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In-Depth Information
11
Unified model for intraplate earthquakes
PRADEEP TALWANI
Abstract
After the development of plate tectonic theory, understanding the genesis of
intraplate earthquakes has been the focus of many studies. I combine the
results of these studies with recently improved seismological and other data
to formulate a unified model for intraplate earthquakes. Intraplate earthquakes
occur within continental interiors in response to a generally uniform, com-
pressional stress field associated with large-scale tectonic forces. The global
pattern of seismic energy release occurs preferentially in failed and passive
rifts, and less frequently on the edges of cratons. Thermo-mechanical model-
ing shows that during basin inversion, rifts preferentially utilize inherited zones
of crustal weakness. As a result, pockets of elevated strain rate and consequently
local stress accumulations occur on discrete structures, which I identify as local
stress concentrators (LSC). These 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 intraplate earthquakes. Commonly observed LSCs
are favorably oriented (relative to the regional stress field) fault bends and
intersections, flanks of shallow plutons and buried rift pillows. Stress build-up
associated with one or more LSCs 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.
11.1 Introduction
Although intraplate earthquakes (IPE) are associated with only about 5% of global seis-
mic energy release, historically these earthquakes account for a disproportionate number
