Unified model for intraplate earthquakes - Intraplate Earthquakes - page 278

stresses described by Zoback ( 1992a ) . These stresses have also been incorporated in more
recent models.
Artyushkov's (1973) suggestion that lithospheric stresses were caused by inhomo-
geneities in regional and global thickness led Mareschal and Kuang ( 1986 ) to speculate
on the role of topography and density heterogeneities in the genesis of IPEs. Turcotte
and Oxburgh ( 1976 ) hypothesized that IPEs were associated with tensional failure in the
lithosphere caused by intraplate stresses generated by different processes. These processes
included changes in latitude of surface plates (membrane stresses), change in temperature
with depth (thermal stresses), addition or removal of overburden, changes in crustal thick-
ness, in addition to the forces associated with the driving mechanism for plate tectonics. The
role of addition and removal of overburden and associated thermal stresses was emphasized
further by Haxby and Turcotte ( 1976 ) .
11.3.2 Perturbation of S T by surface and other processes
The regional compressive stress field due to plate tectonic forces, S T , can be perturbed
on both a regional scale due to surface or other processes, and on a local scale due to
stress build up near LSCs. At many locations of IPEs, S T is perturbed by a regional stress
field due to surface processes, such as deglaciation, erosion, and sedimentation (see, e.g.,
Talwani and Rajendran, 1991 ; Muir-Wood, 2000 ; Mazzotti et al ., 2005 ) or, as suggested by
modeling, by stress transfer to the brittle upper crust of thermal and other stresses caused by
thermal and compositional anomalies in the lower crust and upper mantle (Liu and Zoback,
1997 ; Kenner and Seagall, 2000; Pollitz et al ., 2001 ; Sandiford and Egholm, 2008 ) . For
example, Liu and Zoback ( 1997 ) suggested that the higher heat flow within the NMSZ
resulted in thermal weakening of the lower crust and upper mantle and the higher ductile
strain rates led to stress concentrations. The transmission of these stresses to the upper crust
leads to the observed seismicity. Kenner and Segall ( 2000 ) speculated that sudden tectonic
perturbations and viscous relaxation of a weak lower crust within an elastic lithosphere
caused stress transfer to the overlying crust and the resulting seismicity. Perturbation on a
regional scale can advance or delay the occurrence of an IPE and also alter the style of the
deformation and its location. They further modeled that coseismic slip in the crustal faults
in turn reloads the lower crust, causing cyclic stress transfer and thus accounting for large
repeat earthquakes.
11.3.3 Deglaciation and erosion
Of these various suggestions for regional stress perturbation, the role of surface processes,
especially deglaciation, has been investigated by many authors.
Deglaciation occurred most rapidly around 10,000 years ago and melting was largely
complete by about 6,000 years ago. The crust is still reacting to glacial unloading of
the former ice sheet as it rebounds to a state near isostatic equilibrium (e.g., Steffen
and Kaufmann, 2005 ) . This is manifested by uplift in the deglaciated regions in northern