Earthquake: what are the risks for Cayman ?

 When it comes to natural disaster risks, people in the Cayman Islands tend to think mainly of hurricanes.  This is natural, considering the territory’s historical frequency of close encounters with hurricanes, especially since 2004.
   Most residents however don’t realise that the Cayman Islands lies very close to a very active fault line, making earthquakes a risk as well.
   For a long time there have been no strong and devastating earthquakes around in the Cayman Islands region, so there seemed no reason to give one of Mother Nature’s most feared disasters much thought.
   A 6.8 magnitude earthquake in December 2004 rattled nerves already on edge after the devastation left by Hurricane Ivan just three months earlier.  Because the earthquake only caused minor damage, and because everyone was concentrating on rebuilding after Ivan, there was little thought given to Cayman’s earthquake preparedness.
   That all changed after 12 January 2010 when a 7.0 magnitude earthquake in Haiti killed an estimated 230,000 people, injured some 300,000 more, and made a million people homeless in one of the 10 deadliest natural disasters in the history of the world.
   A week later, the Cayman Islands experienced a 5.9 magnitude earthquake.  Although it was widely felt on Grand Cayman, it caused only minor damage.  Since that time, however, there have been an additional six tremors in the vicinity of Grand Cayman, four of which have occurred in April.
   There was also an 8.8 magnitude earthquake in Chile that killed almost 500 people on 27 February and in mid-April a 7.1 magnitude earthquake in China killed more than 2,000 people.
   Although some religious fundamentalists believe the increase in earthquakes as a sign of the coming apocalypse, earthquakes are quite common and always have been.  The United States Geological Survey, the American government organisation that monitors earthquakes and earthquake risk, states on its website that there is a 100 per cent chance of an earthquake today.
   “Though millions of persons may never experience an earthquake, they are very common occurrences on this planet,” the USGS states. “So today – somewhere – an earthquake will occur.”
   Earthquakes are so common, in fact, that several million of them occur annually and thousands every day.
   The USGS website lists earthquakes in the world over 2.5 magnitude and there are usually dozens of such quakes every day.  The USGS estimates that about 700 earthquakes a year have the potential of causing property damage, death and injury.
   “But fortunately, most of these potentially destructive earthquakes centre in unpopulated areas far from civilization,” the USGS states.
   All of these facts would be of little comfort to those who lost friends or loved ones in earthquakes in Haiti, Chile or China this year. They would also be of little comfort if the Cayman Islands were to feel the brunt of a big earthquake. The big questions are: could that happen here and if it did, how prepared is the Cayman Islands?
   
   Earthquake risk
   Grand Cayman lies just to the north of the Oriente Fracture Zone. The active fault line runs along south-east coast of Cuba to an area just west of Cayman, roughly following the northern edge of the Cayman Trough.
   Over the course of recorded history, there have been a number of powerful earthquakes along the Oriente Fracture Zone, but only the ones near Santiago de Cuba are known to have caused severe damage and deaths.
   The deadly earthquakes near Santiago, Cuba’s second largest city, have occurred on a relatively regular basis. In fact, over the last 432 years, an earthquake of 6.75 magnitude or higher has hit the area once every 80 to 100 years.
    Earthquakes estimated at 6.8 magnitude hit Santiago in 1578 and then again in 1678.  In 1766 an estimated 7.6 magnitude earthquake occurred there; 86 years later in 1852, Santiago was affected by an estimated 7.2 magnitude earthquake; 80 years later, in 1932, an estimated 6.75 magnitude earthquake hit the city, killing up to 1,500 people.
   It’s now been 78 years since the last big earthquake in the Santiago area. In 1999, Cuban scientist Leonardo Alvarez, along with three other scientists, wrote a paper for the United Nations Educational Scientific and Cultural Organisation in which they said “there exists a high probability of occurrence of a [magnitude 7.0] earthquake in the near future on the Oriente Fault system, located immediately south of [Santiago de Cuba].
   United States Geological Survey scientist Uri ten Brink acknowledged the large earthquakes along the Oriente fault in southern Cuba, but he couldn’t say what that could mean for the western edge of fault, near Cayman.
   “I am not sure anyone knows the seismic potential of the [Oriente] segment farther west, closer to the [Cayman] islands,” he said.
   But ten Brink said the relatively high occurrence of earthquakes near the Cayman Islands recently isn’t necessarily a prelude to a large earthquake.
   “Some big earthquakes are preceded by foreshocks, but most [aren’t],” he said.
   Since the recent flurry of tremblers in the Cayman area have all occurred after the 12 January earthquake in Haiti, some scientists have suggested a link.  But ten Brink said a link was not likely.
   “The Haiti earthquake wasn’t that big,” he said, referring to its strength.
   In addition, the earthquake in Haiti occurred along the Enriquillo-Plantain Garden fault, a fault that has its western end near Kingston, Jamaica. Canadian geologist Murrray Roed, author of the book Islands of the Sea: Geologic Stories of the Cayman Islands, also noted that the Cayman Islands belonged to the much more stable North American tectonic, not the Caribbean tectonic plate that includes Jamaica and Haiti.

