Abstract
The 2011 Tohoku earthquake (Mw 9.0) nucleated at 24 km depth along the plate boundary. To understand the earthquake nucleation mechanisms, it is essential to reveal the frictional properties of metamorphic rocks, which are expected to be present at depth within the Tohoku subduction zone. We performed friction experiments on glaucophane schist powders (Franciscan Belt, California) using a rotary shear apparatus. Simulated fault gouges were sheared at temperatures of 22-400oC, effective normal stresses of 25-200 MPa and pore fluid pressures of 25-200 MPa. We conducted velocity-stepping sequences to determine the rate and state parameter (a-b) and investigated how this parameter changed with temperature and effective pressure.
At 22oC, the gouges show a friction coefficient of about 0.75 and positive (a-b) values which decrease to become negative with increasing temperature. At 200oC, the behavior is velocity weakening and shows negative (a-b) values with a background friction of ~0.75. In some cases the samples exhibit stick-slip behavior. At 300oC, friction is ~0.65 and slip is velocity weakening (negative (a-b) value) at low pressure but strengthening at high pressure, showing larger (a-b) values than at 200 oC. The samples show unstable stick- slips under some conditions. (a-b) values slightly decrease at 400oC with a background friction of ~0.7. There is also effective normal stress dependence: (a-b) values are negative at low effective pressure and increase to positive with increasing effective normal stress except those at 200oC which remain negative under almost all pressure conditions. Micro-/nano-structural analyses to decipher the mechanisms that determine the frictional behavior of glaucophane schists are in progress.
Our results suggest that earthquakes can nucleate in glaucophane schists at depths within a temperature range of 100-300oC or/and low effective pressure. The temperature of the Tohoku earthquake hypocenter is reported to be about 160oC, which is in agreement with our experimental results.
At 22oC, the gouges show a friction coefficient of about 0.75 and positive (a-b) values which decrease to become negative with increasing temperature. At 200oC, the behavior is velocity weakening and shows negative (a-b) values with a background friction of ~0.75. In some cases the samples exhibit stick-slip behavior. At 300oC, friction is ~0.65 and slip is velocity weakening (negative (a-b) value) at low pressure but strengthening at high pressure, showing larger (a-b) values than at 200 oC. The samples show unstable stick- slips under some conditions. (a-b) values slightly decrease at 400oC with a background friction of ~0.7. There is also effective normal stress dependence: (a-b) values are negative at low effective pressure and increase to positive with increasing effective normal stress except those at 200oC which remain negative under almost all pressure conditions. Micro-/nano-structural analyses to decipher the mechanisms that determine the frictional behavior of glaucophane schists are in progress.
Our results suggest that earthquakes can nucleate in glaucophane schists at depths within a temperature range of 100-300oC or/and low effective pressure. The temperature of the Tohoku earthquake hypocenter is reported to be about 160oC, which is in agreement with our experimental results.
| Original language | English |
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| Publication status | Published - Jul 2014 |
| Event | Asia Oceania Geosciences Society (AOGS) - Sapporo, Japan Duration: 28 Jul 2014 → 1 Aug 2014 |
Conference
| Conference | Asia Oceania Geosciences Society (AOGS) |
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| Country/Territory | Japan |
| City | Sapporo |
| Period | 28/07/14 → 1/08/14 |