Earthquakes Versus Hurricanes: Measuring DamageNov 04 2014 · 0 comments · NISHKIAN DEAN, Seismic ·0
Earthquakes versus hurricanes…which natural disaster proves to be more damaging to buildings? This is an interesting question to compare and contrast. Each event affects buildings in fundamentally different ways, yet there are some striking similarities, as well. Let’s examine them.
Earthquakes are strong ground movements that result from ruptured crustal faults. And although regions that are seismically active and prone to earthquakes are largely known and geoscientists have mapped at least the potential for strong ground motions throughout the United States, earthquakes are unpredictable. The strength of the ground shaking below a particular building is a function of the distance from the rupture (both depth and distance along the surface), the type of soil the building sits on, and, of course, the size of the rupture/the extent to which the fault fractures during the event. The ground accelerations manifest themselves in forces within the building (remember from science class, F = ma where “m” is the mass of the building and “a” is the ground accelerations). Some of the strongest ground accelerations mapped by the USGS can be found in:
- The New Madrid fault near Memphis, Tennessee at 2.0g (expressed as a multiple of gravity)
- Seismically active California with the highest mapped values at 2.5g in Northern California
- And in the 1994 Northridge Earthquake in Los Angeles where the largest measured ground acceleration was 1.8g recorded at Tarzana, located about 7 km south of the epicenter.
Earthquakes vs. Hurricanes
|Earthquakes||Hurricanes||Which is worse?|
|Can they be predicted?||No. Historically seismic regions are largely known. Scientists are not able to predict when an earthquake may occur.||Yes. While a particular hurricane cannot be predicted per se, the development of a hurricane can be monitored in advance of it landfall.||Earthquakes. They simply can’t be predicted.|
|Which costs more?||Earthquakes represent both a risk to the loss of life and property. Because they cannot be predicted and can occur at any time there is typically significant damage to the buildings and bridges. The annualized cost of earthquake damage in the US is estimated by FEMA to be $5 billion /year.||Because people can be warned and evacuated in advance of a hurricane’s landfall, the greatest risk is to property damage. However, many individuals have chosen not to evacuate and have been injured in hurricanes. NOAA estimates that the annualized cost of hurricane damage in the US is $10 billion /year.||Hurricanes. While the toll on human lives and economic damage could be higher in an earthquake, the annualized financial toll of hurricanes is about twice that of earthquakes given that they occur with higher frequency in more populated areas. However, these are potentially large, infrequent events, which makes them difficult and misleading to annualize. It is estimated that a magnitude 7.8 earthquake along the San Andreas Fault in southern California projected as many as 1,800 fatalities and more than $200 billion in economic losses.|
|Which is more damaging to buildings?||The ground shaking from an earthquake generates inertial damage internally within the building. In an earthquake, damage can range from devastation to a portion of a building’s components to the outright collapse of the building. Fires following earthquakes from broken gas mains can lay total waste to structures that survive the earthquake itself.||The winds of a hurricane place pressure on the outside envelope of a building, putting forces on the exterior components. Damage to the exterior component windows or roof can result in significant damage and, in extreme cases, collapse of the building. Flying debris can cause further damage to buildings and people. Torrential rains following the hurricane can lay waste to the interiors of the buildings.||Tie. Both can cause extreme damage to buildings.|
|Which generates greater flooding?||Undersea landslides and subduction earthquakes generate large vertical ground movements that produce devastating Tsunamis. These monstrous sea waves, measuring up to 30-feet or more in height, come ashore and can devastate large areas of a coastline. This level of flooding was the case in 2011 in Japan (18,550 fatalities) and 2004 in Indonesia (230,210 fatalities).||The storm surge pushed up by a major hurricane can reach heights in excess of 20-feet. Katrina was one of the most devastating hurricanes in the history of the US. It produced catastrophic damage – estimated at $75 billion in the New Orleans area and along the Mississippi coast – and is the costliest U. S. hurricane on record. Storm surge flooding of 25 to 28 feet above normal tide levels was associated with Katrina.||Hurricanes. From a US perspective, the storm surge associated with hurricanes in the Gulf and Atlantic coasts represent a far greater economic risk than the potential for Tsunamis on the northern US Pacific coast.|
Only small areas of the US are exposed to both earthquake and hurricane risks. On the East Coast, Charleston, South Carolina has seen large historic earthquakes and is potentially in the path of hurricanes every season. The Oregon and Washington coasts are exposed to a potential huge subduction zone earthquake and, while not called hurricanes, the coast is battered annually by winter storms with winds in excess of 100-mph.
Earthquakes versus hurricanes, which prove to be more damaging to buildings? While an interesting topic of discussion, it is not a fair question to answer. Both negatively impact communities and their surrounding areas, resulting in property damage, loss of life, and severe economic hardship. It is imperative that we all do our best to prepare for these potential events and encourage our governmental agencies to respond to these disasters when they occur. We can do our part as structural design professionals to produce high quality designs that are resilient to the potentially devastating effects of these natural hazards and work to protect lives as well as investments.
1 SDS from USGS website for ASCE7-10, Site Class B, Type I, II or III, http://geohazards.usgs.gov/designmaps/us/regions.php
2 SDS from USGS website for ASCE7-10, Site Class B, Type I, II or III, http://geohazards.usgs.gov/designmaps/us/regions.php
4 Earthquakes: Risk, Detection, Warning, and Research by Peter Folger, Specialist in Energy and Natural Resources Policy, July 18, 2013, http://www.fas.org/sgp/crs/misc/RL33861.pdf