Unified model for intraplate earthquakes - Intraplate Earthquakes - page 290

zone of structural and rheological weakness in a rift is the optimal location for reactivation
under compressional inversion. These results suggested that pre-existing faults in a rift
would be likely locations for reactivation. In a more detailed study Hansen and Nielsen
( 2003 ) developed a two-dimensional thermo-mechanical continuum model to investigate
the whole sequence of lithospheric rifting and subsequent basin reactivation and inversion
by compression. In this model, rifting was assumed to initiate from a thermal anomaly
imposed at the base of the crust, with mass flux from below and above. The thermal
anomaly is created by elevating the Moho temperature in a small area around the model
center ( Figure 11.3a ) . Allowing for strain hardening, the rifting process is carried out
for 10 Ma during which boundary faults and interior conjugate faults extending to the
brittle-ductile transition develop. The mantle undergoes regional uplift to compensate for
localized crustal thinning and the development of crustal-scale faults significantly weakens
the lithosphere and influences the rift structural style. Compressional stress is applied
after 60 Ma and basin inversion follows as a natural consequence. In their reactivation
model, the inversion preferentially utilizes the inherited zones of crustal weakness. After
compression and post-compressional relaxation, at 100Ma the modeled compressive strains
are preferentially located along the boundary faults, interior through-going and conjugate
faults, and on top of the up-welled mantle in the lower crust ( Figure 11.3b ) . A comparison
of the locations of these pockets of elevated strain rates ( Figure 11.3b ) with models showing
seismicity and associated structures in the Sea of Japan (Figure 4 in Kato et al ., 2009 ; and
reproduced as Figure 9.5 in this volume) and the Kutch rift (Figure 4 in Biswas, 2005 ;
modified and reproduced as Figure 6.3c in this volume) was used to infer the locations
of LSCs within the rift ( Figure 11.3c ) . The comparison led to the identification of a rift
pillow (1), border faults and interior conjugate faults (2 and 3). A through-going fault to
the lower crust based on the model by Iio et al .( 2004 ) was added to the figure (4) together
with a shallow pluton (5) based on the observation of the Osceola pluton in the NMSZ
(Hildenbrand et al ., 2001 ) . Additionally, the rifts are broken and displaced laterally along
transfer faults or accommodation zones, providing additional fault intersections. Shallow
plutons are emplaced at or near these intersections, producing more LSCs.
Next I will compare the results of this model with earlier models proposed to explain
IPEs and with the global distribution of IPEs (Schulte and Mooney, 2005 ; Mooney et al .,
2012 ) to formulate a unified model for intraplate earthquakes.
11.9 Unified model for intraplate earthquakes
11.9.1 The model
Some of the observations and conclusions presented in earlier sections can be summarized
as follows:
There is a generally uniform compressional stress field, S T , in continental regions.
Globally, IPEs occur primarily in rifts and at craton boundaries.
Basin inversion models show how weak structures within a rift are preferentially reacti-
vated by S T .