Foreshock Activity during Laboratory Friction Tests using Meter-scale Rock Specimens Inferred from two Apparatuses with Different Stiffness
Tuesday, 16 December 2014: 8:15 AM
Foreshock activity has been reported for large earthquakes. The generation mechanism of these foreshocks is one of the most important keys to understand the initiation of large earthquakes. To simulate earthquakes in laboratory, Fukuyama et al. (2014) developed a meter-scale biaxial friction apparatus, using a large shaking table at NIED. Two rectangular parallelepiped gabbro samples were prepared. The upper one fixed to the outside floor of the shaking table was 1.5 m long, 0.5 m high and 0.5 m wide, and the lower one that moved with the shaking table was 0.5 m longer. The original loading apparatus had a long arm between outside floor and the loading frame with two hinges. We renovated the apparatus to have a shorter arm without hinges in order to make the apparatus stiffer. Stick-slip events were observed during the friction tests with both apparatuses. In this study, we analyzed acoustic emission events using continuously recorded elastic waves during the tests with these two apparatus. The foreshock activity was quite high for the less stiff apparatus, and occurrence rate of foreshocks increased toward a main stick-slip event. In contrast, the activity for the stiffer apparatus was much lower. We consider that the difference in stiffness in the lab should correspond to the tectonic setting in nature. For inter-plate earthquakes, foreshocks were reported to be active. Some foreshocks were migrating, and occurrence rate of foreshocks increased toward the main shocks (e.g., Bouchon et al., 2011; Kato et al., 2012; Ruiz et al., 2014). On the other hand, foreshock activities of inland earthquakes were low, and quiescence prior to main shocks was observed. Further, they occurred at the close vicinity of the hypocenter of main shock (e.g., Doi and Kawakata, 2012; 2013). Considering that the loading systems for inter-plate earthquakes are less stiff than those for inland earthquakes, the stiffness of the loading system should control the foreshock activity.