Unified model for intraplate earthquakes - Intraplate Earthquakes - page 283

11.4.5 Local stress concentrator model

Talwani and Gangopadhyay ( 2000 ) suggested that one or more of the three LSCs, fault
intersections, shallow plutons, and buried rift pillows were associated with most intraplate
earthquakes. Gangopadhyay and Talwani ( 2003 ) examined seismicity, geophysical, and
geological data for 20M

5.0 intraplate earthquakes to identify the responsible LSCs. They
found that fault intersections were associated with 30% of the events, fault intersections and
plutons together with 35%, i.e., fault intersections were associated with nearly two-thirds
of the events. Plutons alone and rift pillows alone were associated with 20% and 15% of
the events, respectively. In the following section I examine some examples that provide
evidence for the presence of local stress anomalies within the uniform regional tectonic
stress field.

11.5 Evidence of the presence of a local stress anomaly
Zoback ( 1992a ) identified large-scale regional crustal features as sources of second-order
stress fields. Because of superposition of S T by these regional stresses, the resultant stress
field with wavelengths of hundreds to thousands of kilometers is rotated relative to the
direction of S T . Now, with the availability of modern seismic networks, stress field pertur-
bations associated with LSCs and the resulting rotations of S T with wavelengths of tens to
hundreds of kilometers are being recognized and thus providing evidence for their pres-
ence. The orientation and magnitude of the anomalous stress build-up in a discrete volume
around a LSC, S L , and the resulting local rotation, γ ,oftheS T depends on the kind of
stress concentrator and its geometrical relationship with S T ( Figure 11.1 ) .
For a rift pillow, Sonder ( 1990 ) and Zoback ( 1992a ) showed that γ depends on the
angle, α , between the strike of the rift and S T , and the ratio of the differential horizontal
stress to S L ,(S Hmax -S hmin )/S L. The magnitude of S L depends on the mass contrast with
the surrounding volume ( Figure 11.1b ) . For intersecting faults, S L is oriented along the
direction of the shorter of the two intersecting faults (BC in Figure 11.1c ) and γ depends
on the angle, α , between the longer fault AB and S T and the angle between the two faults,
β . The magnitude of S L depends on the lengths of the two faults and the angles α and β
(Gangopadhyay and Talwani, 2007 ) . For plutons, the directions of S L and γ depend on
the orientation of the long axis of an elliptical pluton relative to S T ( Figure 11.1d ) . The
magnitude of S L depends on the size of the pluton and the ratio of its rigidity modulus
to that of the surrounding volume (Campbell, 1978 ) . Next we present some examples of
stress rotation associated with LSCs (see also Table 11.1 ) . Due to uncertainties in the
determination of the orientation of S L from seismicity data, only those cases with γ

15 °

are considered meaningful (Mazzotti and Townend, 2010 ) .

11.5.1 France

In a reanalysis of 40 years (1962-2002) of shallow crustal seismicity ( 12 km) data from

western and central France, including 4,500 events, four with M 5.1 to 5.7, Mazarbaud et al .