Slow/silent slip events in deepSlow/silent slip events in deeper seismogenic portion er seismogenic portion

of subduction zones of subduction zones -- -- mechanism investigation from mechanism investigation from

3D modeling3D 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

Deeper Cascadia subduction interfaceDeeper Cascadia subduction interface((Dragert et al., Science, 2001Dragert 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 .

Nankai trough subduction zone, southwestern Japan Nankai trough subduction zone, southwestern Japan ((Ozawa et a l., JGR, 2001Ozawa 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.

Geometry and modeling set-upGeometry 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 experi

mental data (Blanpied et al., 1991,1995)

Depth-averaged (over 50 km seismogenic zone) Depth-averaged (over 50 km seismogenic zone) slip versus along strike distanceslip versus along strike distance

Lines plotted every 5 years

Aseismic transients features Aseismic transients features ----from simulationsfrom 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;

Log10(V) in m/s

Log10(V) in m/s

A large event starts here!

Aseismic transients features (continuAseismic transients features (continued)ed)

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.

Depth of aseismic transients and Depth of aseismic transients and depth of velocity-weakening to velocity-strdepth of velocity-weakening to velocity-str

engthening transition?engthening transition?

Log10(V) in m/s

Conclusions: 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.