Estimation of the seismic moment and the corner frequency with continuous broadband AE records in a fracturing rock sample
Abstract
The seismic moment (Mo) and the corner frequency (fc) are fundamental parameters which characterize the source properties of the earthquake rupture. In the previous studies, the relationship that Mo is proportional to fc-3 in natural earthquakes has been reported, and the lower limit of this scale was down to about Mw -2 level. In a laboratory, a series of elastic waves radiated from micro cracking (Acoustic Emission; AE) in a rock sample enables us to estimate whether this relationship can be extended down to the AE size events (source size is millimeter order). To compare the source parameters obtained from AE with those from natural earthquakes, AE spectral analysis for the estimation should be done in a similar manner of spectral analysis to natural earthquakes. Unfortunately, the records of narrowband transducers, which have often been used in laboratories, are inadequate for spectral analysis because of their narrow frequency ranges. To solve this problem, Sellers et al. (2003) recorded broadband AE waveforms during a rock fracture experiment and indicated that the same scaling relationship as that for natural earthquakes can be satisfied with AE. However, they carried out triggered recording which hide some events behind the mask times. In this study, we tried to record all events in a fracturing rock under uniaxial conditions with broadband, continuous recording. We prepared ten broadband transducers (sensitive range; 100 kHz - 1,000 kHz), and a cylindrical Westerly granite sample, 100 mm in height and 50 mm in diameter. Six strips with 60 degree intervals parallel to the loading axis of the sample were grounded to mount the transducers. Eight transducers were attached on the side surfaces and the other two were attached inside of metallic vessels placed at the upper and lower ends of the sample. High sampling recording was continued at 20 MS/s per channel, during uniaxial loading which was continued to be controlled even after the peak strength. Owing to the continuous recording, various amplitudes of signal waveforms are found. We estimated the frequency response of the transducer and removed it from the waveform spectrum, hence we obtained displacement waveform spectrum for S wave to estimate fc. Mo is estimated, using S wave velocity (2800 m/s), path length, and low-frequency displacement spectral plateau of S wave spectrum. Corner frequencies of 22 events (-6.7 < Mw < -6.1) were ranging from 160 kHz to 570 kHz, which is consistent with the extended scaling relationship for natural earthquakes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.S41C2204Y
- Keywords:
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- 5104 PHYSICAL PROPERTIES OF ROCKS / Fracture and flow;
- 7200 SEISMOLOGY;
- 7215 SEISMOLOGY / Earthquake source observations