Earthquakes
cause many deaths and injuries worldwide. For example, the 2015 Nepal
earthquake is thought to have killed nearly 9,000 people. This event was only
one of many events contributing to the 750,000 deaths attributable to
earthquakes during the period 1996 to 2015 according to a recent report by the
United Nations.
This is a higher
total than deaths caused by all other natural disasters (e.g. storms, heatwaves
and floods) combined. Earthquakes also cause considerable economic losses. For
example, the 2016 Kumamoto (Japan) earthquake is thought to have caused $42
billion-worth of damage according to the Japanese government. The largest
earthquakes can also have dramatic and permanent effects on the landscape, e.g.
roughly 180km of surface faulting occurred in the 2016 Kaikoura (New Zealand)
earthquake.
The vast
majority of earthquake-related deaths and injuries and economic losses are
caused by damage (including total collapse) to buildings or their contents.
When designing buildings and infrastructure (e.g. bridges and power plants) to
withstand earthquakes, engineers require estimates of the ground motions that
their structure could be subjected to in future earthquakes. These estimates
are computed by engineering seismologists using two sets of mathematical
models.
The first of
these provides an assessment of the rate of occurrence of earthquakes of
different sizes within a few hundred kilometers of the structure, e.g. how
often does a magnitude 6 earthquake occur on a nearby fault? The second model
predicts what the ground motion would be given the occurrence of different
earthquakes, e.g. if the magnitude 6 earthquake occurred what is the expected
ground shaking at the base of the structure? Many hundreds of these so-called
ground-motion models have been published.
One of the
greatest challenges in developing ground-motion models is a lack of recordings
of previous earthquakes, particularly those within the region of interest. This
challenge is particularly acute when considering the largest earthquakes
because they happen so rarely. Generally, less than one earthquake of magnitude
7.2 or larger occurs in the Earth’s crust every year and is recorded
at close distances by modern seismographic networks.
In our recent
study, we collected data from 38 crustal earthquakes of magnitudes 7.2 or
larger that had occurred worldwide over the past 60 years. Many of these data
had been forgotten about and never collated before and hence they had not been
used to derive the most recent ground-motion models. Because of its importance
for seismic design and since it is often the only information available
concerning ground motions in past earthquakes in our study we considered the
maximum horizontal acceleration measured from each of the available
seismograms. The newly-collated data were compared to predictions from eight
ground-motion models that are routinely used to assess the earthquake shaking
that a structure may suffer during its lifetime.
We found that
these eight models provide, on average, a good match to the maximum horizontal
accelerations observed in the largest crustal earthquakes. This is reassuring
for earthquake engineering. The study, however, did show that the ground
motions in some earthquakes (e.g. the 2001 Bhuj earthquake in India) were much
higher than those that would be expected given the magnitude.
The data
collated for this study will be invaluable for the derivation of new
ground-motion models for use in the design of safer structures and consequently
for the reduction of earthquake risk.
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