Source distribution of acoustic emissions during an in-situ direct shear test: Implications for an analog model of seismogenic faulting in an inhomogeneous rock mass

TLDR: In this article, an in-situ direct shear test on a sample composed of a slate-dominant alternation of slate and sandstone was conducted in a survey tunnel for an underground powerhouse in central Japan, and the authors concluded that an initially intact region of rock bounded by the joints and the seam is fractured, generating the acoustic emission.

About: This article is published in Engineering Geology.The article was published on 2010-02-09 and is currently open access. It has received 74 citations till now. The article focuses on the topics: Shear (geology) & Rock mechanics.

Figures

Figure 10 AE count rates and shear load for approximately 22 minutes prior to failure. Bars along the bottom lateral axes indicate occurrence of AE events for which epicenters are shown in Figure 12. (a) From 22 minutes 7 seconds to 9 minutes 37 seconds prior to the failure. (b) From 6 minutes prior to the failure to 4 minutes following the failure. Arrow at around 70 seconds prior to the failure indicates the occurrence time of the AE event for which waveforms are shown in Figure 11.

Figure 4 (a) Plane view and (b) vertical view of the test block for the in-situ direct shear test. Setting positions of the AE sensors (numbers 1 to 8) and displacement gauges (letters i to r) are also shown. Two large open arrows show the directions of vertical load, V, and shear load, S, which were applied to the test block.

Figure 13 Plane view of the test block with time prior to the failure. Front of the test block is indicated by a solid line. Displacement is enlarged in comparison with the block size, as shown in the scale. The block rotation angle, θ, is also shown with an exaggeration of approximately 17.4 times larger than the real angle.

Figure 12 Distribution of all 154 AE epicenters located (open circles). Comparing this figure with Figure 8, the AE clustering region corresponds to the intact region bounded by the two joints J-2 and J-3, and the loosening seam Sm-1. Large open arrow indicates the direction of the applied shear load.

Figure 7 (a) Contour map of the fractured plane. Numerals indicate height in centimeters from the expected shear plane. Zero level is the ground surface around the specimen. Hatched area indicates concave regions lower than the expected shear plane. Large open arrow is the direction of the shear load. (b) Three sectional views along the lines A-A', B-B' and C-C' shown in (a).

Figure 11 Example of recorded waveforms. Arrow on each waveform indicates arrival time that we interpret as the P-wave initial motion.

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Source distribution of acoustic emissions during an in-situ direct shear test: implications for an analog model of seismogenic faulting in an inhomogeneous rock mass" ?

In this paper, an in-situ direct shear test on a slate-dominant alternation of slate and sandstone was conducted in a survey tunnel for an underground powerhouse in central Japan. 

Q2. What is the analog model for an inland earthquake?

If the intact region is considered to correspond to a portion of asperity that causes strong ground motion and the weakened region is considered to correspond to a region of aseismic creep, an in-situ direct shear test could be a good analog model for an inland earthquake fault. 

Q3. How many dB was used in the AE monitoring system?

An AE signal received at each AE sensor was amplified by 100 dB in total: 40 dB in a pre-amplifier and 60 dB in a main amplifier. 

Q4. What is the AE count rate in a subduction zone earthquake?

(1) The AE count rate increase corresponded to the change in thevertical displacement from downward to upward, which indicated the onset of the final failure of a test block, as has been observed in many in-situ direct shear tests. 

Q5. What is the analog model for a subduction zone earthquake?

(6) Large-scale inhomogeneous rock fracturing experiments such asthe in-situ direct shear test have the possibility to provide useful insights as analog models of seismogenic faulting. 

Q6. What is the thickness of the brittle layer?

The thickness of the seismogenic brittle layer is also proportional to the age of the oceanic plate, which then cools the overlying continental crust and deepens the seismogenic depth along the plate interface. 

Q7. How long before the failure was indicated by the sudden decrease in the shear load?

the AE sources were located only for the period of 22 minutes before the final failure, which was indicated by the sudden decrease of the shear load. 

Q8. What is the effect of a relatively smoothed subducted seafloor on the plate interface?

relatively smoothed subducted seafloor seems to have even caused the stress inhomogeneity in the plate interface due to the variety of contact conditions and small scale asperities. 

Q9. What is the main factor controlling earthquake rupture?

the authors believe that the fluid effect would be minor, but large-scale inhomogeneities such as those in their test are the primary factor controlling earthquake rupture. 

Q10. How many AE events were detected at more than four sensors?

Most AE events whose P-wave initial motions were detected at more than four sensors and that satisfied the condition of the standard deviation were recorded just prior to the final failure. 

Q11. How long did the AE event rate increase?

It gradually increased again from around 280 or 300 minutes, and a burst of AE events occurred after 380 min, leading up to the final failure of the test block.