Unified model for intraplate earthquakes - Intraplate Earthquakes - page 277

In the first part I describe the results of earlier studies, which are then integrated to arrive
at the new model. I start with a description of ideas about the present-day lithospheric stress
field in continental regions (Section 11.2), and how it is perturbed on regional (Section 11.3)
and local (Section 11.4) scales. Perturbation on a local scale and the resulting local rotation
of the stress field attest to the presence of the LSCs (Section 11.5). The magnitudes of the
local stress perturbations is discussed in Section 11.6. In Section 11.7, I address the global
distribution of IPEs, and in Section 11.8 I use insights from basin inversion modeling to
develop the unified model, presented in Section 11.9. Some consequences and uses of this
model are discussed in Section 11.10, and the conclusions are presented in the final section.

11.2 Lithospheric stress field

In her seminal study, Mary Lou Zoback ( 1992a ) identified two orders of stress in the
continental lithosphere. The first-order mid-plate stress field, extending uniformly over
thousands of kilometers, S T , is associated with plate tectonic forces. Ziegler (1987) showed
that collision-related major stresses can be transmitted over great distances through conti-
nental and oceanic lithosphere. This continental stress is generally compressional with one
or both horizontal stresses (S Hmax and S hmin) greater than the vertical stress, S V. In a com-
pressional regional stress field, the maximum horizontal stress, S Hmax , can be determined
by direct measurements at shallow depths and its direction inferred from focal mechanisms
at seismogenic depths. Zoback ( 1992a ) showed that in continental interiors the direction
of S Hmax is the same at both depth ranges. Superimposed on S T are second-order stress
fields with wavelengths of hundreds to thousands of kilometers associated with specific
geological and tectonic features. Following Zoback ( 1992a ) , the near-surface perturbing
horizontal deviatoric stress above the LSC is labeled S L (compression is assumed positive).
Locally, this superposition of S T with S L can cause a local rotation of the regional stress
field. The vertical stress due to the regional or local stress perturbation does not cause
stress rotation, but can change the relative stress magnitude (stress regime, Zoback, 1992a ) .
Rotation of the horizontal stress field depends on the angle between S T and (the axis of)
the local structure as well as on the relative magnitudes of S T and S L. For a potentially
detectable rotation of S T (

) the magnitude of the local horizontal uniaxial stress, S L ,
must be greater than about half the magnitude of the horizontal stress difference (S Hmax −
S hmin ). Zoback ( 1992a ) shows that discernible rotations of S T implied that the magnitudes
of both S T and S L were of the same order, hundreds of megapascals.

15 °

11.3 Regional perturbation of the stress field S T
11.3.1 Early ideas about perturbing lithospheric stresses

Before the development of plate tectonics theory, different lithospheric stresses were iden-
tified as potential causes of IPEs. We recognize them now as the second-order perturbing