CGRG Bibliography of Canadian Geomorphology
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Author : Dragert, H.; Wang, K.; Schmidt, M.; Lu, Y.; and Mazzotti, S.
Date : 2002.
Title : Seismology without seismometers: towards characterizing aseismic slip on the deeper Cascadia subduction interface.
Publication : Seismological Society of America 2002, Annual Meeting 17-19 April 2002. Victoria Conference Centre, Victoria, British Columbia
Issue :
Page(s) :
Abstract
Although the unambiguous resolution of multiple "silent slip" events on the Cascadia subduction zone is still sparse, similarities between observed events help to characterize and understand the processes involved. The fact that all surface displacements caused by slips are opposite to the interseismic deformation caused by the locked portion of the subduction zone implies that this slip occurs on the subduction interface. Furthermore, the spatial pattern of the amplitudes of the surface motion due to the slips shows that maximum slip occurs between the 30 to 40 km depth contours of the suducting slab interface, with a sharp downdip but a more diffuse updip boundary suggesting both hydrological and thermal rheological constraints. To date, slip events have been observed predominantly around southeastern Vancouver Island and the eastern Olympic plateau, at times propogating northwest along the plate interface, but (apparently) not occurring to the south, implying a possible structural control. The duration of the slip events at any given site ranges from 5 to 15 days, and the maximum accumulated surface displacements for slip events observed to date is around 6 mm which can be modelled by 3 cm of slip on the slab interface. The observed northwest move-out of a slip event in 1999 indicates a strike-parallel slip propogation of the order of 5 km/day. The duration of slip events is also consistent with this magnitude of slip propagation in an updip direction. It is not clear what limits the magnitude of the slips and their propagation velocity, but it is expected that a megathrust earthquake may begin with such a silent slip. The GPS time series also clearly show that in the period between slips, deformation velocities due to the locked seismogenic zone are augmented substantially at sites that show the largest slip displacements. This is consistent with transient coupling across the deeper interface, having stronger coupling which impedes plate-rate slip for a period of a year or two, followed by aseismic slip which releases this accumulated stress locally. With a denser continuous GPS network, it may become possible to define a "frequency-magnitude" relationship for these slip events which will be key to evaluating the role of aseismic slip in the temporal redistribution of stress and the possible triggering of megathrust earthquakes.
Bibliography of Canadian Geomorphology