Unified model for intraplate earthquakes - Intraplate Earthquakes - page 295

rebound due to deglaciation (GIA) and/or due to erosion, it opposes the ambient regional
vertical stress. In some cases the relative value of the vertical stress changes from S 2 to
S 3 , promoting failure by thrust faulting (see, e.g., Talwani and Rajendran, 1991 ; Zoback,
1992b ; Muir-Wood, 2000 ; Mazzotti et al ., 2005 ; Hurd and Zoback, 2012 ) . Major reverse
faulting observed in higher latitudes (e.g., in Fennoscandia) followed the onset of initial
deglaciation (Wu, 1998 ) . However, the contribution of GIA to the present-day stress field
is less than 10 MPa (Zoback, 1992b ; Wu, 1998 ; Wu and Johnston, 2000 ) . Various authors
have suggested that under the present-day stress field the local GIA may be adequate to
trigger seismicity on critically stressed faults in Eastern Canada (see, e.g., Quinlan, 1984 ;
Mazzotti et al ., 2005 ; Wu and Mazzotti, 2007 ; Mazzotti and Townend, 2010 ) .

11.10 Discussion

Analysis of newly compiled stress data from Central and Eastern United States and south-
eastern Canada “suggests that shear failure on the preferred nodal planes generally do
not require reduced fault friction or elevated pore pressures” (Hurd and Zoback, 2012 ) .
Assuming the normal friction and pore pressure conditions recently confirmed by Hurd and
Zoback ( 2012 ) , Mazzotti and Townend ( 2010 ) estimated 160-250MPa for the differential

stress in the horizontal plane (S L ) for some seismogenic structures in the same study area.
These results support the basic premise of the unified model that intraplate earthquakes
result primarily from stress build-up of hundreds of megapascals on LSCs. They also argue
against models wherein an order of magnitude lower stresses associated with deglaciation,
erosion, or thermal and compositional anomalies in the lower crust and upper mantle were
proposed to be the cause of intraplate earthquakes. However, these small secondary stresses
can modulate S L due to the LSCs and trigger seismicity.
Next, I discuss some observations related to IPEs and their possible explanations based
on the unifying model. These relate to the absence of significant topography in some
regions of IPEs, and the apparent absence of detection of anomalous strain accumulation
in continental regions.

11.10.1 Absence of topography

In some rift basins there is evidence of uplift of tens to hundreds of meters along internal
and border faults. Such is the case in the Kutch Rift Basin, where repeated thrusting along
dipping faults that extend to great depths and are oriented almost orthogonally to S T have
led to the formation of local uplifts and grabens (Biswas, 2005 ; Figure 6.3, this volume).
However, the deep-seated, near-vertical faults in the Reelfoot Rift are not favorably oriented
relative to S T , and there is a general lack of extensive topographic expression over extended
distances. In the NMSZ thrusting is limited to cross faults and major deformation occurs
by strike-slip faulting.
The absence of topography or significant offsets in sedimentary deposits within the
Reelfoot Rift has been interpreted to suggest that present-day seismic activity in the NMSZ