Stronger buildings
   Building codes have a lot to do with how any particular place can handle a strong earthquake.
   Even though Chile’s 8.8-magnitude earthquake was about 500 times stronger than Haiti’s 7.0-magnitude trembler, the Haitian natural disaster killed about 250 times as many people. Much of that difference is attributable to stronger building codes in Chile, where earthquakes are a much more frequent.
   David Champoux, a senior structural engineer with Cayman’s Apec Consulting Engineers Ltd wrote in a report recently that the earthquakes in Haiti and Chile “illustrate the dramatic differences between well-designed and constructed buildings versus poorly constructed buildings.
   “Much like the crumple zones incorporated into automobile design to absorb energy in the event of a head-on collision, the engineer aims to detail joists and reinforced walls which are capable of resisting energy while preventing building collapse,” he said.
   Champoux said the structural parameters adopted by the Cayman Islands Building Control Department are designed to withstand a once-in-475 year earthquake.
   Based on statistical information obtained by Apec from the US-based ABS Consulting Engineers, a 6.8 magnitude earthquake like what occurred near Cayman in 2004 has an average frequency of occurrence of 440 years.  The acceleration forces of that earthquake only reached 75 per cent of the design accelerations required by the current Cayman Islands Building Code.
   But Champoux also pointed out that strong building codes only prescribe seismic design criteria and structural requirements with the intention of life safety.
   “Buildings would not necessarily be expected to remain functional or serviceable after a major earthquake,” he said. “Typically, it would be cost prohibitive and impractical to construct a building that could resist [a rare great earthquake] without any structural damage. Similarly, it would not be economical or aesthetically pleasing to build a car to withstand a 60-mile-per-hour head-on collision without damage.”
   A 6.7 magnitude Northridge earthquake, almost as strong as the earthquake Cayman experienced in 2004, caused serious damage and killed 57 people near Los Angeles in 1994. Besides the large difference in population density, there is another reason Cayman emerged relatively unscathed in the 2004 earthquake and others since.
   “The big difference was that our earthquakes were centred offshore,” Champoux said. “This makes a big difference, just like a hurricane passing south of us would not cause the same level of damage as a direct hit.”
   Structural engineer Ian Washbrook, the principal of Halcrow Yolles International Engineers Ltd. here, said it was a tough question to say how earthquake-resistant buildings were the Cayman Islands.
   “In most cases, unless excluded from the building code and the Building Control Unit, buildings need to be designed by a professionally qualified architect, professionally qualified engineer, or both,” he said.  “Have all buildings been designed this way? Unfortunately, no.  Luckily, most designers and contractors understand that when it comes to masonry and concrete structures, reinforcement is good thing.  Part of this is coming from the fact that designing a hurricane-resistant building somewhat kills two birds with one stone and assists in the strength required for earthquakes.” 
   Washbrook pointed out that Cayman does not have high-rise buildings, large span buildings, or buildings that require complex transfer systems.
   “Generally speaking though, the larger buildings designed by experienced engineers… and built by experienced contractors on the Island will perform as per the code,” he said. “Experienced professional engineers understand lateral systems and detailing requirements.”   
   Champoux said since the January earthquake in Haiti and the seven subsequent tremors in Cayman, clients have expressed greater concern about the structural soundness of their existing and planned buildings, and that his firm has done a number of reports on specific buildings.  The reports, however, have not led to other actions because they “confirmed their buildings were adequate as designed to safely withstand a major earthquake event,” Champoux said.
   Washbrook believes there is always room to improve the standards and procedures when it comes to the building code and earthquakes. In particular, he believes more attention needs to be paid to how soil types impact seismic forces.
   “Generally speaking, the softer the soil, the larger the amplification of the seismic forces,” he said.  “Therefore, in my opinion, geotechnical investigations prepared by a qualified geotechnical engineer should be made mandatory for the buildings that require a professional engineer to design the structure.”
   
