HR: 10:20h
AN: T52A-01 INVITED    [Abstracts]
TI: Structural Control on the Megathrust Slip: the Example of the Ecuador-Colombia Active Margin.
AU: * Collot, J
EM: collot@geoazur.obs-vlfr.fr
AF: IRD/UPMC UMR Geosciences Azur, B.P. 48, Villefranche-sur-mer, 06235, France
AU: Marcaillou, B
EM: boris@jamstec.go.jp
AF: IFREE/JAMSTEC, 2.15 Natsushima-cho, Yokosuka, Kanagawa, 237-0061, Japan
AU: Agudelo, W
EM: william.agudelo@ecopetrol.com.co
AF: ICP, El Limonal km14 Autopista Piedecuesta, Bucarramanga, 1, Colombia
AU: Sage, F
EM: sage@geoazur.obs-vlfr.fr
AF: IRD/UPMC UMR Geosciences Azur, B.P. 48, Villefranche-sur-mer, 06235, France
AU: Ribodetti, A
EM: ribodeti@geoazur.obs-vlfr.fr
AF: IRD/UPMC UMR Geosciences Azur, B.P. 48, Villefranche-sur-mer, 06235, France
AB: Along subduction zones, earthquake nucleation and megathrust slip are controlled by parameters including the rheology of the fore-arc, the magnitude of transient shear stress and the physical properties of the megathrust. Furthermore, geological structures play a major role on stress and strain distribution both across and along the megathrust, and consequently affect the earthquake cycle. A marine geophysical study of the Ecuador-SW Colombia subduction zone, where three large (7.7<Mw<8.2) thrust earthquakes occurred in 1942, 1958 and 1979 following the Mw 8.8, 1906 megathrust event, reveals a correlation between a multi-scale crustal segmentation of the margin, and the spatial distribution and extent of the earthquake rupture zones. On a 500 to >1000-km scale, the subduction trench is segmented from south to north, into NS-, NNE- and NS-trending segments, respectively associated with normal, oblique, and normal plate convergence settings. These segments relate to large-scale, structural domains of both the Nazca Plate (Carnegie Ridge, Panama Basin) and the margin of the South American Plate, which consists of a mosaic of accreted oceanic terranes. The 1906 event likely ruptured the entire NNE-trending, obliquely convergent segment. The slip was blocked southward by the buoyant subducting Carnegie Ridge, and northward by the sharp change in structural trend associated with the accreted Choco arc. On a 100-300-km scale, multichannel seismic reflection and multibeam bathymetry data show that the margin is segmented by transverse crustal faults. The faults correlate with the limits of large earthquake rupture zones, suggesting that transverse faults are weak and contribute to placing a limit on the along strike propagation of the co-seismic slip. On a 20-100-km scale, geophysical data collected on the Nazca Plate reveal oceanic asperities entering the subduction. Downdip, these features may correlate with seismological asperities, which, upon rupture, can trigger large events. On a ~50 km-scale, a crustal splay fault and the underlying updip segment of the plate interface may have respectively controlled the updip coseismic and postseismic slips of the M7.7 1958 earthquake. The splay fault is interpreted as an inverted, landward dipping normal listric fault that developed initially in the oceanic plateau that makes up the margin. Reactivation and inversion of the fault occurred when subduction erosion put the fault into contact with the megathrust. The splay fault separates inner and outer margin wedges. The wedges behave differently during the earthquake cycle, according to their differing velocity structures, and the slip weakening and slip hardening nature of their respective underlying interplate fault segments. It is suggested that during co-seismic slip along the splay fault, the elastic rebound of the inner wedge load the outer wedge with elastic stress. The resulting accumulated strain is likely to be released during the post-seismic phase, by creeping along the upper segment of the megathrust. Combination of the three scales of margin segmentation generates a complex pattern of stress distribution at the plate interface, and shows the importance of geologic structures on the megathrust slip.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3025 Marine seismics (0935, 7294)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting