The rupture process of great subduction earthquakes: the concept of the barrier and asperity models Yoshihiro Kaneko (Presentation based on Aki, 1984; Rundle and Kanamori, 1987; Park and Mori, 2007; Das and Aki, 1977; Madariaga, 1983; Heaton, 1990; Dunham et al., 2003) Ge 277 – Fall, 2007

Two important seismological observations 1.Slip is heterogeneous over a fault plane with several patches of relatively larger slip 2.Rise time at any point is shorter than the duration of an EQ   Rise Time: 15-30s   Rupture velocity: 2.5-3kms   Duration: 80s Ozgun Kunca

Key seismological terminology Barriers = areas of smaller amount of slip in a single EQ Asperities = areas of larger amount of slip in a single EQ Ammon et al, 2005

Barrier vs. asperity models (interpretation by Aki, 1984) Barrier/asperity = regions of higher shear strength

Concept of “barrier” model

Concept of “asperity” model

Summary: barrier vs. asperity models Barrier model Barrier model –Stress roughening process (homogenous -> heterogeneous) –Stable barrier is unbroken in repeated EQs, but weaker ones break in different patterns (different amount of slip in different EQs) –Resulting in characteristic fault length but not a characteristic amount of slip –How do strong barriers behave over time (the region needs to accommodate some slip eventually)? Asperity model Asperity model –Stress smoothening process (heterogeneous -> homogenous) –Consistent with many great interplate earthquakes that show stable slip patches over EQ cycles (both characteristic fault length and amount of slip) –The stability of asperity strength can explain "the time predictable model" –Act as a strong barrier during interseismic(?), or slipping seismically

An evidence for the barrier model: Maximum slip scales with barrier interval barrier interval (km) maximum slip (m) Aki (1984)

Observational studies: Are asperities and barriers stable? Papua New Guinea sequences (Park and Mori, 2007) Papua New Guinea sequences (Park and Mori, 2007) Colombia-Ecuador sequences (Randle and Kanamori, 1987) Colombia-Ecuador sequences (Randle and Kanamori, 1987) Nankai trough sequences (Randle and Kanamori, 1987) Nankai trough sequences (Randle and Kanamori, 1987) Sumatra sequences (J.-P. Avouac, introduction) Sumatra sequences (J.-P. Avouac, introduction)

Tectonic setting Park and Mori (2007) 8 – 15 cm/year Studied Area

Locations of the fault planes and focal mechanisms Park and Mori (2007)

Asperities do not overlap

Park and Mori (2007) Asperities may not be stable in some places The studied area (Papua New Guinea subduction) may be tectonically complicated region (?).

Rundle and Kanamori (2007) Nankai Trough sequences 6.5 cm/year

Rundle and Kanamori (2007) Remark: Slip at a given patch is larger in the largest EQ than that those in individual small EQs

Colombia-Ecuador sequences Rundle and Kanamori (2007)

Remark: Moment is 2.7 – 7 times larger in the largest 1906 EQ than that the sum of the other 3 EQs.

Sumatra subduction zone (J.-P. Avouac, this class) - Are asperities stable over many earthquake cycles? - Is the time predictable model applicable for this region? - Is slip at a given patch greater in the largest EQ than that those in individual small EQs?

Hiroo’s comments

Alternative model (Yoshi, Avouac, Nadia) Feature: - asperities are stable over EQ cycles - time predictable (on average) - the slip at a given point is comparable or greater in the largest EQ than those in small EQs - explains the relation of seismic coupling and asperity