Earth Planets Space, Vol. 61 (No. 6), pp. e21-e24, 2009E-LETTER
Nana Yoshimitsu1, Hironori Kawakata2, and Naoki Takahashi3
1Graduate School of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
2Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
3Sumitomo Mitsui Construction Co., Ltd., Tokyo 160-0023, Japan
(Received March 16, 2009; Revised May 19, 2009; Accepted May 22, 2009; Online published June 26, 2009)
We analyzed temporal changes in the velocity and amplitude of P waves transmitted through a granite sample during a triaxial compression test, with the goal of monitoring the fault formation process associated with open and shear cracking. We used newly developed transducer assemblies for the broadband recording, and we continued to record transmitting waves even after the peak stress occurred. For transmitting P waves with paths parallel to the maximum compressive axis, we found that both the first wave amplitude and the velocity decreased after dilatancy started, and they kept decreasing even after the peak stress. In addition, the large nonlinear decrease in amplitude was associated with a rapid decrease in differential stress, whereas the rate of decrease in velocity remained almost constant. Thus, before the rapid decrease of differential stress, when both the amplitude and the velocity gradually decreased, open cracking was indicated to be dominant. Thereafter, shear cracking was indicated to become dominant in synchronization with the rapid decrease in differential stress. It is suggested that a main fault started to grow around the sample surface and then progressed into the sample interior; this corresponded to the rapid stress decrease. This fault acts as a strong scatterer for P waves that are parallel to the maximum compressive
Key words: Broadband recording, fracturing Westerly granite, triaxial compression test, post-peak stress, transmitting P wave, amplitude change, velocity change, fault formation process.