1. Slow/silent slip events in deeper seismogenic portion of subduction zones -- mechanism investigation from 3D modeling Yajing Liu [1], James R. Rice [1,2] 1 Department of Earth and Planetary Sciences, Harvard University 2 Division of Engineering and Applied Sciences, Harvard University

2. Deeper Cascadia subduction interface (Dragert et al., Science, 2001) Duration of this event: ~ 40 days; Estimated slip rate: ~ 10 -9 to 10 -8 m/s; Estimated migration speed: ~ 6 km/day.

3. Nankai trough subduction zone, southwestern Japan (Ozawa et a l., JGR, 2001) Duration of this event: ~ 1 year; Estimated slip rate: ~ 10 -9 to 10 -8 m/s; Estimated migration speed: tens of km/year. Hirose et al., GRL, 1999; Ozawa et al., Science, 2002; Ozawa et al., GRL, 2003.

4. Geometry and modeling set-up (a-b)<0: seismogenic zone Small perturbations in frictional properties: Assumed (a–b)and a values converted from a typical subduction geotherm, based on temperature dependent experimental data (Blanpied et al., 1991,1995)

5. Depth-averaged (over 50 km seismogenic zone) slip versus along strike distance Lines plotted every 5 years

6. Aseismic transients features -- from simulations Usually start below the less well locked “gap” regions; Have clearly aseismic slip rate: 10 -9 to 10 -8 m/s; Migrate along strike: 1. at depth near or below the downdip end of the seismogenic zone; 2. with representative speed of ~ 20 km/year ; Locking intensity is weakened around those “gap” regions  potential nucleation sites for future large events;

7. Log 10 (V) in m/s

8. Log 10 (V) in m/s A large event starts here!

9. Aseismic transients features (continued) Some events do not migrate along strike for such a long distance, like more than 100 km; Appear ~simultaneously (within a few years) at a wide range of along-strike locations; Show nucleation, “rupture propagation”, and re-locking behaviors, like large seismic events.

11. Depth of aseismic transients and depth of velocity-weakening to velocity- strengthening transition?

12. Log 10 (V) in m/s

13. Conclusions: Small along-strike variations in material frictional properties (or initial conditions) can perturb the system into a non-uniform slip mode. Various features: nucleation site, slip rate, along- strike propagation speed and range, are shown from simulated aseismic transients, and are comparable to observations at natural sites. There is a clear correspondence between the depth range of aseismic transients and the transition in friction behavior.