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

Abstract

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 . 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 À1 to À3 nanostrain/millibar(hPa) and À0.3 to À1.0 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 À5.1 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.

Near-field strain observations of the October 2013 Ruisui, Taiwan, earthquake: source parameters and limits of very short-term strain detection

“…The Institute of Earth Sciences Academia Sinica (IESAS) in cooperation with the Department of Terrestrial Magnetism, Carnegie Institution of Washington, has deployed since 2003, 11 Sacks-Evertson borehole strainmeters (8 SES-1, 3 SES-3 design, (Hsu et al 2015)) at a depth range of 180-270 m along the active Longitudinal Valley fault in eastern Taiwan (Fig. 1).…”

“…The instruments installed along the LVF are completing the few networks deployed worldwide; in particular in Japan (Okada et al 2004), in western USA (Roeloffs 2010), or in western Europe (Bernard et al 2006;Canitano et al 2013, Crescentini et al 1997, for instance. Dilatometers installed in eastern Taiwan have recorded tens of slow earthquakes between 2003 and 2007, and some of them were found to be triggering by typhoons (Hsu et al 2015;Liu et al 2009). …”

“…Here, we focus on the four stations located on the central LV (Hsu et al 2015) between the cities of Ruisui and Chimei (Fig. 1) as they are located about 20-30 km from the epicenter of the 2013 Ruisui shock and open the possibility to pre-strain signatures (Takemoto 1991).…”

“…Note that the instruments have been calibrated against the Earth tides (M 2 constituent) by using the common technique of comparing output data with the modeled tidal signals. As a check on these calibrations, the responses of the stations have been compared to long-period (50 -100 s) surface waves from distant great earthquakes and a good agreement between the ratios of tidal and waveform admittances has been found (Hsu et al 2015). The calibration of borehole strainmeters using the solid Earth tides (or surface waves) means that it includes local structures, somewhat similar to a station correction in seismology.…”

A first modeling of dynamic and static crustal strain field from near-field dilatation measurements: example of the 2013 $$M_w$$ M w 6.2 Ruisui earthquake, Taiwan

“…Thus, strainmeters can supply continuous deformation data at short periods, but could also play a significant role in the attempt of detecting aseismic deformation prior to earthquakes. Strainmeters have recorded crustal strain changes associated with the occurrence of typhoons along the central section of the LVF between 2003-2007(Liu et al 2009Hsu et al 2015).…”