   Tsunami risk
   Having an active fault located underwater about 25 miles away, does put the Cayman Islands at some risk to a tsunami. But ten Brink thinks the kind of faults near Cayman would most likely not cause a tsunami.
   “They are strike-slip faults, which do no cause large vertical offsets,” he said.”There was a 7.5 [magnitude earthquake] on the Swan Island fault… a few years ago that was not tsunamigenic. Having said that, there can be large tsunamis [in Cayman] for two reasons: the earthquake triggers a submarine landslide; [or] the fault geometry is not totally smooth and there may be areas where it causes uplift or subsidence of the sea floor.”
   Former head of the University of West Indies Earthquake Unit Margaret Wiggins-Granderson also thinks a tsunami being generated by either of the faults closest to Cayman is unlikely. In a paper titled ‘Tsunamis and Jamaica’ Wiggins-Granderson explained that the typical crustal movement of the Oriente Fracture Zone is transcurrent, which means it creates horizontal stress not vertical stress. The crustal movement of a another feature south of Grand Cayman called the Cayman Spreading Centre is divergent, meaning two tectonic plates  are moving away from each other.
    “In the present state of motion, which has been the case for the past 10 million years at least, the Oriente Fracture Zone and the Cayman Rise should not generate tsunamigenic earthquakes,” she wrote.
   Wiggins-Granderson also pointed out that tsunamis are far rarer in the Atlantic Ocean than in the Pacific Ocean and that the areas most likely to cause a tsunami in the Atlantic Basin are in the regions  north of Hispaniola, north of Puerto Rico and east of the Lesser Antilles, all relatively far from the Cayman Islands.
   “Earthquakes capable of generating tsunamis do happen along this part of the Caribbean plate margin,” she wrote. “But the consensus among earth scientists, which is based on historical occurrences and current plate motions and seismicity, is that the risk of a [Indonesian tsunami-type] event is low.
   History also indicates the rather low risk of tsunamis. The University of West Indies Seismic Research Unit states on its website that over the past 500 years, there have only been 10 confirmed earthquake generated tsunamis in the Caribbean Basin, which killed an estimated 350 people in total.
   Another factor in generating tsunamis is the strength of the earthquake. Although any underwater earthquake at 6.5 magnitude or more could trigger a tsunami, the most destructive tsunamis have occurred with earthquakes that were 7.5 magnitude or more, a much rarer occurrence.  Wiggins-Granderson said that the maximum credible earthquake along the Caribbean plate boundary is thought to be of magnitude 7.5, based on her estimates and those of other Caribbean region geo-scientists.
   Another factor that would help protect Cayman from a tsunami even if there were one is its bathymetry.  A report produced by the National Disasters Assessment Consulting Group for the Cayman Islands Government in 2009 pointed out that the Islands do not have a continental shelf that shallows gradually toward the coastline, which amplifies tsunami waves.
   “An analogy to the Cayman Islands is the effect observed on Diego Garcia Island in the Indian Ocean during the tsunami of December 26, 2004,” the report stated. “Although the island was on the path of the incoming tsunami waves, the island was essentially unaffected…. Nonetheless, the coast of Africa, downstream from Diego Garcia, suffered considerable damage due to the large tsunami wave generated along the coast. The reason for this is that the island of Diego Garcia is a dormant volcano that rises sharply from the ocean floor. The lack of a sloping continental shelf did not generate the large tsunami waves observed on the continental shorelines.”

1
0

NO COMMENTS