Unified model for intraplate earthquakes - Intraplate Earthquakes - page 288

In various models for the genesis of IPE discussed earlier, regional stress perturbations
due to surface processes or due to anomalies in the lower crust and upper mantle (Section
11.3.3) are a few megapascals at best, at least an order of magnitude lower than the
contribution of LSCs to the local stress perturbations. This magnitude difference suggests
that that, in the present-day stress field, local stress perturbations associated with LSCs are
the likely cause of IPE rather than the smaller regional effect of surface processes, e.g.,
deglaciation and erosion. The latter could, however, provide a trigger for an earthquake.
In the following section I examine the global distribution of IPEs and possible association
with geological features.

11.7 Intraplate earthquakes and rifts

A possible spatial association between IPEs and rifts has been suggested by a number of
earlier studies (see, e.g., Adams and Basham, 1989 ; Johnston, 1989 ) . Johnston and Kanter
( 1990 ) found that globally most of the seismic energy release within intraplate regions
occurs by the reactivation of weak pre-existing structures within failed rifts and passive
margins in response to an ambient compressional stress field. Schulte and Mooney ( 2005 )
reevaluated this correlation by comparing an updated seismicity catalog with a compilation
of “Rifts of the world” (¸ engor and Natal'in, 2005). Using the latter's definition,“ Rifts are
fault-bounded elongate troughs, under and near which the entire thickness of the lithosphere
has been reduced in extension during their formation,” Schulte and Mooney ( 2005 ) found
that failed rifts and passive margins (labeled “interior rifts and rifted continental margins”
by them) together accounted for more than half of all IPEs and 90% of the seismic energy
release. Twelve taphrogens, which are linked chains of rifts and grabens in rifted continental
crust, account for 74% of all events and 98% of the total seismic energy release within
failed rifts. Gangopadhyay and Talwani ( 2003 ) found that IPEs not associated with old rifts
occur primarily in Pre-cambrian crust.

11.7.1 Correlation with deep mantle structure

Mooney et al .( 2012 ) used shear-wave velocity perturbation, δ V S , at a depth of 175 km as a
proxy for lithospheric temperature and composition. δ V S is the perturbation in the measured
shear-wave velocity with reference to the Preliminary Reference Earth Model (Dziewonsky
and Anderson, 1981 ) and was interpreted in terms of cratonic and non-cratonic lithosphere.
They explored possible correlations between the gradient of lithospheric thickness, inter-
preted from δ V S anomalies at 175 km depth, and intraplate seismicity with magnitudes

4.5. They concluded that significant crustal intraplate seismicity is concentrated in rifted
margins (in agreement with Schulte and Mooney, 2005 ) and the edges of cratons as defined
by seismic tomography ( Figure 11.2 ) . As detailed knowledge of the structures associated
with IPEs in the cratonic regions of Australia, Brazil, China, and India improves, we will
be able to identify the nature of the responsible LSCs. In the absence of such detailed
knowledge, the LSCs associated with rift structures will be the focus of this study. In this