Introduction - Intraplate Earthquakes - page 1

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Introduction

PRADEEP TALWANI

The study of intraplate earthquakes began after the development of plate tectonics theory,
which explains the genesis of more than 95% of the global seismic energy release. I use
a more liberal definition of intraplate earthquakes as those that occur within a tectonic
plate away from a plate boundary as contrasted with the more restrictive stable conti-
nental interiors (Johnston, 1989 ) , thereby including earthquakes in locations such as the
Sea of Japan. Because of the rarity of large destructive intraplate earthquakes, a great
disparity in the local geological conditions where they occur, and the wide variation in
the efforts made to study them in different parts of the world, our understanding of their
nature has been slow to evolve. However, in the past four decades great progress has
been made in monitoring of seismicity globally, as well as in diverse approaches to their
study, suitable for local geological and logistical conditions. This topic is an attempt to
improve our understanding about the nature of intraplate earthquakes by bringing together
the results of recent advances. The different chapters in the topic, written by experts in
their fields, are designed in such a way that each describes a different element of the
phenomenon. Together they contribute to a more integrated understanding of intraplate
earthquakes.
This topic describes intraplate earthquakes in eight regions of the world - fromAustralia,
with a relatively low level of seismicity and a short historical record, to China, with a three
millennia long history of destructive earthquakes. It covers regions withmoderate seismicity
(Western Europe and Brazil), New Madrid in the Central United States with a record of
large historical earthquakes, and currently active areas of Eastern Canada, the eastern
margin of the Japan Sea and Kutch, in Western India. The results on numerical modeling
described in one chapter show how these mid-plate earthquakes occur in response to stresses
emanating from plate boundaries. Another chapter presents a unified model to explain how
these stresses build up on discrete structures and lead to such mid-plate earthquakes. Our
changing views in approaches to seismic hazard analysis and conclusions and insights
gained from the various chapters are discussed in the final two chapters.
Compared with the other intraplate regions, the historical record of seismicity in Aus-
tralia spans little more than two centuries, and the instrumental record of seismicity extends