Uncommon disasters

I remember viewing a TV program, many years ago now, that was basically a neat exploration of worst-case scenarios for natural disasters in the US. This program—I recall it being aired around 1990 on cable television, but not the name—was really a lesson in geology and geography as much as anything else, though it did employ some mild scare tactics (after all, it’s television). Scare tactics or no, however, two of the five featured disasters have come to pass: a direct hit to New Orleans by a powerful hurricane (2005) and a powerful earthquake in southern California (1994). A third disaster, the 1989 earthquake in the San Francisco Bay area, is what prompted the creation of the program. Disaster scenarios number four and five have not occurred. What were they?



One involves the Seattle-Tacoma metropolitan area and its rapid development in proximity to Mount Rainier. Rainier is a 14,000-foot volcano that last erupted in 1854. The US Geological Survey claims that “During the past few thousand years, lahars that spanned valley floors well into the now densely populated Puget lowland have recurred, on average, at least every 500 to 1,000 years. There is every reason to expect that future lahars from Mount Rainier will be similar in behavior and frequency of occurrence to past lahars.” If this is so, then several communities in the greater metropolitan area would be in the direct path of a lahar. Some communities, such as Orting, South Hill, Prairie Ridge, and Graham, would almost certainly be affected, even by a moderate event. An event of greater magnitude could possibly reach as far as the Puget Sound coastline itself, an occurrence for which there is good geological evidence. According to the 2000 census, the Seattle-Tacoma metropolitan area had over 3.5 million people, and while ‘The Mountain’ is 50 miles or so from the area, geological evidence of prior lahar events demonstrate that many folks in the greater Seattle area have a false sense of security. After all, the 1980 eruption of Mount St. Helens triggered a lahar that covered a distance as great, but did so in an area that was, relatively, sparsely populated.

The other disaster touched upon in the old TV program concerns a repeat of the 1812 earthquakes that struck the Midwestern US, known as the New Madrid earthquake sequence. The greatest earthquake of this sequence was estimated to be the most powerful ever the strike the continental US in recorded history, registering an estimated 8.0 on the Richter scale. Ground liquefaction and deformation created lakes and changed the course of the Mississippi River, including the creation of temporary water falls and a short-term reversal of the directional flow of the river itself. In 1812, the affected area was essentially frontier land, very sparsely populated, so first-hand accounts are few but very interesting (particularly those of river pilots confused by the changes along the river). Imagining a similar event today, however, the cities of St. Louis and Memphis would likely endure great damage, if not from the severity of the seismic jolt, then from potential flooding events, particularly given the elaborate system of levees and locks in place. Also, given the efficiency with which rock layers east of the Mississippi conduct seismic shocks, it is possible that a similarly great event could effect cities not generally believed to be at risk, cities such as Louisville, Nashville, Indianapolis, and possibly Cincinnati. The likelihood of such a repeat occurrence is not so farfetched, particularly when you consider that the lower Mississippi valley itself is the product of an old rift zone that still exhibits some geologic activity (such as the seismic zones along the river). The majority of the seismic activity in this area tends to occur along and near the river’s path, from just north of Memphis up to the confluence of the Mississippi and Ohio Rivers…between two large concentrations of population and infrastructure.

Most folks are well aware of potentially devastating earthquakes in California, but the risk to Seattle and Memphis due to equally dangerous geologic events is almost as great, if not as well known.