Depth-dependent earthquake
focal mechanism orientation: Evidence for a weak zone in the lower
crust
Bokelmann, G.H.R, Beroza, G.C., Stanford University
The traction free boundary condition across the Earth's surface provides
an opportunity for studying the relationship between stress orientation
and earthquake focal mechanisms because it requires alignment of principal
stress axes with vertical and horizontal orientations. A survey of earthquake
focal mechanisms in Northern California shows that their principal axes are
also closely aligned with the vertical and the horizontal in the upper few
km of the Earth's crust. Thus, the signature of the free surface boundary
condition on stress appears in focal mechanism orientations as well.
The focal mechanism alignment can also be characterized by the relative magnitude of the off-diagonal
elements, $M_{xz}$ and $M_{yz}$, of the seismic moment tensor.
We find significant and systematic depth variations in the
quantity $m_s=\sqrt{ M_{xz}^2+ M_{yz}^2}$, which
relates to the shear traction acting on a horizontal plane
for the special case of perfect alignment between principal stress and focal mechanism
axes.
Values of $m_s$ near the Earth's
surface are small, but increase with depth to a maximum between 5 and 8 km.
At greater depths there is a gradual decrease, which suggests decreasing
horizontal shear traction towards the base of the seismogenic zone. We
interpret this tendency of axes to become oriented near the base of the
seismogenic zone (and its expression in $m_s$) as the signature of a weak zone
in the lower crust. If
correct, this observation would have important implications for the mechanics
of lithospheric deformation.
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