Unified model for intraplate earthquakes - Intraplate Earthquakes - page 282

intersections act as stress concentrators that give rise to seismicity in their vicinity.
Parametric studies by Gangopadhyay and Talwani ( 2007 ) showed that a fault at 45

° ±

° ±

15 , to it are optimal directions

for stress accumulation. This model is scale independent, and has been found applica-
ble in explaining many cases of IPE. On a continental scale, Hildenbrand et al . (1996,
2001) observed that along the 400 km long Reelfoot Rift axis, the only seismically active
zone, the NMSZ, occurs near the 100 km wide intersection zone of the Reelfoot Rift
and the Missouri batholiths. Perhaps the first regional-scale field evidence of stress con-
centration or perturbation in the vicinity of intersecting tectonic features was provided
by Ellis ( 1991 ) . By mapping stress distribution from 1,500 hydraulically fractured wells
in south-central Oklahoma, he found a spatial correlation between the intersection of
major crustal fault zones, the resulting stress distribution, and the contemporary seis-
micity. In another example, Dentith and Weatherstone ( 2003 ) also found that the spa-
tial and temporal distribution of seismicity associated with the M 6.9 Meckering earth-
quake in southwestern Australia closely correlated with the predictions of the intersection
model.
Many strike-slip fault systems consist of numerous discrete en echelon segments.
Another geometrical configuration that acts as a local stress concentrator is a restrain-
ing stepover in an en echelon strike-slip fault system. Two-dimensional quasi-static elastic
analysis of the restraining stepovers by Segall and Pollard ( 1980 ) showed that they store
elastic energy and may be the sites of large earthquakes. We recognize them as potential
local stress concentrators. Association of these stopovers with seismicity has been observed
in theMiddleton Place Summerville seismic zone near Charleston, South Carolina (Talwani,
1999 ) , the NMSZ (Figure 7.11) and the 2001 Kutch earthquake (Figure 6.1).

° relative to S Hmax , with an intersecting fault at 90 35 °

11.4.4 Local shear model

Iio et al .( 2004 ) presented a conceptual “local shear” model to explain the large recurring
IPEs in Japan. They assumed that a seismogenic fault in the brittle crust extends as a ductile
fault zone with low viscosity into the viscoelastic lower crust. This assumption was based on
their inference of aseismic slip (localized shear deformation) on the downward extensions
of seismogenic faults following several earthquakes (Iio and Kobayashi, 2002 ) . In their
conceptual model, plate tectonic forces load the crustal fault and can cause (intraplate)
earthquakes. The stress drop that follows subsequently loads the fault extension in the
viscoelastic lower crust. When the stress in the lower crust relaxes, it reloads the fault in the
upper crust, which is further loaded by S T , leading to the next earthquake. The recurrence
time of the earthquakes in the brittle crust is a function of the viscosity of the lower crust
and the strength of the brittle fault. The presence of fluids can drastically reduce the strength
of the fault. This model presents a simple mechanism for transmitting stresses from the
ductile lower crust to the brittle upper crust, and may be applicable to the models discussed
earlier.