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over a quarter of that time. Therefore, for planners responsible for estimating seismic
hazards, the concern is whether this short record of historical and instrumental seismicity
including twenty earthquakes of M
>
6.0 and seven with surface rupture is truly represen-
tative of the long-term behavior of the seismogenic structures in the continent. Because of
a paucity of dense seismic networks, the earthquake locations are not well constrained, and
a very small fraction of the estimated depths are accurate enough to make tectonic asso-
ciations, thus limiting the ability to accurately identify and outline the buried seismogenic
features.
Clark, McPherson, and Allen (Chapter 2) solve the problem of a short historical and
poorly constrained seismicity record by using a well-preserved geological record to extend
the prehistorical record of past earthquake activity tomore than 10Ma. They note, “Australia
is one of the lowest, flattest, most arid and slowly eroding continents on Earth. Accordingly,
large parts of Australia are favorable for the preservation of tectono-geomorphic features,
such as fault scarps, for tens of thousands to millions of years.” They use a two-pronged
approach in their search for records of paleo and historical (collectively, morphogenic)
earthquakes that have ruptured faults during the past millennium. A geomorphological
study based on aerial photographs was used to select sites and disruption localities for
the second phase of the investigations, which involves trenching for paleoseismological
investigations. Evidence of more than 300 fault scarps and other geomorphic features
dating back to more than 10 Ma was found in the geological record.
Based on the geology and crustal setting, the authors recognized six neotectonic domains
in Australia. For each of those, they analyzed the geomorphic features based on their fault
length and vertical displacement. They interpreted the results in terms of seismological
parameters such as the maximum magnitude, earthquake frequency, and the temporal
pattern of the occurrence of morphogenic earthquakes in different neotectonic domains.
They conclude that the catalog of historical seismicity significantly underestimates the large
earthquake potential in the continent.
Brazil spans most of the mid-plate area in South America and little has been published
on the seismicity characteristics of this large, stable continental region. Two magnitude 6
earthquakes are the largest events in the revised catalog of historical events (from about mid
nineteenth century) and instrumental data (mainly from the past 60 years). Assump¸ ao and
colleagues (Chapter 3) discuss the distribution of seismicity and possible correlations with
the main geological provinces and geophysical features. Despite the short period of the
catalog, which may not be representative of the long-term seismicity (paleoseismological
studies, for example, indicate magnitudes could reach 7, based on liquefaction features in
northeastern Brazil), interesting correlations have been found. A trend of higher seismicity
was found in areas presumed to have thinned lithosphere, as also near craton edges (around
craton keels), even for magnitudes down to 3.5, similar to global correlations for larger
earthquakes. Also, the continental shelf is more seismically active than the average intraplate
region, even for low magnitudes. Flexural stresses associated with gravity highs also were
found to be an important factor in helping explain Brazilian intraplate seismicity. The
new multidisciplinary Brazilian dataset should provide a basis for comparisons with other
intraplate regions and to test models developed in other continents.
