Ultra-micro fractures observed in a fracturing rock sample (acoustic emission; AE) showed some similar statistical features to natural earthquakes. However, the source characteristic of AE was veiled in mystery, and there was a question whether AE can be regarded as an ultra-micro-earthquake or not. To address this issue, we estimated source parameters of AE, using a measurement system which achieved multi-channel (8 channels), broadband (up to 2 MHz), and high-speed (maximum 33 MS/sec) continuous recording (5 hours) under seismogenic pressure conditions (10 MPa confining pressure).

Estimated AE source parameters based on the seismological analyses method satisfied the scaling relationship for natural earthquakes in which seismic moment is inversely proportional to the cube of corner frequency. The results suggest that both millimeter-scale laboratory fractures and kilometer-scale natural earthquakes have self-similar processes, and AE can be interpreted as an ultra-micro-earthquake (Mw~-7). It also demonstrate that laboratory observation is an effective approach in studying natural earthquake generation process.


The relationship between the seismic moment and the corner frequency estimated from the laboratory experiment (gray arrow) with the results from previous studies.

See Yoshimitsu et al. (2014) for a detailed description.
Magnitude -7 level earthquakes: A new lower limit of self-similarity in seismic scaling relationships