Evolution of stress in the oceanic lithosphere and oceanic intraplate earthquakes; implication for aseismic stress release

Abstract:

Understanding stress state in the oceanic lithosphere is one of important clues to reveal dynamics of plate tectonics. Previous studies revealed that “thermal stress” and “ridge-push” have an important role to cause differential stress in the oceanic lithosphere, based on focal mechanism of oceanic intraplate earthquakes and their modeling [e.g. Bratt et al., 1985; Wiens and Stein, 1984]. Recently, various new data and experimental results have become available and these enable us to make more precise modeling and detailed discussion. Therefore, we revisit stress evolution of the oceanic lithosphere to reveal detail of it.

 We make a model of stress evolution in the oceanic lithosphere from oceanic ridge to before subduction, using time integration of stress history in the Lagrangian description. Our model considers thermal stress, ridge-push, and basal resistance as the mechanical sources; and brittle fracture and ductile flow as stress relaxation mechanism.

 Our result shows that focal mechanisms of oceanic intraplate earthquakes basically can be interpreted as a result of the thermal stress effects. Predicted seismic moment release rate (SMR) from our model is almost consistent with observed SMR in the young oceanic lithosphere (<45My). On the other hand, in the old oceanic lithosphere (>45Myr), observed SMR is only ~10% of our model prediction. In the old oceanic lithosphere (>45Myr), our results also indicate that thermal strain rate is less than 10^(-10)/year, and seismic moment release should occur in the upper part (~10 km) of oceanic lithosphere (where <300℃). Thus we propose that very slow (~10^(-10)/year) stress relaxation mechanism may work in the oceanic lithosphere even low temperature condition (<300℃). Dissolution-precipitation creep is candidate mechanism, however, its strain rate under geological condition has not been investigated yet. Our results may constrain on mechanism of very slow stress accumulation and relaxation under low temperature condition.