This week, the week of 28 March-3 April 2019, I’m going to be following (very loosely) the theme of natural disasters. Specifically, I have two earthquake-related pieces and one on the damage caused by Hurricane Harvey back in 2017.
Alaska Earthquake: M6.5
The biggest earthquake this week was a magnitude 6.5 event in Alaska. Normally I would skirt over this but as last week happened to mark the 55th anniversary of the second largest earthquake on record (in Alaska) I thought I would link the two.
I’m the first to admit that the link is tenuous and that there are as many differences as there are similarities, but we can learn from difference. At M9.2, the 1964 earthquake was over 500 times larger than this week’s M6.5 and released a mind-blowing 11,220 times as much energy, which places the week’s earthquake in perspective.
Speaking of perspective, it does no harm to remember that Alaska is so large that earthquakes within the state need not be close in geographical terms and the distance between those of April 2019, in the Aleutian Islands, and March 1964, in Prince William Sound, is staggering — over 5,500km.
Nevertheless, the two share a common tectonic setting, both resulting from the northwards movement of the Pacific plate and its subduction beneath North America. The situation in the west (this week) is relatively straightforward, with the crust being subducted along an ocean trench, while that in the west (1964) is complicated by its relationship to the North American continent and the adjacent lateral faults of the Denali and Queen Charlotte faults.
The 1964 earthquake, while nowhere near as destructive as it could have been, nevertheless cost 131 lives, the majority of them as a result of local and distant (as far away as California) tsunami waves. “There was significant damage covered about 130,000 square kilometers. The area in which it was felt was about 1,300,000 square kilometers (all of Alaska, parts of Canada, and south to Washington). The four minute duration of shaking triggered many landslides and avalanches. Major structural damage occurred in many of the major cities in Alaska.”
The anniversary of the 1964 ‘quake is a regular reminder of the potential for a huge and destructive earthquakes in Alaska. The remoteness of the state means that the impacts are limited — the M6.5 of this week caused no reported damage whereas a tremor of a similar size in, say, LA, could cause significant death and destruction.
Earthquake Early Warnings: How Residents Respond
In remote areas, large earthquakes cause relatively little damage but in others — areas with dense populations — even a relatively small tremor can have huge consequences. This week, a new piece of research from Japan caught my eye. This focussed not on the physical aspects of an earthquake but on human responses.
Although accurate earthquake prediction remains beyond us, sensitive scientific instruments allow the authorities to issue an earthquake early warning (EEW) once an earthquake has occurred. In reality the time available is very limited — “usually only a matter of seconds…” but “In some situations…even … up to 1–2 minutes of warning”.
This doesn’t give much time for residents to escape but it is enough to allow them to take some kind of avoiding action, such as stop machinery, follow earthquake preparedness drills and so on. The fascinating thing about this research is that in the two case studies it covers, the response of the 749 people who were interviewed was not what we might expect.
The study indicated that, rather than use precious seconds to protect themselves physically, residents used the time to prepare themselves mentally for what they knew was about to happen (remember that these warnings relate to an earthquake that has already taken place so there’s no concern of a false alarm) or to look for further information.
The results are perhaps not what we might expect, and the report’s conclusions describe them as “pessimistic” yet the overall conclusion is positive: “they provide researchers and practitioners with important information that can be used to improve EEW messaging, outreach, education, and training, which could help mitigate injury, loss of life, and damage from earthquakes”.
Hurricane Harvey: Assessing the Damage
Hurricane Harvey ripped through the Caribbean in 2017 before hitting Texas, becoming the first hurricane of at least Category 3 to make landfall in the mainland US since 2005, killing at least 88 people and causing over $125 billion dollars of damage according to the National Hurricane Center. Over 200,000 homes were damaged and such was the extent of the storm that many were in areas where structural damage wasn’t expected.
Understanding where, how and to what degree storm damage occurs can be crucial to mitigating the effects of future events. This week I came across an article (in Earth magazine) by the team who were tasked by the USGS with carrying out an assessment of the damage Harvey caused to coastal communities in Texas.
Despite the low-lying nature of the landscape and the extensive floods, the team found that most of the damage was caused by wind rather than water and was sporadic. Although the storm lessened from Category 4 to Category 2, within each of these areas some structures were almost untouched and others severely damaged.
The team identified various factors which influenced the extent of the damage, including the extent of vegetation: “…thick vegetation, such as trees and shrubs, provided buildings and homes with a degree of protection from hurricane-force winds by acting as windbreaks. Damage to structures surrounded by thick vegetation was considerably less compared to structures in the same community that were not surrounded by such barriers”.
The findings of the study might not be much help to those whose property was damaged by Harvey, but there are lessons that can be learned for the future in terms of minimising the damage that future storms might cause to coastal communities.