Risks, Hazards, & Loss
Earthquake hazard is the probability of a certain level of shaking (Peak Ground Acceleration) occurring from an earthquake in a particular area. Earthquake risk is the probability of economic loss occurring from an earthquake in a particular area. The hazard is determined by the USGS National Seismic Hazards Mapping Program.
Some extremes illustrate the differences between risk and hazard. Parts of Alaska are classified as “high hazard” but “low risk”. There are major faults that generate large earthquakes with short recurrence intervals. However, there is very little population or infrastructure to suffer damage. Therefore, the relatively common high ground accelerations cause little economic loss.
By contrast, New York City is a low-to-moderate hazard but a high risk. Large earthquakes are not particularly common (low hazard). However, because building codes do not require much in the way of earthquake resistant design, and because the value of the building stock is very high (vulnerability), even a moderate earthquake could cause significant damage. This is exemplified by the widespread damage from the August 2011 magnitude 5.8 earthquake in Virginia, which caused $200-300 million dollars in damage.
Colorado is rated as low-to-moderate earthquake hazard by the USGS seismic hazard map. The Federal Emergency Management Agency (FEMA) estimates annualized earthquake losses of $5.8 million.
FEMA developed a software package (named HAZUS) that will estimate economic loss from natural hazards in a given area. The program calculates loss two ways: 1) The statistical probability of loss, i.e. the average annual loss for an area (e.g., Colorado’s annualized loss of $5.8 million dollars per year), or 2) a deterministic loss, i.e. what would be the total loss from an earthquake that actually occurred?
The CGS has calculated the deterministic loss from the Maximum Credible Earthquake (MCE) assigned to a number of faults around the state.
Using FEMA’s HAZUS software, the CGS modeled projected earthquake impacts statewide on a county-by-county basis, these reports detail the impact of a variety of earthquake scenarios on human infrastructure.
The Worst-Case reports use the maximum magnitudes that can be generated along a fault of any given length, as calculated by Wells and Coppersmith (1994). These reports inform policy makers, emergency managers and responders of the consequences of an earthquake within each county. Counties can suffer damage even if the fault is located completely outside the county. Because the scientists who generate the National Seismic Hazards Map recognize that we don’t know as much as we would like about when or where an earthquake will strike, the USGS uses a “Random Earthquake” of magnitude 7.0. Therefore, we ran a “Random Earthquake” scenario for each county with a magnitude 7.0 event essentially in the geographic center of each county.