Unified model for intraplate earthquakes - Intraplate Earthquakes - page 297

in the surrounding region (Reddy and Patil, 2008 ) . After the coseismic strain had reduced
to background levels, subsequent GPS and InSAR measurements led to the detection of
up to

13 mm/yr of vertical displacement in regions of continuing aftershock activity
(Rastogi et al ., 2012 ) . Another example of localized elevated coseismic epicentral strain
was provided by the 2 m shortening of railroad tracks in the 1886 Charleston earthquake

(Talwani, 1999 ) . Kato et al .( 2009 ) reported that the 1964 Niigata (M 7.5) and the 1983 Sea
of Japan intraplate earthquakes are located in a zone of contractural strain rates larger than
10 − 7 /yr. This elevated strain away from the plate boundary was detected along the Japan
Sea coast, on a continuous GPS array (Sagiya et al ., 2000 ) . The region had been host to the
1964 Niigata (M 7.5), the 1995 Kobe (M 7.2), and four other M > 7.0 earthquakes since
1847 (Sagiya et al ., 2000 ) .
These observations suggest that in earthquake-prone regions it may be possible to detect
preseismic increases in local strain rates, which may indicate potential locations of large
earthquakes. A refocus of strain measurement strategy may be in order.

11.11 Conclusions

Intraplate earthquakes account for a very small fraction of the Earth's seismic budget.
Although large intraplate earthquakes are much less frequent than their plate-boundary
counterparts, when they do occur they can be associated with large-scale destruction.
Because of their rarity, efforts to study them have been limited. As the results of these efforts
began to accumulate, several features common to intraplate earthquakes were recognized
and used to explain their genesis.
In this chapter I present a review of some earlier ideas and integrate them with more
recent observations to develop a unified model of intraplate earthquakes. These include a
review of both theoretical models and those based on a spatial association of intraplate
earthquakes with identifiable geological features. These geological features, located within
rigid plates where stresses accumulate in response to a regional compressional stress, S T ,
were recognized as LSCs. Pockets of local stress accumulation, S L , at any LSC extend
over wavelengths of tens to hundreds of kilometers and were found to locally change the
direction of S T .AtanyLSCthereisabuild-upofS L which interacts with S T and may
ultimately lead to an intraplate earthquake.
An inventory of M 4.5 intraplate earthquakes showed that they are preferentially

located in old rifts and at boundaries of cratons. Possible reasons for this location preference
were suggested by Ziegler (1987), who in his study of inverted basins in central Europe
noted, “Amongst different types of intra-plate discontinuities, rifts with strongly thinned
crust appear to be prone to early inversion in response to collision related intra-plate
tangential stresses. Most wrench faults, which penetrate much of the crust and possibly
extend into the upper mantle, are also prone to compressional activation”. It thus appears
that the location of intraplate earthquakes is not random, but that there is a solid mechanical
basis for their preferred location in rifts. Models of basin inversion by Nielsen and Hansen
( 2000 ) and Hansen and Nielsen ( 2003 ) illustrate these mechanical underpinnings. Ziegler's