HR: 16:00h
AN: T44A-01    [Abstracts]
TI: A Synoptic Model of Ridge Transform Fault Seismicity
AU: * Boettcher, M S
EM: margaret@whoi.edu
AF: MIT/WHOI Joint Program, Woods Hole Oceanographic Instution MS #24, Clark S. 275B, Woods Hole, MA 02543 United States
AU: Hirth, G
EM: ghirth@whoi.edu
AF: Woods Hole Oceanographic Institution, Woods Hole Oceanographic Institution MS #24, Woods Hole, MA 02543 United States
AU: McGuire, J J
EM: jmcguire@whoi.edu
AF: Woods Hole Oceanographic Institution, Woods Hole Oceanographic Institution MS #24, Woods Hole, MA 02543 United States
AU: Jordan, T H
EM: tjordan@usc.edu
AF: University of Southern California, Department of Earth Sciences University of Southern California, Los Angeles, CA 90089-0740 United States
AB: The size and location of large earthquakes on mid-ocean ridge transform faults (RTFs) can be better understood through a synoptic model that unites the rheology and geology with the seismicity of these faults. In the absence of ocean bottom seismometers on RTFs now is an appropriate time for developing synoptic models to guide up-coming deployments (e.g. NEPTUNE) and direct future investigations of earthquake processes on RTFs. Studies of earthquake focal depths and laboratory friction experiments suggest that the depth extent of oceanic earthquakes is thermally controlled, with the $600^{\circ}$C isotherm as the approximate lower limit. The relatively simple compositional and thermal structure of RTFs, together with the extensive data on high-temperature olivine deformation, allows us to construct a complete rheological model. This includes an aseismic mylonitic zone below the seismogenic layer and likely includes a significant amount of serpentine with velocity and temperature dependent frictional properties. Few large earthquakes occur on RTFs and these events are followed by very few aftershocks. On average, the area ruptured by the largest earthquake scales as the square-root of the area above the 600$^{\circ}$C isotherm. Faulting on RTFs is primarily aseismic, i.e. only $\sim$1/4 of the tectonic offset is accommodated by earthquakes and the remaining 3/4 occurs through steady aseismic creep and slow or silent earthquakes. This aseismic loading drives much of the seismicity and may contribute to the abundance of foreshocks that precede the larger RTF earthquakes. To better understand the physical processes that give rise to the observed seismic characteristics we must integrate our knowledge of the rheological and seismic aspects of RTFs. Our model predicts that RTFs are likely to be weak ($\Delta \sigma < 1$~MPa), but can support high stresses ($\sim$100~MPa) over a narrow depth range in the viscously deforming region beneath the seismogenic zone. We show that when the rheology is combined with the geology and seismicity observed for the Blanco Transform Fault on the Juan de Fuca Ridge, we can start to understand the spatial and size distribution of earthquakes along the fault and identify target locations and problems for future study.
DE: 8159 Rheology--crust and lithosphere
DE: 8164 Stresses--crust and lithosphere
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics and mechanics
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting