Geoscience Reference
In-Depth Information
major seismic hazard: a conclusion potentially at odds with the paleoseismological record
and alternative explanations of the GPS data. Currently, these data have been explained
from a local perspective. I anticipate that new explanations and models will emerge
when these assorted studies are evaluated from the global perspective described in this
topic.
In Western Europe, large earthquakes have been few and far between, but, when they did
occur, even the moderate
M 5 events left a long-lasting impression. In Chapter 8, Camel-
beeck and co-authors present a method of quantitative assessment of the seismic hazard
based on a detailed evaluation of the hazard caused by past earthquakes, especially those in
the past decade. They use the damage caused by past earthquakes as an important source
of information about future seismic vulnerability of a region. They use a computerized,
rich archival database that includes heritage records about the architectural style and kind
of building - church, castle, or ordinary home - the construction style and material, and
information about the underlying soils, to develop a seven-point “damage characteristics”
scale. They illustrate their methodology with an evaluation of the damage following the
Central Belgium earthquake of 1828. Their results show that, in other intraplate regions
where detailed macroscopic data are available for the rare, moderate earthquake, it is pos-
sible to assess the vulnerability and potential seismic hazard to a region posed by future
earthquakes.
Although a spatial association of intraplate earthquakes with rifts has long been sug-
gested, the identification of a causal seismogenic structure within the rift has so far proved
elusive because of a lack of detailed seismicity, geological, and geophysical data. In Chapter
9, Kato describes the results of using a dense seismic network deployed immediately after
three large intraplate earthquakes along the eastern margin of the Japan Sea. Analysis of
the seismicity data together with detailed seismic tomographic studies led to the discovery
of a buried rift with stepwise and tilted block structures, together with a buried rift pillow
in the lower crust with associated weakening fluids. The seismicity was caused by the
reactivation of the pre-existing faults within the ancient rift system. The rift pillow appears
to have acted as a stress concentrator, and the fluids as weakening agents. The identification
of the seismogenic features responsible for the M 6
earthquakes provided validation of
models suggested for the genesis of these intraplate earthquakes.
A significant percentage of intraplate earthquakes are associated with ancient rift zones
and occur hundreds to thousands of kilometers from plate boundaries. One of the important
questions is explaining the source(s) of stresses that are responsible for their occurrence.
Nielsen and Stephenson address this question in Chapter 10. They note that the locations
of the intraplate earthquakes in Western Europe are also the locations of stress inversion
of rift basins. They use numerical thermo-mechanical models of stress inversion of basins
to show that these intraplate earthquakes in Western Europe result from the interaction of
the stresses transmitted from plate boundaries through the lithosphere and the perturbing
“potential energy” stresses. These potential energy stresses are derived from models of
lateral variation in the present-day density structure of the lithosphere, and lateral pres-
sure variations in the mantle below the lithosphere due to density contrasts and related
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