Shaking during earthquakes in geologically older continental regions like central and eastern North America extends for much greater distances than in younger regions like much of western North America. The prevalence of older stone and brick buildings and unreinforced structures in these areas is also seen as a contributing factor making them more susceptible to shaking damage from an earthquake. (1)
In late 2014, scientists installed 27 temporary seismic instruments around Washington D.C., an area perceived to be at a lower risk from earthquake activity. However, these sensitive instruments recorded 30 earthquakes from the US and around the world during the 10 months they were in place.
According to new research published by the U.S. Geological Survey (USGS), the recordings revealed that significant differences in the underlying geology was causing a localised amplification of seismic waves resulting in greater damage to the predominantly stone and brick buildings in some areas than expected.
This localised amplification and the response of buildings to the frequency of shaking by the shallow layers of sediment beneath Washington, DC was in their opinion likely responsible for the damaging levels experienced during the 5.8 magnitude Virginia earthquake in August 2011, despite the earthquake being a distant 130km and only moderate in size. This earthquake was the most damaging in the area since 1886, triggering rockfalls up to 245km away and forcing the first automatic shutdown of an US Nuclear Power Plant.
The research concludes that there are many other central and eastern North American cities sitting on similar geological conditions to those of Washington, DC, where ground shaking amplification could also occur.
The USGS publishes National Seismic Hazard Maps which look at the potential areas likely to experience damage from natural or human-induced earthquakes. They are used widely by the insurance industry to set earthquake insurance premiums and by reinsurance companies to evaluate their risk to major disasters.
This article was attributed and provided by PG International