HR: 1340h
AN: T53A-1108    [Abstracts]
TI: Seismotectonics of accretive versus erosive subduction zones - insights from analog seismic cycle simulation
AU: * Rosenau, M
EM: rosen@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam (GFZ), Telegrafenberg, Potsdam, 14473, Germany
AU: Bachmann, R
EM: raik@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam (GFZ), Telegrafenberg, Potsdam, 14473, Germany
AU: Oncken, O
EM: oncken@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam (GFZ), Telegrafenberg, Potsdam, 14473, Germany
AB: Accretive and erosive subduction zones differ both in their forearc structure and seismic release character. For instance the greatest historical megathrust earthquakes concentrated along accretive margins, tectonically characterized by forearc shortening (e.g. Sumatra, Southern Chile, Alaska), whereas erosive margins, tectonically characterized by forearc extension (e.g. Peru, Kuriles), have often been the locus of tsunami earthquakes (i.e. slow and shallow events). Here we investigate the implied link between internal forearc deformation and megathrust seismogenesis and its implications for seismic hazard in subduction zones. We interpret quasi two- dimensional plastoelastic (allowing deformation to localize, permanent shortening dominates) and elastoplastic (elastic deformation dominates, minor internal deformation) granular wedge models as analogs of accretive and erosive subduction forearcs, respectively, overlying a rate-state frictional plate interface which represents a seismogenic megathrust. Experimental observations support current hypotheses that internal forearc deformation is controlled by stress changes associated with the megathrust seismic cycle: Consistent with the theory of dynamic Coulomb wedges, coseismic compression at the updip limit of great earthquakes triggers shallow postseismic forearc deformation. Plastic shortening of the outer forearc wedge and shallow afterslip both are interpreted as transient postseismic relaxation mechanisms with the first being dominant in plastoelastic/accretive settings and the second being dominant in elastoplastic/erosive settings. Interseismically, permanent crustal shortening localizes in both settings above the downdip limit of great earthquakes and may lead to uplift of a coastal cordillera. Longterm coastal uplift rates at elastoplastic/erosive margins are about one order of magnitude lower than in plastoelastic/accretive settings, and associated with permanent crustal extension above the seismogenic zone. Extension here appears as an effect of postseismic relaxation coeval to interseismic reloading. Internal shortening of plastoelastic/accretive margins reduces the subduction velocity stretching the recurrence intervals. We found, however, no evidence for that longer recurrence intervals lead to larger earthquake slip (e.g. due to stronger healing effects). Consequently, the seismic moment release rate tends to be smaller at accretive than at erosive margins, at least in the 2D case presented here. Since megathrust earthquakes of magnitude 8 and higher gain their size not primarily by increasing coseismic slip but by unzipping larger parts of the plate interface along strike of the subduction zone, the historical concentration of great events along accretive margins, if statistically significant, is probably a 3D effect associated with lateral rupture propagation. These findings may have important implications for seismic hazard in subduction zones since (1) forearc anatomy is suggested to reflect the seismogenic structure at depth, for instance in accretive settings, (2) erosive settings tend to release elastic energy by shallow megathrust afterslip and thus have a generally higher risk of tsunami earthquakes than accretive margins and (3) erosive margins are characterized by more frequent megathrust earthquakes than accretive margins. Very great earthquakes along accretive margins may be triggered either by a longer wavelength along-strike-segmentation of the subduction interface or by more synchronized seismic cycles of neighboring segments compared to erosive margins thus boosting lateral rupture propagation during a single giant event.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8104 Continental margins: convergent
DE: 8123 Dynamics: seismotectonics
SC: Tectonophysics [T]
MN: 2007 Fall Meeting