Introduction - Intraplate Earthquakes - page 6

convection. They show that the main source of intraplate stress is that derived from plate
boundaries, and “primary inversion” occurs only when it combines favorably with poten-
tial energy stresses in combination with the existence of favorably oriented pre-existing
lithosphere-scale weaknesses.
Talwani (Chapter 11) presents a unified model, based on an integration of the results
of earlier studies aimed at explaining how stresses transmitted from plate boundaries
cause intraplate earthquakes. These stresses were found to cause local stress accumu-
lation on discrete structures, which were identified as local stress concentrators. These
local stress concentrators are located in both the upper and lower crust within the rift,
and their reactivation occurs in the present-day compressional stress field in the form of
earthquakes. Commonly observed local stress concentrators are favorably oriented (relative
to the regional stress field) fault bends and fault intersections, flanks of shallow plutons
and buried rift pillows. Stress build-up associated with one or more local stress concen-
trator interacts with and produces a potentially detectable local rotation of the regional
stress field with wavelengths of tens to hundreds of kilometers. A local rotation of the
regional stress field provides evidence of local stress increase and thus potentially suggests
the location of future intraplate earthquakes. This model provides a framework for poten-
tially testing for and assigning the cause of intraplate earthquakes at other locations in the
world.
One of the important elements in the study of earthquakes in any region is to assess
the potential seismic hazard posed by future earthquakes. The traditional approach used in
intraplate regions, especially in the construction of critical facilities, is probabilistic seismic
hazard analysis; a methodology imported from plate boundary regions. As better seismicity
and other data become available for intraplate regions, it is becoming apparent that some of
the assumptions on which the standard seismic hazard analysis is based may not be valid for
intraplate regions, especially the assumption of stationarity of the seismic source. Hough
(Chapter 12) discusses the problems of applying the standard seismic hazard methodology
in the case of the Central and Eastern United States, especially in the case of the NewMadrid
seismic zone. Among some of the problems noted by Hough are that GPS results show
little or no resolvable deformation in the source zones, and paleoseismological evidence
suggests large Holocene and late Pleistocene earthquakes in other regions. In addition, there
is evidence for short-term temporal clustering of seismicity with longer-termmigration, and
estimating earthquake rates from short historical records also is problematic. The results
of this study serve as a warning to scientists and engineers tasked with assessing seismic
hazards in intraplate regions about the wisdom of using standard probabilistic seismic
hazard analysis in intraplate regions.
In the final chapter, Talwani presents a synthesis of conclusions from a study of the
chapters in this topic. These new data and case histories from different parts of the world
support earlier ideas of a global pattern in the location of the larger intraplate earthquakes
in old rift structures. Various case histories show evidence that large earthquakes do not
recur at the same locations but in fact jump from one fault to another. The different chapters
illustrate both the similarities and the differences in the nature of intraplate earthquakes