Abstract
There is a coupling relationship between surrounding rock stress, deformation, and fracture evolution, especially in the microdynamics of the crust caused by mining activities and earthquakes. Previous research has investigated many cases regarding the coseismal water level responses and proposed a method to calculate the aquifer parameters by tidal analysis. However, to date, measurement of the degree of rock damage in the field has not been reported. Quantifying the fracture characteristics is essential for accurate evaluation of rock stability. This study has analyzed the relationship between the seismograms and hydroseismograms in response to the Mw 7.8 Solomon Islands earthquake and the Mw 7.8 Kaikōura earthquake, both events occurring in 2016. The calculated and measured changes in water level in the X10 well were fitted in order to study the relationships among the volumetric strain, the deviatoric strain, and the oscillations in the pore pressure. Then, we further estimate the degree of rock damage and the hydraulic characteristics of the aquifer. The results showed that the values for the rock damage parameter, 0.662 < αD < 0.754, and the Skempton coefficient, −0.100 < A < 0.026, estimated for the Solomon Islands earthquake signified higher damage and dilatancy in the X10 well. Also, the respective values for the parameters, 0.293 < αD < 0.363 and 0.226 < A < 0.251, calculated for the Kaikōura earthquake signified a lower degree of rock damage. It is concluded that the changes in the pore pressure were influenced by both the volumetric strain and the deviatoric strain. The degree of rock damage and the hydraulic properties of the aquifer estimated from the water level fluctuations in the wells which were induced by the seismic waves represent the actual aquifer characteristics.
Highlights
Understanding the mechanical and hydrological properties of rock is important for engineering, including mining, sources evaluation, landslide stability assessment, infrastructure stability, reservoir geomechanics, and so on [1,2,3,4]
Estimation of the Degree of Rock Damage by Seismic Analysis. e amplitudes of the oscillations in the water level and the changes in the pore pressure are affected by the wavelength and frequency of the specific seismic waves
The Solomon Islands earthquake and the Kaiko ura earthquake were selected. e time series selected was divided into 10-day interval and a 5day sliding window. e tidal factors obtained by BAYTAPG in each interval did not change with time, which indicated that the aquifer characteristics were stable. e tidal factor estimated for the Solomon Islands earthquake was 4.167 × 105 m, and that for the Kaiko ura earthquake was 4.831 × 105 m
Summary
Understanding the mechanical and hydrological properties of rock is important for engineering, including mining, sources evaluation, landslide stability assessment, infrastructure stability, reservoir geomechanics, and so on [1,2,3,4]. An empirical damage-mechanism model is introduced and is widely used in geophysics and engineering for irreversible deformation [5]. E rock damage is described by 0 < αD < 1 which is influenced by aquifer properties and stress state [6]. High damage value (αD > 0.8) means that the rock is highly fractured and its mechanical properties are low while its permeability is high. Low damage value (αD < 0.2) means stiff and strong rock with low permeability. Measuring the rock damage in the field is still impossible
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