Revisiting borehole strain, typhoons, and slow earthquakes using quantitative estimates of precipitation‐induced strain changes

Taiwan experiences high deformation rates, particularly along its eastern margin where a shortening rate of about 30 mm/yr is experienced in the Longitudinal Valley and the Coastal Range. Four Sacks‐Evertson borehole strainmeters have been installed in this area since 2003. Liu et al. (2009) proposed that a number of strain transient events, primarily coincident with low‐barometric pressure during passages of typhoons, were due to deep‐triggered slow slip. Here we extend that investigation with a quantitative analysis of the strain responses to precipitation as well as barometric pressure and the Earth tides in order to isolate tectonic source effects. Estimates of the strain responses to barometric pressure and groundwater level changes for the different stations vary over the ranges −1 to −3 nanostrain/millibar(hPa) and −0.3 to −1.0 nanostrain/hPa, respectively, consistent with theoretical values derived using Hooke's law. Liu et al. (2009) noted that during some typhoons, including at least one with very heavy rainfall, the observed strain changes were consistent with only barometric forcing. By considering a more extensive data set, we now find that the strain response to rainfall is about −5.1 nanostrain/hPa. A larger strain response to rainfall compared to that to air pressure and water level may be associated with an additional strain from fluid pressure changes that take place due to infiltration of precipitation. Using a state‐space model, we remove the strain response to rainfall, in addition to those due to air pressure changes and the Earth tides, and investigate whether corrected strain changes are related to environmental disturbances or tectonic‐original motions. The majority of strain changes attributed to slow earthquakes seem rather to be associated with environmental factors. However, some events show remaining strain changes after all corrections. These events include strain polarity changes during passages of typhoons (a characteristic that is not anticipated from our estimates of the precipitation transfer function) that are more readily explained in terms of tectonic‐origin motions, but clearly the triggering argument is now weaker than that presented in Liu et al. (2009). Additional on‐site water level sensors and rain gauges will provide data critical for a more complete understanding, including the currently unresolved issue of why, for some typhoons, there appears to be a much smaller transfer function for precipitation‐induced strain changes.