Geoscience Reference
In-Depth Information
pockets of elevated strain-rate build-up provide evidence that the spatial and temporal
pattern of strain accumulation is different from that for plate boundaries.
To explain some of our current observations, different elements of earlier models were
integrated into a unified model for intraplate earthquakes (Chapter 11). In this simple,
testable model stresses can accumulate at local stress concentrators and react with, as
well as locally rotate, the maximum horizontal stress direction. Seismic tomography has
made it possible to identify these stress concentrators (Chapters 6 and 9). These local
stress concentrators are of different kinds and of diverse sizes, varying from tens to hun-
dreds of kilometers across (Figure 11.3c). Compressional jogs between en echelon faults
also act as stress concentrators and have been identified in regions of large earthquakes,
e.g., in the New Madrid seismic zone (Figure 7.10), the Kutch Rift Basin (between KMF
and NWF in Figure 6.13) and in the Middleton Place Summerville Seismic Zone of the
can, in some cases, be associated with a detectable rotation of the local maximum hori-
zontal stress direction (Chapter 11). Future studies need to be designed to test this model
and seek additional evidence for the local stress concentrator and the associated rotation
of S
Hmax
.
13.3 Future studies
The role of fluids as weakening agents was not addressed in this topic. However, the
presence of fluids has been detected by seismic tomography near two source zones
(Chapters 6 and 9), and its role in the seismogenesis of intraplate earthquakes merits further
study.
The unified model (Chapter 11) is based on a synthesis of current ideas and observa-
tions. It provides a framework for studying intraplate earthquakes. Both field studies and
theoretical modeling are needed to test its viability and predictability. As additional and
more focused data become available, new and better models will evolve to explain the
phenomenon of intraplate earthquakes. How new data can help this evolution is shown by
the following example.
A few years after the 2001 Bhuj earthquake, the large coseismic strain rates had returned
to their background levels. More than 6 years after the mainshock, two earthquakes
with magnitudes 4.1 and 4.7 occurred just south of the Kutch Mainland fault (KMF in
Figure 6.13), about 25 to 30 km southwest of the 2001 earthquake. InSAR data collected
between 2004 and 2007 and between 2007 and 2010 detected a local pocket of elevated
vertical strain rate (16-27 mm/yr) in about a 200 sq. km area surrounding the 2007 epi-
centers (Figure 6.17). Does this pocket of increased strain rate surrounding the M 4
+
epicenters signify that stresses are building up on a new, hitherto unidentified local stress
concentrator and the potential location of a larger future earthquake? To test this idea, an
array of additional GPS receivers and seismometers are being deployed in the vicinity of
the region of increased uplift rate. This example shows that with further improvements
in instrumentation and analysis techniques and with detailed, focused, complementary
