Midnight Sun

West coast geologic hazards are more numerous than east coast since on top of the threat of tsunami’s from the Pacific Ring, we have local earthquakes, landslides, liquefaction other seismic related hazards to deal with.

But all of that aside – tsunamis. We have experienced 2 tsunamis since we have lived in San Clemente, California (22 years); the first from the 2010 Chilean magnitude 8.8; the second from the 2011 magnitude 8.9 Japanese earthquake. Both tsunamis were small in Dana Point, California harbor (<1.5m) but interesting. Since 1812, there have been 14 observed tsunamis along the west coast impacting California. The most destructive tsunami in recent recorded US history hit Crescent City, California in 1964, the result of the magnitude 9.2 Alaska Good Friday earthquake. Twelve people died in that event.

During that approximate same period (1840-2014) there have been 6 tsunamis on the east coast of the United States with the most likely source being earthquakes along the Lesser Antilles subduction zone (although not always).

For west coasters, the two biggest tsunami threats are from Alaska earthquakes and from  a quake along the Cascadia subduction zone. A magnitude 9.1 quake in Alaska would create larger waves in Northern California — between 10 and 23 feet — but the damage could be greater in Southern California because the region has more coastal development and fewer coastal cliffs. A 3-10 foot tsunami would inundate parts of Long Beach (ports of Los Angeles and Long Beach), Huntington Beach and Harbor and Newport Beach as well as traveling back up coastal rivers.

The difference in east coast and west coast tsunamis is topography and tsunami height. The east coast is a relatively low-lying flat coastal plain (at least south of Connecticut) and the west coast is frequently cliff bounded getting lower in southern California (Orange County and San Diego County). Therefore, west coast damage from tsunamis is predictably more focused in harbors, and coastal river planes. On the east coast, the tsunami threat is less focused and inundation would tend to spread through low lying areas. Moreover, east coast earthquake generated tsunamis tend to be smaller with lower overall earthquake magnitudes than along the Pacific Rim.

However, the east coast has potential for tsunamis from at least two other sources; submarine landslides along the continental shelf and landslides from the Canary Island volcanoes, most notably the Cumbre Vieja volcano.

So where does that leave us? Risk analyses for exposure to any geologic hazard is complex for a variety of reasons, not the least of which is that the time element of when the next event will occur is not well defined. However, in looking at it solely from historical data, it seems to me that the west coast is more prone to geologic hazards such as earthquakes and tsunamis than the east cost. Damage on the west coast is more likely to be from an actual earthquake event occurring more or less locally and damage on the east coast from a mega-tsunami type of event occurring some distance from the impacted area. West coast damage will be more localized (San Francisco will not be seismically impacted by a LA earthquake). East coast damage will be more wide spread resulting from a mega-tsunami.

My thought is that to minimize damage from geologic hazards on both the west and east coast, buy inland, on bedrock above 300 feet above mean sea level, not below a cliff and at least 50 feet above a stream or river. -LOL.

The Cumbre Vieja volcano in the Canary Islands has received a lot of attention as a potential mega-tsunami generator. The premise is that it is a steep sided, active volcanic island with identified zones of weakness running approximately north-south that if she blows may result in a large, fast moving landslide failure on the southwest side of the island. This is termed a flank collapse. Early work suggested that this flank collapse could cause a tsunami event that would reach the southeast US about 8-11 hours after the failure. Along the Carolina and Florida coast with a complete rapid detachment of the landslide, initial modeling suggested 25-30 m of water will come ashore. (http://www.geo.arizona.edu/~andyf/LaPalma/doomsday.html)

With time, additional analyses and additional capabilities, these numbers have changed significantly. In the most recent studies (Harris, Grilli et.al. 2013) the times are shorter and the water height smaller. We are now at 6.9 hours and about 3.25 meters in Florida.

Cumbre Vieja has erupted several times in the recent past, in 1585, 1646, 1677, 1712, 1949 and 1971. The magma source for this volcano is deep and there has been only relatively minor seismic activity on the island. The next eruptions may tell the tale. If they are relatively low energy eruptions through existing vents, the opportunity for a flank collapse is low but the opportunity for gathering more information is high. If the next event is significant, keep the boats handy if you are a US East Coaster.