Earthquakes in USVI
The tragedy in Haiti got me to wondering about earthquakes and the USVI. Hurricanes, and hurricane preparedness are things I would associate with life on island, but what about earthquakes? One of the things I've heard contributed to the devastation in Haiti was the poor building quality. I know from looking at real estate listings that many homes are built into hillsides so the question arises do the building codes on the USVI address the possibility of earthquakes as well as hurricanes?
BTW, did anyone feel the quake on January 31st? I'm assuming if you did that it would have been mentioned on the board, or maybe not. I only know about it because I went onto the USGS web site to see when the last quake was. Here is a link if you want to see for yourself:
During the coverage of Haiti, they would show destroyed buildings. I didn't see much/any rebar in the crumbled concrete. I'm no expert, but I believe that reinforced concrete helps a lot. Although, I think a 7.0 quake would be REALLY tough to protect against.
I didn't see any coverage of the hills surrounding the city, so I couldn't tell if there were landslides.
There will be a lot to learn from that quake...
CA and VI quakes feel different -- twisting earth in CA versus shaking in the VI. I've owned homes in both places, on a fault in both cases, and am unconcerned, as is my VI house insurer which covers earthquake damage. You can't compare the VI, which actually has building codes, to Haiti, where anything goes.
Oy! In my desire to escape snow I'd ruled out CA in large part because of wild fires, mud slides, and... earthquakes. Thinking hurricanes were the worst of it, the USVI were looking really good. Being a New Yorker I'm used to Nor'easters, but believe the only acceptable ground shaking is when a subway train passes underfoot! I didn't feel so bad either after reading the first few replies, but the last two! I went to the second link provided by Bombi, and what STXBob says is true. I had no idea.
If it hadn't snowed here last night. And if snow wasn't expected again tomorrow. And if we weren't only about half way through Winter I would probably quit this board.
Thanks for the info.
After the Haiti quake, the local paper (Avis, I think) published an article with an expert saying that a "big one" could hit the VI any time. We get them about every 200 years, and the last one was 1867.
Geologically speaking you cannot lump St. Croix in with the northern islands, the seismic hazard risk is much lower than St. Thomas and the BVIs. The seismic zone of immediate concern to St. Croix is the Anegada Passage off our northern shore, which has been modeled to produce a magnitude 6.5 or greater every 300 years. Unlike Hati, where the fault-zone extends onshore, there is virtually no chance of a large magnitude shallow quake epicentered on St. Croix. The destructive power of an earthquake is based upon the peak acceleration of the ground during the event, this ground motion quickly loses intensity with distance from the focus. On Haiti even with their poor construction codes, moderate damage to structures (cracked walls, etc.) extended about 50 miles from the epicenter. So it would be very improbable for a large quake in the Anegada Passage (with a typically deep focus) to recreate the type of devastation that occurred on Haiti here on St. Croix.
The reason why the 1867 earthquake was so devastating to the island was most likely not from the shaking, but instead from the accompanying tsunami, and a repeat of a tsunami is also an unlikely event. The Anegada Passage is a transtensional fault-zone which means it transfers energy between the Muertos Trough, south of PR to the Puerto Rican Trench, north of STT and thus does not create large vertical displacements of the crust like an extensional or compressional fault zone. To generate a large and devastating tsunami you need a vertical displacement of the water column. The most likely cause of the 1867 tsunami was not the quake itself but a large underwater landslide within the Anegada passage, this has been argued from the results of side scanning sonar that has mapped the bathymetery of the basin.
To summarize, if you want to minimize any potential loss of property or life from earthquake hazards in the USVI, live in a modern one story structure on St. Croix, above 80ft of elevation, on the South Eastern quarter of the island (any large tsunami will be generated from the N or W). If your home fits this description I'll gladly take your money and write you an insurance policy for earthquake damages.
Thanks for that summary AllMashUp... very timely for me. I studied the article that Bombi referenced. It got me thinking about hanging out on beaches of STX. Would I have any warning of a tsunami, I wondered? Especially if the probable earthquake loci are so close. A little more googling around has got me thinking:
- do all tsunamis produce receding seas before the tsunami arrives (what causes the receding seas)?
- if not, what is the probability that we'd have a tsunami that does not first have receding seas (what types of earthquakes produce tsunamis without initial receding seas)?
- with initial receding seas, how much time would you have to get to higher ground (is there a correlation with size of tsunami and amount of receding; given our distance from probable loci, would we have minutes or seconds)?
Any thoughts on this, or where to find the answers?
Good questions, a tsunami is a wave and like all waves they have a troughs and crests, the question becomes which of the two is generated first, since that will be the leading edge of the tsunami. Most tsunami's are generated by the vertical displacement of the ocean floor resulting from a thrust fault where ocean floor rises on one side of the fault (hanging wall) while dropping on the other (foot wall). This generates an initial positive wave (a crest) on the hanging wall side while a negative wave forms first on the foot wall, and the wave then propagates out from the origin. In the case of the Sumatra Tsunami of '04 you had the receding seas portelling the tsunami on the Indonesian coast, while on the Sri Lankan, Indian and African coast there was no receding ocean since the positive crest hit first. The visulalization is shown in the below link. You can stop the animation by hitting the stop button on your broweser, try refreshing the page and then stopping immidiately to see the first crests and troughs generated.
That the website is linked from here, which has a collection of different tsunami animations.
When you have an earthquake that generates from an underwater landslide, I would imagine that since your taking the ocean floor under relatively shallow water and dropping it down into a deeper canyon, you are producing a "hole" of water on the ocean surface above the , thus a negative trough in all directions to begin with, kinda like the opposite of dropping a stone in a pool where you are pushing the water out in all directions to begin with, thus creating a positive crest in all directions.
To summarize, most likely "thrust fault" tsunami you have a 50/50 chance of seeing a receding ocean, an underwater water landslide probably a greater chance (though I haven't seen any wave models to substantiate this claim), Meteor impact tsunami no chance.
The time you have once you do see a receding wave is dependent alot upon the nature of the ocean floor along the coast, as well as the distance that the wave has traveled from the origin. In the deep ocean a tsunami only has a vertical profile of a few meters and is travelling upwards to 500mph, as it travels towards a landmass the shallower water depth forces the energy in the wave up increasing its amplitude and shortening its period as well as slowing the wave due to greater friction. I honestly don't know what type of coastal configuration produces waves of the longest period, but I have read that the it can be anywhere between 10 minutes and two hours between subsequent waves.
If it was me and I was laying on a beach and saw the sea sucked out with no warning I'd try to climb up rather then run.
Regarding earthquake insurance,
don`t use any insurer who uses Crawford VI for claims adjusting(don`t pay claims).
I also have a friend who used to work for them & quit as they thought it was disgusting how they were treating the insured.
I would suspect hurricane would be treated the same way but I can`t say firsthand they would(I CAN say firsthand they don`t pay for earthquake damage even if you are insured).
Yes, I have felt tremors here many times over the years on STX.
To anwer question in the orginal post: didn't feel the January 31 tremor, but January 22 one around dawn woke me up, north shore of STX.
Verified it later in the day on the USGS earthquake site, so it was not a truck or my imagination. It was pin drop quiet and the bed shook.