Unified model for intraplate earthquakes - Intraplate Earthquakes - page 294

North China, Liu et al . ( 2011 ) documented the absence of a repeating major earthquake
at any location in the Wiehe and Shanxi rifts. These observations of multiple events on
different faults in the Kutch Rift, at distant locations in the Reelfoot Rift, and non-repeat
earthquakes in the rifts in North China are in accord with the unified model wherein different
LSCs can be sequentially activated within a rift.

11.9.3 Areal dimensions of local stress changes

Local stress concentrators and the associated surface area affected by S L are of smaller
areal dimensions, with wavelengths of tens to hundreds of kilometers ( Table 11.1 ) , that
is, an order of magnitude smaller than the areas affected by perturbing secondary regional
stresses associated with regional-scale features identified by Zoback ( 1992a ) . However,
the local stresses S L associated with these LSCs have magnitudes comparable to S T as
evidenced by the observed rotation of local S T (next section).

11.9.4 Local rotation of the regional stress field S T and magnitude of S L
As discussed in Section 11.5, the interaction of S L with S T causes a local rotation in the
direction of S T which depends on the type of LSC, its orientation, and the ratio of the
differential horizontal stress to S L ( Figure 11.1 ) . S L grows with time, and for optimal
conditions the rotation γ is detectable from seismicity data. A stress rotation of > 15

°
associated with a perturbing stress suggests that its magnitude is hundreds of megapascals.
The various examples of stress rotation (Section 11.5) associated with different LSCs thus
suggest that the related stress accumulation on them is adequate to trigger IPEs. The actual
magnitude of the stress accumulation will depend on the nature of the LSC, its geometry,
and the duration of the stress accumulation.

11.9.5 Local deviatoric stress

Zoback ( 1992a ) presented an example of a secondary deviatoric stress associated with
lithospheric thinning in the East African Rift system. Assuming normal frictional and pore
pressure distributions, she showed a regional extension in the upper brittle crust of S L =
− 180 MPa. Mazarbaud et al .( 2005 ) presented examples of local extensional deviatoric

stress associated with Amorian Massif and Massif Central in northern and central France.
Their results suggest that the local stress associated with these LSCs is extensional and of the
order of hundreds of megapascals extending over wavelengths of a few hundred kilometers
( Table 11.1 ) .

11.9.6 Effect of deglaciation

Perturbing regional secondary stresses act over a large region and interact with local stress
changes in the vicinity of LSCs. When this regional perturbing stress is due to elastic