Abstract
We found some obvious abnormal strain induced by heavy rainfall from borehole strainmeters deployed in Western Taiwan. The strain induced by rainfall can be divided into two parts, one is the quick response for extra loads of rainwater on the ground, and another one is the slow response for rainwater infiltrating into the strata. The quick and slow rainfall responses of areal strain data are analyzed using the technique of recursive digital filtering. Moreover, the rainfall impact functions of the studied stations are calculated using deconvolution. We found, in most cases, the response strain will reach the maximum in half an hour after heavy rainfall, and then show an exponential decay, it might persist more than 200 h depending on the hydrogeological condition around the station. Whereas the river flowing beside the station will help accelerating the runoff dispersion and reducing rainfall decay time in the hill or mountain region. We also compare the results after calibration in term of isotropic and vertical coupling individually. We found that the response strains are smaller in vertical coupling rather than isotropic coupling. The effects of debris avalanches caused by intensive rainfall in the mountain areas can be viewed as two types of rock deformation: generated only under the influence of rainfall and generated by the increased load in the river channels due to rainfall-induced landslides or debris flow. When the cumulative rainfall exceeds a certain threshold, the strain response curves show a noticeable anomaly likely due to the effects of the debris flow events in places prone to landslides.
Highlights
Strainmeters feature high precision and wide frequency ranges and are able to record the crustal deformation caused by tectonic activities
The analysis of the rainfall quick response (RQR) corresponding to the rainfall load was conducted on relatively heavy rainfall events during the sampling period
RQR obtained by convolution of the average rainfall impact function and rainfall are shown on the right
Summary
Strainmeters feature high precision and wide frequency ranges and are able to record the crustal deformation caused by tectonic activities. Apart from the application of observations and research of seismic precursors, strainmeters are frequently utilized to observe and study volcanic activities, slow earthquakes, and tremors [1,2,3]. 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 hectopascal (hPa) and heavy rainfalls resulting in geohazards [8,9]. Typhoons represent one of the major hydrological factors triggering significant transient deformation in terms of atmospheric events in Taiwan [5,6]
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