series of imbricated thrust faults located in the accretionary prism on the leading edge of the forearc basin due to plate convergence, forming the fold-and-thrust belt in Taiwan expressed by a classic critical taper theory [15; 16; 17] with a detachment located approximately 7-8 km below the fold-and-thrust belt of Western Taiwan [18]. The uplifting rate can reach to 17.4 mm/yr in the Central Range according to precise leveling data [19]. The ongoing mountain building process has generated the steep mountain ranges on Taiwan Island [20] and a high denudation rate of the mountainous area [21; 22]. For example, the Zengwun River in Southern Taiwan yields about 25 Mt of sediment every year at a riverbed erosion rate >16 mm/yr [21]. Furthermore, frequently earthquakes often lead to large-scale landslides [23] and severe river incision [24]. Thus, the fluvial response to rapid episodic erosion by earthquakes and typhoons is a key issue for transient deformation occurred in orogenic belt in Taiwan. The Central Geological Survey (CGS) in Taiwan has deployed 11 Gladwin Tensor Strain Meters (GTSM) in Western Foothills to observe strain variation (the abbreviations of these stations are JING, SLIN, SANS, CINT, PFMT, BMMT, TAIS, TSUN, DARB, RNT and RST respectively, the locations are shown in Figure 1) since 2004. In addition to borehole strainmeters (BSMs), barometers, water pressure transducers, rain gauges, and GPS timers are used to detect the crustal deformation at each station. The strainmeters are buried underground at a depth of 200 m and designed to measure axial strain in four different orientations. The stations are distributed in three major networks (Taipei, Hsinchu, and Jianan) for long-term continuous observations of crustal strain caused by the seismic, fault activity and earth and ocean tides. Figure 1: Tectonic map and locations of installed borehole strainmeters in this study (red circles). Subduction along the Ryukyu and Manila trenches shown as a solid line with triangles to the overriding plate. Red arrow represents directions and convergence rate between the volcanic and continental margin. Installed borehole strain meter (BSM) divided into three networks (show as blue block in the bottom diagrams). Major faults are shown in the small diagrams. Relief map in the vicinity of the borehole strainmeters discussed in this paper is shown in the Figure S1. The period of aseismic strain during the seismogenic process varies between one tenth of a second and several decades.