What can seismology tell us about lower crustal deformation
in a plate boundary system?
Bokelmann, G.H.R., Beroza, G.C., Stanford University
The nature of lower crustal deformation is an important aspect of the
plate boundary system and plays a key role in the buildup of tectonic
stresses that lead to earthquakes. Despite this, the rheology of the
lower crust is not well understood.
Geodetic observations suffer from fundamental ambiguities in their
ability to discriminate between competing models of the lower crust.
Generally, a wide range of crustal deformation models can satisfy
geodetic data, in which the deeper parts range from uniform viscoelastic
layers (horizontal models) to narrow vertical zones characterized by
transient slip (vertical models).
Seismological techniques are available for studying lower crustal
structure, either on larger length scales or on small length scales
(e.g., laminated reflection bands). These can perhaps be argued to
constrain composition and perhaps to indirectly argue for certain
deformation behaviour, but seismology has the potential to contribute
more directly to this problem by constraining the geometry of the
stress field at depth. Studying moment tensors (focal mechanisms)
along the plate boundary in Northern California we find that their
principle axes are closely aligned with the vertical and the horizontal,
near the surface and again near the base of the seismogenic crust. Near
the surface, this is what we expect for the principal axes of the stress
tensor from the free-surface boundary condition. We can understand the
alignment near the base as a consequence of a viscously relaxing lower
crust, which leads to relaxation of horizontal shear following large
earthquakes. If the observations are made long time after the last big
event (interval>> relaxation time), stress differences in the lower
crust should have relaxed. Observations of this relaxation from seismological
observations may help constrain the viscosity and geometry of deformation
in the lower crust.
(Invited Talk, Seismological Society of America 2000, San Diego)
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