Abstract
The occurrence of large earthquakes on passive continental margins poses the challenge of explaining the causes of earthquakes not directly related to plate boundary processes. Here, we suggest several features of models for passive margin seismicity based on inferences from the distribution and mechanisms of seismicity in the zone along the Canadian Atlantic coast The concentration of earthquakes along the margin suggests that they are related to the reactivation of faults remaining from the continental rifting. The variation in focal mechanisms across the margin suggests a spatially varying stress field. Although the later feature may not be a general characteristic, the empiricism that passive margin seismicity seems most evident on recently deglaciated margins suggests that deglaciation is at least partially responsible for the earthquakes.
Several sources of stress may contribute to passive margin seismicity. Flexural stresses due to the removal of ice loads, and spreading stresses due to the different densities of continental and oceanic lithosphere, can give stress differences of tens of MPa across a margin and normal to it. These stresses could cause the change from deviatoric compression landward to deviatoric extension seaward suggested by the Baffin Bay focal mechanisms. Stresses of comparable magnitude, but compressive everywhere, can result from the combination of integrated plate driving forces, “ridge push” and basal drag. The resultant compression direction can often be approximately normal to the margin.
Although these stresses can explain many features of passive margin seismicity, it is natural to ask what effects the major (~10 km) sediment loads along margins may have. These loads can give rise to flexural stresses of hundreds of MPa, an order of magnitude greater than anticipated from the other causes. These stresses might then be expected to dominate the others, giving rise to seismicity along all passive margins, with no preference for deglaciated ones. Similarly, local variations in sediment loading should at least partially mask the effects of the other stress sources.
We consider two possible explanations for this paradox, both based on the rheology of the passive margin lithosphere. In one model, the stresses due to long term sediment loading are relaxed since the lithosphere acts as a viscoelastic material. Simple calculations for a typical passive margin loading history, in which sediment is deposited over long periods of time throughout the margin’s history, imply that the flexural stresses can be reduced by an order of magnitude. Only very rapid sedimentation allows stress to accumulate faster than it relaxes, suggesting that only in extreme cases can sediment loading result in seismicity. In the second model, due to the depth dependant rheology of the lithosphere, a brittle region overlying a ductile one, the flexural stresses are somewhat reduced, but seismicity occurs only in response to the recent stress increment. In this model the long term sediment load contributes to the stress but does not induce seismicity.
“Nearly all stable masses exhibit marginal features which are seismically active.”
-Seismicity of the Earth, Gutenberg and Richter
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Stein, S., Cloetingh, S., Sleep, N.H., Wortel, R. (1989). Passive Margin Earthquakes, Stresses and Rheology. In: Earthquakes at North-Atlantic Passive Margins: Neotectonics and Postglacial Rebound. NATO ASI Series, vol 266. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-2311-9_14
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DOI: https://doi.org/10.1007/978-94-009-2311-9_14
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