This week, the week of 7-13 March, 2019, marks the eighth anniversary of one of the largest, most destructive and potentially far-reaching earthquakes of the twenty-first century. That’s one of the items that kept coming up on my news feed this week, but there were plenty of others. We’re also looking at how an ancient fault reactivated and caused a deadly earthquake in Italy and, on a different note, the risks that climate change may pose to a rare and precious ecosystem.
A Sobering Anniversary: Tohoku-oki 2011
On 11 March 2011, the southern end of the Japan Trench, where the Pacific plate subducts beneath the Okhotsk section of the North American plate, was the location of a major, and significant earthquake.
At M9.1, the earthquake was jointly the largest of the century so far with the Boxing Day tremor of 2004. The total casualties (the NOAA gives the figures as: “15,890 deaths and 2,590 missing and presumed deaths; and 6,152 injuries in 12 Japanese prefectures. The tsunami also caused one death in Papua, Indonesia and one death in Klamath River, California”), though considerable, were far less than they might have been in an area less well prepared.
Other stats about the earthquake are no less alarming. Maximum wave height at sea was recorded as around nine metres and the waves reached land almost 40m above sea level, penetrating several kilometres inland. The event is perhaps most notorious for the inundation of parts of the Fukushima nuclear power plant.
Perhaps a key thing to take from this devastating earthquake was its unpredictability. Major earthquakes aren’t unexpected and Fukushima had been planned with the possibility in mind. The World Nuclear Association reports that the site was built to resist tsunami heights of 5.7 metres. In the event, those that arrived were almost three times higher.
The 2011 earthquake wasn’t just larger than its predecessors: it also ruptured in a different way, affecting an area 400km along the trench and 200m to landward. It’s an illustration of how much more we have to learn about earthquake processes in order to plan for them.
If there’s any comfort it’s that the recurrence interval for earthquakes (the average time between them) of this magnitude in this area is of the order of 1000 years. But the Tohoku-oki earthquake may well have lessons for elsewhere.
Old Faults Resurfacing: The Mount Vettore Earthquake
Another recent and damaging earthquake is on that struck in central Italy in 2016. Though not large in global terms, at M6.2 it was large for its location, and it killed 299 people.
The earthquake occurred on the Mount Vettore fault, one of very many that are associated with the macro-scale tectonics of the closure of the Mediterranean basin and the uplift of the Apennine Mountains. Crucially, though the fault was known, seismologists considered it to be dormant, or inactive, and therefore not a significant threat.
The earthquake changed all that, and prompted a reassessment of the seismic risk to the area. This week a new study was published which looks closely at much earlier earthquake activity on the fault using a technique called palaeoseismology — identification of the dates and magnitudes of previous earthquakes through geological techniques and thereby producing an estimate of recurrence intervals and refining the risk of a future event.
Paleoseismology is important because the very large earthquakes often have recurrence intervals that extend beyond written or oral records — and so, in both this case and in the case of 2011, the fact that we know of no major earthquake does not mean that it will never happen or that it has never occurred.
The conclusions of the study were that there have been “six surface faulting events (including the 2016 one) which occurred in the past 9 kyr, with an average return period of 1.8 ± 0.3 kyr, for Mw ≥ 6.6 earthquakes”. And, to reinforce the value of paleoseismology, it adds that: “Since the largest earthquakes that threaten Italy, and many other region of the globe, recur hundreds or thousands of year apart, only paleoseismology can provide long‐term rates of occurrence needed by the national map that forecast seismic shaking at various probability levels”.
Climate Change and Arctic Ecosystems
It’s always been self-evident that a change in climate will affect (usually but not necessarily adversely) the more marginal ecosystems. Flora and fauna which are intolerant of forms might benefit from a warmer world, with their ranges (the territory they occupy) expanding. At the other end of the scale, those which don’t like too much heat may be lost.
Obviously this is an oversimplification and Earth has many habitats whose environmental requirements are highly specific and sensitive to change. This week my eye was caught by a report on change in the Arctic, a region where “Temperatures are rising faster … than any other place on Earth” and where a range of plants “have adapted to survive in a climate where the annual average temperature is below zero, where the soil is permanently frozen, and there is total darkness for up to six months of the year”.
The darkness will remain, of course, but changes in temperature, the thawing of the permafrost with associated water logging, and an increase in temperature will promote the spread of plants more usually found further south. “It is likely that the unique and diverse Arctic tundra landscape will change to a more uniform vegetation, dominated by shrubs.”
Loss of vegetation not only means the possible extinction of certain plants, but also increases the vulnerability of other species that depend upon them. The report covered a single example in the Arctic — but there will be others worldwide.