## 2 Specimen, Experimental Setup, and Acoustic Emission Monitoring
### Acoustic Emission Monitoring  
Acoustic emission signals from the 16 PZTs were pre-amplified by 45 dB and then digitized at a sampling rate of 25 MHz and a sample length of 4096 words (approximately 160 microseconds) for each event. The A/D system had a dynamic range of 16 bits, 256 mega-words of onboard memory for each channel, and three working modes: single-event mode, multi-event mod, and continuous mode.  
Figure 1: (**a**) Photograph of the test specimen. (**b**) Specim with piezoelectric transducers (PZT) and cross-type strain gauges mounted. (**c**) Specim and endpieces covered with silicone sealant. (**d**) Specim-cover assembly. (**e**) Plot showing the distributions of PZTs (solid circles) and cross-type strain gauges (crosses). (**f**) Photograph of the load frame used in the test. The maximum amplitudes of AE signals from PK1 and PK2 were recorded by two peak detectors and were used for the calculation of seismic \(b\)-value.  
In the single-event mode, the digitized waveform data of a triggered AE event were immediately transferred to the hard disk of the host PC. Since the system was able to record at most 10 events per second, the single-event mode was applied during the earlier stages of the experiment, when the AE rate was lower than 10 events per second. Once AE occurred more frequently, the system was switched to the multi-event mode, under which the system could record up to approximately 5000 events per second. All digitized waveform data were stored in the onboard memory, which had a capacity of 65,536 events under the aforementioned sampling conditions. Waveform data were finally transferred to the hard disk either when the onboard memory was full or the experiment was completed.  
In addition to waveform recording, AE signals from two selected sensors (#3 and #10 in Figure 1e) were amplified by 20 dB and then fed to peak detectors (dynamic range: 100 dB) for event counting and recording the maximum amplitude of each event. The noise level for AE signals was approximately 40 dB after applying the 20-dB preamplifier. Thus, the threshold for the peak detectors was set to 45 dB, resulting in an effective dynamic range of 55 dB (corresponding to a relative magnitude range of from 0 to 2.75). A complicated triggering logic was designed for recording waveforms. First, three pairs of neighboring sensors were selected as triggering sensors (Figure 1e). In order to avoid biased triggering, the three pairs needed to be homogeneously distributed on the sample surface. Then, an OR logic was used within each pair, and an AND logic was used among the three pairs. Finally, the triggering threshold level was determined, which was approximately 10 times higher than that of the peak detectors. As a result, the waveform data constituted a sub-set of relatively large events among the AE count data. Higher thresholds and multiple homogeneously distributed trigger channels ensured that waveform data were collected for AE events without significant bias throughout the specimen volume. Note that the system captured most AE events, other than a short period of dynamic rupturing, in which AE occurred at a very high rate and AE signals seriously overlapped. Since the present study focused on pre-failure AE activity, event loss was not a problem.  
An automatic switching sub-system was used to sequentially connect selected sensors (#6 to #13 in the present study) to a pulse generator in order to generate ultrasonic pulses for velocity measurement. During the velocity measurement, signals from the source sensor were recorded by other sensors. The time required to complete a measurement of the seven sources was less than 1 s, and losing AE events during velocity measurement was very rare.