Cross-Scale Model of the Subduction Seismic Cycle: Parameter Sensitivity Study

Abstract:

We model seismic cycle of great megathrust earthquakes using finite element numerical technique that employs elasto-visco-plastic rheology consistent with laboratory data of crustal and mantle rocks and is capable to describe both geological and seismic-cycle time-scale deformation of lithosphere (see accompanied abstract by Sobolev and Muldashev). Here we present details of model design and parameter sensitivity analysis.

We prepare model setup by modeling subduction process for several millions of years, with the different assumptions for initial lithospheric structure and temperature of the overriding and subducting plates and kinematic boundary conditions. As a result we obtain the subduction model with appropriate geometry and stress distribution. We then substitute static friction at the plate interface by rate and state friction law and employ adaptive time-step integration procedure that varies time step from 10^-6 year at instability (earthquake), and gradually increases it to 5 years during postseismic relaxation.

We study sensitivity of model in 2D to the magnitude of static friction, rate and state parameters (a, b and Lc) and viscosity in subduction channel and demonstrate agreement with theoretical expectations and observations. In particular we obtain almost linear relation between the earthquake period and stress drop from one hand and the rate and state parameter (b-a) from another, and realistic values of stress drop of few MPa for the typical great earthquakes. The model also shows classic instable behavior at low Lc (<10-20 cm) and conditionally stable behavior at high Lc (>20-25 cm).

Next we investigate dependency of seismic moment (and average slip) of model earthquake on rupture area in 3D. We obtain relations close to the theoretical expectation for the average stress drop of earthquake weakly dependent from its magnitude.

Finally we show consistency of our modelling results with observed surface deformation during Great Tohoku Earthquake of 2011 at day to 1 year time scale.