Strainmeter
Strain changes revealed in borehole strainmeter array, eastern Taiwan
Author : Jiun-Ting Lin
Advisor : Ya-Ju Hsu & Li Zhao
ABSTRACT
Over hundreds of GPS sites and seismometers have been installed in Taiwan for the purpose of monitoring crustal deformation and fault activities . In order to study the Longitudinal Valley Fault, Academia Sinica has deployed eleven borehole strainmeters in eastern Taiwan since 2002. The goal aims to investigate fault activities which are not able to detect by GPS and seismometers. Recent studies have found slow/silent earthquakes in various tectonic environments. The high resolution records from borehole strainmeters make the detection of subtle strain possible; while the high accuracy make them very sensitive to ambient noise factors such as precipitation, hydrological, and pressure effects. Characterizing strain changes with different factors is very important to distinguish between rainfall-induced fault creep and slip-rate changes of tectonic origin.
INTRODUCTION
Plate boundary Observatory in Taiwan is an array composed of mainly GPS instruments. However, GPS and seismometers have poor sensitivity on the time periods of 102-105 sec. Borehole strainmeter are designed to fill this gap. The eleven Sacks-Evertson borehole strainmeter had deployed at a depth range of 170–270 m in the Longitudinal Valley and the Costal Range of eastern Taiwan since 2002 (see below (right)).
DATA
To ensure a high accuracy of about 10-10 strain, the instrument have to reset itself, called valve opening, which make the data discontinuous. Grout curing & hole relaxation are the two effect depend on different material and can be model by equation:
exponential terms represent the grout curing and hole relaxation decay, respectively. A0 denotes the initial value, C1*t is the long term trend. Because initial value is hard to evaluate, we than solve this by differential form.
Effects of rainfall & barometric?
Could the strain change relating to the effect of rainfall loading or even air pressure?
These effects could be corrected by auto-regression (AR) method (right).
The tide effect
Seismic related strain change and synthetic model
The order of the transfer coefficients obtain from earth tides and synthetic straingrams are both on the order 10-11~10-12,which are consistent with strainmeter instrument response.
CONCLUSION
Borehole strainmeter is able to detect subtle strain changes associated with slow earthquakes or strain variations prior to earthquakes (see Liu, C., Linde, A. T., & Sacks, I. S. (2009). Slow earthquakes triggered by typhoons. Nature, 459(7248), 833-836.), but there are still several problems haven’t be solved. For example, the strain responses to precipitation or ground water level variations. Further investigation is needed to understand the influence of local hydrological contribution to observed strain changes and fault slip behaviors.