Unified model for intraplate earthquakes - Intraplate Earthquakes - page 292

Within rifts, pockets of elevated modeled strains are identifiable with locations of LSCs.
In response to S T , around the LSC there is temporal accumulation of local stress, S L ,
which can lead to IPEs.
The influence of S L extends over wavelengths of tens to hundreds of kilometers.
An increase in S L can cause a local rotation in the direction of S T.
The magnitude of S L can be comparable to that of S T , hundreds of megapascals.
The structural style and timing of IPEs at any LSC can be modulated by the smaller
second-order regional stresses such as those due to GIA, erosion, and lower crust-upper
mantle thermal anomalies.

The observations and conclusions listed above address different aspects of IPEs and the
factors that contribute to their genesis. Individually, most of these observations have been
known for a long time. I combine them with the results of improved observational data to
develop a testable, unified model for intraplate earthquakes.
Intraplate earthquakes occur in continental regions characterized by a uniform compres-
sional stress field, S T , extending over thousands of kilometers (Zoback, 1992a ) . This stress
field is primarily associated with strike-slip faulting, i.e., S Hmax > S V

S hmin (e.g., in
continental United States), unless perturbed by a secondary stress field due to regional or
local features (Talwani and Rajendran, 1991 ; Zoback,1992a, b). There is a global pattern
of seismic energy release by IPEs in response to S T . It preferentially occurs in failed (or
interior) rifts and passive (or rifted) margins (see, e.g., Johnston and Kanter, 1990 ; Schulte
and Mooney, 2005 ) . Most of the seismic energy release not associated with failed rifts
occurs on the edges of cratons (Mooney et al ., 2012 ) . Nearly half of the smaller IPEs (M
< 4.5) occur outside the failed rifts, and their contribution to the global seismic energy
release from IPEs is negligible (Schulte and Mooney, 2005 ) . Consequently, in this model I
will focus on the larger (M 4.5) events.

Thermo-mechanical modeling by Hansen and Nielsen ( 2003 ) demonstrated that large
strain accumulations are localized within rifts during their formation and reactivation. These
high strain accumulations occur on discrete structures that act as local stress accumulators
or concentrators. These LSCs are located in both the upper and the lower crust. Their reacti-
vation in the form of IPEs occurs in the present-day compressional stress field. Commonly
observed LSCs are favorably oriented, relative to S T , fault bends and intersections, flanks of
shallow plutons, and buried rift pillows (Talwani and Gangopadhyay, 2000 ; Gangopadhyay
and Talwani, 2003 ) . Local stresses build up on these discrete LSCs lying within the failed
rifts or at the edge of cratons and cause IPEs. Next I explore some of the features of S L and
its bearing on the genesis of IPEs.

11.9.2 Build-up of S L and sequential fault reactivation
Local stresses accumulate at the LSCs in response to the regional stress field S T .Atany
LSC, the local stress, S L , grows with time until it reaches a critical threshold, and interacts
with S T to generate an earthquake. At any time the stress accumulation can occur on